Structural, chemical, and electrical characterization of III-V materials grown by low temperature molecular beam epitaxy.

摘要

The successful application of Gallium Arsenide (GaAs) grown at low molecular beam epitaxy (MBE) temperatures in electro-optical devices has promoted research to unravel the relationships between the structure and the electrical properties of similar III-V materials. The advantage of growth at low temperatures is the incorporation of excess group V element in the lattice. Low temperature (LT) GaAs usually has 1-2% excess As in the lattice. These point defects are expected to form deep level states in the band gap thus dominating the conductivity by carrier trapping. Annealing of LT GaAs at typical MBE growth temperatures (450-600{dollar}spcirc{dollar}C) for about 10 minutes has been shown to produce phosphorus precipitation that acts as buried Schottky barriers possessing "spherical" depletion or "space-charge" regions. It is believed that when these depletion regions overlap, the layer becomes highly resistive and semi-insulating. Carrier lifetimes in such LT material are very short (in the sub-picosecond range) and electronic transport is mainly by variable range hopping conduction. The high resistivity and the short carrier lifetimes have initiated the use of LT GaAs in field effect transistor (FET) buffer layers and ultra-fast optical switches.; This research focuses on analyzing the nanostructure, chemistry and morphology of LT Indium Phosphide (InP) synthesized under a variety of growth conditions with the intent of obtaining a correlation with relevant electrical properties. Work along the lines of the equivalent LT GaAs system has been carried out and a comparison has been drawn between As-based and P-based systems. Transmission electron microscopy (TEM) and allied techniques of imaging, spectroscopy and microdiffraction have been employed to characterize hyperstoichiometric InP grown by low temperature MBE, using solid indium and P{dollar}sb2{dollar} (from phosphine) for elemental sources. Observations of structural defects, elemental phosphorus and indium precipitates, and related lattice parameter variations in the LT layers have been made. Lattice strain in the epitaxial layers and corresponding variations in the stoichiometry have been investigated. The trends in the microstructural properties and chemistry of the as-grown material as a result of annealing have been analyzed and correlated with variations in electro-optical properties like conductivity and photoluminescence. Structural and electrical characterization has been performed on LT GaAs layers in order to elucidate the relationship between the trends in the nanostructural characteristics and those in the electrical behavior of the material upon annealing. Comparisons with As-based systems, particularly LT GaAs, have been established with similarities as well as some key differences in the properties. While a correlation between a moderate enhancement of the resistivity and crystalline phosphorus precipitation in the annealed material was observed, highly resistive layers were not formed. The absence of high resistivity material in the case of LT InP could be explained by the presence of indium or phosphorus antisite defects.