The strain state on InN quantum dots (QDs) over GaN/sapphire substrates was analyzed by transmission electron microscopy. Changes in the in-plane lattice parameter of uncapped and capped InN QD heterostructures have been measured using moire fringe analysis. The uncapped QDs are almost completely relaxed, due to a misfit dislocation network present at the InN/GaN interface without generating any threading dislocations inside the QDs. In addition, a low-temperature-GaN capping process on InN QDs heterostructures was evaluated. Although this deposition avoids the InN decomposition, it modifies the QDs' morphology, decreases both the aspect ratio and, consequently, the plastic relaxation of the heterostructure.

A 10 nm thick epitaxially grown AIN cap has been used to protect the surface of a GaN epilayer both during Eu ion implantation and the subsequent high-temperature annealing. The 15 K photoluminescence (PL) intensity of the intra-4f Eu transition increases by two orders of magnitude when the annealing temperature is increased from 1000 to 1300 degrees C. High-resolution PL spectra reveal that the emission lines due to the D-5(0)-F-7(2) transition exhibit different dependencies on the annealing temperature in the studied annealing range. PL excitation measurements demonstrate band edge absorption by the GaN host at 356 nm, together with a broad excitation band centred at similar to 385 nm. The PL spectra of the D-5(0)-F-7(2) transition selectively excited by above band-gap absorption and by this broad excitation band are noticeably different. The first peak at 620.8 nm is suppressed when exciting below the GaN band gap. This demonstrates differing energy transfer processes for the different Eu luminescent peaks and is direct evidence for at least two kinds of different Eu sites in the host with distinct optical activation. Temperature dependent PL and PLE demonstrate that one of the two Eu-centres does not contribute to the room temperature luminescence. (c) 2005 Elsevier B.V. All rights reserved.

We have investigated the crystalline quality of thulium-doped GaN obtained either by in situ doping during MBE growth or by ion-beam implantation of MOCVD GaN layers. Both types of samples display the typical sharp intra-4f shell emission lines of Tm3+ ions in the blue and infrared spectral regions. The Raman spectra of the MBE samples indicate a good crystalline quality, showing the narrow E-2 and A(1) phonon peaks characteristic of GaN, even for the highest Tin concentrations. In contrast, Raman peaks associated with vacancy-related defects, as well as low-frequency bands due to disorder-activated modes can be observed in the Raman spectra of the implanted samples. These results indicate that, for the implantation doses required to achieve Tm emission, some residual disorder remains in the implanted GaN layers which is not observed in Tm-doped MBE samples with higher Tm concentration. (c) 2005 Elsevier B.V. All rights reserved.

GaN epilayers grown by MOCVD were implanted with Tm and Eu under different implantation conditions, in order to optimize the implantation parameters of fluence, implantation temperature and energy. Rutherford backscattering spectrometry in the channelling mode (RBS/C), scanning electron microscopy (SEM), transmission electron microscopy (TEM), temperature-dependent photoluminescence (PL) and room temperature cathodoluminescence (CL) were used to study structural and optical properties of the samples. Different annealing methods are compared.;According to RBS/C measurements, Tm at low fluences is found to incorporate entirely on substitutional Ga-sites while for higher fluences the substitutional fraction decreases. Increasing the fluence leads to increasing damage levels and finally to the formation of a partially amorphous, nanocrystalline surface layer. The luminescence intensity increases with fluence at first. However., for fluences in excess of 2 x 10(15) Tm/cm(2) it decreases again, showing a strong correlation between structural and optical properties.;Implanting at higher temperature inhibits the formation of the nanocrystalline layer and increases both the substitutional fraction and the luminescence intensity.;We also found that the presence of a thin AIN cap protects the sample during post-implant high temperature annealing and prevents the formation of a nanocrystalline surface layer during the implantation even for high fluences. The intensity of Eu-related CL near 622 nm at room temperature increases by a factor of 40 within the studied annealing range from 1000 to 1300 degrees C. (c) 2005 Elsevier B.V. All rights reserved.

In this report we present our recent investigation on the co-existence of superconducting and semiconducting properties in InN grown on sapphire (0001) by the use of MBE and MOCVD methods. Magneto-transport measurements were done using InN with a carrier density n. from 4 x 10(17) to 4 x 102 cm 3 in the temperature range from 20 mK to 2 K and under the magnetic field (B) up to 23 T-c As a result, even InN samples with n, as low as 4 x 10(17) cm(-3) showed superconductivity at T-c = 0.12 K. The vortex solid of InN was melted by thermal fluctuation and by the external field, similarly to the observation in layered high T-c superconductors. By the Shubnikov-de Haas oscillation measurements it was found that there were substantial carriers spread in the a-b plane which played an essential role not only for the occurrence of the superconductivity, but also for many other unexpected electronic and optical properties of InN. Based on these results we present a possible mechanism of superconductivity in terms of the interaction between the conduction electrons and the fixed d-electrons of In atoms spread in the a-b plane. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We have applied a slow positron beam to study InN samples grown by metal-organic vapor-phase epitaxy with different V/III molar ratios (3300-24000) and at different growth temperatures (550-625 degrees C). Indium vacancies were identified in samples grown at V/III ratios below 4000. Their concentration is in the 10(17)cm(-3) range. No strong dependence of vacancy concentration on the molar ratio was observed. At low V/III ratios, however, In droplets and vacancy clusters are formed near the substrate interface. The elevated growth temperature enhances the In vacancy formation, possibly due to limited sticking of In on the growth surface close to the decomposition temperature. (c) 2006 American Institute of Physics.