@article{baumann_nemanich_1998, title={Characterization of copper-diamond (100), (111), and (110) interfaces: Electron affinity and Schottky barrier}, volume={58}, ISSN={["1550-235X"]}, DOI={10.1103/physrevb.58.1643}, abstractNote={In this study ultraviolet photoemission spectroscopy was employed to correlate the electron affinity and Schottky barrier height of Cu films on type-IIb ($p$-type) diamond (100), (111), and (110) surfaces. Furthermore, field emission measurements were correlated with the effective electron affinity of the samples. Prior to deposition the diamond samples were cleaned by various annealings and plasma treatments in ultrahigh vacuum. Annealing the diamond substrates to 1150 \ifmmode^\circ\else\textdegree\fi{}C resulted in adsorbate-free surfaces with a positive electron affinity. A negative electron affinity (NEA) was induced after depositing 1 \AA{} of Cu on the clean surface. The Schottky barrier heights for the clean surfaces ranged from 0.30 eV for the (111) surface to 0.70 eV for the (100) surface. Depositing Cu onto H-terminated surfaces exhibiting a NEA still resulted in a NEA on all surfaces. However, the Schottky barrier heights were larger, ranging from 0.50 eV for the (111) surface to 0.90 eV for the (100) and (110) surfaces. The metal-induced NEA has been found to be stable to exposure to air. Following a 500 \ifmmode^\circ\else\textdegree\fi{}C annealing an oxygen-terminated (100) surface with a positive electron affinity was obtained. Cu deposition resulted in a positive electron affinity and the largest Schottky barrier height with 1.60 eV. A field emission threshold field of 79 V/\ensuremath{\mu}m was obtained for an oxygen-terminated diamond (100) surface. Values of 20, 25, and 53 V/\ensuremath{\mu}m were measured for Cu on clean, H- and O-terminated surfaces, respectively. Based on these experiments, it is suggested that chemisorbed species such as H or O on diamond surfaces cause an increase in the Schottky barrier as well as in the field emission threshold field after Cu deposition.}, number={3}, journal={PHYSICAL REVIEW B}, author={Baumann, PK and Nemanich, RJ}, year={1998}, month={Jul}, pages={1643–1654} }
 @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{baumann_nemanich_1998, title={Electron affinity and Schottky barrier height of metal-diamond (100), (111), and (110) interfaces}, volume={83}, ISSN={["0021-8979"]}, DOI={10.1063/1.366940}, abstractNote={The electron emission properties of metal–diamond (100), (111), and (110) interfaces were characterized by means of UV photoemission spectroscopy (UPS) and field-emission measurements. Different surface cleaning procedures including annealing in ultrahigh vacuum (UHV) and rf plasma treatments were used before metal deposition. This resulted in diamond surfaces terminated by oxygen, hydrogen, or free of adsorbates. The electron affinity and Schottky barrier height of Zr or Co thin films were correlated by means of UPS. A negative electron affinity (NEA) was observed for Zr on any diamond surface. Co on diamond resulted in NEA characteristics except for oxygen-terminated surfaces. The lowest Schottky barrier heights were obtained for the clean diamond surfaces. Higher values were measured for H termination, and the highest values were obtained for O on diamond. For Zr, the Schottky barrier height ranged from 0.70 eV for the clean to 0.90 eV for the O-terminated diamond (100) surface. Values for Co ranged from 0.35 to 1.40 eV for clean- and O-covered (100) surfaces, respectively. The metal-induced NEA proved to be stable after exposure to air. For the oxygen-terminated diamond (100) surface a field-emission threshold of 79 V/μm was measured. Zr or Co deposition resulted in lower thresholds. Values as low as 20 V/μm were observed for Zr on the clean diamond (100) surface. Results for Zr or Co on H- or O-terminated surfaces were higher. H or O layers on diamond tend to cause an increase in the Schottky barrier height and the field-emission threshold field of Zr– and Co–diamond interfaces. The value of the electron affinity and Schottky barrier were correlated with work function and different initial surface preparation. The results were largely consistent with a model in which the vacuum level was related to the metal work function and the measured Schottky barrier.}, number={4}, journal={JOURNAL OF APPLIED PHYSICS}, author={Baumann, PK and Nemanich, RJ}, year={1998}, month={Feb}, pages={2072–2082} }
 @article{baumann_nemanich_1998, title={Electron emission from metal-diamond (100), (111) and (110) interfaces}, volume={7}, ISSN={["0925-9635"]}, DOI={10.1016/S0925-9635(97)00256-2}, abstractNote={Electron emission characteristics of Cu, Co or Zr films on diamond (100), (111) and (110) surfaces were measured by employing ultraviolet photoemission spectroscopy (UPS) and field emission measurements. Prior to metal deposition, the diamond substrates were terminated with oxygen, hydrogen or were free of adsorbates. Deposition of thin Cu or Co films induced a NEA on clean and H-terminated surfaces. A positive electron affinity was observed for Cu or Co on oxygenated surfaces, and depositing thin Zr films resulted in a NEA on all surfaces considered. UPS can be used to correlate the electron affinity and Schottky barrier height. Schottky barriers of metals on clean surfaces were the lowest, whereas they were the highest on oxygen-covered surfaces. Values for the Schottky barrier height ranged from 0.70 eV to 1.60 eV for Cu, 0.35 eV to 1.40 eV for Co and 0.70 eV to 0.95 eV for Zr. A field emission threshold of 79 V μm−1 was measured for oxygenated (100) surfaces. The lowest value of 20 V μm−1 was observed for Zr on the clean (100) surface. For all the metals studied, it was found that a lower Schottky barrier height results in a lower electron affinity, and a lower electron affinity results in a lower field emission threshold.}, number={2-5}, journal={DIAMOND AND RELATED MATERIALS}, author={Baumann, PK and Nemanich, RJ}, year={1998}, month={Feb}, pages={612–619} }
 @article{nemanich_baumann_benjamin_nam_sowers_ward_ade_davis_1998, title={Electron emission properties of crystalline diamond and III-nitride surfaces}, volume={130}, ISSN={["0169-4332"]}, DOI={10.1016/s0169-4332(98)00140-8}, abstractNote={Wide bandgap semiconductors have the possibility of exhibiting a negative electron affinity (NEA) meaning that electrons in the conduction band are not bound by the surface. The surface conditions are shown to be of critical importance in obtaining a negative electron affinity. UV-photoelectron spectroscopy can be used to distinguish and explore the effect. Surface terminations of molecular adsorbates and metals are shown to induce an NEA on diamond. Furthermore, a NEA has been established for epitaxial AlN and AlGaN on 6H–SiC. Field emission measurements from flat surfaces of p-type diamond and AlN are similar, but it is shown that the mechanisms may be quite different. The measurements support the recent suggestions that field emission from p-type diamond originates from the valence band while for AlN on SiC, the field emission results indicate emission from the AlN conduction band. We also report PEEM (photo-electron emission microscopy) and FEEM (field electron emission microscopy) images of an array of nitride emitters.}, number={1998 June}, journal={APPLIED SURFACE SCIENCE}, author={Nemanich, RJ and Baumann, PK and Benjamin, MC and Nam, OH and Sowers, AT and Ward, BL and Ade, H and Davis, RF}, year={1998}, month={Jun}, pages={694–703} }
 @article{baumann_nemanich_1998, title={Surface cleaning, electronic states and electron affinity of diamond (100), (111) and (110) surfaces}, volume={409}, ISSN={["0039-6028"]}, DOI={10.1016/S0039-6028(98)00259-3}, abstractNote={The effects of cleaning natural type IIb diamond (100), (111) and (110) samples by annealing and hydrogen – or deuterium plasma exposure were investigated by means of ultraviolet photoemission spectroscopy (UPS). Different wet chemical cleaning processes (a conventional chromic acid clean and an electrochemical etch) and a H plasma exposure have been employed to clean natural type IIb semiconducting diamond C(100) wafers. The effects of these processes on the diamond surface have been assessed and compared. As evidenced by Auger electron spectroscopy (AES), an oxygen free surface could be obtained following vacuum annealing to 900°C for the electrochemical process compared to 1050°C for the chromic acid etch. In addition, the technique of atomic force microscopy demonstrated the presence of oriented pits on the surface of samples that were electrochemically etched for long times at high currents. After a H plasma exposure the negative electron affinity (NEA) peak in the UPS spectra doubled in intensity. An anneal to 1100°C resulted in the removal of the sharp NEA feature. A second H plasma treatment resulted in the reappearance of the NEA peak similar to that after the first H plasma exposure. A (2×1) reconstructed low energy electron diffraction pattern was observed subsequent to the anneals as well as the H plasma treatments. The fact that a NEA can be induced or removed repeatedly by means of a H plasma exposure or annealing at 1100°C, respectively, provides evidence to correlate the appearance of a NEA with the presence of a monohydride terminated surface. Corresponding effects were found for (111) and (110) surfaces. A NEA could be induced by a H plasma and removed by annealing at 900 or 800°C for diamond (111) or (110) surfaces, respectively. Following a deuterium plasma exposure the diamond surfaces exhibited a NEA like the ones treated by a hydrogen plasma. Higher annealing temperatures were necessary to remove the NEA for deuterium due to the isotope effect. Values of 79 and 81 V μm−1 were measured for the field emission threshold of the oxygen terminated C(100) and C(110) surfaces, respectively. A value of 25 V μm−1 was determined for the hydrogen terminated C(110) surface.}, number={2}, journal={SURFACE SCIENCE}, author={Baumann, PK and Nemanich, RJ}, year={1998}, month={Jul}, pages={320–335} }
 @article{baumann_bozeman_ward_nemanich_1997, title={Characterization of metal-diamond interfaces: Electron affinity and Schottky barrier height}, volume={6}, ISSN={["0925-9635"]}, DOI={10.1016/S0925-9635(96)00601-2}, abstractNote={In this study, the electron affinity and Schottky barrier height of thin Cu and Zr films on diamond (100) substrates were correlated by means of UV photoemission spectroscopy (UPS) measurements. Prior to metal deposition the diamond crystals were cleaned by a 1150°C or 500°C anneal in UHV, and the surfaces were characterized by AES and AFM. This resulted in surfaces terminated with oxygen or free of chemisorbed species. By means of UPS it was found that whether a metal did induce a negative electron affinity (NEA) on a diamond surface was dependent on the surface preparation before metal deposition and on the metal work function. In particular, the Schottky barrier height for clean surfaces was lower than for surfaces terminated by oxygen. Metal-diamond interfaces exhibiting a NEA had a lower Schottky barrier height than those exhibiting a positive electron affinity. These effects were attributed to different interfacial layers. Field emission measurements were performed before and after metal deposition. For all cases a reduction in the threshold electric field was observed upon metal overgrowth.}, number={2-4}, journal={DIAMOND AND RELATED MATERIALS}, author={Baumann, PK and Bozeman, SP and Ward, BL and Nemanich, RJ}, year={1997}, month={Mar}, pages={398–402} }
 @article{baumann_nemanich_1997, title={Comparison of electron affinity and Schottky barrier height of zirconium and copper-diamond interfaces}, volume={15}, DOI={10.1116/1.589444}, abstractNote={In this study, the evolution from diamond surfaces to metal–diamond interfaces has been examined. The electron affinity and the Schottky barrier height of a few Å thick films of Zr and Cu deposited in ultrahigh vacuum (UHV) onto IIb substrates were correlated. Prior to metal deposition, the diamond surfaces have been cleaned by different anneals and plasma treatments in UHV, and the surfaces were characterized by Auger electron spectroscopy and atomic force microscopy. The initial surfaces were terminated with oxygen, or free of chemisorbed species. Ultraviolet photoemission spectroscopy was employed to determine whether the samples exhibited a positive electron affinity or a negative electron affinity (NEA) before and after metal deposition. For Zr, the Schottky barrier height was found to change very little with the presence or absence of chemisorbed species at the interface. A NEA was observed for Zr on diamond independent of the surface termination. However, for Cu, the surface cleaning prior to metal deposition had a more significant effect. The Schottky barrier height changed strongly depending on the chemical species at the interface. A NEA was only detected for Cu on clean diamond surfaces. The differences between Zr on the one hand and Cu on the other are correlated with differences in interface chemistry and structure.}, number={4}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B}, author={Baumann, PK and Nemanich, RJ}, year={1997}, pages={1236–1240} }