Determination of the surface structures of the GaAs(001)-(2×4) As-rich phase

Field-ion scanning tunneling microscopy (STM) and in situ reflection high-energy electron diffraction (RHEED) have been used to study the atomic structures of the As-rich GaAs(001) surfaces grown by molecular-beam epitaxy and migration-enhanced epitaxy. The (2×4) α, β, and γ phases and the c(4×4) phase have been investigated in a systematic manner, controlling the As surface coverage. The high-resolution STM images show that the (2×4) α, β, and γ phases all have the same unit structure in the outermost surface layer, which consists of two As dimers and two As dimer vacancies. Various structure models proposed for the (2×4) phases are examined based on the STM observations and dynamical calculation of the RHEED spot intensities. We now propose the following model: The α phase is the two-As-dimer model proposed by Farrell and Palmstrom with relaxation incorporated by Northrup and Froyen. The surface coverage of the α phase is 0.5 ML of As. The β phase is the two-As-dimer model proposed by Chadi. The surface coverage of the β phase is 0.75 ML of As. The so-called γ phase is characterized by its small domains separated by open areas. The domains consist of the same local structure as the β phase and the open areas between them have a disordered As double-layer structure similar to that of the c(4×4) phase. The γ phase is, thus, no more than the mixture of the β phase and the c(4×4) phase with the surface As coverage varying between 1.75 and 0.75 ML depending on the growth conditions. Our structure model for the As-rich GaAs(001) surface is consistent with most of the observations reported.