Growth and evaluation of GaN grown on patterned sapphire substrates

We report the microstructure and the optical properties of gallium nitride (GaN) epilayers grown on a lens-shaped patterned sapphire substrate (PSS) by using metalorganic chemical vapor deposition (MOCVD) for various growth times. The lens shaped pattern was used to reduce the threading dislocation density and to improve the optical emission efficiency. The scanning electron microscope (SEM) image shows a flat and smooth surface for the GaN grown on the PSS for 80 min, which could be achieved by lateral growth from the trench region. Prom the double crystal X-ray diffraction (DCXRD) spectra, the full width at half maximum (FWHM) value was found to decrease with increasing growth time. The FWHM of the sample grown for 80 min was 473.5 arc sec. This indicates that there is an improvement in the crystalline quality of the GaN grown on the PSS as the growth time increases. From the Raman spectra, the shift of the A1(LO) and E2 ^high phonon mode frequencies towards a higher wavelnumber was observed for GaN grown on PSS as compared to GaN grown on an unpatterned sapphire substrate (UPSS). The high Raman peak intensity of the GaN epilayers using the patterned substrate with a low FWHM indicates that there is an improvement in the quality of the GaN compared to the layer grown on an unpatterned substrate. From the photoluminescence (PL) spectra, an increase in the band edge emission intensity and a decrease in the defect related yellow luminescence were observed for GaN on the PSS as the growth time increased. From the PL spectra, the FWHM was 82.2 meV at a peak position of 363.9 nm for the sample grown for 80 min. It is clearly seen that threading dislocations can be reduced by lateral growth, thereby improving the light emission efficiency by internal light reflection on the lens surface for the GaN grown on the PSS.