A review was presented of scanning tunnelling microscopic studies of Fermi-level pinning on the surface of n-type and p-type (001)GaAs. The samples were grown by using molecular beam epitaxy, and exhibited a (2 x 4)/c(2 x 8) surface reconstruction. Scanning tunnelling microscopy showed that, on the surface of highly-doped n-type GaAs(001), there was a high density of kinks in the dimer-vacancy rows of the (2 x 4) reconstruction. These kinks were found to be surface acceptors, with approximately 1 electron per kink. The kinks formed in exactly the numbers which were required to pin the Fermi level of n-type GaAs(001) at an acceptor level which was close to the mid-gap; regardless of the doping level. The Fermi-level position was confirmed by tunnelling spectroscopy. No similar surface donor states were found on p-type GaAs(001). In this case, the Fermi-level pinning resulted from so-called intrinsic surface defects, such as step edges. Since this intrinsic defect density was independent of doping, the Fermi level of p-type GaAs(001) moved down the band-gap towards the valence band at high doping levels. Tunnelling spectroscopy of p-type GaAs(001) which was doped with 1019/cm3 of Be showed that the Fermi level was 0.15eV above the valence band maximum.

M.D.Pashley: Philosophical Transactions of the Royal Society A, 1993, 344[1673], 533-43