Electronic Properties of Heterostructures and Defects in Compound Semiconductors

This thesis deals with the electronic characteristics of semiconductor heterostructures and with the electrical properties of semiconductors which are used in the growth of heterostructures. Chapter 1 presents the background for the two techniques primarily used in the research described in this thesis: inelastic tunneling spectroscopy and deep -level transient spectroscopy. Chapter 2 describes electrical measurements which were made on heterostructures composed of the compound semiconductors AlAs and GaAs. The mechanisms for current transport perpendicular to one or more AlAs layers sandwiched between two degenerate GaAs layers were studied, with an emphasis on elastic and inelastic tunneling through the AlAs layers at low temperatures. Measurements on metalorganic chemical vapor deposition grown samples with a single, p-type AlAs barrier indicated that thermionic emission was the dominant mechanism for current transport over the barrier at room temperature. At low temperatures, leakage currents dominated if the AlAs layer was thicker, while tunneling currents were dominant in the samples with thinner AlAs barriers. Electron self-energy effects due to the coupling of electrons and optical phonons in the GaAs, and the inelastic-excitation of longitudinal optical phonons in the AlAs were observed in the low temperature tunneling currents. This was the first observation of these effects in the AlAs/GaAs system. There was a significant difference between samples with n-type or p-type AlAs barriers. A possible explanation for these differences based on band bending in the AlAs barriers is given. Negative resistance regions were observed in the I-V curves of samples with multiple AlAs layers, indicating the presence of resonant tunneling effects. Chapter 3 presents an investigation of the deep -level defect structure of the compound semiconductor CdTe using the technique of deep-level transient spectroscopy. Nominally undoped, Cu-doped and In-doped CdTe crystals were studied both before and after anneals in Cd-vapor, Te-vapor, or in a purified H(,2) ambient. Some deep levels were seen in all of the n-type CdTe samples. Levels were also observed which were common to all of the p-type crystals. These levels were attributed to native crystal defects or to impurities which are commonly incorporated into CdTe. Levels specific to certain crystals were attributed to unidentified impurities. . . . (Author's abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UMI.