Electron depletion at InAs free surfaces: Doping-induced acceptorlike gap states

Abstract

For almost all $n$-type zinc-blende III-V semiconductor free surfaces a depletion layer is observed. InAs is an exception, since $n$-type InAs free surfaces exhibit electron accumulation, due to donorlike surface states which pin the Fermi level far above the conduction band minimum. High-resolution electron-energy-loss spectroscopy (HREELS) has been used to investigate the free surfaces of slightly degenerate $(n\ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}{10}^{17}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3})$ and highly degenerate $(n\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{18}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3})$ InAs. From HREEL studies of these InAs samples, an electron accumulation and depletion layer were observed, respectively. The Fermi-level pinning mechanism at free surfaces is discussed in terms of the position of the bulk Fermi level with respect to the branch-point energy $({E}_{B})$. Depending on the location of the bulk Fermi level, amphoteric defects are incorporated into the surface, which if ionized facilitate the band bending that results in the surface Fermi level becoming pinned close to ${E}_{B}$. Occupied cation-on-anion antisite defects and unoccupied anion-on-cation antisite defects at the surface are considered to be the ionized surface states responsible for the depletion and accumulation layer profiles, respectively.