Reduced Residual Stress and Enhanced Performance of GaN-Based LEDs Prepared by Liquid Phase Deposition Silicon Oxide-Nano Patterned Sapphire Substrate

Abstract

A simple liquid phase deposition (LPD) method was used to introduce nano SiO 2 on sapphire substrates to fabricate nano-patterned sapphire substrates with top oxide layers (MNPSS). The X-ray diffraction (XRD) rocking curves and etching pit density (EPD) analyses show that the quality of MNPSS-GaN was better than that of GaN grown on flat sapphire substrates (FLAT-GaN). The photoluminescence (PL) spectrums showed a blueshift of MNPSS-GaN peak position compared with FLAT-GaN. The analyses of XRD reciprocal space map (RSM) revealed that this shift was caused by the residual stress.High-brightness GaN-based light-emitting diodes (LEDs) have been widely used in a solid state lighting field. Many techniques have been developed for improving LED performance. Currently, the patterned sapphire substrate (PSS) (especially nano-sized PSS) has attracted much attention because it can improve both internal quantum efficiency (IQE) and light extraction efficiency (LEE). 1-6 This is because with increase in growth time, laterally grown GaN caused the threading dislocations to bend toward the patterns. However, it is still hard to control the indium fraction and the residual stress during the epitaxial process. 7 In this study, liquid phase deposition (LPD) was used to introduce nano-sized PSS. 8 After growth of the GaN layer, the crystallinity, indium fraction and the residual stress were investigated. ExperimentalTwo kinds of c-plane sapphire substrates were used to grow GaN. Samples designated as "FLAT" are flat sapphire substrates, while "MNPSS" are nano-patterned sapphire substrates with top oxide layers prepared by LPD method. 8 For MNPSS, substrate was immersed in mixed solution of H 2 SiF 6 saturated with silica gel and H 3 BO 3 (0.01 mol/l). The related LPD reactions on sapphire were listed as follows:LPD-SiO 2 can be formed after the dehydration of OH-bonded siloxane oligomer [SiF m (OH) 4−m ] by catalytic reaction. Substrates were then annealed at 900 • C in N 2 ambient for 30 minutes to improve SiO 2 quality. To fabricate the MNPSS, sample was then etched in hot H 3 PO 4 -based solutions at 270 • C for 60 second.After the cleaning process, GaN-based LED structures were grown by metal-organic chemical vapor deposition (MOCVD). The crystal quality and optical performance of GaN were characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron-microscopy (TEM), X-ray diffraction (XRD) and photoluminescence (PL). Results and DiscussionThe SEM image of MNPSS is shown in Fig. 1a. The MNPSS structure comprised a hexagonal pyramid with an oxide-covered top c-plane. 2,9 The height of pattern was 200-300 nm (measured by AFM), while the diameter of hemisphere oxide (LPD-SiO 2 ) was 250-300 nm. * Electrochemical Society Active Member. z E-mail: SermonWu@StanfordAlumni.org After growth of the GaN layer, the nature of GaN crystal qualities were analysed by XRD rocking curves and etching pit density (EPD). For XRD rocking curves, both (002) and (102) reflections were shown in Table I. The...