Close-spaced Vacuum Sublimation Research Articles

Photoluminescence of CdTe thin films doped with Yb

In this paper, for the first time, we investigated the influence of ytterbium doping on the radiative recombination processes in CdTe thin films deposited on a glass substrate by means of the close-spaced vacuum sublimation technique. This allowed to determine the nature and energy structure of the intrinsic defects, residual impurities and complex acceptor centers in the films. It was found that the optical spectra of such films reflect the processes of formation of free excitons and radiative recombination with the participation of bound excitons as well as donor-acceptor pairs. For the first time, emission associated with intracenter optical transitions of Yb3+ ions in CdTe was detected and its nature was established. CdTe: Yb thin films have been shown to have very high crystalline and optical quality. It is established to be a result of a strong decrease in the concentration of shallow acceptor centers associated with residual atoms and cadmium vacancies. This is due to the formation of complex centers with the participation of Yb3+ ions and the mentioned acceptors, i.e by gettering of residual atoms in CdTe:Yb thin films. In addition, in this case we can expect the conversion of their conductivity from p-to n-type. These results open a unique opportunity to control the crystalline and optical quality, as well as the conductivity type of semiconductor crystals II-VI by doping them with rare earth elements.

Integrated Wafer Scale Growth of Single Crystal Metal Films and High Quality Graphene.

We report on an approach to bring together single crystal metal catalyst preparation and graphene growth in a combined process flow using a standard cold-wall chemical vapor deposition (CVD) reactor. We employ a sandwich arrangement between a commercial polycrystalline Cu foil and c-plane sapphire wafer and show that close-spaced vacuum sublimation across the confined gap can result in an epitaxial, single-crystal Cu(111) film at high growth rate. The arrangement is scalable (we demonstrate 2″ wafer scale) and suppresses reactor contamination with Cu. While starting with an impure Cu foil, the freshly prepared Cu film is of high purity as measured by time-of-flight secondary ion mass spectrometry. We seamlessly connect the initial metallization with subsequent graphene growth via the introduction of hydrogen and gaseous carbon precursors, thereby eliminating contamination due to substrate transfer and common lengthy catalyst pretreatments. We show that the sandwich approach also enables for a Cu surface with nanometer scale roughness during graphene growth and thus results in high quality graphene similar to previously demonstrated Cu enclosure approaches. We systematically explore the parameter space and discuss the opportunities, including subsequent dry transfer, generality, and versatility of our approach particularly regarding the cost-efficient preparation of different single crystal film orientations and expansion to other material systems.

Close-spaced sublimation of SnS absorber layers and SnS/CdS heterojunction solar cells with Mo and Ti back metal contacts

Tin sulfide is a promising semiconductor candidate to replace the currently used absorber materials in thin film photovoltaic such as Cu(In,Ga)(S,Se)2 and CdTe devices, which use toxic and scarce elements. However, numerous challenges still remain to approach comparable efficiencies for SnS-based solar cells. In this report, we address the following: (i) impact of growth conditions on phase purity and crystal quality of SnS thin films deposited by close-spaced vacuum sublimation technique; (ii) based on optimized SnS growth conditions, we further fabricate tin sulfide solar cells using traditional CdS buffer and i-ZnO/ZnO:Al bilayers and investigate the impact of molybdenum and titanium back metal contacts on photovoltaic parameters of resulting devices; (iii) finally, using device simulations, we reveal the impact of minority carrier lifetime in the SnS absorber and recombination velocity at the SnS/CdS heterointerface on solar cell parameters, demonstrating that these are the main factors affecting the performance of SnS-based photovoltaic devices.

The chemical, phase composition and optical properties of Zn<sub>x</sub>Cd<sub>1-x</sub>S films obtained by close spaced vacuum sublimation

The chemical, phase composition and optical properties of Zn<sub>x</sub>Cd<sub>1-x</sub>S films obtained by close spaced vacuum sublimation

Formation of SnS phase obtained by thermal vacuum annealing of SnS2 thin films and its application in solar cells

We demonstrate a simple approach to fabricate single-phase SnS thin films by thermal vacuum annealing of SnS2 layers obtained by the close-spaced vacuum sublimation method. It was found that the initial non-annealed SnS2 films exhibit typical chemical composition for SnS2 with ratio of Sn/S = 0.49. The structural quality of the SnS2 phase was studied by X-ray diffraction and Raman spectroscopy measurements. In particular, it was established that films have a hexagonal 2H-SnS2 crystal structure. The field emission scanning electron microscope analysis of the surface and cross-section shows plate-like crystallites with an average size of 2 µm. Annealing of the SnS2 samples was carried out at 300, 400, and 500 °C for 30, 60, and 90 min for each temperature, and at 600 °C for 30 min. It was shown that concentration of S gradually decreases with increasing annealing temperature and time. The samples annealed at 500 °C for 30, 60, and 90 min demonstrate a typical SnS composition ratio of Sn/S = 0.96. Further, by using X-ray diffraction, Raman, and energy dispersive X-ray analysis, we found that annealing of the samples at 500 °C for 30, 60, and 90 min provides a phase transition from hexagonal SnS2 to orthorhombic SnS. The shorter annealing time and temperature leads to the mixed SnS, Sn2S3, and SnS2 phase composition. The shape and size of plate-like crystallites remains the same after annealing. However, randomly distributed nano-pores were observed. Transmittance and reflectance measurements of the SnS2 and SnS films show both direct and indirect optical band gaps in the materials. For SnS films a large absorption coefficient of 104–105 cm−1 above the band gap was found. The current-voltage characteristic of the ITO/SnS2/Sn structure shows small rectification current, while the current-voltage curve of ITO/SnS/Sn is linear. The dark resistivity was found to be 1.01 × 105 Ω cm and 1.18 × 103 Ω cm for SnS2 and SnS films, respectively. The heterojunction structure was obtained by annealing of the SnS2 deposited on the ITO/CdS structure to obtain the SnS phase. The p-SnS/n-CdS heterojunction shows weak photovoltaic response under illumination at AM 1.5 conditions, namely, open circuit voltage (Voc), short circuit current density (Jsc), and fill factor (FF) of 0.35 V, 34.08 µA/cm2, and 0.42, respectively.

Photoluminescence of high optical quality CdS:Dy thin films deposited by close-spaced vacuum sublimation

CdS:Dy polycrystalline films were deposited by close-spaced vacuum sublimation (CSVS) method using glass slides as substrates. The films are highly dispersed with the grain size from smaller than 0.1 μm to 1.0 μm. The CdS:Dy films are n-type and only correspond to single wurtzite phase. The crystallites are preferentially oriented with (002) planes parallel to the substrate surface. Analysis of PL spectra, caused by the emission of donor-acceptor pairs lets us determine the nature and energy structure of the intrinsic defects and residual impurities in the films. It is important to note that the PL and reflection spectra observe the narrow lines of bound and free excitons. This allows us to found bandgaps of these films quite accurately. A correlation between optical and structural properties was established. The results of the structural and PL measurements showed that the CdS:Dy polycrystalline films are of highly crystal and optical quality.

Electrical, structural and optical properties of Cd1-xZnxTe thick polycrystalline films

Thick polycrystalline Cd1-xZnxTe films with different Zn concentration were deposited by the close-spaced vacuum sublimation method on ITO-coated glass substrates. In order to evaluate the usefulness of the films as a base layer in X-ray detector, the study of their electrical properties and photoresponse was carried out. Structural, microstructural properties and phase composition of the films were investigated using X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, and Raman spectroscopy. It was found that films have cubic zinc-blende crystal lattice; the microstructural analysis showed that crystalline quality (e.g. coherent scattering domains size and microstrains level) of the films is decreasing with increasing of Zn concentration. Raman measurements showed only CdTe- and ZnTe-related vibrational modes, which indicates the absence of tellurium inclusions at the surface.The temporal photoresponse of the detector was described with the help of the analytical model, taking into account the presence of two types of recombination centers. As a result, the parameters of the recombination centers and their possible influence on the current flow in the films were established.

The surface morphology, structural properties and chemical composition of Cd1−xZnxTe polycrystalline thick films deposited by close spaced vacuum sublimation

Thick polycrystalline Cd1−xZnxTe films with x ranged from 0.37 to 0.80 were obtained by the close spaced vacuum sublimation method. In order to investigate properties of the films structural, PIXE and Raman studies were carried out. Determination of chemical composition of the films by EDS, PIXE and XRD has shown good correlation of results. Raman spectroscopy reveals the relation between zinc concentration and vibrational properties of the films. Studies of the spatial distribution of the chemical elements on the film surface by micro-PIXE and micro-Raman spectroscopy have shown that films are uniform and free of secondary phases such as CdTe, ZnTe and Te inclusions.

Substructural Properties and Anisotropic Peak Broadening in Zn1−x Mn x Te Films Determined by a Combined Methodology Based on SEM, HRTEM, XRD, and HRXRD

Lattice deformation and extended defects such as grain boundaries and dislocations affect the crystalline quality of films and can dramatically change material’s properties. In particular, magnetic and optoelectronic properties depend strongly on these structural and substructural characteristics. In this paper, a combined methodology based on SEM, HRTEM, XRD, and HRXRD measurements is used to determine and assess the structural and substructural characteristics of films. This combined methodology has been applied to Zn1−x Mn x Te films grown on glass substrates by close-spaced vacuum sublimation. Nevertheless the methodology can be applied to a wide variety of materials and could become a useful characterization method which would be particularly valuable in semiconductor growth field. The knowledge of the structural and substructural characteristics can allow not only the optimization of growth parameters, but also the selection of specific samples having the desired characteristics (crystallite size, minimum dislocation content, etc.) for high-quality technological devices.

Plasmonic Effects in Tin Disulfide Nanostructured Thin Films Obtained by the Close-Spaced Vacuum Sublimation

This work reports the investigation of plasmonic effects in tin disulfide (SnS2) nanostructured films obtained by the close-spaced vacuum sublimation method (CSS). Structural properties and phase composition of SnS2 films were studied with the help of field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), X-ray diffraction (XRD) and Raman spectroscopy. Surface morphology and optical polarization properties of SnS2 films deposited at different substrate temperature were investigated by modulation-polarization spectroscopy (MPS). Surface plasmon resonances (SPR) with localized and polariton types were observed by measuring of angular and spectral characteristics of polarization difference. Radiative and non-radiative modes of surface plasmons have been analyzed at different light incident angles. The influence of surface morphology on resonant parameters of different types of SPR was studied. Correlation between the experimental results and theoretical calculations was established. The refractive and absorption indexes were found for the SnS2 films at different substrate temperature.

Laser-induced SnS2-SnS phase transition and surface modification in SnS2 thin films

A thin film of SnS2 obtained by close-spaced vacuum sublimation was irradiated by an Nd:YAG laser (λ = 532 nm) using two intensities of laser radiation of 8.5 MW/cm2 and 11.5 MW/cm2. It was shown that laser irradiation leads to evaporation of sulphur from the surface, and the formation of SnS and Sn2S3 phases. The study of samples’ cross-section by energy dispersive X-ray analysis reveals that in the case of irradiation at 8.5 MW/cm2 intensity, the SnS layer is formed only at the surface of the initial SnS2 thin film. The application of more intensive radiation of 11.5 MW/cm2 leads to changes in chemical composition for the entire thin film. The formation of the predominant SnS phase, which includes a small amount of Sn2S3, was confirmed by the X-ray diffraction and Raman spectroscopy methods, as well as by measurements of optical reflectance and transmittance spectra. It was established that laser irradiation of the samples leads to the coalescence of grains accompanied by smoothing of the surface. The current-voltage characteristics of the ITO/SnxSy/Al samples show an ohmic behaviour in the case of non-irradiated intensity samples; for irradiated samples, the diode behaviour of I-V curves was observed. This is considered as evidence of the formation of p-SnS/n-SnS2 heterojunction by laser irradiation.

Composition dependence of structural and optical properties of Cd1−xZnxTe thick films obtained by the close-spaced sublimation

This paper reports the results of structural, photoluminescence and Raman characterization of thick Cd1−xZnxTe films with different zinc concentration obtained by the close spaced vacuum sublimation method. The analysis of the X-rays patterns allows us to determine the effect of the zinc concentration on crystal quality of the films. It was found that samples with x ≈ 0.10 and x ≈ 0.32 have high crystal quality. However, with increasing of zinc concentration the crystal quality decreases. This result was confirmed by the photoluminescence study. Namely, the significant degradation of optical properties for the samples with high zinc concentration (x > 0.32) was observed. Raman spectroscopy reveals the relation between zinc concentration and vibrational properties of the films. Also, the micro-Raman method shows that obtained films are uniform and free of tellurium inclusions.

Structural and microstructural properties of Cd1-xZnxTe films deposited by close spaced vacuum sublimation

Structural and microstructural properties of Cd1-xZnxTe films deposited by close spaced vacuum sublimation

Structural and electrical properties of SnS2 thin films

The effect of substrate temperature on the structural and electrical properties, phase composition, and surface morphology of tin disulfide thin (SnS2) films obtained by the close-spaced vacuum sublimation (CSS) method was studied.Scanning electrical microscope (SEM) images of the samples showed that all of the films were polycrystalline with an average grain size of 0.7–1.2 μm. The average thickness of the thin films was 1 μm.Energy dispersive spectroscopy (EDS) analysis showed that all layers had close to stoichiometric atomic composition. Namely, the concentrations of tin and sulfur were 35 and 65% respectively.X-ray diffraction (XRD) study indicated that the samples obtained at 473–723 K mostly contained hexagonal phase SnS2 with high texture along the (002) crystallographic plane. The values of the lattice constants (a and c) of SnS2 thin films increase monotonically with substrate temperature from 0.3637 to 0.3647 nm and from 0.5703 to 0.5743 nm, respectively.Investigation of the SnS2 films by Raman spectroscopy confirmed the results of XRD studies, namely that the layers have single-phase hexagonal structure of 2H polytype.Studies of the electrical properties of SnS2 thin films showed that the conductivity of the films changed from 1.8 × 10−4 to 10−7 (Om⋅cm)−1. Analysis of the I–V characteristics in the space-charge limited current (SCLC) mode made it possible to define (Et1 = (0.52–0.55), Et2 = (0.46–0.49), Et3 = (0.43–0.45), and Et4 = (0.35–0.39) eV), the localized states energy depths in the band gap of the SnS2 thin films. The concentration of these localized states exceeds 1.31 × 1014 ≿m−3. Also, from the measurements of temperature dependent conductivity, several localized states with activation energies of 0.25 and 0.26 eV states were determined.

Photoluminescence of CdZnTe thick films obtained by close-spaced vacuum sublimation

Polycrystalline Cd1−xZnxTe thick films with thicknesses of about 30μm have been deposited on a Mo coated glass substrate by means of close-spaced vacuum sublimation technique. X-ray diffraction measurements have shown that the films obtained have only cubic zinc blende phase. The influence of Zn concentration on the photoluminescence (PL) spectra of Cd1−xZnxTe films was investigated. This let us determine the nature and energy structure of the intrinsic defects and residual impurities in the films. The presence of the most intense acceptor bound exciton A°X-line for x=0.10 and the lines of localized excitons (x=0.32−0.44) in PL spectra of Cd1−xZnxTe films indicates their fairly good optical quality as well as the p-type conductivity. There were also other intensive broad PL bands, caused by the recombination of donor–acceptor pairs involving complex acceptor centers, extended defects of dislocation type, and microstress in the films. It was also established a correlation between the broadening of exciton lines and the values of microstress in Cd1−xZnxTe thick films. Taking into account the energy position of exciton lines, the concentration dependence of the band gap for the Cd1−xZnxTe thick films is presented.

Influence of substrate temperature on structural and optical properties of bismuth oxide thin films deposited by close-spaced vacuum sublimation

Bi2O3 thin films were deposited on ultrasonically-cleaned glass and mica substrates by close-spaced vacuum sublimation technique. Films surface morphology was studied using scanning electron microscopy (SEM). Structural study based on the transmission-electron microscopy (TEM) and selected-area electron diffraction (SAED) analysis has been shown that deposited films were polycrystalline with face-centered cubic structure. Optical study was carried out by spectral photometry analysis in the wavelengths range λ = 320–900 nm using the optical transmittance and absorbance measurements. For determination optical band gap Eg the Tauc plot was used and the band gap energy Eg is determined in the range of 3.50–3.62 eV, respectively. Fourier-transform infra-red (FTIR) analysis shown that obtained films are well-crystalline and have a good optical quality.

Morphological, structural, compositional properties and IR-spectroscopy of CdSe films deposited by close-spaced vacuum sublimation

The polycrystalline CdSe films were deposited by the close-spaced vacuum sublimation technique at the different substrate temperatures (373–873 K). Surface morphology, grain size and growth mechanism of the films were determined by the scanning electron microscopy. The X-ray diffraction analysis of structural and sub-structural properties of the films was carried out to study their phase composition and growth texture. The main structural parameters of thin films, such as texture, lattice parameter, grain size, scattering domain size and micro-stress level have been determined in the work depending on the condensation film conditions. RBS and FTIR analysis shows that obtained films in general are homogenous and pure. As a result, the growth conditions of CdSe polycrystalline films with good crystal quality were determined.

Influence of deposition conditions on morphological, structural, optical and electro-physical properties of ZnSe films obtained by close-spaced vacuum sublimation

ZnSe thin films were deposited on well-cleaned glass substrates by the close-spaced vacuum sublimation technique. Various structural, sub-structural and optical properties have been investigated through scanning electron microscopy (SEM), X-ray diffraction (XRD), spectral photometry, Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy. Electro-physical studies were performed using an ITO/ZnSe/In “sandwich” structure. The correlation between the films structure and their optical and electro-physical properties was estimated. The evaluated films were fine-crystalline, with their grain size depending on the substrate temperature. The optical band gap was estimated through optical measurements and the high optical qualities of the ZnSe films were confirmed by Raman and FTIR analyses.

Effect of CdCl2 annealing treatment on structural and optoelectronic properties of close spaced sublimation CdTe/CdS thin film solar cells vs deposition conditions

CdTe/CdS thin films were deposited by close-spaced vacuum sublimation at substrate temperatures 250–500 °C. The effect of substrate temperature in conjunction with CdCl2 heat treatment on the properties of CdTe/CdS thin films and devices is reported. X-ray diffraction measurements of structural properties of the films were carried out to study their phase composition and texture. Surface morphology, grain size and growth mechanism of the films were determined by the scanning electron microscopy. The photoluminescence measurements allowed us to establish the correlation between the point and extended defect structure on the one hand and the growth conditions on the other. The CdTe/CdS solar cells fabricated at high substrate temperature, exhibited improved electrical characteristics compared to cells fabricated at low substrate temperature, yielding a high efficiency.

Study of the correlation between structural and photoluminescence properties of CdSe thin films deposited by close-spaced vacuum sublimation

CdSe polycrystalline films were deposited by a close-spaced vacuum sublimation method at different substrate temperatures (Ts) using glass slides as substrates. At Ts≤673K the films have a structure with strong dispersion of grain size (d) (from 0.1 to 0.3μm). In this case the layer-by-layer mechanism determines the growth process of the layers. For Ts=873K they have a columnar-like structure with a clear growth texture and the average grain size d=3–4μm. The films obtained at Ts>473K are n-type and only correspond to a single wurtzite phase. The crystallites are preferentially oriented with the (102) planes parallel to the substrate. At lower temperatures the films are bi-phase. The microstress level in CdSe films obtained at Тs=873K (0.5×10−3) is considerably smaller than for the films deposited at Тs=773K (4.0×10−3). Increase of the value of Ts improves the stoichiometry of CdSe films. Analysis of the low-temperature photoluminescence (PL) spectra let us determine the nature and energy of point and extended defects in the investigated films. It was shown that the films contain Na(Li) and P residual impurities. The results of the structural and PL measurements showed that the CdSe polycrystalline films are of fairly good crystal and optical quality for Ts=873K and can be suitable for various applications.