Optical properties of wide bandgap III-nitrides, zinc oxide and III-N/zinc oxide heterostructures
Cheung, Maurice C-K
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Wide bandgap III-Nirtride (III-N) materials have attracted much research in the past decade. This have been driven by the desire to develop UV and blue sources for high capacity optical storage and by the desire to develop highly efficient solid-state lighting using nitride-based light emitting diodes. It has already been demonstrated that these devices have higher efficiency than incandescent lighting as well as a significantly longer operation lifetime (> 10, 000 hours is expected). However, the currently marketed white light LEDs rely on a layer of phosphor to convert the UV/blue emission to visible white light with specific emission spectra. It is preferable to developed a solid-state light emitting device that can directly covert the electrical energy to visible light with a controllable emission spectrum. Recently, in collaboration with the Georgia Institute of Technology, we have actively pursued light emitting devices that have a phosphor-less white emission. This device is based on III-Nitride materials combined with ZnO layers. The device is a heterojunction device that consists of a p-GaN/InGaN/ZnO heterostructure that can be readily fabricated into a diode. Specifically, in this thesis we have investigated the nature of the white emission using continuous wave and time-resolved photoluminescence. Specifically, in this dissertation, we will present a detailed investigation of individual components and the composite device. This will include the basic properties of InGaN thin films, focusing on the effects of indium segregation on the optical properties and carrier dynamics. We will also present an optical study on InGaN thin films grown on an alternative substrate: lithium gallate (LGO). In addition, we will present the optical properties of four ZnO thin films and directly correlate those properties to their electrical properties. This is followed by the presentation of detailed optical studies of the novel p-GaN/InGaN/ZnO LED, as well as a discussion of the working principle of the device. To conclude the research sections, we present some of the results of our recent optical studies on p-GaN nanorods formed on InGaN/ZnO layers. These nanorods can potentially be used in future phosphor-less white light LEDs to improve device efficiency, as well as a means to emission wavelength (color) control. Finally, the thesis ends with a brief conclusion and discussion of future works.