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We present the results of a systematic study designed to investigate the physical mechanisms involved in Fermi level pinning on III–V semiconductor surfaces. This study compares the results of surface sensitive photoemission spectroscopy performed during the initial stages of Schottky barrier formation (sub to several monolayer coverages of metals, as well as nonmetals) to the results of device electrical measurements for thick metal coverages (∼1000 Å). These results give strong support for the basic premises of the unified defect model (UDM), as first proposed in 1979. However, several revisions were found to be important in correcting and/or sharpening several details of the UDM. As proposed in 1979, we find that the same native defect energy levels are created independent of the foreign atoms deposited on the clean GaAs surface. However, the small but finite range in pinning positions in GaAs and InP and the correlation that high (low) barrier heights are formed for high (low) electronegativity metals on n-type substrates indicates that electronic interactions between the foreign atoms and the GaAs can determine the charge state of the defects which are found to pin the Fermi level. The donor at 0.1 eV from the conduction band minimum (CBM) originally associated with a native defect of InP in 1979, was shown to be due to a level in the oxide on the surface of the InP. The reassignment of the energy levels of the native defects in InP to an acceptor level at CBM-0.3 eV and a donor level at CBM-0.5 eV was found to be more consistent with the available experimental data.
Newman et al. (Tue,) studied this question.