Theory of scanning tunneling microscopy of defects on semiconductor surfaces

We present a detailed theory of scanning tunneling microscopy (STM) of point defects located near the surface of semiconductors. We derive several conditions required to get a permanent current through the gap states of a defect and we deduce general rules concerning the current spectroscopy of defects with one or two ionization levels in the gap. We obtain that these ionization levels may be completely invisible in STM spectroscopy. In the particular case of low-temperature-grown GaAs containing a high density of arsenic-related defects, we show that the observation of gap states in STM spectroscopy is related to the conductivity of the material due to thermally activated hopping between defects. We propose the use of STM to measure the rates of capture and emission of carriers by surface defects and we describe two methods to perform it.