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

• Published in: IEEE journal of quantum electronics Vol. 44; no. 12; pp. 1211 - 1218
• Main Authors: Jinn-Kong Sheu, Ming-Lun Lee, Lu, Y.S., Shu, K.W.
• Format: Journal Article
• Language: English
• Published: New York IEEE 01.12.2008; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Annealing; Conductive films; Ga-doped zinc oxide; Gallium nitride; gallium oxide; Indium tin oxide; Indium tin oxides; Light emitting diodes; Optical films; Refractive index; Rough surfaces; Sputtering; Surface roughness; transparent contact layer; Zinc oxide
• ISSN: 0018-9197; 1558-1713
• DOI: 10.1109/JQE.2008.2002101

In this study, Ga-doped ZnO (GZO) thin films were deposited on a sapphire substrate utilizing a magnetron sputtering approach. ZnO and Ga 2 O 3 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 (~90%) in visible light and low resistivity (~5.3 x 10 -4 Omega-cm) when they were annealed at a temperature of 600-800deg 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~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.