The emission spectrum of pulsed laser deposited GaN and its powder precursor

Thin films of wurtzite GaN have been grown on heated sapphire substrates in reactive atmospheres of nitrogen or ammonia by pulsed laser deposition (PL D) using KrF excimer laser ablation of compressed and sintered powder targe ts of GaN. We report here a comparative study of the films and their powder precursor by means of low temperature photoluminescence (PL) spectroscopy, cathodoluminescence (CL) imaging and scanning electron microscopy (SEM). G aN powder manufactured by Cerac shows several well-defined PL features. Alt hough the free exciton is absent, two relatively sharp bands appear at 3.46 1 and 3.410 eV, in addition to the familiar donor-acceptor pair band near 3 .2 eV and the well-known yellow band. SEM imaging reveals relatively poor c rystallinity in the micropowder. PLD films prepared from the Cerac powder s how a completely different set of sharp features, between 3.360 and 3.160 e V. Thus it is clear that PLD is not just a means of stoichiometric material transfer, but also leads to modification of structural and electronic prop erties. The spectroscopy of these sharp lines, observed previously in GaN s amples prepared by vapour phase epitaxial techniques, provides some interes ting clues to their origin. (C) 1999 Elsevier Science S.A. All rights reser ved.