Optical properties of GaN pyramids

Picosecond time-resolved photoluminescence ~PL! spectroscopy has been used to investigate the optical properties of GaN pyramids overgrown on hexagonal-patterned GaN~0001! epilayers on sapphire and silicon substrates with AlN buffer layers. We found that: ~i! the release of the biaxial compressive strain 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 grown under identical conditions; ~ii! in the GaN pyramids on GaN/AlN/sapphire substrate, strong band edge transitions with much narrower linewidths than those in the GaN epilayer have been observed, indicating the improved crystalline quality of the overgrown pyramids; ~iii! PL spectra taken from different parts of the pyramids revealed that the top of the pyramid had the highest crystalline quality; and ~iv! the presence of strong band-to-impurity transitions in the pyramids were primarily due to the incorporation of the oxygen and silicon impurities from the SiO2 mask. © 1999 American Institute of Physics. @S0003-6951~99!02609-1#