Development and confirmation of the unified model for Schottky barrier formation and MOS interface states on III-V compounds

Abstract

The object of the work reported here was to develop an understanding on an atomic basis of the interactions between semiconductors and metal or oxygen overlayers which determine the electronic characteristics of the interface, e.g. the Schottky barrier heights and the density and the energy position of states at oxide-semiconductor interfaces. The principal experimental tool used by ourselves was photoemission excited by monochromatized synchrotron radiation (10 eV< hv<300 eV). Extreme surface sensitivity is obtained by tuning the synchrotron radiation so that the minimum escape depth is obtained for the excited electrons of interest. In this way only the last two or three atomic layers of the solid are sampled. By changing hv, core levels or valence bands can be studied. The Fermi level position E fs at the surface can be directly determined using a metallic reference. GaAs, InP and GaSb were studied. On a properly cleaved surface there are no surface states in the semiconductor band gap—thus, no pinning of E fs. Pinning of E fs can then be monitored as metal or oxygen is added to the surface, starting from submonolayer quantities. Two striking results are obtained: (1) the pinning position is independent of the adatom, whether it is oxygen or one of a wide range of metals, and (2) the pinning is completed by much less than a monolayer of adatoms. These results cannot rationally be explained by the hypothesis that the pinning is due to the levels produced directly by the adatoms. Rather, they suggest strongly that the adatoms disturb the semiconductor surface indirectly, forming defect levels. This is supported by the appearance of the semiconductor atoms in the metal and by the disordering of the semiconductor surface by submonolayer quantities of oxygen. Since these basic experiments have been reported previously they are only briefly reviewed here. When metal or oxygen is added under very gentle conditions, the following levels are formed (all energies are relative to the conduction band minimum). Semiconductor Acceptor Donor GaAs 0.65 eV 0.85 eV InP 0.45 eV 0.1 eV GaSb 0.5 eV Below VBM where VBM denotes the valence band maximum.