ADSORPTION OF NA ON THE GAAS(110) SURFACE STUDIED BY THE FIELD-ION-SCANNING-TUNNELING-MICROSCOPY

The geometry and the coverage-dependent growth structures formed by Na deposition on the GaAs(110) surface at room temperature were studied using a field-ion scanning tunneling microscope. When the coverage is low, Na adatoms reside on the bridge site encompassing one Ga and two As surface atoms to form linear chains along the [110BAR] direction. The Na-Na nearest-neighbor distance in this low-density chain structure is 8 angstrom. At slightly increased coverage, the Na chains began to disorder. Some of them were packed closer to form domains showing a local 2 x 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 of three-dimensional disordered clusters. The saturation coverage of Na was determined to be approximately 0.1 ML (1 ML = 2 Na per substrate unit cell).