F. Sánchez-Quesada

Optical analysis of absorbing thin films: application to ternary chalcopyrite semiconductors

The refractive index n and the absorption coefficient a of radio frequency sputtered CuGaSe(2) and CuInSe(2) thin films were obtained by means of transmissivity (T) and reflectivity (R) measurements at normal incidence. The optical properties were determined from the rigorous expressions for the transmission and the reflection in an air/film/(glass)substrate/air multilayer system. The solutions to this system of equations are not unique, and the physically meaningful solution is identified by trying different thicknesses in the numerical approach. Usually, nonacceptable n dispersion curves are found for all thicknesses. To be able to obtain a good n dispersion curve and, therefore, a correct absorption coefficient, we propose a simple modification of the equations for R and T through a factor called the coherence factor (CF). Because of the surface roughness and the nonuniformity of n and alpha, the light rays that reflect internally in the interface between the substrate and the film have a random difference in opt cal path. The CF accounts for this effect. This modification leads to an unambiguous and accurate determination of the optical properties and thickness of thin films for all wavelengths where transmission is not negligible. The CF is shown to be greatly dependent on the thickness of the film. This method can be used even when the R and T spectra do not have interference fringes. This method is applied successfully to the optical analyses, in the 0.4-2.5-mum wavelength range, of CuInSe(2) and CuGaSe(2) ternary chalcopyrite thin films deposited onto glass substrates by radio-frequency sputtering.

Structural and optical properties of r.f.-sputtered CdS thin films

Abstract R.f.-sputtered CdS thin films have been produced from an intrinsic CdS target and their structural and optical properties have been studied to determine the influence of the sputtering conditions on these properties. Scanning electron microscopy showed that the grain size of the polycrystalline structures varied between 500 and 3000–4000 A, controlled by the target voltage (500–1600 V) or the substrate temperature (60–300 °C). Optical measurements showed that the absorption edge was shifted towards higher wavelengths when the substrate bias voltage increased or when the gas pressure decreased; band gap values obtained ranged between 2.23 and 2.38 eV.

CuInSe2 thin films produced by rf sputtering in Ar/H2 atmospheres

Structural, compositional, optical, and electrical properties of CuInSe2thin filmsgrown by rf reactive sputtering from a Se excess target in Ar/H2 atmospheres are presented. The addition of H2 to the sputtering atmospheres allows the control of stoichiometry of films giving rise to remarkable changes in the film properties. Variation of substrate temperature causes changes in film composition because of the variation of hydrogen reactivity at the substrate. Measurements of resistivity at variable temperatures indicate a hopping conduction mechanism through gap states for films grown at low temperature (100–250 °C), the existence of three acceptor levels at about 0.046, 0.098, and 0.144 eV above valence band for films grown at intermediate temperature (250–350 °C), and a pseudometallic behavior for film grown at high temperatures (350–450 °C). Chalcopyrite polycrystalline thin films of CuInSe2 with an average grain size of 1 μm, an optical gap of 1.01 eV, and resistivities from 10− 1 to 103 Ω cm can be obtained by adding 1.5% of H2 to the sputtering atmosphere and by varying the substrate temperature from 300 to 400 °C.

Deposition dependence of r.f.-sputtered CdS films

Abstract The variations in the electrical and structural properties of r.f.-sputtered CdS thin films with the adjustable variables of the sputtering system were analysed. The films were produced from an undoped high purity target in an argon atmosphere. The variables systematically modified were the substrate temperature, the r.f. power, the argon pressure and the operation mode (which was either floating or biased). The films were polycrystalline with grain sizes that increased with temperature, the mean value being 1000 A. The room temperature resistivity varied between 102 and 108 Ω cm, showing a clear dependence on the substrate temperature and the concentration of impurities present in the discharge chamber.

Temperature and bias effects on the electrical properties of CdS thin films prepared by r.f. sputtering

Abstract The electrical properties of r.f.-sputtered CdS thin films were studied from the point of view of the relationship between the sputtering process parameters and the resistivity and mobility of the deposited films. The processing parameters studied were the power or voltage applied to the target (in the range 0.24–2.82 W cm -2 or equivalently 500–1600 V), the substrate temperature (60–300 °C), the gas pressure (2–30 mTorr) and the substrate bias voltage (from -200 to 0 V). The interdependence of the substrate temperature and the input power as well as the relation between the self-bias voltage and the gas pressure were evaluated. The paper focuses on the conditions governing the electrical properties of the films. Resistivities from 10 to 10 8 Ω cm and the mobilities in the range 2–6 cm 2 V -1 s -1 were obtained depending on the production conditions.

Lithium insertion in Ba2YCu3O7-y

Novel materials, LixBa2YCu3O7-y(0≤x≤2), have been obtained by reaction of Ba2YCu3O7-y with n-butyl lithium. The most striking feature of these materials is the coexistence of both ionic conductivity and superconductivity for 0<x<0.5. Transmission electron microscopy (TEM) and electron diffraction show the lithiation to occur anisotropically.

METAL-INSULATOR TRANSITION IN SRRUO3 INDUCED BY ION IRRADIATION

We have studied the effect of He+ irradiation on the electrical resistivity and Curie temperature of ferromagnetic SrRuO3 thin films. An evolution from metallic to insulating behavior is observed when He+ ion fluence is increased, suggesting a metal-insulator transition. Damage by ion irradiation produces a strong decrease of the Curie temperature. On the other hand, no significant change in T-c (similar to 160 K) takes place in fresh samples grown at different substrate temperatures. We discuss the possible correlation between structural changes induced by irradiation, which reflect in an increase of the pseudocubic lattice parameter, and the observed depression of T-c.

Ionic conductivity of lithium inserted Ba2YCu3O7−y

Abstract Dielectric response experiments, varying the temperature of lithium inserted Ba 2 YCu 3 O 7−y samples, have been performed to search an ionic conduction mechanism. The study showed that Ba 2 YCu 3 O 7−y becomes ionic conductor by lithiation at temperatures higher than 400 K. The activation energies for the electronic and ionic conduction processes resulted to be 0.17 and 0.7 eV, respectively.

Heat treatment of rf sputtered CdS films for solar cell applications

Abstract Annealings in H 2 and N 2 atmospheres of CdS thin films produced by rf sputtering from undoped targets have been performed to improve the electrical and optical properties. As-grown films had a resistivity of 5–500 Ω cm (depending on the growth conditions), a mobility of 4–6 cm 2 /(V s), and an optical transmission of 60% in the visible spectrum for a 6 μm thick sample. After annealing at 200°C in an H 2 or N 2 atmosphere for 20 min the resistivity decreased about two orders of magnitude for all analyzed samples. This reduction is due to an increase in the carrier concentration and Hall mobility. The optical transmittance increased by 70% after annealing at 200°C in the same atmosphere for 5 h. Simultaneously the absorption edge shifted toward shorter wavelength and the band gap increased from 2.36 to 2.39 eV. All results can be explained in terms of desorption of impurities (probably oxygen) from the grain boundaries, as suggested for the irrelevant structural changes observed by SEM.

USE OF KRAMERS-KRONIG TRANSFORMS FOR THE TREATMENT OF ADMITTANCE SPECTROSCOPY DATA OF P-N JUNCTIONS CONTAINING TRAPS

The use of Kramers–Kronig transforms is proposed for the treatment of admittance spectroscopy data of junctions when significant shunt conductance or series resistance is present. An algorithm has been implemented to calculate the transformations numerically and the validity of the method developed has been tested using simulated data. Two experimental systems, p‐n junctions into InP made by ion implantation, and atomic‐layer‐epitaxy‐grown CdS/CdTe heterojunctions, have been characterized using this procedure.