We present a study by transmission electron microscopy (TEM) of the strain state of individual InN quantum dots (QDs) grown on GaN substrates. Moire fringe and high resolution TEM analyses showed that the QDs are almost fully relaxed due to the generation of a 60 degrees misfit dislocation network at the InN/GaN interface. By applying the Geometric Phase Algorithm to plan-view high-resolution micrographs, we show that this network consists of three essentially non-interacting sets of misfit dislocations lying along the angle(1) over bar2 (1) over bar0 > directions. Close to the edge of the QD, the dislocations curve to meet the surface and form a network of threading dislocations surrounding the system.

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.

Comparative studies have been carried out on the cathodoluminescence (CL) and photoluminescence (PL) properties of GaN implanted with Tin and GaN co-implanted with Tin and a low concentration of Er. Room temperature CL spectra were acquired in an electron probe microanalyser to investigate the rare earth emission. The room temperature CL intensity exhibits a strong dependence on the annealing temperature of the implanted samples. The results of CL temperature dependence are reported for blue emission (similar to 477 nm) which is due to intra 4f-shell electron transitions ((1)G(4)-> H-3(6)) associated with Tm3+ ions. The 477 nm blue CL emission is enhanced strongly as the annealing temperature increases up to 1200 degrees C. Blue PL emission has also been observed from the sample annealed at 1200 degrees C. To our knowledge, this is the first observation of blue PL emission from Tin implanted GaN samples. Intra-4f transitions from the D-1(2) level (similar to 465 nm emission lines) of Tm3+ ions in GaN have been observed in GaN:Tm films at temperatures between 20-200 K. We will discuss the temperature dependent Tm3+ emission in both GaN:Tm,Er and GaN:Tm samples.

The effect of a low temperature GaN capping layer on the structural properties of InN quantum dots is reported. InN quantum dots grown onto sapphire using GaN buffer, with and without low temperature GaN capping layer, have been investigated by atomic force microscopy and transmission electron microscopy. The analysis revealed hexagonal shape quantum dots in both samples. Moreover, the GaN capping layer gives rise to a reduction in the dots aspect ratio. Moire fringe patterns, obtained in planar view geometry, were used to analyse the strain relaxation of the InN quantum dots. The moire pattern measurements demonstrated that the uncapped InN quantum dot is almost fully relaxed. These results are related to the formation of a 60 degrees misfit dislocation network in the InN/GaN interface. However, the capping layer not only generated a change of the aspect ratio, but also a decrease in the heterostructure plastic relaxation. The reason of this effect is discussed. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Lattice recovery of Eu-implanted GaN has been studied by means of Raman scattering under UV excitation. GaN epilayers implanted at 300 keV with doses ranging from 2 x 10(14) to 4 x 10(15) cm(-2) and subsequently annealed at 1000 degrees C for 20 min show an increasing degree of disorder as the implantation dose increases. Higher temperature annealings up to 1300 degrees C were also investigated in samples having an AlN capping layer. Disorder related modes, observed in samples annealed at 1000 degrees C, disappear at higher annealing temperatures, indicating an incomplete lattice recovery at 1000 degrees C. The Raman scattering spectra show resonant A(1)(LO) multiphonon scattering up to sixth order, whose relative intensities depend on the implantation dose. The intensity ratios between multiphonon peaks observed for the highest implantation doses suggest a spread of the resonance, which could be related to a heterogeneous strain distribution, also indicative of incomplete lattice recovery. (c) 2006 Elsevier Ltd. All rights reserved.

The structural properties of nanometric AlN caps, grown on GaN to prevent dissociation during high temperature annealing after Eu implantation, have been characterized by scanning electron microscopy and electron probe microanalysis. The caps provide good protection up to annealing temperatures of at least 1300 degrees C, but show localized failure in the form of irregularly shaped holes with a lateral size of 1-2 mu m which extend through the cap into the GaN layer beneath. Compositional micrographs, obtained using wavelength dispersive x-ray analysis, suggest that these holes form when GaN dissociates and ejects through cracks already present in the as-grown AlN caps due to the large lattice mismatch between the two materials. Implantation damage enhances the formation of the holes during annealing. Simultaneous room temperature cathodoluminescence mapping showed that the Eu luminescence is reduced in N-poor regions. Hence, exposed GaN dissociates first by outdiffusion of nitrogen through AlN cracks, thereby opening a hole in the cap through which Ga subsequently evaporates. (c) 2006 American Institute of Physics.

Indium nitride under high pressure (up to 50 GPa) was analyzed by means of Raman spectroscopy. The wurtzite to rocksalt phase transition was evidenced at the pressure of 13.5 +/- 0.5 GPa and the pressure dependence of vibration modes of both structures was investigated, leading to the determination of linear pressure coefficients and mode Gruneisen parameters. Influence of the pressure dependence of the energy gap on the spectra intensity is also discussed.