Isothermal Closed Space Sublimation Research Articles

Photoinduced growth of the crystalline phase of tellurium on a 1T′-MoTe2 matrix

Due to the growing demand for miniaturization and energy efficiency in modern electronic devices, there is a renewed interest for optoelectronic memories and sensors based on 2D materials. In particular, the molybdenum ditelluride (MoTe2) is one of the most promising materials for applications in nonvolatile phase-change memory devices, as its properties can be controlled by visible-light illumination. Among the several ways to synthesize MoTe2, the molybdenum oxide tellurization through isothermal close space sublimation (CSS) annealing in gas atmosphere is a simple and low-cost effective method for large-scale production of devices based on this layered material. Therefore, the understanding of the physical properties of MoTe2 thin films produced by this technique is crucial for future applications. Surprisingly, our results indicate that there is a photoinduced growth of the crystalline phase of tellurium on the 1T′-MoTe2 matrix even when the power density of the laser is low. From Raman spectroscopy investigations, we were able to show that nanometer-sized tellurium crystallites work as seed sites for the photocrystallization of tellurium. By assuming that the overall crystallization process is described by a kinetic approach that is based on the Kolmogorov–Johnson–Mehl–Avrami theory, our results indicate that the process is governed by an anisotropic organization of the tellurium atoms in helical structures during the crystal growth.

Modeling and growth of graded composition interdiffused multilayers in the CdTe-ZnTe system

A theoretical model of the Cd/Zn interdiffusion during the growth of different sequences of ZnTe/CdTe layers was developed. The model can predict the values of the growth parameters needed to obtain desired composition profiles from specific ZnTe/CdTe multilayer growth sequences. With these simulations as a guide, CdxZn1-xTe graded composition samples with different Cd profiles and thicknesses were grown. A good agreement was found, demonstrating the possibility of preparing different and predictable Cd content profiles in CdxZn1-xTe graded thin films by the simple isothermal close space sublimation method.

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.

Enhanced ZnTe infiltration in porous silicon by Isothermal Close Space Sublimation

Isothermal Close Space Sublimation (ICSS) technique was used for embedding porous silicon (PS) films with ZnTe. It was studied the influence of the preparation conditions and in particular of a chemical etching step before the ZnTe growth, on the composition profile and final porosity of ZnTe embedded PS. The structure of the embedded material was determined by X-ray diffraction analysis while the thickness of the samples was determined by scanning electron microscopy (SEM). Rutherford backscattering (RBS) and Energy Dispersive (EDS) spectrometries allowed determining the composition profiles. We conclude that the etching of the PS surface before the ZnTe growth has two main effects: increasing the porosity and enhancing the reactivity of the inner surface. It was observed that both effects benefit the filling process of the pores. Since RBS and EDS cannot detect the porosity in the present system, we explore the evolution of porosity by the fitting of the UV–VIS reflectance spectra. The atomic percent determined with this method was in relatively good agreement with that obtained from the RBS and EDS measurements.

Isothermal close space sublimation for II-VI semiconductor filling of porous matrices

Isothermal close space sublimation, a simple and low-cost physical vapour transport technique, was used to infiltrate ZnTe and CdSe semiconductors in porous silicon. The structure of the embedded materials was determined by X-ray diffraction analysis while Rutherford backscattering spectra allowed determining the composition profiles of the samples. In both cases, a constant composition of the II-VI semiconductors throughout the porous layer down to the substrate was found. Resonance Raman scattering of the ZnTe samples indicates that this semiconductor grows in nanostructured form inside the pores. Results presented in this paper suggest that isothermal close space sublimation is a promising technique for the conformal growth of II-VI semiconductors in porous silicon.

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.

Isothermal close space sublimation of CdTe/ZnTe heterostructures in vacuum conditions

AbstractThin films and structures of ZnTe and CdTe have been grown by isothermal close space sublimation using alternated exposure of single crystalline substrates to elemental Zn, Te and Cd sources. The results show that the use of vacuum conditions promotes the transport of vapors towards the surface and then efficient growth at lower temperatures. Films thickness ranged between 200 and 600 nm for 50 cycles samples. ZnTe and CdTe films were obtained even at temperatures as low as 310 °C. This is important because low temperatures worsen the inter‐ diffusion processes; as a consequence we have obtained relatively abrupt interfaces in the ZnTe/CdTe system. Its compositional, structural and optical properties are presented. These results offer new possibilities of this low cost technique for growing heterostructures. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth of ZnSe(1−x)Tex epilayers by isothermal closed space sublimation

ZnSe ( 1 − x ) Te x (x∼0.06) epilayers were grown on GaAs(001) substrates at 350 °C by isothermal closed space sublimation (ICSS) technique. The epitaxial growth was performed in low-pressure helium atmosphere (∼0.1 mbar) by sequential exposures of the substrate to vapors of a solid solution of selenium-tellurium and elemental zinc. The use of a mixed source is proposed in order to regulate the partial vapor pressure of the constituents by composition. Strain and composition of the ZnSe(1−x)Tex epilayers were extracted from high resolution x-ray reciprocal space mapping. Structural investigations show a reasonably good crystalline quality of the epilayers. Good reproducibility of composition and control of thickness were obtained although atomic layer epitaxy regimen was not achieved. A growth rate of 1.3 monolayers/cycle was ascribed to multilayer adsorption and the existence of an efficient transport of SeTe in graphite under thermodynamic conditions of ICSS. Both Raman and photoluminescence characterizations suggest the existence of random alloy epilayers with larger composition disorder in the mesoscopic scale than those obtained by molecular beam epitaxy.

On the isothermal closed space sublimation growth of CdSe using a mixed source for selenium

The use of a selenium–tellurium (SeTe) mixed source in the isothermal close space sublimation growth of CdSe epilayers is considered. The epitaxial growth was performed in flowing helium by sequential exposures of the substrate to vapors of the mixed SeTe source and elemental cadmium at temperatures within 350–410 °C. In spite of the mixed source (proposed to decrease the partial pressure of Se), tellurium incorporation was small and CdSe x Te (1− x) ( x∼0.98) epilayers were obtained. X-ray diffraction reciprocal space mapping shows the existence of hexagonal inclusions mainly on the (1 1 1) facets of the cubic phase. Material deposition on areas of the graphite crucible exposed to the sources, contamination of the Cd source and large growth rates suggest the existence of a selenium transport process via graphite. This transport might be the result of the combination of selenium deposition on graphite with a subsequent activated desorption of selenium under cadmium exposure. It affects Cd source purity and growth kinetic bringing on a modification of the usual atomic layer deposition regimen; however, a reproducible growth rate of the epilayers was obtained.

(Co, Zn)O compound obtained from ZnTe vapor deposition on Co/Si substrates

(Co, Zn)O compound has been obtained by a non-expensive synthesis route. ZnTe thin films were obtained by isothermal close space sublimation on the Co thin layer previously sputtered on silicon substrates. After the annealing process in humid ambient cobalt atoms diffusion and Zn oxidation were obtained besides partial Te evaporation. The detailed characterization of the samples by using XRD, RBS, AFM, XPS, VSM and MFM techniques point to the formation of room temperature ferromagnetic CoxZn(1−x)O phase (x<0.15). This ferromagnetic behavior is mainly attributed to Co atoms substituting Zn atoms in the ZnO network.

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.

Nanoporous Anodic Alumina as Template and Mask for Functional Nanostructures Fabrication

AbstractNanoporous Anodic Alumina Films (NAAF) have been used for growing a lot of nanostructure functional materials. Of particular interest is the NAAF conversion into membranes (NAAM) with different highly controlled pore diameter and distribution to be used as templates and masks to grow a wide variety of nanomaterials. This work constitutes an approach to a review of our latest results regarding the use of NAAF and NAAM as templates in which II-VI semiconductor, functional oxides, hard materials and magnetic nanowires have been grown. The growth techniques and methods used include Isothermal Close Space Sublimation (ICSS), Magnetron Sputtering, electroplating and sol-gel. Ion Beam Irradiation (IBI) combined with different NAAM as masks has been also used for Titania substrates functionalization.

Growth of CdSe and CdTe crystals shaped by thick alumina membranes

CdSe and CdTe semiconductors were deposited on single-crystal substrates by means of the simple isothermal closed space sublimation (ICSS) method using thick commercial alumina membranes as masks. By placing 60-nm-thick alumina membranes between the substrates and the incoming vapors, CdSe and CdTe small crystals were grown coherently on the surface of GaAs following the pattern of the membranes, whereas CdSe crystals with random crystalline orientations grew over Si substrates. Simultaneously, CdSe crystals grew inside the 200 nm diameter pores, along their complete length, from one surface of the membrane to the other. The growth of CdTe inside the pores was not efficient, and only few isolated crystals were observed. Electron microscope images and X-ray diffraction patterns allowed determination of the morphology and the structure of the grown material. Optical reflectance was used to verify the characteristics of CdSe thin films used as a reference.

ZnTe/Si heterostructures grown by isothermal closed space sublimation

ZnTe films with different thickness were grown onto (001) Si substrates using isothermal closed space sublimation by alternated sublimation of Zn and Te sources. As a result ZnTe polycrystalline films were obtained with a strong [111] orientation as revealed by X-ray diffraction patterns. The reason for this polycrystalline nature of the samples comes from incomplete removal of SiO 2 from the Si substrate surface. A preferential adsorption of Te in the first stages of the growth was concluded from Rutherford backscattering spectra analysis.

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.

Structure and growth rate of ZnTe films grown by isothermal closed space sublimation

Results on isothermal closed space sublimation (ICSS) of ZnTe using different experimental conditions are presented. Wide ranges of exposure and purge times were used to study the properties of the adsorption and desorption processes in this material. Moreover, the atmosphere and the type of substrates were also varied. It was found that epitaxial ZnTe layers are always obtained in either [1 1 1] or [1 0 0] oriented GaAs substrates regardless the used gas atmosphere. In contrast, samples grown on [1 0 0] oriented Si substrates showed powder-like X-ray diffraction features with a strong preferential orientation in the [1 1 1] direction. In both kinds of substrates, there were obtained growth rates considerably larger than one monolayer per cycle for samples grown with large exposure times to the sources. The influence of different process like gas absorption in the substrate holder, gas transport in the substrate compartment and multilayer adsorption on these large growth rates is discussed.

Structural characterization of epitaxial Cd 1− xZn xTe semiconductor thin films by ion beam techniques

We report on the structural characterization of ZnTe, CdTe and Cd 1− x Zn x Te epilayers grown on GaAs(1 0 0) by isothermal closed space sublimation epitaxy. A detailed study of the heterostructures based on conventional Rutherford backscattering spectrometry, ion channeling and nuclear reaction analysis allows to determine the thickness, composition, impurities, lateral homogeneity and interface structure of the grown thin films. The growth rates were determined for the extreme composition (ZnTe, CdTe). Stoichiometry and lateral homogeneity were observed in ZnTe and CdTe films. Low carbon contamination homogeneously distributed in the film was present and no oxygen contamination of the samples was observed. The channeling analysis of ZnTe films allowed to evaluate the thickness of the interface damaged region.

CdTe Epitaxial Growth by Isothermal Closed Space Configuration

The growth and structural characterization of CdTe epitaxial films on GaAs(100) substrates are reported. The samples were obtained by a novel isothermal closed space sublimation (ICSS) 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 and Te) at isothermal conditions employing a closed space geometry. The influence of the growth parameters on the composition and the thickness of the films is discussed.