The effectiveness of (NH4)2S
x
treatment on the (100) surface of GaP, (Al, Ga)As, InP and InAs was studied in comparison to that on GaAs by means of Auger electron spectroscopy (AES) and reflection high-energy electron diffraction (RHEED). It was concluded that the existence of sulfur atoms bonded to semiconductors prevents the adsorption of oxygen. This phenomenon brings about the metal-dependent Schottky barrier fabricated on the (NH4)2S
x
-treated surfaces, implying the reduction in the interface state density. The structure and effect of the (NH4)2S
x
-treated surface of III-V compounds are qualitatively the same.
The chemistry of the (NH4)2Sx-treated n-GaAs (100) surfaces has been studied using synchrotron radiation photoemission spectroscopy. Ga 3d, As 3d, and S 2p photoemission spectra are measured before and after annealing in vacuum with a photon energy of about 210 eV, where S 2p core level spectra can be sensitively detected. It is found that Ga-S, As-S, and S-S bonds are formed on the as-treated GaAs surfaces, and that stable Ga-S bonds become dominant after annealing at 360 °C for 10 min in vacuum. The thickness of the surface sulfide layer is reduced from about 0.5 to 0.3 nm by annealing. The surface Fermi- level position of the as-treated surfaces is determined to be about 0.8 eV below the conduction band minimum, which is about 0.1 eV closer to the valence band maximum than that of the untreated surfaces. A Fermi-level shift of 0.3 eV toward a flat band condition is also observed after annealing. It is found that the Ga-S bonding plays an important role in passivating GaAs surfaces.
MIS capacitors prepared on the (NH4)2S-treated GaAs substrate showed a marked reduction in the density of the dominant pinning levels near 0.6 eV below the conduction band. The annealing effect on the interface characteristics was also investigated. Analyses by means of secondary ion mass spectroscopy (SIMS) and Auger electron spectroscopy (AES) indicate that sulfur atoms at the interface stabilize the oxygen-free GaAs surface both electronically and thermally.
A model is presented to explain why and how the GaAs surface is passivated and stabilized by (NH4)2S
x
treatment. Natural oxide, together with a thin layer of GaAs, is removed and the fresh GaAs surface is covered with a monoatomic layer of sulfur which has no dangling bonds. On this surface, foreign atoms are prohibited from chemical adsorption. Thus, the As and Ga vacancies which are, according to Spicer's “unified defect model”, the cause of the interface trap level are not introduced. The present model of the passivation successfully interprets different experimental data on the surfaces treated with solutions of sodium sulfide and ammonium sulfide with and without excess sulfur.
AlN films have been successfully deposited by plasma CVD for the first time. Al was supplied by trimethyl Al (TMA) with H2 or N2 carrier gas, and N was supplied as NH3 through separate gas lines. The deposition rate depends upon the TMA supply, and is essentially independent of the NH3 flow rate. The composition of the deposited films was almost AlN, although a small amount of oxygen was always detected. A better film was obtained for the H2 carrier gas than for N2 carrier gas. X-ray diffraction profiles of the deposited films exhibited no crystalline AlN diffracting peaks, suggesting that the films are not crystallized, but the infrared and ultraviolet absorption spectra exhibited the presence of the Al–N bond (650/cm) and an optical band gap of 5.55 eV. The refractive index was about 1.9. These results suggest that the plasma-deposited films possess dominant AlN properties even though they are not crystalline.
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