Spectroscopy

Optical absorption and photoluminescence (PL) measurements were performed on a series of hydrogenated silicon-nitrogen thin films deposited by plasma enhanced chemical vapor deposition covering a wide compositional range with the aim to find how absorption and emission processes are correlated. Our analysis indicates that the observed broadening of the PL spectra is linked with the existence of band tail states due to topological disorder, as confirmed by the spectral dependence of the PL decay. Moreover, nitrogen alloying reduces the temperature quenching of luminescence and increases the radiative recombination rate. Such phenomena are ascribed to the formation of wide tails in the density of states of N-rich samples and, in turn, to a lower carrier diffusion.

GaN films were grown by pulsed laser deposition (PLD on different crystalline substrates using a KrF excimer laser to ablate a hexagonal phase GaN target in a reactive atmosphere of ammonia. Films with small homogeneously distributed granular structures over the entire sample surface were obtained, The microstructure and surface morphology of the deposited layers were characterized by X-ray diffraction (XRD, atomic force microscopy (AFM) and Raman spectroscopy (RS). XRD reveals that the structure of the GaN layer is predominantly wurtzite. AFM images reveal that all the deposited layers have a relatively smooth surface, while RS confirmed the predominant presence of hexagonal GaN with a high polycrystalline character. Analysis of the results obtained for samples grown under different conditions, such as the substrate temperatures in the growth chamber as well as different substrates used, helps to define better the experimental conditions of the growth process of PLD-GaN films. (C) 2000 Elsevier Science S.A. All rights reserved.

The polymorphism of WO, powder samples, resulting from mild mechanical treatments and from temperature changes between 30 K and room temperature, has been investigated by using Raman spectroscopy and X-ray diffraction. A transition from the monoclinic (I) gamma-phase to the triclinic delta-phase after moderate mechanical treatments has been observed for untreated powder, just what happens when the same samples are rapidly cooled to low-temperature. Evidences of the low temperature monoclinic (II) polar epsilon-phase have been found at room temperature in samples after a stronger milling treatment. The sequence of the low-temperature phase transitions appears to be strongly dependent on the mechanical history of the powders. A new low-temperature N-phase has been observed below about 200 K in different samples: it is the main phase in commercial untreated powders, having the monoclinic (I) gamma-phase at room temperature, but constitutes only a small fraction in moderately treated powders, having the triclinic delta-phase at room temperature. (C) 1999 Academic Press.

The luminescent properties of GaN thin films grown by pulsed laser deposition have been studied to understand the nature of the luminescent centres and the recombination dynamics. The films were grown on heated sapphire substrates using KrF excimer laser ablation of GaN in a reactive atmosphere of nitrogen. At low temperature the continuous wave (CW) blue luminescence of the samples grown shows two sharp lines attributed to excitonic recombination localized at extended defects. An analysis of the temperature dependence of photoluminescence (PL) lifetimes assesses the relative contributions of radiative and non-radiative recombination in the centres responsible for these emissions. The measurement of room temperature nanosecond radiative lifetimes for these lines supports the excitonic attribution of the luminescence. (C) 1999 Elsevier Science S.A. All rights reserved.

Photoluminescence measurements in all-porous-silicon optical microcavities (PSM) are reported over a wide temperature range. Both continuous wave and time resolved measurements have been performed. The microcavity is formed by an all porous silicon Fabry-Perot filter made by two distributed Bragg reflectors separated by a lambda-thick PS cavity layer. The luminescence properties of PSM are changed with respect to those of PS: a temperature independent narrowing in the emission line shape, a different temperature dependence of the emission intensity, and a fractional shortening of the luminescence decay time over the 50-300 K temperature interval are achieved. The PSM luminescence properties are explained by the spatial redistribution of the spontaneous emission, by an effective refractive index probed by the photon mode confined in the cavity layer and by the coupling between the singlet exciton state and the photon mode confined in the cavity layer. The saturation of the absorption of the distributed Bragg reflector is also addressed. (C) 1999 American Institute of Physics. [S0021-8979(99)04503-X].

The relationship between structural transformations and colour centres creation is discussed for deeply coloured hydrogen tungsten bronzes and for pure WO(3) powders, acquiring less intense colour after mechanical treatments of variable duration. A comparative study on coloration is made also for mixed compounds (1-x)WO(3-y)center dot xReO(2), where an evidence of a resonance effect for a particular Raman band at 970 cm(-1), attributed to the color centres, is observed. Besides, it is found that even moderate milling treatments result in a quite different structural evolution of tungsten trioxide upon cooling.

An extensive investigation of the temperature dependence of Raman spectra has been carried out on WO3 powders from room temperature to 800 degrees C. In particular the orthorhombic-to-tetragonal phase transition occurring at about 740 degrees C has been studied for the first time. The Raman active mode at 710 cm(-1) of the orthorhombic phase disappears from the spectrum at temperature below the phase transition point and the Raman activity in the tetragonal phase results very low. A comparative study of hydrogenated tungsten bronzes HxWO3 (x less than or equal to 0.23), where the same transition sequence is driven by an increase of the proton concentration from x = 0 to 0.23, reveals similar behaviour of the high frequency Raman active modes, which are better resolved due to the weaker anharmonic interactions. (C) 1999 Elsevier Science B.V. All rights reserved.

Radiative recombinations in amorphous Si1-xNx:H-based nanometric multilayer structures have been studied by stationary and time-resolved photoluminescence measurements at room temperature. Such structures show higher photoluminescence efficiency than reference bulk amorphous silicon nitride films. The enhancement is attributed to the localization of electron-hole Fairs induced by the multilayer structure. Fast monomolecular recombination processes are shown, with lifetimes dependent on the emission energy. Analysis of photoluminescence and absorption spectra shows a low density of defects and no intermixing at the multilayer heterointerfaces. Such features open the potential for the investigated structures to function as fast optoelectronic devices. [S0163-1829(99)13635-X].

The precipitation of germanium nanocrystals in amorphous carbon-germanium films allows for the development of innovative devices, but the accurate control of both size and size distribution of Ge quantum dots in these matrices still constitutes a challenging step. In this paper, both the structure and morphology of amorphous carbon-germanium films (a-Ge1-xCx), deposited by d.c. magnetron sputtering onto silicon substrates and annealed in vacuum at temperatures up to 550 degrees C, are investigated by Raman spectroscopy and scanning electron microscopy. The main features of Raman spectra obtained from carbon-rich films (x > 0.43) are the D and G bands, characteristic of graphitic carbon films. The ratio between the intensities of the bands, I-D/I-G, increases with the annealing temperature, suggesting a progressive increase of the graphitic domains within the films. Raman spectra obtained in the low frequency region from both as-deposited and annealed germanium-rich films (x < 0.43) show broad bands associated with transverse acoustic and transverse optic Ge-Ge modes. Cre-Ge optic modes merge up into a well-shaped peak at 300 cm(-1) in the germanium-richest sample, and underwent annealing treatment at 550 degrees C, thus indicating the precipitation of crystalline Ge. Scanning electron microscopy analysis shows an apparently uniform nucleation of Ge crystallites at the sample surface. Microprobe Raman scattering results suggest the formation of a nearly homogeneous distribution of Ge nanocrystals in germanium-rich films annealed at 550 degrees C. (C) 1999 Elsevier Science S.A. All rights reserved.

Room-temperature resonant second harmonic generation is demonstrated in a ZnSe bulk microcavity with Si3N4/SiO2 Bragg reflectors. The resonance occurs at the second harmonic wavelength in the blue-green spectral region and yields an enhancement of one order of magnitude in the second harmonic process. Tunability of the resonant effect between 480 and 500 nm is achieved by varying the angle of incidence. (C) 1999 American Institute of Physics. [S0003-6951(99)01514-4].

Continuous-wave photoluminescence (PL) and time-resolved photoluminescence of gallium nitride layers grown by pulsed laser deposition are compared. The temperature dependence of the photoluminescence decay time and the PL-integrated intensity allows a determination of radiative and nonradiative time constants of GaN. We find that luminescence peaks centered at 3.360 and 3.305 eV at low temperature can be attributed to recombination of excitons localized at extended defects. The photoluminescence radiative lifetime at room temperature is on the order of tens of ns. (C) 1998 American Institute of Physics. [S0003-6951(98)00549-X].

Optical and photoluminescence characterizations were performed on nanometric multilayer structures based on amorphous silicon nitrogen alloys. Evidences are shown that the radiative efficiency of multilayers increases with respect to single-layer structures. This is ascribed to a strong electron-hole pair localization and a low heterointerfaces defect density. Time-resolved photoluminescence measurements yield fast recombination with an energy-dependent lifetime due to hopping processes. Finally, the performance of an electroluminescent device based on multilayers is presented. (C) 1999 Elsevier Science B.V. All rights reserved.

Pure ground tungsten trioxide WO3 and (1-x) WO3-y. xReO(2) mixtures were studied by x-ray absorption spectroscopy, x-ray powder diffraction and Raman spectroscopy in comparison with hydrogen bronzes HxWO3 and hydrogenated calcium tungstate CaWO4:H. It was found that a grinding of pure WO3 leads to a decrease of the crystallites size and a development of the bluish coloration. The color change was found to be reversible under moderate heat treatment or after storage in oxidizing atmosphere and is attributed to the reduced W5+ ions, located at the surface of freshly ground powder. The (1-x) WO3-y. xReO(2) mixtures were found to be composed of monoclinic/orthorhombic WO3 and orthorhombic ReO2 phases with a grain boundary containing reduced W5.7+ ions which are mainly responsible for the compound color at low rhenium ion concentrations. In both cases, the W(6-z+) (0<z less than or equal to 1) color centers are responsible for strong optical absorption resulting in the dramatic decrease of the total Raman intensity. The structural models of free surface in pure ground WO3 and bulk WO3/ReO2 intragrain boundary in (1-x) WO3-y. xReO(2) mixtures are proposed and discussed. (C) 1998 American Institute of Physics. [S0021-8979(98)07322-8].