G. Contreras-Puente

Raman studies in CdS thin films in the evolution from cubic TO hexagonal phase

Abstract 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.

Graphite-like bonding induced in hydrogenated amorphous carbon films with high nitrogen content

In the present work we analyze the effect of nitrogen incorporation on the structural properties of hydrogenated amorphous carbon films. Photoluminescence, Raman and optical studies indicate that the high nitrogen content induces the formation of sixfold fused rings clusters (graphite layers).

Physical properties of CdTe:Cu films grown at low temperature by pulsed laser deposition

CdTe:Cu films were grown by pulsed laser deposition on Corning glass slides at a substrate temperature of 300u2009°C. The thin films were grown using CdTe and Cu2Te powders, varying the Cu2Te concentration from 3 to 10u2009wt. %. The structural, compositional, optical, and electrical properties were analyzed as a function of the nominal copper concentration. X-ray diffraction shows that films have CdTe cubic phase. The compositional analysis indicates that CdTe:Cu films grown with lower Cu content have Te excess, on the other hand, films with higher Cu content have Te deficiencies. The electrical measurements showed that CdTe:Cu films grown with low Cu content present lowest resistivity.

Photoluminescence Studies of Semiconducting Polycrystalline CdTe Films

We report the first systematic measurements of the temperature dependence of the photoluminescence (PL) in the range of 10–300 K in CdTe. The experiments were carried out on semiconducting CdTe films of high quality grown by a modified close-spaced vapour transport (CSVT) technique. Several luminescence bands were observed, one around 1.4 eV showing a temperature-independent behaviour, and another band located around 1.52 eV, at 300 K, showing a strong temperature dependence. The excitonic origin of this band at low temperatures has been confirmed by the dependence of the PL intensity on the excitation intensity. A band-to-band luminescent recombination component at high temperatures is also observed. This study allows us to elucidate the nature and basic physical properties of the bound exciton as well as the temperature dependence of the band gap.

Optical phonons in ZnxCd1−xSe thin films

Abstract 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.

Photoluminescence Study of Gallium Nitride Thin Films Obtained by Infrared Close Space Vapor Transport

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.

Thin film cadmium telluride charged particle sensors for large area neutron detectors

Thin film semiconductor neutron detectors are an attractive candidate to replace 3He neutron detectors, due to the possibility of low cost manufacturing and the potential for large areas. Polycrystalline CdTe is found to be an excellent material for thin film charged particle detectors—an integral component of a thin film neutron detector. The devices presented here are characterized in terms of their response to alpha and gamma radiation. Individual alpha particles are detected with an intrinsic efficiency of >80%, while the devices are largely insensitive to gamma rays, which is desirable so that the detector does not give false positive counts from gamma rays. The capacitance-voltage behavior of the devices is studied and correlated to the response due to alpha radiation. When coupled with a boron-based neutron converting material, the CdTe detectors are capable of detecting thermal neutrons.

Raman scattering by phonons in heavily doped semiconductors

We present in this work experimental results of the Full Width at Half Maximum (FWHM) and frequency shift of the Raman phonon in heavily doped Ge:P, as a function of the temperature. Phonon confinement is found due to the disorder induced by the impurities in the crystal.

Photovoltaic modules processing of CdS/CdTe by CSVT in 40 cm 2

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. Two independent series of photovoltaic modules have been processed: in the first series the CdS was processed by the technique of Chemical Bath Deposition (CBD), the CdTe was deposited by the conventional technique of Closed Spaced Vapor Transport and the device consists of a photovoltaic module constituted of 9 independent solar cells with approximate 3 cm2 contact area. Of the total of these 9 solar cells, 6 have almost the same l-V characteristics in dark as well as in illumination AM1.5 (100 mW/cm2), with an average efficiency of 5.9 ± 0.3%, and the three remaining cells have approximately 3% of photovoltaic efficiency. The maximum power of each solar cell is of 18.5 mW. The power obtained by the prototype of photovoltaic module is approximately of 105 mW or 0.105 W in 18 cm2, which could scale up to 58 W/m2. The second series consists of photovoltaic modules, totally processed in a new CSVT system (completely designed, machined and assembled in our workshop and laboratory) including 9 interconnected solar cells to each another, with 3 cm2 of contact area, approximately.

Effects of Magnetic Field Configuration on Rf Sputtering for CdS/CdTe Solar Cells

We report studies of solar cells prepared by rf planar magnetron sputtering in which the films were deposited using magnetic field structures ranging from approximately balanced to strongly unbalanced in the “type II” configuration. For films grown with the unbalanced configurations, we find much stronger photoluminescence and much better cell performance than for the balanced configuration. The CdTe films show differences in electrical performance depending on magnetic field as well. These effects are interpreted as arising from the enhanced electron and ion bombardment of the film growth interface for the unbalanced magnetrons. Using two unbalanced magnetrons we have fabricated an all-rf-sputtered cell with NREL-verified efficiency of 11.66% at air mass 1.5 illumination.