Piezoelectricity and indium segregation in III-nitride heterostructure devices
Sweeney, Paul M
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Significant progress has been made in the development of LEDs with emission from the UV to the red. Moreover, AlGaN/InGaN/GaN UV and blue lasers with long operational lifetimes are commercially available but remain expensive. Much research interest has focused on three mechanisms affecting III-Nitride device performance: phase-segregation resulting from In clustering in InGaN alloys, piezoelectricity in InGaN resulting from the lattice mismatch between InGaN and the GaN substrate, and spontaneous polarization of AlGaN resulting from the asymmetry in the tetrahedral sub-structure of the wurtzite lattice. It is of significant technological importance to determine the relative contribution of each of these processes to the emission efficiencies in these nitride materials. In this thesis, comparative time-resolved and CW photoluminescence measurements of the emission from InGaN/GaN, InGaN/AlGaN, and AlInGaN/AlGaN heterostructures is presented. The first studies presented are focused on understanding the carrier dynamics associated with piezoelectricity and indium segregation. This first study was limited to InGaN quantum wells with GaN barriers in which spontaneous polarization can be neglected. Time-resolved photoluminescence measurements of these structures were performed to ascertain the magnitude of the energy shifts and carrier lifetimes of these structures. The magnitude of the observed energy shifts can be explained by phenomenological models that include only piezoelectricity. However, consistent with indium segregation, quantum wells with the same nominal indium concentrations and well width are observed to have drastically differing peak emission energy and linewidths. The focus of the second set of studies was an InGaN single quantum well (SQW) with AlGaN barriers and a strain engineered AlInGaN SQW with AlGaN barriers. For these samples, spontaneous polarization, resulting from the AlGaN barriers, must be included in any models. However, the in-well strain in the strain engineered AlInGaN SQW is significantly reduced by a careful match of the lattice constant of the substrate and well, therefore the piezoelectric field can be neglected. The results of these studies imply that phase segregation produces regions of radiative recombination centers that can be optically saturated on the low energy (long wavelength) side of the observed emission with long carrier lifetimes. The significance of these measurements and the effects of these mechanisms on device performance will be presented. Finally, future research to further determine the contributions of each mechanism will be discussed.