Doctor of Philosophy, The Ohio State University, 2004, Electrical Engineering

The epitaxial integration of III-V semiconductors with Si is of interest for photovoltaics since Si substrates offer a lighter, stronger, and cost effective platform for device production. By using compositionally step-graded SiGe layers to 100% Ge, the 4% lattice-mismatch between Si and GaAs and In 0.49 Ga 0.51 P is accommodated; this method has produced record low threading dislocation densities (TDD) of 1x10 6 cm -2 in fully relaxed the Ge/SiGe/Si (SiGe) substrates. In this dissertation, this method of III-V/Si integration is used for the development of GaAs and In 0.49 Ga 0.51 P single junction (SJ) solar cells and In 0.49 Ga 0.51 P /GaAs dual junction (DJ) solar cells, integrated on a Si platform. As such, we report that the minority carrier electron lifetime in p-type GaAs grown on Si is lower than that of holes in n-type GaAs at a given TDD and is a consequence of the higher mobility of electrons. This lower lifetime produced higher reverse saturation currents and lower open-circuit voltages for n+/p compared to p+/n configuration GaAs cells grown on SiGe with the same TDD. The higher performance of the p+/n GaAs/Si cell, by virtue of its higher open-circuit voltage, has demonstrated a record terrestrial efficiency of 18.1% and has been produced in areas up to 4 cm 2 with no degradation in cell performance. In 0.49 Ga 0.51 P SJ cells were integrated on Si substrates and an increase in depletion region recombination component of the reverse saturation current with TDD was also measured. A p+/n polarity preference for In 0.49 Ga 0.51 P on Si was demonstrated, although, the lower mobility of both carriers in In 0.49 Ga 0.51 P compared to GaAs, suggests a greater TDD tolerance. Based on these SJ results, the first realization of an In 0.49 Ga 0.51 P/GaAs DJ cell on Si with an output voltage greater than 2 V was demonstrated. A comparison with an identical DJ cell on GaAs found that the DJ cell on Si retained 91% of open-circuit voltage and 99% of short-circuit cu (open full item for complete abstract)

Committee: Steven Ringel (Advisor) Subjects: Physics, Condensed Matter

Doctor of Philosophy, The Ohio State University, 2004, Physics

SiC is a polytypic material that may crystallize in many different close-packing sequences with cubic, hexagonal, or rhombohedral Bravais lattices. All SiC polytypes have wide bandgaps ranging from 2.39 eV in cubic SiC to 3.023 a€“ 3.330 eV in common hexagonal polytypes. This, as well as many other properties favorable to electrical applications, makes SiC a very promising material in electronic device fabrication. However, the many lattice stacking sequences may impair the stability of SiC devices. In the hexagonal 4H polytype, it has been found that thin cubic SiC inclusions may be formed due to stacking fault expansion, and it has been proposed that the inclusions may behave as quantum wells because of the lower bandgap of cubic SiC. We performed ultra-high vacuum ballistic electron emission microscopy (BEEM) measurements on n-type 4H-SiC samples containing double-stacking-fault cubic inclusions to characterize the electrical properties of individual inclusions. Thin Pt films are deposited in ultra-high vacuum on the sample surfaces to form Schottky contacts. A Schottky barrier height of ~1.01 eV is observed over the inclusions in a background of normal 4H-SiC barrier height of 1.54 eV, which directly confirms the cubic inclusions support two-dimensional propagating quantum well states, and the 0.53 eV lowering of barrier height indicates the two-dimensional conduction band minimum is located at ~0.53 eV below the conduction band minimum of bulk 4H-SiC. We also used BEEM to study the Schottky contact between Pt and p-type 4H-SiC, and observed a second transmission channel in the BEEM spectrum that suggests a split-off valence band at ~0.11 eV below the valence band maximum. We also measured the barrier heights of p-type and n-type Schottky contacts prepared under identical conditions and the results suggest the existence of an interfacial layer. An earlier study of threading dislocations in GaN using BEEM is also described. Although threading dislocations in GaN (open full item for complete abstract)

Committee: Jonathan Pelz (Advisor) Subjects: Physics, Condensed Matter

Doctor of Philosophy, The Ohio State University, 2002, Electrical Engineering

The III-Nitride material system has proven extremely valuable for semiconductor device applications. The ability to grow high quality AlGaN/GaN that can be used for RF device applications is largely due to the commercial success of the implementation of p-type doping in GaN for optical devices. Even high quality GaN has relatively large defect densities. GaN devices are still able to achieve impressive performance, but not consistently. The variation in material quality, including deep-level defects and non-uniformities introduced by processing and growth, have deleterious effects on microwave device performance. These variations and the inability to control them reduce yield and reliability thus making AlGaN/GaN devices difficult to produce commercially. The purpose of this work is to characterize and contribute to the understanding of defects in AlGaN/GaN device systems and their effects on microwave device performance both DC and RF. The effects of device fabrication and surface processing on these defects have also been characterized. Low Energy Electron-Excited Nano-luminescence (LEEN) Spectroscopy has been used to characterize radiative defects in the AlGaN/GaN material system on a microscopic scale and compare them with electrical measurements on HEMT's and TLM structures. Salient features commonly observed in the LEEN spectra include donor-bound excitons in GaN at ~3.43 eV, donor-acceptor pair transitions (DAP) at ~3.30 eV, yellow luminescence (YL) centered at ~2.20 eV, AlGaN donor-bound exciton emission, and associated phonon replicas. These measurements have been used to successfully correlate contact and sheet resistance with DAP, YL, and AlGaN near-band edge emission spectral features within a given wafer and between wafers. The effects of ultra-high vacuum processing with Argon sputtering and rapid thermal annealing on defects observed with LEEN spectra have been documented. Microscopic LEEN analysis has also been performed on working microwave devices (open full item for complete abstract)

Committee: Leonard Brillson (Advisor) Subjects: