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Photoluminescence (PL) studies in GaN thin films grown by infrared close space vapor transport (CSVT-IR) in vacuum are presented in this work. The growth of GaN thin films was done on a variety of substrates like silicon, sapphire and fused silica. Room temperature PL spectra of all the GaN films show near band-edge emission (NBE) and a broad blue and green luminescence (BL, GL), which can be seen with the naked eye in a bright room. The sample grown by infrared CSVT on the silicon substrate shows several emission peaks from 2.4 to 3.22 eV with a pronounced red shift with respect to the band gap energy. The sample grown on sapphire shows strong and broad ultraviolet emission peaks (UVL) centered at 3.19 eV and it exhibits a red shift of NBE. The PL spectrum of GaN films deposited on fused silica exhibited a unique and strong blue-green emission peak centered at 2.38 eV. The presence of yellow and green luminescence in all samples is related to native defects in the structure such as dislocations in GaN and/or the presence of amorphous phases. We analyze the material quality that can be obtained by CSVT-IR in vacuum, which is a high yield technique with simple equipment set-up, in terms of the PL results obtained in each case.

Structural, morphological and optical properties of CdS films, deposited by the pulsed laser ablation technique on SnO2:F/glass substrates, were studied as a function of time deposition. Up to our knowledge the properties of these films grown onto conducting polycrystalline substrates have not been reported before. The photoluminescence measurements showed the existence of an exciton peak at room temperature, as an indication of good crystalline quality of the layers. A thermal treatment was applied to the samples in a 100 mT Ar atmosphere at 400 °C for 30 min after the deposition of a CdCl2 layer on them. Films characterization was carried out through X-ray diffraction, photoluminescence and atomic force microscopy measurements. The structural, optical and morphological properties as a function of growth parameters are presented and discussed. Thermal treatment makes uniform the grain size in the samples, with a tendency to increase the grain size average. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper, we present a comparative study of photoluminescence and its relation to the structural properties of cadmium sulphide (CdS) thin films grown by chemical bath deposition (CBD), close spaced vapour transport, laser ablation (LA) and sputtering. Taking into account that the physical properties of CdS thin films depend upon the growth technique and the optimization of the deposition conditions for each technique, we show that the best crystal perfection occurs for LA-CdS films since the main photoluminescence peak at low temperature is due to bound excitons (2.53 eV). As expected, the films with the worst crystalline quality are those grown by CBD, ascertained by the photoluminescence band around 1.72 eV due to sulfur vacancies ('red band') without the corresponding exciton band. The photoluminescence results are correlated to x-ray diffraction measurements that confirm the above results.

CdS polycrystalline thin films prepared onto glass substrates by the chemical bath deposition method at 80°C showed cubic crystalline structure (β-CdS). Upon thermal annealing in Ar + S2 flux at different temperatures in the range 200–510°C, the crystalline structure evolution of layers from the cubic modification (as-grown sample) to hexagonal phase (annealed at highest temperature samples) has been observed. At around 300°C the critical point of the phase transition has been experimentally determined to occur. Raman spectroscopy measurements were carried out at room temperature for as-grown and annealed layers. The A1(LO) + E1(LO) modes at 305 cm−1 and replicas appear for as-grown and annealed samples. The E1(TO) and the A1(TO) modes were also observed. A classic simple approach is proposed in order to explain the presence of the TO modes.

A calculation is presented on the evolution of longitudinal and transverse optical phonons as a function of the concentration parameter x in the system ZnxCd1−xSe. The calculation is based in an isodisplacement model of the atoms which include interactions to first and second neighbors. The model is fitted to Raman measurements carried out on thin-film samples grown by Close Spaced Vapor Transport (CSVT). Comparison to the experimental data allows to conclude that the x-dependence of these optical modes corresponds to an intermediate-mode behavior.

Raman scattering (RS) is a very important experimental tool to characterize the optical modes and another elementary excitations of materials. Among other issues it can determine for example the degree of crystalline quality and point defects like local modes. Therefore GaN - thin films and related compounds for photovoltaic purposes and as processed by several systems have been measured by this technique. The films were grown by Molecular Beam Epitaxy (MBE), Close Spaced Vapor Transport (CSVT) and Laser Ablation (LA) with the use of optimal growth parameters and substrates. Gallium nitride crystallizes in the wurtzite structure with 4 atoms in the unit cell and presents 7 allowed Raman modes of A 1 , E 1 , and E 2 symmetries. In this work we present and discussed our Raman experiments where particularly the detection of the E 2 and A 1 modes are illustrated in these nitride semiconductor compounds.

We present in this work the fabrication of photovoltaic modules of the CdS/CdTe type, the first ones processed in Mexico in large area (40 cm2 and contact area of 3 cm2); these photovoltaic modules are constituted of the n-type cadmium sulfide (CdS) semiconductor, as the window material, and the p-type cadmium telluride (CdTe) semiconductor as the absorbing material. The back contacts were deposited by vacuum thermal evaporation. The electrical characteristics were determined by the measurement of current on voltage I vs. V.

In this paper, a comparative study of the properties of CdS thin films grown by laser ablation (LA), close space vapour transport (CSVT), sputtering (Sp) and chemical bath deposition (CBD) is presented, taking into account that the physical properties of CdS thin films depend upon the growth technique, and the optimisation of the growth conditions for each technique. We summarize and discuss our results on growth kinetics, electrical, optical, structural and morphological characteristics, in order to have a more complete evaluation of those techniques for making CdS thin films to be applied to solar cells.

A calibrated standard resistance (R/sub s/)is indispensable in order to perform reliable temperature measurements with a Standard Platinum Resistance Thermometer (SPRT). Since in many instances the calibration of such R/sub s/ is difficult to achieve, a novel method for such calibration using two SPRTs, a bridge and a bath is proposed.

We present calculations of the compositional dependence of LO and TO phonon frequencies for the cubic and hexagonal structures of the Zn x Cd 1− x Se ternary semiconducting system. The theory is based on the modified random-element displacement model (MREI). The zone center optical phonons show a variety of patterns for their mode behavior.

"In this work we present the results of the monitoring and statistics of a grid connected photovoltaic (PV) system located on the roof of the building occupied by the “Instituto de Ciencia y Tecnolog´ıa del Distrito Federal” (ICyTDF), at the historical center of Mexico-City (DF). The on-grid connected photovoltaic system has 6150Watts peak (Wp). This system ensures a daily mean reduction of 30 kW.h in the consumption of electric energy at the ICyTDF facilities; which results in a significant saving of fossil fuels and avoiding the emission of greenhouse gases. This system can generate an annual average of 10.8 MW.h of electrical energy, equivalent to burning 0.87 tonnes of oil which would lead to the emission of approximately 2.1 tonne/year of CO2 into the atmosphere. The ecological impact of the photovoltaic system installed in the ICyTDF is equivalent to a forest area with more than 700 adult trees (7 hectares of forest). We also report the analysis of the PV-performance in real time of several commercial solar modules, manufactured with different materials and technologies, these results may give an indication of the comparative performance of the technologies under the environmental conditions of M´exico City. Four different solar modules made of monocrystalline silicon, polycrystalline silicon, amorphous silicon and CdS/CdTe technologies were tested. The outcome of this study is that a better performance was noted for modules made of amorphous silicon technology under temperature variations typical of Mexico City’s environmental conditions."