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 donor states. In fact, the measured tunneling current results from a pure tunneling current between the energy levels of the STM tip and the As-Ga energy level E-t followed by an exchange of carriers between E-t and the bands or between E-t and donor states of neighbor point defects. We determine the influence of both contributions on the current. We show that hopping conduction is required to explain the observation of the midgap states in tunneling spectroscopy. (C) 2000 American Institute of Physics. [S0003-6951(00)02620-6].