New materials preparation techniques such as molecular beam epitaxy (MBE) provide dramatically improved possibilities for interface control and novel device structures. However, compared with our knowledge of Schottky barriers or semiconductor-insulator interfaces, our microscopic information on the basic science underlying semiconductor heterojunctions is very limited. Insight into the growth, extent and composition of such interfaces is needed to illuminate the origins of the electronic and optoelectronic behavior. Most recent experimental studies have not properly monitored the detailed structural characteristics of the heterojunctions being measured.

In this paper we discuss the scientific exploration of the interrelationship between growth and electronic properties of hetero-interfaces prepared by MBE. In situ low energy electron diffraction and synchrotron radiation photoemission were performed on germanium interfaces with GaAs and AlAs polar and non-polar surfaces. The individual surface and subsurface bonding interactions (rather than bulk thermodynamic properties of the semiconductors) are shown to control the formation and electrical properties of hetero-interfaces of either germanium on the compound or the inverse deposition sequence. Simple arguments based on bulk semiconductor heats of formation incorrectly predict the relative stability of interfaces in the lattice-matched Ge/GaAs/AlAs system. Rather the formation of a GeAsx surface layer provides a lower energy free surface during MBE growth and causes arsenic diffusion from the interface layer to lower the free energy when a pure germanium surface begins to form. Significant variations in the band edge offsets of heterostructures are observed when the semiconductor-semiconductor interface microstructure changes with substrate orientation and overlayer crystallinity. Neither the common electron affinity difference “rule” nor recent tight-binding models for determining heterojunction barriers allows for effects of interface microstructure. Measurement and analysis of such “materials-related” aspects of semiconductor interfaces are central to the understanding and control of device characteristics.

These studies are partially funded by the U.S. Office of Naval Research (L. R. Cooper), Contract N00014-81-C-0696. Part of this work was performed at SSRL and Stanford Linear Accelerator Center, supported by National Science Foundation Grant DMR 77–27489 and the U.S. Department of Energy.