A. Iribarren

Optical and structural evidence of the grain-boundary influence on the disorder of polycrystalline CdTe films

We report the linear behavior of the band-tail parameter as a function of the reciprocal of the grain size in polycrystalline CdTe. On the other hand, the study of the full width at half maximum of the x-ray diffraction peak shows a similar behavior, which indicates that the disorder increases as grain size diminishes. A theoretical analysis justifies that the behavior is ruled by the contribution of the grain-boundary traps, and the trap concentration is calculated. Both results constitute experimental evidences of the grain-boundary disorder, which was quantified, and demonstrate that it is caused by the extension of the grain-boundary effect into the grain.

Chemical and phase composition of SnOx:F films grown by DC reactive sputtering

Abstract Using X-ray diffraction and X-ray photoelectron spectroscopy we studied the structural variations of SnO x :F films grown by DC reactive sputtering. We used a metallic tin target and an Ar–O 2 –Freon plasma. We found that a mixture of compounds, i.e. SnO, Sn 3 O 4 , Sn 2 O 3 and SnO 2 were formed in the films. From the analysis of the deconvoluted peaks of the X-ray diffractograms and from the X-ray photoelectron spectroscopy results, we obtained the Sn 2+ /Sn and Sn 4+ /Sn molar fractions present in the films. The dependence of the Sn 2+ /Sn and Sn 4+ /Sn molar fraction with oxygen content was fitted by applying a combinatory model. The Sn 2+ /Sn and Sn 4+ /Sn molar fractions obtained from XRD and XPS were compared with theoretical values and showed good agreement. When the cathode voltage used to grow the films is varied, the Sn 4+ /Sn molar fraction shows two well-defined regions, related to the SnO and SnO 2 stoichiometries. This is related to the Sn 2+ →Sn 4+ transition in the cathode, as described in a previously proposed phase diagram for DC reactive sputter deposition of SnO x :F films. Sn 2+ and Sn 4+ are both present in the films at low current densities, and SnO and SnO 2 are the only species found in these conditions. Sn 2 O 3 and Sn 3 O 4 are formed at current densities over 30 mA cm −2 , when the growth takes place near the transition region in the phase diagram. The growth rates are also affected and closely related with the variations of the cathode voltage in this region.

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.

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.

COMPOSITION MIXTURE PROBABILISTIC MODEL IN THE FORMATION OF SEMICONDUCTOR MATERIALS OBTAINED BY RANDOM GROWTH TECHNIQUES

During the obtaining of semiconductor materials by some growth techniques the film structure forms randomly according to how the species arrive to the subtract. If the film is a material with three or more elements they can organize in several compounds, which have only local order and even become amorphous. As a consequence the physico-chemical parameters manifest non-typical behaviors other than in pure materials. In the present work we develop a dynamical-probabilistic model, which describes quantitatively such composition mixture and was applied on the behavior of the absorption profiles of CdTeO films grown by radio frequency (rf) sputtering with different oxygen concentrations. The model can be applied to films obtained by other growth techniques.

CHEMICAL COMPOSITION AND CRYSTALLINE PHASES IN F-DOPED TIN OXIDE FILMS GROWN BY DC REACTIVE SPUTTERING

We study by x-ray diffraction (XRD) the structural variations on a series of SnOx:F films grown by dc reactive sputtering from a metallic tin target in an Ar-O2-Freon plasma. We found that the films tend to be crystalline when the stoichiometry approaches to that of SnO or SnO2, being amorphous in between. We fitted the x-ray diffractograms and found that films are composed by a mixture of compounds, i.e. SnO, Sn3O4, Sn2O3 and SnO2, given by the simultaneous presence of Sn+2 and Sn+4. From the analysis of the deconvoluted areas under the x-ray diffractograms we calculate the Sn+2/Sn and Sn+4/Sn molar fraction present in the films. The same calculations are done for the x-ray photoelectron spectroscopy (XPS) results. By applying a combinatory model we fitted the general behavior of SnOx films with different oxygen content versus the Sn+2/Sn and Sn+4/Sn molar fraction. Both XRD and XPS results are compared with the theoretical curve, showing a well agreement.

Reaction kinetics of the corn pericarp during the nixtamalization

We studied by grazing incidence X-ray diffraction the behavior of the amorphous and crystalline diffractogram areas of corn pericarp samples taken during a typical nixtamalization process. The pericarp amorphous part suffers two dissolution processes, one fast, due to the dissolution of the water- and alkali-soluble parts of the pericarp, and other slow, associated with the hemicelluloses dissolution. During the steeping stage the crystalline part of the pericarp reflects the transformation of the native cellulose under the alkaline treatment into cellulose II, meanwhile, the amorphous part increases due to the alkaline cellulose transformation into amorphous II-type cellulose. Assuming first-order reactions we fitted the area behaviors in the steeping stage and found the reaction rates from native cellulose into alkaline cellulose and from alkaline cellulose into cellulose II to be k 1 ; 0.027 min m 1 and k 2 ; 0.00077 min m 1 respectively.