Close-spaced Sublimation Research Articles

Self-powered, high-speed Sb2Se3/Si heterojunction photodetector with close spaced sublimation processed Sb2Se3 layer

Antimony selenide (Sb2Se3) has received great attention recently due to its excellent carrier transport capacity and special one-dimensional crystal structure. The Sb2Se3-based bulk heterojunctions have been applied for photovoltaic with great success, but its potential application to photodetector has not been explored yet. In this work, a self-powered, high-speed Sb2Se3/Si heterojunction photodetector is fabricated and investigated, where the Sb2Se3 layer is deposited through close spaced sublimation (CSS) technique. The photodetector exhibits detection performance with a responsivity of 0.34 A/W at zero bias voltage and a broad spectral range from 300 to 1100 nm. Ultrafast response rise and fall times of 86 and 96 μs, respectively, are obtained, which is significantly better than previous reports of Sb2Se3-based photodetectors. The high crystallographic quality of the CSS-processed Sb2Se3 thin films and the built-in field at the Sb2Se3/Si interface is responsible to the high response speed of Sb2Se3/Si heterostructural photodetectors. The high-speed and broadband response properties make the Sb2Se3/Si heterojunction photodetector have potential applications for next-generation optoelectric devices.

Structural and chemical evolution of the CdS:O window layer during individual CdTe solar cell processing steps

Oxygenated cadmium sulfide (CdS:O) is often used as the n-type window layer in high-performance CdTe heterojunction solar cells. The as-deposited layer prepared by reactive sputtering is XRD amorphous, with a bulk composition of CdS0.8O1.2. Recently it was shown that this layer undergoes significant transformation during device fabrication, but the roles of the individual high temperature processing steps was unclear. In this work high resolution transmission electron microscopy coupled to elemental analysis was used to understand the evolution of the heterojunction region through the individual high temperature fabrication steps of CdTe deposition, CdCl2 activation, and back contact activation. It is found that during CdTe deposition by close spaced sublimation at 600 °C the CdS:O film undergoes recrystallization, accompanied by a significant (∼30%) reduction in thickness. It is observed that oxygen segregates during this step, forming a bi-layer morphology consisting of nanocrystalline CdS adjacent to the tin oxide contact and an oxygen-rich layer adjacent to the CdTe absorber. This bilayer structure is then lost during the 400 °C CdCl2 treatment where the film transforms into a heterogeneous structure with cadmium sulfate clusters distributed randomly throughout the window layer. The thickness of window layer remains essentially unchanged after CdCl2 treatment, but a ∼25 nm graded interfacial layer between CdTe and the window region is formed. Finally, the rapid thermal processing step used to activate the back contact was found to have a negligible impact on the structure or composition of the heterojunction region.

CdTe-Based Photoanode for Oxygen Evolution from Water under Simulated Sunlight.

This study investigated the properties of a photoanode fabricated by depositing a p-type CdTe thin film on a CdS-coated FTO substrate (CdTe/CdS/FTO) via close-space sublimation. This CdTe/CdS/FTO electrode was found to work as a photoanode with a long absorption edge wavelength of 830 nm. In a CdTe-based photoanode such as this, the p-n junction formed at the CdTe/CdS interface promotes charge separation of photoexcited carriers and forces photogenerated holes to move toward the photoanode surface to promote oxidation reactions on the electrode surface. A MoOx buffer layer was also found to play a crucial role in facilitating the transfer of photogenerated holes to surface reaction sites through decreasing the energy barrier at the interface between the CdTe and a surface protective layer. A biphotoelectrode photoelectrochemical cell composed of a CdTe-based photoanode and a CdTe-based photocathode exhibited a solar-to-hydrogen conversion efficiency of 0.22% without an external voltage in response to illumination by AM 1.5G light.

Cd 1-x Zn x Te photodetectors with transparent conductive ZnO contacts

High quality Cd1-xZnxTe (CZT) films were prepared using the close-spaced sublimation (CSS) technique. CZT film UV (ultraviolet) photodetectors were fabricated with B and Ga co-doped ZnO (BGZO) transparent conductive interdigitated contacts. The contact properties of BGZO/CZT were investigated by the transmission line model (TLM). The results indicate that a good ohmic contact is formed between BGZO and CZT with a very low contact resistivity of about 0.26 Ω·cm2. Compared with CZT photodetectors with Au contacts, the detectors with BGZO contacts show a higher value of UV photo response.

Raman Characterisation of Cd1-xZnx Te Thick Polycrystalline Films Obtained by the Close-Spaced Sublimation

Raman Characterisation of Cd_1-xZn_x Te Thick Polycrystalline Films Obtained by the Close-Spaced Sublimation

The investigation of Ga-doped ZnO as an interlayer for ohmic contact to Cd1−xZnxTe films

In this work, high quality Cd1−xZnxTe films were prepared on fluorine doped tin oxide (FTO) glass substrates by close-spaced sublimation (CSS) method. A low resistivity sputtered Ga-doped ZnO (GZO) film was used as an interlayer between Au electrodes and Cd1−xZnxTe films try to reduce the contact resistance and contribute to bring about a better Ohmic contact. Circular transmission line model (CTLM) was adopted to investigate the effects of GZO intermediate layer on the contact properties of Au/GZO/Cd1−xZnxTe structure. The results show a low contact resistivity of 0.37Ωcm2 for Au/GZO contacts on Cd1−xZnxTe films. Cd1−xZnxTe film radiation detectors were also fabricated using Au/GZO contacts and an energy resolution of about 28% was obtained from a 60KeV 241Am γ-ray source for the first time.

Nucleation of Cu2Te layer by a closed space sublimation method toward the growth of Te based chalcopyrite

Nucleation of Cu-Te layers was performed by the closed space sublimation method using various source materials, source temperatures, and Si substrates with different surface orientations. The objective was to produce nuclei layers with high quality for use as nucleation centers for CuGaTe2. The grown samples were evaluated by x-ray diffraction and scanning electron microscopy. Cu2Te deposits were obtained using either a CuTe or Cu2Te source, but the latter gave a higher area coverage of Cu2Te nuclei. Highly oriented nuclei were obtained when they were grown on Si (001) at a source temperature of 640 °C and substrate temperature of 590 °C. When the source temperature was raised to 750 °C and the corresponding substrate temperature was 700 °C, nonuniform but highly oriented nuclei were obtained. Both nuclei layers exhibited a strong preference for (0001) orientation. The crystallographic features of the Cu2Te nuclei formed on Si (111) were similar to those of the Cu2Te nuclei formed on Si (001).

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.

Study of Cd1–xSnxTe Thin Films Prepared by Closed Space Sublimation

Study of Cd_1–<i>x</i>Sn_<i>x</i>Te Thin Films Prepared by Closed Space Sublimation

Reduced electrical performance of Zn enriched ZnTe nanoinclusion semiconductors thin films for buffer layer in solar cells

Closed space sublimation (CSS) technique was employed to deposit thin films of zinc telluride (ZnTe) on a glass substrate under high vacuum. Two sets of ZnTe thin films and Zn enriched ZnTe thin films were prepared for comparative study. The enrichment for Zn onto the as-deposited ZnTe thin films was done by the novel manner of layer by layer deposition with subsequent annealing. X-ray diffraction (XRD) studies revealed before and after the enrichment of Zn the preferred orientation is [1 1 1] having cubic phase. The lattice constant was found to be increased and the crystallite size decreased 28 nm to 24 nm after the enrichment of Zn. A morphological study was carried out through a scanning electron microscope (SEM). For Zn enriched samples the average grain size is smaller as compared to ZnTe thin films. The local compositions of Zn and Te were confirmed by energy dispersive x-rays (EDX) from 51 atomic % of as-deposited ZnTe thin films to 68 atomic % in Zn enriched ZnTe thin films. The Zn enriched samples have a slight decrease in optical transmission in UV–VIS–NIR range as compared to the as-deposited ZnTe thin films. Due to the deposition of Zn there is a very small change in optical band gap energy. A four-probe technique was used to study electrical properties of as-deposited and Zn-enriched ZnTe thin films. These results shows that the as-deposited samples had the resistivity of 106 Ω · cm. For Zn enrichment samples resistivity increases from 106 Ω · cm to 108 Ω · cm, which shows that Zn-enriched samples are not suitable for back contact of II–VI solar cells. X-rays photoelectron spectroscopy (XPS) was used to confirm the elemental compositions and its bonding strength before and after the enrichment of Zn.

Low temperature ferromagnetic properties of CdS and CdTe thin films**Project supported by the National Natural Science Foundation of China (Grant No. 61474103) and the Chinese Scholarship Council (CSC) Fellowship for H. Tariq Masood and Z. Muhammad.

The magnetic property in a material is induced by the unpaired electrons. This can occur due to defect states which can enhance the magnetic moment and the spin polarization. In this report, CdS and CdTe thin films are grown on FTO glass substrates by chemical bath deposition and close-spaced sublimation, respectively. The magnetic properties, which are introduced from oxygen states, are found in CdS and CdTe thin films. From the hysteresis loop of magnetic moment it is revealed that CdS and CdTe thin films have different kinds of magnetic moments at different temperatures. The M–H curves indicate that from 100 K to 350 K, CdS and CdTe thin films show paramagnetism and diamagnetism, respectively. A superparamagnetic or a weakly ferromagnetic response is found at 5 K. It is also observed from ZFC/FC curves that magnetic moments decrease with temperature increasing. Spin polarized density functional calculation for spin magnetic moment is also carried out.

CdTe/CdS solar cells with CdTe grown at low vacuum

CdTe thin films were grown on FTO/CdS substrate using a close space sublimation technique. The temperature of CdTe source was kept constant at 650 °C while the substrate temperature was varied from 500 °C to 580 °C under a constant pressure of 10−2 Torr. Subsequently, the films were activated in the atmosphere of CdCl2 vapor at different temperatures from 390 °C to 410 °C for 10 min. After the pressure of CdTe were explored of vacuum, 0.3, 0.5, 1 and 3 Torr. It was found that CdTe grown at 3 Torr exhibited 10.6% of efficiency, and approximately 80% of QE. The XRD analysis showed that the crystalline phase is cubic with preferential orientation in (111) plane. In the present work, we have demonstrated that it is possible to obtain device efficiencies above 10% with a chamber pressure of 3 Torr during CdTe film deposition and without using commonly utilized atmospheres, such as He-O2 or Ar-O2 for the CdTe growth and device activation. The present work contributes significantly in the development of low cost and highly efficient CdTe/CdS solar cells fabricated under low vacuum.

Physical Properties of CdS&lt;sub&gt;x&lt;/sub&gt;Te&lt;sub&gt;1-x &lt;/sub&gt;Thin Films Prepared by Close Spaced Sublimation Method

CdSxTe1-x (0 ≤ x ≤ 1) thin films were prepared on clear quartz glass substrate by close spaced sublimation method using stoichiometric mixed powders of pure CdS and CdTe compounds. Crystal structure of CdSxTe1-x thin films was cubic structure with a preferential orientation of (111) plane when the S mole ratio less than 0.2. However, For the composition 0.2 ≤ x ≤ 0.8, the cubic and the hexagonal phases were found to coexist in the system and the films became less preferentially oriented whereas CdS thin films formed in the hexagonal phase with a preferential orientation of (002) plane. SEM micrographs showed the grain size decreased when the S content increased. As the S content increased, the energy gap value of CdSxTe1-x thin films varied from 1.48 eV (for CdTe) to 2.25 eV (for CdS). Dark electrical sheet resistance of CdSxTe1-x thin films decreased as a function of S content to minimum value about 2.24×107 Ω/sq for x=0.8 and then slightly increased to 4.13×107 Ω/sq for x=1.0. From the transient photoconductivity measurements, the highest photosensitivity value about 60.0 was obtained for the films with x = 0.8.

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.

Elemental, structural and optical properties of nanocrystalline Zn 1−x Cu x Se films deposited by close spaced sublimation technique

Abstract The elemental composition, film thickness and concentration depth profiles of as-deposited and annealed Zn 1−x Cu x Se films were studied by the Rutherford backscattering spectrometer (RBS) technique. The films were deposited on glass substrates by close spaced sublimation (CSS) technique. As-deposited films of about 250–300 nm thickness were then annealed in air at temperatures of 200 °C and 400 °C for 1 h. Structural characterization including crystal structure, crystal orientation, stacking fault energy ( Ү SFE ) and surface morphology were carried out by using X-ray diffraction (XRD) and atomic force microscopy (AFM). XRD studies revealed that the fabricated films are polycrystalline with a zinc-blende structure and a strong (111) texture plane. Surface roughness was observed to be enhanced with annealing temperature with a decrease in stacking fault energy ( Ү SFE ). Spectroscopic ellipsometry has been utilized for the estimation of band gap energy (E g ) and dielectric constant (e 1 ). Band gap energy of the film increased with increasing annealing temperature while the dielectric constant decreased.

Photoelectrochemical hydrogen evolution from water on a surface modified CdTe thin film electrode under simulated sunlight

Photoelectrochemical hydrogen evolution from water using CdTe photocathodes prepared by close spaced sublimation was investigated. A CdTe thin film deposited on a fluorine-doped tin oxide-coated (FTO) glass plate acted as the photocathode. Surface modification of this photocathode with Pt resulted in an increase in the cathodic photocurrent from only 0.01 to 0.14 mA cm−2 at 0.1 VRHE. Further surface modification with an approximately 80 nm-thick CdS layer dramatically increased both the cathodic photocurrent and the onset potential, from 0.14 to 6.0 mA cm−2 at 0.1 VRHE and from 0.2 to 0.6 VRHE, respectively, under simulated sunlight. The CdTe photocathode showed stoichiometric hydrogen evolution from water, with a half-cell solar to hydrogen conversion efficiency of 0.9% at approximately 0.2 VRHE.

The effects of Cu-doped ZnTe intermediate layer on the Ohmic contact to CdZnTe films

Polycrystalline CdZnTe films with thickness about 300μm were deposited by close spaced-sublimation (CSS) method. The as-deposited CdZnTe samples were polished by mechanical polishing (MP) process. Cu-doped ZnTe films were sputtered by RF magnetron sputtering as an intermediate layer to improve the performance of Ohmic contact of Au electrodes to CdZnTe films. The effect of Cu-doped ZnTe intermediate layers on the electrical characteristics of Au/CdZnTe and Au/ZnTe:Cu/CdZnTe was investigated by the circular transmission line model (CTLM). The results indicate a good Ohmic contact of Au/ZnTe:Cu/CdZnTe with low contact resistivity of about 0.79Ω·cm2.

Annealing of Thin-film Cadmium Telluride Photovoltaics Using Holographically Controlled Laser Processing

Abstract CdTe-based thin film solar cells currently represent one of the fastest growing PV technologies, with a superior combination of efficiency, energy payback time and lifecycle environmental impact. However, the current post-deposition annealing treatment is still an energy intensive step of the manufacturing process. A novel method is presented for annealing of CdTe using a high-power diode laser (35 W, 808 nm) for thermal post-processing, combined with holographic optical elements (HOE's) for laser beam heat flow control. The advantage of a laser for annealing lies in its ability to selectively heat only the surface of the CdTe solar cell; improving energy efficiency, process speed and energy resilience. Heat transfer simulations were used to predict the effects of different laser irradiance profiles on the annealing process thermal cycle influence the experimental design and predict optimal laser irradiance profiles. Variations in power and process speed on as-deposited and MgCl 2 -treated close-space sublimated (CSS) CdTe samples have been performed. The results were characterised using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Optical properties were analysed with a spectrophotometer and ellipsometric spectroscopy (SE). The laser annealing treatment was found to be effective in promoting Chlorine diffusion and improving the optical and morphological properties of CdTe thin film devices.

Optical spectroscopic analysis of annealed Cd1−xZnxSe thin films deposited by close space sublimation technique

Abstract Cd1−xZnxSe (x = 0, 0.40 and 1) thin films were deposited on a glass substrate at room temperature by closed space sublimation method. Optical investigation has been performed using spectrophotometry and ellipsometry. It has been found that for as deposited films the optical band gap increased and the optical constants decreased with increasing Zn content. To improve the optical properties of Cd1−xZnxSe thin films annealing effect at 400 °C was taken into consideration for various Zn contents. It was observed that the optical transmittance and band gap decreased while optical constants increased with increasing Zn content after annealing. The effects of composition and annealing on the optical dispersion parameters Eo and Ed were investigated using a single effective oscillator model. The calculated value of the average excitation energy Eo obeys the empirical relation (Eo = Eg/2) obtained from the single oscillator model.

CdCl2 Treatment-Induced Enhanced Conductivity in CdTe Solar Cells Observed Using Conductive Atomic Force Microscopy.

The rear surfaces of CdTe photovoltaic devices without back contacts, grown by close-spaced sublimation (CSS), were analyzed using conductive atomic force microscopy (C-AFM). As-deposited and CdCl2-treated CdTe samples were compared to clarify the effect of the treatment on charge flow through grains and grain boundaries. The CdCl2-treated samples exhibit a more homogeneous and enhanced current flow across the grains as compared to the as-deposited samples. The grain boundaries show variable current. Under high bias, grain boundaries dominate current flow when the main junction is reverse biased and with the conducting current in reverse breakdown. Under the opposite bias conditions, where the contact of the conductive tip to the surface is reverse biased and under breakdown conditions, the current flow is uniform with little contrast between grains and grain boundaries. The results are interpreted as resulting from the improved crystallinity of the CdTe with reduced p-type doping along the grain boundaries.