Distribution of S in (NH ₄) ₂S _X-treated GaAs surfaces

After treating GaAs using HCl- and (NH ₄) ₂S _X-solutions, we investigated the atomic distribution and the chemical bonding states of the treated surfaces by using angle-resolved X-ray photoelectron spectroscopy and low-energy electron diffraction (LEED). The HCl-treated GaAs showed an Asrich surface containing elemental As, but the S-passivated GaAs surface had As-S bonds instead of elemental As. With the (NH ₄) ₂S _X-treated GaAs, a more pronounced oscillation of the photoelectron intensities of Ga and As was observed than was observed with the HCl-treated one due to the angular effect. The spots observed in the LEED pattern for the S-passivated GaAs surface indicated that it had a (2×1)-reconstructed structure with a regular distribution of As-S bonds. After in-situ annealing under ultra-high vacuum conditions, As-S bonds changed to Ga-S bonds completely, and the photoelectron intensities of S showed a pronounced oscillation according to the polar angle. The polar-angle distribution showed that S dispersed and penetrated until the third layer and statistically occupied the As site in the zinc-blende structure. The LEED pattern of S-passivated GaAs after annealing showed a weak regular structure. This could be explained by anion exchange between As and S or by S occupying the vacant As sites formed during the anneal.