Ga-Doped ZnO Transparent Conductive Oxide Films Applied to GaN-Based Light-Emitting Diodes for improving Light Extraction Efficiency

In this study, Ga-doped ZnO (GZO) thin films were deposited on a sapphire substrate utilizing a magnetron sputtering approach. ZnO and Ga2O3 targets were employed as the sputtering sources during a cosputtering deposition. After thermal annealing in nitrogen ambient conditions, the electrical resistivity and optical transparency of the GZO films were analyzed in detail. The GZO films exhibited high transparency (similar to 90%) in visible light and low resistivity (similar to 5.3 x 10(-4) Omega-cm) when they were annealed at a temperature of 600-800 degrees C. Although the utilization of indium tin oxide (ITO) serving as the transparent contact layer (TCL) in conventional GaN-based light-emitting diodes (LEDs) is a well accepted technology, ZnO-based TCLs with a high refractive index of around 2.0 would render another advantage when a roughening process is performed on the surface. In other words, since packaged LEDs are generally encapsulated using epoxy with a refractive index of around 1.5, surface roughening performed on ITO TCL would thus result in only a minor improvement in light extraction because the typical refractive index of an ITO film prepared by our e-beam evaporator is around 1.7. In this study, GaN-based LEDs that utilized ITO/GZO composite oxide films as a TCL were also demonstrated. The light output power of an LED (LED-C) with a textured ITO/GZO composite TCL is markedly improved by 42% and 48% of magnitude as compared to LEDs with a planar GZO TCL (LED-A) and a ITO/GZO composite TCL (LED-B), respectively. This enhancement is due to the fact that a ZnO-based TCL with a higher refractive index (n similar to 2.0) allows further. enhancement of light extraction through the creation of a textured structure on the TCL that is deposited on the top surface of LEDs.