| Indium gallium nitride (InGaN) based quantum well structures have been successfully commercialized in recent years into bright blue and green light emitting diodes (LEDs); however, despite advances in device performance, materials issues and inherent physical effects continue to hinder their performance. An incomplete understanding of the variables that affect quantum efficiency in the disordered system hampers optimization of quantum well parameters and operating conditions.; InGaN is a disordered alloy composed of the III-nitride binaries GaN and the poorly understood InN. With increasing mole fraction of InN, the structural, optical, and electrical properties InGaN containing structures become increasingly unstable and unpredictable due to the emergence of spinodal decomposition. On one hand, the presence of indium rich nanoclusters and potential fluctuations are responsible for a relatively high optical efficiency despite high threading dislocation densities (109--1010/cm 2), but a large polarization field (> 1 MV/cm) is also known to reduce both the transition energy via the quantum confined Stark effect and the recombination efficiency by spatially reducing wavefunction overlap. The overarching goal is to gain understanding of the complex interaction between nanoscopic variations in field/composition and the macroscopic polarization field.; Continuous wave photoluminescence (PL) and time-resolved PL are used to study emission characteristics including peak energy, spectral width, and recombination lifetime as a function of external variables such as temperature and injection density. The optical results from an ensemble of emitting states are compared to high resolution transmission electron microscopy and high resolution valence electron energy loss spectroscopy to reconcile the Stokes shift - the redshift of the emission from absorption and the calculated ground state transitions. Modeling of the various transitions is self-consistently performed with an implied InN bandgap of 1.9 eV.; The presence of exciton localization centers is demonstrated both directly and indirectly, and the varying nature of localization effects on the PL properties is shown for different sample sets. Localization via in-plane variations of composition is distinguished from localization due to minute variations in composition or due to interfacial fluctuations. The temperature dependence of exciton dimensionality, emission peak width, and radiative lifetime are demonstrated as keys to distinguishing the recombination mechanism at various temperatures and injection densities.; Throughout this work, quantum mechanical wavefunction, bandstructure, and potential modeling are used to predict important considerations such as device efficiency, transition energies, and various field effects. |
