New and unified model for Schottky barrier and III–V insulator interface states formation

For n- and p-doped III–V compounds, Fermi-level pinning and accompanying phenomena of the (110) cleavage surface have been studied carefully using photoemission at hν≲300 eV (so that core as well as valence band levels could be studied). Both the clean surfaces and the changes produced, as metals or oxygen are added to those surfaces in submonolayer quantities, have been examined. It is found that, in general, the Fermi level stabilizes after a small fraction of a monolayer of either metal or oxygen atoms have been placed on the surface. Most strikingly, Fermi-level pinning produced on a given semiconductor by metals and oxygen are similar. However, there is a strong difference in these pinning positions depending on the semiconductor: The pinning position is near (1) the conduction band maximum (CBM) for InP, (2) midgap for GaAs, and (3) the valence band maximum (VBM) for GaSb. The similarity in the pinning position on a given semiconductor produced by both metals and oxygen suggests that the states responsible for the pinning resulted from interaction between the adatoms and the semiconductor. Support for formation of defect levels in the semiconductor at or near the surface is found in the appearance of semiconductor atoms in the metal and in disorder in the valence band with a few percent of oxygen. Based on the available information on Fermi energy pinning, a model is developed for each semiconductor with two different electronic levels which are produced by removal of anions or cations from their normal positions in the surface region of the semiconductors. The pinning levels have the following locations, with respect to the VBM: GaAs, 0.75 and 0.5 eV; InP, 0.9 and 1.2 eV (all levels + 0.1 eV). The first energy given is assocaited with a missing anion and the second with a missing cation. For GaSb, only an acceptor due to a missing Sb has been located at 0.1 eV. Our work is found to correlate well with that on practical Schottky barriers. A detailed comparison is made with interface state positions and densities found by others on practical MIS structures, and it is suggested that the large density of these states on III–V’s as compared to Si is due to extrinsic interface states created by stoichiometric deficits of the III–V semiconductor. For GaAs, the dominant state is found at 0.7 eV and is associated with an As deficit. For InP, the major interface level is about 0.1 eV below the CBM. These positions are in good agreement with the existing data obtained from a wide variety of samples.