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Coverage-Dependent Growth Structures of Sodium on the GaAs(110) Surface

The geometry and the coverage-dependent growth structures formed by Na deposition on the GaAs(110) surface were studied by a field-ion scanning tunneling microscope (FI-STM) in an ultrahigh-vacuum. In the low-coverage regime at 300 K, Na adatoms reside on a bridge site encompassing one Ga and two As surface atoms to form linear chains along the substrate [11̄0] direction. The Na-Na nearest-neighbor distance in this low-density chain structure is 0.8 nm. With slightly larger Na coverages, the chains became disordered. Some of them were packed closer to form domains showing a local 2×2 structure. None of high-density 2D ordered structures or low-density zigzag chains were 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 adsorption on the GaAs(110) surface was determined to be about 0.1 ML (1 ML=2 Na per substrate unit cell). The I-V curve recorded over the clean surface was consistent with published data for n-GaAs(110), while no evidence of metallic characteristics was found in the I-V curves recorded over the various Na-covered surfaces.