Since the last few years due to many optoelectronic and photonic applications an increased interest in rare earth (RE) doped semiconductors has been observed. One of the effective techniques of RE incorporation in GaN is ion implantation, however it contributes to creation of a considerable amount of structural defects. Their negative impact can be removed by post implantation high temperature annealing which results in optical activation of rare earth ions. Unfortunately high temperature annealing leads to the decomposition of the GaN surface. In order to avoid this decomposition, a number of solutions have been proposed i.e. use of GaN proximity cap, annealing under high N2 overpressure (>10×1010 Pa), and recently implantation through a thin AlN cap layer epitaxially grown on top of GaN. By Rutherford backscattering and channeling (RBS/C), it was shown that the presence of the AlN layer allowed decreasing the implantation damage. It also provided an efficient way to protect the GaN surface for high temperature annealing.
In this work, we investigate the structure of a 10 nm AlN layer grown on GaN by MOCVD. The influence of RE implantation and subsequent high temperature annealing have been studied. Our aim is to understand the protective role of AlN cap during implantation process and high temperature annealing. Scanning electron microscopy (SEM) observations allow us to report on the morphological evolution of the surface. Structural analysis at the atomic scale of the layers and the interface between AlN and GaN is done by the use of high resolution transmission electron microscopy (HRTEM). It is shown that a number of defects like surface pinholes may contribute to the relaxation of the mismatch between AlN and GaN. These defects need to be avoided as their evolution during implantation and annealing leads to significant degradation of the layer.

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