@article{yang_zeman_nemanich_2007, title={Coarsening dynamics of nanoscale Ti-silicide islands on Si surfaces}, volume={50}, DOI={10.3938/jkps.50.575}, abstractNote={The late-stage coarsening dynamics of nanoscale Ti-silicide (TiSi2) islands on Si surfaces is explored in ultra high vacuum (UHV) by using ultraviolet photoelectron emission microscopy (UVPEEM). The UV-PEEM is employed for real-time, in-situ monitoring of the nanostructure dynamics and evolution at high temperatures. Continuous annealing at ∼1150 ◦C leads to an increase in the size of the initially nucleated silicide islands and to a reduction in the number density. By monitoring the relative position and the size of individual islands, we find that islands grow through Ostwald ripening and attractive migration and coalescence (AMC). The AMC is a new coarsening process where nearby islands are observed to migrate directly towards each other and subsequently coalesce. This process has been attributed to a growth-decay flow of the island edges driven by a non-uniform chemical potential around the islands. The non-uniform chemical potential results from a varying adatom surface concentration induced by local variations in size, number, and location of the neighboring islands. Significant shape distortions of the coarsening islands, migration of each island towards the center of mass of a group of islands, and screening of attractive migration of islands with a continuous Ti flux support our suggested model for the AMC mechanism. The mass exchange between the coarsening TiSi2 islands in the AMC process is explained in terms of a Ti adatom surface-diffusion-limited process.}, number={3}, journal={Journal of the Korean Physical Society}, author={Yang, W. C. and Zeman, M. and Nemanich, R. J.}, year={2007}, pages={575–580} }
 @misc{edwards_allen_haglund_nemanich_redlich_simon_yang_2005, title={Applications of free-electron lasers in the biological and material sciences}, volume={81}, ISSN={["1751-1097"]}, DOI={10.1562/2004-11-08-IR-363R.1}, abstractNote={Abstract Free-Electron Lasers (FELs) collectively operate from the terahertz through the ultraviolet range and via intracavity Compton backscattering into the X-ray and gamma-ray regimes. FELs are continuously tunable and can provide optical powers, pulse structures and polarizations that are not matched by conventional lasers. Representative research in the biological and biomedical sciences and condensed matter and material research are described to illustrate the breadth and impact of FEL applications. These include terahertz dynamics in materials far from equilibrium, infrared nonlinear vibrational spectroscopy to investigate dynamical processes in condensed-phase systems, infrared resonant-enhanced multiphoton ionization for gas-phase spectroscopy and spectrometry, infrared matrix-assisted laser-desorption–ionization and infrared matrix-assisted pulsed laser evaporation for analysis and processing of organic materials, human neurosurgery and ophthalmic surgery using a medical infrared FEL and ultraviolet photoemission electron microscopy for nanoscale characterization of materials and nanoscale phenomena. The ongoing development of ultraviolet and X-ray FELs are discussed in terms of future opportunities for applications research.}, number={4}, journal={PHOTOCHEMISTRY AND PHOTOBIOLOGY}, author={Edwards, GS and Allen, SJ and Haglund, RF and Nemanich, RJ and Redlich, B and Simon, JD and Yang, WC}, year={2005}, pages={711–735} }
 @article{yang_rodriguez_gruverman_nemanich_2005, title={Photo electron emission microscopy of polarity-patterned materials}, volume={17}, ISSN={["1361-648X"]}, DOI={10.1088/0953-8984/17/16/012}, abstractNote={This study presents variable photon energy photo electron emission microscopy (PEEM) of polarity-patterned epitaxial GaN films, and ferroelectric LiNbO3 (LNO) single crystals and PbZrTiO3 (PZT) thin films. The photo electrons were excited with spontaneous emission from the tunable UV free electron laser (FEL) at Duke University. We report PEEM observation of polarity contrast and measurement of the photothreshold of each polar region of the materials. For a cleaned GaN film with laterally patterned Ga- and N-face polarities, we found a higher photoelectric yield from the N-face regions compared with the Ga-face regions. Through the photon energy dependent contrast in the PEEM images of the surfaces, we can deduce that the threshold of the N-face region is less than ∼4.9 eV while that of the Ga-face regions is greater than 6.3 eV. In both LNO and PZT, bright emission was detected from the negatively poled domains, indicating that the emission threshold of the negative domain is lower than that of the positive domain. For LNO, the measured photothreshold was ∼4.6 eV at the negative domain and ∼6.2 eV at the positive domain, while for PZT, the threshold of the negative domain was less than 4.3 eV. Moreover, PEEM observation of the PZT surface at elevated temperatures displayed that the domain contrast disappeared near the Curie temperature of ∼300 °C. The PEEM polarity contrast of the polar materials is discussed in terms of internal screening from free carriers and defects and the external screening due to adsorbed ions.}, number={16}, journal={JOURNAL OF PHYSICS-CONDENSED MATTER}, author={Yang, WC and Rodriguez, BJ and Gruverman, A and Nemanich, RJ}, year={2005}, month={Apr}, pages={S1415–S1426} }
 @article{rodriguez_yang_nemanich_gruverman_2005, title={Scanning probe investigation of surface charge and surface potential of GaN-based heterostructures}, volume={86}, ISSN={["1077-3118"]}, DOI={10.1063/1.1869535}, abstractNote={Scanning Kelvin probe microscopy (SKPM) and electrostatic force microscopy (EFM) have been employed to measure the surface potentials and the surface charge densities of the Ga- and the N-face of a GaN lateral polarity heterostructure (LPH). The surface was subjected to an HCl surface treatment to address the role of adsorbed charge on polarization screening. It has been found that while the Ga-face surface appears to be unaffected by the surface treatment, the N-face surface exhibited an increase in adsorbed screening charge density (1.6±0.5×1010cm−2), and a reduction of 0.3±0.1V in the surface potential difference between the N- and Ga-face surfaces.}, number={11}, journal={APPLIED PHYSICS LETTERS}, author={Rodriguez, BJ and Yang, WC and Nemanich, RJ and Gruverman, A}, year={2005}, month={Mar} }
 @misc{samokhvalov_garguilo_yang_edwards_nemanich_simon_2004, title={Photoionization threshold of eumelanosomes determined using UV free electron laser - Photoelectron emission microscopy}, volume={108}, ISSN={["1520-5207"]}, DOI={10.1021/jp046701q}, abstractNote={The application of UV-free electron laser photoelectron emission microscopy (UV-FEL PEEM) to measure the threshold photoelectron spectrum and photoionization potential for human eumelanosomes is described. The origin of potential artifacts and the limitations of the technique are discussed and their potential effects on the measured photoionization potential are quantified. The UV-FEL-PEEM images collected on human eumelanosomes isolated from black hair show that the organelle is photoionized by UV-B radiation. The photoionization threshold is determined to be 4.6 ± 0.2 eV. This result provides new insight into the origin of the differences between the photoionization and oxygen photoconsumption action spectra for eumelanins.}, number={42}, journal={JOURNAL OF PHYSICAL CHEMISTRY B}, author={Samokhvalov, A and Garguilo, J and Yang, WC and Edwards, GS and Nemanich, RJ and Simon, JD}, year={2004}, month={Oct}, pages={16334–16338} }
 @article{yang_rodriguez_gruverman_nemanich_2004, title={Polarization-dependent electron affinity of LiNbO3 surfaces}, volume={85}, ISSN={["1077-3118"]}, DOI={10.1063/1.1790604}, abstractNote={Polar surfaces of a ferroelectric LiNbO3 crystal with periodically poled domains are explored using UV-photoelectron emission microscopy (PEEM). Compared with the positive domains (domains with positive surface polarization charges), a higher photoelectric yield is found from the negative domains (domains with negative surface polarization charges), indicating a lower photothreshold and a corresponding lower electron affinity. The photon-energy-dependent contrast in the PEEM images of the surfaces indicates that the photothreshold of the negative domains is ∼4.6eV while that of the positive domains is greater than ∼6.2eV. We propose that the threshold difference between the opposite domains can be attributed to a variation of the electron affinity due to opposite surface dipoles induced by surface adsorbates.}, number={12}, journal={APPLIED PHYSICS LETTERS}, author={Yang, WC and Rodriguez, BJ and Gruverman, A and Nemanich, RJ}, year={2004}, month={Sep}, pages={2316–2318} }
 @article{yang_ade_nemanich_2004, title={Shape stability of TiSi2 islands on Si (111)}, volume={95}, ISSN={["1089-7550"]}, DOI={10.1063/1.1636526}, abstractNote={The evolution of the shape and size of TiSi2 islands on Si (111) surfaces is explored with real time ultraviolet photoelectron emission microscopy. During continuous deposition of Ti at elevated temperatures, individual islands in a dilute surface distribution grow larger without island–island interactions. As they increase in size, symmetric islands transform into elongated shaped islands with high length-to-width aspect ratios. An extremely elongated island shows a ratio of 85:1 and is ∼17 μm long and ∼0.2 μm wide. The individual elongated islands have different widths regardless of their length. The width of the growing islands is determined at the initial transition stage and remains essentially constant with increasing length. We propose that the various widths of the elongated islands are determined by the degree of strain relaxation, possibly through the nucleation of dislocations at the island interface. In addition, it is found that the elongated islands display a prism-like shape or a truncated prism-like shape. We propose that the shape evolution of the elongated islands is related to both strain relaxation and growth kinetics.}, number={3}, journal={JOURNAL OF APPLIED PHYSICS}, author={Yang, WC and Ade, H and Nemanich, RJ}, year={2004}, month={Feb}, pages={1572–1576} }
 @article{yang_zeman_ade_nemanich_2003, title={Attractive migration and coalescence: A significant process in the coarsening of TiSi2 islands on the Si(111) surface}, volume={90}, ISSN={["0031-9007"]}, DOI={10.1103/physrevlett.90.136102}, abstractNote={The dynamics and coarsening of TiSi2 islands on Si(111) surfaces are studied in real time with photoelectron emission microscopy. A significant fraction of events are observed in which nearby islands move attractively toward each other and subsequently coalesce. It is proposed that attractive island migration is due to the growth-decay flow of the island edges driven by a nonuniform surface concentration around the islands. The local surface concentration is induced by the neighboring islands. This coarsening mechanism should significantly affect the evolution of the island distribution.}, number={13}, journal={PHYSICAL REVIEW LETTERS}, author={Yang, WC and Zeman, M and Ade, H and Nemanich, RJ}, year={2003}, month={Apr} }
 @article{fitting_zeman_yang_nemanich_2003, title={Influence of strain, surface diffusion and Ostwald ripening on the evolution of nanostructures for erbium on Si(001)}, volume={93}, ISSN={["1089-7550"]}, DOI={10.1063/1.1557787}, abstractNote={This study explores the evolution of nanoscale islands and wire structures during deposition and surface ripening. Ultraviolet photoelectron emission microscopy has been employed to study the real time growth process of individual erbium silicide nanostructures on Si(001) surfaces at temperatures up to 1050 °C. During the initial island formation process compact islands form and some undergo a shape transition to elongated islands oriented along the 〈110〉 directions of the Si substrate. The initial island formation is driven by the surface and interface energies of the silicide/Si structure. The widths of the growing islands remain essentially constant while the lengths increase. The observed elongated islands are ∼150 nm wide, which is larger than the width of prior reported erbium silicide nanowire structures. We propose that the ∼150 nm elongated islands are partially relaxed, possibly through the formation of misfit dislocations. The results indicate a temperature regime where island growth is mainly governed by surface diffusion of the deposited Er adatoms and a higher temperature regime where Ostwald ripening contributes to the island morphology.}, number={7}, journal={JOURNAL OF APPLIED PHYSICS}, author={Fitting, L and Zeman, MC and Yang, WC and Nemanich, RJ}, year={2003}, month={Apr}, pages={4180–4184} }
 @article{yang_rodriguez_park_nemanich_ambacher_cimalla_2003, title={Photoelectron emission microscopy observation of inversion domain boundaries of GaN-based lateral polarity heterostructures}, volume={94}, DOI={10.1063/1.1618355}, abstractNote={An intentionally grown GaN film with laterally patterned Ga- and N-face polarities is studied using in situ UV-photoelectron emission microscopy (PEEM). Before chemical vapor cleaning of the surface, the emission contrast between the Ga- and N-face polarities regions was not significant. However, after cleaning the emission contrast between the different polarity regions was enhanced such that the N-face regions exhibited increased emission over the Ga-face regions. The results indicate that the emission threshold of the N-face region is lower than that of the Ga face. Moreover, bright emission was detected from regions around the inversion domain boundaries of the lateral polarity heterostructure. The PEEM polarity contrast and intense emission from the inversion domain boundary regions are discussed in terms of the built-in lateral field and the surface band bending induced by the polarization bound surface charges.}, number={9}, journal={Journal of Applied Physics}, author={Yang, W. C. and Rodriguez, B. J. and Park, M. and Nemanich, R. J. and Ambacher, O. and Cimalla, V.}, year={2003}, pages={5720–5725} }
 @article{ballarotto_siegrist_phaneuf_williams_yang_nemanich_2001, title={Photon energy dependence of contrast in photoelectron emission microscopy of Si devices}, volume={78}, ISSN={["0003-6951"]}, DOI={10.1063/1.1376151}, abstractNote={We investigate the variation in doping-induced contrast with photon energy in photoelectron emission microscopy images of Si pn devices using a free-electron laser as a tunable monochromatic light source. Photoyield is observed from p-doped regions of the devices for photon energies as low as 4.5 eV. Band tailing is the dominant effect contributing to the low energy photoyield from the heavily doped p regions. The low intensity tail from the n regions, however, may be from surface states.}, number={22}, journal={APPLIED PHYSICS LETTERS}, author={Ballarotto, VW and Siegrist, K and Phaneuf, RJ and Williams, ED and Yang, WC and Nemanich, RJ}, year={2001}, month={May}, pages={3547–3549} }
 @article{jeon_jung_kim_yang_nemanich_2000, title={Effects of a Ta interlayer on the phase transition of TiSi2 on Si(111)}, volume={88}, DOI={10.1063/1.1286725}, abstractNote={This study examines the effects of a thin Ta interlayer on the formation of TiSi2 on Si(111) substrate. The Ta interlayer was introduced by depositing Ta and Ti films sequentially on an atomically clean Si(111) substrate in an ultrahigh vacuum (UHV) system. Samples of 100 Å Ti with 5 and 10 Å Ta interlayers were compared to similar structures without an interlayer. After deposition, the substrates were annealed for 10 min, in situ, at temperatures between 500 and 750 °C in 50 °C increments. The TiSi2 formation with and without the Ta interlayer was analyzed with an X-ray diffractometer, Auger electron spectroscopy (AES), Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and a four-point probe. The AES analysis data showed a 1:2 ratio of Ti:Si in the Ti-silicide layer and indicated that the Ta layer remained at the interface between TiSi2 and the Si(111) substrate. The C 49–C 54 TiSi2 phase transition temperature was lowered by ∼200 °C. The C 49–C 54 TiSi2 phase transition temperature was 550 °C for the samples with a Ta interlayer and was 750 °C for the samples with no Ta interlayer. The sheet resistance of the Ta interlayered Ti silicide showed lower values of resistivity at low temperatures which indicated the change in phase transition temperature. The C 54 TiSi2 displayed different crystal orientation when the Ta interlayer was employed. The SEM and TEM micrographs showed that the TiSi2 with a Ta interlayer significantly suppressed the tendency to islanding and surface agglomeration.}, number={5}, journal={Journal of Applied Physics}, author={Jeon, H. and Jung, B. and Kim, Y. D. and Yang, W. C. and Nemanich, R. J.}, year={2000}, pages={2467–2471} }
 @article{brown_boney_matthews_srinivasan_schetzina_nohava_yang_krishnankutty_2000, title={UV-specific (320-365 nm) digital camera based on a 128x128 focal plane array of GaN/AlGaN p-i-n photodiodes}, volume={5}, DOI={10.1557/s1092578300000065}, abstractNote={An ultraviolet-specific (320-365 nm) digital camera based on a 128×128 array of backside-illuminated GaN/AlGaN p-i-n photodiodes has been successfully developed. The diode structure consists of a base n-type layer of AlGaN (~23% Al) followed by undoped and then p-type GaN layers deposited by metal organic vapor phase epitaxy. Double-side polished sapphire wafers serve as transparent substrates. Standard photolithographic, etching, and metallization procedures were employed to fabricate the devices. The fully-processed photodiode array was hybridized to a silicon readout integrated circuit (ROIC) using In bump bonds for electrical contact. The UV camera was operated at room temperature at frame rates ranging from 15 to 240 Hz. A variety of UV scenes were successfully recorded with this configuration.}, number={6}, journal={MRS Internet Journal of Nitride Semiconductor Research}, author={Brown, J. D. and Boney, J. and Matthews, J. and Srinivasan, P. and Schetzina, J. F. and Nohava, T. and Yang, W. and Krishnankutty, S.}, year={2000}, pages={1–12} }
 @article{jung_kim_jeon_yang_nemanich_1999, title={Reduction of the transition temperature of C54TiSi(2) through a Ta interlayer}, volume={35}, number={1999 Dec.}, journal={Journal of the Korean Physical Society}, author={Jung, B. and Kim, Y. D. and Jeon, H. and Yang, W. and Nemanich, R. J.}, year={1999}, pages={S769–773} }
 @article{ade_yang_english_hartman_davis_nemanich_litvinenko_pinayev_wu_madey_1998, title={A free electron laser-photoemission electron microscope system (FEL-PEEM)}, volume={5}, ISSN={["0218-625X"]}, DOI={10.1142/S0218625X98001596}, abstractNote={ We report first results from our effort to couple a high resolution photoemission electron microscope (PEEM) to the OK-4 ultraviolet free electron laser at Duke University (OK-4/Duke UV FEL). The OK-4/Duke UV FEL is a high intensity source of tunable monochromatic photons in the 3–10 eV energy range. This tunability is unique and allows us to operate near the photoemission threshold of any samples and thus maximize sample contrast while keeping chromatic berrations in the PEEM minimal. We have recorded first images from a variety of samples using spontaneous radiation from the OK-4/ Duke UV FEL in the photon energy range of 4.0–6.5 eV. Due to different photothreshold emission from different sample areas, emission from these areas could be turned on (or off) selectively. We have also observed relative intensity reversal with changes in photon energy which are interpreted as density-of-state contrast. Usable image quality has been achieved, even though the output power of the FEL in spontaneous emission mode was several orders of magnitude lower than the anticipated full laser power. The PEEM has achieved a spatial resolution of 12 nm. }, number={6}, journal={SURFACE REVIEW AND LETTERS}, author={Ade, H and Yang, W and English, SL and Hartman, J and Davis, RF and Nemanich, RJ and Litvinenko, VN and Pinayev, IV and Wu, Y and Madey, JMJ}, year={1998}, month={Dec}, pages={1257–1268} }
 @article{yang_jedema_ade_nemanich_1997, title={Correlation of morphology and electrical properties of nanoscale TiSi2 epitaxial islands on Si (001)}, volume={308}, ISSN={["1879-2731"]}, DOI={10.1016/S0040-6090(97)00431-8}, abstractNote={The morphology and electrical properties of nanoscale epitaxial islands of TiSi2 are explored. The TiSi2 islands are prepared by ultra-high vacuum (UHV) deposition of ultra-thin Ti (0.3–2 nm) on both smooth and roughened Si (001) substrates. The roughened substrates are prepared by etching with atomic H produced in a plasma. The island formation is initiated by annealing to 800–1000°C. The morphologies of the substrate before and after island formation are examined by atomic force microscopy (AFM). In particular, the influence of surface-roughness on both the formation of islands and the size distribution of islands is investigated. Islands with a lateral dimension of ~35 nm and a vertical dimension of ~2.5 nm on a roughened substrate (RMS=12 nm) were observed, with a uniform distribution of 120 nm spacing between the islands. It was found that for similar processing conditions the size distribution of islands formed on a rough surface was smaller than islands formed on smooth surfaces. The results are discussed in terms of surface energy, diffusion and the strain field around the islands. The island structures can affect the electrical characteristics of the interface and the Schottky barrier was obtained from diodes formed with a Pt layer deposited over the islanded interface. The Schottky barrier was lowest for interfaces with the smaller TiSi2 islands.}, number={1997 Oct. 31}, journal={THIN SOLID FILMS}, author={Yang, W and Jedema, FJ and Ade, H and Nemanich, RJ}, year={1997}, month={Oct}, pages={627–633} }