Theory of scanning tunneling microscopy of defects on semiconductor surfaces

We present a detailed theory of scanning tunneling microscopy (STM) of poin t defects located near the surface of semiconductors. We derive several con ditions required to get a permanent current through the gap states of a def ect and we deduce general rules concerning the current spectroscopy of defe cts with one or two ionization levels in the gap. We obtain that these ioni zation levels may be completely invisible in STM spectroscopy. In the parti cular case of low-temperature-grown GaAs containing a high density of arsen ic-related defects, we show that the observation of gap states in STM spect roscopy is related to the conductivity of the material due to thermally act ivated hopping between defects. We propose the use of STM to measure the ra tes of capture and emission of carriers by surface defects and we describe two methods to perform it.