R. Ramírez-Bon

Optical properties of PbS thin films chemically deposited at different temperatures

Abstract PbS thin films were deposited on glass slide substrates using the chemical bath deposition technique. The films were obtained in a reaction bath at temperatures of 10, 15, 20, 25 and 30 °C. The structure and surface morphology of the films were studied by X-ray diffraction and by atomic force microscopy measurements. The optical properties were determined from spectroscopy measurements of ellipsometry, transmission and reflection, in the energy range of 240–840 nm. In order to analyze the ellipsometry measurements, two models for the dielectric function of the films were considered, the Bruggemans effective medium approximation and the Lorentz oscillator expression. From this analysis, the complex dielectric function e ( E )= e 1 ( E )+ ie 2 ( E ), thickness, roughness and void fraction of the films were examined as a function of temperature deposition. With the model obtained in the ellipsometry analysis, the optical spectra of reflection and transmission were calculated and compared with the measured spectra finding a good agreement.

Impedance spectroscopy studies on SnO2 films prepared by the sol–gel process

Abstract The stannic chloride was used as the starting material to prepare undoped SnO 2 films. Films with different thickness were made by multiple applications of the precursor solutions using a dip coating apparatus. The as-prepared and heat treated, in air, films were analyzed using impedance spectroscopy as a function of the thickness or number of layers constituting the films. The impedance spectra of the films represented in the complex plane showed two well-defined but partially convoluted semicircles. The impedance spectra were analyzed using the brick layer model, generally employed for polycrystalline materials. X-ray diffraction, optical transmission and reflection as well as atomic force microscopy measurements were also used in the characterization of the films. The results show that the as-deposited SnO 2 films are amorphous and that the annealing treatments produce their crystallization and densification. The impedance spectra provided quantitative information about the densification process, which is in agreement with the qualitative information obtained by the atomic force microscopy measurements.

Impact of Gate Dielectric in Carrier Mobility in Low Temperature Chalcogenide Thin Film Transistors for Flexible Electronics

Cadmium sulfide thin film transistors were demonstrated as the n-type device for use in flexible electronics. CdS thin films were deposited by chemical bath deposition (70°C) on either 100 nm HfO 2 or SiO 2 as the gate dielectrics. Common gate transistors with channel lengths of 40-100 μm were fabricated with source and drain aluminum top contacts defined using a shadow mask process. No thermal annealing was performed throughout the device process. X-ray diffraction results clearly show the hexagonal crystalline phase of CdS. The electrical performance of HfO 2 /CdS-based thin film transistors shows a field effect mobility and threshold voltage of 25 cm 2 V -1 s -1 and 2 V, respectively. Improvement in carrier mobility is associated with better nucleation and growth of CdS films deposited on HfO 2 .

Structural, optical and electrical characterization of In/CdS/glass thermally annealed system

Abstract Chemical bath deposited CdS thin films coated with an evaporated indium thin film were analyzed by electrical, optical absorption, spectroscopic ellipsometry, X-ray and AFM measurements. As-deposited and thermally annealed (250–400°C) samples were evaluated. Heat treatments promote the formation of an external In 2 O 3 layer, as revealed by X-ray data. Indium atoms diffuse to the CdS layer from an intermediate indium layer. CdS doped with In with resistivities of approximately 10 −1 –10 −2 Ω/cm were obtained after annealing the In/CdS/glass system at 350 and 400°C. From the ellipsometry spectra, the evolution of the initial two-layer structure, In/CdS/glass, with thermal annealing was investigated. The analysis shows a more complex structure than the In 2 O 3 /In/CdS/glass structure previously proposed.

Oxygen diffusion in silicon oxide films produced by different methods

Thin films of silicon oxide were prepared on copper substrates using several techniques, such as DC sputtering, sol-gel and microwave decomposition of gases (SiH4 and O2). The sol-gel films were made using a conventional mixture of tetraethoxysilane (TEOS), water and ethanol, with various TEOS/water ratios. The oxygen diffusion coefficient is found from the thickness of the copper oxide layer formed at the copper–SiO2 interface, which was estimated from reflectivity and Auger spectroscopy measurements. Thermal treatments at various temperatures allowed us to determine the temperature dependence of the diffusion coefficient. It is found that the diffusion coefficient depends on the method of preparation of the SiO2 layer, which could be related to differences in the structure of the oxide layer. This conclusion is confirmed by the dielectric breakdown measurements. It follows from the results obtained that the oxygen diffusion constants and the dielectric breakdown field could be the indicators of the structural quality of silica.

Effects of Cd vacancies on the electrical properties of polycrystalline CdTe sputtered films

Polycrystalline CdTe films were grown on 7059 Corning glass substrates at room temperature by sputtering from a pure CdTe target and by co-sputtering from a composite CdTe‐Cd target. The electrical, optical and structural properties of the films were analyzed as a function of their Cd concentration. It was found that the films grown from the pure CdTe target were Cd-deficient with electrical resistivity of about 10 7 ‐10 8 Vcm and those grown from the CdTe‐Cd target were Cd-rich with electrical resistivity of the order of 10 3 Vcm. Dark electrical current vs 1/KBT measurements showed that the CdTe films with low cadmium content (,50 at.%) exhibit an unusual current peak in the range of 1/KBTa 40‐60 eV 21 . When the cadmium composition in the CdTe films approaches 50 at.%, this effect vanishes and the current has an exponential behavior for 1/ KBT . 40 eV 21 . The current peak was associated with the presence of Cd vacancies in the CdTe lattice. q 2001 Elsevier

The effects of porosity on optical properties of semiconductor chalcogenide films obtained by the chemical bath deposition

This paper is dedicated to study the thin polycrystalline films of semiconductor chalcogenide materials (CdS, CdSe, and PbS) obtained by ammonia-free chemical bath deposition. The obtained material is of polycrystalline nature with crystallite of a size that, from a general point of view, should not result in any noticeable quantum confinement. Nevertheless, we were able to observe blueshift of the fundamental absorption edge and reduced refractive index in comparison with the corresponding bulk materials. Both effects are attributed to the material porosity which is a typical feature of chemical bath deposition technique. The blueshift is caused by quantum confinement in pores, whereas the refractive index variation is the evident result of the density reduction. Quantum mechanical description of the nanopores in semiconductor is given based on the application of even mirror boundary conditions for the solution of the Schrödinger equation; the results of calculations give a reasonable explanation of the experimental data.

ZnO/CdS bilayers prepared by concurrent deposition from a chemical bath

ZnO/CdS bilayer films have been prepared by the chemical bath deposition (CBD) technique from a single reaction bath. The bilayer films were obtained by using solution concentrations of 1 M for ZnSO4 and 0.1 M for CdCl2. The pH was adjusted by the use of an ethanolamine‐ammonia system. A solution 1 M of thiourea was used as the source of sulfur ions. The thickness of each layer in the resultant film were 2 and 0.3 mm for ZnO and CdS, respectively. We report here the optical and structural characterization of this ZnO/CdS bilayer system. q 2001 Elsevier Science Ltd. All rights reserved.

Qualitative evaluation of sol-gel SiO2 as a protective layer for soft surfaces

Abstract In this paper, it is shown that SiO 2 sol–gel coatings deposited on glass and acrylic substrates (polymethyl methacrylate) considerably improved the mechanical resistance of the surface of these materials. We have used new methods to test the mechanical resistance and to measure the damage produced to the surface of the coated and bare substrates. The qualitative method employed to evaluate the mechanical resistance of the bare and coated surfaces consists of exposing the samples to the constant impacts and friction of zirconia balls. For that, the samples were fixed to the inner wall of a ball mill containing the zirconia balls. It is observed that during the mechanical treatment, the bare acrylic substrates lose their transparency, due to a large amount of surface defects created by the mechanical treatment. The presence of the defects is monitored measuring the transmitted light from a laser beam. Long exposure times to the mechanical test show that small amounts of materials are removed from the surface of the coating.

Presence of oxygen in the lattice of CdTe thin films

Polycrystalline CdTe thin films, with oxygen concentrations (x) in the range of 0.01–15 at. %, were grown at room temperature on 7059 Corning glass by means of the rf sputtering method. For low oxygen concentrations (x⩽0.3 at. %) the CdTe develops compressive stress during growth, making the films mechanically unstable. The stress normally relaxes when films are exposed to air, making stable films. In some cases, in films with low oxygen concentration, the stress relaxes abruptly immediately after they are exposed to air, and this results in either some fractures or in a loud explosive cracking of the film. In the latter case, the film is reduced to small pieces and violently dispersed over a wide area. Surface images of the fractures, obtained by atomic force microscopy, show detachment and liftup of the films in the regions adjacent to fractures. This indicates that the films were originally under compressive stress. Values of x in the bulk of the films were measured by means of Auger mass spectroscopy....