We report several new results in hydrogen sulfide (H2S) treatment of a GaAs (001) substrate. Surface reconstruction and morphology were investigated by in situ reflection high energy electron diffraction (RHEED) and ex situ atomic force microscopy (AFM) in terms of the annealing temperature and the H2S irradiation sequence. A (4 × 3) GaAs surface was obtained by annealing the substrate under H2S irradiation (4 × 10-7 Torr). The surface was atomically flat, i.e., large terraces with monolayer steps were clearly observed. A (2 × 6) S-terminated GaAs surface was obtained by irradiation H2S at 300°C on a Ga-terminated surface, which was formed by annealing at 580°C in high vacuum. The molecular beam epitaxy (MBE) growth of ZnSSe-based semiconductors on the (4 × 3) surface results in high quality structures such as a novel ZnSSe/ZnMgSSe tensile-strained quantum well (QW).