Virtual gap states and Fermi level pinning by adsorbates at semiconductor surfaces

The pinning positions of the Fermi level observed after the deposition of metal or semiconductor atoms and the adsorption of oxygen or hydrogen on clean (110) surfaces of III–V compound semiconductors are compared with the charge-neutrality levels of the virtual gap states of these compounds. Close agreement is found for GaAs, GaSb, and InAs independent of the adsorbate while both GaP and InP give deviations of up to 0.3 eV depending on the adsorbates. The latter differences are explained by additional defects of donor type which may be phosphorous vacancies. It is concluded that the virtual gap states, which were first discussed in this context by Heine, are predominantly determining the position of the Fermi level at adsorbate-covered semiconductor surfaces.