A microspectroscopic study of cap damage in annealed RE-doped AlN-capped GaN
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A microspectroscopic study of cap damage in annealed RE-doped AlN-capped GaN E. Nogales1,*, K. Lorenz2, K. Wang1, I.S. Roqan1, R.W. Martin1, K.P. O’Donnell1, E. Alves2, S. Ruffenach3 and O. Briot3 1 Department of Physics, University of Strathclyde, Glasgow G4 0NG, United Kingdom 2 Instituto Tecnológico e Nuclear, Sacavém, Portugal 3 Groupe d’Etudes des Semiconducteurs, Univ. Montpellier II, Montpellier, France ABSTRACT Integrated AlN nanocaps are used to protect gallium nitride epilayers during high temperature annealing treatments following high-energy implantation of rare earth (RE) ions. Cracks formed in thicker caps due to the lattice mismatch between AlN and GaN lead to the creation of microscopic surface defects at annealing temperatures higher than around 1200 ºC. GaN dissociates locally to produce holes in the caps. Simultaneous cathodoluminescence/wavelength dispersive X-ray microanalysis in a modified electron probe microanalyzer allows study of the compositional and light emission variations near these microscopic defects. The intensity of the 5D0 – 7F2 transition related emission is enhanced and spectral changes can be observed, which indicate changes in the structure and/or composition of a very thin layer that forms the walls of holes in the caps. We also report some preliminary observations on the influence of the annealing atmosphere (nitrogen or ammonia) on cap damage.
INTRODUCTION Rare earth doped GaN has been widely studied in recent years for optoelectronic applications due to the robustness of this semiconductor and its large bandgap [1]. Favennec et al. showed that semiconductors with larger gaps show lower thermal quenching of RE luminescence [2]. The main drawback of implantation doping is that lattice damage created in the process has to be removed by applying a subsequent annealing treatment. Total removal of the implantation damage and a complete activation of dopants in GaN may require temperatures as high as 1500 ºC. Unfortunately, unprotected GaN tends to dissociate at annealing temperatures higher than around 1000 ºC in the N2 atmosphere normally used for such treatments [3-5]. In order to protect the implanted material at temperatures higher than 1000 ºC, Zolper et al. introduced the use of sputtered AlN caps which can withstand heat treatment, under certain conditions, up to 1400 ºC [5]. However, localized failure of the caps normally appears above 1200 ºC, which degrades the samples. In an attempt to design more robust caps, a study of their failure mechanisms has recently been carried out [6]. The presence of cracks in as-grown AlN caps, caused by the lattice mismatch between GaN and AlN (~ 2.4%), is found to be the main cause of cap failure during annealing. Localized dissociation of GaN occurs with loss of nitrogen through the extended defects and creates a pitted surface with holes about a micron deep and up to some microns wide. It has also been shown that in microscopic regions with high local loss of N, the RE related emission intensity decreases markedly [6]
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