Characterization of GaN/InGaN multiple quantum wells grown on sapphire substrates by nano-scale epitaxial lateral overgrowth technique

Abstract

High-quality GaN/InGaN multiple quantum wells (MQWs) were fabricated on nano-scale epitaxial lateral overgrown (NELO) GaN layers. To grow the NELO layer first a 200nm-thick SiO2 was deposited on a 2μm-thick undoped GaN layer. A 10nm-thick Ni layer was then deposited on the SiO2 film followed by an annealing process to form nano-scale Ni clusters. These Ni clusters will serve as a dry etch mask for the underlying SiO2 layer, resulting in the formation of SiO2 islands of diameter and inter-distance of 300 and 200nm, respectively. Undoped NELO GaN layer of thickness 2μm was grown on the template with SiO2 growth mask using metal organic chemical vapor deposition technique. A 2μm-thick n-GaN epilayer and a 5-period GaN/InGaN MQWs were grown on top of the NELO layer. It is found that the overgrown GaN epilayers exhibit a significant reduction in threading dislocation (TD). From the atomic force microscopy characterizations, the TD density reduces from 3×108 to 6×107cm−2 by utilizing the NELO technique. Optical properties of the MQWs deposited on the NELO layer (type N) were characterized by temperature-dependent photoluminescence (PL). The results are compared to a control structure (type C) grown in the same growth run as the type N structures but without the NELO layer. It is found that type N sample exhibits three-fold improvement in PL intensity at room temperature. The increase in external quantum efficiency arises from both enhanced extraction efficiency and internal quantum efficiency. Detailed temperature-dependent PL studies were conducted to evaluate the relative improvement in internal quantum efficiency to account for the improved material quality when MQWs were grown on top of NELO epilayers.