Interfaces of semiconducting molecular beam epitaxial films

Abstract

Detailed microstructural properties of thin film interfaces are systematically explored by the combination of synchroton-radiation-excited photoelectron spectroscopy with controlled sample preparation by molecular beam deposition. This paper is a discussion of (1) the forces controlling the formation of thin film interfaces on compound semiconductor surfaces, (2) the reactions, interdiffusion and subsurface microstructural rearrangements which can occur, and (3) implications for the fundamental electronic behavior of the resulting junctions. Epitaxial and disordered thin films of germanium, gallium, gold and aluminum on both non-polar (110) and polar (100) surfaces of GaAs and AlAs are considered. It is clearly seen that local interatomic interactions at or near the surface control the interface microstructure. Bulk equilibrium compound thermodynamic values ( e.g. compound heat of reaction) predict the opposite behavior from that observed for both metal and semiconductor thin films. The thin film interface is sensitive to temperature and/or time, surface stoichiometry and surface structure. The interface diffusion process is distinct from interfacial reactions which occur during the deposition, condensation and growth of the thin film on the surface. The interface microstructure controls the band edge discontinuity for Ge-GaAs(110) interfaces. The common electron affinity difference rule is hardly ever a correct description of the barrier height. An extrinsic process, remarkably similar to that proposed by Spicer for metal thin film interfaces, also occurs for compound semiconductor heterojunctions.