Surface structure of GaAs(001)-(2 × 4) α, β and γ phases

The As-rich GaAs(001)-(2 × 4) α, β and γ phases grown by molecular-beam epitaxy (MBE) and migration enhanced epitaxy (MEE) have been investigated by field ion-scanning tunneling microscopy and in-situ reflection high-energy electron diffraction (RHEED). The high-resolution STM images show that the 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. The structure models are examined based on the STM observations and dynamical RHEED calculation. We propose a new structure model: The α phase is the two-As-dimer model proposed by Farrell and Palmstrom with relaxation incorporated by Northrup and Froyen. The β phase is the two-As-dimer model proposed by Chadi. The γ phase consists of the local structure same as the β phase and the open areas with a disordered As double-layer structure similar to that of the c(4 × 4) phase.