Optical properties of GaN pyramids

Picosecond time-resolved photoluminescence (PL) spectroscopy has been used to investigate the optical properties of GaN pyramids overgrown on hexagona l-patterned GaN(0001) epilayers on sapphire and silicon substrates with AlN buffer layers. We found that: (i) the release of the biaxial compressive s train in GaN pyramids on GaN/AlN/sapphire substrate led to a 7 meV redshift of the spectral peak position with respect to the strained GaN epilayer gr own under identical conditions; (ii) in the GaN pyramids on GaN/AlN/sapphir e substrate, strong band edge transitions with much narrower linewidths tha n those in the GaN epilayer have been observed, indicating the improved cry stalline quality of the overgrown pyramids; (iii) PL spectra taken from dif ferent parts of the pyramids revealed that the top of the pyramid had the h ighest crystalline quality; and (iv) the presence of strong band-to-impurit y transitions in the pyramids were primarily due to the incorporation of th e oxygen and silicon impurities from the SiO2 mask. (C) 1999 American Insti tute of Physics. [S0003-6951(99)02609-1].