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|Title: ||InGaN/GaN quantum wells grown on polar and nonpolar GaN substrates|
|Authors: ||Lai, Kun-Yu Alvin|
|Advisors: ||Mark Johnson, Committee Co-Chair|
metal-organic chemical vapor deposition
light emitting diode
|Issue Date: ||3-Jun-2009|
|Discipline: ||Electrical Engineering|
|Abstract: ||Nonpolar (m-plane or a-plane) gallium nitride (GaN) is predicted to be a potential substrate material to improve luminous efficiencies of nitride-based quantum wells (QWs). Numerical calculations indicated that the spontaneous emission rate in a single In0.15Ga0.85N/GaN QW could be improved by ~2.2 times if the polarization-induced internal field was avoided by epitaxial deposition on nonpolar substrates.
A challenge for nonpolar GaN is the limited size (less than 10Ãƒâ€”10 mm2) of substrates, which was addressed by expansion during the regrowth by Hydride Vapor Phase Epitaxy (HVPE). Subsurface damage in GaN substrates were reduced by annealing with NH3 and N2 at 950 Ã‚ÂºC for 60 minutes. It was additionally found that the variation of m-plane QWsÃ¢â‚¬â„¢ emission properties was significantly increased when the substrate miscut toward a-axis was increased from 0Ã‚Âº to 0.1Ã‚Âº.
InGaN/GaN QWs were grown by Metalorganic Chemical Vapor Deposition (MOCVD) on c-plane and m-plane GaN substrates. The QWs were studied by cathodoluminescence spectroscopy with different incident electron beam probe currents (0.1 nA ~ 1000 nA). Lower emission intensities and longer peak wavelengths from c-plane QWs were attributed to the Quantum-confined Stark Effect (QCSE). The emission intensity ratios of m-plane QWs to c-plane QWs decreased from 3.04 at 1 nA to 1.53 at 1000 nA. This was identified as the stronger screening effects of QCSE at higher current densities in c-plane QWs.
To further investigate these effects in a fabricated structure, biased photoluminescence measurements were performed on m-plane InGaN/GaN QWs. The purpose was to detect the possible internal fields induced by the dot-like structure in the InGaN layer through the response of these internal fields under externally applied fields. No energy shifts of the QWs were observed, which was attributed to strong surface leakage currents.|
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