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 (PLD) using KrF excimer laser ablation of compressed and sintered powder targets 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). GaN powder manufactured by Cerac shows several well-defined PL features. Although the free exciton is absent, two relatively sharp bands appear at 3.461 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 crystallinity in the micropowder. PLD films prepared from the Cerac powder show a completely different set of sharp features, between 3.360 and 3.160 eV. Thus it is clear that PLD is not just a means of stoichiometric material transfer, but also leads to modification of structural and electronic properties. The spectroscopy of these sharp lines, observed previously in GaN samples prepared by vapour phase epitaxial techniques, provides some interesting clues to their origin.