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We report on the current properties of Al1-x InxN (x approximate to 0.18) layers lattice- matched ( LM) to GaN and their specific use to realize nearly strain- free structures for photonic and electronic applications. Following a literature survey of the general properties of AlInN layers, structural and optical properties of thin state- of- the- art AlInN layers LM to GaN are described showing that despite improved structural properties these layers are still characterized by a typical background donor concentration of ( 1 - 5) x 10(18) cm(-3) and a large Stokes shift (similar to 800 meV) between luminescence and absorption edge. The use of these AlInN layers LM to GaN is then exemplified through the properties of GaN/ AlInN multiple quantum wells ( QWs) suitable for near- infrared intersubband applications. A built- in electric field of 3.64MVcm(-1) solely due to spontaneous polarization is deduced from photoluminescence measurements carried out on strain- free single QW heterostructures, a value in good agreement with that deduced from theoretical calculation. Other potentialities regarding optoelectronics are demonstrated through the successful realization of crack- free highly reflective AlInN/ GaN distributed Bragg reflectors ( R > 99%) and high quality factor microcavities ( Q > 2800) likely to be of high interest for short wavelength vertical light emitting devices and fundamental studies on the strong coupling regime between excitons and cavity photons. In this respect, room temperature ( RT) lasing of a LM AlInN/ GaN vertical cavity surface emitting laser under optical pumping is reported. A description of the selective lateral oxidation of AlInN layers for current confinement in nitride- based light emitting devices and the selective chemical etching of oxidized AlInN layers is also given. Finally, the characterization of LM AlInN/ GaN heterojunctions will reveal the potential of such a system for the fabrication of high electron mobility transistors through the report of a high two- dimensional electron gas sheet carrier density ( n(s) similar to 2.6 x 10(13) cm(-2)) combined with a RT mobility mu(e) similar to 1170 cm(2) V-1 s(-1) and a low sheet resistance, R similar to 210 Omega square.

We report on a clear experimental demonstration of the presence of biexciton recombination in high quality GaN/AlGaN quantum wells, thus opening the way to relevant improvements of the performances of GaN based devices. The samples have been grown by metalorganic vapor phase epitaxy optimizing the deposition conditions to strongly reduce the Al fluctuations in the barriers. This leads to a photoluminescence broadening of only 5 meV at a temperature of 10 K for an Al content in the barriers of 5%. The biexciton binding energy if found to be of the order of 9 meV. The recombination kinetics show that the exciton and biexciton components are not thermalized at low temperature allowing to extract the intrinsic lifetimes of the two components. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The effect of the type of the silica gel pore structure on the surface properties of the Ni-silica gel catalyst precursors for the vegetable oil hydrogenation process has been examined applying N-2 sorption and X-ray photoelectron spectroscopy techniques. The nickel catalyst precursors with identical composition (SiO2/Ni = 1.0) has been synthesized by precipitation of Ni(NO3)(2) center dot 6H(2)O solution with Na2CO3 solution on the three types of silica gel with different pore structures. It is shown that the usage of the silica gel supports with different texture as source of SiO2 causes different location of Ni-species into the support pores and on the external surface area. The XPS data confirm the formation of surface species with different strength of interaction and different dispersion. These surface characteristics of the precursors will predetermine the formation of the active nickel metallic phase as well as the mass transfer of the reactants and products to and from the catalytic sites.

We report on the dependence on temperature of the homogeneous and inhomogeneous broadening of the fundamental transition of single polar GaN/AlN quantum dots (QDs). Stranski-Krastanov QDs have been grown by molecular beam epitaxy using NH3 as a nitrogen source, with a very low surface density. Low temperature (LT) microphotoluminescence measurements have been performed on 200 nm wide mesas in order to isolate the luminescence of single QDs. The linewidth is found to vary from 590 mu eV at 4 K up to 1350 mu eV at 65 K in a dot of 6 monolayer height. Though the LT linewidth is still dominated by spectral diffusion, the temperature dependent broadening up to 50 K is mainly accounted for by interactions between excitons and acoustic phonons through a coupling coefficient value nearly two orders of magnitude larger than its counterpart in InAs QDs. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The influence of thermally excited vibrations on vibronic spectra (absorption, CARS excitation profile and resonance background) has been modelled for polyatomic molecules in solution. It is shown that for a model describing some polymethine compounds, the spectra depend only slightly on temperature except in the case of a resonance background and an excitation profile of vibrations with a relatively high value of the origin shift parameter (Δ > 1). The investigations of low-frequency vibrations by resonance CARS to giving information about some molecular parameters is proposed.

Crack-free lattice-matched Al 0.85 In 0.15 N/GaN heterostructures were grown on sapphire substrates with barrier thicknesses up to 100 nm which exhibit very high polarization-induced electron sheet density ( cm −2 ) located at the heterointerface. These layers have been further processed as high electron mobility transistors (HEMTs). Optical characterization of these structures was carried out by photoluminescence and microphotoluminescence (PL) for different biased voltages. The insertion of an InGaN back-barrier unambiguously reveals that spatially direct optical recombinations occur within the AlInN alloy. Since the GaN excitonic bandgap is very sensitive to local temperature changes, the PL technique allows mapping very precisely the actual local temperature distribution in biased HEMT devices. For a gate length of 1.5 m temperatures up to 1130 K were found at a drain-source voltage of 20 V thus indicating the presence of a hot phonon bath.

Coherent Raman spectra for resonance pumping conditions and a three-frequency variant were claculated for polyatomic molecules. A model based on density of states and displaced harmonic oscillators was used. The contributions of different intermediate vibrational levels in the electronic state S1 to the resonance Raman susceptiblity χ and the interference of χ with a non-resonant electronic background are discussed. It is shown that many intermediate levels belonging to S1 and corresponding to two excited vibrations (the relevant Raman vibration and either one of the others) influence the spectrum significantly, in contrast to the case with absorption. The asymmetry of the Raman line is due mainly to the interference with a non-resonant background. The method used takes into account the influence of the interference of neighbouring lines on the line shape and also the possible deformation of the lines by the dispersion of the Raman polarizability. The results are in qualitative agreement with known experimental data for some dyes.

Model calculations of coherent Raman spectra under resonance pumping conditions for polyatomic molecules have been carried out. The influence of the resonance background, which originates from the ‘extra’ resonance term of the resonance vibronic transition S0S1, is demonstrated. According to the calculations, the coherent anti-Stokes Raman scattering (CARS) spectra for ωa = 2ωp−ωs are slightly changed when the resonance background is taken into account. The background contribution to the coherent Stokes Raman scattering (CSRS) for ωs = 2ωp−ωa is higher. It is shown that a line reshaping associated with the concentration changes contains information about the relaxation parameters of the molecules, especially about the dephasing Γ.

In the treatment of absorption spectra and Raman excitation profiles of polyatomic molecules, relaxation constants appear as ‘soft’ parameters. It is shown that the spectra of non-linear Raman scattering under resonance pumping conditions are sensitive to variations in the relaxation constants and the number of active vibrations, especially when ‘extra’ resonances are included. This is demonstrated by calculations of coherent Stokes Raman scattering spectra carried out using the density of states model and displaced independent harmonic oscillators. The use of non-linear Raman spectra for checking the reliability of the model descriptions of S0–S1 transitions is proposed.

A polysilicon sealed pad oxide local oxidation process has been investigated. The oxidation mask in this method has a polysilicon frame formed along the perimeter of a conventional mask pattern by a self-aligned technique. This technology is completely free of “bird's beak” formation which is particularly important for increasing the packing density.