UV-transparent glass electrodes for high-efficiency nitride-based LEDs

Abstract

Nitride-based UV LEDs are promising replacements for conventional UV lamps1 because of their higher energy efficiency, longer lifetime, and greater reliability. However, the external quantum efficiency of UV LEDs is currently much lower than that of visible LEDs. This difference is mainly due to the light absorption that occurs in the p-type gallium nitride (p-GaN) contact layer and the metal electrode layers. In deep-UV LEDs, absorption becomes an even greater problem.2 One possible solution to this fundamental issue is to obtain a direct ohmic contact to p-type aluminum gallium nitride (p-AlGaN). This can be achieved using UV-transparent conductive electrodes (TCEs), thus avoiding absorption and increasing device efficiency. Prior to our work, no solution had been found to overcoming the trade-off between high electrical conductivity and high optical transmittance. Indeed, these properties have generally been considered mutually exclusive. In recent years, some groups have reported the use of metal nanowires, metal nanomeshes, graphene, carbon nanotubes, metal oxides, and conductive polymers as replacements for conventional indium tin oxide (ITO),3, 4 but these efforts are still under way. We have proposed a universal method for producing TCEs using wide bandgap (WB) materials such as silicon oxides and nitrides.5 Glass-based TCEs (G-TCEs) enable effective current injection from a metal to a WB semiconductor (e.g., p-type AlGaN under bias) via conducting filaments (CFs) that are formed by the electrical breakdown (EBD) that occurs in the G-TCE. In these devices, high transmittance is maintained even in the deep-UV region (i.e., more than 95% at a wavelength of 280nm). To achieve this, we developed a G-TCE using aluminum nitride (AlN) as a unique solution and implemented the resultant Figure 1. (a) Schematic view of a lateral-type aluminum gallium nitride—(Al)GaN—based LED with aluminum nitride (AlN)-based glass transparent conducting electrodes (G-TCEs), after electrical breakdown (EBD). This magnified image shows that current can be injected via conductive filaments (CFs), which are formed in the AlN layer after EBD, and can subsequently spread through the device via thin indium-tin-oxide (ITO) buffer layers. (b) Current-voltage characteristics measured for the AlN-based G-TCE, before (red) and after (blue) EBD. The inset shows conductive atomic force microscopy images taken for the AlN top layer before (left) and after (right) EBD at 1V with a compliance current of 10nA.