The origin of Ga2O3 passivation for reconstructed GaAs(001) surfaces

Ab initio molecular-orbital calculations are used to study the origin of the Ga2O3 passivation mechanism for GaAs(100) reconstructed surfaces. Two cluster models an used to simulate the main features of reconstructed and oxygen chemisorbed GaAs(100) surfaces. The simulation results show that the reduction in the density of surface states located within the bulk energy gap derives from the initial near-bridge-bonded O atoms. The calculated electronic energy spectra reveal that the surface-state energy gap lies completely outside of the bulk energy gap in distinct contrast to the case for S passivation. At the optimized geometry, each surface Ga atom (situated beneath the adsorbed O) is distorted by 0.40 Angstrom from its ideal position, resulting in a strained surface. O atoms are almost buried in the GaAs(100) surface; each is located 0.30 and 0.25 Angstrom above the reconstructed GaAs(100) surface, respectively. The O-Ga bond length is 1.63 Angstrom and the Ga-O-Ga bond angle is 157.4 degrees. Each O atom deviates from the bridge position by 0.11 and 0.19 Angstrom from the vertical position, respectively. This causes further deposition to result in the formation of an amorphous oxide film, which provides an effective protection layer against further oxidation of the near-bridge-site oxidized GaAs surface. The calculated electronic structure and local density of states also reflect a large charge accumulation near the adsorbed O atoms. [S0021-8979(98)03211-3].