Influence of the Annealing Ambient on Structural and Optical Properties of Rare Earth Implanted GaN
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Influence of the Annealing Ambient on Structural and Optical Properties of Rare Earth Implanted GaN K. Lorenz1, E. Nogales2, R. Nédélec3, J. Penner3, R. Vianden3, E. Alves1, R.W. Martin2, K.P. O`Donnell2 1 Instituto Tecnológico e Nuclear, EN10, 2686-953 Sacavém, Portugal 2 Department of Physics, University of Strathclyde, Glasgow, G4 0NG, U.K. 3 HISKP, University of Bonn, 53115 Bonn, Germany ABSTRACT GaN films were implanted with Er and Eu ions and rapid thermal annealing was performed at 1000, 1100 and 1200 ºC in vacuum, in flowing nitrogen gas or a mixture of NH3 and N2. Rutherford backscattering spectrometry in the channeling mode was used to study the evolution of damage introduction and recovery in the Ga sublattice and to monitor the rare earth profiles after annealing. The surface morphology of the samples was analyzed by scanning electron microscopy and the optical properties by room temperature cathodoluminescence (CL). Samples annealed in vacuum and N2 already show the first signs of surface dissociation at 1000 ºC. At higher temperature, Ga droplets form at the surface. However, samples annealed in NH3+N2 exhibit a very good recovery of the lattice along with a smooth surface. These samples also show the strongest CL intensity for the rare earth related emissions in the green (for Er) and red (for Eu). After annealing at 1200 ºC in NH3+N2 the Eu implanted sample reveals the channeling qualities of an unimplanted sample and a strong increase of CL intensity is observed. INTRODUCTION Doping GaN with optically active rare earth (RE) elements allows the production of electroluminescent emitters that cover the entire visible spectral range [1]. Ion implantation is a powerful technique to introduce ions in a reproducible way with a defined concentration profile. However, for GaN this method still suffers from the incomplete annealing of the resultant lattice damage and the dissociation of the surface at high temperatures [2]. It was shown previously that the emission intensity of Eu-implanted GaN increases strongly with rising annealing temperature [3]. However, higher annealing temperatures require an increase of the partial nitrogen pressure to suppress the loss of N from the sample. To prevent surface dissociation, several approaches were reported in literature including the use of a proximity cap (i.e. a piece of unimplanted GaN placed face to face with the sample), the predeposition of a capping layer or the annealing in high N2 overpressures [2-4]. The annealing atmosphere can influence the stability of the surface and thus the recovery of the crystal. Using a NH3 containing atmosphere is closer to the growth conditions in the MOCVD reactor and can help to stabilize the surface. However, at the same time highly reactive hydrogen can damage the surface; etching effects were observed for heat treatment of GaN in NH3 at temperatures above 800 ºC [5]. Emission properties can also be influenced by incorporated hydrogen. Lozykowski et al. [6] reported an improvement in luminescence intensity for
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