We have made direct experimental observation of a new mechanism for Fermi-level pinning in Si-doped (001) GaAs. Scanning-tunneklng-microscope images show that high levels of Si doping cause the GaAs(001)-(2×4) unit cells to reorder by the formation of kinks in the dimer vacancy rows. We are able to show that these kinks are surface acceptors, which form in the required numbers fo compensate exactly the GaAs surface region. We find that in the depleted surface layer all the incorporated Si atoms are donors up to Si concentrations of at least 1×${10}^{19}$ ${\mathrm{cm}}^{\mathrm{-}3}$.

• Received 26 July 1991

DOI:https://doi.org/10.1103/PhysRevLett.67.2697