DIFFERENT FERMI-LEVEL PINNING BEHAVIOR ON N-TYPE AND P-TYPE GAAS(001)

Scanning-tunneling-microscopy studies of both the n- and p-type GaAs(0 01)-(2 X 4)/c(2 X 8) surfaces show important differences in the Fermi- level pinning behavior of n- and p-type material. It has been shown pr eviously that Fermi-level pinning on the n-type GaAs(001) surface resu lts from the formation of kinks in the dimer-vacancy rows of the (2 X 4)/c(2 X 8) surface reconstruction. These kinks form in the required n umber to pin the Fermi level close to midgap at all doping levels. On p-type GaAs(001) we now show that no similar surface donor state forms . As a result, at high p-doping levels (10(19) cm-3 Be), the Fermi lev el determined by tunneling spectroscopy is found to be within 150 meV of the valence-band maximum. At lower p-doping levels the Fermi level moves towards midgap as determined by the density of ''intrinsic'' sur face defects such as step edges, and missing unit cells.