Date

2013

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Electrical Engineering

First Adviser

Tansu, Nelson

Other advisers/committee members

Bartoli, Filbert J.; Ding, Yujie; Stavola, Michael J.

Abstract

III-Nitride semiconductors have significant applications for lasers and energy-efficient technologies including solid state lighting. Specifically, the use of InGaN alloy is of great interest as visible light emitting diodes (LEDs) active region. Conventional LEDs employ InGaN quantum wells (QWs) grown on GaN templates, which lead to large QW strain from the lattice mismatch between InGaN QW and GaN substrate / barriers. Our works have pursued the design of InGaN QWs with large optical matrix element to address the charge separation issue, resulting in 3X enhanced efficiency for green-emitting LEDs. In addition to employing large overlap QWs design, my research work has extended the approach by using ternary InGaN substrate for realizing QWs with reduced strain and polarization fields in the QWs. For green- and red-emitting InGaN QWs on ternary substrate, the spontaneous emission rates were found as ~3 times of the conventional approach.In contrast to the progress in visible nitride emitters, advances have only been realized for ultraviolet (UV) LEDs recently. The pursuit of efficient UV lasers has been limited to 1) growth challenges of high quality AlGaN gain media; 2) lack of understanding in gain characteristics of the QW employed for UV laser. My work has pointed out the first time about the physical challenge of the AlGaN QWs, which is related to the valence subbands crossover in high Al-content AlGaN QWs gain media. The valence subbands crossover is of key importance to realize large transverse-magnetic polarized gain for deep-UV lasers. We have also proposed the novel AlGaN-delta-GaN QW structure, which led to large transverse-electric polarized gain for mid-UV lasers.Furthermore, the high power density requirements in III-Nitride devices lead to the demand of solid state cooling technology, particularly for nitride-based thermoelectric materials that can be integrated with GaN devices. Our works presented the high thermoelectric figure of merit Z*T value from lattice-matched AlInN alloy grown by metalorganic chemical vapor deposition (MOCVD), which represent the record Z*T value reported for any III-nitride semiconductors. In addition, we have proposed the novel nanostructure engineering of three-layer superlattice for ~2-times enhanced thermoelectric properties for solid state cooling applications.

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