Scanning tunneling microscopy and spectroscopy of arsenic antisite defects in GaAs

Abstract

The scanning tunneling microscope (STM) is used to study thin layers of low-temperature-grown (LT) GaAs, containing about 1×1020 cm-3 arsenic-related defects. The studies are performed in cross-section, on the (110) cleavage edge of the LT-layers. Four types of defect images are observed, and are interpreted as arising from a single type of defect located at varying depths below the surface. Spectroscopy of the defect reveals an intense band of midgap states, which is identified with the EL2 level in GaAs. The defect is found to have full tetrahedral symmetry, with its core residing on the Ga-sublattice. Thus, the defect is identified as an isolated arsenic antisite (As on a Ga site), which is believed to be the dominant defect in LT-GaAs. Satellite features in the STM images are observed at distances of 15 angstroms away from the defect core, and are attributed to wave-function tails of the antisite defect. Observed gaps in the spectrum of midgap states are interpreted in terms of on-site and off-site Coulomb interactions (Hubbard U and Coulomb gap respectively).