R. Castro-Rodríguez

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 .

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.

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.

X-ray photoelectron spectroscopy study of CdTe oxide films grown by rf sputtering with an Ar-NH3 plasma

Abstract CdTe oxide films were prepared on glass substrates using the rf-sputtering technique and a controlled plasma of Ar–NH 3 . Films were studied by X-ray photoelectron spectroscopy and X-ray diffraction. The changes in chemical composition as a function of NH 3 partial pressure during deposition indicate that oxygen incorporates in the films up to approximately 62 at.%, while the Cd and Te contents decrease from ∼43 to 19 at.%. At NH 3 partial pressure of 1.3×10 −2 Pa, the Te–Cd bonds are strongly reduced and the Te in the films is mainly bonded to oxygen. The Cd MNN X-ray Auger feature shows a shift in energy of approximately 0.8 eV as a function of NH 3 partial pressure. This shift appears to be related to the change in Cd bonding from Cd–Te to Cd–O. Films prepared at NH 3 partial pressure of 4×10 −4 Pa present crystallinity with a [111] cubic CdTe orientation, while those prepared at higher NH 3 partial pressure show an amorphous structure. The amorphous material formed at NH 3 partial pressure saturation appears to be mainly amorphous CdTeO 3 .

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.

Discharge diagnosis and controlled deposition of SnOx:F films by dc-reactive sputtering from a metallic tin target

Abstract Fluorine-doped tin oxide (SnO x :F) films on glass substrates have been prepared by reactive magnetron DC-sputtering in Ar–O 2 –Freon plasma from a metallic tin target. We studied the relationship between the discharge characteristics and film properties and found the discharge conditions in which the films belong to the SnO 2 or to the SnO stoichiometry, by proposing a phase diagram for this system. We showed that the film stoichiometry is closely related to the tin valence changes at the target. We found that there is a narrow voltage band and a limit value in the oxygen content where the films have SnO 2 stoichiometry; outside this region SnO films are obtained. We studied the crystallinity and the optical properties of the SnO 2 films. The results show that crystalline films have cassiterite-like diffraction patterns with preferred orientation in the (110) planes. The crystallinity of the films is improved when the discharge voltage and oxygen content increase. A thermal annealing of the amorphous films at 500°C leaves them in the crystalline cassiterite-like form. The films have a transmittance of ∼80% in the visible region of the spectrum and a uniform thickness.

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.

S diffusion at the CdTe/CdS interface grown by rf magnetron sputtering

CdTe films were grown by radio-frequency magnetron sputtering (rf-sputtering) on CdS substrates in the growth temperature range between 250 and 400°C. The stoichiometry of the CdTe/CdS interface was observed by the Auger electron spectroscopy (AES), and Auger depth profiles demonstrate that the CdTe/CdS interface was not abrupt. The mechanism of the interface diffusion as well as the parameters necessary for its description were determined.