Characteristic Comparison of GaN Grown on Patterned Sapphire Substrates Following Growth Time

Abstract

Gallium nitride (GaN) epilayers were grown on a patterned sapphire substrate (0001) (PSS) using metalorganic chemical vapor deposition (MOCVD) for various growth time from 10 to 80 min. Structural characteristics, surface morphology and optical properties of the GaN epilayer were investigated using double crystal X-ray diffraction (DCXRD), scanning electron microscopy (SEM) and photoluminescence (PL). A lens shaped pattern was formed on the sapphire substrate to reduce threading dislocation density and also to improve the optical emission efficiency by internal reflection on the lens. From the DCXRD rocking spectrum, full width at half maximum (FWHM) was decreased with increasing growth time. The FWHM of the sample grown at 80 min was 473.48 arcsec which indicates good crystalline quality. From the SEM images, full coalescence takes place at 80 min growth time. It was observed that the threading dislocation densities were reduced as the growth time increased thereby obtaining a smooth and defect free GaN epilayer at high growth time. From the PL spectra, a decrease in the FWHM and an increase in the peak position were observed as the growth time was increased. Similarly, the yellow luminescence was decreased as the growth time increased and disappeared at growth time 80 min. From the band edge emission spectra, FWHM was 82.2 meV at peak position 363.9 nm for the sample grown at 80 min which indicates a low threading dislocation density. Our results show that by using PSS for growing GaN epilayer with high growth time, threading dislocation density could be reduced thereby improving the crystalline and optical quality of the GaN layers.