Electrical characterization of nonstoichiometric GaAs grown at low temperature by molecular beam epitaxy.

摘要

GaAs grown by molecular beam epitaxy at low substrate temperatures is highly nonstoichiometric (NS), with as much as 2% excess As incorporated in otherwise good quality, single crystal material. The as-grown material is thermally unstable: during a postgrowth anneal the excess As precipitates out of solid solution, resulting in a two-phase composite material. If the anneal is at a sufficiently high temperature {dollar}({lcub}sim{rcub}600spcirc C),{dollar} NS-GaAs becomes semi-insulating, which has led to its use in devices such as field effect transistors and photodetectors. The primary objective of this dissertation was to understand electrical transport in structures containing NS-GaAs films, especially at high electric fields.; Using a combination of temperature-dependent current-voltage measurements and transmission electron microscopy on bulk films, and by studying the effects of trapped charge on an underlying doped GaAs channel, we show that one of two deep donor bands controls the electrical properties of NS-GaAs films, depending on the anneal temperature. Both bands are associated with excess As-related point defects, not As precipitates. A direct measurement of the density of trap states in the top half of the band gap also shows no evidence for precipitates being directly responsible for "pinning" the Fermi level near midgap in semi-insulating NS-GaAs. Depending on the growth, anneal, and measurement temperatures, transport in NS-GaAs at low fields is often found to be dominated by hopping conduction via the deep donor states. Thus, the predominant transport mechanism is different on either side of a GaAs/NS-GaAs homojunction, which leads to diode-like behavior and causes unusual field screening effects unless high doping in the GaAs allows direct tunneling of free carriers to the hopping band in the NS-GaAs. Due to a weak field-dependence of the hopping mechanism, high field transport in bulk NS-GaAs films is shown to be dominated by conduction band electrons. The I-V characteristics of bulk films are explained using a model based on space-charge-limited transport theory and modified to take into account drift velocity saturation and field-dependent trapping effects.