Isothermal Close Space Sublimation Technique Research Articles

Infiltration of CdTe nano crystals into a ZnO wire vertical matrix by using the isothermal closed space technique

A ZnO nanorod structure was grown by the hydrothermal method and interpenetrated with CdTe using the isothermal closed space sublimation technique. The obtained structure was studied by using the Rutherford backscattering spectrometry (RBS), Scanning Electron Microscopy (SEM), High Resolution Transmission Electron Microscopy (HRTEM). The X-ray Diffraction (XRD) technique confirmed the presence of CdTe nanocrystals (NCs) of very small size formed on the surface and in the interspaces between the ZnO nanorods. The RBS observations together with the SEM observations give information on the obtained structure. Finally the photoluminescence studies show a strong energy confinement effect on the grown CdTe NCs.

Graded composition CdxZn1−xTe films grown by Isothermal Close Space Sublimation technique

Graded composition CdxZn1−xTe films were prepared by growing several alternate layers of CdTe and ZnTe by the Isothermal Close Space Sublimation technique. The thickness of both kinds of layers was modified along the structure to produce an increase of the average concentration of CdTe towards the surface of the films. Due to Zn/Cd inter-diffusion during the growth process the sequence of layers was converted into a relatively smooth graded composition film. According to X-ray diffraction characterization the layers grew coherently with the (100) oriented GaAs substrates although they showed a relatively high mosaicity. θ−2θ plots show very wide diffraction peaks as expected from variable composition samples. The band gap grading effect in light absorption was also verified through transmission measurements, using transparent substrates. Graded composition profiles of the thin films were confirmed by x-ray photoelectron and secondary ion mass spectroscopies. Moreover, quantitative Cd, Zn and Te profiles were obtained by the analysis of Rutherford backscattering spectra of the samples. This analysis showed a CdTe molar fraction profile ranging from approximately x=0.8 at the surface of the sample and x=0.35 at the interface with the substrate. The possibility of growing graded composition layers using a simple and cost-effective procedure was demonstrated. This could be interesting in technological applications like CdxZn1−xTe layers of variable composition in CdS/CdTe solar cells.

Intense white luminescence in ZnTe embedded porous silicon

Porous silicon layers were embedded with ZnTe using the isothermal close space sublimation technique. The presence of ZnTe was demonstrated using cross-sectional energy dispersive spectroscopy maps. ZnTe embedded samples present intense room temperature photoluminescence along the whole visible range. We ascribe this PL to ZnTe nanocrystals of different sizes grown on the internal pore surface. Such crystals, with different orientations and sizes, were observed in transmission electron microscopy images, while transmission electron diffraction images of the same regions reveal ZnTe characteristic patterns.

Vapour transport and inner walls covering in large aspect ratio macropores

A model for the covering of the inner surfaces of narrow and large aspect ratio tubes is developed. It considers diffusive vapour transport toward the bottom of the tubes and self-regulated adsorption at their walls. According to the growth conditions and the tubes geometry, Knudsen diffusion approximation is used. Pressure and coverage profiles, calculated for different geometries and sticking coefficients, allowed discriminating two well defined regimes. In these regimes, surface reaction or vapour diffusion are the corresponding limiting processes; then, a simple equation to estimate the walls covering time as a function of the growth parameters and tubes geometry is proposed. This model allowed revealing the main features guiding the deposition of CdSe semiconductor within commercial alumina membranes by the Isothermal Close Space Sublimation technique.

Growth of ZnTe and CdTe films using the isothermal close space sublimation technique in vacuum environment

Epitaxial and polycrystalline films of CdTe and ZnTe have been obtained on GaAs and Si substrates, by using isothermal close space sublimation. Contrary to previous experiments which were performed in a gas environment, in the present work the films were grown in vacuum. Due to this environment, transport of molecules from the source to the growing surface was significantly enhanced and larger growth rates were obtained. This is important for two reasons. First, it was possible to grow relatively thick films in a vacuum environment, without making an excessive number of cycles. Second, the films could be grown at lower temperatures than those required when using an inert gas environment with the subsequent advantage of decreasing inter-diffusion processes.

CdSe epitaxial films and nanostructures grown by the isothermal closed space sublimation technique

The use of the isothermal closed space sublimation technique for growing CdSe very thin films and nanostructures is reported. In this technique, the samples are grown by exposing a substrate alternately to elemental sources of Cd and Se. The whole system is at the same temperature, so the only driving force for the film growth is the vapour pressure difference between the elemental source and the film surface. This is due to the difference in chemical potential between the pure elements and the elements when forming the compound. Between subsequent exposures the surface is exposed to the carrier vapour. In this step, re-evaporation of the non-stable outermost part of the films can be controlled. This procedure leads to a self-regulated growth rate in which the size of the deposited materials is controlled very accurately. In this way, nanostructures and epitaxial thin films of CdSe were grown on single-crystal substrates. For the nanostructure growth a nanoporous anodic alumina membrane (AAM) was interposed between the elemental sources and the substrate. Optical and structural properties of the structures are reported. The structural characterization using x-ray diffraction and high resolution electron microscopy reveals the epitaxial quality of the films and the chemical composition and morphology of the semiconductor filled AAM samples.

Growth of Cd (1− x) Zn xTe epitaxial layers by isothermal closed space sublimation

The growth and structural characterization of Cd 1− x Zn x Te epitaxial films on (1 0 0 ) GaAs substrates is reported. The samples were obtained by a novel isothermal closed space sublimation technique, which was previously used for the growth of ZnTe in atomic layer epitaxy regime. For growing the films, a GaAs substrate is exposed alternately to the elemental sources (Cd, Zn, Te) at isothermal condition employing a closed space geometry. Eleven different ZnTe : CdTe cycle combinations were done to obtain different Zn compositions. The Zn molar fractions were measured by X-ray and transmission electron diffraction patterns that also revealed the epitaxial quality of the films. Influence of the growth parameters on the composition and the thickness of the films is discussed.

Low temperature growth of epitaxial CdSe thin films by an isothermal closed space sublimation technique using two elemental sources

We present a novel low-cost technique for the epitaxial growth of thin films. Using a closed space isothermal vapor phase technique, we have grown CdSe thin films on GaAs (1 0 0) substrates. The epitaxial growth is performed by exposing the substrate to Cd and Se vapor from elemental sources in a cycled manner. The thermodynamic and kinetic aspects of the process are discussed. X-ray diffraction patterns confirm the epitaxial growth of CdSe films on the GaAs substrates. AFM images were obtained to study the film morphologies.