Lattice order in thulium-doped GaN epilayers: In situ doping versus ion implantation S. Herna ´ndez a, * , R. Cusco ´ b , L. Artu ´s b , E. Nogales a , R.W. Martin a , K.P. OÕDonnell a , G. Halambalakis c , O. Briot c , K. Lorenz d , E. Alves d a Department of Physics, University of Strathclyde, Glasgow G4 0NG, Scotland, UK b Institut Jaume Almera (CSIC), Lluı ´s Sole ´ i Sabarı ´s s.n., 08028 Barcelona, Spain c GES, Universite ´ de Montpellier II, 34095 Montpellier, France d ITN, Estrada Nacional 10, 2686-953 Sacave ´m, Portugal Available online 9 November 2005 Abstract We have investigated the crystalline quality of thulium-doped GaN obtained either by in situ doping during MBE growth or by ion- beam implantation of MOCVD GaN layers. Both types of samples display the typical sharp intra-4f shell emission lines of Tm 3+ ions in the blue and infrared spectral regions. The Raman spectra of the MBE samples indicate a good crystalline quality, showing the narrow E 2 and A 1 phonon peaks characteristic of GaN, even for the highest Tm concentrations. In contrast, Raman peaks associated with vacancy-related defects, as well as low-frequency bands due to disorder-activated modes can be observed in the Raman spectra of the implanted samples. These results indicate that, for the implantation doses required to achieve Tm emission, some residual disorder remains in the implanted GaN layers which is not observed in Tm-doped MBE samples with higher Tm concentration. Ó 2005 Elsevier B.V. All rights reserved. PACS: 85.40.Ry; 78.30.j; 63.50.+x Keywords: Rare earth doping; Ion beam implantation; Raman scattering 1. Introduction Over the last few years, rare-earth (RE) doped III-nit- rides have been extensively studied because of their poten- tial applications in full colour emission devices [1]. The intra-4f shell electronic transitions of the RE ions give rise to sharp emission lines with small energy dispersion (Dk 6 0.5 nm), and it is possible to cover the near-ultravio- let to near-infrared spectral range by selecting the RE spe- cies. In GaN-based systems, the RE 3+ ions mainly occupy Ga substitutional positions [2,3]. While the intra-4f shell transitions involved in RE emission are dipole-forbidden in isolated ions, the crystal field of the site occupied by the RE 3+ ions affects their transition probabilities by mix- ing states of opposite parity [4]. Moreover, it has been sug- gested that deep defect levels participate in the energy transfer process from the host to the RE 3+ ions [5]. There- fore, the structure of the local environment of RE 3+ ions plays a crucial role in the determination of the optical lumi- nescence efficiency of RE doped semiconductors. Raman scattering is a powerful technique for assessing crystalline quality in a non-destructive way, and provides information about local and extended defects in the struc- ture. To our knowledge, only one Raman study has been published on implantation-induced disorder in RE doped GaN [6]. In that work, additional phonon modes were detected in the Raman spectra of GaN after implantation of Er 3+ ions. Those modes had been previously identified with disorder and Ga and N vacancies, and they appear in the Raman spectra of implanted samples irrespective of the implanted species [7–9]. 0925-3467/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.09.021 * Corresponding author. E-mail address: sergi.hernandez@strath.ac.uk (S. Herna ´ndez). www.elsevier.com/locate/optmat Optical Materials 28 (2006) 771–774