MSE PhD Prospectus Defense of Denis Nothern

3:30 pm on Thursday, December 5, 2013
5:30 pm on Thursday, December 5, 2013
15 Saint Mary's Street, Rm 105
TITLE: Development of Deep UV LEDs based on AlGaN Quantum Wells and Quantum Dots ABSTRACT: The development of deep UV LEDs, based on AlGaN alloys, is currently a topic of worldwide activity. The realization of efficient deep UV LEDs will provide solutions to a number of industrial and medical applications, including: water / air / food sterilization, surface disinfection, free-space and non-line-of-sight communication, counterfeit detection and fluorescence identification of biological / chemical agents. An additional motivation is the development of AlGaN alloys for additional deep UV optoelectronic devices such as lasers, solar-blind detectors, and electroabsorption modulators. The majority of deep UV LEDs reported so far in the literature are grown on sapphire substrates and their design consists of AlGaN quantum wells (QWs) embedded into an AlGaN pn-junction with the n-type layer on the sapphire. These devices suffer from poor internal quantum efficiency (IQE) due to a high concentration of threading defects originating from the large lattice mismatch (~14%) between the sapphire substrate and AlGaN alloys. Furthermore, these devices suffer from poor carrier injection efficiency due to the difficulties associated with doping AlGaN p-type. Finally, they also suffer from poor light extraction efficiency. In this research project, a novel design of deep UV LEDs is being developed on p-SiC substrates with the active region based on either AlGaN QWs or AlGaN QDs. This new class of DUV LEDs is expected to overcome the problems associated with current generation devices. All three factors that contribute to the external quantum efficiency (EQE) of a DUV LED are being addressed. Specifically, the AlGaN based DUV LEDs will be grown on SiC, which has a better lattice mismatch (~1%) to AlGaN alloys. This together with a growth mode that promotes band structure potential fluctuations is expected to lead to IQE in excess of 70%. Injection efficiency is addressed by employing degenerately doped p-SiC to circumvent the difficulties associated with p-type doping of AlGaN. High efficiency light extraction will be accomplished by employing p-type AlGaN distributed Bragg reflectors (DBRs) to prevent absorption in the SiC, and by extracting the light from the top n-AlGaN surface, which can be textured by reactive ion etching to produce light extraction efficiencies in excess of 70%. Individual layers (n-AlGaN, AlGaN QWs, and DBRs) have been produced by plasma assisted MBE and characterized in situ by reflected high energy electron diffraction (RHEED) and ex situ by x-ray diffraction, scanning electron microscopy, atomic force microscopy, photoluminescence, and reflectivity. Furthermore, prototype p-AlGaN DBR structures with reflectivity in excess of 50% have been produced, as well as UV LED devices incorporating some of the elements of the proposed design. In addition to developing such DUV LEDs based on AlGaN QWs, I will investigate the growth of AlGaN quantum dots by droplet epitaxy for use in the active region of the devices. This method involves the formation of liquid Al-Ga droplets, followed by conversion to AlGaN quantum dots by exposure to active nitrogen. The method has been demonstrated for binary compounds such as GaN. However, in forming AlGaN QDs by this method, a host of new issues must be addressed. This includes the solubility of Al-Ga alloys and active nitrogen at different temperatures and compositions, as well as the surface energy of the liquid alloy droplets. Issues such as these will be the subject of the current investigation. COMMITTEE: Advisor: Theodore Moustakas, MSE/ECE; Anna Swan, MSE/ECE; Karl Ludwig, MSE/Physics; Dmitris Pavlidis, ECE