@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{pozina_edwards_bergman_monemar_bremser_davis_2001, title={Time-resolved photoluminescence in strained GaN layers}, volume={183}, number={1}, journal={Physica Status Solidi. A, Applications and Materials Science}, author={Pozina, G. and Edwards, N. V. and Bergman, J. P. and Monemar, B. and Bremser, M. D. and Davis, R. F.}, year={2001}, pages={151–155} }
 @article{pozina_edwards_bergman_paskova_monemar_bremser_davis_2001, title={Time-resolved spectroscopy of strained GaN/AlN/6H-SiC heterostructures grown by metalorganic chemical vapor deposition}, volume={78}, ISSN={["0003-6951"]}, DOI={10.1063/1.1350421}, abstractNote={Temperature-dependent time-resolved photoluminescence measurements were performed on GaN film/AlN buffer/6H–SiC substrate heterostructures grown by metalorganic chemical vapor deposition. The overlying GaN layers were under tension, as estimated from the free A exciton (FEA) position. The recombination lifetimes were determined for the FEA and for the neutral-donor-bound exciton (D0X). We observed that the recombination lifetime for the FEA has the same value of 40–50 ps in all the layers, whereas the recombination time for the D0X varies for different samples. We observed that the recombination lifetimes for D0X have a clear dependence on the position of FEA, i.e., the recombination lifetime increases with decreasing strain in the layers. We discuss the results in term of the hole states involved in the donor-bound exciton recombination.}, number={8}, journal={APPLIED PHYSICS LETTERS}, author={Pozina, G and Edwards, NV and Bergman, JP and Paskova, T and Monemar, B and Bremser, MD and Davis, RF}, year={2001}, month={Feb}, pages={1062–1064} }
 @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{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{buyanova_wagner_chen_edwards_monemar_lindstrom_bremser_davis_amano_akasaki_1999, title={Electronic structure of the 0.88-eV luminescence center in electron-irradiated gallium nitride}, volume={60}, number={3}, journal={Physical Review. B, Condensed Matter and Materials Physics}, author={Buyanova, I. A. and Wagner, M. and Chen, W. M. and Edwards, N. V. and Monemar, B. and Lindstrom, J. L. and Bremser, M. D. and Davis, R. F. and Amano, H. and Akasaki, I.}, year={1999}, pages={1746–1751} }
 @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} }
 @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{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{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{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} }
 @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{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} }