Victor Sosa

Comparison of properties of CdS thin films grown by two techniques

Abstract Polycrystalline cadmium sulfide (CdS) thin films were deposited on glass substrates by chemical bath deposition (CBD) and close-spaced sublimation (CSS) techniques. The typical deposition temperatures between these techniques are quite different. The CdS thin films deposited by CBD were prepared using two methods of bath agitation: magnetic and ultrasonic agitation. We found that films deposited with ultrasonic agitation presented a cleaner surface with minor presence of contaminants, similar to the obtained for CSS films, as demonstrated by Auger analysis. Properties of the CdS films such as morphology, optical transmission, crystallinity and band gap energy, are discussed in order to compare them in both techniques. We obtain that crystallinity of CdS films depends strongly on the temperature used for deposition. In agreement with several works, films prepared by CBD technique presented a cubic structure, while films grown by CSS technique exhibited an hexagonal symmetry.

Effect of indium tin oxide substrate roughness on the morphology, structural and optical properties of CdS thin films

Abstract Indium tin oxide (ITO) coatings of glass substrates were etched with hydrochloric acid in order to obtain different root-mean-square roughness ( R ITO ) on its surface. The effect of R ITO on the morphology, structural and optical properties of CdS films deposited was investigated. Polycrystalline cadmium sulfide thin films were deposited on ITO/glass substrates by chemical-bath deposition (CBD) at 358 K, and studied by atomic force microscopy (AFM). Roughness of CdS films ( R CdS ) showed a nearly linear increase with R ITO . The thickness of CdS films was investigated by Auger Electron Spectroscopy (AES) and showed an increment with R CdS . X-ray diffraction results showed that CdS films have a cubic zincblende structure with a (111) preferred orientation. The measured residual strain of the CdS films showed an initial increse with R ITO reaching a maximum point at 15±2 nm, and after that exhibited a decreasing dependence. The optical band gap E o of the CdS films obtained from transmittance measurements did not depend on R ITO .

First stages of growth of CdS films on different substrates

Cadmium sulfide films were grown on glass, silicon, and ITO substrates by chemical-bath deposition (CBD) at 358 K, and studied by atomic force microscopy, Auger electron microscopy and X-ray diffraction. CdS film growth initiates immediately and thickness increases with deposition time. The rms roughness of CdS films grown for 1 to 10 min (thicknesses of 10 to 70 nm) was measured by atomic force microscopy. The roughness of films grown on ITO remains similar to that of the substrate regardless of deposition time. On the other hand, the roughness of CdS films grown on glass and silicon increases with deposition time, finally reaching a roughness similar to that of films grown on ITO. Auger profiles show that the CdS/substrate interface is not abrupt and depends on the substrate. Substrate roughness plays an important role during the initial CdS growth process. X-ray studies show that silicon substrates are not appropriate for CdS film growth by CBD. The most appropriate substrate is ITO.

Experimental evidence of compositional mixture in CdTeO films grown by radio-frequency sputtering

An analysis of the absorption spectra of amorphous CdTeO films grown by radio-frequency sputtering has been carried out. The behavior and the shape of the spectra suggest the presence of CdxTeyOz variable-composition compounds ranging between CdTe and CdTeO with 67 at. % oxygen. We present a model based on bond statistics, which describes the absorption profile behavior of this and other materials with similar optical behavior, and models of the composition of such films.

Levitation force between a small magnet and a superconducting sample of finite size in the Meissner state

Abstract The repulsion force between a cylindrical superconductor in the Meissner state and a small permanent magnet was calculated under the assumption that the superconductor was formed by a continuous array of dipoles distributed in the finite volume of the sample. After summing up the dipole–dipole interactions with the magnet, we obtained analytical expressions for the levitation force as a function of the superconductor–magnet distance, radius and thickness of the sample. We analyzed two configurations, with the magnet in a horizontal or vertical orientation.

Low- and high-frequency vibration isolation for scanning probe microscopy

A study of the vibration isolation system in scanning probe microscopes (SPMs) to reduce external noises in a wide range of frequencies is presented. For the low-frequency isolation case, a pneumatic system based on a cylindrical plastic tube with an elastic membrane for damping is studied. The theoretical model and the experimental results obtained from it to isolate noises above 2 Hz are discussed. For the high-frequency isolation, a design based on stacked metallic sheets, with cylindrical elastomers (viton) between them, is simulated. The effect of the elastomer geometry when it is used in a real case by means of the transfer function of the vibration system is discussed. From the results, we are able to predict and optimize the performance of SPMs with regard to noise isolation.

Strain gradients in polycrystalline CdS thin films

The strain gradient through an 800-nm thick CdS thin film, deposited on an ITO substrate by chemical bath deposition CBD . has been analyzed by using grazing incidence X-ray diffraction GIXD . The main X-ray diffraction peak found in the CdS film . . was separated in the cubic- 111 and hexagonal- 002 reflections by a multi-Gaussian fit. We measured the d-spacings of the . . h- 002 and c- 111 phases mixed in the CdS film as a function of the penetration depth of the incidence X-ray beam. Afterwards, . . we calculated their respective strain. Both the c- 111 and h- 002 strain tend gradually to reach a top value from the surface to w . .x the depth of the films. These strain magnitudes were correlated with the misfit parameter between the CdS c- 111 and h- 002 phases and the ITO substrate. From the experimental results we concluded that strain is therefore induced by the mismatch and by other effects, as grain size of the CdS film, and ITO substrate roughness. Q 2000 Elsevier Science S.A. All rights reserved.

Auger electron spectroscopy study of silver incorporation in epitaxial Y–Ba–Cu–O superconducting films grown on MgO by laser ablation

We have studied by Auger electron spectroscopy the incorporation of silver in epitaxial YBa2Cu3O7 (YBCO) thin films fabricated by pulsed laser deposition using YBCO+Ag 10% targets. Films were grown on MgO substrates. The silver was found in a thin layer, of approximately 50 nm in depth. X‐ray photoelectron spectroscopy confirmed that the Ag is in its metallic form.

Screening technique measurement of ac imaginary susceptibility and critical current density of YBa2Cu3O7−δ films

The screening technique was used to study systematically the ac susceptibility of superconducting YBa2Cu3O7−δ thin films in the temperature range of 32–83 K. We used the Fourier transform of the pick-up signal, coming from the secondary coil, to estimate the critical current density. From the experimental data we observed regularities in the imaginary part of ac susceptibility, and confirmed that the flux creep mechanism is dominant.

Comparative analysis of the determination of Jc of YBCO films at different temperatures and magnetic fields by means of the shielding technique.

Abstract Imaginary susceptibility of YBCO films was studied between 32 and 83 K as a function of magnetic field (0 to 350 Oe). No maximum in the curve of susceptibility was observed at low temperatures (i50 K), this made that this criterion for the determination of the critical current density could not be applied. The same fact occurred in the curves of the third harmonic against magnetic field. Therefore, to determine Jc we proposed the criterion of taking the maximum of the third/first harmonic ratio. It was observed that the Jc values determined with the three criteria tend to coincide as they approach to the critical temperature.