H. Arizpe-Chávez

Quantum confinement effects in CdTe nanostructured films prepared by the RF sputtering technique

Abstract Two kinds of CdTe nanostructured films were prepared by the RF sputtering technique. The first kind of film was composed of a mixture of CdTe and CdTeO3 crystallites and was obtained from thermal annealing of amorphous oxygenated CdTe sputtered films. The second one constituted by low oxygen doped CdTe crystallites were deposited at a substrate temperature of 500°C. The films were characterized by means of X-ray diffraction, optical absorption and Auger electron spectroscopy. The results show that the fundamental absorption edge of the films is shifted toward higher energy as a consequence of the small size of the CdTe crystallites, which constitute them. The absorption edge of the films versus CdTe crystallite radius relation is analyzed in terms of the models by Efros and Efros and by Kayanuma. Due to the discrepancies of the experimental data with these models, a numerical analysis made with a variational method was implemented. This method considers the different environment surrounding the CdTe crystallites in the two kinds of films studied.

Characterization of Cds clusters in zeolite-A grown in alkaline solution

CdS clusters were synthesized in A type zeolite by reaction in alkaline aqueous solution at temperatures from 30 to 70 °C. The optical properties of the samples were studied by diffuse reflectance and photoluminescence spectroscopy. Their crystalline structure and morphology were studied by X-ray diffraction and by transmission and scanning electron microscopy. We found that at lower temperatures the CdS clusters are encapsulated in the zeolite cages. We compared the properties of these clusters with those encapsulated in the cages of zeolites X and Y, prepared by similar methods. CdS clusters smaller than the CdS exciton diameter are also formed outside the cages in the zeolite matrix. The size of these clusters increases with temperature producing a red-shift of the absorption edge in the optical absorption spectra.

Cubic to hexagonal phase transition in CdTe polycrystalline thin films by oxygen incorporation

Abstract Polycrystalline oxygenated cadmium telluride thin films, with oxygen contents ranging from 0 to 15 at%, were grown by the reactive radio frequency sputtering technique. The structure of samples was studied using X-ray diffraction. The energy band gaps of the samples were obtained from optical absorption measurements. It was found that the cubic crystalline structure of as-deposited samples changes to the crystalline hexagonal CdTe phase and then to an amorphous CdTe:O structure as the oxygen content was increased from 0 to 15 at%. Evidence of this phase transition was obtained from X-ray diffraction and optical absorption measurements. For the films with low oxygen content an increase in the interplanar distances was observed, yielding to a reduction of the band gap of the material. Samples containing hexagonal CdTe nanocrystals show larger energy band gaps, as compared with those having the cubic CdTe phase.

CDTE NANOSTRUCTURES PREPARED BY THERMAL ANNEALING

Homogeneous thin films of a‐CdTe:O were thermal annealed in an Ar flux. The as‐grown samples have an amorphous structure. The annealed films crystallize to a mixture of both CdTe and CdTeO3. For low annealing temperatures (less than 100 °C) it is possible to get CdTe crystallites with sizes smaller than 14 nm. In this work we report measurements of x‐ray diffraction and optical absorption spectra of these films. The x‐ray diffraction patterns of the films were used to study the CdTe crystallization process as a function of the annealing temperature. The CdTe crystallite size of the films was determined from the diffraction patterns using the Debye–Scherrer formula. It was observed that the CdTe crystallite size increases with annealing temperature. The optical absorption spectra of the films with the smallest CdTe crystallite size show how the absorption band edge of CdTe is shifted by the effects of the crystallite size.

Structural and optical studies in a‐CdTe:O annealed films

Thin films of stable crystalline phases of the Cd–Te–O ternary system were prepared by thermal annealing in an Ar flux of amorphous CdTe:O films. The composition of the annealed films depended on the initial oxygen content in the film, the temperature and duration of the annealing process. The annealed films were characterized by means of x‐ray diffraction spectroscopy and by optical absorption spectroscopy. The results show that for low oxygen content in the as grown film and high annealing temperature, the CdTe cubic phase is the predominant crystalline phase in the annealed film. For intermediate oxygen content in the as grown film, the result obtained after the annealing process is a composite material consisting in the mixture of both CdTe and CdTeO3 crystallites. The variation of the annealing temperature produces changes in the size of the crystallites of both crystalline phases. It also can change the CdTe to CdTeO3 proportion in the films. For low annealing temperature, the size of the CdTe cryst...

Presence of oxygen in the lattice of CdTe thin films

Polycrystalline CdTe thin films, with oxygen concentrations (x) in the range of 0.01–15 at. %, were grown at room temperature on 7059 Corning glass by means of the rf sputtering method. For low oxygen concentrations (x⩽0.3 at. %) the CdTe develops compressive stress during growth, making the films mechanically unstable. The stress normally relaxes when films are exposed to air, making stable films. In some cases, in films with low oxygen concentration, the stress relaxes abruptly immediately after they are exposed to air, and this results in either some fractures or in a loud explosive cracking of the film. In the latter case, the film is reduced to small pieces and violently dispersed over a wide area. Surface images of the fractures, obtained by atomic force microscopy, show detachment and liftup of the films in the regions adjacent to fractures. This indicates that the films were originally under compressive stress. Values of x in the bulk of the films were measured by means of Auger mass spectroscopy....

Optical and structural properties of CdTe‐CdTeO3 nanocrystalline composites

We report x‐ray diffraction and optical absorption measurements of CdTe‐CdTeO3 nanocrystalline composites prepared by thermal annealing of homogeneous a‐CdTe:O thin films. The as‐grown samples showed an amorphous nature. Films with higher oxygen content crystallized upon annealing, in a mixture of both CdTe and CdTeO3 crystallites. For annealing temperatures below 100 °C we obtained composite materials having CdTe crystallites with sizes smaller than 14 nm. The size of the CdTe crystallites was determined from the x‐ray diffraction patterns of the films by using the Debye‐Scherrer broadening formula. Quantum effects were observed in these films as a shift in the CdTe absorption band edge to higher energies.