Synchrotron radiation-based x-ray photoemission spectroscopy was used to study the surface Fermi level position within the band gap for thin metal overlayers of Au, Al, Ni, Ti, Pt, and Pd on n-GaN and p-GaN. Nonequilibrium effects were taken into account by measuring the Fermi edge of the metal overlayer. There are two different behaviors observed for the six metals studied. For Au, Ti, and Pt, the surface Fermi level lies about 0.5-eV higher in the gap for n-type than for p-type GaN. For Ni, Al, and Pd, the surface Fermi level position is independent of doping, but varies from one metal to the other. Results for Ni, Pd, and Al fit a modified Schottky-Mott theory, while Au, Ti, and Pt demonstrate a more complex behavior. Atomic force microscopy was used along with photoemission to investigate the growth mode of each metal on the GaN surface.
The nitridation of c-plane sapphire within the hydride vapor phase epitaxy system was systematically studied as a function of time and ammonia partial pressure using ex situ x-ray photoelectron spectroscopy, reflection high-energy electron diffraction, and atomic force microscopy. During the nitridation process, nitrogen was incorporated into the sapphire surface. There were two different nitrogen chemical bonding states, which can be attributed to N-Al bonds and nitrogen in oxygen-rich environment ͑'N-O'͒. As the nitridation continued, the N 1s intensity increased while the O 1s intensity decreased indicating the growth of a nitrogen-rich layer. The sapphire nitridation process can be modeled as a diffusion couple of AlN and Al 2 O 3 , where N 3Ϫ and O 2Ϫ interdiffuse in the rigid Al 3ϩ framework. Nitrogen diffuses into sapphire and substitutes for oxygen to bond with aluminum. The bond substitution is accompanied by structural changes where the AlN in-plane direction is rotated 30°with respect to the sapphire direction. The replaced oxygen diffuses out to the surface, combines with hydrogen and desorbs as H 2 O. The overall nitridation rate is determined by the slower of the two moving anions. From the x-ray photoelectron spectroscopy data, the chemical diffusion coefficient of nitrogen (D N ) and oxygen (D O ), were estimated. D N was found to be higher than D O , which suggested that the overall nitridation rate was controlled by the diffusion of oxygen to the surface. After nitridation, no protrusions were observed on the surface and no significant changes in the surface roughness were measured when compared to the as-received sapphire.
The effects of surface chemical treatments and metal deposition on the InN surface are studied via synchrotron-based photoemission spectroscopy. Changes in the In 4d core level as well as the valence band spectra are reported. The surface Fermi level position, E F , relative to the valence band maximum was determined for both Au and Ti Schottky barriers. E F lies at an energy of 0.7 eV above the valence band maximum for Au deposited on annealed InN and 1.2 eV above the valence band maximum for Ti deposited on Ar-sputtered InN. These results that the surface Fermi level lays at or above the conduction band maximum when a value of InN band gap of 0.7-0.9 eV is assumed.
The etching of sapphire substrates using H 2 SO 4 , H 3 PO 4 , and a 3:1 H 2 SO 4 :H 3 PO 4 mixture, as a function of temperature and etching time, was systematically studied using atomic force microscopy. The sapphire preparation by liquid-based etchings was compared with H 2 etching at 1100°C and air-annealing at 1400°C. In liquid-based treatments, the smoothest, pit-free surface was obtained by etching in pure H 2 SO 4 at 300°C for 30 min. Sulfuric acid etching at higher temperatures or for longer periods generated an insoluble mixture of Al 2 (SO 4 ) 3 and Al 2 (SO 4 ) 3 •17H 2 O crystalline deposits on the surface. Phosphoric acid and the 3:1 H 2 SO 4 :H 3 PO 4 mixture, which is the routinely employed chemical treatment for sapphire preparation, etched the sapphire preferentially at defect sites and resulted in pit formation on the surface. Sapphire treatment using H 2 at 1100°C did not remove the surface damage. Air annealing the sapphire at 1400°C for 1 h produced an atomically smooth surface consisting of a terrace-and-step structure. The results of this study were described in terms of the chemistry of the sapphire etching process.
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