North-Holland, Amsterdam DEFECTIVE HETEROJUNCTION MODELS

Fermi-level pinning behavior has been observed at the free surface, oxide interface, metal interface, MBE grown surface, stop-regrown homojunction, and misfit-dislocation pinned heterojunction of GaAs. Theories of such behavior are numerous and disparate. Theories of ideal heterojunction band offsets are less diverse, but have still not converged to a single mechanism. Recent studies of heterojunctions suggest that the conduction-band offsets are rela-tively independent of interface Fermi-level position, including situations in which the interface Fermi-level appears to be strongly "pinned". In "ideal " heterojunctions, the conduction-band offsets and bulk doping determine interface Fermi-level location; among other results, this mechanism allows the two-dimensional electron gas at modulation-doped AlGaAs-GaAs heterojunctions. If "pinned " heterojunctions involve charge densities comparable to those infer-red for Schottky barriers, then the pinning interface states should set up a dipole sufficient to alter the band offsets; the interfacial band alignment should then be dominated by the alignment of the pinning states, rather than that of the bulk bands. The experimentally suggested lack of sensitivity of band offsets to changes in pinning at heterojunction interfaces suggests that the mechanisms involved in band line-ups at "ideal " heterojunctions may be related to thos