Mark C. Benjamin

Observation of a negative electron affinity for heteroepitaxial AlN on α(6H)‐SiC(0001)

This study demonstrates the presence of a negative electron affinity (NEA) surface on AlN was grown on α(6H)‐SiC. Heteroepitaxial AlN was grown on α(6H)‐SiC(0001) substrates by molecular beam epitaxy techniques. The surface electronic states were characterized by ultraviolet photoemission obtained at surface normal. The observation of a sharp spectral feature at the lowest energy of the emitted electrons is an indication of a surface with a negative electron affinity. In addition, the trend of the NEA feature was examined as a function of annealing. The surface Fermi level is found to be near the middle of the AlN gap, and a possible band alignment between the AlN and SiC is presented.

Observation of a negative electron affinity for boron nitride

This study reports UV‐photoemission (UPS) measurements made on boron nitride crystals and thin films. The materials examined are commercial grade c‐BN powder and thin films of BN deposited with ion beam assisted e‐beam evaporation and laser ablation. The thin film samples examined exhibited varying amounts of sp3 (cubic) and sp2 (hexagonal, amorphous) bonding as determined by FTIR measurements. The UPS measurements displayed the spectral distribution of the low energy photoemitted electrons and the total energy width of the spectra. These characteristics can be related to the electron affinity. The measurements on several of the BN powder and thin film samples revealed features in the emission spectra which are indicative of a negative electron affinity (NEA) surface.

Negative electron affinity surfaces of aluminum nitride and diamond

Abstract The electron affinity of diamond and AlGaN surfaces are studied by UV photoemission spectroscopy. It is shown that H terminated diamond surfaces exhibit a negative electron affinity while oxide terminated surfaces exhibit a positive electron affinity. In addition, thin metal layers can also induce a NEA on both (100) and (111) surfaces of diamond. Photoemission results of AlGaN alloy films grown on 6HSiC indicate a negative electron affinity for as-prepared and air exposed surfaces with high Al concentrations.

UV photoemission study of heteroepitaxial AlGaN films grown on 6H-SiC

Abstract This study presents results of UV photoemission measurements of the surface and interface properties of heteroepitaxial AlGaN on 6H-SiC. Previous results have demonstrated a negative electron affinity of AlN on 6H-SiC. In this study Al x Ga 1− x N alloy films were grown by organometallic vapor phase epitaxy (OMVPE) and doped with silicon. The analytical techniques included UPS, Auger electron spectroscopy, and LEED. All analysis took place in an integrated UHV transfer system which included the analysis techniques, a surface processing chamber and a gas source MBE. The OMVPE alloy samples were transported in air to the surface characterization system while the AlN and GaN investigations were prepared in situ. The surface electronic states were characterized by surface normal UV photoemission to determine whether the electron affinity was positive or negative. Two aspects of the photoemission distinguish a surface that exhibits a NEA: (1) the spectrum exhibits a sharp peak in the low kinetic energy region, and (2) the width of the spectrum is hv - E g . The in situ prepared AlN samples exhibited the characteristics of a NEA while the GaN and Al 0.13 Ga 0.87 N samples did not. The Al 0.55 Ga 0.45 N sample shows a low positive electron affinity. Annealing of the sample to > 400°C resulted in the disappearance of the sharp emission features, and this effect was related to contaminant effects on the surface. The results suggest the potential of nitride based cold cathode electron emitters.

Dependence of (0001) GaN/AlN valence band discontinuity on growth temperature and surface reconstruction

X ray and ultraviolet photoelectron spectroscopies have been used to determine the heterojunction valence band discontinuity at the (0001) GaN/AlN interface. Type I discontinuity values of 0.5±0.2 eV were determined for GaN grown on AlN at 650 °C and 0.8±0.2 eV for GaN grown on AlN at 800 °C. These values are critically evaluated with respect to film quality, the results of other experimental studies, and theory.

Electron emission properties of crystalline diamond and III-nitride surfaces

Abstract 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.

Valence band discontinuity, surface reconstruction, and chemistry of (0001), (0001̄), and (11̄00) 2H–AlN/6H–SiC interfaces

A detailed examination of the valence band discontinuity (ΔEv) formed at the (0001), (0001), and (1100) interfaces between 2H–AlN and 6H–SiC has been conducted using x-ray and UV photoelectron spectroscopies. The ΔEv was observed to range from 0.6–2.0 eV depending on the growth direction (i.e., AlN on SiC vs SiC on AlN), as well as the crystallographic orientation, cut of the SiC substrate (i.e., on versus off axis), and SiC surface reconstruction and stoichiometry. A ΔEv of 1.4–1.5 eV was observed for AlN grown on (3×3) (0001)Si6H–SiC on-axis substrates; a ΔEv of 0.9–1.0 eV was observed for off-axis substrates with the same surface reconstruction. The values of ΔEv for AlN grown on (√3×√3)R30°(0001) 6H–SiC on-and-off-axis substrates were 1.1–1.2 eV. A larger valence band discontinuity of 1.9–2.0 eV was determined for 3C–SiC grown on (0001) 2H–AlN. Smaller values of ΔEv of 0.6–0.7 and 0.8–0.9 eV were observed for AlN grown on on-axis (0001)C and (1100)6H–SiC substrates, respectively.

Accurate Sheet Resistance Measurement on Ultra-Shallow Profiles

Comparison of state-of-the-art zero-penetration sheet resistance tools on ultra-shallow Boron CVD layers on top of a medium doped As layer.

Negative) electron affinity of AlN and AlGaN alloys

The electron affinity of a semiconductor defines the relationship of the vacuum level and the semiconductor band structure. It is dependent on the atomic orbitals of the material and the surface termination. We report experimental and theoretical results that support the presence of a negative electron affinity on AlN and the Al rich AlGaN alloys. The GaN surface is found to exhibit a (positive) electron affinity of 3.3eV. The experimental measurements employ UV-photoemission spectroscopy on in situ gas-source MBE samples and on CVD samples. Theoretical results indicate that the (negative) electron affinity of AlN depends sensitively on the surface reconstruction and adatom termination. The experimental dependence of the electron affinity on alloy concentration is presented. The results indicate that AlGaN alloys with band gap similar or greater than that of diamond will exhibit a negative electron affinity. Field emission results are reported, and the characteristics are similar to those obtained from a diamond film. Issues related to cold cathode electronic devices based on NEA surfaces are noted.

Ex Situ and in Situ Methods for Oxide and Carbon Removal from AlN and GaN Surfaces

Exposure to numerous acids and bases and UV/O 3 oxidation were used to determine the best ex situ cleaning techniques for the (0001) surfaces of AIN and GaN. HF and HCI were the most effective in removing the oxide from AIN and GaN, respectively. However, AES and XPS revealed the surfaces to be terminated with F and CI which inhibited re-oxidation prior to loading into vacuum. TPD showed mat temperatures of 650 and 850°C are necessary to thermally desorb the CI and F, respectively. UV/O 3 oxidation in air was not effective in removing hydrocarbons from either surface but was effective for oxide growth. In situ remote hydrogen plasma exposure at 450°C removed halogens and hydrocarbons remaining after ex situ cleaning of both AIN and GaN surfaces; however, oxide free surfaces could not be achieved. Thermal desorption of hydrocarbons from GaN in UHV was achieved at 650°C. Complete thermal desorption of the surface oxide in UHV was only achieved at temperatures > 800°C where some GaN decomposition occurred. Annealing GaN in NH 3 at 700°C reduced the surface oxide without loss of surface stoichiometry.