Characteristics of GaN-based LEDs using Ga-doped or In-doped ZnO transparent conductive layers grown by atomic layer deposition

Abstract

Ga-doped ZnO (GZO) and In-doped ZnO (IZO) films were prepared by atomic layer deposition (ALD), and the ALD-grown GZO (or IZO) films with (or without) N2 annealing were employed to serve as transparent conducting layers (TCLs) in InGaN/GaN (multiple quantum well) MQW LEDs. Based on θ-to-2θ X-ray diffraction (XRD) analyses, the N2-annealed GZO was found to show almost the same lattice constant c as ZnO does, while the lattice constant c of a N2-annealed IZO was detected to be larger than that of the ZnO. It appears that the implementation of N2-annealed ALD-grown GZO (or IZO) in an InGaN/GaN MQW LED allows to enable light extraction and forward voltage reduction of the LED under certain conditions. At 20mA operating condition, the 400°C N2-annealed n-GZO-coated and the 600°C N2-annealed n-IZO-coated InGaN/GaN MQW LEDs were found to exhibit optimized forward voltages of 3.1 and 3.2V, respectively, with the specific contact resistances of the n-GZO/p-GaN and n-IZO/p-GaN contacts being 4.1×10−3 and 8.8×10−3Ω-cm2. By comparing with an InGaN/GaN MQW LED structure having a commercial-grade indium tin oxide (ITO) TCL, the 400°C N2-annealed n-GZO-coated InGaN/GaN MQW LED shows an increment of light output power of 15% at 20mA. It is believed that the enhanced light extraction of the n-GZO-coated InGaN/GaN MQW LED is due to a higher refractive index of n-GZO than that of ITO along with a comparable optical transmittance of n-GZO to that of ITO.