ARSENIC-DEFICIENT GAAS(001)-(2X4) SURFACES - SCANNING-TUNNELING-MICROSCOPY EVIDENCE FOR LOCALLY DISORDERED (1X2) GA REGIONS

Scanning tunneling microscopy (STM) has been used to study GaAs(001)-( 2X4) surfaces grown by molecular-beam epitaxy. Although reflection hig h-energy electron diffraction always showed a characteristic (2x4) pat tern, STM images indicated significant differences in the composition of the surfaces depending on the nature of the quenching conditions. A previously unreported, locally disordered (1X2) structure was observe d under As-deficient conditions. This was found extending from step ed ges and in missing-dimer holes, where the top layer of As had been rem oved to expose the second layer. The atoms in this exposed layer are i dentified as Ga and they form short rows in the [110] direction. The t wofold periodicity is due to a vacancy structure, with Ga atoms locate d at alternate sites along the [110] direction. These results may rati onalize some of the recent controversies regarding the composition and structure of the As-terminated (2X4) surface prepared by decapping me thods.