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 2 describes electrical measurements which were made on heterostructures composed of the compound semiconductors AlAs and GaAs. Specifically, 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. Tunneling currents occur because the conduction band offset between AlAs and GaAs causes the AlAs to act as a barrier to electrons in the GaAs. Samples composed of single or multiple layers of AlAs sandwiched between GaAs layers were grown by metal organic chemical vapor deposition (MOCVD) and by molecular beam epitaxy (MBE). Electron transport perpendicular to the AlAs barriers was studied as a function of temperature, doping, and layer thickness by making I-V, first derivative (dI/dV), and second derivative d2I/dV2 measurements on these samples. The I-V curves give information about current transport mechanisms. If the dominant mechanism is tunneling, the I-V curves reflect mostly elastic tunneling currents, Structure in the derivatives of the I-V curves indicates the presence of inelastic and resonant tunneling processes. The elastic tunneling measurements give an understanding of the structure of the barriers since these measurements depend on barrier thickness, barrier spacings, and barrier height. Inelastic tunneling measurements can be used to identify the fundamental excitations in the tunneling barrier which can couple to the tunneling electrons; thus, inelastic measurements give additional information about the properties of the barrier. First and second derivatives were measured using modulation techniques.
The main results of this study were the identification of the dominant current transport mechanisms across the AlAs barriers as a function of temperature and AlAs layer thickness, the observation of inelastic tunneling currents due to the excitation of phonons, and the observation of resonant tunneling currents. Measurements on MOCVD 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 barrier was thicker than approximately 100 Å, 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 tunneling current through samples with 50 Å thick AlAs barriers. This was the first observation of these effects in the AlAs/GaAs system. Measurements were also made on MOCVD grown samples with a single, n-type AlAs barrier. I-V curves for these samples did not have the expected dependence on AlAs layer width. Reproducible structure was still present in the second derivative spectra. A possible explanation for the differences between samples with n-type and p-type barriers which is based on band bending in the AlAs barrier is given. Measurements on MOCVD grown samples with AlxGa1-xAs barriers were made with results similar to those for pure AlAs barriers. Negative resistance regions were observed in the I-V curves of samples with multiple AlAs layers, indicating the presence of resonant tunneling effects. Tunneling measurements on MBE grown structures with a single AlAs barrier did not give reproducible or consistent results.
Chapter 3 presents an investigation of the deep-level defect structure of CdTe using the technique of deep-level transient spectroscopy (DLTS). Layered structures composed of the compound semiconductors CdTe and HgTe or of the alloy Hg1-xCdxTe may have interesting properties. To realize these properties it is important to understand the electrical characteristics of CdTe and HgTe.The electronic properties of CdTe and HgTe are complicated by the fact that native defects may dominate the electrical characteristics of the crystals. An understanding of the deep-level defect structure of CdTe is, thus, important. DLTS measurements can be used to determine the energy of a deep-level with respect to the band edges, the concentration of the level, and its carrier capture cross section. DLTS measurements are made by monitoring changes in the capacitance of a diode caused by capturing carriers at levels in the depletion region of the diode and then thermally emitting the carriers back to the conduction and valence edges.
Measurements were made on a variety of CdTe crystals. Nominally undoped, Cu-doped and In-doped CdTe crystals were studied. Some of the crystals were observed before and after anneals in Cd-vapor, Te-vapor, or in a purified H2 ambient. Characteristics of deep levels which are seen in all of the n-type CdTe samples are presented. These levels were attributed to native crystal defects or to impurities which are commonly incorporated into CdTe. Levels were also observed which were common to all of the p-type crystals. The same explanation was given for the origin of these traps. A few levels which were specific to certain crystals were also observed and were attributed to unidentified impurities. Deep states were present in the In-doped CdTe which were not observed until the sample was illuminated with above band gap light at low temperatures. Other levels were induced to appear by stressing the CdTe crystals. In general, anneal conditions had a large effect on the concentrations of both shallow and deep levels in the crystals, but did not alter which deep levels were present and their relative concentrations. Modest sample heating (400K) during the process of making DLTS measurements could change the amplitude of levels, sometimes causing new levels to appear or previously observed levels to no longer be observed.
