Scanning tunnelling microscopic studies were made of Fermi-level pinning on the surface of both n- and p-type GaAs(001). The samples were all grown by molecular beam epitaxy and have a (2 x 4)/c(2 x 8) surface reconstruction. The scanning tunnelling microscopy had shown 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 one electron per kink. The kinks formed in exactly the required number to pin the Fermi-level of n-type GaAs(001) at an acceptor level close to mid gap, irrespective of doping level. The Fermi-level position was confirmed with tunnelling spectroscopy. No similar surface donor states were found on p-type GaAs(001). In this case Fermi-level pinning results from `intrinsic' surface defects such as step edges. Since this intrinsic defect density was independent of doping, at high doping levels the Fermi-level on p-type GaAs(001) moves down in the band gap towards the valence band. Tunnelling spectroscopy on p-type GaAs(001) doped 1019/cm3 with Be showed the Fermi-level to be 150mV above the valence band maximum.

STM Studies of Fermi-Level Pinning on the GaAs(001) Surface. M.D.Pashley, G.P.Srivastava: Philosophical Transactions of the Royal Society, 1993, 344[1673], 533-43