Effects of Bi on the Morphology and Atomic Structure of III-V Semiconductor Surfaces.

Abstract

Use of Bi in III-V semiconductor films in recent years has a variety of applications. Bi lowers the bandgap, introduces a large spin-orbit coupling, and preserves electron mobility, enabling low bandgap, high mobility and novel spintronic devices. Bi is also a nearly ideal surfactant, smoothing the surface, creating the right conditions for a sharp, high quality interface. However, the mechanism for this behavior and the atomic surface reconstructions are poorly characterized, both for surface- and bulk-incorporated Bi. This dissertation consists of four studies to investigate this behavior. The first half explores the effects of Bi as an ideal surfactant. The first study is an experimental scanning tunneling microscopy characterization of the Bi/GaAs surface, revealing the same reconstruction appears for the observed (1x3), (2x3), and (4x3) reflective high-energy electron diffraction patterns. Steps become wider on the micron length scale from the induced smoothing, owing to the increase of opposite direction step edges on the nanometer length scale. The second study is a combined cluster expansion aand density functional theory (DFT) analysis of the Bi/GaAs xvi reconstructions. The (4x3) reconstruction was found to stabilize into a variety of compositions in the presence of Bi. Monte Carlo analysis of the (4x3) reconstruction shows a strong propensity for surface disorder even at temperatures well below Bi deposition temperatures. The second half focuses on the surface effects of incorporated Bi. In the third study, several GaSbBi films were grown as a function of Ga, Sb, and Bi growth rates. Biphasic droplets were observed, with sub-droplets, facets, and etching into the film. X-ray diffraction and Rutherford backscatter measurements showed a con- current increase in Bi and As concentration, indicating a previously unseen strain auto-compensation mechanism. The fourth study is a cluster expansion/DFT char- acterization of the Bi/GaSb surface system as a proxy for the GaSbBi surface. In par- ticular, the c(2x10) reconstruction was investigated for the bare GaSb and Bi/GaSb systems, where the instability of this reconstruction was established in a rigorous manner. Finally, the Bi-induced (2x1) reconstruction was found to be stable in the Bi/GaSb system, consistent with other Bi/III-V systems.