@article{le_kim_kim_aspnes_2024, title={Deconvolution of the biexciton structure of monolayer MoSe<sub>2</sub> in spectroscopic ellipsometric data: a comparison of maximum-entropy methods}, volume={6}, ISSN={["1976-8524"]}, DOI={10.1007/s40042-024-01123-6}, journal={JOURNAL OF THE KOREAN PHYSICAL SOCIETY}, author={Le, Long V. and Kim, Tae Jung and Kim, Young Dong and Aspnes, D. E.}, year={2024}, month={Jun} }
 @article{le_deijkers_kim_wadley_aspnes_2023, title={Noise reduction and peak detection in x-ray diffraction data by linear and nonlinear methods}, volume={41}, ISSN={["2166-2754"]}, url={https://doi.org/10.1116/6.0002526}, DOI={10.1116/6.0002526}, abstractNote={Considerable progress has been made in the last few years in removing white noise from visible–near-ultraviolet (UV/VIS) spectra while leaving information intact. For x-ray diffraction, the challenges are different: detecting and locating peaks rather than line shape analysis. Here, we investigate possibilities of state-of-the-art UV/VIS methods for noise reduction, peak detection, and peak location applied to x-ray diffraction data, in this case, data for a ZrO2 −33 mol. % TaO4 ceramic. The same advantages seen in UV/VIS spectroscopy are found here as well.}, number={4}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B}, author={Le, Long V. V. and Deijkers, Jeroen A. A. and Kim, Young D. D. and Wadley, Haydn N. G. and Aspnes, David E. E.}, year={2023}, month={Jul} }
 @article{gao_aspnes_franzen_2022, title={Classical Model of Surface Enhanced Infrared Absorption (SEIRA) Spectroscopy}, volume={1}, ISSN={["1520-5215"]}, DOI={10.1021/acs.jpca.1c08463}, abstractNote={The molecule-plasmon interaction is the key to the mechanisms of surface enhanced infrared absorption (SEIRA) and surface enhanced Raman scattering (SERS). Since plasmons are well described by Maxwell's equations, one fundamental treatment involves the classical interpretation of infrared absorption and resonance Raman spectroscopies. We can understand the molecule-plasmon interaction using electromagnetic theory if the classical field effect on a transition dipole moment or transition polarizability is properly described. In previous work, we derived the Raman excitation profile of a model molecule using a classical driven spring attached to a charged mass with a perturbative force constant due to vibrational oscillations. In this study we generalize the interactions of plasmons with molecules by considering the N2O asymmetric stretch SEIRA signal on a Dy doped CdO (CdO:Dy) film. This semiconductor has tunable plasmon dispersion curves throughout the near-and mid-infrared that can interact directly with vibrational absorption transitions. We have demonstrated this using the Kretschmann configuration with a CaF2 prism and a MgO substrate. The model predicts the phase behavior of SEIRA. The calculated enhancement factor relative to an Au control is 6.2, in good agreement with the value of 6.8 ± 0.5 measured under the same conditions.}, journal={JOURNAL OF PHYSICAL CHEMISTRY A}, author={Gao, Yuan and Aspnes, D. E. and Franzen, Stefan}, year={2022}, month={Jan} }
 @article{le_kim_kim_aspnes_2022, title={Decoding 'Maximum Entropy' Deconvolution}, volume={24}, ISSN={["1099-4300"]}, DOI={10.3390/e24091238}, abstractNote={For over five decades, the mathematical procedure termed “maximum entropy” (M-E) has been used to deconvolve structure in spectra, optical and otherwise, although quantitative measures of performance remain unknown. Here, we examine this procedure analytically for the lowest two orders for a Lorentzian feature, obtaining expressions for the amount of sharpening and identifying how spurious structures appear. Illustrative examples are provided. These results enhance the utility of this widely used deconvolution approach to spectral analysis.}, number={9}, journal={ENTROPY}, author={Le, Long V and Kim, Tae Jung and Kim, Young Dong and Aspnes, David E.}, year={2022}, month={Sep} }
 @article{le_kim_kim_aspnes_2022, title={Detection of the Biexciton of Monolayer WS2 in Ellipsometric Data: A Maximum-Entropy Success}, volume={10}, ISSN={["1521-3951"]}, DOI={10.1002/pssb.202200271}, abstractNote={For nearly 50 years, the Burg/Andersen (BA) maximum‐entropy (ME) deconvolution procedure has been used to sharpen, and therefore accentuate, weak features in spectra, optical and otherwise. It is shown that the BA procedure can be further enhanced to yield even sharper features. Using this approach, the authors establish the existence of the biexciton in ellipsometric data of monolayer WS2 at 50 K, whereas differentiation and standard BA analysis cannot.}, journal={PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS}, author={Le, Long V and Kim, Tae Jung and Kim, Young Dong and Aspnes, David E.}, year={2022}, month={Oct} }
 @article{le_kim_aspnes_2022, title={Eliminating noise from spectra by linear and nonlinear methods}, volume={761}, ISSN={["1879-2731"]}, DOI={10.1016/j.tsf.2022.139515}, abstractNote={Eliminating noise from spectra has been a goal in spectroscopy from its beginning. With recent advances, the goal of removing white noise completely from spectra with no deleterious side effects is now within sight. This review provides necessary background and summarizes the current state of the art.}, journal={THIN SOLID FILMS}, author={Le, L. V. and Kim, Y. D. and Aspnes, D. E.}, year={2022}, month={Nov} }
 @article{le_kim_aspnes_2022, title={Eliminating white noise in spectra: A generalized maximum-entropy approach}, volume={132}, ISSN={["1089-7550"]}, url={https://doi.org/10.1063/5.0100140}, DOI={10.1063/5.0100140}, abstractNote={In linear filtering, high-frequency (white) noise is reduced by apodization, which is the attenuation or elimination of high-order Fourier coefficients followed by an inverse transformation. Unfortunately, apodization requires compromises to be made among noise leakage, information loss, and Gibbs oscillations. These shortcomings are avoided with the corrected maximum-entropy (CME) procedure, but this procedure applies only to Lorentzian or approximately Lorentzian features. We develop a generalized maximum-entropy method based on partial Hilbert transforms that allows CME to be applied to any spectrum, thereby eliminating white-noise completely with no deleterious side effects. As Hilbert transforms are exact Kramers–Kronig replicas of the original endpoint-discontinuity-corrected segment, new spectral processing opportunities are also realized.}, number={7}, journal={JOURNAL OF APPLIED PHYSICS}, author={Le, L. V. and Kim, Y. D. and Aspnes, D. E.}, year={2022}, month={Aug} }
 @article{le_kim_kim_aspnes_2022, title={Reducing or eliminating noise in ellipsometric spectra}, volume={8}, ISSN={["1976-8524"]}, DOI={10.1007/s40042-022-00554-3}, journal={JOURNAL OF THE KOREAN PHYSICAL SOCIETY}, author={Le, Long V. and Kim, Tae Jung and Kim, Young Dong and Aspnes, David E.}, year={2022}, month={Aug} }
 @article{le_kim_kim_aspnes_2021, title={Maximum-entropy revisited: Optimal filtering of spectra}, volume={129}, ISSN={["1089-7550"]}, url={https://doi.org/10.1063/5.0051602}, DOI={10.1063/5.0051602}, abstractNote={In spectroscopy, the objective is to obtain information by analyzing spectra that ideally are undistorted and noise-free. In standard Fourier-space filtering, this goal cannot be achieved because of apodization, which forces a trade-off among errors arising from distortion, noise leakage, and Gibbs oscillations. We show that low-order coefficients can be preserved and apodization, and its associated errors eliminated with the corrected maximum-entropy (M-E) filter obtained here. Although the Burg derivation begins as M-E, by making certain assumptions the Burg approach yields a procedure that deconvolves (sharpens) structure in spectra, thereby violating the basic M-E principle of leaving the low-order coefficients intact. The corrected solution preserves these data and projects the trends established by them into the white-noise region in a model-independent way, thereby eliminating apodization and its associated errors. For a single Lorentzian line, the corrected M-E approach has an exact analytic solution, which reveals not only how M-E performs its extension but also why it works particularly well for line shapes resulting from first-order decay processes. The corrected M-E filter is quantitatively superior to any previous filtering method, including recently proposed high-performance linear filters, yet requires only minimal computational effort. Examples, including multiple differentiation, are provided.}, number={22}, journal={JOURNAL OF APPLIED PHYSICS}, publisher={AIP Publishing}, author={Le, L. V. and Kim, T. J. and Kim, Y. D. and Aspnes, D. E.}, year={2021}, month={Jun} }
 @article{gao_aspnes_franzen_2020, title={Classical Correlation Model of Resonance Raman Spectroscopy}, volume={124}, ISSN={["1520-5215"]}, DOI={10.1021/acs.jpca.0c04401}, abstractNote={A classical correlation model (CCM), based on forces instead of potentials, is developed and applied to resonance Raman scattering to provide a foundation for further advances in understanding the effects of fields and vibronic perturbations on the optical properties of materials by a simple, yet versatile, description. The model consists of a charge connected by a classical spring to a surface and driven by an external electric field. The spring represents the charge cloud of the electrons and the transition strength, and the surface represents the nucleus or molecule. Molecular vibrations are assumed to be many-body effects that change the configuration and hence modify the spring constant directly, as opposed to all previous classical models of Raman scattering, and opposed to the anisotropic bond model (ABM) of nonlinear optics, by adding anharmonic terms to the potential. The resulting expression agrees exactly with quantum mechanical models of resonance Raman scattering in the limit of weak electron-phonon coupling, and it agrees well when the coupling becomes strong. The result is a classical derivation of Kramers-Heisenberg-Dirac scattering theory. We show that the difference between classical and quantum approaches lies only in the interpretation of the prefactor. In particular, the Raman excitation profile shows excellent agreement with all other methods of calculation. By comparing complementary classical and quantum solutions of the same complex system, understanding of both is enhanced.}, number={44}, journal={JOURNAL OF PHYSICAL CHEMISTRY A}, author={Gao, Y. and Aspnes, D. E. and Franzen, S.}, year={2020}, month={Nov}, pages={9177–9186} }
 @article{sachet_aspnes_maria_franzen_2020, title={Critical Test of the Interaction of Surface Plasmon Resonances with Molecular Vibrational Transitions}, volume={124}, ISSN={["1520-5215"]}, DOI={10.1021/acs.jpca.9b10835}, abstractNote={We determine the absorption spectra of a gas due to evanescent plasmonic electromagnetic fields in a system where surface interactions (physisorption and chemisorption) are demonstrably negligible. The plasmonic host material, degenerate semiconductor CdO:Dy, has high mobility (366 - 450 cm2/Vs) and carrier density (0.6 - 3.5 x 1020 cm-3) and therefore supports low-loss surface plasmon resonances in the mid-IR. This high mobility layer gives the highest resolution observed in a plasmonic conducting layer in the infrared, higher than gold and rivaling silver at optical frequencies in the resolution of spectral features relative to the plasmon energy. This high resolution permits new understanding of the nature of the interaction of emerging fields with molecular transitions. Using different carrier concentrations, the resonance condition of the surface plasmon polariton (SPP) frequency (ω_SPP) and N2O vibrational absorption spectral frequency (ω_(N_2 O)) can be controlled, thereby allowing a critical test of field-molecule interactions. Experiment and theory both indicate a dispersive N2O line shape for ω_SPPω_(N_2 O) and an abrupt change between the two when the resonance condition ω_SPP=ω_(N_2 O) is reached. A first-order expansion of the Airy equation describes this behavior analytically. The SPP surface enhancement is 6.8±0.5 on resonance, less than enhancements observed in other systems, but in agreement with recent quantitative reports of surface enhanced infrared reflection absorption spectroscopy (SEIRA). Our results show that interactions of infrared SPPs with molecular vibrations are in the weak coupling limit, and that enhancements comparable those reported for noble metals are achievable.}, number={9}, journal={JOURNAL OF PHYSICAL CHEMISTRY A}, author={Sachet, Edward and Aspnes, D. E. and Maria, J-P and Franzen, Stefan}, year={2020}, month={Mar}, pages={1744–1753} }
 @article{le_kim_kim_aspnes_2020, title={External removal of endpoint-discontinuity artifacts in the reciprocal-space analysis of spectra}, volume={20}, ISSN={["1878-1675"]}, DOI={10.1016/j.cap.2019.11.004}, abstractNote={We present a systematic method of removing endpoint-discontinuity artifacts in the Fourier analysis of spectral segments, enabling the more accurate extraction of information. This principal-component-removal approach differs from a previous version by using extrapolated (or extended) data outside rather than inside the spectral range. This not only allows coefficients to be accessed to the white-noise limit with no distortion of the segment, but also generates interpolated coefficients for improved analytic insight. Examples are provided.}, number={1}, journal={CURRENT APPLIED PHYSICS}, author={Le, V. L. and Kim, T. J. and Kim, Y. D. and Aspnes, D. E.}, year={2020}, month={Jan}, pages={232–236} }
 @article{le_kim_aspnes_2020, title={Quantitative assessment of linear noise-reduction filters for spectroscopy}, volume={28}, ISSN={["1094-4087"]}, DOI={10.1364/OE.411768}, abstractNote={Linear noise-reduction filters used in spectroscopy must strike a balance between reducing noise and preserving lineshapes, the two conflicting requirements of interest. Here, we quantify this tradeoff by capitalizing on Parseval’s Theorem to cast two measures of performance, mean-square error (MSE) and noise, into reciprocal- (Fourier-) space (RS). The resulting expressions are simpler and more informative than those based in direct- (spectral-) space (DS). These results provide quantitative insight not only into the effectiveness of different linear filters, but also information as to how they can be improved. Surprisingly, the rectangular (“ideal” or “brick wall”) filter is found to be nearly optimal, a consequence of eliminating distortion in low-order Fourier coefficients where the major fraction of spectral information is contained. Using the information provided by the RS version of MSE, we develop a version that is demonstrably superior to the brick-wall and also the Gauss-Hermite filter, its former nearest competitor.}, number={26}, journal={OPTICS EXPRESS}, author={Le, Long V and Kim, Young D. and Aspnes, David E.}, year={2020}, month={Dec}, pages={38917–38933} }
 @article{le_kim_kim_aspnes_2019, title={Combined interpolation, scale change, and noise reduction in spectral analysis}, volume={37}, ISBN={2166-2746}, DOI={10.1116/1.5120358}, abstractNote={The authors present a simple, convenient, and accurate noise-reduction approach for interpolating spectra, in particular, for converting those available as discrete points equally spaced in wavelength, acquired, for example, by a photodiode-array detector, to equivalent spectra equally spaced in energy, as needed for analysis. Based on continuum mathematics, the algorithm uses Gaussian kernels and capitalizes on the fact that trapezoidal-rule integration is accurate to fourth order in the ratio of point separation to width when applied to Gaussian functions. The approach can be expanded to perform differentiation and other operations. Examples include false-data verification, wavelength-to-energy conversion of near-bandgap interference oscillations of a GaN film, and calculation of the second energy derivative of monolayer MoS2 in the exciton region.}, number={5}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B}, author={Le, Van L. and Kim, Tae J. and Kim, Young D. and Aspnes, David E.}, year={2019}, month={Sep} }
 @article{kim_park_byun_le_nguyen_nguyen_kim_song_aspnes_2019, title={Dielectric Functions and Critical Points of GaAsSb Alloys}, volume={74}, ISSN={["1976-8524"]}, DOI={10.3938/jkps.74.595}, abstractNote={We report the pseudodielectric functions and the critical points of GaAsxSb1−x ternary alloy films. Data were obtained by performing spectroscopic ellipsometry on 1-μm-thick films grown on (001) GaAs by using molecular beam epitaxy. Artifacts from surface contaminants, including oxide overlayers, were minimized by using in-situ chemical cleaning, leading to accurate representations of the bulk dielectric responses of these materials. We determined the energies of the E1, E1+Δ1, E′0, E′0+Δ′0, E2, E′2, and E′1 critical points from numerically calculated second energy derivatives, as well as their compositional dependences by using lineshape fitting.}, number={6}, journal={JOURNAL OF THE KOREAN PHYSICAL SOCIETY}, author={Kim, Tae Jung and Park, Han Gyeol and Byun, Jun Seok and Le, Van Long and Nguyen, Hoang Tung and Nguyen, Xuan Au and Kim, Young Dong and Song, Jin Dong and Aspnes, David E.}, year={2019}, month={Mar}, pages={595–599} }
 @article{aspnes_le_kim_2019, title={Linear and nonlinear filtering of spectra}, volume={37}, ISSN={["2166-2746"]}, DOI={10.1116/1.5118230}, abstractNote={The authors investigate linear and nonlinear methods of reducing noise while preserving information in spectra, optical and otherwise. The optimum linear and nonlinear approaches are Gauss–Hermite and maximum-entropy, respectively. However, intelligent processing still requires an initial assessment of the data in reciprocal space.}, number={5}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B}, author={Aspnes, David E. and Le, Van L. and Kim, Young D.}, year={2019}, month={Sep} }
 @article{morales_stoute_yu_aspnes_dickey_2016, title={Liquid gallium and the eutectic gallium indium (EGaIn) alloy: Dielectric functions from 1.24 to 3.1 eV by electrochemical reduction of surface oxides}, volume={109}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/1.4961910}, DOI={10.1063/1.4961910}, abstractNote={Liquid metals based on gallium are promising materials for soft, stretchable, and shape reconfigurable electromagnetic devices. The behavior of these metals relates directly to the thicknesses of their surface oxide layers, which can be determined nondestructively by ellipsometry if their dielectric functions ε are known. This paper reports on the dielectric functions of liquid gallium and the eutectic gallium indium (EGaIn) alloy from 1.24 to 3.1 eV at room temperature, measured by spectroscopic ellipsometry. Overlayer-induced artifacts, a continuing problem in optical measurements of these highly reactive metals, are eliminated by applying an electrochemically reductive potential to the surface of the metal immersed in an electrolyte. This technique enables measurements at ambient conditions while avoiding the complications associated with removing overlayers in a vacuum environment. The dielectric responses of both metals are closely represented by the Drude model. The EGaIn data suggest that in the absence of an oxide the surface is In-enriched, consistent with the previous vacuum-based studies. Possible reasons for discrepancies with previous measurements are discussed.}, number={9}, journal={APPLIED PHYSICS LETTERS}, publisher={AIP Publishing}, author={Morales, Daniel and Stoute, Nicholas A. and Yu, Zhiyuan and Aspnes, David E. and Dickey, Michael D.}, year={2016}, month={Aug} }
 @article{aspnes_2015, title={Bond Models in Linear and Nonlinear Optics}, volume={9584}, ISSN={["1996-756X"]}, DOI={10.1117/12.2188400}, abstractNote={Bond models, also known as polarizable-point or mechanical models, have a long history in optics, starting with the Clausius-Mossotti relation but more accurately originating with Ewald’s largely forgotten work in 1912. These models describe macroscopic phenomena such as dielectric functions and nonlinear-optical (NLO) susceptibilities in terms of the physics that takes place in real space, in real time, on the atomic scale. Their strengths lie in the insights that they provide and the questions that they raise, aspects that are often obscured by quantum-mechanical treatments. Statics versions were used extensively in the late 1960’s and early 1970’s to correlate NLO susceptibilities among bulk materials. Interest in NLO applications revived with the 2002 work of Powell et al., who showed that a fully anisotropic version reduced by more than a factor of 2 the relatively large number of parameters necessary to describe secondharmonic- generation (SHG) data for Si(111)/SiO<sub>2</sub> interfaces. Attention now is focused on the exact physical meaning of these parameters, and to the extent that they represent actual physical quantities.}, journal={ULTRAFAST NONLINEAR IMAGING AND SPECTROSCOPY III}, author={Aspnes, D. E.}, year={2015} }
 @article{yu_yu_cai_li_gurarslan_peelaers_aspnes_walle_nguyen_zhang_et al._2015, title={Exciton-dominated Dielectric Function of Atomically Thin MoS2 Films}, volume={5}, ISSN={["2045-2322"]}, DOI={10.1038/srep16996}, abstractNote={Abstract}, journal={SCIENTIFIC REPORTS}, author={Yu, Yiling and Yu, Yifei and Cai, Yongqing and Li, Wei and Gurarslan, Alper and Peelaers, Hartwin and Aspnes, David E. and Walle, Chris G. and Nguyen, Nhan V. and Zhang, Yong-Wei and et al.}, year={2015}, month={Nov} }
 @article{rowe_aspnes_pinczuk_yu_2015, title={Manuel Cardona Castro obituary}, volume={68}, ISSN={["1945-0699"]}, DOI={10.1063/pt.3.2695}, abstractNote={Manuel Cardona Castro passed away unexpectedly on 2 July 2014 at the Max Planck Institute for Solid State Research in Stuttgart, Germany, where he had worked since 1971. Cardona’s main interests were in optical spectroscopy and light scattering. Among the methods he employed were modulation spectroscopy, in which he was an early leader, and inelastic light-scattering techniques using both visible lasers and x rays generated by synchrotron radiation.Manuel Cardona CastroMAX PLANCK INSTITUTE FOR SOLID STATE RESEARCH/CARMEN-M. MÜLLERPPT|High resolutionCardona and his collaborators applied those techniques to study electrons and phonons in bulk semiconductors, semiconductor micro- and nanostructures, tailor-made isotopically pure materials, and high-Tc superconductors in the normal and superconducting states. In addition to his many experimental achievements, Cardona also contributed to theoretical semiconductor physics. He was an innovator in applying the k ∙ p approximation to understand the full-zone band structure of electrons, their effective masses, and g-factors. His other contributions to theory include the computation of the deformation potential mechanism of the electron–phonon interaction and the renormalization of the bandgap of elemental semiconductors, such as diamond, due to electron–phonon interactions.Cardona was born in Catalonia, Spain, on 7 September 1934. As a young boy he traveled throughout Europe and learned several languages. That knowledge became one of his well-known skills; often he would stop in the middle of a conversation in one language and change to another spoken by a visitor. After obtaining a master’s in physics in 1955 from the University of Barcelona, Cardona joined the University of Madrid as an instructor for electronics. In 1956 he was awarded a graduate fellowship to Harvard University to work under William Paul. He investigated the dielectric properties of semiconductors—in particular, germanium and silicon. With that work as his thesis, he received a PhD in applied physics from Harvard in 1959.That same year Cardona joined RCA Laboratories in Zürich as a member of the technical staff. There he extended his reflectance work to III–V semiconductors. In 1961 he moved to RCA Labs in Princeton, New Jersey, where he continued to work on the optical properties of semiconductors and started investigating their microwave properties. He introduced a notation to label peaks in the reflectance spectra of semiconductors according to the specific parts of the energy-band structure in momentum space where the peaks originate. That notation also highlights peaks that are related via spin–orbit splitting. His notation has since become the standard method for labeling reflectance spectra of semiconductors.Between 1964 and 1971, Cardona was a member of the physics faculty of Brown University. He spent the summer of 1965 as a visiting professor at the University of Buenos Aires under the auspices of the Ford Foundation. Between 1969 and 1970, he received a John S. Guggenheim Memorial Scholarship, which allowed him to spend a year at the German Electron Synchrotron in Hamburg, Germany, where he studied the vacuum-UV optical properties of semiconductors. In 1971 he moved to Stuttgart as one of the founding directors of the recently created Max Planck Institute for Solid State Research. His activities there came to have a global impact. Although he retired in 2000, he continued to collaborate daily with numerous scientists throughout Europe, the Americas, and elsewhere until his death.Cardona authored or coauthored more than 1300 publications in international journals and is one of the eight most cited physicists since 1970. He wrote or edited 10 monographs on solid-state physics and cowrote a textbook on semiconductors. Beginning in 1972 Cardona served on the editorial boards of seven journals, including being the editor-in-chief of Solid State Communications from 1992 to 2004. He received the 1984 Frank Isakson Prize for Optical Effects in Solids and the 1997 John Wheatley Award from the American Physical Society, the 2001 Mott Medal and Prize from the Institute of Physics, the 2004 Blaise Pascal Medal in Physics from the European Academy of Sciences, and numerous other prestigious honors.In his memory, a special symposium entitled “Electrons, Phonons, and Photons in Semiconductors” was recently held at the Max Planck Institute for Solid State Research. The gathering was attended by more than 100 of his former students, postdoctoral fellows, and collaborators from around the world. The physics community and many others who have known him will miss him immensely and remember him for his encyclopedic knowledge of semiconductors and superconductors, his sense of humor, and most of all, his warm and generous personality.© 2015 American Institute of Physics.}, number={2}, journal={PHYSICS TODAY}, author={Rowe, Jack and Aspnes, Dave and Pinczuk, Aron and Yu, Peter Y.}, year={2015}, month={Feb}, pages={58–58} }
 @article{aspnes_choi_2014, title={Combined direct- and reciprocal-space approach for converting spectra to energy scales with negligible loss of information}, volume={571}, ISSN={["1879-2731"]}, DOI={10.1016/j.tsf.2013.11.028}, abstractNote={Extracting information from spectra is best done in reciprocal space, provided that baseline effects, information, and noise can be localized in low-, intermediate-, and high-index coefficients, respectively. In optical spectroscopy this occurs if data are equally spaced in energy E. We develop and quantify a procedure to generate such spectra from any initial distribution of wavelengths λ. Reciprocal-space analysis shows that this transformation can be accomplished with negligible loss of information. The procedure can be applied to any system where the Fourier coefficients of the result can be separated as noted above.}, journal={THIN SOLID FILMS}, author={Aspnes, D. E. and Choi, S. G.}, year={2014}, month={Nov}, pages={506–508} }
 @article{kim_hwang_byun_aspnes_lee_song_liang_chang_park_choi_et al._2014, title={Dielectric functions and interband transitions of InxAl1 (-) P-x alloys}, volume={14}, ISSN={["1878-1675"]}, DOI={10.1016/j.cap.2014.06.026}, abstractNote={We report pseudodielectric functions <ε> from 1.5 to 6.0 eV of InxAl1 − xP ternary alloy films. Data were obtained by spectroscopic ellipsometry on 1.2 μm thick films grown on (001) GaAs substrates by molecular beam epitaxy. Artifacts were minimized by real-time assessment of overlayer removal, leading to accurate representations of the bulk dielectric responses of these materials. Critical-point (CP) energies were obtained from numerically calculated second energy derivatives, and their Brillouin-zone origins identified by band-structure calculations using the linear augmented Slater-type orbital method.}, number={9}, journal={CURRENT APPLIED PHYSICS}, author={Kim, T. J. and Hwang, S. Y. and Byun, J. S. and Aspnes, D. E. and Lee, E. H. and Song, J. D. and Liang, C. -T. and Chang, Y. -C. and Park, H. G. and Choi, J. and et al.}, year={2014}, month={Sep}, pages={1273–1276} }
 @article{kim_byun_hwang_park_kang_park_kim_aspnes_2014, title={Parameterization of the dielectric functions of InGaSb alloys}, volume={14}, ISSN={["1878-1675"]}, DOI={10.1016/j.cap.2014.03.010}, abstractNote={Analytic expressions are presented that accurately represent the dielectric functions ε = ε1 + iε2 of In1−xGaxSb from 1.5 to 6 eV. We used the parametric model, which portrays ε as a sum of polynomials and can accommodate the asymmetric nature of critical point transitions. The ε spectra were obtained previously by spectroscopic ellipsometry for x = 0.000, 0.102, 0.305, 0.473, 0.684, and 1.000. The ε data are successfully reconstructed and parameterized by eight polynomials. With the interpolation of parameters of ε spectra, we can determine ε as a continuous function of Ga composition and energy over the entire composition range 0 ≤ x ≤ 1. These results should be useful for device design and in situ monitoring of deposition.}, number={5}, journal={CURRENT APPLIED PHYSICS}, author={Kim, Tae Jung and Byun, Jun Seok and Hwang, Soon Yong and Park, Han Gyeol and Kang, Yu Ri and Park, Jae Chan and Kim, Young Dong and Aspnes, David E.}, year={2014}, month={May}, pages={768–771} }
 @article{aspnes_2014, title={Spectroscopic ellipsometry - Past, present, and future}, volume={571}, ISSN={["0040-6090"]}, DOI={10.1016/j.tsf.2014.03.056}, abstractNote={Since its initial development in the early 1970s, spectroscopic ellipsometry (SE) has evolved to become the primary technique for determining the intrinsic and structural properties of homogeneous and inhomogeneous materials in bulk and thin-film form, including properties of surfaces and interfaces. As an indispensible nondestructive approach for determining critical dimensions in integrated-circuit technology, its economic impact has been enormous. I review the development of theory as well as instrumentation, from the perspective of someone who has worked in SE essentially from its beginning. I provide comments about its present state, note some unresolved issues, then consider possible improvements and predict how the field is likely to evolve.}, journal={THIN SOLID FILMS}, author={Aspnes, D. E.}, year={2014}, month={Nov}, pages={334–344} }
 @article{hwang_kim_byun_barange_diware_kim_aspnes_yoon_song_2013, title={Analytic representation of the dielectric functions of InAsxSb1-x alloys in the parametric model}, volume={547}, ISSN={["0040-6090"]}, DOI={10.1016/j.tsf.2012.11.088}, abstractNote={We report expressions that allow the dielectric functions ε = ε1 + iε2 from 1.5 to 6.0 eV of InAsxSb1 − x alloys over the entire composition range 0 ≤ x ≤ 1 to be calculated analytically. We base our work on the parametric model (PM), which describes the dielectric functions of semiconductor materials as a sum of Gaussian-broadened polynomials. Our reference ε spectra are those that we obtained previously by spectroscopic ellipsometry for the specific compositions x = 0.000, 0.127, 0.337, 0.491, 0.726, and 1.000. The PM reconstructions are in excellent agreement with the data, and with the interpolations provided here, the model is extended to arbitrary compositions. We expect these results to be useful in a number of contexts, for example for the design of optoelectronic devices.}, journal={THIN SOLID FILMS}, author={Hwang, S. Y. and Kim, T. J. and Byun, J. S. and Barange, N. S. and Diware, M. S. and Kim, Y. D. and Aspnes, D. E. and Yoon, J. J. and Song, J. D.}, year={2013}, month={Nov}, pages={276–279} }
 @article{kim_yoon_byun_hwang_aspnes_shin_song_liang_chang_barange_et al._2013, title={Interband transitions and dielectric functions of InGaSb alloys}, volume={102}, ISSN={["0003-6951"]}, DOI={10.1063/1.4795622}, abstractNote={We report pseudodielectric functions of In1−xGaxSb ternary alloy films from 1.5 to 6.0 eV determined by spectroscopic ellipsometry. Artifacts were minimized by real-time assessment of overlayer removal, leading to accurate representations of the bulk dielectric responses of these materials. Critical-point (CP) energies were obtained from numerically calculated second energy derivatives, and their Brillouin-zone origins identified by band-structure calculations. The E2′ and E2 + Δ2 CP energies cross with increasing In content as a result of increasing spin-orbit splitting Δ2.}, number={10}, journal={APPLIED PHYSICS LETTERS}, author={Kim, T. J. and Yoon, J. J. and Byun, J. S. and Hwang, S. Y. and Aspnes, D. E. and Shin, S. H. and Song, J. D. and Liang, C. -T. and Chang, Y. -C. and Barange, N. S. and et al.}, year={2013}, month={Mar} }
 @article{reynolds_reynolds_mohanta_muth_rowe_everitt_aspnes_2013, title={Shallow acceptor complexes in p-type ZnO}, volume={102}, ISSN={["1077-3118"]}, DOI={10.1063/1.4802753}, abstractNote={We show that N-doped ZnO films grown on sapphire can exhibit significant (∼1018 cm−3) room-temperature p-type behavior when sufficient nitrogen (N) is incorporated and the material is annealed appropriately. Substitutional N on the oxygen (O) sublattice is a deep acceptor; however, shallow acceptor complexes involve N, H, and zinc vacancies (VZn). Combining secondary ion mass spectrometry, Raman-scattering, photoluminescence, and Hall-effect data, we establish the evolution of N from its initial incorporation on a Zn site to a final shallow acceptor complex VZn_NO_H+ with an ionization energy of ca. 130 meV. This complex is responsible for the observed p-type behavior.}, number={15}, journal={APPLIED PHYSICS LETTERS}, author={Reynolds, J. G. and Reynolds, C. L., Jr. and Mohanta, A. and Muth, J. F. and Rowe, J. E. and Everitt, H. O. and Aspnes, D. E.}, year={2013}, month={Apr} }
 @article{aspnes_2013, title={Spectroscopic ellipsometry-A perspective}, volume={31}, ISSN={["1520-8559"]}, DOI={10.1116/1.4809747}, abstractNote={Since its initial development in the early 1970s, spectroscopic ellipsometry (SE) has become the primary technique for determining optical properties of materials. In addition to the other historic role of ellipsometry, determining film thicknesses, SE is now widely used to obtain intrinsic and structural properties of homogeneous and inhomogeneous materials in bulk and thin-film form, including properties of surfaces and interfaces. Its nondestructive capability for determining critical dimensions has made SE indispensible in integrated-circuits technology. The present work is aimed at those who are unfamiliar with SE but may feel that it could provide useful information in specific situations. Accordingly, the author gives some background and basic theory, and then illustrates capabilities with various applications. Coverage of the topic is necessarily limited, but references to more complete treatments are provided.}, number={5}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A}, author={Aspnes, D. E.}, year={2013}, month={Sep} }
 @article{gokce_gundogdu_aspnes_2012, title={Control of the oxidation kinetics of H-terminated (111)Si by using the carrier concentration and the strain: a second-harmonic-generation investigation}, volume={60}, ISSN={["1976-8524"]}, DOI={10.3938/jkps.60.1685}, abstractNote={We discuss recent results regarding the effects of strain, carrier type and concentration on the oxidation of H-terminated (111)Si. Second-harmonic-generation data show that this is a two-stage process where the H of the “up” bonds of the outermost Si layer is replaced by OH, followed by O insertion into the “back” bonds. These data provide additional detailed information about both stages. In particular, directional control of the in-plane surface chemistry by using the applied uniaxial stress provides new opportunities for interface control.}, number={10}, journal={JOURNAL OF THE KOREAN PHYSICAL SOCIETY}, author={Gokce, B. and Gundogdu, K. and Aspnes, D. E.}, year={2012}, month={May}, pages={1685–1689} }
 @article{nelson_sandin_dougherty_aspnes_rowe_diebold_2012, title={Optical and structural characterization of epitaxial graphene on vicinal 6H-SiC(0001)-Si by spectroscopic ellipsometry, Auger spectroscopy, and STM}, volume={30}, ISSN={["2166-2754"]}, DOI={10.1116/1.4726199}, abstractNote={The authors report results of spectroscopic ellipsometry (SE) measurements in the near-IR, visible, and near-UV spectral ranges using a Woollam dual rotating-compensator ellipsometer, analyzing data in terms of both epitaxial graphene and interface contributions. The SiC samples were cleaned by standard methods of CMP and HF etching prior to mounting in UHV and growing epitaxial graphene by thermal annealing at ∼1400 °C. Most samples were vicinally cut 3.5° off (0001) toward [11−20]. STM measurements show that the initial regular step edges were replaced by somewhat irregular edges after graphene growth. From growth-temperature and Auger data the authors estimate that the graphene is ∼3–4 ML thick. The authors find significant differences among the spectral features of the interface “buffer” layer and those of graphene. Specifically, the hyperbolic-exciton peak reported previously at ∼4.5 eV in graphene shifts to a similarly shaped peak at ∼4 eV in the interface buffer layer. The authors attribute this shift to a significant component of sp3 bonded carbon in the buffer, which occurs in addition to the sp2 bonded carbon that is present in the graphene layer. SE data in the terahertz range obtained by Hoffman et al. [Thin Solid Films 519, 2593 (2011)] show that the mobility values of graphene grown on the carbon face of SiC vary with proximity to the substrate. This leads to the question as to whether an interface layer at the Si face has properties (i.e., dielectric function/complex refractive index) that are different from and/or affect those of the graphene layers.}, number={4}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B}, author={Nelson, Florence and Sandin, Andreas and Dougherty, Daniel B. and Aspnes, David E. and Rowe, Jack E. and Diebold, Alain C.}, year={2012}, month={Jul} }
 @article{choi_schilfgaarde_aspnes_norman_olson_peshek_levi_2011, title={Above-band-gap dielectric functions of ZnGeAs2: Ellipsometric measurements and quasiparticle self-consistent GW calculations}, volume={83}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.83.235210}, abstractNote={We report the above-band-gap dielectric-function spectra ! a = ! a1 + i! a2 of single-crystal ZnGeAs2 grown epitaxially on (001)GaAs, and study it theoretically. After surface overlayers were removed chemically to minimize artifacts, pseudodielectric-function spectra ! ! "=! ! 1 "+ i! ! 2 " were acquired ellipsometrically from 1.5 to 6.0 eV with the sample at room temperature. The! a spectra were then extracted by multilayer analysis. The procedureensuresthattheresultisacloseapproximationtothea-axiscomponentofthedielectric-functiontensor ! =! 1 +i! 2 ofZnGeAs2.Thedataexhibitnumerousspectralfeaturesassociatedwithcriticalpoints.Theenergies of these critical points are determined accurately by fitting standard line shapes to second energy derivatives of the data obtained by a combined method of spectral analysis. We compare our results to the predictions of quasiparticle self-consistent GW calculations. Good agreement is achieved for the major critical-point features, and their probable origins are identified.}, number={23}, journal={PHYSICAL REVIEW B}, author={Choi, S. G. and Schilfgaarde, M. and Aspnes, D. E. and Norman, A. G. and Olson, J. M. and Peshek, T. J. and Levi, D. H.}, year={2011}, month={Jun} }
 @inproceedings{ghong_han_chung_byun_aspnes_kim_ihm_cheong_2011, title={Analysis of surface roughness of critical-dimension structures using spectroscopic ellipsometry}, url={http://dx.doi.org/10.1063/1.3666489}, DOI={10.1063/1.3666489}, abstractNote={As critical dimension of integrated‐circuits shrink to 100 nm and below, observation of critical dimension roughness will become increasingly important. In this article, we analyze roughness of grating structure that fabricated on a Si wafer using standard photolithography, and characterize roughness of CD by rigorous coupled‐wave analysis and effective medium approximation method with spectroscopic ellipsometry.}, publisher={AIP}, author={Ghong, T. H. and Han, S.-H. and Chung, J.-M. and Byun, J. S. and Aspnes, D. E. and Kim, Y. D. and Ihm, Jisoon and Cheong, Hyeonsik}, year={2011} }
 @article{gokce_gundogdu_adles_aspnes_2011, title={Back-reflection Second-harmonic Generation of (111)Si: Theory and Experiment}, volume={58}, ISSN={["1976-8524"]}, DOI={10.3938/jkps.58.1237}, abstractNote={We consider second-harmonic generation (SHG) from a (111) surface of a tetrahedrally bonded semiconductor illuminated at normal incidence by a focused pump beam of Gaussian cross section as a model of SHG by focused beams. Calculations are done in the anisotropic bond model (ABM) and the results are applied to Si. The unit-cell configuration is simple enough for the calculations to be done analytically, so the results can be compared directly to similar calculations done for amorphous material. Although the differences in unit-cell symmetry occur on the atomic scale, they lead to large differences in the spatial distribution of the emerging radiation. Lateral focusing, which might be expected to increase the bulk contribution to SHG by increasing the lateral field gradient, has little effect; the spatial-dispersion contribution remains dominated by the phase term. Focusing does not inhibit backscattered SHG from the bulk, although our data on the oxidation of H-terminated (111)Si clearly show that in some cases the interface contribution dominates by a wide margin.}, number={5}, journal={JOURNAL OF THE KOREAN PHYSICAL SOCIETY}, author={Gokce, B. and Gundogdu, K. and Adles, E. J. and Aspnes, D. E.}, year={2011}, month={May}, pages={1237–1243} }
 @article{gokce_aspnes_lucovsky_gundogdu_2011, title={Bond-specific reaction kinetics during the oxidation of (111) Si: Effect of n-type doping}, volume={98}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.3537809}, DOI={10.1063/1.3537809}, abstractNote={It is known that a higher concentration of free carriers leads to a higher oxide growth rate in the thermal oxidation of silicon. However, the role of electrons and holes in oxidation chemistry is not clear. Here, we report real-time second-harmonic-generation data on the oxidation of H-terminated (111)Si that reveal that high concentrations of electrons increase the chemical reactivity of the outer-layer Si-Si back bonds relative to the Si-H up bonds. However, the thicknesses of the natural oxides of all samples stabilize near 1 nm at room temperature, regardless of the chemical kinetics of the different bonds.}, number={2}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Gokce, B. and Aspnes, D. E. and Lucovsky, G. and Gundogdu, K.}, year={2011}, month={Jan}, pages={021904} }
 @article{gokce_aspnes_gundogdu_2011, title={Effect of strain on bond-specific reaction kinetics during the oxidation of H-terminated (111) Si}, volume={98}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.3567528}, DOI={10.1063/1.3567528}, abstractNote={Although strain is used in semiconductor technology for manipulating optical, electronic, and chemical properties of semiconductors, the understanding of the microscopic phenomena that are affected or influenced by strain is still incomplete. Second-harmonic generation data obtained during the air oxidation of H-terminated (111) Si reveal the effect of compressive strain on this chemical reaction. Even small amounts of strain manipulate the reaction kinetics of surface bonds significantly, with tensile strain enhancing oxidation and compressive strain retarding it. This dramatic change suggests a strain-driven charge transfer mechanism between Si–H up bonds and Si–Si back bonds in the outer layer of Si atoms.}, number={12}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Gokce, Bilal and Aspnes, David E. and Gundogdu, Kenan}, year={2011}, month={Mar}, pages={121912} }
 @article{gokce_adles_aspnes_gundogdu_2011, title={Measurement and Control of In-Plane Surface Chemistry During Oxidation of H-Terminated (111)Si}, volume={1399}, ISSN={["0094-243X"]}, DOI={10.1063/1.3666321}, abstractNote={We demonstrate both directional control and measurement of the oxidation of H‐terminated (111)Si. Control is achieved through externally applied strain, with strained back bonds oxidizing faster than unstrained ones. Real‐time measurement is achieved by second‐harmonic generation (SHG), with SHG anisotropy data analyzed with the anisotropic bond‐charge model of nonlinear optics. Anisotropic oxidation also results in structural changes, which appear as rotations of the average orientations of the back bonds from their unperturbed directions.}, journal={PHYSICS OF SEMICONDUCTORS: 30TH INTERNATIONAL CONFERENCE ON THE PHYSICS OF SEMICONDUCTORS}, author={Gokce, Bilal and Adles, Eric J. and Aspnes, David E. and Gundogdu, Kenan}, year={2011} }
 @article{aspnes_2011, title={Plasmonics and effective-medium theories}, volume={519}, ISSN={["0040-6090"]}, DOI={10.1016/j.tsf.2010.12.081}, abstractNote={I discuss the close connection between the description of plasmons, which are charge excitations localized at the boundaries between dissimilar regions in composite materials, and effective-medium theories (EMTs), which describe the macroscopic dielectric response of such materials. The requirements for the validity of an EMT are also considered.}, number={9}, journal={THIN SOLID FILMS}, author={Aspnes, D. E.}, year={2011}, month={Feb}, pages={2571–2574} }
 @article{ghong_han_chung_byun_kim_aspnes_2011, title={Roughness Analysis of the Critical Dimension by Using Spectroscopic Ellipsometry}, volume={58}, ISSN={["0374-4884"]}, DOI={10.3938/jkps.58.1426}, abstractNote={In semiconductor electronics applications, observation of the linewidth roughness (LWR) or the critical dimension roughness will become increasingly important. In this article, we study to analyze the roughness of a grating structure by using spectroscopic ellipsometry. A simple 1D patterned c-Si grating structure was measured from the zeroth-order diffraction response at a fixed angle of incidence, which result agreed with a theoretical prediction made by using a rigorous coupled-wave analysis (RCWA) calculation with an effective-medium approximation (EMA) for a structure with roughness.}, number={5}, journal={JOURNAL OF THE KOREAN PHYSICAL SOCIETY}, author={Ghong, T. H. and Han, S. -H. and Chung, J. -M. and Byun, J. S. and Kim, Y. D. and Aspnes, D. E.}, year={2011}, month={May}, pages={1426–1428} }
 @article{aspnes_2010, title={Bond models in linear and nonlinear optics}, volume={247}, ISSN={["1521-3951"]}, DOI={10.1002/pssb.200983937}, abstractNote={Abstract}, number={8}, journal={PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS}, author={Aspnes, D. E.}, year={2010}, month={Aug}, pages={1873–1880} }
 @article{adles_aspnes_2010, title={Chemical-etch-assisted growth of epitaxial zinc oxide}, volume={28}, ISSN={["0734-2101"]}, DOI={10.1116/1.3305814}, abstractNote={The authors use real-time spectroscopic polarimetric observations of growth, and a chemical model derived therefrom, to develop a method of growing dense, two-dimensional zinc oxide epitaxially on sapphire by metal organic chemical vapor deposition. Particulate zinc oxide formed in the gas phase is used to advantage as the deposition source. Their real-time data provide unequivocal evidence that a seed layer is required, unwanted fractions of ZnO are deposited, but these fractions can be removed by cycling between brief periods of net deposition and etching. The transition between deposition and etching occurs with zinc precursor concentrations that only differ by 13%. These processes are understood by considering the chemistry involved.}, number={4}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A}, author={Adles, E. J. and Aspnes, D. E.}, year={2010}, pages={689–692} }
 @article{yoon_kim_jung_aspnes_kim_kim_chang_shin_song_2010, title={Dielectric functions and interband transitions of In1-xAlxSb alloys}, volume={97}, ISSN={["0003-6951"]}, DOI={10.1063/1.3488827}, abstractNote={Pseudodielectric functions ⟨ε⟩ of In1−xAlxSb ternary alloy films were determined from 1.5 to 6.0 eV by spectroscopic ellipsometry. Overlayer effects were minimized by performing in situ chemical etching to more accurately determine intrinsic bulk dielectric responses. Critical-point (CP) energies of structures were determined from numerically calculated second energy derivatives. Where necessary, Brillouin-zone origins were identified by electronic band structure calculations done with the linear augmented Slater-type orbital method. These calculations also showed increasing separation of the E2 and E2′ CP structures with increasing Al-composition}, number={11}, journal={APPLIED PHYSICS LETTERS}, author={Yoon, J. J. and Kim, T. J. and Jung, Y. W. and Aspnes, D. E. and Kim, Y. D. and Kim, H. J. and Chang, Y. C. and Shin, S. H. and Song, J. D.}, year={2010}, month={Sep} }
 @article{choi_aspnes_fuchser_martinez-tomas_munoz sanjose_levi_2010, title={Ellipsometric study of single-crystal gamma-InSe from 1.5 to 9.2 eV}, volume={96}, ISSN={["0003-6951"]}, DOI={10.1063/1.3420080}, abstractNote={We report the component E⃑⊥ĉ of the pseudodielectric-function tensor ⟨ε(E)⟩=⟨ε1(E)⟩+i⟨ε2(E)⟩ of γ-phase single-crystal InSe, obtained from 1.5 to 9.2 eV by vacuum-ultraviolet spectroscopic ellipsometry with the sample at room temperature. Overlayer artifacts were reduced as far as possible by measuring fresh surfaces prepared by cleavage. Accurate critical-point energies of observed structures were obtained by a combined method of spectral analysis.}, number={18}, journal={APPLIED PHYSICS LETTERS}, author={Choi, S. G. and Aspnes, D. E. and Fuchser, A. L. and Martinez-Tomas, C. and Munoz Sanjose, V. and Levi, D. H.}, year={2010}, month={May} }
 @article{gokce_adles_aspnes_gundogdu_2010, title={Measurement and control of in-plane surface chemistry during the oxidation of H-terminated (111) Si}, volume={107}, ISSN={0027-8424 1091-6490}, url={http://dx.doi.org/10.1073/pnas.1011295107}, DOI={10.1073/pnas.1011295107}, abstractNote={In-plane directional control of surface chemistry during interface formation can lead to new opportunities regarding device structures and applications. Control of this type requires techniques that can probe and hence provide feedback on the chemical reactivity of bonds not only in specific directions but also in real time. Here, we demonstrate both control and measurement of the oxidation of H-terminated (111) Si. Control is achieved by externally applying uniaxial strain, and measurement by second-harmonic generation (SHG) together with the anisotropic-bond model of nonlinear optics. In this system anisotropy results because bonds in the strain direction oxidize faster than those perpendicular to it, leading in addition to transient structural changes that can also be detected at the bond level by SHG.}, number={41}, journal={Proceedings of the National Academy of Sciences}, publisher={Proceedings of the National Academy of Sciences}, author={Gokce, Bilal and Adles, Eric J. and Aspnes, David E. and Gundogdu, Kenan}, year={2010}, month={Sep}, pages={17503–17508} }
 @article{ghong_han_chung_byun_kim_aspnes_kim_park_kim_2010, title={Nondestructive analysis of coated periodic nanostructures from optical data}, volume={35}, ISSN={["1539-4794"]}, DOI={10.1364/ol.35.000733}, abstractNote={Optical data are essential for the accurate nondestructive determination of profiles of periodic structures in integrated-circuit technology. In rigorous coupled-wave analysis, the sample is generally modeled as layers consisting of a single material and the ambient. We extend present capabilities to the analysis of structures with overlayers and demonstrate our approach by determining quantitatively the thicknesses of top, sidewall, and bottom oxides of deliberately and naturally oxidized structures.}, number={5}, journal={OPTICS LETTERS}, author={Ghong, T. H. and Han, S-H. and Chung, J-M. and Byun, J. S. and Kim, T. J. and Aspnes, D. E. and Kim, Y. D. and Park, I. H. and Kim, Y-W.}, year={2010}, month={Mar}, pages={733–735} }
 @article{liu_aspnes_2010, title={Thickness inhomogeneities and growth mechanisms of GaP heteroepitaxy by organometallic chemical vapor deposition}, volume={28}, ISSN={["0734-2101"]}, DOI={10.1116/1.3442805}, abstractNote={The authors report comparative studies of the deposition of GaP on (001) GaAs, as-polished (001)Si, SiO2-coated (001)Si, nanoscopically roughened (001)Si, and polycrystalline GaP surfaces by organometallic chemical vapor deposition using trimethylgallium (TMG) and phosphine (PH3) sources. The thicknesses of the GaP films increase or decrease exponentially toward the edge of wafers. This functional dependence implies one-dimensional gas-phase diffusion of a reactive species, possibly H–P=Ga–CH3, generated by heterogeneous catalysis according to the reactivity of the different surfaces to the decomposition of PH3. Deposition on (001)Si depends on the type of nanoscopic roughness of the substrate.}, number={4}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A}, author={Liu, X. and Aspnes, D. E.}, year={2010}, pages={583–589} }
 @article{liu_aspnes_2009, title={Analytical solution of thickness variations in selective area growth by organometallic chemical vapor deposition}, volume={94}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.3157266}, DOI={10.1063/1.3157266}, abstractNote={Using conformal mapping, we derive an analytic expression for the thickness d of a deposited film as a function of the distance r from a mask edge in selective area growth by organometallic chemical vapor deposition (OMCVD). Adjacent to the mask d∼r−1/2, which is clearly different from the exponential dependence observed for large planar surfaces. The result provides a means of distinguishing growth mechanisms in OMCVD.}, number={25}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Liu, X. and Aspnes, D. E.}, year={2009}, month={Jun}, pages={253112} }
 @article{arwin_aspnes_2009, title={Follow the light: Ellipsometry and polarimetry}, volume={62}, ISSN={["0031-9228"]}, DOI={10.1063/1.3141950}, abstractNote={By exploiting the polarization and wavelength dependence of optical responses, ellipsometers and polarimeters furnish accurate information about interfaces, materials, and even biological systems.}, number={5}, journal={PHYSICS TODAY}, author={Arwin, Hans and Aspnes, David E.}, year={2009}, month={May}, pages={70–71} }
 @article{kim_yoon_hwang_aspnes_kim_kim_chang_song_2009, title={Interband transitions of InAsxSb1-x alloy films}, volume={95}, ISSN={["1077-3118"]}, DOI={10.1063/1.3216056}, abstractNote={We report pseudodielectric-function spectra from 1.5 to 6.0 eV of InAsxSb1−x ternary alloys obtained by spectroscopic ellipsometry. Energies of the E1, E1+Δ1, E0′, E0′+Δ0′, E2, E2+Δ2, E2′, E2′+Δ2, and E1′ critical points (CPs) were obtained from numerically calculated second derivatives, and identifications established by band-structure calculations using the linear augmented Slater-type orbital method. The E2, E2+Δ2, E2′, and E2′+Δ2 CP structures cross with increasing As-composition. Two saddle-point transitions, Δ5cu-Δ5vu and Δ5cl−Δ5vu, are found for InSb.}, number={11}, journal={APPLIED PHYSICS LETTERS}, author={Kim, T. J. and Yoon, J. J. and Hwang, S. Y. and Aspnes, D. E. and Kim, Y. D. and Kim, H. J. and Chang, Y. C. and Song, J. D.}, year={2009}, month={Sep} }
 @article{franzen_rhodes_cerruti_gerber_losego_maria_aspnes_2009, title={Plasmonic phenomena in indium tin oxide and ITO-Au hybrid films}, volume={34}, ISSN={["1539-4794"]}, DOI={10.1364/OL.34.002867}, abstractNote={The observation of surface-plasmon resonances in indium tin oxide (ITO) thin films is complemented with the effects of hybrid ITO/Au conducting layers where charge densities can be tuned. Where carrier densities are similar (ITO and nanoparticle Au), the plasmonic behavior is that of a monolithic ITO thin film. Where the carrier density of one layer is much greater than that of the other (ITO and Au metal), boundary conditions lead to cancelation of the surface plasmon. In the latter case a capacitivelike plasmon resonance is observed for sufficiently thin films.}, number={18}, journal={OPTICS LETTERS}, author={Franzen, Stefan and Rhodes, Crissy and Cerruti, Marta and Gerber, Ralph W. and Losego, Mark and Maria, Jon-Paul and Aspnes, D. E.}, year={2009}, month={Sep}, pages={2867–2869} }
 @article{kim_yoon_kim_aspnes_klein_ko_kim_elarde_coleman_2008, title={Analysis of interface layers by spectroscopic ellipsometry}, volume={255}, ISSN={["1873-5584"]}, DOI={10.1016/j.apsusc.2008.07.005}, abstractNote={Abstract We investigate the relative validity of the Bruggeman effective-medium approximation and several alloy models to describe interfaces in the analysis of spectroscopic ellipsometric data of laminar samples, using data obtained on an Al x Ga 1− x As multilayer sample fabricated specifically for this purpose. The investigation highlights the types of errors that result from the use of inappropriate models. Optimum results are obtained with the alloy model where the graded-composition regions are approximated with multilayer stacks.}, number={3}, journal={APPLIED SURFACE SCIENCE}, author={Kim, T. J. and Yoon, J. J. and Kim, Y. D. and Aspnes, D. E. and Klein, M. V. and Ko, D. -S. and Kim, Y. -W. and Elarde, V. C. and Coleman, J. J.}, year={2008}, month={Nov}, pages={640–642} }
 @article{adles_aspnes_2008, title={Application of the anisotropic bond model to second-harmonic generation from amorphous media}, volume={77}, ISSN={["1098-0121"]}, DOI={10.1103/physrevb.77.165102}, abstractNote={As a step toward analyzing second-harmonic generation SHG from crystalline Si nanospheres in glass, we develop an anisotropic bond model ABM that expresses SHG in terms of physically meaningful parameters and provide a detailed understanding of the basic physics of SHG on the atomic scale. Nonlinear-optical NLO responses are calculated classically via the four fundamental steps of optics: evaluate the local field at a given bond site, solve the force equation for the acceleration of the charge, calculate the resulting radiation, then superpose the radiation from all charges. Because the emerging NLO signals are orders of magnitude weaker and occur at wavelengths different from that of the pump beam, these steps are independent. Paradoxically, the treatment of NLO is therefore simpler than that of linear optics LO, where these calculations must be done self-consistently. The ABM goes beyond previous bond models by including the complete set of underlying contributions: retardation RD, spatial-dispersion SD, and magnetic MG effects, in addition to the anharmonic restoring force acting on the bond charge. Transverse as well as longitudinal motion is also considered. We apply the ABM to obtain analytic expressions for SHG from amorphous materials under Gaussian-beam excitation. These materials represent an interesting test case not only because they are ubiquitous but also because the anharmonic-force contribution that dominates the SHG response of crystalline materials and ordered interfaces vanishes by symmetry. The remaining contributions, and hence the SHG signals, are entirely functions of the LO response and beam geometry, so the only new information available is the anisotropy of the LO response at the bond level. The RD, SD, and MG contributions are all of the same order of magnitude, so none can be ignored. Diffraction is important in determining not only the pattern of the emerging beam but also the phases and amplitudes of the different terms. The plane-wave expansion that gives rise to electric quadrupole magnetic dipole effects in LO appears here as retardation. Using the paraxial-ray approximation, we reduce the results to the isotropic case in two limits, that where the linear restoring force dominates glasses and that where it is absent metals. Both forward- and backscattering geometries are discussed. Estimated signal strengths and conversion efficiencies for fused silica appear to be in general agreement with data where available. Predictions that allow additional critical tests of these results are made.}, number={16}, journal={PHYSICAL REVIEW B}, author={Adles, E. J. and Aspnes, D. E.}, year={2008}, month={Apr} }
 @article{rhodes_cerruti_efremenko_losego_aspnes_maria_franzen_2008, title={Dependence of plasmon polaritons on the thickness of indium tin oxide thin films}, volume={103}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.2908862}, DOI={10.1063/1.2908862}, abstractNote={The evolution of polariton features with increasing thickness in p-polarized (TM) reflectance spectra of indium tin oxide (ITO) thin films deposited on BK7 glass reveals the nature of plasmons in conducting thin films without interference from band-to-band transitions or the tendency of very thin films to form islands, both of which are complicating factors with the noble metals Au and Ag. Although the dependence on energy, film thickness, and angle of incidence is complex, these features are accurately described by the three-phase (substrate/overlayer/ambient) Fresnel model using only the Drude free-electron representation for the dielectric function of the ITO film. For film thicknesses less than 80nm the relevant excitation is a one-dimensional screened-bulk plasmon (SBP) that corresponds to charge transfer across the entire film. The associated SBP polariton (SBPP) occurs at the energy of the SBP and is relatively independent of the angle of incidence. For film thicknesses greater than 120nm, the relevant excitation is the surface plasmons (SP). The associated surface plasmon polariton (SPP) exhibits the usual strong dependence of energy on the angle of incidence. For larger thicknesses this structure gradually weakens, in agreement with theory. No other collective excitations are observed. The optimum thicknesses for the SPP in ITO is 160nm, whereas the SBPP is observed only when the film thickness is less than 70nm. The SBPP exhibits many of the features that make the SPP attractive for both science and technology, but has not been observed previously. Our results show that ITO films, in particular, and conducting-metal-oxide films in general provide new opportunities for investigating plasmons in conductors and obtaining new insights into plasmons, plasmon polaritons, and related optical phenomena.}, number={9}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Rhodes, C. and Cerruti, M. and Efremenko, A. and Losego, M. and Aspnes, D. E. and Maria, J.-P. and Franzen, S.}, year={2008}, month={May}, pages={093108} }
 @article{choi_aspnes_stoute_kim_kim_chang_palmstrom_2008, title={Dielectric properties of InAsP alloy thin films and evaluation of direct- and reciprocal-space methods of determining critical-point parameters}, volume={205}, ISSN={["1862-6300"]}, DOI={10.1002/pssa.200777848}, abstractNote={Abstract}, number={4}, journal={PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE}, author={Choi, S. G. and Aspnes, D. E. and Stoute, N. A. and Kim, Y. D. and Kim, H. J. and Chang, Y. -C. and Palmstrom, C. J.}, year={2008}, month={Apr}, pages={884–887} }
 @article{liu_kim_aspnes_2008, title={Investigation of heteroepitaxy on nanoscopically roughened (001)Si by real-time spectroscopic polarimetry}, volume={5}, ISSN={1610-1634 1610-1642}, url={http://dx.doi.org/10.1002/pssc.200777896}, DOI={10.1002/pssc.200777896}, abstractNote={Abstract}, number={5}, journal={physica status solidi (c)}, publisher={Wiley}, author={Liu, X. and Kim, I.-K. and Aspnes, D. E.}, year={2008}, month={May}, pages={1312–1315} }
 @article{jung_kim_yoon_kim_aspnes_2008, title={Model dielectric functions for AlxGa1-xAs alloys of arbitrary compositions}, volume={104}, ISSN={["1089-7550"]}, DOI={10.1063/1.2952536}, abstractNote={Many optical models have been used to construct analytic composition-dependent dielectric functions of AlxGa1−xAs alloys. However, these models incorporate various unphysical assumptions to improve their fits to data. Here, we provide the parameters needed to calculate dielectric functions of AlxGa1−xAs for 1.5≤E≤6.0 eV and 0≤x≤1 by means of the parametric model of Johs et al. [Thin Solid Films 313–314, 137 (1998)], which eliminates these problems. A representative example concerning interface analysis is discussed, where it is necessary to construct a dielectric function of an alloy of essentially arbitrary composition.}, number={1}, journal={JOURNAL OF APPLIED PHYSICS}, author={Jung, Y. W. and Kim, T. J. and Yoon, J. J. and Kim, Y. D. and Aspnes, D. E.}, year={2008}, month={Jul} }
 @misc{aspnes_2008, title={Normal incidence rotating compensator ellipsometer}, volume={7,355,708}, number={2008 Apr. 8}, publisher={Washington, DC: U.S. Patent and Trademark Office}, author={Aspnes, D. E.}, year={2008} }
 @article{yoon_ghong_byun_kim_aspnes_kim_chang_song_2008, title={Optical properties of InxAl1-xAs alloy films}, volume={92}, ISSN={["1077-3118"]}, DOI={10.1063/1.2909546}, abstractNote={Pseudodielectric functions ⟨ε⟩ of InxAl1−xAs ternary alloy films were determined from 1.5to6.0eV by spectroscopic ellipsometry. We minimized overlayer effects by performing wet-chemical etching to more accurately determine intrinsic bulk dielectric responses. Energies of the E1, E1+Δ1, E0′, E2, E2+Δ2 and E2′ critical points (CPs) were identified by band structure calculations of the linear augmented Slater-type orbital method. These calculations also showed a crossing of the E0′ and E2 CP structures with increasing In composition and a new saddle point in the AlAs band structure.}, number={15}, journal={APPLIED PHYSICS LETTERS}, author={Yoon, J. J. and Ghong, T. H. and Byun, J. S. and Kim, Y. D. and Aspnes, D. E. and Kim, H. J. and Chang, Y. C. and Song, J. D.}, year={2008}, month={Apr} }
 @article{ghong_kim_jung_kim_aspnes_2008, title={Overlayer effects in the critical-point analysis of ellipsometric spectra: Application to InxGa1-xAs alloys}, volume={103}, ISSN={["1089-7550"]}, DOI={10.1063/1.2902502}, abstractNote={We investigate the effect of incomplete removal of semiconductor overlayers on critical-point (CP) parameters determined from the analysis of ellipsometric spectra. An approximate analytic expression shows that CP energies and broadening parameters should be relatively unaffected for isolated CPs if the dielectric response of the overlayer varies slowly with energy. The results are confirmed by model calculations for InAs, which show that the energies of the E1 and E1+Δ1 CP structures that are commonly used for compositional analysis of semiconductor alloys are relatively unaffected. We also analyze overlayer-removal data for a series of InxGa1−xAs alloy samples. Consistent with the above, the amplitudes and phases are affected significantly for all CPs, while the energies of the well-separated E1 and E1+Δ1 transitions are relatively invariant. The results show that accurate values of composition can be obtained from the analysis of the E1 and E1+Δ1 CP structures, even if complete removal of overlayers is not achieved.}, number={7}, journal={JOURNAL OF APPLIED PHYSICS}, author={Ghong, T. H. and Kim, T. J. and Jung, Y. W. and Kim, Y. D. and Aspnes, D. E.}, year={2008}, month={Apr} }
 @article{adles_aspnes_2008, title={The anisotropic bond model of nonlinear optics}, volume={205}, ISSN={1862-6300 1862-6319}, url={http://dx.doi.org/10.1002/pssa.200777846}, DOI={10.1002/pssa.200777846}, abstractNote={Abstract}, number={4}, journal={physica status solidi (a)}, publisher={Wiley}, author={Adles, E. J. and Aspnes, D. E.}, year={2008}, month={Apr}, pages={728–731} }
 @article{asar_aspnes_2008, title={The nearly aligned rotating-monoplate compensator}, volume={205}, ISSN={["1862-6300"]}, DOI={10.1002/pssa.200777871}, abstractNote={Abstract}, number={4}, journal={PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE}, author={Asar, M. and Aspnes, D. E.}, year={2008}, month={Apr}, pages={739–742} }
 @article{liu_aspnes_2008, title={Thickness inhomogenities in the organometallic chemical vapor deposition of GaP}, volume={93}, ISSN={["0003-6951"]}, DOI={10.1063/1.3029742}, abstractNote={We analyze exponential lateral-thickness variations observed in the growth of GaP on (001) GaAs, thermally generated SiO2, (001) Si, and nanoscopically roughened Si surfaces by organometallic chemical vapor deposition, using as a reference the polycrystalline GaP deposited on the Mo susceptor surrounding the 2in. wafers. We find these variations to be due to differences in the chemical reactivities of the various surfaces toward the generation of a precursor, probably a H–P=Ga–CH3 dimer adduct, by heterogeneous catalysis followed by desorption and diffusion through the gas phase.}, number={20}, journal={APPLIED PHYSICS LETTERS}, author={Liu, X. and Aspnes, D. E.}, year={2008}, month={Nov} }
 @article{kim_aspnes_2007, title={Analytic determination of n, k, and d of an absorbing film from polarimetric data in the thin-film limit}, volume={101}, ISSN={["1089-7550"]}, DOI={10.1063/1.2434004}, abstractNote={We obtain a solution of the three-phase model in the limit d∕λ⪡1, where the complex refractive index ñ=n+iκ and thickness d of an isotropic film on an isotropic substrate are given analytically at any single wavelength λ from polarimetric data Δρ∕ρ and ΔR∕R, where ρ is the complex reflectance ratio and R is either the p- or s-polarized reflectance. We describe several procedures for extending the range of validity of the solution. Analysis of correlations shows that the uncertainty δ(ΔR∕R) of ΔR∕R is significantly more important than the δ(Δρ∕ρ) of Δρ∕ρ, which allows us to obtain an expression for the uncertainties δn, δκ, and δd of n, κ, and d, and to identify conditions that optimize the determination of the layer parameters. We find that the relative uncertainties δn∕n and δd∕d are not equal, as would be expected if they were determined by the optical thickness nd measured by ellipsometry, but that ΔR∕R breaks the connection. We verify our results by measurements of H2O reversibly physisorbed on oxidized GaAs, finding, for example, that for our conditions δκ is determined more accurately than δn, and δn more accurately than δd. These data and model calculations show that fluctuations in parameters, particularly d, are asymmetric, leading in principle to inaccurate average values. However, we show that the importance of the ΔR∕R data together with the remaining high correlation between n and d allows us to define a characteristic curve that can be used to correct the results for this nonlinearity. Finally, we extend our analysis to determine the orthogonal linear combinations of n, κ, and d that the data actually determine, which explains why the data fit the characteristic curve so well. Our results will be useful in various contexts for the analysis of films less than 1nm thick, for example, in applications involving preparation of next-generation electronic and optoelectronic devices with complicated multilayer structures, real-time control of deposition, and the identification of physisorbed and chemisorbed layers on the monolayer scale.}, number={3}, journal={JOURNAL OF APPLIED PHYSICS}, author={Kim, I. K. and Aspnes, D. E.}, year={2007}, month={Feb} }
 @article{choi_palmstrom_kim_aspnes_kim_chang_2007, title={Dielectric functions and electronic structure of InAsxP1-x films on InP}, volume={91}, ISSN={["0003-6951"]}, DOI={10.1063/1.2766682}, abstractNote={The authors present room-temperature pseudodielectric function spectra ⟨ε⟩ of InAsxP1−x films grown on (001) InP by chemical beam epitaxy. A wet-chemical etching procedure was used to remove overlayers and obtain the best approximation to the bulk dielectric responses ε of the films. By line shape fitting, we determine the compositional dependences of the E1, E1+Δ1, E0′, E2, and E2′ critical-point energies. The results are in good agreement with calculations of the electronic structure done by the linear augmented Slater-type orbital method.}, number={4}, journal={APPLIED PHYSICS LETTERS}, author={Choi, S. G. and Palmstrom, C. J. and Kim, Y. D. and Aspnes, D. E. and Kim, H. J. and Chang, Yia-Chung}, year={2007}, month={Jul} }
 @article{jung_ghong_kim_aspnes_2007, title={Effect of overlayers on critical-point parameters in the analysis of ellipsometric spectra}, volume={91}, ISSN={["1077-3118"]}, DOI={10.1063/1.2784187}, abstractNote={As the complete removal of overlayers may not be possible in general, the authors investigate the effect of incomplete removal on critical-point parameters in the analysis of ellipsometric data. Using an approximate analytic expression, they show that energies and broadening parameters are much less affected by overlayers than amplitudes and phases. These conclusions are confirmed by false-data calculations for GaAs and overlayer-removal data for CdTe.}, number={12}, journal={APPLIED PHYSICS LETTERS}, author={Jung, Y. W. and Ghong, T. H. and Kim, Y. D. and Aspnes, D. E.}, year={2007}, month={Sep} }
 @article{liu_kim_aspnes_2007, title={Initial stages of GaP heteroepitaxy on nanoscopically roughened (001)SI}, volume={25}, ISSN={["1071-1023"]}, DOI={10.1116/1.2750345}, abstractNote={The authors report real-time spectroscopic-polarimetric determinations of the initial phase of GaP heteroepitaxy by organometallic chemical vapor deposition on nanoscopically roughened (NR) (001)Si substrates, where polarimetry measurements are also used to quantify roughness. The authors compare the results with analogous data for GaP homoepitaxy and the initial phase of GaP heteroepitaxy on (001)GaAs. The large density of nucleation sites on NRSi significantly improves film continuity relative to nonroughened vicinal (001)Si substrates, but conditions that are typically used to grow GaP on (001)III-V surfaces generate metallic Ga, indicating that NRSi is more efficient at decomposing trimethylgallium than either GaP or GaAs.}, number={4}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B}, author={Liu, X. and Kim, I. K. and Aspnes, D. E.}, year={2007}, pages={1448–1452} }
 @article{kim_ghong_kim_aspnes_klein_ko_kim_elarde_coleman_2007, title={Investigation of effective-medium approximation, alloy, average-composition, and graded-composition models for interface analysis by spectroscopic ellipsometry}, volume={102}, ISSN={["1089-7550"]}, DOI={10.1063/1.2781519}, abstractNote={We critically test the capabilities of the effective-medium approximation (EMA) and alloy models to describe multilayer samples with gradual interfaces by analyzing spectroscopic ellipsometric (SE) data of two AlGaAs samples grown expressly for this purpose. The dielectric functions ε of the interfaces are calculated in the EMA and alloy models, and the interfaces themselves simulated either as a single layer of Al0.5Ga0.5As or a stack of layers of AlxGa1−xAs with x increasing or decreasing between 0.1 and 0.9 in increments of 0.1. The EMA essentially fails completely for either interface representation. For the alloy model the stepwise-graded representation is significantly better, not only simulating the data more accurately but also yielding thicknesses in essential agreement with those obtained by cross-sectional transmission electron microscopy. The results highlight the types of errors that are encountered with the different models, and show that the analysis of SE data can provide information about these interfaces.}, number={6}, journal={JOURNAL OF APPLIED PHYSICS}, author={Kim, T. J. and Ghong, T. H. and Kim, Y. D. and Aspnes, D. E. and Klein, M. V. and Ko, D-S. and Kim, Y-W. and Elarde, V. C. and Coleman, J. J.}, year={2007}, month={Sep} }
 @misc{aspnes_2007, title={Normal incidence rotating compensator ellipsometer}, volume={7,173,700}, number={2007 Feb. 6}, publisher={Washington, DC: U.S. Patent and Trademark Office}, author={Aspnes, D. E.}, year={2007} }
 @article{ghong_kim_aspnes_klein_ko_kim_elarde_coleman_2006, title={Interface analysis of an AlGaAs multilayer system by using spectroscopic ellipsometry}, volume={48}, number={6}, journal={Journal of the Korean Physical Society}, author={Ghong, T. H. and Kim, Y. D. and Aspnes, D. E. and Klein, M. V. and Ko, D. S. and Kim, Y. W. and Elarde, V. and Coleman, J.}, year={2006}, pages={1601–1605} }
 @article{lucovsky_fulton_ju_stoute_tao_aspnes_luening_2006, title={Suppression of Jahn-Teller term-split band edge states in the x-ray absorption spectra of non-crystalline Zr silicates and Si oxynitride alloys, and alloys of ZrO2 with Y2O3}, volume={75}, ISSN={["0969-806X"]}, DOI={10.1016/j.radphyschem.2006.05.004}, abstractNote={Jahn–Teller (J–T) term-split states in nanocrystalline transition metal and trivalent rare earth elemental and complex oxides reduce the band gap, and tunnelling barrier height at interfaces with crystalline Si substrates. These states are identified by x-ray absorption spectroscopy and spectroscopic ellipsometry. Alloys for suppression of J–T d-state degeneracy removal are identified as: (i) non-crystalline Zr/Hf silicates and Si oxynitrides and (ii) ZrO2–Y2O3 alloys with high concentrations of randomly distributed O-vacancies that promote cubic crystalline symmetry.}, number={11}, journal={RADIATION PHYSICS AND CHEMISTRY}, author={Lucovsky, G. and Fulton, C. C. and Ju, B. S. and Stoute, N. A. and Tao, S. and Aspnes, D. E. and Luening, J.}, year={2006}, month={Nov}, pages={1591–1595} }
 @article{brinkley_powell_aspnes_2006, title={Systematic approach for analyzing reflectance-difference spectra: Application to silicon-dielectric interfaces}, volume={88}, ISSN={["1077-3118"]}, DOI={10.1063/1.2204844}, abstractNote={We describe a combinatorial approach for analyzing reflectance-difference/reflectance-anisotropy (RD/RA) spectra that avoids the need to establish null orientations experimentally, suppresses experimental artifacts, signal averages, and allows RD/RA spectra to be assessed systematically for secondary contributions at principal angles different from that of the dominant contribution. Application to rapid-thermal-annealed oxidized and nitrided vicinal (111) Si-dielectric interfaces demonstrates the effectiveness of the procedure.}, number={20}, journal={APPLIED PHYSICS LETTERS}, author={Brinkley, M. K. and Powell, G. D. and Aspnes, D. E.}, year={2006}, month={May} }
 @article{kim_aspnes_2006, title={Toward n kappa d spectroscopy: Analytic solution of the three-phase model of polarimetry in the thin-film limit}, volume={88}, ISSN={["1077-3118"]}, DOI={10.1063/1.2203967}, abstractNote={We find an analytic solution of the three-phase (substrate/overlayer/ambient) model of polarimetry in the thin-film limit, which allows overlayer thicknesses d and refractive indices ñ=n+iκ to be determined from measured changes Δρ∕ρ of the complex reflectance ratio ρ and ΔRp∕Rp or ΔRs∕Rs of the p- or s-polarized reflectances Rp or Rs, from a quadratic equation without the need for potentially unstable numerical methods. We also find a transformation of the data that extends the range of accuracy by up to an order of magnitude without introducing additional mathematical complications. The results are illustrated by the application to a layer of physisorbed H2O on oxidized GaAs, and show that the wavelength-by-wavelength spectroscopy of adsorbed monolayers is now within range of existing polarimetric technology.}, number={20}, journal={APPLIED PHYSICS LETTERS}, author={Kim, I. K. and Aspnes, D. E.}, year={2006}, month={May} }
 @article{lucovsky_hong_fulton_stoute_zou_nemanich_aspnes_ade_schlom_2005, title={Conduction band states of transition metal (TM) high-k gate dielectrics as determined from X-ray absorption spectra}, volume={45}, ISSN={["0026-2714"]}, DOI={10.1016/j.microrel.2004.11.038}, abstractNote={This paper uses X-ray absorption spectroscopy to study the electronic structure of the high-k gate dielectrics including TM and RE oxides. The results are applicable to TM and rare earth (RE) silicate and aluminate alloys, as well as complex oxides comprised of mixed TM/TM and TM/RE oxides. These studies identify the nature of the lowest conduction band d∗ states, which define the optical band gap, Eg, and the conduction band offset energy with respect to crystalline Si, EB. Eg and EB scale with the atomic properties of the TM and RE atoms providing important insights for identification high-k dielectrics that meet performance targets for advanced CMOS devices.}, number={5-6}, journal={MICROELECTRONICS RELIABILITY}, author={Lucovsky, G and Hong, JG and Fulton, CC and Stoute, NA and Zou, Y and Nemanich, RJ and Aspnes, DE and Ade, H and Schlom, DG}, year={2005}, pages={827–830} }
 @article{choi_palmstrom_kim_cooper_aspnes_2005, title={Dielectric functions of AlxGa1-xSb (0.00 <= x <= 0.39) alloys from 1.5 to 6.0 eV}, volume={98}, ISSN={["1089-7550"]}, DOI={10.1063/1.2134890}, abstractNote={Room-temperature pseudodielectric function spectra ⟨ε(ω)⟩=⟨ε1(ω)⟩+i⟨ε2(ω)⟩ of AlxGa1−xSb (0.00⩽x⩽0.39) alloys were measured from 1.5to6.0eV by spectroscopic ellipsometry. Effects of surface overlayers on the measured spectra have been minimized as far as possible by performing in situ chemical etching on samples immediately before the measurements rather than correcting the measured data mathematically for the presence of overlayers. Analysis of second energy derivatives numerically calculated from the measured data yielded the critical-point energies of the E1, E1+Δ1, E0′, E2, and E1′ interband transitions. Dependence of the critical-point energies on composition x is obtained.}, number={10}, journal={JOURNAL OF APPLIED PHYSICS}, author={Choi, SG and Palmstrom, CJ and Kim, YD and Cooper, SL and Aspnes, DE}, year={2005}, month={Nov} }
 @article{peng_aspnes_2005, title={Dipole-radiation model for terahertz radiation from semiconductors}, volume={86}, ISSN={["0003-6951"]}, DOI={10.1063/1.1937992}, abstractNote={We present a simple dipole-radiation model for the generation of terahertz radiation from semiconductors under short-pulse excitation. Using a one-dimensional force model applied to bond charges and free carriers, we evaluate both current-surge and frequency-mixing contributions to obtain analytic expressions that provide an integrated description of previous results.}, number={21}, journal={APPLIED PHYSICS LETTERS}, author={Peng, HJ and Aspnes, DE}, year={2005}, month={May} }
 @article{choi_srivastava_palmstrom_kim_cooper_aspnes_2005, title={Optical properties of (GaSb)(3n)(AlSb)(n) (1 <= n <= 5) superlattices}, volume={23}, ISSN={["2166-2746"]}, DOI={10.1116/1.1881552}, abstractNote={We report room-temperature pseudodielectric function spectra ⟨ε(ω)⟩=⟨ε1(ω)⟩+i⟨ε2(ω)⟩ from 1.5 to 6.0 eV of a series of (GaSb)3n(AlSb)n(1⩽n⩽5) short-period superlattices (SLs) and the corresponding ternary alloy, Al0.3Ga0.7Sb, as determined by spectroscopic ellipsometry. An in situ chemical etching procedure was performed to minimize the effect of surface oxides and other overlayers. For small periods (n⩽2), the dielectric responses of the SLs are similar to those of the random alloy. For larger n, structures due to the E1, E1+Δ1, E0′, E2, and E1′ interband transitions can be seen, with the E1, E1+Δ1, and E0′ structures showing a clear SL-period dependence. Two additional features were also observed near the E1+Δ1 structure for n⩾4.}, number={3}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B}, author={Choi, SG and Srivastava, SK and Palmstrom, CJ and Kim, YD and Cooper, SL and Aspnes, DE}, year={2005}, pages={1149–1153} }
 @misc{aspnes_2005, title={Real-time diagnostics for metalorganic vapor phase epitaxy}, volume={242}, ISSN={["0370-1972"]}, DOI={10.1002/pssb.200541109}, abstractNote={Abstract}, number={13}, journal={PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS}, author={Aspnes, DE}, year={2005}, month={Nov}, pages={2551–2560} }
 @article{peng_adles_wang_aspnes_2005, title={Relative bulk and interface contributions to optical second-harmonic generation in silicon}, volume={72}, ISSN={["1098-0121"]}, DOI={10.1103/physrevb.72.205203}, abstractNote={Using the simplified bond-hyperpolarizability model, we obtain analytic expressions for the first-forbidden spatial dispersion, magnetic dipole/electric quadrupole bulk contributions to second-harmonic generation for centrosymmetric materials. Applying these to oxidized Si, we show theoretically and by comparison to experiment that the relative bulk contribution near 800 nm is minor, less than half that of the interface, but that the coherent superposition of bulk and interface contributions is important and cannot be neglected.}, number={20}, journal={PHYSICAL REVIEW B}, author={Peng, HJ and Adles, EJ and Wang, JFT and Aspnes, DE}, year={2005}, month={Nov} }
 @article{ebert_aspnes_2004, title={Biplate artifacts in rotating-compensator ellipsometers}, volume={455}, ISSN={["0040-6090"]}, DOI={10.1016/j.tsf.2004.01.033}, abstractNote={Data obtained on rotating-compensator ellipsometers using biplate retarders often exhibit small interference oscillations whose periods are not obviously related to any of the dimensions of the biplate. We show that these artifacts are due to a non-linear second-order interference effect that originates with the boundary condition on the outgoing wave and depends critically on the spacing of the air gap between plates. The latter characteristic makes them component-specific and hence difficult to control in production. They can be eliminated by filling the air gap with an index-matching medium, or by replacing the biplate with a single-plate device. Degree-of-polarization data are in agreement with theoretical predictions.}, number={2004 May 1}, journal={THIN SOLID FILMS}, author={Ebert, K and Aspnes, DE}, year={2004}, month={May}, pages={779–783} }
 @misc{aspnes_opsal_2004, title={Broadband spectroscopic rotating compensator ellipsometer}, volume={6,831,743}, number={2004 Dec. 14}, publisher={Washington, DC: U.S. Patent and Trademark Office}, author={Aspnes, D. E. and Opsal, J.}, year={2004} }
 @article{peng_aspnes_2004, title={Calculation of bulk third-harmonic generation from crystalline Si with the simplified bond hyperpolarizability model}, volume={70}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.70.165312}, abstractNote={We apply the simplified bond hyperpolarizability model to calculate bulk third-harmonic generation from tetrahedrally bonded crystals of cubic symmetry, comparing the results to data for crystalline Si. We find very good agreement between model predictions and data. Only scaling is needed for the $ps$ and $sp$ polarization combinations, and scaling and offsets for the $pp$ and $ss$ combinations. In particular, the anisotropies themselves are predicted with no adjustable parameters. The need to adjust scaling and offset separately for $pp$ and $ss$ shows that the transverse as well as the longitudinal hyperpolarizability must be included. When used as an adjustable fitting parameter the angle of incidence agrees satisfactorily with that calculated from Snell's law, further emphasizing the validity of the model and the essentially bulk origin of the response. We obtain analytic expressions for both longitudinal and transverse contributions for on-axis (111), (001), and (110) orientations, and establish the connection between the microscopic bulk hyperpolarizabilities and the macroscopic nonvanishing components of the fourth-rank susceptibility tensor.}, number={16}, journal={PHYSICAL REVIEW B}, author={Peng, HJ and Aspnes, DE}, year={2004}, month={Oct} }
 @article{mori_aspnes_2004, title={Comparison of the capabilities of rotating-analyzer and rotating-compensator ellipsometers by measurements on a single system}, volume={455}, ISSN={["0040-6090"]}, DOI={10.1016/j.tsf.2003.12.037}, abstractNote={We take advantage of a spectroscopic ellipsometer that can operate in both rotating-analyzer (RAE) and rotating-compensator (RCE) modes to compare directly the capabilities of the two configurations. The extra information available from the additional Fourier component of the transmitted intensity for the RCE provides considerable advantages both with respect to identifying artifacts and obtaining more accurate data. For example, our comparison revealed a previously unsuspected polarization-leakage artifact that affects RAE data to as much as several percent. Comparative measurements on Ge and ZnO show the RCE has greatly reduced sensitivity to artifacts in general, and particularly those resulting from wavelength-averaging interference effects in thin films.}, number={2004 May 1}, journal={THIN SOLID FILMS}, author={Mori, T and Aspnes, DE}, year={2004}, month={May}, pages={33–38} }
 @article{aspnes_2004, title={Expanding horizons: new developments in ellipsometry and polarimetry}, volume={455}, ISSN={["0040-6090"]}, DOI={10.1016/j.tsf.2003.12.038}, abstractNote={This is the eighth in the aperiodic series of ellipsometry conferences, and the third devoted specifically to spectroscopic ellipsometry. I discuss the evolution of the field mainly from a technological perspective, and the changes that must be made in analytic procedures as accuracy requirements are increased from 1 to 0.1%. The current interest in Mueller-matrix spectroscopy motivates a discussion on how to include depolarization artifacts in Mueller-matrix calculations from first principles. Representative applications include extensions to the far infrared and vacuum ultraviolet, the determination of all elements of the Mueller matrix with a stationary sample, and the determination of critical dimensions by scatterometry.}, number={2004 May 1}, journal={THIN SOLID FILMS}, author={Aspnes, DE}, year={2004}, month={May}, pages={3–13} }
 @article{flock_kim_asar_kim_aspnes_2004, title={Integrated rotating-compensator polarimeter for real-time measurements and analysis of organometallic chemical vapor deposition}, volume={455}, ISSN={["0040-6090"]}, DOI={10.1016/j.tsf.2004.01.069}, abstractNote={We describe a single-beam rotating-compensator rotating-sample spectroscopic polarimeter (RCSSP) integrated with an organometallic chemical vapor deposition (OMCVD) reactor for in-situ diagnostics and control of epitaxial growth, and report representative results. The rotating compensator generates Fourier coefficients that provide information about layer thicknesses and compositions, while sample rotation provides information about optical anisotropy and therefore surface chemistry. We illustrate capabilities with various examples, including the simultaneous determination of 〈ε〉 and α10 during exposure of (001)GaAs to TMG, the heteroepitaxial growth of GaP on GaAs, and the growth of (001)GaSb with TMG and TMSb. Using a recently developed approach for quantitatively determining thickness and dielectric function of depositing layers, we find the presence of metallic Ga on TMG-exposed (001)GaAs. The (001)GaSb data show that Sb deposition is self-limiting, in contrast to expectations.}, number={2004 May 1}, journal={THIN SOLID FILMS}, author={Flock, K and Kim, SJ and Asar, M and Kim, IK and Aspnes, DE}, year={2004}, month={May}, pages={639–644} }
 @article{asar_aspnes_2004, title={Optical anisotropy relevant to rotating-compensator polarimeters: application to the monoplate retarder}, volume={455}, ISSN={["0040-6090"]}, DOI={10.1016/j.tsf.2003.12.044}, abstractNote={We analyze the optical properties of the monoplate compensator under misalignment conditions. The coupling between the boundary conditions and the secular equation in the crystal frame leads to a quartic secular equation, and certain representations of the mode vectors vanish for particular combinations of parameters. Misalignment affects mainly odd harmonics in the transmitted intensity. The even harmonics, which carry sample information, are affected only to second order. Results are in good agreement with experiment.}, number={2004 May 1}, journal={THIN SOLID FILMS}, author={Asar, M and Aspnes, DE}, year={2004}, month={May}, pages={50–53} }
 @article{ihn_kim_kim_aspnes_kossut_2004, title={Optical properties of Cd1-xMgxTe (x=0.00, 0.23, 0.31, and 0.43) alloy films}, volume={84}, ISSN={["1077-3118"]}, DOI={10.1063/1.1639506}, abstractNote={Pseudodielectric functions 〈ε〉 of Cd1−xMgxTe ternary alloy films of compositions x=0.00, 0.23, 0.31, and 0.43 have been determined from 1.5 to 6.0 eV by spectroscopic ellipsometry. We obtain approximations to the bulk dielectric functions ε by performing wet-chemical etching to remove overlayers and using parametric modeling to remove interference oscillations below the fundamental band gap. The values of the E0, E0+Δ0, E1, E1+Δ1, E2, and E0′ critical point energies and their x dependences at room temperature were determined from numerically calculated second energy derivatives of these data.}, number={5}, journal={APPLIED PHYSICS LETTERS}, author={Ihn, YS and Kim, TJ and Kim, YD and Aspnes, DE and Kossut, J}, year={2004}, month={Feb}, pages={693–695} }
 @article{aspnes_2004, title={Optimizing precision of rotating-analyzer and rotating-compensator-ellipsometers}, volume={21}, ISSN={["1084-7529"]}, DOI={10.1364/JOSAA.21.000403}, abstractNote={I investigate the dependence of shot-noise-limited uncertainties of the ellipsometric parameters psi, and delta for the rotating-analyzer ellipsometer (RAE) and the rotating-compensator ellipsometer (RCE) of the polarizer-sample-compensator-analyzer type. The development is general and takes into account correlations among the Fourier coefficients of the transmitted intensity, in particular the average intensity, which is necessarily correlated with all other coefficients through normalization. The results are expressed in terms of the traditional uncertainties delta(psi) and delta(delta) of the ellipsometric parameters psi and delta, respectively, although a more appropriate measure of uncertainty is the differential area 2delta(psi) x sin psi(delta)delta on the unit-radius Poincaré sphere. Numerical results for broadband operation from 1.5 to 6.0 eV with a Si sample show that the optimum measurement conditions for both configurations occur when the intensity of light reflected from the sample is approximately balanced between the TE and the TM modes, and, for the RCE, when the analyzer azimuth is essentially equal to that of the polarizer. Under typical broadband operating conditions in which components cannot be optimized on a wavelength-by-wavelength basis, the RCE is better at determining delta, whereas the RAE is better at determining psi. The approach is easily generalized to other configurations and other types of experimental uncertainty, both random and systematic.}, number={3}, journal={JOURNAL OF THE OPTICAL SOCIETY OF AMERICA A-OPTICS IMAGE SCIENCE AND VISION}, author={Aspnes, DE}, year={2004}, month={Mar}, pages={403–410} }
 @article{ihn_kim_ghong_kim_aspnes_kossut_2004, title={Parametric modeling of the dielectric functions of Cd1-xMgxTe alloy films}, volume={455}, ISSN={["0040-6090"]}, DOI={10.1016/j.tsf.2004.01.015}, abstractNote={We report of parameters by which one can construct dielectric functions of Cd1−xMgxTe alloy films for any composition x (0≤x≦0.43) in the energy range 1.5–6.0 eV using the parametric semiconductor model. The parametric model describes analytically the dielectric functions of semiconductor materials as a sum of Gaussian-broadened polynomials and provides a reasonably accurate representation. The dielectric function spectra we used as a basis were obtained on films with Mg compositions x=0.00, 0.23, 0.31 and 0.43, with overlayer effects minimized by chemical etching. The appearance of well-defined interference–oscillation patterns below the E0 bandgap and the excellent separation of the E2 peaks confirm that the dielectric functions constructed in this work represent the best room-temperature optical responses of Cd1−xMgxTe alloys (0.00≤x=0.4) from 1.5 to 6.0 eV to date.}, number={2004 May 1}, journal={THIN SOLID FILMS}, author={Ihn, YS and Kim, TJ and Ghong, TH and Kim, YD and Aspnes, DE and Kossut, J}, year={2004}, month={May}, pages={222–227} }
 @article{kim_flock_asar_kim_aspnes_2004, title={Real-time characterization of GaSb homo- and heteroepitaxy}, volume={22}, ISSN={["1071-1023"]}, DOI={10.1116/1.1771669}, abstractNote={We examine the homo- and heteroepitaxial growth of moderately thick (∼700 nm) layers of GaSb with the objectives of optimizing growth conditions and determining the initial phase of heteroepitaxy on (001)GaAs. Real-time spectroscopic ellipsometry (RTSE) data show that the (001)GaSb surface degrades immediately in excess trimethylgallium (TMG), but both (001)GaSb and As-terminated (001)GaAs surfaces are stable in trimethylantimony (TMSb). The surface-dimer contribution to the optical-anisotropy (OA) signal of (001)GaSb is small and generally masked by structural (roughness) effects, hence it is not useful for determining surface stoichometry. However, we show that laser light scattering (LLS) data do allow the V/III ratio to be fine-tuned during growth to minimize macroscopic roughness. TEM micrographs show that our GaSb/GaAs heterointerface is relatively defect-free except for the necessary local accommodation of lattice mismatch. The initial phase of heteroepitaxy on (001)GaAs occurs here as a coexistence of separate regions of GaAs and GaSb.}, number={4}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B}, author={Kim, S and Flock, KL and Asar, M and Kim, IK and Aspnes, DE}, year={2004}, pages={2233–2239} }
 @article{ghong_kim_kim_aspnes_2004, title={Spectroscopic ellipsometric analysis of interfaces: Comparison of alloy and effective-medium-approximation approaches to a CdMgTe multilayer system}, volume={85}, ISSN={["1077-3118"]}, DOI={10.1063/1.1779965}, abstractNote={We discuss the accuracy and detectability of interface layers in the analysis of ellipsometric spectra in the CdxMg1−xTe system. Using parametric-alloy and effective-medium-approximation (EMA) representations to simulate interfaces in a single-quantum-well structure, we show that EMA analysis overestimates thicknesses of alloy interfaces by more than a factor of 3. While detailed results will clearly depend on the nature of the epitaxial materials involved, the results suggest that analyses of interfaces by the EMA should be done with caution.}, number={6}, journal={APPLIED PHYSICS LETTERS}, author={Ghong, TH and Kim, TJ and Kim, YD and Aspnes, DE}, year={2004}, month={Aug}, pages={946–948} }
 @article{hansen_peng_aspnes_2003, title={Application of the simplified bond-hyperpolarizability model to fourth-harmonic generation}, volume={21}, ISSN={["1071-1023"]}, DOI={10.1116/1.1593057}, abstractNote={We show that the simplified bond-hyperpolarizability model (SBHM), previously used to accurately describe anisotropies observed in second-harmonic generation from Si–dielectric interfaces, also describes anisotropies observed in fourth-harmonic generation (FHG). FHG data from (001)Si–SiO2 interfaces show two contributions: one from the intrinsic response of the interface bonds and the second from roughness. SBHM calculations yield excellent agreement with scaled ps and ss polarization data and the phases of the pp and sp equivalents with no adjustable parameters. The SHBM also precisely reproduces the anisotropy data for samples with systematic variations of interface roughness, and predicts relative amplitudes consistent with measured fields to factors of about 2.}, number={4}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B}, author={Hansen, JK and Peng, HJ and Aspnes, DE}, year={2003}, pages={1798–1803} }
 @misc{aspnes_opsal_2003, title={Broadband spectroscopic rotating compensator ellipsometer}, volume={6,650,415}, number={2003 Nov. 28}, publisher={Washington, DC: U.S. Patent and Trademark Office}, author={Aspnes, D. E. and Opsal, J.}, year={2003} }
 @article{ihn_kim_kim_aspnes_kossut_2003, title={Dielectric functions of Cd1-xMgxTe alloy films by uusing spectroscopic ellipsometry}, volume={43}, number={4}, journal={Journal of the Korean Physical Society}, author={Ihn, Y. S. and Kim, T. J. and Kim, Y. D. and Aspnes, D. E. and Kossut, J.}, year={2003}, pages={634–637} }
 @article{kim_ghong_kim_kim_aspnes_mori_yao_koo_2003, title={Dielectric functions of InxGa1-xAs alloys}, volume={68}, ISSN={["1098-0121"]}, DOI={10.1103/physrevb.68.115323}, abstractNote={We present room-temperature pseudodielectric function spectra $〈\ensuremath{\varepsilon}〉=〈{\ensuremath{\varepsilon}}_{1}〉+i〈{\ensuremath{\varepsilon}}_{2}〉$ of ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ films grown on (100) InP by solid-source molecular-beam epitaxy. A wet-chemical etching procedure was used to remove overlayers and obtain the best approximation to the bulk dielectric responses $\ensuremath{\varepsilon}={\ensuremath{\varepsilon}}_{1}+i{\ensuremath{\varepsilon}}_{2}$ of the films. By lineshape fitting we determined the dependences of the ${E}_{1}{,E}_{1}+{\ensuremath{\Delta}}_{1}{,E}_{0}^{\ensuremath{'}}{,E}_{0}^{\ensuremath{'}}+{\ensuremath{\Delta}}_{0}^{\ensuremath{'}},$ and ${E}_{2}$ critical-point energies on x. Using a parametric semiconductor model we represent these spectra analytically to allow $〈\ensuremath{\varepsilon}〉$ to be calculated as a continuous function of x. These results are expected to be useful for design purposes, for example of nanostructures and multilayer systems involving ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}.$}, number={11}, journal={PHYSICAL REVIEW B}, author={Kim, TJ and Ghong, TH and Kim, YD and Kim, SJ and Aspnes, DE and Mori, T and Yao, T and Koo, BH}, year={2003}, month={Sep} }
 @article{ihn_ghong_kim_kim_aspnes_yao_koo_2003, title={Optical properties of InGaAs alloy films in the E-2 region by spectroscopic ellipsometry}, volume={42}, number={2003 Feb}, journal={Journal of the Korean Physical Society}, author={Ihn, Y. S. and Ghong, T. H. and Kim, Y. D. and Kim, S. J. and Aspnes, D. E. and Yao, T. and Koo, B. H.}, year={2003}, pages={S242–245} }
 @article{aspnes_hansen_peng_powell_wang_2003, title={Simplified bond-hyperpolarizability model of second- and fourth-harmonic generation: application to Si-SiO2 interfaces}, volume={240}, ISSN={["1521-3951"]}, DOI={10.1002/pssb.200303825}, abstractNote={Abstract}, number={3}, journal={PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS}, author={Aspnes, DE and Hansen, JK and Peng, HJ and Powell, GD and Wang, JFT}, year={2003}, month={Dec}, pages={509–517} }
 @article{ghong_kim_kim_kim_aspnes_choi_yu_2003, title={Study of the dielectric function of ZnS by spectroscopic ellipsometry}, volume={42}, number={2003 Feb}, journal={Journal of the Korean Physical Society}, author={Ghong, T. H. and Kim, T. J. and Kim, Y. D. and Kim, S. J. and Aspnes, D. E. and Choi, Y. D. and Yu, Y. M.}, year={2003}, pages={S238–241} }
 @misc{aspnes_opsal_2002, title={Broadband spectroscopic rotating compensator ellipsometer}, volume={6,449,043}, number={2002 Sep. 10}, publisher={Washington, DC: U.S. Patent and Trademark Office}, author={Aspnes, D. E. and Opsal, J.}, year={2002} }
 @article{blickle_flock_dietz_aspnes_2002, title={Pseudodielectric function of ZnGeP2 from 1.5 to 6 eV}, volume={81}, ISSN={["0003-6951"]}, DOI={10.1063/1.1492022}, abstractNote={We report pseudodielectric function data 〈ε〉=〈εa1〉+i〈εa2〉 and 〈ε〉=〈εc1〉+i〈εc2〉 for the optically uniaxial material ZnGeP2, critical point energies of structures in these data, and dielectric function data for the natural oxide. Annealing reduces the values of the peaks of 〈εa2〉.}, number={4}, journal={APPLIED PHYSICS LETTERS}, author={Blickle, V and Flock, K and Dietz, N and Aspnes, DE}, year={2002}, month={Jul}, pages={628–630} }
 @article{kim_ihn_kim_kim_aspnes_yao_shim_koo_2002, title={Pseudodielectric functions of InGaAs alloy films grown on InP}, volume={81}, ISSN={["0003-6951"]}, DOI={10.1063/1.1509093}, abstractNote={We present room-temperature pseudodielectric function spectra 〈ε〉 of InxGa1−xAs films grown on (100) InP by solid-source molecular-beam epitaxy. A wet-chemical etching procedure is used to remove overlayers and obtain the best approximation to the bulk dielectric responses ε of the films. By line shape fitting, we determined the x dependences of the E1 and E1+Δ1 critical-point energies and that of the Δ1 bowing parameter. The results are in good agreement with the predictions of a universal tight-binding model.}, number={13}, journal={APPLIED PHYSICS LETTERS}, author={Kim, TJ and Ihn, YS and Kim, YD and Kim, SJ and Aspnes, DE and Yao, T and Shim, K and Koo, BH}, year={2002}, month={Sep}, pages={2367–2369} }
 @article{powell_wang_aspnes_2002, title={Simplified bond-hyperpolarizability model of second harmonic generation}, volume={65}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.65.205320}, abstractNote={We show that the anisotropies of second-harmonic-generation (SHG) intensities of vicinal (111) and (001)Si-SiO 2 interfaces can he described accurately as dipole radiation originating from the anharmonic motion of band charges strictly along bond directions. This simplified bond-hyperpolarizability model not only substantially simplifies the description of SHG, but also provides a microscopically physical and mathematically more efficient picture of the process than those found in standard phenomenological treatments employing tensor or Fourier coefficients. Using this approach we obtain an analytic solution for the expected response of (111) terraces, and by comparing to data show that the effective angles of incidence and observation for the (111)Si-SiO 2 interface are not those measured in the laboratory hut correspond to those refracted at the air-SiO 2 interface. For (111) vicinal interfaces at 765 nm SHG absorption is found to occur mainly for the step bond. The success of this formulation indicates that in many, if not most, cases the description of SHG may be simpler than that of the linear-optical response.}, number={20}, journal={PHYSICAL REVIEW B}, author={Powell, GD and Wang, JF and Aspnes, DE}, year={2002}, month={May} }
 @article{wang_powell_johnson_lucovsky_aspnes_2002, title={Simplified bond-hyperpolarizability model of second harmonic generation: Application to Si-dielectric interfaces}, volume={20}, ISSN={["2166-2746"]}, DOI={10.1116/1.1493783}, abstractNote={We show that the anisotropies of second-harmonic-generation (SHG) intensities of singular and vicinal (111) and (001)Si–dielectric interfaces can be described accurately as dipole radiation originating from the anharmonic motion of bond charges parallel to the bond directions. This simplified bond-hyperpolarizability model not only provides a simpler and mathematically more efficient representation of SHG, but also allows a direct physical interpretation at the bond level, which was lacking in previous approaches. Application to oxidized and nitrided Si–SiO2 interfaces provides new insight into bonding that occurs at these interfaces as well as the origin of SHG.}, number={4}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B}, author={Wang, JFT and Powell, GD and Johnson, RS and Lucovsky, G and Aspnes, DE}, year={2002}, pages={1699–1705} }
 @misc{aspnes_opsal_faton_2002, title={Thin film optical measurement system and method with calibrating ellipsometer}, volume={6,411,385}, number={2002 Jun. 25}, publisher={Washington, DC: U.S. Patent and Trademark Office}, author={Aspnes, D. E. and Opsal, J. and Faton, J. T.}, year={2002} }
 @article{bang_lee_kim_kim_aspnes_yu_o_choi_2001, title={Above bandgap optical properties of ZnS and ZnS1-xTex alloys grown by using hot-wall epitaxy}, volume={39}, number={3}, journal={Journal of the Korean Physical Society}, author={Bang, C. Y. and Lee, M. S. and Kim, T. J. and Kim, Y. D. and Aspnes, D. E. and Yu, Y. M. and O, B. S. and Choi, Y. D.}, year={2001}, pages={462–465} }
 @misc{aspnes_opsal_2001, title={Broadband spectroscopic rotating compensator ellipsometer}, volume={6,320,657}, number={2001 Nov. 20}, publisher={Washington, DC: U.S. Patent and Trademark Office}, author={Aspnes, D. E. and Opsal, J.}, year={2001} }
 @article{yoo_aspnes_2001, title={Elimination of endpoint-discontinuity artifacts in the analysis of spectra in reciprocal space}, volume={89}, ISSN={["0021-8979"]}, DOI={10.1063/1.1368391}, abstractNote={Reciprocal-space analysis offers several advantages for determining critical point parameters in optical and other spectra, for example the separation of baseline effects, information, and noise in low-, medium-, and high-index Fourier coefficients, respectively. However, endpoint-discontinuity artifacts can obscure much of the information when segments are isolated for analysis. We developed a procedure for eliminating these artifacts and recovering buried information by minimizing in the white-noise region the mean-square deviation between the Fourier coefficients of the data and those of low-order polynomials, then subtracting the resulting coefficients from the data over the entire range. We find that spectral analysis is optimized if no false data are used, i.e., when the number of points transformed equals the number of actual data points in the segment. Using fractional differentiation we develop a simple derivation of the variation of the reciprocal-space coefficients with index n for Lorentzian and Gaussian line shapes in direct space. More generally, we show that the definition of critical point energies in terms of phase coherence of the Fourier coefficients allows these energies to be determined for a broad class of line shapes even if the direct-space line shapes themselves are not known. Limitations for undersampled or highly broadened spectra are discussed, along with extensions to two- or higher-dimensional arrays of data.}, number={12}, journal={JOURNAL OF APPLIED PHYSICS}, author={Yoo, SD and Aspnes, DE}, year={2001}, month={Jun}, pages={8183–8192} }
 @misc{aspnes_law_2001, title={Ellipsometer and polarimeter with zero-order plate compensator}, volume={6,181,421}, number={2001 Jan. 30}, publisher={Washington, DC: U.S. Patent and Trademark Office}, author={Aspnes, D. E. and Law, J. Y.}, year={2001} }
 @article{ebert_bell_flock_aspnes_2001, title={Investigation and control of MOVPE growth by combined spectroscopic ellipsometry and reflectance-difference spectroscopy}, volume={184}, ISSN={["0031-8965"]}, DOI={10.1002/1521-396x(200103)184:1<79::aid-pssa79>3.0.co;2-b}, abstractNote={The detailed characterization of epitaxial growth by metal organic vapor phase epitaxy (MOVPE) and its closed-loop feedback control at the sample level require a combination of thin-film, near-surface, and surface-sensitive techniques to determine layer thicknesses and compositions, the composition of the most recently deposited material, and surface chemistry, respectively. These data can be obtained nondestructively by spectroscopic ellipsometry (SE) and reflectance-difference (-anisotropy) spectroscopy (RDS/RAS). We describe the first unified optical system, basically a rotating-polarizer ellipsometer (RPE) integrated within a modified commercial rotating-sample MOVPE reactor, that performs both SE and RDS simultaneously in a single optical path. We provide examples of its use, showing in particular that GaP can intermix with Si during the initial stages of heteroepitaxy, and demonstrating sample-driven closed-loop feedback control of epitaxy through the fully automatic deposition of an InxGa1—xP parabolic quantum well. These results illustrate capabilities of the presented configuration and its potential for future use.}, number={1}, journal={PHYSICA STATUS SOLIDI A-APPLIED RESEARCH}, author={Ebert, M and Bell, KA and Flock, K and Aspnes, DE}, year={2001}, month={Mar}, pages={79–87} }
 @article{ebert_aspnes_2001, title={Investigation of noise in a spectrometer system using a short-arc source}, volume={72}, ISSN={["0034-6748"]}, DOI={10.1063/1.1384423}, abstractNote={We show that noise can be reduced significantly at certain frequencies in spectrometer systems that use arc-lamp sources if the image of the lamp is oriented perpendicular to the entrance slit of the spectrometer. This appears to be a result of aligning the direction of arc wander along, rather than across, the slit. Data on a specific system show a reduction in the appropriate Fourier component of the power density spectrum by a factor of 4.}, number={8}, journal={REVIEW OF SCIENTIFIC INSTRUMENTS}, author={Ebert, M and Aspnes, DE}, year={2001}, month={Aug}, pages={3477–3479} }
 @article{aspnes_2001, title={Linear and nonlinear optical spectroscopy of surfaces and interfaces}, volume={188}, ISSN={["1862-6319"]}, DOI={10.1002/1521-396x(200112)188:4<1353::aid-pssa1353>3.0.co;2-m}, abstractNote={I review some of the basic physics that determines the optical properties of surfaces and interfaces, proceeding from the optical properties of bulk material through photon-driven localization to microscopic formulations at the bond level. Bond models may provide more insight than band models for interpreting processes that take place within a monolayer or so of the interface, as for example second-harmonic generation in crystals with inversion symmetry.}, number={4}, journal={PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE}, author={Aspnes, DE}, year={2001}, month={Dec}, pages={1353–1360} }
 @article{hyun_aspnes_cuomo_2001, title={Nondestructive measurement of a glass transition temperature at spin-cast semicrystalline polymer surfaces}, volume={34}, ISSN={["0024-9297"]}, DOI={10.1021/ma0012797}, abstractNote={ADVERTISEMENT RETURN TO ISSUECommunication to the...Communication to the EditorNEXTNondestructive Measurement of a Glass Transition Temperature at Spin-Cast Semicrystalline Polymer SurfacesJ. Hyun, D. E. Aspnes, and J. J. CuomoView Author Information Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27695-7907 Cite this: Macromolecules 2001, 34, 8, 2395–2397Publication Date (Web):March 9, 2001Publication History Received21 July 2000Revised1 November 2000Published online9 March 2001Published inissue 1 April 2001https://doi.org/10.1021/ma0012797Copyright © 2001 American Chemical SocietyRequest reuse permissions Article Views208Altmetric-Citations18LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit Read OnlinePDF (97 KB) Get e-AlertscloseSUBJECTS:Glass transition,Optical properties,Plasma,Polymers,Positron emission tomography Get e-Alerts}, number={8}, journal={MACROMOLECULES}, author={Hyun, J and Aspnes, DE and Cuomo, JJ}, year={2001}, month={Apr}, pages={2395–2397} }
 @article{lindquist_jarrendahl_peters_zettler_cobet_esser_aspnes_henry_edwards_2001, title={Ordinary and extraordinary dielectric functions of 4H-and 6H-SiC from 3.5 to 9.0 eV}, volume={78}, ISSN={["0003-6951"]}, DOI={10.1063/1.1369617}, abstractNote={We report ordinary (ε⊥c axis) and extraordinary (ε∥c axis) dielectric function data of 4H– and 6H–SiC from 3.5 to 9.0 eV. These data, which were obtained by with spectroscopic ellipsometry, are also compared to recently reported ab initio calculations. Critical point energies were found using real and reciprocal space analysis.}, number={18}, journal={APPLIED PHYSICS LETTERS}, author={Lindquist, OPA and Jarrendahl, K and Peters, S and Zettler, JT and Cobet, C and Esser, N and Aspnes, DE and Henry, A and Edwards, NV}, year={2001}, month={Apr}, pages={2715–2717} }
 @article{kim_koo_lee_kim_aspnes_jonker_2001, title={Spectroscopic ellipsometric study of the dielectric function of ZnSe and its overlayer}, volume={39}, number={2001 Dec}, journal={Journal of the Korean Physical Society}, author={Kim, T. J. and Koo, M. S. and Lee, M. S. and Kim, Y. D. and Aspnes, D. E. and Jonker, B. T.}, year={2001}, pages={S372–375} }
 @article{seong_bang_kim_wang_aspnes_koo_yao_2001, title={Spectroscopic ellipsometry study of InGaAs alloy films grown on InP}, volume={39}, number={2001 Dec}, journal={Journal of the Korean Physical Society}, author={Seong, G. Y. and Bang, C. Y. and Kim, Y. D. and Wang, J. and Aspnes, D. E. and Koo, B. H. and Yao, T.}, year={2001}, pages={S389–392} }
 @misc{aspnes_opsal_faton_2001, title={Thin film optical measurement system and method with calibrating ellipsometer}, volume={6,304,326}, number={2001 Oct. 16}, publisher={Washington, DC: U.S. Patent and Trademark Office}, author={Aspnes, D. E. and Opsal, J. and Faton, J. T.}, year={2001} }
 @misc{aspnes_opsal_2000, title={Broadband spectroscopic rotating compensator ellipsometer}, volume={6,134,012}, number={2000 Oct. 17}, publisher={Washington, DC: U.S. Patent and Trademark Office}, author={Aspnes, D. E. and Opsal, J.}, year={2000} }
 @article{rossow_aspnes_2000, title={Characterization of Al(x)Ga(1-x)N-compound layers by reflectance difference spectroscopy}, volume={177}, ISSN={["1862-6300"]}, DOI={10.1002/(sici)1521-396x(200001)177:1<157::aid-pssa157>3.0.co;2-p}, abstractNote={We discuss the application of reflectance-difference (anisotropy) spectroscopy (RDS/RAS) in the characterization of AlxGa1—xN-compound layers grown on sapphire and 6H-SiC. The optical method RDS measures optical anisotropies in normal incidence. While for cubic materials the optical anisotropy is related to surfaces and interfaces, in the case of hexagonal materials a bulk optical anisotropy can also be observed because of a possible reduction of symmetry by a tilt of the c-axis with respect to the surface normal. Our simulations suggest that the RDS signal of AlxGa1—xN-compound layers originates from a bulk optical anisotropy. The spectra consist of interference features below the bandgap, a derivative feature at the bandgap, and no signal or a broad feature above the bandgap. We discuss how layer properties such as thickness, tilt of c-axis, composition and strain can be derived from RDS spectra.}, number={1}, journal={PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE}, author={Rossow, U and Aspnes, DE}, year={2000}, month={Jan}, pages={157–163} }
 @article{aspnes_mantese_bell_rossow_2000, title={Coherence effects and time dependences of the optical response of surfaces and interfaces of optically absorbing materials}, volume={220}, ISSN={["1521-3951"]}, DOI={10.1002/1521-3951(200007)220:1<709::aid-pssb709>3.0.co;2-d}, abstractNote={Components in the optical spectra of surfaces and interfaces that are related to energy and lifetime derivatives of the bulk dielectric response can be understood if final-state coherence effects that are ignored in conventional quasistatic approaches are retained. The theory shows that the finite penetration depth of light in optically absorbing materials generates wave packets that are localized to the same region as the photon field. This eliminates the energy-conservation and causality violations of the conventional picture and in particular allows the surface or interface potential to influence energy gaps and broadening parameters. Since the localized packets are only stable dynamically, optical absorption is an intrinsically nonlocal process. The formulation also provides a quantitative physical basis for Lorentzian broadening, which is usually treated phenomenologically.}, number={1}, journal={PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS}, author={Aspnes, DE and Mantese, L and Bell, KA and Rossow, U}, year={2000}, month={Jul}, pages={709–715} }
 @article{lee_kim_powell_aspnes_lee_peiris_furdyna_2000, title={Dielectric function and bowing parameter of Zn1-xMgxSe and Zn1-xBexSe alloys}, volume={37}, ISSN={["0374-4884"]}, DOI={10.3938/jkps.37.1012}, number={6}, journal={JOURNAL OF THE KOREAN PHYSICAL SOCIETY}, author={Lee, H and Kim, IY and Powell, J and Aspnes, DE and Lee, S and Peiris, F and Furdyna, JK}, year={2000}, month={Dec}, pages={1012–1016} }
 @article{koo_kim_lee_oh_kim_yoo_aspnes_jonker_2000, title={Dielectric function of epitaxial ZnSe films}, volume={77}, ISSN={["0003-6951"]}, DOI={10.1063/1.1328098}, abstractNote={We examine various ZnSe spectra to obtain that which best represents the dielectric response ε of ZnSe. The measured evolution of pseudodielectric function 〈ε〉 data with chemical etching shows that the natural overlayer on ZnSe can be modeled accurately only if we assume that it contains amorphous Se. Hence previous assumptions made in correcting 〈ε〉 mathematically are not correct, and data obtained on stripped samples yield the best representation of ε.}, number={21}, journal={APPLIED PHYSICS LETTERS}, author={Koo, MS and Kim, TJ and Lee, MS and Oh, MS and Kim, YD and Yoo, SD and Aspnes, DE and Jonker, BT}, year={2000}, month={Nov}, pages={3364–3366} }
 @article{hyun_barletta_koh_yoo_oh_aspnes_cuomo_2000, title={Effect of Ar+ ion beam in the process of plasma surface modification of PET films}, volume={77}, ISSN={["0021-8995"]}, DOI={10.1002/1097-4628(20000822)77:8<1679::AID-APP4>3.0.CO;2-F}, abstractNote={In general, plasma modified polymer surfaces tend to show short aging time and rapid hydrophobic recovery after treatment. To prevent reorientation from the surface to the bulk, appropriate crosslinking is necessary among the polymer chains. In this work, an Ar+ ion beam was used to provide crosslinking to the surface. Crosslinking was shown by spectroscopic ellipsometry, AFM, and FTIR. Contact angle measurements were performed to see the aging of the modified surfaces. The surface modified with Ar+ ion beam followed by RF plasma treatment exhibited reduced chain mobility and a highly stable hydrophilic surface. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1679–1683, 2000}, number={8}, journal={JOURNAL OF APPLIED POLYMER SCIENCE}, author={Hyun, J and Barletta, P and Koh, K and Yoo, S and Oh, J and Aspnes, DE and Cuomo, JJ}, year={2000}, month={Aug}, pages={1679–1683} }
 @article{yoo_aspnes_lastras-martinez_ruf_konuma_cardona_2000, title={High-resolution spectroscopy with reciprocal-space analysis: Application to isotopically pure Si}, volume={220}, ISSN={["1521-3951"]}, DOI={10.1002/1521-3951(200007)220:1<117::aid-pssb117>3.0.co;2-4}, abstractNote={We discuss a new Fourier-transform approach that has recently been developed to optimize the determination of critical point parameters in optical spectra. In this approach, segments of direct (energy or frequency) space spectra are Fourier transformed into reciprocal (Fourier-inverse) space, and the endpoint-discontinuity artifacts that result are eliminated by subtracting corresponding coefficients of low-order Legendre polynomials determined by least-squares fitting these coefficients to the transformed data in the white-noise region. We apply this approach to determine the extremely small effect of isotopic mass on the energy of the E1 critical point of crystalline Si from low-temperature spectroscopic ellipsometric data.}, number={1}, journal={PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS}, author={Yoo, SD and Aspnes, DE and Lastras-Martinez, LF and Ruf, T and Konuma, M and Cardona, M}, year={2000}, month={Jul}, pages={117–125} }
 @article{ebert_bell_yoo_flock_aspnes_2000, title={In situ monitoring of MOVPE growth by combined spectroscopic ellipsometry and reflectance-difference spectroscopy}, volume={364}, ISSN={["1879-2731"]}, DOI={10.1016/S0040-6090(99)00920-7}, abstractNote={Comprehensive characterization of epitaxial growth by metal organic vapor phase epitaxy (MOVPE) requires a combination of thin-film, near-surface-, and surface-sensitive techniques to determine layer thicknesses and compositions, composition of the most recently deposited material, and surface chemistry, respectively. These data can be obtained non-destructively by spectroscopic ellipsometry (SE) and reflectance-difference (-anisotropy) spectroscopy (RDS/RAS). Here we describe the first unified optical system, basically a rotating-polarizer ellipsometer (RPE) integrated into a modified commercial rotating-sample MOVPE reactor, that performs both SE and RDS simultaneously with a single optical path. Data are obtained in parallel from 240 to 840 nm with a high-speed 16-bit photodiode array (PDA) at a repetition rate greater than 2 Hz and a precision of ±0.0001. We provide examples of its use, and show in particular that GaP intermixes with Si during the initial stages of heteroepitaxy. Capabilities of the presented configuration and its potential for future investigations are discussed.}, number={1-2}, journal={THIN SOLID FILMS}, author={Ebert, M and Bell, KA and Yoo, SD and Flock, K and Aspnes, DE}, year={2000}, month={Mar}, pages={22–27} }
 @article{lastras-martinez_ruf_konuma_cardona_aspnes_2000, title={Isotopic effects on the dielectric response of Si around the E-1 gap}, volume={61}, ISSN={["1550-235X"]}, DOI={10.1103/physrevb.61.12946}, abstractNote={The effect of isotopic composition on the dielectric function of silicon from 3.1 to 3.7 eV has been investigated using spectroscopic ellipsometric data obtained on ${}^{28}\mathrm{Si},$ natural $\mathrm{Si} {(}^{\mathrm{nat}}\mathrm{Si},{M}_{\mathrm{nat}}=28.09 \mathrm{amu}),$ and ${}^{30}\mathrm{Si}$ crystals. At low temperatures, the energies of the ${E}_{0}^{\ensuremath{'}}$ and ${E}_{1}$ interband transitions, which occur in the energy range under study, become mass dependent through the dependence of the electron-phonon interaction and the lattice parameter on the average isotopic mass. We determine the mass dependence of critical point energies and other optical parameters as accurately as possible by analyzing the ellipsometric data in reciprocal (Fourier-inverse) rather than direct (frequency) space. The obtained dependence of the critical point energy versus isotope mass $[\ensuremath{\Delta}{E}_{1}/\ensuremath{\Delta}M=+1.9(4) \mathrm{meV}/\mathrm{amu}]$ is in reasonable agreement with estimated values obtained from the temperature dependence of ${E}_{1}$ in natural silicon.}, number={19}, journal={PHYSICAL REVIEW B}, author={Lastras-Martinez, LF and Ruf, T and Konuma, M and Cardona, M and Aspnes, DE}, year={2000}, month={May}, pages={12946–12951} }
 @misc{aspnes_ebert_2000, title={Method of reducing noise generated by arc lamps in optical systems employing slits}, volume={6,411,381}, number={2000 May 2}, publisher={Washington, DC: U.S. Patent and Trademark Office}, author={Aspnes, D. E. and Ebert, M.}, year={2000} }
 @article{edwards_bremser_batchelor_buyanova_madsen_yoo_welhkamp_wilmers_cobet_esser_et al._2000, title={Optical characterization of wide bandgap semiconductors}, volume={364}, ISSN={["0040-6090"]}, DOI={10.1016/S0040-6090(99)00903-7}, abstractNote={Our work primarily concerns the characterization of wide-gap III–V nitride semiconductors, nondestructively and at variable temperature, with spectroscopic ellipsometry (SE) and reflectometry in the spectral range from 1.5 to 6 eV. In the case of GaN, there are three main concerns associated with such data: (a) the quantification of the dispersion of the index of refraction with energy; (b) the removal of surface overlayers in real-time; and (c) the determination of the variation of valence bands with biaxial stress and the quantification of residual stress in thin films. The SE and reflectance capabilities provide (1) broadband spectra from 1.5 to 6 eV, which yield information about (a) below the bandgap and (b) above it, and (2) high resolution spectra (less than 1 meV at 3.4 eV) in the vicinity of the gap (3.3–3.6 eV), which enables (c). Here we will discuss issues concerning the relation of (c) to GaN material and growth parameters, though similar data for other wide bandgap materials will be discussed where relevant. Specifically, optimal heterostructure design for potential valence band engineering applications will be discussed in the context of trends in residual stress as a function of film thickness, growth temperature and substrate orientation for GaN/AlN/6H-SiC heterostructures. Standard heterostructures are mostly compressive for samples less than about 0.7 μm thick, are tensile up to about 2 μm and then abruptly become less tensile with stress values near 1 kbar thereafter. Additionally, these trends can be circumvented for moderately thick (∼2 μm) GaN layers (normally >2 kbar, tensile) by the introduction of a ‘buried interface’ approach; namely, a strain mediating layer (SML) above the standard high-temperature AlN buffer layer designed to yield a range of compressive stresses from 0 to 2 kbar. The strain characteristics but also the growth rates of subsequently deposited nitride layers can be modulated by changing the growth parameters of the SML. This is achieved by in situ techniques during crystal growth without degrading the optical and structural properties of the deposited layer, as confirmed by XRD, SEM, PL, and AFM data taken on the overlying GaN layers. These results are interpreted in terms of coefficient of thermal expansion data for the layers and data concerning the planarization of GaN layers and growth behavior in non-(0001) directions.}, number={1-2}, journal={THIN SOLID FILMS}, author={Edwards, NV and Bremser, MD and Batchelor, AD and Buyanova, IA and Madsen, LD and Yoo, SD and Welhkamp, T and Wilmers, K and Cobet, C and Esser, N and et al.}, year={2000}, month={Mar}, pages={98–106} }
 @article{choi_kim_yoo_aspnes_woo_kim_2000, title={Optical properties of AlxGa1-xP (0 <= x <= 0.52) alloys}, volume={87}, ISSN={["0021-8979"]}, DOI={10.1063/1.372011}, abstractNote={We report optical properties of AlxGa1−xP (0⩽x⩽0.52) alloys grown by gas source molecular-beam epitaxy on S-doped GaP(001) substrates. Room-temperature pseudodielectric function spectra from 1.5 to 6.0 eV were obtained by spectroscopic ellipsometry. By applying the parabolic-band critical point model to numerically calculated second energy derivatives of these spectra, we obtained accurate room-temperature values of the E1, E0′, E2, and E2′ critical point energies and their dependence on composition x.}, number={3}, journal={JOURNAL OF APPLIED PHYSICS}, author={Choi, SG and Kim, YD and Yoo, SD and Aspnes, DE and Woo, DH and Kim, SH}, year={2000}, month={Feb}, pages={1287–1290} }
 @article{edwards_madsen_robbie_powell_jarrendahl_cobet_esser_richter_aspnes_2000, title={Real-time assessment of overlayer removal on 4H-SiC surfaces: Techniques and relevance to contact formation}, volume={338-3}, ISBN={["*************"]}, ISSN={["0255-5476"]}, DOI={10.4028/www.scientific.net/msf.338-342.1033}, abstractNote={We applied real-time spectroscopic ellipsometric (SE) measurements to assess the removal of overlayer material from 4H-SiC Si- and C-face surfaces in order to investigate the final step of an other ...}, number={3}, journal={SILICON CARBIDE AND RELATED MATERIALS - 1999 PTS, 1 & 2}, author={Edwards, NV and Madsen, LD and Robbie, K and Powell, GD and Jarrendahl, K and Cobet, C and Esser, N and Richter, W and Aspnes, DE}, year={2000}, pages={1033–1036} }
 @article{edwards_jarrendahl_aspnes_robbie_powell_cobet_esser_richter_madsen_2000, title={Real-time assessment of selected surface preparation regimens for 4H-SiC surfaces using spectroscopic ellipsometry}, volume={464}, ISSN={["1879-2758"]}, DOI={10.1016/S0039-6028(00)00689-0}, abstractNote={Spectroscopic ellipsometry (SE) was used to assess the removal of overlayer material from 4H–SiC (0001) and (0001) [Si- and C-face] surfaces in real time and, in particular, the critical final step of an otherwise standard RCA cleaning regimen commonly used to prepare SiC surfaces for contact formation. The treatments selected [buffered hydrofluoric acid (HF), concentrated HF, and dilute HF] removed 4–40 Å of effective SiO2 overlayer thickness from these surfaces. The concentrated HF treatment yielded the best surface, i.e. that with the most abrupt transition region between bulk and surface and with the most oxide material removed. A fourth treatment regimen (sequential application of methanol, water, and 5% HF in methanol) was also developed for comparison with the full RCA clean.}, number={1}, journal={SURFACE SCIENCE}, author={Edwards, NV and Jarrendahl, K and Aspnes, DE and Robbie, K and Powell, GD and Cobet, C and Esser, N and Richter, W and Madsen, LD}, year={2000}, month={Sep}, pages={L703–L707} }
 @article{bell_ebert_yoo_flock_aspnes_2000, title={Real-time optical characterization of heteroepitaxy by organometallic chemical vapor deposition}, volume={18}, ISSN={["0734-2101"]}, DOI={10.1116/1.582323}, abstractNote={Heteroepitaxy of GaP on Si(100) and GaAs(100) is investigated under organometallic chemical vapor deposition conditions using combined spectroscopic ellipsometry (SE) and non-normal-incidence reflectance-difference (-anisotropy) spectroscopy. Real-time monitoring greatly assists in identifying optimum starting surfaces for heteroepitaxy since prolonged exposure to PH3 results in roughening of Si(100) and GaAs(100) surfaces, in agreement with previous work. Real-time SE data of GaP on Si indicate that under our conditions GaP and Si interpenetrate as optically identifiable materials over the first 75 Å, suggesting that either trimethylgallium or a reaction by-product can act as a catalyst for the formation of Si{111} facets.}, number={4}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A}, author={Bell, KA and Ebert, M and Yoo, SD and Flock, K and Aspnes, DE}, year={2000}, pages={1184–1189} }
 @article{bell_ebert_yoo_flock_aspnes_2000, title={Real-time optical techniques and QMS to characterize growth in a modified commercial OMVPE reactor}, volume={29}, ISSN={["0361-5235"]}, DOI={10.1007/s11664-000-0104-6}, number={1}, journal={JOURNAL OF ELECTRONIC MATERIALS}, author={Bell, KA and Ebert, M and Yoo, SD and Flock, K and Aspnes, DE}, year={2000}, month={Jan}, pages={106–111} }
 @article{rossow_mantese_aspnes_2000, title={Surface-induced optical anisotropy of Si and Ge}, volume={18}, ISSN={["2166-2746"]}, DOI={10.1116/1.1306309}, abstractNote={We compare surface-induced optical anisotropy spectra measured by reflectance difference/anisotropy spectroscopy of Si and Ge. Our previous work on Si has shown that we can roughly distinguish two types of contributions to the optical response of surfaces: direct contributions involving surface state transitions and indirect contributions in which the surface modifies the bulk response. We demonstrate here that this view is valid for Ge as well. For Si the indirect surface contributions exhibit three extreme, basic line shapes, which are all related to the bulk dielectric function εb or nanostructured material. We demonstrate that this line shape analysis can also be applied to Ge.}, number={4}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B}, author={Rossow, U and Mantese, L and Aspnes, DE}, year={2000}, pages={2229–2231} }
 @article{lee_kim_powell_aspnes_lee_peiris_furdyna_2000, title={Visible-near ultraviolet ellipsometric study of Zn1-xMgxSe and Zn1-xBexSe alloys}, volume={88}, ISSN={["0021-8979"]}, DOI={10.1063/1.373750}, abstractNote={We report pseudodielectric function data 〈ε〉=〈ε1〉+i〈ε2〉 of Zn1−xMgxSe and Zn1−xBexSe samples grown on GaAs substrates. The data were obtained from 1.5 to 6.0 eV using spectroscopic ellipsometry. Critical point parameters were obtained by fitting model line shapes to numerically calculated second energy derivatives of 〈ε〉, from which the bowing parameters and spin-orbit-splitting Δ1 of the E1 and E1+Δ1 gaps were obtained. A transfer of oscillator strength from E1+Δ1 to E1 with increasing Mg and Be composition and a positive bowing of these threshold energies are attributed to the k-linear interaction, which is large in small-band gap semiconductors.}, number={2}, journal={JOURNAL OF APPLIED PHYSICS}, author={Lee, H and Kim, IY and Powell, J and Aspnes, DE and Lee, S and Peiris, F and Furdyna, JK}, year={2000}, month={Jul}, pages={878–882} }
 @article{mantese_xue_sakurai_aspnes_1999, title={Analysis of high-index Si(001) surfaces by reflectance difference spectroscopy}, volume={17}, ISSN={["1520-8559"]}, DOI={10.1116/1.581867}, abstractNote={We report surface-induced optical anisotropy spectra of high-index Si(115), (114), and (113) surfaces obtained using reflectance difference spectroscopy. Air-oxidized surfaces show sharp derivative-type features that are step-induced and located near the critical point energies of bulk Si, consistent with those of lower-index Si(001) surfaces. Clean reconstructed surfaces are characterized by a broad feature near 3 eV that tends to decrease in amplitude upon H exposure and a step-induced structure near the (E0′,E1) transition of bulk Si. In contrast, H exposure of Ge-covered surfaces tends to sharpen and enhance lower-energy structures. The derivative-type features located near the bulk critical point energies of Si can be described in terms of electronic states localized by the finite penetration depth of light.}, number={4}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A}, author={Mantese, L and Xue, QK and Sakurai, T and Aspnes, DE}, year={1999}, pages={1652–1656} }
 @misc{aspnes_opsal_1999, title={Broadband spectroscopic rotating compensator ellipsometer}, volume={5,973,787}, number={1999 Oct. 26}, publisher={Washington, DC: U.S. Patent and Trademark Office}, author={Aspnes, D. E. and Opsal, J.}, year={1999} }
 @article{cardona_lastras-martinez_aspnes_1999, title={Comment on "Ab initio calculation of excitonic effects in the optical spectra of semiconductors"}, volume={83}, ISSN={["1079-7114"]}, DOI={10.1103/PhysRevLett.83.3970}, abstractNote={A Comment on the Letter by Stefan Albrecht, Lucia Reining, Rodolfo Del Sole, and Giovanni Onida, Phys. Rev. Lett. 80, 4510 (1998). The authors of the Letter offer a Reply.Received 30 April 1999DOI:https://doi.org/10.1103/PhysRevLett.83.3970©1999 American Physical Society}, number={19}, journal={PHYSICAL REVIEW LETTERS}, author={Cardona, M and Lastras-Martinez, LF and Aspnes, DE}, year={1999}, month={Nov}, pages={3970–3970} }
 @article{kim_kim_yoo_aspnes_kossut_1999, title={Dielectric function of Cd0.57Mg0.43Te alloy film studied by ellipsometry}, volume={34}, number={1999 June}, journal={Journal of the Korean Physical Society}, author={Kim, T. J. and Kim, Y. D. and Yoo, S. D. and Aspnes, D. E. and Kossut, J.}, year={1999}, pages={S496–498} }
 @article{aspnes_yoo_1999, title={High-resolution spectroscopy with reciprocal-space analysis}, volume={215}, ISSN={["0370-1972"]}, DOI={10.1002/(sici)1521-3951(199909)215:1<715::aid-pssb715>3.0.co;2-g}, abstractNote={Optical spectra can be analyzed more accurately for critical point energies and other parameters in reciprocal space than in direct space because baseline effects, information, and noise are effectively isolated in the low-, medium-, and high-index Fourier coefficients, respectively. However, the requirement that the spectral segment being analyzed is periodic usually gives rise to large contributions to the coefficients from the value and slope discontinuities at the wrapped ends of the segment. Previously, the effect of these discontinuities was reduced with false data in direct space. Here, we show that these artifacts can be removed more effectively in reciprocal space, and that the process is optimized when the number of data points in the segment equals the number of points in the transformation, i.e., when no false data are required. Our approach reduces reciprocal-space analysis of optical spectra to a routine process, as demonstrated by application to crystalline Si.}, number={1}, journal={PHYSICA STATUS SOLIDI B-BASIC RESEARCH}, author={Aspnes, DE and Yoo, SD}, year={1999}, month={Sep}, pages={715–723} }
 @article{mantese_bell_aspnes_rossow_1999, title={Photon-induced localization in optically absorbing materials}, volume={253}, ISSN={["0375-9601"]}, DOI={10.1016/S0375-9601(98)00953-0}, abstractNote={We show that components of surface- and interface-related optical spectra that are related to derivatives of their bulk dielectric functions are due to a dynamic photon-induced localization of the initial and final states. Localization is described by correlation effects that arise from the finite penetration depth of light in optically absorbing materials, and lead to a substantially different perspective of optical absorption than that given by conventional theory.}, number={1-2}, journal={PHYSICS LETTERS A}, author={Mantese, L and Bell, KA and Aspnes, DE and Rossow, U}, year={1999}, month={Mar}, pages={93–97} }
 @article{ayars_aspnes_moyer_paesler_1999, title={Proximal electromagnetic shear forces}, volume={196}, DOI={10.1046/j.1365-2818.1999.00596.x}, abstractNote={We perform a simple model calculation to estimate the electromagnetically induced shear force caused by a current dissipation when a charged tip is moved parallel to a conducting material. For parameters typical in shear force imaging, the force is many orders of magnitude below reported values. Thus, proximal electromagnetic tip–sample forces can be neglected in discussions of shear force imaging.}, number={1}, journal={Journal of Microscopy}, author={Ayars, E. and Aspnes, D. E. and Moyer, P. and Paesler, M. A.}, year={1999}, pages={59–60} }
 @article{edwards_batchelor_buyanova_madsen_bremser_davis_aspnes_monemar_1999, title={Relaxation phenomena in GaN/ AlN/ 6H-SiC heterostructures}, volume={4S1}, DOI={10.1557/s1092578300002830}, abstractNote={We have developed a method to modulate the strain state (normally > 4 kbar, tensile) of moderately thick (∼2 μm) GaN based structures grown on 6H-SiC to a range 0 to -2 kbar of compressive stresses by introducing a strain-mediating layer (SML) above the standard high-temperature AlN buffer layer. The strain characteristics of subsequently deposited nitride layers can be modulated by changing the growth parameters of the SML layer. This is achieved by in-situ techiniques during crystal growth without degrading the optical and structural properties of the deposited layers.}, number={G3.78}, journal={MRS Internet Journal of Nitride Semiconductor Research}, author={Edwards, N. V. and Batchelor, A. D. and Buyanova, I. A. and Madsen, L. D. and Bremser, M. D. and Davis, R. F. and Aspnes, D. E. and Monemar, B.}, year={1999} }
 @misc{aspnes_opsal_faton_1999, title={Thin film optical measurement system and method with calibrating ellipsometer}, volume={5,900,939}, number={1999 May 4}, publisher={Washington, DC: U.S. Patent and Trademark Office}, author={Aspnes, D. E. and Opsal, J. and Faton, J. T.}, year={1999} }
 @article{yoo_edwards_aspnes_1998, title={Analysis of optical spectra by Fourier methods}, volume={313}, ISSN={["0040-6090"]}, DOI={10.1016/S0040-6090(97)00801-8}, abstractNote={Results of a systematic investigation of reciprocal-space analysis, applied to optical spectroscopy, are reported. Filtering procedures for removing baseline and noise artifacts are more effective in reciprocal- than in real-space. Also, correlations among parameters are reduced and the functional dependence of real-space lineshapes need be known a priori only in very general terms. We apply reciprocal-space analysis to achieve accurate values of critical point energies for the E1 and E1+Δ1 transitions of GaAs from ellipsometric spectra and to locate critical point energies in low-temperature reflectance data of GaN. We show that data may easily be corrected for systematic artifacts such as the monochromator slit width, an optimum slit width can be defined, and the degree of improvement needed to achieve a particular level of performance can be predicted.}, number={1998 Feb.}, journal={THIN SOLID FILMS}, author={Yoo, SD and Edwards, NV and Aspnes, DE}, year={1998}, month={Feb}, pages={143–148} }
 @article{leng_opsal_chu_senko_aspnes_1998, title={Analytic representations of the dielectric functions of crystalline and amorphous Si and crystalline Ge for very large scale integrated device and structural modeling}, volume={16}, ISSN={["0734-2101"]}, DOI={10.1116/1.581137}, abstractNote={Various representations of the dielectric functions of semiconducting materials have been proposed. We compare five different models for representing the dielectric functions of crystalline (c-) Si, amorphous (a-) Si, and c-Ge from 1.5 to 6.0 eV. The four-oscillator critical point with parity plus one-dimensional tight-binding model (CPP-1D) best represents c-Si. Different models best represent a-Si and c-Ge. These representations provided a basis for modeling variations encountered in semiconductor manufacturing.}, number={3}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A-VACUUM SURFACES AND FILMS}, author={Leng, J and Opsal, J and Chu, H and Senko, M and Aspnes, DE}, year={1998}, pages={1654–1657} }
 @article{leng_opsal_chu_senko_aspnes_1998, title={Analytic representations of the dielectric functions of materials for device and structural modeling}, volume={313}, ISSN={["0040-6090"]}, DOI={10.1016/S0040-6090(97)00799-2}, abstractNote={Analytic representations of the dielectric function ε are needed for the analysis of optical data of complex materials and structures. Here, we examine various harmonic-oscillator-based representations of the dielectric functions of the silicon-related materials, crystalline Si, amorphous Si, and silicon nitride. For crystalline semiconductors we develop a new representation with a prefactor proportional to ω−2, the expected response for materials with wavefunctions that are eigenfunctions of the momentum operator.}, number={1998}, journal={THIN SOLID FILMS}, author={Leng, J and Opsal, J and Chu, H and Senko, M and Aspnes, DE}, year={1998}, month={Feb}, pages={132–136} }
 @article{opsal_fanton_chen_leng_wei_uhrich_senko_zaiser_aspnes_1998, title={Broadband spectral operation of a rotating-compensator ellipsometer}, volume={313}, ISSN={["0040-6090"]}, DOI={10.1016/S0040-6090(97)00769-4}, abstractNote={We show that a rotating-compensator ellipsometer (RCE) with a zero-order retarder can be used for broadband spectroscopy, for example from 200 to 800 nm, when the compensator retardation δ is known as a function of wavelength and sample properties are determined by least-squares methods. The resulting instrument offers the standard advantages of an RCE and is no less sensitive than a rotating-analyzer or -polarizer ellipsometer under worst-case conditions, where the sin (2ωt) component vanishes at δ=180°C. Sensitivity and operation are illustrated by application to the measurement of crystalline Si.}, number={1998}, journal={THIN SOLID FILMS}, author={Opsal, J and Fanton, J and Chen, J and Leng, J and Wei, L and Uhrich, C and Senko, M and Zaiser, C and Aspnes, DE}, year={1998}, month={Feb}, pages={58–61} }
 @article{mantese_bell_rossow_aspnes_1998, title={Interpretation of critical point energy shifts in crystalline Si by near-surface localization of excited electronic states}, volume={313}, ISSN={["0040-6090"]}, DOI={10.1016/S0040-6090(97)00883-3}, abstractNote={Various surface-optical spectra can be described in terms of energy derivatives of the bulk dielectric function. The spectra unequivocally indicate that critical point energies obtained from optical data are not necessarily equal to bulk values and that surface chemical and structural terminations are at least contributing factors. We invoke localization and transition-lifetime arguments to describe these effects. Existing surface-optical calculations do not address these contributions, which may explain in part why discrepancies remain between theory and experiment.}, number={1998 Feb.}, journal={THIN SOLID FILMS}, author={Mantese, L and Bell, KA and Rossow, U and Aspnes, DE}, year={1998}, month={Feb}, pages={557–560} }
 @article{rossow_mantese_aspnes_1998, title={Lineshapes of surface induced optical anisotropy spectra measured by RDS/RAS}, volume={123}, ISSN={["0169-4332"]}, DOI={10.1016/S0169-4332(97)00544-8}, abstractNote={In this paper we discuss lineshapes of reflectance-difference spectroscopy (RDS/RAS) spectra of various surfaces. This work aims toward a basic understanding of the surface optical response. We find that in some cases RDS spectra are given by the bulk dielectric function or its energy derivative. In the former case screening effects such as the surface local field effect are dominant. In the latter case the surface modifies the bulk electronic structures and we find that the excited states must be localized near the surface. Even in the simplest case where the anisotropy originates at the surface the optical anisotropy in general depends on the dielectric function of the bulk as well as that of the surface.}, number={1998 Jan.}, journal={APPLIED SURFACE SCIENCE}, author={Rossow, U and Mantese, L and Aspnes, DE}, year={1998}, month={Jan}, pages={237–242} }
 @article{aspnes_mantese_bell_rossow_1998, title={Many-body and correlation effects in surface and interface spectra of optically absorbing materials}, volume={170}, number={2}, journal={Physica Status Solidi. A, Applications and Materials Science}, author={Aspnes, D. E. and Mantese, L. and Bell, K. A. and Rossow, U.}, year={1998}, pages={199–210} }
 @article{aspnes_dietz_1998, title={Optical approaches for controlling epitaxial growth}, volume={130}, ISSN={["1873-5584"]}, DOI={10.1016/s0169-4332(98)00085-3}, abstractNote={Optical spectroscopy of surface and near-surface regions has advanced to the stage where detailed measurements can be made and analyzed in real time. Here, we discuss reported and potential applications of optical probes for sample-driven closed-loop feedback control of semiconductor epitaxy. Parameters that have been controlled include temperature, thickness in both deposition and etching, and composition, including continuously graded compositions. Although considerable progress has been made, much remains to be done before these techniques become viable production tools.}, number={1998 June}, journal={APPLIED SURFACE SCIENCE}, author={Aspnes, DE and Dietz, N}, year={1998}, month={Jun}, pages={367–376} }
 @article{woo_han_choi_lee_kim_lee_kim_kang_choi_kim_et al._1998, title={Optical characterization of GaAs/AlAs short period superlattices}, volume={43-4}, ISSN={["0167-9317"]}, DOI={10.1016/S0167-9317(98)00173-7}, abstractNote={We present optical studies of a series of GaAs/AlAs short period superlattices grown by molecular beam epitaxy. The structural properties were examined by X-ray diffraction measurements. The quantum confinement of the electronic and the vibronic states were observed in the low temperature photoluminescence and Raman scattering experiments, respectively. Spectroscopic ellipsometric (SE) measurements were also performed. We found a new structure at the lower E2 peak, which is the best resolution of the E2 structure in these SLs so far obtained by SE.}, number={1998 Aug.}, journal={MICROELECTRONIC ENGINEERING}, author={Woo, DH and Han, IK and Choi, WJ and Lee, S and Kim, HJ and Lee, JI and Kim, SH and Kang, KN and Choi, SG and Kim, YD and et al.}, year={1998}, month={Aug}, pages={265–270} }
 @article{aspnes_mantese_bell_rossow_1998, title={Photon-induced localization and final-state correlation effects in optically absorbing materials}, volume={16}, ISSN={["1071-1023"]}, DOI={10.1116/1.590176}, abstractNote={Two consequences of the absorption of light in optically absorbing materials that appear not to have been recognized previously are: (1) localization of the final electron and hole states involved in the absorption process into wave packets and (2) propagation of these wave packets with their respective group velocities. We demonstrate the existence of these phenomena by applying first-order time-dependent perturbation theory to a simple model that can be solved analytically even when correlations that are ordinarily discarded in the random phase approximation are retained. This approach provides a natural explanation of components in surface- and interface-optical spectra that are related to energy derivatives of the bulk dielectric function εb and apparent differences in nominally bulk critical point energies Eg and broadening parameters Γ depending on surface conditions.}, number={4}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B}, author={Aspnes, DE and Mantese, L and Bell, KA and Rossow, U}, year={1998}, pages={2367–2372} }
 @article{rossow_lindner_lubbe_aspnes_zahn_1998, title={Reflectance difference spectroscopy spectra of clean (3 x 2), (2 x 1), and c(2 x 2) 3C-SiC(001) surfaces: New evidence for surface state contributions to optical anisotropy spectra}, volume={16}, ISSN={["1071-1023"]}, DOI={10.1116/1.590174}, abstractNote={In this study we applied the surface sensitive optical method reflectance difference spectroscopy to investigate the optical response of clean cubic 3C-SiC(001) surfaces. The main reconstructions, the Si-rich (3×2) and (2×1) as well as the C-rich c(2×2) were prepared by annealing in Si flux. For (3×2) we find a strong spectral feature at 4.2 eV that is not observed for the other two reconstructions. Since the direct gap of 3C-SiC is at higher photon energies the 4.2 eV feature must be related to the surface. Angle resolved ultraviolet photoelectron spectroscopy measurements indicate surface states around the X̄ point of the unreconstructed surface Brillouin zone. We conclude that the 4.2 eV feature is associated with this surface state and suggest that it is linked to the E2 gap of bulk Si at 4.2 eV.}, number={4}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B}, author={Rossow, U and Lindner, K and Lubbe, M and Aspnes, DE and Zahn, DRT}, year={1998}, pages={2355–2357} }
 @article{edwards_yoo_bremser_horton_perkins_weeks_liu_stall_kuech_davis_et al._1998, title={Spectroscopic ellipsometry and low-temperature reflectance: complementary analysis of GaN thin films}, volume={313}, ISSN={["0040-6090"]}, DOI={10.1016/S0040-6090(97)00815-8}, abstractNote={Abstract We report spectroscopic ellipsometry (SE) and low-temperature reflectance data on epitaxial GaN thin-film samples covering the widest range of tensile and compressive stress (−3.8 to 3.5 kbar) thus far. SE allows us to assess the preparation of smooth and abrupt GaN surfaces by chemical treatments in real time, and, coupled with the reflectance data, the E d n /d E contribution to dispersion, which is important for laser action. The reflectance data explicitly show the non-linear behavior of the B-A and C-A splittings vs. the energy of the A exciton. Lineshape ambiguities that hindered previous interpretations have been resolved with reciprocal space analysis, allowing us to obtain band parameters such as Δ SO =17.0±1 meV and Δ CF =9.8±1 meV with increased confidence.}, number={1998 Feb.}, journal={THIN SOLID FILMS}, author={Edwards, NV and Yoo, SD and Bremser, MD and Horton, MN and Perkins, NR and Weeks, TW and Liu, H and Stall, RA and Kuech, TF and Davis, RF and et al.}, year={1998}, month={Feb}, pages={187–192} }
 @article{bell_mantese_rossow_aspnes_1998, title={Systematic differences among nominal reference dielectric function spectra for crystalline Si as determined by spectroscopic ellipsometry}, volume={313}, ISSN={["0040-6090"]}, DOI={10.1016/S0040-6090(97)00804-3}, abstractNote={Results of the first systematic investigation of differences among reference-quality ellipsometrically measured pseudodielectric function 〈ε〉 spectra of crystalline Si are discussed. These data are nominally used to approximate the bulk dielectric function of this material for optical modeling. In addition to the expected influence of residual overlayers, we identify surface-local-field and energy-derivative effects even between spectra obtained for seemingly identical H-terminated surfaces. Model calculations indicate that these effects account for nearly all differences among spectra studied. The isotropic contribution to the surface-local-field effect is also reported.}, number={1998 Feb.}, journal={THIN SOLID FILMS}, author={Bell, KA and Mantese, L and Rossow, U and Aspnes, DE}, year={1998}, month={Feb}, pages={161–166} }
 @misc{aspnes_opsal_faton_1998, title={Thin film optical measurement system and method with calibrating ellipsometer}, volume={5,798,837}, number={1998 Aug. 25}, publisher={Washington, DC: U.S. Patent and Trademark Office}, author={Aspnes, D. E. and Opsal, J. and Faton, J. T.}, year={1998} }
 @article{edwards_bremser_davis_batchelor_yoo_karan_aspnes_1998, title={Trends in residual stress for GaN/AlN/6H-SiC heterostructures}, volume={73}, ISSN={["1077-3118"]}, DOI={10.1063/1.122597}, abstractNote={We discuss trends in residual stress as a function of film thickness, growth temperature, and substrate orientation for GaN/AlN/6H–SiC heterostructures. Films are mostly compressive for samples less than about 0.7 μm thick, are tensile up to about 2 μm, then abruptly become less tensile with stress values near 1 kbar thereafter. We interpret this as a successive relief of lattice mismatch and thermal stresses culminating in a catastrophic relief by unknown mechanisms at moderate thicknesses. These data indicate that relaxation processes in these heterostructures are not as well understood as previously supposed.}, number={19}, journal={APPLIED PHYSICS LETTERS}, author={Edwards, NV and Bremser, MD and Davis, RF and Batchelor, AD and Yoo, SD and Karan, CF and Aspnes, DE}, year={1998}, month={Nov}, pages={2808–2810} }
 @article{kim_ko_choi_yoo_aspnes_jonker_1997, title={Above bandgap dielectric function of epitaxial ZnSe layers}, volume={31}, number={4}, journal={Journal of the Korean Physical Society}, author={Kim, Y. D. and Ko, Y. D. and Choi, S. G. and Yoo, S. D. and Aspnes, D. E. and Jonker, B. T.}, year={1997}, pages={L553–555} }
 @article{choi_kim_yoo_duk_miotkowski_i._ramdas_1997, title={Ellipsometric studies of Cd1-xMgxTe (0<=x<=0.5) alloys}, volume={71}, ISSN={["1077-3118"]}, DOI={10.1063/1.119511}, abstractNote={The determination of the above band gap optical properties of zincblende Cd1−xMgxTe (0⩽x⩽0.5) ternary alloys are reported on. Using the parabolic-band critical point model, room-temperature critical point energies of the E0, E0+Δ0, E1, E1+Δ1, E2, and E0′ interband transitions from numerically calculated second energy derivatives of ellipsometric spectra were obtained. The presence of two distinct structures in the E2 feature for x>0 was also observed.}, number={2}, journal={APPLIED PHYSICS LETTERS}, author={Choi, S. G. and Kim, Y. D. and Yoo, Sang and Duk, Aspnes and Miotkowski, David E. and I. and Ramdas, A. K.}, year={1997}, month={Jul}, pages={249–251} }
 @article{mantese_bell_rossow_aspnes_1997, title={Evidence of near-surface localization of excited electronic states in crystalline Si}, volume={15}, ISSN={["2166-2746"]}, DOI={10.1116/1.589438}, abstractNote={Surface- and interface-related spectra, obtained either directly by techniques such as reflectance-difference (-anisotropy) spectroscopy or indirectly by subtracting pseudodielectric function spectra obtained ellipsometrically on surfaces with different chemical termination, exhibit features related to energy derivatives of the bulk dielectric function. We argue that these spectra provide direct evidence that the excitations involved are localized both in space and time. These data unequivocally indicate that critical point energies obtained from above-band-gap ellipsometric or reflectrometric optical spectra are not necessarily equal to bulk values, and that surface chemical and structural termination is at least one contributing factor. Present surface-optical calculations do not include these effects, which may explain, in part, remaining discrepancies between theory and experiment.}, number={4}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B}, author={Mantese, L and Bell, KA and Rossow, U and Aspnes, DE}, year={1997}, pages={1196–1200} }
 @article{bremser_perry_zheleva_edwards_nam_parikh_aspnes_davis_1997, title={Growth, doping and characterization of AlxGa1-xN thin film alloys on 6H-SiC(0001) substrates}, volume={6}, ISSN={["0925-9635"]}, DOI={10.1016/S0925-9635(96)00626-7}, abstractNote={Thin films of AlxGa1 − xN (0.05≤x≤0.96) having smooth surfaces were deposited directly on both vicinal and on-axis 6H-SiC(0001) substrates. Cross-sectional transmission electron microscopy of Al0.13Ga0.87N revealed stacking faults near the SiC/nitride alloy interface and numerous threading dislocations. Energy dispersive analysis, Auger electron spectroscopy (AES) and Rutherford backscattering were used to determine the compositions. These were paired with their respective cathodoluminescence (CL) near band-edge emission energies. A negative bowing parameter was determined. The CL emission energies were similar to the bandgap energies obtained by spectroscopic ellipsometry. Field emission AES of the initial growth of Al0.2Ga0.8N revealed an Al-rich layer near the interface. N-type (Si) doping was achieved for AlxGa1 − xN for 0.12≤x≤0.42.}, number={2-4}, journal={DIAMOND AND RELATED MATERIALS}, author={Bremser, MD and Perry, WG and Zheleva, T and Edwards, NV and Nam, OH and Parikh, N and Aspnes, DE and Davis, RF}, year={1997}, month={Mar}, pages={196–201} }
 @inproceedings{rossow_edwards_bremser_kern_liu_davis_aspnes_1997, title={In-plane optical anisotropies of Al(x)Ga(1-x)N films in their regions of transparency}, DOI={10.1557/proc-449-835}, abstractNote={ABSTRACT}, booktitle={III-V nitrides: symposium held December 2-6, 1996, Boston, Massachusetts, U.S.A.  (Materials Research Society symposia proceedings ; v. 449)}, publisher={Pittsburgh, Pa.: Materials Research Society}, author={Rossow, U. and Edwards, N. V. and Bremser, M. D.. and Kern, R. S. and Liu, H. and Davis, R. F. and Aspnes, D. E.}, year={1997}, pages={835–840} }
 @inproceedings{hinds_aspenes_lucovsky_1997, title={Low pH chemical etch route for smooth H-terminated Si(100) and study of subsequent chemical stability}, booktitle={Environmental, safety, and health issues in IC production: Symposium held December 4-5, 1996, Boston, Massachusetts, U.S.A. (Materials Research Society Symposium proceedings, no. 447)}, publisher={Pittsburgh, PA: Materials Research Society}, author={Hinds, B. J. and Aspenes, D. E. and Lucovsky, G.}, year={1997}, pages={191–196} }
 @article{bachmann_hopfner_sukidi_miller_harris_aspnes_dietz_tran_beeler_ito_et al._1997, title={Molecular layer epitaxy by real-time optical process monitoring}, volume={112}, DOI={10.1016/S0169-4332(96)00975-0}, abstractNote={In this paper we consider modern methods of optical process monitoring and control in the context of atomic layer epitaxy. One specific method, p-polarized reflectance spectroscopy (PRS), is chosen to assess details of layer-by-layer growth. We show that PRS monitoring under conditions of steady-state growth by pulsed chemical beam epitaxy (PCBE) can achieve the deposition of molecular layers of GaP on silicon (100) deposited with a precision of 5%, which can be improved by reducing the growth rate and increasing the period of time averaging of the reflectance data. Since in the nucleation period prior to formation of a contiguous heteroepitaxial film inhomogeneous surface chemistry and roughening complicates the modeling of the overgrowth process, advances in both experimental methods and theory are required for extending the control to non-steady-state growth conditions. Results of simultaneous single-wavelength PR monitoring and laser light scattering measurements in conjunction with atomic force microscopy studies of short period heteroepitaxial overgrowth processes are presented. The extension of PRS to the monitoring of organometallic chemical vapor deposition at higher pressures is also discussed.}, number={1997 Mar.}, journal={Applied Surface Science}, author={Bachmann, K. J. and Hopfner, C. and Sukidi, N. and Miller, A. E. and Harris, C. J. and Aspnes, D. E. and Dietz, N. A. and Tran, Hien and Beeler, S. C. and Ito, K. and et al.}, year={1997}, pages={38–47} }
 @inproceedings{aspnes_dietz_rossow_bachmann_1997, title={Multilevel approaches toward monitoring and control of semiconductor epitaxy}, DOI={10.1557/proc-448-451}, abstractNote={Abstract}, booktitle={Control of semiconductor surfaces and interfaces: Symposium held December 2-5, 1996, Boston, Massachusetts, U.S.A. (Materials Research Society Symposium proceedings, no. 448)}, publisher={Pittsburgh, PA: Materials Research Society}, author={Aspnes, D. E. and Dietz, N. and Rossow, U. and Bachmann, K. J.}, year={1997}, pages={451–462} }
 @article{aspnes_1997, title={Real-time optical analysis and control of semiconductor epitaxy: Progress and opportunity}, volume={101}, ISSN={["0038-1098"]}, DOI={10.1016/S0038-1098(96)00447-4}, abstractNote={Various optical techniques have been developed in the last few years for real-time analysis of surfaces and near-surface regions during semiconductor epitaxy. These techniques are providing new insights into microscopic mechanisms of crystal growth and new opportunities for several levels of closed-loop feedback control of epitaxy, including control at the product (sample) level. The latter is made possible by new data-reduction algorithms that return the dielectric response of the most recently deposited material even if nothing is known about the underlying sample structure. Examples are provided, and opportunities and challenges discussed.}, number={2}, journal={SOLID STATE COMMUNICATIONS}, author={Aspnes, DE}, year={1997}, month={Jan}, pages={85–92} }
 @article{edwards_yoo_bremser_zheleva_horton_perkins_weeks_liu_stall_kuech_et al._1997, title={Spectral analysis of above-, below-, and near-bandedge phenomena in GaN thin films}, volume={50}, ISSN={["0921-5107"]}, DOI={10.1016/s0921-5107(97)00151-7}, abstractNote={We report spectroscopic ellipsometry (SE) and low-temperature reflectance data on epitaxial GaN thin film samples covering the widest range of tensile and compressive stress (−3.8–3.5 kbar) thus far. SE allows us to assess the preparation of smooth and abrupt GaN surfaces by chemical treatments in real time, and, coupled with the reflectance data, the E dn/dE contribution to dispersion, which is important for laser action. The reflectance data explicitly show the nonlinear behavior of the B–A and C–A splittings versus the energy of the A exciton. Lineshape ambiguities that hindered previous interpretations have been resolved with reciprocal space analysis, allowing us to obtain band parameters such as ΔSO=17.0±1 meV and ΔCF=9.8±1 meV with increased confidence.}, number={1-3}, journal={MATERIALS SCIENCE AND ENGINEERING B-SOLID STATE MATERIALS FOR ADVANCED TECHNOLOGY}, author={Edwards, NV and Yoo, SD and Bremser, MD and Zheleva, T and Horton, MN and Perkins, NR and Weeks, TW and Liu, H and Stall, RA and Kuech, TF and et al.}, year={1997}, month={Dec}, pages={134–141} }
 @article{kim_choi_klein_yoo_aspnes_xin_furdyna_1997, title={Spectroscopic ellipsometric characterization of undoped ZnTe films grown on GaAs}, volume={70}, ISSN={["0003-6951"]}, DOI={10.1063/1.118289}, abstractNote={We report highly accurate dielectric function data for ZnTe. These data were made possible by the high quality of the heteroepitaxial material and the development of a chemical etching procedure for producing abrupt surfaces on ZnTe; they provided the first observation of the E0+Δ0 structure in ZnTe by spectroscopic ellipsometry and evidence for several contributions to the E2 structure. Accurate critical point energies were obtained by Fourier analysis.}, number={5}, journal={APPLIED PHYSICS LETTERS}, author={Kim, YD and Choi, SG and Klein, MV and Yoo, SD and Aspnes, DE and Xin, SH and Furdyna, JK}, year={1997}, month={Feb}, pages={610–612} }
 @article{choi_kim_klein_yoo_aspnes_xin_furdyna_1997, title={Spectroscopic ellipsometric study of Zn(1-x)Mn(x)Te films grown on GaAs}, volume={31}, number={1}, journal={Journal of the Korean Physical Society}, author={Choi, S. G. and Kim, Y. D. and Klein, M. V. and Yoo, S. D. and Aspnes, David E. and Xin, S. H. and Furdyna, J. K.}, year={1997}, pages={202–205} }
 @article{choi_kim_yoo_aspnes_rhee_woo_woo_kim_kang_1997, title={Spectroscopic ellipsometry study of GaAs/AlAs superlattices and Al(0.5)Ga(0.5)As alloy}, volume={30}, number={suppl.}, journal={Journal of the Korean Physical Society}, author={Choi, S. G. and Kim, Y. D. and Yoo, S. D. and Aspnes, David E. and Rhee, S. J. and Woo, J. C. and Woo, D. H. and Kim, S. H. and Kang, K. N.}, year={1997}, pages={108–112} }
 @article{bell_mantese_rossow_aspnes_1997, title={Surface and interface effects on ellipsometric spectra of crystalline Si}, volume={15}, DOI={10.1116/1.589440}, abstractNote={We present the first systematic investigation of the differences among reference-quality ellipsometrically measured pseudodielectric function 〈ε〉 spectra of crystalline Si, which are nominally used to approximate the bulk dielectric function of this material. In addition to the expected influence of residual overlayers, we identify surface-local-field and energy-derivative effects, the latter representing shifts between bulk and measured critical point energies, as well as changes in excited-carrier lifetimes due to the surface. Model calculations indicate that these four effects account for nearly all differences among spectra studied, although a second-energy-derivative component appears at the E1 transition in some cases. The isotropic contribution to the surface-local-field effect is observed for the first time.}, number={4}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B}, author={Bell, KA and Mantese, L and Rossow, U and Aspnes, DE}, year={1997}, pages={1205–1211} }
 @article{edwards_yoo_bremser_weeks_nam_davis_liu_stall_horton_perkins_et al._1997, title={Variation of GaN valence bands with biaxial stress and quantification of residual stress}, volume={70}, ISSN={["0003-6951"]}, DOI={10.1063/1.119089}, abstractNote={Low-temperature reflectance data on epitaxial GaN thin-film samples covering the widest range of tensile and compressive stress (−3.8–3.5 kbar) thus far explicitly show the nonlinear behavior of the B–A and C–A splittings versus the energy of the A exciton. Lineshape ambiguities that hindered previous interpretations have been resolved with reciprocal-space analysis, allowing us to obtain band parameters such as ΔSO=17.0±1meV with increased confidence.}, number={15}, journal={APPLIED PHYSICS LETTERS}, author={Edwards, NV and Yoo, SD and Bremser, MD and Weeks, TW and Nam, OH and Davis, RF and Liu, H and Stall, RA and Horton, MN and Perkins, NR and et al.}, year={1997}, month={Apr}, pages={2001–2003} }
 @inproceedings{edwards_yoo_bremser_horton_perkins_weeks_liu_stall_kuech_davis_et al._1997, title={Variation of GaN valence bands with biaxial stress: quantification of residual stress and impact on fundamental band parameters}, DOI={10.1557/proc-449-781}, abstractNote={ABSTRACT}, booktitle={III-V nitrides: symposium held December 2-6, 1996, Boston, Massachusetts, U.S.A.  (Materials Research Society symposia proceedings ; v. 449)}, publisher={Pittsburgh, Pa.: Materials Research Society}, author={Edwards, N. V. and Yoo, S. D. and Bremser, M. D. and Horton, M. N. and Perkins, N. R. and Weeks, T. W. and Liu, H. and Stall, R. A. and Kuech, T. F. and Davis, R. F. and et al.}, year={1997}, pages={781–786} }
 @misc{aspnes_1994, title={Extraction of spatially varying dielectric function from ellipsometric data}, volume={5,277,747}, number={1994 Jan. 11}, publisher={Washington, DC: U.S. Patent and Trademark Office}, author={Aspnes, D. E.}, year={1994} }
 @article{aspnes_1993, title={Minimal-data approaches for determining outer-layer dielectric responses of films from kinetic reflectometric and ellipsometric measurements}, volume={10}, ISSN={1084-7529 1520-8532}, url={http://dx.doi.org/10.1364/josaa.10.000974}, DOI={10.1364/josaa.10.000974}, abstractNote={For stratified samples where material is being uniformly deposited or removed at a known rate, I show that the dielectric function ∊o of the outermost region is determinable exactly and analytically at any given wavelength from the value and the thickness derivative of the complex reflectances for p-polarized, s-polarized, or normally incident light without any knowledge of the underlying structure. This minimal-data approach greatly simplifies analysis compared with the standard procedure, in which dielectric functions are determined sequentially from a combination of new data and previously established sample parameters. It is also robust, eliminating cumulative error and error propagation that can cause conventional analysis to fail. An interferometric method for acquiring complex-reflectance data is proposed, although to achieve the necessary level of accuracy with present technology would be a formidable challenge. For ellipsometric measurements these technical obstacles do not exist, but an equivalent exact solution is not possible. However, I develop a common-pseudosubstrate approximation (CPA) that in applications to semiconductor crystal growth is accurate to better than 0.1%. The minimal-data approach also provides new insights about how sample parameters are determined from measured optical functions. For example, to determine deposition rates one needs to establish the second derivative (curvature) as well, which places additional constraints on measurement accuracy and/or the amount of data required. Also, the small-term expansion of the ellipsometrically determined pseudodielectric function 〈∊〉, originally derived as a thin-film limit of the three-phase model, is shown to be more generally valid. This result provides a theoretical basis for the direct analysis of several phenomena, including interface mixing, from 〈∊〉 data obtained during epitaxial growth. Using the CPA, I derive expressions that allow one to assess whether the performance of a given ellipsometer is adequate for growth control. Finally, the influence of selvage layers on determined values of ∊0 is briefly discussed.}, number={5}, journal={Journal of the Optical Society of America A}, publisher={The Optical Society}, author={Aspnes, D. E.}, year={1993}, month={May}, pages={974} }
 @misc{aspnes_quinn_1992, title={Ellipsometric control of material growth}, volume={5,091,320}, number={1992 Feb. 25}, publisher={Washington, DC: U.S. Patent and Trademark Office}, author={Aspnes, D. E. and Quinn, D. E.}, year={1992} }
 @article{aspnes_quinn_tamargo_pudensi_schwarz_brasil_nahory_gregory_1992, title={Growth of AlxGa1−xAs parabolic quantum wells by real‐time feedback control of composition}, volume={60}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.107419}, DOI={10.1063/1.107419}, abstractNote={Epitaxial AlxGa1−xAs structures whose compositions x vary continuously with thickness according to a given input function have been grown by chemical-beam epitaxy under closed-loop ellipsometric control. 200- and 500-Å parabolic quantum wells analyzed by photoreflectance and secondary-ion mass spectrometry, respectively, show that actual compositions follow target values to within 0.02 in x. Growth of the 200-Å profile was controlled using compositions ellipsometrically determined for the outermost running 3.1 Å (∼1 monolayer) of depositing material.}, number={10}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Aspnes, D. E. and Quinn, W. E. and Tamargo, M. C. and Pudensi, M. A. A. and Schwarz, S. A. and Brasil, M. J. S. P. and Nahory, R. E. and Gregory, S.}, year={1992}, month={Mar}, pages={1244–1246} }
 @misc{aspnes_bhat_colas_florez_harbison_studna_1990, title={Optical control of deposition of crystal monolayers}, volume={4,931,132}, number={1990 Jun. 5}, publisher={Washington, DC: U.S. Patent and Trademark Office}, author={Aspnes, D. E. and Bhat, R. and Colas, E. G. and Florez, L. T. and Harbison, J. P and Studna, A. A.}, year={1990} }
 @misc{aspnes_1985, title={Cylindrical grating monochromator for synchrotron radiation}, volume={4,492,466}, number={1985 Jan. 8}, publisher={Washington, DC: U.S. Patent and Trademark Office}, author={Aspnes, D. E.}, year={1985} }
 @article{aspnes_studna_1983, title={Dielectric functions and optical parameters of Si, Ge, GaP, GaAs, GaSb, InP, InAs, and InSb from 1.5 to 6.0 eV}, volume={27}, ISSN={0163-1829}, url={http://dx.doi.org/10.1103/physrevb.27.985}, DOI={10.1103/physrevb.27.985}, abstractNote={We report values of pseudodielectric functions $〈\ensuremath{\epsilon}〉=〈{\ensuremath{\epsilon}}_{1}〉+i〈{\ensuremath{\epsilon}}_{2}〉$ measured by spectroscopic ellipsometry and refractive indices $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{n}=n+ik$, reflectivities $R$, and absorption coefficients $\ensuremath{\alpha}$ calculated from these data. Rather than correct ellipsometric results for the presence of overlayers, we have removed these layers as far as possible using the real-time capability of the spectroscopic ellipsometer to assess surface quality during cleaning. Our results are compared with previous data. In general, there is good agreement among optical parameters measured on smooth, clean, and undamaged samples maintained in an inert atmosphere regardless of the technique used to obtain the data. Differences among our data and previous results can generally be understood in terms of inadequate sample preparation, although results obtained by Kramers-Kronig analysis of reflectance measurements often show effects due to improper extrapolations. The present results illustrate the importance of proper sample preparation and of the capability of separately determining both ${\ensuremath{\epsilon}}_{1}$ and ${\ensuremath{\epsilon}}_{2}$ in optical measurements.}, number={2}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Aspnes, D. E. and Studna, A. A.}, year={1983}, month={Jan}, pages={985–1009} }
 @misc{aspnes_studna_1983, title={Method of preparing semiconductor surfaces}, volume={4,380,490}, number={1983 Apr. 19}, publisher={Washington, DC: U.S. Patent and Trademark Office}, author={Aspnes, D. E. and Studna, A. A.}, year={1983} }
 @article{aspnes_1982, title={Local‐field effects and effective‐medium theory: A microscopic perspective}, volume={50}, ISSN={0002-9505 1943-2909}, url={http://dx.doi.org/10.1119/1.12734}, DOI={10.1119/1.12734}, abstractNote={Standard textbook derivations of the Clausius–Mossotti (Lorentz–Lorenz) relation tend to obscure the physical origin of local-field effects by proceeding from the macroscopic dielectric function of the equivalent homogeneous system to the microscopic parameters of the model. The microscopic and macroscopic aspects can be made clearer by reversing the order, that is, by first obtaining the microscopic solution and then implementing the definition of macroscopic quantities as averages of their microscopic counterparts. This approach also leads naturally into a treatment of effective-medium theory and the description of the dielectric response of heterogeneous materials.}, number={8}, journal={American Journal of Physics}, publisher={American Association of Physics Teachers (AAPT)}, author={Aspnes, D. E.}, year={1982}, month={Aug}, pages={704–709} }
 @misc{aspnes_1982, title={Method for optical monitoring in materials fabrication}, volume={4,332,833}, number={1982 Jul. 1}, publisher={Washington, DC: U.S. Patent and Trademark Office}, author={Aspnes, D. E.}, year={1982} }
 @misc{adams_aspnes_bagley_1982, title={Method for producing devices comprising high density amorphous silicon or germanium layers by low pressure CVD technique}, volume={4,357,179}, number={1982 Nov. 2}, publisher={Washington, DC: U.S. Patent and Trademark Office}, author={Adams, A. C. and Aspnes, D. E. and Bagley, B. G.}, year={1982} }
 @article{aspnes_1982, title={Optical properties of thin films}, volume={89}, ISSN={0040-6090}, url={http://dx.doi.org/10.1016/0040-6090(82)90590-9}, DOI={10.1016/0040-6090(82)90590-9}, abstractNote={In this paper we discuss the connection between the microstructure of a heterogeneous thin film and its macroscopic dielectric response ε. Effective medium theory is developed from a solution of the Clausius-Mossotti problem from basic principles. The solution is generalized to obtain the Lorentz-Lorenz. Maxwell Garnett and Bruggeman expressions. The connection between microstructure and absolute limits to the allowed values of the dielectric response of two-phase composites is reviewed. The form of these limits for two-phase composites of known composition and two- or three-dimensional isotropy can be used to derive simple expressions for ε and also for the average fields within each phase. These results are used to analyze dielectric function spectra of semiconductor films for information about density, polycrystallinity and surface roughness. Examples illustrating the detection of unwanted overlayers and the real-time determination of nucleation growth are also given.}, number={3}, journal={Thin Solid Films}, publisher={Elsevier BV}, author={Aspnes, D.E.}, year={1982}, month={Mar}, pages={249–262} }
 @misc{aspnes_1976, title={Measurement of thin films by polarized light}, volume={3,985,447}, number={1976 Oct. 12}, publisher={Washington, DC: U.S. Patent and Trademark Office}, author={Aspnes, D. E.}, year={1976} }
 @article{aspnes_studna_1975, title={High Precision Scanning Ellipsometer}, volume={14}, ISSN={0003-6935 1539-4522}, url={http://dx.doi.org/10.1364/ao.14.000220}, DOI={10.1364/ao.14.000220}, abstractNote={We describe the design, construction, alignment, and calibration of a photometric ellipsometer of the rotating-analyzer type. Data are obtained by digital sampling of the transmitted flux with an analog-to-digital converter, followed by Fourier transforming of the accumulated data with a dedicated minicomputer. With an operating mechanical rotation frequency of 74 Hz, a data acquisition cycle requires less than 7 msec. The intrinsic precision attainable is high because precision is limited only by shot noise or intrinsic source instabilities, even when relatively weak continuum lamps are used as light sources. Precision may be improved by accumulating the data for consecutive cycles at a fixed wavelength. The system allows complex reflectance ratios to be determined as continuous functions of wavelength from the near infrared to the near ultraviolet spectral range. Data reduction programs can be modified to calculate complex refractive index or dielectric function spectra, or film thicknesses and refractive indices, as well as the usual ellipsometric parameters tanpsi, cosDelta.}, number={1}, journal={Applied Optics}, publisher={The Optical Society}, author={Aspnes, D. E. and Studna, A. A.}, year={1975}, month={Jan}, pages={220} }
 @article{aspnes_1966, title={Electric-Field Effects on Optical Absorption near Thresholds in Solids}, volume={147}, ISSN={0031-899X}, url={http://dx.doi.org/10.1103/physrev.147.554}, DOI={10.1103/physrev.147.554}, abstractNote={The weak-field effective-mass-approximation calculations of the absorption coefficient in the presence of an electric field for direct and indirect transitions at a normal (${M}_{0}$) threshold are extended to an arbitrary orientation of the electric field in an anisotropic solid. To do this, a systematic method of evaluating density-of-states integrals arising in the electroabsorption or Franz-Keldysh effect is presented. Certain integrals obtained in prior calculations at the ${M}_{0}$ threshold which have not been previously evaluated are given in closed form. This method is also used to derive the change in absorption coefficient, $\ensuremath{\Delta}\ensuremath{\alpha}(\mathcal{E})$, occuring at the saddle-point edges ${M}_{1}$ and ${M}_{2}$, as well as the edge ${M}_{3}$, for an arbitrary field direction in an anisotropic solid. It is shown that there is a direct correlation between $\ensuremath{\Delta}\ensuremath{\alpha}(\mathcal{E})$ for the ${M}_{0}$ and ${M}_{3}$ edges, and also for the ${M}_{1}$ and ${M}_{2}$ thresholds. Reduced masses of opposite signs, as in the ${M}_{1}$ and ${M}_{2}$ edges, give rise to two branches in $\ensuremath{\Delta}\ensuremath{\alpha}(\mathcal{E})$.}, number={2}, journal={Physical Review}, publisher={American Physical Society (APS)}, author={Aspnes, David E.}, year={1966}, month={Jul}, pages={554–566} }