GEOMETRIC AND ELECTRONIC-STRUCTURES OF NAGAAS

This article reports structural and electronic properties of sodium ad sorbed on room-temperature GaAs(110) surface as observed with scanning tunneling microscopy in an ultrahigh vacuum. In the low-coverage regi me, Na adatoms reside on the bridge site encompassing one Ga and two A s surface atoms to form linear chains along the [110BAR] direction. Th e Na-Na nearest-neighbor distance in this low-density chain structure is 8 angstrom. When the Na coverage was increased to approximately 0.0 9 ML (1 ML = 2 Na per substrate unit cell), the chains became slightly disordered. Some of them were packed closer to form domains with loca l 2 X 2 structure. None of high-density two-dimensional ordered struct ures or low-density zigzag chains was observed, in contrast to the Cs/ GaAs (110) system. Additional Na adsorption resulted in the formation of three-dimensional disordered clusters. The saturation coverage of N a adsorption on the GaAs (110) surface was determined to be approximat ely 0.1 ML. No evidence of metallic characteristics was found in the I -V curves recorded over the various Na-covered surfaces.