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 relatively 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 inferred 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 those mechanisms involved in Fermi-level "pinned" systems. A simple mechanism is that of work function matching, in which the transition to "pinned" behavior involves the generation of a new material at the interface; since the work function difference between heterojunction materials is unaffected, the band offset would likewise be unaffected. The effective work function model explains the pinning phenomenon on the basis of anion clusters, which have been observed at all classes of pinned interfaces involving III-V compounds. The application of other models to both pinned and unpinned interfaces is less clear; more information is required. Pinning models which involve interface state densities within each semiconductor must address the lack of sensitivity of band offset to different interface Fermi-level locations.