| Gallium nitride is a leading candidate material for the development of short wavelength light emitters. Because of the lack of a well lattice-matched substrate such as single crystalline GaN, however, defects are a major factor limiting the progress of this material system. This thesis concentrates on studying the deep gap states found in both undoped and doped GaN. Optical techniques have been utilized because of their ability to easily probe levels deep within the wide band gap of GaN.; First, photoconductivity spectroscopy was performed on a large number of samples in order to scan both radiative and non-radiative states within the gap. In all samples measured, a wide tail of states was observed extending deep into the gap. This tail was found in both p-type and n-type material and was attributed to structural defects in the film.; The focus was next directed toward the ubiquitous 2.2 eV photoluminescence peak found in undoped GaN films. A series of measurements, including cathodoluminescence, temperature dependent photoluminescence, and infrared photoluminescence quenching, was performed to probe this transition. These measurements led to a new self-consistent model of the yellow transition in undoped GaN. The luminescence was found to be the result of the recombination of an electron trapped at a shallow donor state with a hole trapped at a deep donor level about 860 meV above the valence band edge. Strong lattice relaxation was also found to be involved in this recombination.; Finally, the role of doping with magnesium, which is the pre-eminent p-type dopant in GaN, was studied via photoluminescence spectroscopy. In order to differentiate between discrete levels and phonon replica, the temperature dependence of the PL spectra was examined. This procedure revealed a new PL peak attributable to magnesium. A model of magnesium incorporation is presented which explains the total PL spectrum in Mg-doped GaN by assuming that magnesium impurities take both substitutional lattice sites. |
