Enhancing Light Extraction Efficiency of the AlGaN Deep Ultraviolet Light-emitting Diodes by Utilizing Strong TM Polarized Emission and p-AlGaN Transparent Region

Abstract

AlGaN-based deep ultraviolet (UV) light-emitting diodes (LEDs) have been attracting increasing attentions for the wide range of applications due to their compact in size, energy-efficient and environmentally friendly. However, their external quantum efficiency (EQE) is still too low to replace widely used highly toxic low-pressure mercury lamps because of their extremely poor light extraction efficiency (LEE). Thus, increasing the LEE of the deep-UV LEDs is very important to improve the EQE, and thereby widely utilize them as deep-UV light sources in the different areas. In this dissertation, the effective approaches to overcome inherently poor LEE of the AlGaN-based deep-UV LED are proposed. A different number of arrays of micro-ring deep-UV LED having an inclined sidewall at the outer perimeter and a p-GaN-removed inner circle of the micro-ring, together with MgF2/Al omnidirectional reflectors have been fabricated. The micro-ring deep-UV LED shows remarkably higher light output power by 70% than the reference, consistent with the calculated result, as well as a comparable turn-on and operational voltages, which are attributed to the effective extraction of strong TM polarized anisotropic emission and the reduction of the absorption loss by the p-GaN contact layer, simultaneously. Moreover, we showed the light output power from the micro-ring LEDs is increased and the operational voltage is reduced while the size of micro-ring is getting smaller owing to the effective extraction of the TM polarized light and better ohmic contact formation by increased n-contact areas. Furthermore, micro-ring with additional circular p-contact at the center of the ring and reduced p-contact size were fabricated. These LEDs show higher light output power as compared with that of the micro-ring LEDs due to the combining effects of increasing the current spreading throughout the active region and further reduction in the deep-UV photon loss at the top layer. Finally, the three terminal semiconductor device known as light emitting triodes (LETs) were fabricated to investigate the holes injection efficiency, together with LEE.