Characterization of InGaN/GaN Multiple Quantum Wells Grown on Sapphire Substrates by Nano-scale Epitaxial Lateral Overgr
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Characterization of InGaN/GaN Multiple Quantum Wells Grown on Sapphire Substrates by Nano-scale Epitaxial Lateral Overgrowth Technique W.K. Fong, K.K. Leung, and Charles Surya Department of Electronic and Information Engineering and Photonics Research Centre The Hong Kong Polytechnic University Hong Kong ABSTRACT High-quality InGaN/GaN multiple quantum wells (MQWs) were fabricated on nanoscale epitaxial lateral overgrown (NELO) GaN layers which was prepared using nanometer-scale SiO2 islands, with an average diameter and interdistance of 300nm and 200nm respectively, as the growth mask. The active region of the device consists of five periods of GaN/InGaN MQWs were grown on top of the NELO layer using MOCVD technique. It is observed that some of the dislocations from the undoped GaN were blocked by the SiO2 growth mask and typical threading dislocation (TD) density found in the NELO samples is ~7.5×107cm-2. Significant improvement in the electroluminescence (EL) is observed which is believed to partly arise from the improvement in the internal quantum efficiency (ηi). The experimental data on the temperature dependence of the photoluminescence (PL) were fitted to a proposed model using LevenbergMarquardt approximation. Based on our analyses it is found that the relative improvement in ηi at 300K over a control device grown in the same growth condition but without the NELO layer to a NELO device is only 0.59. It is generally accepted that TD is the non-radiative recombination center which affects the IQE. Therefore, room-temperature IQE values also support that NELO device exhibits lower TD density. INTRODUCTION The high threading dislocation (TD) density has shown to have significant implications on the optoelectronic and reliability properties of the devices. Such high TD density arises from the large lattice constants and thermal expansion coefficients mismatches between GaN and the sapphire substrates [1-3]. It is important to develop novel growth techniques to reduce the TD density. The epitaxial lateral overgrown (ELO) technique has been widely used in vapor phase epitaxy technology to accomplish low TD density. For the one-step ELO (1S-ELO) process [4], coherent GaN still follows the high TD density as the underneath GaN template. Since the typical widths of the dielectric growth mask, with SiN or SiO2, and the window are 7 µm and 4 µm respectively, an LED die will cover the highly defective coherent GaN region which can significantly reduce the lifetime of the device. Another ELO technique is the two-step ELO (2SELO) [5] process in which the dislocation density can be further reduced and limited to the coalescence edge. However, the drawback of 2S-ELO is that relatively thick ELO-GaN, depending on the width of dielectric growth mask, is needed for full coalescence resulting in significant bowing of the wafer and hence high non-uniformity in the material quality over the 2inch wafer. In this paper, we report on the investigation of a novel LED structure in which the MQW structure is grown on top of a NELO laye
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