Phonon anomalies due to strong-electronic correlations in layered organic metals

We show how the coupling between the phonons and electrons in a strongly correlated metal can result in phonon frequencies that have a nonmonotonic temperature dependence. Dynamical mean-field theory is used to study the Hubbard-Holstein model that describes the kappa-(BEDT-TTF)(2)X [where BEDT-TTF is bis-(ethylenedithia-tetrathiafulvalene)] family of superconducting molecular crystals. The crossover with increasing temperature from a Fermi liquid to a bad metal produces phonon anomalies that are relevant to recent Raman scattering and acoustic experiments.

Er(3+) doped phosphoniobate glasses and planar waveguides: structural and optical properties

Phosphoniobate glasses with composition (mol%) (100-x) NaPO(3)-xNb(2)O(5) ( x varying from 11 to 33) were prepared and characterized by means of thermal analysis, Fourier transform infrared spectroscopy, Raman scattering and (31)P nuclear magnetic resonance. The addition of Nb(2)O(5) to the polyphosphate base glass leads to depolymerization of the metaphosphate structure. Different colors were observed and assigned as indicating the presence of Nb(4+) ions, as confirmed by electron paramagnetic resonance measurements. The color was observed to depend on the glass composition and melting temperature as well. Er(3+) containing samples were also prepared. Strong emission in the 1550 nm region was observed. The Er(3+4)I(15/2) emission quantum efficiency was observed to be 90% and the quenching concentration was observed to be 1.1 mol%( 1.45 x 10(20) ions cm(-3)). Planar waveguides were prepared by Na(+)-K(+)-Ag(+) ion exchange with Er(3+) containing samples. Optical parameters of the waveguides were measured at 632.8, 543.5 and 1550 nm by the prism coupling technique as a function of the ion exchange time and Ag(+) concentration. The optimized planar waveguides show a diffusion depth of 5.9 mu m and one propagating mode at 1550 nm.

Thermodynamic pathway for the formation of SnSe and SnSe2 polycrystalline thin films by selenization of metal precursors

In this work, tin selenide thin films (SnSex) were grown on soda lime glass substrates by selenization of dc magnetron sputtered Sn metallic precursors. Selenization was performed at maximum temperatures in the range 300 °C to 570 °C. The thickness and the composition of the films were analysed using step profilometry and energy dispersive spectroscopy, respectively. The films were structurally and optically investigated by X-ray diffraction, Raman spectroscopy and optical transmittance and reflectance measurements. X-Ray diffraction patterns suggest that for temperatures between 300 °C and 470 °C, the films are composed of the hexagonal-SnSe2 phase. By increasing the temperature, the films selenized at maximum temperatures of 530 °C and 570 °C show orthorhombic-SnSe as the dominant phase with a preferential crystal orientation along the (400) crystallographic plane. Raman scattering analysis allowed the assignment of peaks at 119 cm−1 and 185 cm−1 to the hexagonal-SnSe2 phase and those at 108 cm−1, 130 cm−1 and 150 cm−1 to the orthorhombic-SnSe phase. All samples presented traces of condensed amorphous Se with a characteristic Raman peak located at 255 cm−1. From optical measurements, the estimated band gap energies for hexagonal-SnSe2 were close to 0.9 eV and 1.7 eV for indirect forbidden and direct transitions, respectively. The samples with the dominant orthorhombic-SnSe phase presented estimated band gap energies of 0.95 eV and 1.15 eV for indirect allowed and direct allowed transitions, respectively.

A study of ternary Cu2SnS3 and Cu3SnS4 thin films prepared by sulfurizing stacked metal precursors

Thin films of Cu2SnS3 and Cu3SnS4 were grown by sulfurization of dc magnetron sputtered Sn–Cu metallic precursors in a S2 atmosphere. Different maximum sulfurization temperatures were tested which allowed the study of the Cu2SnS3 phase changes. For a temperature of 350 ◦C the films were composed of tetragonal (I -42m) Cu2SnS3. The films sulfurized at a maximum temperature of 400 ◦C presented a cubic (F-43m) Cu2SnS3 phase. On increasing the temperature up to 520 ◦C, the Sn content of the layer decreased and orthorhombic (Pmn21) Cu3SnS4 was formed. The phase identification and structural analysis were performed using x-ray diffraction (XRD) and electron backscattered diffraction (EBSD) analysis. Raman scattering analysis was also performed and a comparison with XRD and EBSD data allowed the assignment of peaks at 336 and 351 cm−1 for tetragonal Cu2SnS3, 303 and 355 cm−1 for cubic Cu2SnS3, and 318, 348 and 295 cm−1 for the Cu3SnS4 phase. Compositional analysis was done using energy dispersive spectroscopy and induced coupled plasma analysis. Scanning electron microscopy was used to study the morphology of the layers. Transmittance and reflectance measurements permitted the estimation of absorbance and band gap. These ternary compounds present a high absorbance value close to 104 cm−1. The estimated band gap energy was 1.35 eV for tetragonal (I -42m) Cu2SnS3, 0.96 eV for cubic (F-43m) Cu2SnS3 and 1.60 eV for orthorhombic (Pmn21) Cu3SnS4. A hot point probe was used for the determination of semiconductor conductivity type. The results show that all the samples are p-type semiconductors. A four-point probe was used to obtain the resistivity of these samples. The resistivities for tetragonal Cu2SnS3, cubic Cu2SnS3 and orthorhombic (Pmn21) Cu3SnS4 are 4.59 × 10−2 cm, 1.26 × 10−2 cm, 7.40 × 10−4 cm, respectively.

Growth pressure dependence of Cu2ZnSnSe4 properties

In this work, we show a set of growth conditions, for the two step process, with which the growth of CZTSe is successful and reproducible. The properties of the best CTZSe thin films grown by this method were examined by SEM/EDS, XRD, Raman scattering, AFM/EFM, transmittance and reflectance measurements, photoluminescence (PL) measurements and hot point probe. A broad emission band was observed in the photoluminescence spectrum of the CZTSe thin film. The band gap energy was estimated to be around 1.05 eV at room temperature, using the transmittance and reflectance data, and CZTSe samples show p-type conductivity with the hot point probe. The different characterization techniques show that we could grow single phase CZTSe thin films with our optimized process conditions.

Influence of selenization pressure on the growth of Cu2ZnSnSe4 films from stacked metallic layers

Cu2ZnSnSe4 (CZTSe) is a p-type semiconductor with a high absorption coefficient, 104 to 105 cm-1, and is being seen as a possible replacement for Cu(In,Ga)Se2 in thin film solar cells. Yet, there are some fundamental properties of CZTSe that are not well known, one of them is its band gap. In order to resolve its correct value it is necessary to improve the growth conditions to ensure that single phase crystalline thin films are obtained. One of the problems encountered when growing CZTSe is the loss of Sn through evaporation of SnSe. Stoichiometric films are then difficult to obtain and usually there are other phases present. One possible way to overcome this problem is to increase the pressure of growth of CZTSe. This can be done by introducing an atmosphere of an inert gas like Ar or N2. In this work we report the results of morphological, structural and optical studies of the properties of CZTSe thin films grown by selenization of DC magnetron sputtered metallic layers under different Ar pressures. The films are analysed by SEM/EDS, Raman scattering and XRD.

Cu2ZnSnS4 solar cells prepared with sulphurized dc-sputtered stacked metallic precursors

In the present work we report the details of the preparation and characterization results of Cu2ZnSnS4 (CZTS) based solar cells. The CZTS absorber was obtained by sulphurization of dc magnetron sputtered Zn/Sn/Cu precursor layers. The morphology, composition and structure of the absorber layer were studied by scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction and Raman scattering. The majority carrier type was identified via a hot point probe analysis. The hole density, space charge region width and band gap energy were estimated from the external quantum efficiency measurements. A MoS2 layer that formed during the sulphurization process was also identified and analyzed in this work. The solar cells had the following structure: soda lime glass/Mo/CZTS/CdS/i-ZnO/ZnO:Al/Al grid. The best solar cell showed an opencircuit voltage of 345 mV, a short-circuit current density of 4.42 mA/cm2, a fill factor of 44.29% and an efficiency of 0.68% under illumination in simulated standard test conditions: AM 1.5 and 100 mW/cm2.

Growth and characterization of Cu2ZnSn(S,Se)4 thin films for solar cells

Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) with their band gap energies around 1.45 eV and 1.0 eV, respectively, can be used as the absorber layer in thin film solar cells. By using a mixture of both compounds, Cu2ZnSn(S,Se)4 (CZTSSe), a band gap tuning may be possible. The latter material has already shown promising results such as solar cell efficiencies up to 10.1%. In this work, CZTSSe thin films were grown in order to study its structure and to establish the best growth precursors. SEM micrographs reveal an open columnar structure for most samples and EDS composition profiling of the cross sections show different selenium gradients. X-ray diffractograms show different shifts of the kesterite/stannite (1 1 2) peak, which indicate the presence of CZTSSe. From Raman scattering analysis, it was concluded that all samples had traces of CZTS and CZTSSe. The composition of the CZTSSe layer was estimated using X-ray diffraction and Raman scattering and both results were compared. It was concluded that Se diffused more easily in precursors with ternary Cu–Sn–S phases and metallic Zn than in precursors with ZnS and/or CZTS already formed. It was also showed that a combination of X-ray diffraction and Raman scattering can be used to estimate the ratio of S per Se in CZTSSe samples.

Growth of the [110] oriented TiO2 nanorods on ITO substrates by sputtering technique for dye-sensitized solar cells

TiO2 films have been deposited on ITO substrates by dc reactive magnetron sputtering technique. It has been found that the sputtering pressure is a very important parameter for the structure of the deposited TiO2 films. When the pressure is lower than 1 Pa, the deposited has a dense structure and shows a preferred orientation along the [101] direction. However, the nanorod structure has been obtained as the sputtering pressure is higher than 1 Pa. These nanorods structure TiO2 film shows a preferred orientation along the [110] direction. The x-ray diffraction and the Raman scattering measurements show both the dense and the nanostructure TiO2 films have only an anatase phase, no other phase has been obtained. The results of the SEM show that these TiO2 nanorods are perpendicular to the ITO substrate. The TEM measurement shows that the nanorods have a very rough surface. The dye-sensitized solar cells (DSSCs) have been assembled using these TiO2 nanorod films prepared at different sputtering pressures as photoelectrode. And the effect of the sputtering pressure on the properties of the photoelectric conversion of the DSSCs has been studied.

Preparation and characterization of dye-sensitized TiO2 nanorod solar cells

TiO2 nanorodswere prepared by DC reactive magnetron sputtering technique and applied to dye-sensitized solar cells (DSSCs). The length of the TiO2 nanorods was varied from 1 μm to 6 μm. The scanning electronmicroscopy images showthat the nanorods are perpendicular to the substrate. Both the X-ray diffraction patterns and Raman scattering results show that the nanorods have an anatase phase; no other phase has been observed. (101) and the (220) diffraction peaks have been observed for the TiO2 nanorods. The (101) diffraction peak intensity remained constant despite the increase of nanorod length, while the intensity of the (220) diffraction peak increased almost linearly with the nanorod length. These nanorods were used as the working electrodes in DSSCs and the effect of the nanorod length on the conversion efficiency has been studied. An optimumphotoelectric conversion efficiency of 4.8% has been achieved for 4 μm length nanorods.

Multi-stacks of epitaxial GeSn self-assembled dots in Si: Structural analysis

We report on the growth and structural and morphologic characterization of stacked layers of self-assembled GeSn dots grown on Si (100) substrates by molecular beam epitaxy at low substrate temperature T = 350 °C. Samples consist of layers (from 1 up to 10) of Ge0.96Sn0.04 self-assembled dots separated by Si spacer layers, 10 nm thick. Their structural analysis was performed based on transmission electron microscopy, atomic force microscopy and Raman scattering. We found that up to 4 stacks of dots could be grown with good dot layer homogeneity, making the GeSn dots interesting candidates for optoelectronic device applications.

A systematic study of the structural and magnetic properties of Mn-, Co-, and Ni-Doped Colloidal Fe3O4 nanoparticles

A series of colloidal MxFe3-xO4 (M = Mn, Co, Ni; x = 0–1) nanoparticles with diameters ranging from 6.8 to 11.6 nm was synthesized by hydrothermal reaction in aqueous medium at low temperature (200 °C). Energy-dispersive X-ray microa-nalysis and inductively coupled plasma spectrometry confirms that the actual elemental compositions agree well with the nominal ones. The structural properties of obtained nanoparticles were investigated by using powder X-ray diffraction, Raman scattering, Mössbauer spectroscopy, and electron microscopy. The results demonstrate that our synthesis technique leads to the formation of chemically uniform single-phase solid solution nanoparticles with cubic spinel structure, confirming the intrinsic doping. Magnetic studies showed that, in comparison to Fe3O4, the saturation magnetization of MxFe3-xO4 (M = Mn, Ni) decreases with increasing dopant concentration, while Co-doped samples showed similar saturation magnetizations. On other hand, whereas Mn- and Ni-doped nanoparticles exhibits superparamagnetic behavior at room temperature, ferromagnetism emerges for CoxFe3-xO4 nanoparticles, which can be tuned by the level of Co doping.

The effect of substrate on high-temperature annealing of GaN epilayers: Si versus sapphire

We have studied the effects of rapid thermal annealing at 1300¿°C on GaN epilayers grown on AlN buffered Si(111) and on sapphire substrates. After annealing, the epilayers grown on Si display visible alterations with craterlike morphology scattered over the surface. The annealed GaN/Si layers were characterized by a range of experimental techniques: scanning electron microscopy, optical confocal imaging, energy dispersive x-ray microanalysis, Raman scattering, and cathodoluminescence. A substantial Si migration to the GaN epilayer was observed in the crater regions, where decomposition of GaN and formation of Si3N4 crystallites as well as metallic Ga droplets and Si nanocrystals have occurred. The average diameter of the Si nanocrystals was estimated from Raman scattering to be around 3¿nm. Such annealing effects, which are not observed in GaN grown on sapphire, are a significant issue for applications of GaN grown on Si(111) substrates when subsequent high-temperature processing is required.

Analysis of S-rich CuIn(S,Se)2 layers for photovoltaic applications: Influence of the sulfurization temperature on the crystalline properties of electrodeposited and sulfurized CuInSe2 precursors

This paper reports the microstructural analysis of S-rich CuIn(S,Se)2 layers produced by electrodeposition of CuInSe2 precursors and annealing under sulfurizing conditions as a function of the temperature of sulfurization. The characterization of the layers by Raman scattering, scanning electron microscopy, Auger electron spectroscopy, and XRD techniques has allowed observation of the strong dependence of the crystalline quality of these layers on the sulfurization temperature: Higher sulfurization temperatures lead to films with improved crystallinity, larger average grain size, and lower density of structural defects. However, it also favors the formation of a thicker MoS2 interphase layer between the CuInS2 absorber layer and the Mo back contact. Decreasing the temperature of sulfurization leads to a significant decrease in the thickness of this intermediate layer and is also accompanied by significant changes in the composition of the interface region between the absorber and the MoS2 layer, which becomes Cu rich. The characterization of devices fabricated with these absorbers corroborates the significant impact of all these features on device parameters as the open circuit voltage and fill factor that determine the efficiency of the solar cells.

Microstructure and secondary phases in coevaporated CuInS2 films: Dependence on growth temperature and chemical composition

The microstructure of CuInS2-(CIS2) polycrystalline films deposited onto Mo-coated glass has been analyzed by Raman scattering, Auger electron spectroscopy (AES), transmission electron microscopy, and x-ray diffraction techniques. Samples were obtained by a coevaporation procedure that allows different Cu-to-In composition ratios (from Cu-rich to Cu-poor films). Films were grown at different temperatures between 370 and 520-°C. The combination of micro-Raman and AES techniques onto Ar+-sputtered samples has allowed us to identify the main secondary phases from Cu-poor films such as CuIn5S8 (at the central region of the layer) and MoS2 (at the CIS2/Mo interface). For Cu-rich films, secondary phases are CuS at the surface of as-grown layers and MoS2 at the CIS2/Mo interface. The lower intensity of the MoS2 modes from the Raman spectra measured at these samples suggests excess Cu to inhibit MoS2 interface formation. Decreasing the temperature of deposition to 420-°C leads to an inhibition in observing these secondary phases. This inhibition is also accompanied by a significant broadening and blueshift of the main A1 Raman mode from CIS2, as well as by an increase in the contribution of an additional mode at about 305 cm-1. The experimental data suggest that these effects are related to a decrease in structural quality of the CIS2 films obtained under low-temperature deposition conditions, which are likely connected to the inhibition in the measured spectra of secondary-phase vibrational modes.