Influence of selective GaN islands on optical properties of GaN films grown on sapphire substrate

We report on the growth of GaN films on sapphire substrates by low-pressure metalorganic vapor phase epitaxy using selective GaN islands as buffer layer. These islands are produced by a silane (SiH4) treatment of the sapphire surface at high temperature, followed by a low temperature GaN buffer deposition. A photoluminescence (PL) study demonstrates that this growth process significantly enhances the luminescence emission of the donor bound exciton (D deg. X) recombination and leads to a very narrow peak with a full width at half maximum (FWHM) of 4 meV. This width is about 30% smaller as compared with a standard process using a low temperature buffer layer. Changes in PL peak energies due to the residual strain were linked to the growth mode of the GaN epilayers. Photo-electrochemical (PEC) etching in aqueous solutions of KOH was applied to the GaN epilayers. 'Whisker-like' structures were observed on the surface of etched GaN samples by scanning electron microscope (SEM). These structures result from distribution of dislocations in GaN films. Their density was reduced from 6x109 cm-2 in standard GaN films to 8x108 cm-2 in the GaN layers grown on the selective islands