Scanning tunneling spectroscopy of oxygen adsorbates on the GaAs(110) surface

Abstract

Spatially resolved measurements of the tunneling current versus applied voltage are obtained for oxygen adsorbed on the GaAs(110) surface, using a scanning tunneling microscopy (STM). Effects with different length scales are observed, arising from two sources of charge in the system: negatively charged adsorbates and a positively charged space charge layer. The space charge layer produces band bending on n-type material, which is observed as a shift in the onsets for tunneling out of and into the valence and conduction bands, respectively. We analyze these shifts using theoretical calculations of the tunneling current through surface space charge layers. Directly above an oxygen adsorbate on n-type material, an enhancement in tunneling is observed near the top of the valence band edge, while a corresponding decrease is observed near the bottom of the conduction band. This local behavior is identified as arising from changes is the surface density-of-states produced by the Coulomb potential of the charged adsorbate. On p-type material, no band bending is observed, and the STM images are indicative of neutral adsorbates. This difference between the n and p-type results is attributed to the existence of acceptor states in the band gap.