Atomic structures on a GaAs(001) surface grown by molecular beam epitaxy

A unique apparatus for in-situ atomic-resolution study of solid state structures grown by molecular beam epitaxy (MBE) is developed, in which a scanning tunneling microscope (STM) is combined with an MBE chamber within the same vacuum system. The utility of the apparatus is demonstrated by examining atomic structures on a molecular-beam-epitaxial GaAs(001) surface over a wide range of [As]/[Ga] ratios. By varying the As surface coverage, the 2 × 4-α, β, γ and c(4 × 4) phases are examined in detail. High-resolution STM images indicate that 2 × 4-α, β and γ phases in the outermost surface layer have essentially the same unit cell consisting of two As dimers and two As dimer vacancies. Using the STM images, reflection high-energy electron diffraction (RHEED) patterns and dynamical RHEED calculations, the existing structural models for the 2 × 4 phases are analysed and a new model of the As-rich GaAs(001) surface is proposed, found to be consistent with most of the previous observations.