The optical properties of hetero-polarization GaN/AlGaN (12% Al and 16.5% Al) are studied by photoluminescence (PL), photoreflectance (PR) and photoluminescence excitation (PLE) at low temperature. This material system is known to exhibit very strong internal electric fields and we show that PR spectroscopy is one of the best techniques to get a quantitative measurement of such fields, as it is a direct all-optical measurement. Electric field strength derived from FKO is approximately 120 kV/cm in AlGaN (12% Al) and 850 kV/cm in a 4.2 nm GaN QW. These values are in good agreement with other indirect determinations realized on the same samples by PL and PLE spectroscopy, but which involve computer simulations that need an accurate knowledge of geometrical and composition parameters of the samples. (C) 2003 Elsevier B.V. All rights reserved.

We report the observation of a broad absorption in the 1.25 eV region that is typical of thin InN films. Such a feature we attribute to light absorption at the energy of the fundamental direct band gap of InN, while we attribute the low energy 650-750 meV photoluminescence to an extrinsic recombination process analogous to the process that give the blue band in AlN and the yellow band in GaN. The substantial broadening is attributed to both the residual doping, and strain. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

GaN epilayers grown by metal organic chemical vapor deposition (MOCVD) were implanted with Tm and Eu ions with different energies and fluences and at different temperatures in order to optimize the implantation conditions. The recovery of the implantation damage was studied using both rapid thermal annealing and furnace annealing with nitrogen overpressure of 4 x 10(5) Pa. Rutherford backscattering spectrometry in the channeling mode (RBS/C) was used to monitor the evolution of damage introduction and recovery in the Ga-sublattice and transmission electron microscopy (TEM) was carried out for further structural analysis. The RBS/C spectra as well as TEM images show two different damage regions, one at the surface arising from an amorphous surface layer and another one deeper in the crystal coinciding with the end of range of the implanted ions. For implantation with 150 keV at room temperature, even for fluences as low as 3 x 10(14) at/cm(2), a thin amorphous surface layer, which becomes thicker with increasing implantation fluence, was observed by TEM. High temperature annealing of these highly damaged layers often results in loss of the amorphous layer and accumulation of the implanted species at the surface rather than a regrowth of the crystal. It was possible to prevent the formation of an amorphous layer by implanting at 500 degreesC. In those samples a large part of the lattice damage was removed during annealing at 1000 degreesC and the recovery of the lattice is similar for both applied annealing methods. (C) 2004 Elsevier Ltd. All rights reserved.

We have studied the optical properties of Eu doped GaN thin films. We have grown high quality Eu doped GaN thin films by using Gas Source Molecular Beam Epitaxy (GSMBE), with 1.4% Eu concentration. The Full Width at Half Maximum (FWHM) of the X-ray diffraction in an omega scan was found to be 288 arcsecs. Low Eu concentration (0.08%) doped GaN thin films were grown, where Eu-related photoluminescence at 622 and 613 nm was detected using above band-gap excitation at 2 K. For high Eu concentration of 30% GaN:Eu crystal photoluminescence (PL) and cathodoluminescence (CL) spectra show strong and intense transitions at 622 and 664 nm, but also at 593 nm for CL spectra, with a similar transition observed from the low Eu concentration sample. (C) 2004 Published by Elsevier Ltd.

Studies of lattice dynamics devoted to wurtzite InN are presented. Raman scattering experiments on both InN thin films and nanometric islands grown by Metal-Organic Vapor Phase Epitaxy (MOVPE) were performed at room temperature. From the Raman spectra recorded from InN films under hydrostatic pressure up to 13 GPa, linear pressure coefficients and the corresponding Gruneisen parameters for both E-2 and A(1) (LO) phonons were extracted for the wurtzite structure up to 11 GPa, close to the starting pressure of the hexagonal to rock-salt phase transition of InN. Spectra at higher pressure suggest that InN undergoes a gradual phase transition, and the reverse transition exhibits a strong hysteresis effect during the downstroke. Then, we discuss recent results on large single InN islands grown on GaN buffer layers, obtained by spatially resolved micro-Raman measurements. The magnitude of the residual strain is estimated, using a recent determination of phonon deformation potentials. It is found to vary linearly as a function of island height. (C) 2004 Elsevier Ltd. All rights reserved.

We have grown by metal-organic chemical vapour deposition a thick layer of indium nitride (1.4 mum) on a sapphire substrate using appropriate growth parameters. The substrate was then removed and the layer reduced to powder by a soft mechanical method. The resulting material was used to obtain an X-ray powder diffraction profile for completely relaxed indium nitride, i.e. without strain due to the lattice mismatch with sapphire. The diffraction measurement was made with a Phillips XPERT-Pro diffractometer, without slits, and collected using CuKalpha radiation at 300 K.;The powder pattern contains many (hkl) peaks, depending upon two lattice parameters, a and c, for the wurtzite crystal structure. A two-dimensional least squares procedure was adapted to minimize the fitting errors on the experimental data. The lattice parameters were accurately extracted and will be given here. (C) 2004 Elsevier Ltd. All rights reserved.

This study presents the MOVPE growth of InN films onto different substrate materials, including sapphire, nitrided or not, GaN and AlN buffer layers deposited onto sapphire, and Si(111).;For InN growth onto nitrided sapphire, different growth parameters were investigated in order to determine the best growth conditions. We found that a low V/III molar ratio has to be used in order to increase the growth rate. A light nitridation treatment gives the best electrical properties: mirror like layers with a mobility of 800 cm(2)/V s were obtained. At room temperature, reflectivity experiments show the existence of a transition at 1.2 eV, while photoluminescence appears around 0.8 ev.;Using the same growth conditions onto GaN buffers (with thicknesses ranging from 15 to 1000 A), we found that the best mobilities are obtained above a given buffer thickness.;By comparing also with AlN buffer layers and silicon substrates, we found that our previous conclusion still holds; lightly nitrided sapphire substrate leads to the best electrical properties and morphology. (C) 2004 Elsevier Ltd. All rights reserved.