Voltage-dependent imaging and spatially-resolved spectroscopic studies were made, of the (110) surface of Te-doped single crystals, by using low-temperature scanning tunnelling microscopy. A large fraction of the observed defects were identified as being Te dopant atoms; which could be observed down to the fifth sub-surface layer. At negative sample voltages, the dopant atoms were surrounded by Friedel charge-density oscillations. Spatially resolved spectroscopy of dopant atoms and defect-free areas of the (110) surface revealed the presence of conductance peaks within the semiconductor band-gap. The appearance of the peaks could be linked to charges which resided on states that were localized within the tunnel junction area. It was shown that these localized states could be present on the doped surface as well as at the scanning tunnelling microscope tip.

Scanning Tunnelling Microscopy and Spectroscopy at Low Temperatures of the (110) Surface of Te-doped GaAs Single Crystals A.Depuydt, C.Van Haesendonck, N.S.Maslova, V.I.Panov, S.V.Savinov, P.I.Arseev: Physical Review B, 1999, 60[4], 2619-26