@article{hartman_naniwae_petrich_nemanich_davis_2005, title={Photo-electron emission and atomic force microscopies of the hydrogen etched 6H-SiC(0001) surface and the initial growth of GaN and AlN}, volume={242}, ISSN={["0169-4332"]}, DOI={10.1016/j.apsusc.2004.09.021}, abstractNote={Photo-emission electron microscopy (PEEM) and atomic force microscopy (AFM) have been used to characterize the surfaces of hydrogen etched 6H-SiC(0 0 0 1) wafers and the microstructure of the initial stages of growth of GaN and AlN on these surfaces via molecular beam epitaxy. The PEEM images were obtained using a free electron laser as the photon source. A stepped structure was evident in these images of the surfaces etched at 1600–1700 °C for 15 min. Comparison with the AFM images revealed that emission was occurring from the intersection of the steps and the terraces. Images of the initial stages of deposition of the GaN thin films at 700 and 800 °C revealed three-dimensional island growth. The degree of coalescence of these films was dependent upon the step structure: regions containing steps having unit cell height exhibited complete or nearly complete coalescence; regions containing steps with half unit cell height showed voids in the films parallel to the steps. PEEM of the initial stages of growth of AlN revealed immediate nucleation and rapid coalescence during deposition at 900 °C, except in areas on the substrate surface containing steps having half unit cell height. Incomplete coalescence and pits were also observed in the latter areas.}, number={3-4}, journal={APPLIED SURFACE SCIENCE}, author={Hartman, JD and Naniwae, K and Petrich, C and Nemanich, R and Davis, RF}, year={2005}, month={Apr}, pages={428–436} }
 @article{hartman_roskowski_reitmeier_tracy_davis_nemanich_2003, title={Characterization of hydrogen etched 6H-SiC(0001) substrates and subsequently grown AlN films}, volume={21}, ISSN={["0734-2101"]}, DOI={10.1116/1.1539080}, abstractNote={Wafers of n-type, 6H–SiC(0001) with (ND–NA)=(5.1–7.5)×1017 and 2.5×1018 were etched in a flowing 25%H2/75%He mixture within the range of 1500–1640 °C at 1 atm. Equilibrium thermodynamic calculations indicated that the presence of atomic hydrogen is necessary to achieve etching of SiC. Atomic force microscopy, optical microscopy, and low energy electron diffraction of the etched surface revealed a faceted surface morphology with unit cell and half unit cell high steps and a 1×1 reconstruction. The latter sample also exhibited a much larger number of hexagonal pits on the surface. Annealing the etched samples under ultrahigh vacuum (UHV) at 1030 °C for 15 min resulted in (1) a reduction of the surface oxygen and adventitious hydrocarbons below the detection limit of Auger electron spectroscopy, (2) a (√3×√3)R30° reconstructed surface and (3) a Si-to-C peak-to-peak height ratio of 1.2. By contrast, using a chemical vapor cleaning (CVC) process consisting of an exposure to 3000 Langmuir (L) of silane at 1030 °C for 10 min under UHV conditions resulted in a (3×3) surface reconstruction, a Si-to-C ratio of 3.9, and islands of excess silicon. Continued annealing of the latter material for an additional 10 min at 1030 °C resulted in a (1×1) LEED pattern with a diffuse ring. Films of AlN grown via MOCVD at a sample platter temperature of 1274 °C for 15 min on hydrogen etched wafers having a doping concentration of 8.7×1017 cm−3 and cleaned via annealing had a rms roughness value of ≈0.4 nm.}, number={2}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A}, author={Hartman, JD and Roskowski, AM and Reitmeier, ZJ and Tracy, KM and Davis, RF and Nemanich, RJ}, year={2003}, pages={394–400} }
 @article{platow_oh_nemanich_sayers_hartman_davis_2002, title={TiC nanoisland formation on 6H-SiC(0001)(Si)}, volume={91}, ISSN={["0021-8979"]}, DOI={10.1063/1.1465121}, abstractNote={Spontaneous formation of titanium carbide nanoislands on silicon carbide substrates has been studied with scanning tunneling microscopy and x-ray absorption near-edge spectroscopy. Scratch-free and atomically flat 6H–SiC(0001)Si substrates were prepared by high temperature hydrogen etching. The surfaces were subsequently cleaned by in situ ultrahigh vacuum annealing. Titanium carbide nanoislands were formed by titanium deposition and annealing at 950 °C. The average width (10–50 nm) and separation of the Ti islands was controlled by varying the titanium coverage (0.1–0.3 nm) and the annealing time (1–20 min). At the lowest coverage, the islands were uniformly distributed over the surface, while at higher coverage the islands tended to collect at the substrate step edges.}, number={9}, journal={JOURNAL OF APPLIED PHYSICS}, author={Platow, W and Oh, J and Nemanich, RJ and Sayers, DE and Hartman, JD and Davis, RF}, year={2002}, month={May}, pages={6081–6084} }
 @article{platow_nemanich_sayers_hartman_davis_2001, title={Growth of epitaxial CoSi2 on 6H-SiC(0001)(Si)}, volume={90}, ISSN={["1089-7550"]}, DOI={10.1063/1.1412842}, abstractNote={Epitaxial growth of (111)-oriented CoSi2 has been achieved on a scratch-free 6H-SiC(0001)Si substrate. The surface was prepared using atmospheric hydrogen etching and ultrahigh vacuum Si cleaning. A high-quality CoSi2 thin film was obtained by a modified template method and co-deposition of Co and Si at 550 °C. The structure and morphology of the film is studied by means of reflection high electron energy diffraction, x-ray absorption fine structure, x-ray diffraction, and atomic force microscopy.}, number={12}, journal={JOURNAL OF APPLIED PHYSICS}, author={Platow, W and Nemanich, RJ and Sayers, DE and Hartman, JD and Davis, RF}, year={2001}, month={Dec}, pages={5924–5927} }
 @article{hartman_naniwae_petrich_ramachandran_feenstra_nemanich_davis_2000, title={Photo-emission electron microscopy (PEEM) of cleaned and etched 6H-SiC(0001)}, volume={338-3}, ISBN={["*************"]}, ISSN={["0255-5476"]}, DOI={10.4028/www.scientific.net/msf.338-342.353}, journal={SILICON CARBIDE AND RELATED MATERIALS - 1999 PTS, 1 & 2}, author={Hartman, JD and Naniwae, K and Petrich, C and Ramachandran, V and Feenstra, RM and Nemanich, RJ and Davis, RF}, year={2000}, pages={353–356} }
 @article{ward_hartman_hurt_tracy_davis_nemanich_2000, title={Schottky barrier height and electron affinity of titanium on AIN}, volume={18}, ISSN={["1071-1023"]}, DOI={10.1116/1.1303733}, abstractNote={Approximately 100 or 1000 Å of AlN was deposited on the (0001)Si-face of on-axis n-type 6H–SiC. The surfaces were examined by ultraviolet photoemission spectroscopy (UPS) utilizing the He I α (21.2 eV) and the He II α (40.8 eV) excitation. Experimental difficulties are discussed. Titanium was deposited on the clean surface of in situ grown AlN. The titanium–AlN interface was also characterized with UPS. Two approaches are presented to identify the valence band maximum (VBM) and the electron affinity χ of the clean surface of AlN was found to be either 0 to 1 eV depending upon the position of the valence band edge. The same assumptions were applied to the analysis of the Ti/AlN interface and, for the case of χ=0 eV, the position of the valence band maximum is 3.4 eV below the position of the Fermi level. For the case of χ=1 eV, the position of the valence band maximum is 4.4 eV below the position of the Fermi level. Therefore, the p-type Schottky barrier height of titanium on AlN is measured to be 3.4±0.2 or 4.4±0.2 eV for χ=0 eV and χ=1 eV, respectively. Independent of the selection of the valence band maximum, the observed Schottky barrier differed from that predicted by the Schottky–Mott model by 1.5±0.2 eV.}, number={4}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B}, author={Ward, BL and Hartman, JD and Hurt, EH and Tracy, KM and Davis, RF and Nemanich, RJ}, year={2000}, pages={2082–2087} }
 @article{nemanich_english_hartman_sowers_ward_ade_davis_1999, title={Imaging electron emission from diamond and III-V nitride surfaces with photo-electron emission microscopy}, volume={146}, ISSN={["0169-4332"]}, DOI={10.1016/S0169-4332(99)00021-5}, abstractNote={Wide bandgap semiconductors such as diamond and the III–V nitrides (GaN, AlN, and AlGaN alloys) exhibit small or even negative electron affinities. Results have shown that different surface treatments will modify the electron affinity of diamond to cause a positive or negative electron affinity (NEA). This study describes the characterization of these surfaces with photo-electron emission microscopy (PEEM). The PEEM technique is unique in that it combines aspects of UV photoemission and field emission. In this study, PEEM images are obtained with either a traditional Hg lamp or with tunable UV excitation from a free electron laser. The UV-free electron laser at Duke University provides tunable emission from 3.5 to greater than 7 eV. PEEM images of boron or nitrogen (N)-doped diamond are similar to SEM of the same surface indicating relatively uniform emission. For the N-doped samples, PEEM images were obtained for different photon energies ranging from 5.0 to 6.0 eV. In these experiments, the hydrogen terminated surface showed more intense PEEM images at lower photon energy indicating a lower photothreshold than annealed surfaces which are presumed to be adsorbate free. For the nitrides, the emission properties of an array of GaN emitter structures is imaged. Emission is observed from the peaks, and relatively uniform emission is observed from the array. The field at the sample surface is approximately 10 V/μm which is sufficient to obtain an image without UV light. This process is termed field emission electron microscopy (FEEM).}, number={1-4}, journal={APPLIED SURFACE SCIENCE}, author={Nemanich, RJ and English, SL and Hartman, JD and Sowers, AT and Ward, BL and Ade, H and Davis, RF}, year={1999}, month={May}, pages={287–294} }
 @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{nemanich_baumann_benjamin_english_hartman_sowers_ward_1998, title={Characterization of electron emitting surfaces of diamond and III-V nitrides}, volume={8}, number={4}, journal={Diamond Films and Technology}, author={Nemanich, R. J. and Baumann, P. K. and Benjamin, M. C. and English, S. L. and Hartman, J. D. and Sowers, A. T. and Ward, B. L.}, year={1998}, pages={211–223} }
 @article{ward_nam_hartman_english_mccarson_schlesser_sitar_davis_nemanich_1998, title={Electron emission characteristics of GaN pyramid arrays grown via organometallic vapor phase epitaxy}, volume={84}, ISSN={["0021-8979"]}, DOI={10.1063/1.368775}, abstractNote={Selective growth of arrays of silicon-doped GaN (Si:GaN) pyramids for field emitter applications has been achieved. The electron emission characteristics of these arrays has been measured using techniques such as field emission, field emission energy distribution analysis (FEED), photoemission electron microscopy (PEEM), and field emission electron microscopy (FEEM). The field emission current–voltage (I–V) results indicate an average threshold field as low as 7 V/μm for an emission current of 10 nA. It is suggested that the low threshold field value is a consequence of both the low work function of Si:GaN and the field enhancement of the pyramids. The results of the FEEM and FEED measurements indicate agreement with the field emission I–V characteristics. The FEED results indicate that the Si:GaN pyramids are conducting, and that no significant ohmic losses are present between the top contact to the array and the field emitting pyramids. The PEEM and FEEM images show that the emission from the arrays is uniform over a 150 μm field of view.}, number={9}, journal={JOURNAL OF APPLIED PHYSICS}, author={Ward, BL and Nam, OH and Hartman, JD and English, SL and McCarson, BL and Schlesser, R and Sitar, Z and Davis, RF and Nemanich, RJ}, year={1998}, month={Nov}, pages={5238–5242} }