Arsenic-deficient GaAs(001)-(2×4) surfaces: Scanning-tunneling-microscopy evidence for locally disordered (1×2) Ga regions

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Abstract
Scanning tunneling microscopy (STM) has been used to study GaAs(001)-(2×4) surfaces grown by molecular-beam epitaxy. Although reflection high-energy electron diffraction always showed a characteristic (2×4) pattern, STM images indicated significant differences in the composition of the surfaces depending on the nature of the quenching conditions. A previously unreported, locally disordered (1×2) structure was observed under As-deficient conditions. This was found extending from step edges and in missing-dimer holes, where the top layer of As had been removed to expose the second layer. The atoms in this exposed layer are identified as Ga and they form short rows in the [1¯10] direction. The twofold periodicity is due to a vacancy structure, with Ga atoms located at alternate sites along the [110] direction. These results may rationalize some of the recent controversies regarding the composition and structure of the As-terminated (2×4) surface prepared by decapping methods.