J.L. Hernández-Rojas

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.

Capacitance measurements of p-n junctions: depletion layer and diffusion capacitance contributions

A set of capacitance measurements is proposed to identify the different contributions to the junction capacitance (diffusion capacitance and depletion layer capacitance) of p-n Si diodes. By measuring the C-f and C-V characteristics of Si commercial diodes, we fully characterize the AC behaviour of such devices. At reverse bias voltages, only the depletion layer capacitance is present and it is frequency independent in our range of measurement (up to 13 MHz). From C-V characteristics, we deduce a linearly graded junction nature and a built-in value of 0.59 ± 0.02 V. At forward bias voltages and frequencies lower than 100 kHz, both the diffusion and the depletion layer capacitances contribute to the junction capacitance. A simple calculation allows us to obtain a value for the average minority carrier lifetime of tau = (4 ± 2) × 10-6 s. Finally, a voltage dependence of exp(qV/2kT) is deduced for the diffusion capacitance.

Chalcopyrite CuGaxIn1−xSe2 semiconducting thin films produced by radio frequency sputtering

CuGaxIn1−xSe2 thin films have been deposited by rf sputtering from three targets with different (Ga,In) content (x=0.25, x=0.5, and x=0.75). A structural, compositional, optical, and electrical study has been carried out for films grown at substrates temperatures higher than 350 °C. We have successfully obtained chalcopyrite single phase stoichiometric films. Very sharp absorption edges are obtained, with band gaps of 1.12, 1.35, and 1.51 eV for x=0, x=0.5, and x=0.75, respectively.

Optical spectroscopic study of the growth dynamics of radio‐frequency‐sputtered YBa2Cu3O7−x thin films

An optical spectroscopic study of the plasma produced during rf sputtering of an YBa2Cu3O7−x target was performed to analyze two basic properties of the deposition process: resputtering effects and oxidation mechanisms. Strong emissions of all the species above a value of the target voltage were found. These observations are associated to a strong secondary electron emission of the target which originates a negative self‐bias of the substrate and a subsequent resputtering by argon cations. The addition of different amounts of oxygen to the discharge reveals that preoxidation in the gas phase may decrease the oxygen content in the films: the oxidation of the films is dominated by atomic oxygen.

Stoichiometry control over a wide composition range of sputtered CuGaxIn(1−x)Se2

Films of CuGaxIn(1−x)Se2 (CGIS) have been grown by rf sputtering from stoichiometric single targets with different Ga/In ratios. Adjusting growth temperature and argon pressure we are able to deposit films with a wide range of Cu contents: From CGIS Cu‐poor (16 at. %) to Cu2Se. Reevaporation of (Ga,In)2Se3 binaries is observed when substrate temperature is increased at a constant argon pressure (20 mTorr). An increase in Ar pressure from 5 to 150 mTorr at a growth temperature of 450 °C, produces a decrease in Cu atomic percentage from 24% to 16% due to a preferential diffusion of Cu sputtered atoms in the plasma. The relevant film properties of the analyzed films are found to be ruled by the Cu content. Graded composition absorbers with adequate physical properties for the fabrication of photovoltaic devices are grown with a proper choice of growth parameters.

Texture improvement of sputtered YBa2Cu3O7−x films on MgO (100) with a SrTiO3 buffer layer

Abstract SrTiO 3 buffer layers have been grown on MgO (100) substrates to provide a better match to RF sputtered YBa 2 Cu 3 O 7- x films. This heterostructure allows a highly textured growth to be achieved over thickness as high as 1 μm. The granularity of films grown without buffer layer on MgO (100) and YSZ (100) has been shown to be thickness dependent affecting the critical current density: J c at 77 K is typically 3 × 10 5 A/cm 2 in 1000 A thin films while it reduces to 10 3 A/cm 2 in μm films. A of how the improvement of lattice matching makes the critical current density increase from 10 3 A/cm 2 for 1 μm films grown on YSZ to 4 × 10 5 A/cm 2 for films grown with a SrTiO 3 buffer layer.

Effect of sputtering parameters on the oxygenation dynamics of 123 superconducting thin films

Abstract The oxygenation dynamics of RF sputtered ReBa 2 Cu 3 O 7− x (Re=rare earth) films has been investigated. Oxygen and oxide species have been monitored in the plasma with the glow discharge optical spectroscopy technique and oxygen incorporation in the film has been estimated by AC mutual inductance response and X-ray diffraction. The oxygen partial pressure has been varied from 10% to 50% of the total discharge pressure and the deposition temperature from 550°C to 805°C. A careful control of these deposition parameters allows one to change the film physical properties from a polycrystalline structure and poor superconducting behavior to a highly c -oriented texture, to a sharp superconducting transition ( T c =90 K and ΔT c J c (>10 5 A /cm 2 at 77 K).

Granularity effects in transport properties of 123 superconducting thin films

Abstract Superconducting 123 thin films have been produced by RF sputtering with different morphologies. A study is presented of the transport properties (J c (T,H)) and the AC diamagnetic screening characteristics for the different samples. Granular films show critical currents two orders of magnitude smaller than textured films at 77 K. This has been explained in terms of the presence of oxygen deficient phases in the intergranular regions.

Emission line intensities in an r.f. sputtering glow discharge system

Abstract An in situ analysis of an r.f. discharge is carried out. The relationship between the discharge parameters, composition and growth rate of the deposited thin films and the glow discharge optical spectroscopy signal is presented. Three semiconducting single-phase chalcopyrite targets of Cu(Ga, In)Se2 were sputtered in an Ar atmosphere. The composition of the discharge was measured and it correlated with the composition of the deposited thin films. To be able to extend the determination of the film properties to different discharge conditions we have studied the variation in the optical emission with the Ar pressure and the r.f. power. The behaviour of the optical emission intensities as a function of both the Ar pressure and discharge power is explained in terms of the variation in the electron energy distrobution function.