Epitaxial Interfaces of III-V Heterostructures: Atomic Resolution, Composition Fluctuations and Doping

Abstract

Epitaxially grown III-V semiconductor multilayers are studied by cross-sectional scanning tunneling microscopy (STM) on the (110) plane cleaved in ultrahigh vacuum (UHV). The group III (empty state) or the group V (filled state) sublattice are selectively imaged by tunneling into the conduction states or out of the valence states. The heterojunctions can be clearly identified with atomic resolution in both sublattices and are defined with an accuracy of ±1 unit cell. Atomic resolution topographs of the group III (Al-Ga) sublattice in AlGaAs are shown and the charge density corrugations reflect the composition fluctuations on the atomic scale. The definition and “roughness” of the interface on the atomic scale in the AlGaAs-GaAs heterojunction are displayed. Tunneling spectroscopy across GaAs/AlGaAs junctions displays a transition in the valence band which occurs over a length scale of approximately 3.5 nm. The extent of this electronic transition is discussed and a quantitative interpretation of the I-V curves is made, including tip-induced bandbending in the semiconductor. The role of the local doping concentration is discussed in context with spectroscopy. In addition, the observation of the electronic signature of individual doping sites is reported