Defect-assisted tunneling current: A revised interpretation of scanning tunneling spectroscopy measurements

Scanning tunneling microscopy (STM) is used to study low temperature grown GaAs layers. Excess As gives rise to a high concentration of As antisites ( As-Ga). On these point defects, tunneling spectroscopy reveals a band of do nor states. In fact, the measured tunneling current results from a pure tun neling current between the energy levels of the STM tip and the As-Ga energ y level E-t followed by an exchange of carriers between E-t and the bands o r between E-t and donor states of neighbor point defects. We determine the influence of both contributions on the current. We show that hopping conduc tion is required to explain the observation of the midgap states in tunneli ng spectroscopy. (C) 2000 American Institute of Physics. [S0003-6951(00)026 20-6].