Geometric and electronic structures of Na/GaAs

This article reports structural and electronic properties of sodium adsorbed on room-temperature GaAs( 110) surface as observed with scanning tunneling microscopy in an ultrahigh vacuum. In the low-coverage regime, Na adatoms reside on the bridge site encompassing one Ga and two As surface atoms to form linear chains along the [110] direction. The Na-Na nearest-neighbor distance in this low-density chain structure is 8 A. When the Na coverage was increased to ∼0.09 ML (1 ML=2 Na per substrate unit cell), the chains became slightly disordered. Some of them were packed closer to form domains with local 2 × 2 structure. None of high-density two-dimensional ordered structures or low-density zigzag chains was observed, in contrast to the Cs/GaAs( 110) system. Additional Na adsorption resulted in the formation ofthree-dimensional disordered clusters. The saturation coverage ofNa adsorption on the GaAs (110) surface was determined to be ∼0.1 ML. No evidence of metallic characteristics was found in the I-V curves recorded over the various Na-covered surfaces.