MSE PhD Final Oral Defense of Wei Zhang

8:00 am on Thursday, March 6, 2014
10:00 am on Thursday, March 6, 2014
8 Saint Mary's Street, Rm 901
TITLE: DEVELOPMENT OF ALUMINUM GALLIUM NITRIDE BASED OPTOELECTRONIC DEVICES OPERATING IN DEEP UV AND TERAHERTZ SPECTRAL RANGES ABSTRACT: In this research project I have investigated the AlGaN alloys and their quantum structures for applications in deep UV and terahertz spectral regions. For the deep UV emitter applications they were grown by rf plasma-assisted molecular beam epitaxy on 4H-SiC, 6H-SiC and c-plane sapphire substrates. In the growth of AlGaN /AlN multiple quantum wells on SiC substrates, the AlGaN wells were grown under excess Ga, far beyond than what is required for the growth of stoichiometric AlGaN films, which resulted in liquid phase epitaxy growth mode. Due to the statistical variations of the excess Ga on the growth front we found that this growth mode leads to films with lateral variations in the composition and thus band structure potential fluctuations. Transmission electron microscopy shows that the well in such structures are not homogeneous but have the appearance of quantum dots. We find by temperature dependent photoluminescence measurements that the multiple quantum wells with band structure potential fluctuations emitting at 240 nm have a room temperature internal quantum efficiency as high as 68%. Furthermore, they were found to have a maximum net modal optical gain of 118 cm-1 at a transparency threshold corresponding to 1.4 x 1017 cm-3 excited carriers. We attribute this low transparency threshold to population inversion of only the regions of the potential fluctuations rather than of the entire matrix. Some prototype deep UV emitting LED structures were also grown by the same method on sapphire substrates. Optoelectronic devices for terahertz light emission and detection, based on intersubband transitions in III-nitride semiconductor quantum wells, were grown on single crystal c-plane GaN substrates. Growth conditions such the ratio of group III to active nitrogen fluxes, which determines the appropriate Ga-coverage for atomically smooth growth without requiring growth interruptions were employed. Emitters designed in the quantum cascade structure were fabricated into mesa-structuredevices and the I-V characterization at 20 K indicates sequential tunneling with electroluminescence emission at about 10 THz. Similarly, Far-infrared photoconductive detectors were grown by the same method. Photocurrent spectra centered at 23 ┬Ám (13 THz) are resolved up to 50 K, with responsivity of approximately 7 mA/W. COMMITTEE: Advisor: Theodore Moustakas, MSE/ECE; Roberto Paiella, MSE/ECE; Jonathan Klamkin, MSE/ECE; Karl Ludwig, Physics; Chair: Shyamsunder Erramilli, MSE/BME/Physics