M. Varela

Optical properties of co-evaporated CuInSe2 thin films

Thin films of CuInSe2 within a wide composition range around stoichiometry have been deposited by Cu, In and Se co-evaporation. Their optical properties in the near-infrared have been studied in relation to the composition and the deposition process parameters. The refractive index and the absorption coefficient below the absorption edge, depend on the Cu-In percentages ratio and show a minimum at stoichiometry. Moreover, the refractive index increases with the deposition temperature. Beyond the absorption edge the absorption is due to an allowed direct transition and a forbidden direct transition with gap energies around 1.0 and 1.2 eV, respectively. The values of the optical gaps depend on the Cu-In percentages ratio and show a minimum at stoichiometry. These dependences have been correlated with the variations of the crystalline lattice constant with the Cu-In ratio.

Crystalline properties of co-evaporated CuInSe2 thin films

Abstract CuInSe 2 thin films were deposited onto glass substrates by the co-evaporation of copper, indium and selenium. Scanning electron microscopy and X-ray diffractometry were used to study the morphology and crystalline properties of these films with respect to their composition and the parameters of the deposition process. The crystalline phases, the crystallite orientation and the grain size depend on the substrate temperature and the rate of deposition of the films. However, the lattice constant depends on the composition and shows a maximum for stoichiometric films.

Optical properties of indium doped CdS thin films

Abstract Thin films of polycrystalline In doped CdS with a wide range of indium concentrations ( 10 17 −10 21 cm −3 ) have been deposited by indium and CdS coevaporation. Their optical properties in the visible and near-infrared region have been determined by optical transmission measurements and correlated to the indium concentration and substrate temperature. The refractive index presents a maximum at an indium concentration in the range 10 19 −10 20 cm −3 The energy dependence of the absorption edge on indium concentration is due to the Burnstein-Moss shift. The width of the exponential tail in the absorption coefficient increases with indium concentration and softens the abrupt abrorption edge of the CdS thin films.These dependences have been correlated with the incorporated quantity of indium into the CdS crystallites.

Electrical conductivity of polycrystalline CuInSe2 thin films

CuInSe2 films with different compositions around stoichiometry have been deposited by coevaporation of Cu, In and Se. In p-type films the electrical conductivity is largely dependent on the Cu-In percentage ratio and increases from 10-6 to 102 S cm-1 when the ratio increases from 0.66 to 1.58. The thermal activation energy is also correlated to this ratio, and decreases from 0.36 to 0.009 eV. The results are analogous to those obtained in polycrystalline silicon and are qualitatively explained by the grain boundary carrier trapping theory.

Superconducting YBa2Cu3O7 films deposited on Si (100) substrates with CeO2 buffer layers by laser ablation

Abstract Superconducting YBa 2 Cu 3 O 7 (YBCO) thin films have been deposited on Si (100) substrates with CeO 2 buffer layers by laser ablation. The films were characterized by X-ray diffraction (XRD), secondary ion mass spectrometry (SIMS) and four-point electrical resistivity. Secondary ion mass spectrometry results indicate low interdiffusion between Si, CeO 2 and YBCO. The best YBCO films are textured with c -axis perpendicular to the substrate and their resistance exhibits a normal state metallic behavior with zero resistance around 84 K. The crystalline and electrical properties of YBCO films were related with the substrate temperature. A minimum substrate temperature is necessary to deposit good superconducting YBCO films. Moreover, there is a maximu temperature to obtain c -orientation, although the electrical properties are not degraded yet.

Electrical transport properties of polycrystalline CuInSe2 films

Abstract Electrical conductivity and Hall effect measurements were made on CuInSe 2 films prepared by coevaporation. The films were characterized as grown and after annealing in a 90% N 2 + 10% H 2 atmosphere. The hole density was in the interval 10 13 –10 19 cm −3 . The conductivity activation energy ranged from 0.03 to 0.5 eV in the range 300–400 K. The hole density decreased and the mobility increased with the annealing process. Grain boundary trapping models were considered in the analysis of the results. Compensation effects, partial or total depletion of the grains, and variations in the compensation ratio and the trap density with annealing were observed.

YBa2Cu3O,7 − x superconducting thin films by sequential evaporation on alumina substrates

Abstract Superconducting Y-Ba-Cu-O thin films have been formed on alumina substrates by sequential evaporation of ten Cu Y 2 O 3 BaF 2 layered structures and subsequent heat treatments. The best film showed a superconducting onset temperature of 92 K and a zero resistance at 50 K. The film properties were studied using electrical measurements, scanning electron microscopy, x-ray diffraction and secondary ion mass spectrometry.

Crystalline properties of In-Doped CdS thin films

Abstract Polycrystalline indium-doped CdS thin films have been deposited onto glass by vacuum evaporation from two crucibles of CdS and indium. The influence of the indium concentration and the substrate temperature on the morphology and the crystalline properties of the films has been studied. The films crystallize in the hexagonal phase and show a columnar structure perpendicular to the substrate. The grain size increases with the substrate temperature and with the indium concentration. The interplanar spacing between the (002)H planes also increases with the indium concentration, but, when it is higher than 7×1019 cm-3, indium agglomerates appear in the films. A decrease of the grain size and (002)H interplanar spacing results with further increases in indium concentration.

Laser Ablation Deposited YBa 2 Cu 3 O x , Thin Films on YSZ/Si(100)

Superconducting YBa 2 Cu 3 O x (YBCO) thin films have been deposited on Si(100) substrates with yttria-stabilized zirconia (YSZ) buffer layers by laser ablation. Buffers have been obtained by laser ablation as well. The films have been characterized by scanning electron microscopy, x-ray diffractometry, secondary ion mass spectrometry, and four-contact electrical resistivity measurements. Secondary ion mass spectrometry results indicate very low interdiffusion between Si, YSZ and YBCO. The best YBCO films are textured with c axis perpendicular to the substrate and their resistance shows a normal state metallic behavior with zero resistance at temperatures higher than 80 K. The properties of YBCO films have been related with the substrate temperature and oxygen partial pressure during deposition.

Superconducting Y-Ba-Cu-O thin films on silicon and Al2O3 substrates

Abstract YBaCuO superconducting thin films have been prepared by sequential evaporation of copper, Y 2 O 3 and BaF 2 layers on Si[100] and polycrystalline Al 2 O 3 substrates. Such sequences were evaporated 1, 10 or 30 times on Si[100] with a 0.5 μm ZrO 2 buffer layer. The effects of different buffer layers were studied for Al 2 O 3 substrates by means of the deposition of ZrO 2 , copper, silver and BaTiO 3 layers with thicknesses of 20, 100 and 500 nm. The film properties were studied using electrical measurements, scanning electron microscopy, X-ray diffractometry, energy-dispersive X-ray spectrometry and secondary ion mass spectrometry. The best properties were obtained for the obtained on ZrO 2 /Si[100] with a single evaporation sequence. The studies of the different buffers reveal a strong dependence of the film properties on the nature and thickness of the buffer, with the best results for films deposited on a silver layer.