InN quantum dots (QDs) on GaN (0001) grown by metalorganic vapor phase epitaxy onto a sapphire substrate were studied by transmission electron microscopy (TEM). We found that the nucleation of InN QDs on GaN is directly related to the presence of threading dislocations (TDs) in the center of the QDs. The TEM analysis revealed that the TDs finish at the InN/GaN interface and they are pure edge dislocations. Therefore, spiral growth models cannot explain nucleation of these QDs. Although controlling edge TDs constitute a possible approach to determine the QD density, a better approach may be an increase in the material growth rate in order to enter the diffusion-limited growth mode, where growth is not sensitive to surface heterogeneities. (c) 2005 American Institute of Physics.

The growth of indium nitride quantum dots are studied. An investigation of the dot built-in strain, at the nanometric scale, performed using the synchrotron radiation is presented. These results are correlated with transmission electron microscopy images showing the dot structure. From these results, together with a thermodynamical analysis, it is shown that the dot growth mechanism is not the usual Stranski-Krastanov growth mode, but is more related to the BCF (Burton Cabrera Franck) growth mode.

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.

In order to determine the angular geometry that satisfies quasi-phase matching conditions for enhanced second-harmonic generation (SHG), the equi-frequency surfaces of the resonant photonic modes (that lie above the light line) of a one-dimensional GaN photonic crystal have been experimentally and theoretically studied as a function of frequency, angle of incidence, and azimuthal direction. Enhancement of the SHG has been observed when the angular configuration satisfies the quasi-phase matching conditions, i.e., when both the fundamental and second-harmonic fields coincide with resonant modes of the photonic crystal. The SHG enhancement achieved to the double resonance was 5000 times with respect to the unpatterned GaN layer. A smaller, but still substantially enhanced SHG level was also observed when the fundamental field is coupled into a resonant mode, while the second-harmonic field is not.

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.

Erbium photoluminescence in GaN:Er was studied with above-band-gap excitation, provided by a He-Cd (lambda = 325 nm) laser and below-band-gap excitation by a tunable Ti-Sa laser. The spectra obtained with these two lasers exhibit different spectral shapes. When both lasers are used at the same time, we observe that the Er3+ photoluminescence induced by each of the lasers is partly quenched by the illumination of the other laser. In this experiment, one of the lasers is modulated and a lockin amplifier is used to filter the corresponding photoluminescence signal. The spectra recorded this way are found to be linear combinations of spectra obtained with each of the lasers used separately. This effect is explained by the presence of defects mediating the excitation towards the Er3+ ions. These defects act as electron traps, which can be populated by one specific laser excitation and are photo-ionized by the other laser leading to a large quenching of Er3+ emission. (C) 2004 Elsevier Ltd. All rights reserved.

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.