Materials Science and Engineering B59 (1999) 133 – 136 The emission spectrum of pulsed laser deposited GaN and its powder precursor P.G. Middleton a , K.P. O’Donnell a, *, C. Trager-Cowan a , D. Cole b , M. Cazzanelli b , J. Lunney b a Department of Physics and Applied Physics, Uniersity of Strathclyde, Glasgow, UK b Department of Physics, Trinity College, Dublin, Ireland Abstract 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. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Photoluminescence spectroscopy; Catholuminescence imaging ; Scanning electron microscopy 1. Introduction Pulsed laser deposition (PLD) offers several advan- tages in terms of efficiency and cost when compared to conventional routes of crystal growth such as molecu- lar beam and vapour phase epitaxies. While PLD is already the preferred technique for the epitaxial growth of refractory materials used in high-tempera- ture superconductivity, its application to semiconduc- tors has been sporadic. Only a few papers [1] describe the application of the technique to the III-nitride ma- terial systems that are currently causing a revolution in blue – green optoelectronics. In the present work we discuss the properties of a PLD-grown GaN film and its powder precursor. We show that the PLD process is not simply a means of transferring material from source to substrate, and highlight the changes in both the structural and elec- tronic properties of the material which take place dur- ing crystal growth. 2. Experimental details The target pellets for the GaN film deposition were prepared from 99.99% pure GaN powder manufac- tured by Cerac. The powder was pressed into pellets at a pressure of 7 ×10 8 Pa. This was followed by sinter- ing at 620°C for 12 h in a 1270 mbar N 2 atmosphere to produce a robust target for deposition. A KrF excimer laser was employed for laser ablation, focused to provide a fluence of 2.8 J cm -2 on the target at a repetition rate of 10 Hz. The Al 2 O 3 (11 – 21) substrate, positioned 35 mm from the target, was held at a temperature of 720°C throughout. The chamber was filled with nitrogen at 0.13 mbar to promote stoichio- metric growth. * Corresponding author. Tel.: +141-548-3365; fax: +141-552- 2891. E-mail address: k.p.odonnell@strath.ac.uk (K.P. O’Donnell) 0921-5107/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII:S0921-5107(98)00332-8