Scanning tunnelling microscopy was used to study low-temperature grown layers. It was noted that excess As gave rise to a high concentration of As antisites, AsGa. Tunnelling spectroscopy revealed a band of donor states at these point defects. The measured tunnelling current resulted from a pure tunnelling current between the energy levels of the scanning tunnelling microscope tip and the AsGa energy level, followed by an exchange of carriers between the latter energy level and the bands, or between that energy level and donor states of neighbouring point defects. It was shown that hopping conduction was required in order to explain observations of the mid-gap states in tunnelling spectroscopy.

Defect-Assisted Tunnelling Current - a Revised Interpretation of Scanning Tunnelling Spectroscopy Measurements B.Grandidier, X.De la Broïse, D.Stiévenard, C.Delerue, M.Lannoo, M.Stellmacher, J.Bourgoin: Applied Physics Letters, 2000, 76[21], 3142-4