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Optical studies of GaN-based light emitting structures

Lam, Jack Biu
Scope and Method of Study: The optical properties of group-III Nitrides were systematically studied over a temperature range of 10 to 300 K. A large variety of light sources including femtosecond, picosecond, and nanosecond laser systems as well as continuous wave light sources were used to optically excite the samples in this study. The sample emission was detected with photomultiplier tubes, CCD cameras, and a streak camera. Particular emphasis was put on high-excitation density phenomena and the stimulated emission properties in the samples. Issues related to gain mechanisms, carrier localization, and hot carrier dynamics were addressed.
Findings and Conclusions: Gain mechanisms in AlGaN epilayer were examined, and conclusion was made that over the entire temperature range of 10 to 300 K, recombination of an electron hole plasma is responsible for optical gain. HVPE-grown GaN epilayer and GaN/AlGaN double heterostructure were shown to have good photoluminescence efficiency and stimulated emission properties similar to that of a high quality MOCVD-grown sample, making them excellent candidate for UV - visible light emitting applications. Incorporation of a small amount of In into GaN layers resulted in the familiar carrier localization that are well known to have existed in InGaN structures, and lead to efficient photo emission process. In femtosecond pump-probe and time-resolved photoluminescence spectroscopy of an InGaN sample, emission of two successive LO phonon was clearly observed. As hot carriers relax toward the band edge, optical gain was observed, resulting in stimulated emission of the sample. Stimulated emission was attributed to recombination of an electron hold plasma from band to band transition.