Study of the Al-grading effect in the crystallisation of chalcopyrite CuIn1−xAlxSe2 thin films

Chalcopyrite CuIn_1−xAl_xSe₂ (CIAS) thin films with an atomic ratio of Al/(In + Al) = 0.4 were grown by a two-stage process onto soda-lime glass substrates. The selenisation was carried out at different temperatures, ranging from 400 °C to 550 °C, for metallic precursors layers evaporated with two different sequences. The first sequence, C1, was evaporated with the Al as the last layer, while in the second one, C2, the In was the last evaporated element. The optical, structural and morphological characterisations led to the conclusion that the precursors sequence determines the crystallisation pathway, resulting in C1 the best option due to the homogeneity of the depth distribution of the elements. The influence of the selenisation temperature was also studied, finding 540 °C as the optimum one, since it allows to achieve the highest band gap value for the C1 sequence and for the given composition.     

Effect of substrate temperature on ac conduction properties of amorphous and polycrystalline GaSe thin films

X-ray diffraction analysis of GaSe thin films used in the present investigation showed that the as-deposited and the one deposited at higher substrate temperature are in amorphous and polycrystalline state, respectively. The alternating current (ac) conduction properties of thermally evaporated films of GaSe were studied ex situ employing symmetric aluminium ohmic electrodes in the frequency range of 120-10⁵ Hz at various temperature regimes. For the film deposited at elevated substrate temperature (573 K) the ac conductivity was found to increase with improvement of its crystalline structure. The ac conductivity (σ_ac) is found to be proportional to (ω^s) where s < 1. The temperature dependence of ac conductivity and the parameter, s, is reasonably well interpreted by the correlated barrier-hopping (CBH) model. The maximum barrier heights W_m calculated from ac conductivity measurements are compared with optical studies of our previous reported work for a-GaSe and poly-GaSe thin films. The distance between the localized centres (R), activation energy (ΔE_σ) and the number of sites per unit energy per unit volume N(E_F) at the Fermi level were evaluated for both a-GaSe and poly-GaSe thin films. Goswami and Goswami model has been invoked to explain the dependence of capacitance on frequency and temperature.     

Raman spectroscopy for quality control and process optimization of chalcopyrite thin films and devices

We will demonstrate in this paper that Raman scattering of visible light is a versatile tool both for research and industrial process monitoring of thin chalcopyrite films for solar cells. Thin films of Cu(In, Ga)(S,Se)₂ (CIGSSe) are produced by rapid thermal processing of stacked elemental Cu-In-Ga-Se layers. The Raman investigations are accompanied by grazing incidence X-ray diffraction (GI-XRD) and X-ray florescence (XRF) measurements. GI-XRD measurements confirm that the films show a two-fold elemental gradient: a sulfur gradient from the top and a Ga gradient from the CIGSSe/Mo interface. By Rietveld refinement of the GI-XRD spectra of the surface-near (∼ 100nm) ratio of sulfur to selenium can be obtained which corresponds well to the intensity ratio of the two Raman A1 modes of CuInS₂ and CuInSe₂. The asymmetric line shape of both XRD diffractograms and Raman spectra is attributed to the sulfur gradient. In addition we show that the intensity ratio of the satellite Raman B and E modes shows a correlation with the Cu to In + Ga ratio obtained by XRF.     

Structural characterization and novel optical properties of defect chalcopyrite ZnGa2Te4 thin films

Stoichiometric thin film samples of the ternary ZnGa₂Te₄ defect chalcopyrite compound were prepared and characterized by X-ray diffraction technique. The elemental chemical composition of the prepared bulk material as well as of the as-deposited film was determined by energy-dispersive X-ray spectrometry. ZnGa₂Te₄ thin films were deposited, by conventional thermal evaporation technique onto highly cleaned glass substrates. The X-ray and electron diffraction studies revealed that the as-deposited and the annealed ZnGa₂Te₄ films at annealing temperature t_a ≤ 548 K are amorphous, while those annealed at t_a ≥ 573 K (for 1 h), are polycrystalline. The optical properties of the as-deposited films have been investigated for the first time at normal incidence in the spectral range from 500 to 2500 nm. The refractive index dispersion in the transmission and low absorption region is adequately described by the Wemple–DiDomenico single oscillator model, whereby, the values of the oscillator parameters have been calculated. The analysis of the optical absorption coefficient revealed an in-direct optical transition with energy of 1.33 eV for the as-deposited sample. This work suggested that ZnGa₂Te₄ is a good candidate in solar cell devices as an absorbing layer.     

Effect of the substrate on the electrodeposition of Bi2Te3−ySey thin films

The potentiostatic electrodeposition of n-type Bi₂Te_3−ySe_y thermoelectric films onto stainless steel and gold substrates from nitric acid aqueous solutions has been carried out at room temperature. The cathodic process during the electrodeposition of Bi₂Te_3−ySe_y films was investigated by cyclic voltammetric experiments. The structure and surface morphology of Bi₂Te_3−ySe_y films deposited on both substrates were characterized by X-ray diffraction (XRD) and environment scanning electron microscopy (ESEM) coupled with energy dispersive spectroscopy (EDS). Electrical and thermoelectric properties of as-deposited films were also measured at room temperature. The results show that the reduction process under the same depositing conditions on gold and stainless steel substrates is very different. On gold substrates, H₂SeO₃ in the electrolyte is firstly reduced to elemental Se, and then the deposited Se reacts with HTeO₂⁺ and Bi³⁺ to form Bi₂Te_3−ySe_y alloy. On stainless steel substrates, HTeO₂⁺ in the electrolyte is firstly replaced by elemental Fe to produce elemental Te, and subsequently the generated Te reacts with H₂SeO₃ and Bi³⁺ to form Bi₂Te_3−ySe_y alloy. Analysis of ESEM show that the surface morphology of the films electrodeposited on gold substrates is more compact than that on stainless steel substrates. The XRD patterns indicate that the films electrodeposited on both substrates exhibit preferential orientation along (1 1 0) plane, but the relative peak intensity of (0 1 5) and (2 0 5) planes on stainless steel substrates is stronger than that on gold substrates. The Seebeck coefficient and electrical resistivity of the films deposited on stainless steel substrates are higher than that on gold substrates.     

Hysteresis,structural and composition characteristics of deposited thin films by reactively sputtered titanium target in Ar/CH4/N2 gas mixture

Abstract

Hysteresis, crystal structure and chemical composition of thin films deposited through reactive sputtering of titanium metal target in Ar/CH₄/N₂ gas mixture have been investigated. The transition from metallic to compound sputtering mode was clearly seen as the reactive gases (CH₄ and N₂) flowrate concentration first increased and subsequently decreased. Abrupt cathode current drop from 273 mA to reach a minimum value of 195 mA was observed upon addition of nitrogen gas from 0 to 10% flowrate concentration to the Ar/CH₄ gas mixture. This was also accompanied by an abrupt change in reactive gas partial pressure. Exploration of the deposition rate and film thickness showed that it decreased from 4·5 to 1·5 nm min^?1 and from 140 to 40 nm as the N₂ flowrate concentration increased from 1·5 to 7·5% at 5·5%CH₄ flowrate concentration respectively. X-ray diffraction and X-ray photoelectron spectroscopy analyses of the deposited films confirmed the formation of titanium carbide and carbonitride phases as the methane and nitrogen gas concentrations in the sputtering gas were increased.     

Stabilization of γ-La2S3 by alkali metal ion doping

La₃S₄ is known to crystallize in a defect cubic Th₃P₄ structure. The effect of alkali metal ion doping in the lacunar La_3−xS₄ structure has been studied. Compounds with molecular formula La_3−xA_xS₄ (A = Li, Na and K) with varying A/La ratios (0.1, 0.15, 0.2, 0.25 and 0.3) have been synthesized by gas-solid reaction method. The band gap increases with increase in dopant concentration. The optical properties show an increase in yellowness of the doped compositions when compared to the parent phase.     

The influence of orientation on the photoluminescence behavior of ZnO thin films obtained by chemical solution deposition

A new type of ZnO thin films synthesized from chemical solution deposition at low temperature has been presented. X-ray powder diffraction and field emission scanning electron microscopy investigation reveal that the novel structured ZnO film is uniform and its [0001] direction is parallel to the substrate. The photoluminescence spectrum of this film shows strong ultraviolet band-gap emission and weak defect-related visible emission comparing to that of [0001]-oriented film, indicating high crystal quality of the non-[0001]-oriented ZnO film.     

Some studies on successive ionic layer adsorption and reaction (SILAR) grown indium sulphide thin films

Indium sulphide (In₂S₃) thin films were grown on amorphous glass substrate by the successive ionic layer adsorption and reaction (SILAR) method. X-ray diffraction, optical absorption, scanning electron microscopy (SEM) and Rutherford back scattering (RBS) were applied to study the structural, optical, surface morphological and compositional properties of the indium sulphide thin films. Utilization of triethanolamine and hydrazine hydrate complexed indium sulphate and sodium sulphide as precursors resulted in nanocrystalline In₂S₃ thin film. The optical band gap was found to be 2.7 eV. The film appeared to be smooth and homogeneous from SEM study.     

Novel electrochemical technique: Grain control in preparation of polycrystalline BaMoO4 film

BaMoO₄ films have been prepared by a novel electrochemical technique without impressed current on molybdenum substrates at room temperature in barium hydroxide aqueous solution. In order to form desired surface morphology and grain size, the BaMoO₄ crystal grains have been controlled effectively by adjusting ion-concentration and adding adequate surfactant. The as-prepared BaMoO₄ films have been characterized by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD). SEM and XRD results indicate that the morphology, grain size, phase and crystallinity were significantly developed.     

Homoleptic single molecular precursors for the deposition of platinum and palladium chalcogenide thin films

Platinum and palladium dithio/diselenoimidodiphosphinato complexes have been synthesized and used as single source precursors for the deposition of platinum and palladium chalcogenide thin films by the Low Pressure Chemical Vapour Deposition technique. Pure PtSe₂ thin films were deposited at varying temperatures from the [Pt{N(SePⁱPr₂)₂}₂] complex. However, only Pt films were obtained from the sulfur analogue. The palladium diselenoimidodiphosphinato complex gave palladium selenide films with different stochiometries depending on the growth temperature. The corresponding sulfur complex gave PdS₂ films with a regular platelet-like morphology.     

Synthesis of highly crystalline silver dendrites microscale nanostructures by electrodeposition

Highly crystalline dendritic silver nanostructures have been synthesized by electrodeposition with the assistance of triblock copolymer P123. In the silver dentrites, the diameter of the trunk is around 100 nm with its length up to 40 μm, and the length of its branches can reach 10 μm. Selected area electron diffraction patterns show that the main trunk and side branches both grow along <211> directions and the leaves grow along <11-1> directions of the cubic Fm3m structure. The effects of the growth conditions such as the concentrations of reagents, the current density, and the electrode distance on the morphology of silver dendrites have also been investigated. It is found that the concentrations of AgNO₃ and P123 play significant roles in the growth of the silver dendrites.     

Characteristics of highly orientated BiFeO3 thin films on a LaNiO3-coated Si substrate by RF sputtering

BiFeO₃ (BFO) films were grown on LaNiO₃-coated Si substrate by a RF magnetron sputtering system at temperatures in the range of 300-700 °C. X-ray reflectivity and high-resolution diffraction measurements were employed to characterize the microstructure of these films. For a substrate temperature below 300 °C and at 700 °C only partially crystalline films and completely randomly polycrystalline films were grown, whereas highly (001)-orientated BFO film was obtained for a substrate temperature in the range of 400-600 °C. The crystalline quality of BFO thin films increase as the deposition temperature increase except for the film deposited at 700 °C. The fitted result from X-ray reflectivity curves show that the densities of the BFO films are slightly less than their bulk values. For the BFO films deposited at 300-600 °C, the higher the deposition temperature, the larger the remnant polarization and surface roughness of the films present.     

Epitaxial multi-component rare-earth oxide: A high-k material with ultralow mismatch to Si

New gate dielectric substitute for high-k application requires well matched lattice parameters and an atomically defined interface with Si for optimal performance. Using molecular beam epitaxy technique, we have grown on Si(111) crystalline rare-earth oxide ultrathin films, (Gd_xNd_1 − x)₂O₃ (GNO), a multi-component material that is superior to either of its binary host oxides. By carefully characterizing its crystal structure, we have found that the epitaxial GNO film exhibits a single bixbyite cubic structure with ultralow lattice mismatch to Si, which is indistinguishable even by the powerful synchrotron radiation. This structural perfection could make the GNO a promising high-k material in future devices.     

Room-temperature preparation of highly crystallized CaWO4 film by a galvanic cell method

Highly crystallized CaWO₄ film has been prepared directly by a galvanic cell method on tungsten substrates in calcium hydroxide aqueous solution without impressed current at room temperature (25 °C). The scanning electron microscopy (SEM) and X-ray diffraction (XRD) results reveal that the crystallized film has a scheelite-type tetragonal structure, uniform and homogeneous surface. The film shows only the blue emission of 447.5 nm with the excitation light of 250 nm at room temperature. The formation mechanism of CaWO₄ film under the simple electrochemical process has been discussed. This method could resolve the repulsion of the electric field on the anode for the mass transfer. The crystal growth in the solution is freer.     

Solid-state reactions to synthesize nanostructured lead selenide semiconductor powders by high-energy milling

Both solid-solid and gas-solid reactions have been traced during high-energy milling of Se and PbO powders under vial (P, T) conditions in order to synthesize the PbSe phase. Chemical and thermodynamic arguments are postulated to discern the high-energy milling mechanism to transform PbO-Se micropowders onto PbSe-nanocrystals. A set of reactions were evaluated at around room temperature. Therefore an experimental campaign was designed to test the nature of reactions in the PbO-Se system during high-energy milling.     

Formation of novel morphologies of aragonite induced by inorganic template

A glass-slice was used as a template to induce formation and assembly of aragonite. Thermodynamic theory was applied to explain the production of the aragonite. Transformation of three-dimensional nucleation to template-based two-dimensional surface nucleation caused the production of aragonite. Hemisphere, twinborn hemisphere and flower-shaped particles were produced by direction of the glass-slices. Planes were always appeared in these as-synthesized samples because the nucleation and the growth of these samples were adsorbed at the surfaces of the glass-slices. The formation mechanism of the as-formed sample was proposed. Compared with organic template, the present study provides a facile method to apply inorganic template to prepare functional materials.     

Effects of composition on the phase transformation behavior of anatase TiO2 in photocatalytic ceramics

The present work focuses on the phase transformation mechanism of TiO₂ nanoparticles in photocatalytic ceramics. TiO₂ nanoparticles were mixed with fused quartz, glaze No.1, and glaze No.2 respectively and heat-treated. The phase transformation behavior of anatase TiO₂ was analyzed by XRD. The results show that the phase transformation behavior of anatase TiO₂ in photocatalytic ceramics is highly dependent on the glaze compositions rather than the crystal size effect, which is significantly different with TiO₂ alone. The phase transformation from anatase to rutile always starts near the softening temperature and finishes below the sphere temperature of the glaze. The higher flux contents in glaze the lower phase transformation temperature. The significant retarding effects of silica and phosphate on the anatase-to-rutile phase transformation appear negligible when modified TiO₂ are applied in glaze. The eutectic liquid is essential for the phase transformation behavior of anatase TiO₂ in photocatalytic ceramics.