The MBE growth of zinc selenide/gallium arsenide heterostructures and characterization of epitaxial zinc selenide/gallium arsenide heterointerfaces
Abstract
The ZnSe bandgap of 2.67 eV as compared to (Al,Ga)As having a 2.0 eV bandgap for an Al mole fraction of 0.5, as well as a close lattice match to GaAs (0.27% mismatch), and compatible thermal expansion coefficients, indicate that ZnSe could provide an alternative to (Al,Ga)As for a variety of device applications. Unlike III-V homovalent heterostructures, ZnSe/GaAs has a chemical valence mismatch at the interface. However, despite the valence difference across the interface, it is possible to obtain interface state densities comparable to (Al,Ga)As by choosing the appropriate GaAs epilayer surface stoichiometry prior to the nucleation of ZnSe, or, alternatively, by post-growth annealing. For this study, pseudomorphic and nearly pseudomorphic ZnSe epilayers are grown on GaAs epilayers in a modular MBE having separate growth chambers for II-VI and III-V compounds. Prior to nucleation of ZnSe, the GaAs epilayer temperature is raised to obtain a specific GaAs RHEED pattern corresponding to a particular surface stoichiometry. In a series of experiments, ZnSe is nucleated on GaAs epilayers having surface reconstruction of c(4 x 4), (2 x 4), (4 x 6), and (4 x 3). The electrical properties of the ZnSe/GaAs heterointerfaces are investigated by C-V measurements of metal-insulator-semiconductor capacitor structures in which high resistivity ZnSe layers act as a pseudoinsulator. Starting from a c(4 x 4) surface, the electrical characteristics of the interface improves as the GaAs surface becomes increasingly As deficient. The ZnSe/GaAs interfaces exhibiting the lowest interface state densities are associated with the appearance of an interfacial layer of Ga$\sb2$Se$\sb3$. TEM dark field and high resolution images used to evaluate the microstructure indicate the presence of two monolayers of coherent zincblende Ga$\sb2$Se$\sb3$ at ZnSe/GaAs heterovalent interfaces formed on As-deficient GaAs epilayers. An in-situ XPS has been used to study the chemical bonding at the interfaces which have the low interface state density. The comparison between Se 3d core level features from the interfacial region, from Ga$\sb2$Se$\sb3$ epilayers, and from ZnSe epilayers, clearly indicate the same Se bonding for the interfacial region and the Ga$\sb2$Se$\sb3$ epilayer.
Degree
Ph.D.
Advisors
Gunshor, Purdue University.
Subject Area
Electrical engineering
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