Ammonia-free chemically deposited CdS films as active layers in thin film transistors

In this work we have grown CdS thin films using an ammonia-free chemical bath deposition process for the active layer in thin film transistors. The CdS films were deposited substituting sodium citrate for ammonia as the complexing agent. The electrical characterization of the as-deposited CdS-based thin film transistors shows that the field effect mobility and threshold voltage were in the range of 0.12-0.16 cm²V⁻¹ s⁻¹ and 8.8-25 V, respectively, depending on the channel length. The device performance was improved considerably after thermal annealing in forming gas at 250 °C for 1 h. The mobility of the annealed devices increased to 4.8-8.8 cm²V⁻¹ s⁻¹ and the threshold voltage decreased to 8.4-12 V. I_on/I_off for the annealed devices was approximately 10⁵-10⁶.     

CdS thin films growth by ammonia free chemical bath deposition technique

Cadmium Sulfide CdS thin films were deposited by chemical bath deposition technique using ethanolamine as complexing agent instead of commonly used ammonia to avoid its toxicity and volatility during film preparation. In order to investigate the film growth mechanism samples were prepared with different deposition times. A set of substrates were dropped in the same bath and each 30 minutes a sample is withdrawn from the bath, by this way all the obtained films were grown in the same condition. The films structure was analyzed by X rays diffraction. In early stage of growth the obtained films are amorphous, with increasing the deposition time, the films exhibits a pure hexagonal structure with (101) preferential orientation. The film surface morphology was studied by atomic force microscopy. From these observations we concluded that the early growth stage starts in the 3D Volmer-Weber mode, followed by a transition to the Stransky-Krastanov mode with increasing deposition time. The critical thickness of this transition is 120 nm. CdS quantum dots were formed at end of the film growth. The optical transmittance characterization in the UV-Visible range shows that the prepared films have a high transparency ranging from 60 to 80% for photons having wavelength greater than 600 nm.     

Shallow bath chemical deposition of CdS thin film

Cadmium sulfide thin film was grown by shallow chemical bath deposition technique. This technique used a highly conducted hot plate to heat the substrate, while using a shallow bath for higher thermal gradients. As a result, large area uniformity could be achieved and the homogeneous nucleation was suppressed. More importantly, the solution used was greatly reduced, which is crucial for cost reduction in practice. The effects of temperature and shaking on the growth kinetics and film properties were investigated. The reaction activation energy was obtained to be 0.84 eV, and was not affected much by shaking indicating that the deposition is essentially reaction controlled. Furthermore, the films deposited at low or high temperature conditions had better photoconductivity.     

Optical and structural properties of chemically deposited CdS thin films on polyethylene naphthalate substrates

CdS thin films were deposited on polyethylene naphthalate substrates by means of the chemical bath deposition technique in an ammonia-free cadmium-sodium citrate system. Three sets of CdS films were grown in precursor solutions with different contents of Cd and thiourea maintaining constant the concentration ratios [Cd]/[thiourea] and [Cd]/[sodium citrate] at 0.2 and 0.1 M/M, respectively. The concentrations of cadmium in the reaction solutions were 0.01, 7.5 × 10⁻³ and 6.8 × 10⁻³ M, respectively. The three sets of CdS films were homogeneous, hard, specularly reflecting, yellowish and adhered very well to the plastic substrates, quite similar to those deposited on glass substrates. The structural and optical properties of the CdS films were determined from X-ray diffraction, optical transmission and reflection spectroscopy and atomic force microscopy measurements. We found that the properties of the films depend on both the amount of Cd in the growth solutions and on the deposition time. The increasing of Cd concentration in the reaction solution yield to thicker CdS films with smaller grain size, shorter lattice constant, and higher energy band gap. The energy band gap of the CdS films varied in the range 2.42-2.54 eV depending on the precursor solution. The properties of the films were analyzed in terms of the growth mechanisms during the chemical deposition of CdS layers.     

Structural and optical properties of nanocrystalline CdS thin films prepared using microwave-assisted chemical bath deposition

Cadmium sulfide (CdS) nanocrystalline thin films were prepared using the microwave-assisted chemical bath deposition method onto glass substrates at 80 °C. Aqueous solutions of either cadmium chloride or cadmium acetate and thiourea were used as sources of Cd²⁺ and S²⁻ ions, respectively. Two sets of samples with different concentrations were prepared. A microwave oven was used as a heating source to synthesize the nanocrystalline CdS thin films. The prepared thin films had a good adhesion with no pinholes. These films were examined for their structural and surface morphologies by X-ray diffraction (XRD), scanning electron microscopy, and atomic force microscopy. The optical properties were investigated using UV-vis spectrophotometer, photoluminescence, and Raman spectroscopy. Particle size values obtained from XRD were compared with these calculated using effective mass models. The values of optical band gaps according to optical transmission measurements decreased as the ion source molar concentration increased.     

Properties of thin TiN films deposited onto stainless steel by an in-line dry coating process

A new coil-coating pilot plant, capable of utilizing ion plating, sputtering and plasma-assisted chemical vapor deposition (PACVD) processes, independently or in series, was developed and optimum conditions for TiN, TiC, Al_xO_y, SiO_x and Cr coating were established. This paper is mostly concerned with the results of characterization (conducted in parallel by the authors′ two institutions) of TiN films deposited by ion plating or sputtering onto type-304 stainless steel strips. In particular, the dependence of the basic properties such as chemical composition, structure, adhesion, and color on the coating process are discussed with respect to anti-corrosion, anti-wear, and decorative applications. TiN coatings with a very attractive gold coloration were obtained; they performed well in wear testing, but did not show satisfactory corrosion resistance. However, it was found that the latter can be improved significantly by depositing a SiO_x, top layer by PACVD above the TiN coating. Thus the in-line dry coating processes are capable of producing highly functional steel surfaces with decorative color and high corrosion resistance.     

Influence of substrates on the structural and optical properties of chemically deposited CdS films

Cadmium sulfide (CdS) films were chemically deposited on glass, polycarbonate (PC), polyethylene terephthalate (PET), and Si wafer. Effects of substrate types on the structural and optical properties of the films were investigated. There is a preferential orientation of the crystallites in the film grown on the glass along the c-axis (perpendicular to the plane of the substrate) producing a strong hexagonal (0 0 2) or cubic (1 1 1) peak, regardless of the presence of ITO coating. However, such preferential orientation decreases or disappears when the deposition was made onto PC or PET substrates. The crystallinity of CdS films on glass and Si is better than that of the other ones. The average transmittance of the films on PC and PET is about 50% and 55%, respectively, and increases up to 70% for glass substrate. The improvement of the transmittance was obtained from ITO-coated substrates.     

Chemical bath deposition of indium sulphide thin films: preparation and characterization

Indium sulphide (In₂S₃) thin films have been successfully deposited on different substrates under varying deposition conditions using chemical bath deposition technique. The deposition mechanism of In₂S₃ thin films from thioacetamide deposition bath has been proposed. Films have been characterized with respect to their crystalline structure, composition, optical and electrical properties by means of X-ray diffraction, TEM, EDAX, optical absorption, TRMC (time resolved microwave conductivity) and RBS. Films on glass substrates were amorphous and on FTO (flourine doped tin oxide coated) glass substrates were polycrystalline ( phase). The optical band gap of In₂S₃ thin film was estimated to be 2.75 eV. The as-deposited films were photoactive as evidenced by TRMC studies. The presence of oxygen in the film was detected by RBS analysis.     

Chemical bath deposition of CdS highly-textured, columnar films

Chemical bath deposition (CBD) is one of the most common techniques for depositing CdS films. While there have been many studies on these films, and considerable characterization of their morphologies, most of this characterization has been by either X-ray diffraction or plan-view electron microscopy. With the exception of epitaxial films deposited on single crystal substrates, there has been little characterization of the cross-sectional structure of CBD CdS films. We show how, using a CdSO₄ bath and ethylenediamine as complexant, dense, columnar films of predominantly cubic CdS can be very reproducibly obtained. The initial growth is disordered, but preferential growth perpendicular to the polar face results in highly textured growth. A similar, if somewhat less ordered, morphology is obtained from a commonly-used ammonia bath using CdCl₂ as the source of Cd. Although not explicitly recognized, chloride baths in the literature exhibited sharp X-ray diffraction peaks and this is now connected with the growth mode these baths have in common with ethylenediamine baths.     

Properties of tantalum oxide thin films grown by atomic layer deposition

Thin tantalum oxide films were deposited using atomic layer deposition from TaCl₅ and H₂O at temperatures in the range 80–500 °C. The films deposited at temperatures below 300 °C were predominantly amorphous, whereas those grown at higher temperatures were polycrystalline containing the phases TaO₂ and Ta₂O₅. The oxygen to tantalum mass concentration ratio corresponded to that of TaO₂ at all growth temperatures. The optical band gap was close to 4.2 eV for amorphous films and ranged from 3.9 to 4.5 eV for polycrystalline films. The refractive index measured at λ = 550 nm increased from 1.97 to 2.20 with an increase in growth temperature from 80 to 300 °C. The films deposited at 80 °C showed low absorption with absorption coefficients of less than 100 cm⁻¹ in the visible region.     

Effect of CdCl2 annealing treatment on thin CdS films prepared by chemical bath deposition

In order to study the CdS recrystallization mechanism, a comparative study was carried out on thin films prepared by chemical bath deposition. The CdS films were annealed in air with or without a CdCl₂ coating layer. In-situ Raman spectra obtained during the annealing showed that both the air- and the CdCl₂-annealing did not cause rearrangement of the neighboring atoms in the CdS clusters below ~ 300 °C. CdS thin film was partially oxidated to CdO and CdSO₄ on the cluster surface when annealed in air. The oxides and the sulfur stoichiometric deficiency prevented the clusters to coalesce at higher temperatures. Coating thin CdS film with a thin CdCl₂ layer protected it from oxidation during annealing in air and promoted formation of Cl_S and V_Cd point defects in CdS. The anti-oxidation was attributed to the incorporation of a significant amount of Cl into CdS to form the Cl_S, which prevented the oxygen in-diffusion and chemical bonding during the annealing. The anti-oxidation at the CdS nano-crystalline surface and the point defects formed in the CdS promoted coalescence of the neighboring clusters without the need of long-range redistribution of the atoms. Large CdS grains with good crystalline quality formed through recrystallization during the CdCl₂ heat treatment, which provided the solid basis for the subsequent CdTe growth and high efficient CdS/CdTe solar cell fabrication.     

Deposition and optical studies of silicon carbide nitride thin films

Thin films of silicon carbide nitride (SiCN) have been prepared by reactive radioactive frequency (r.f.) sputtering using SiC target and nitrogen as the reactant gas. Deposition rates are studied as a function of deposition pressures and argon-nitrogen flow ratios. The optical absorption studies indicated the band edge shifting of the films when the nitrogen ratios are increased during deposition. Fourier transform infrared spectroscopy (FTIR) analysis on the films indicated several stretching modes corresponding to SiC, SiN and CN compositions.     

Analysis of the early growth mechanisms during the chemical deposition of CdS thin films by spectroscopic ellipsometry

Chemically deposited CdS thin films were analyzed in this work by means of the spectroscopic ellipsometry technique. The CdS thin films were deposited from an ammonia-free process at short durations in order to obtain information about the layer microstructure and kinetic growth process. We found that the conditions of the ammonia-free reaction solution promote the ion-by-ion deposition process at the early growth stages yielding a compact, high refraction index and highly crystalline oriented CdS layers. Using a concentration of 1.82 mg/ml of cadmium in the reaction solution, the resulting films possess a double layer microstructure which consists of an inner compact layer and an external porous one. The inner layer is developed during the first 15 min of deposition time and it reaches a thickness around of 80 nm. After this time and on this inner layer of CdS, it grows an external porous layer whose thickness increases with the deposition time. The formation of the CdS compact layer at the early stages is related with the ion-by-ion growth mechanism. The subsequent CdS porous layer is formed during the cluster-by-cluster growth stage at longer deposition times. By reducing the cadmium concentration in reaction solution down to 0.76 mg/ml, maintaining constant molar ratio concentrations of Cd/complexing and Cd/thiourea, the chemically deposited CdS films develop only the inner compact layer with a thickness of about 80 nm after 35 min of deposition time.     

Electrophoretic deposition of CdSe nanocrystal films onto dielectric polymer thin films

The electrophoretic deposition of cadmium selenide (CdSe) nanocrystal films from a stable hexane suspension onto a dielectric polymer film is reported. The electrodes were prepared by spin casting a ~ 30 nm thick smooth, defect-free, polystyrene film on silicon substrates. Scanning electron microscopy and atomic force microscopy showed that the CdSe films deposited atop polystyrene possessed morphology comparable to CdSe films deposited on the bare silicon electrodes. Factors affecting deposition, such as nanocrystal charging in suspension and wetting of electrode surfaces, are reviewed. This approach to preparing nanocrystal films onto homogeneous dielectric layers will facilitate the fabrication of novel polymer-nanocrystal composites.     

Transparent conductive ZnO:B films deposited by magnetron sputtering

This paper focuses on the preparation of boron doped ZnO (ZnO:B) films prepared by nonreactive mid-frequency magnetron sputtering from ceramic target with 2 wt.% doping source. Adjusting power density, ZnO:B film with low resistivity (1.54 × 10^− 3 Ω cm) and high transparency (average transparency from 400 to 1100 nm over 85%) was obtained. Different deposition conditions were introduced as substrate fixed in the target center and hydrogen mediation. Hall mobility increased from 11 to above 26 cm²/V·s, while carrier concentration maintained almost the same, leading to low resistivity of 6.45 × 10^− 4 Ω cm. Transmission spectra of ZnO:B films grown at various growth conditions were determined using a UV-visible-NIR spectrophotometer. An obvious blue-shift of absorption edge was obtained while transmittances between 600 nm and 1100 nm remained almost the same. Optical band baps extracted from transmission spectra showed irregular enhancement due to the Burstein-Moss effect and band gap renormalization. Photoluminescence spectra also showed a gradual increase at UV emission peak due to free exciton transition near band gap. We contributed this enhancement in both optical band gap and UV photoluminescence emission to the lattice structure quality melioration.     

Deposition and properties of CBD and CSS CdS thin films for solar cell application

We deposited cadmium sulfide (CdS) thin films using the chemical-bath deposition (CBD) and close-spaced sublimation (CSS) techniques. The films were then treated in CdCl₂ vapor at 400 °C for 5 min. The CSS CdS films had hexagonal structure, and good crystallinity. The CdCl₂ treatment did not produce major changes, but there was a decrease in the density of planar defects. The untreated CBD CdS films had cubic structure and poorer crystallinity than the CSS films. After the CdCl₂ treatment, these films recrystallized to the hexagonal phase, resulting in better crystallinity and a lower density of planar defects. The conformal coverage and the presence of bulk oxygen are the key issues in making the CBD films more suitable for photovoltaic applications.     

Fabrication and characterization of Fe3O4 thin films deposited by reactive magnetron sputtering

Fe-O thin films with different atomic ratio of iron to oxygen were deposited on glass and thermally oxidized silicon substrates at temperatures of 300, 473 and 593 K, by reactive magnetron sputtering in Ar+O₂ atmosphere. The composition and structure of the thin films were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and electrical resistivity. It was found from XRD that with increasing the oxygen partial pressure in the working gas, the crystalline structure of the Fe-O films deposited at the substrate temperature of 473 K gradually changed from α-Fe, amorphous Fe-O, Fe₃O₄, γ-Fe₂O₃ to Fe_21.34O₃₂. The structure and chemical valence of the Fe₃O₄ films were analyzed by electron microscopy and XPS, respectively.     

CdS thin films doped with metal-organic salts using chemical bath deposition

CdS thin films doped with metal-organic salts were grown on glass substrates at 90 °C by the chemical bath deposition technique. Metal-organic salts such as zinc acetate, chromium acetylacetonate, ammonium fluoride, aluminum nitrate, tin acetate and indium acetate were used. The chemical bath was prepared with cadmium acetate, ammonium acetate, thiourea and ammonium hydroxide. In the case of un-doped films, the S/Cd ratio was varied by changing the thiourea in the range 1-12. The best optical, structural and electrical properties were found for S/Cd = 2. The doped films were prepared by always keeping the ratio S/Cd constant at 2. The band gap (E_g) of doped and un-doped films was evaluated from transmittance spectra, where films with lower sulfur concentration exhibited higher E_g. X-ray analysis showed that both un-doped and doped films were polycrystalline with preferential orientation along the (111) direction and with the zincblende structure in all cases. The dark electrical results showed that CdS doped with Zn (1 at.%) exhibited the lowest resistivity values of 10 Ω cm.     

Preparation of porous CdIn2S4 photocatalyst films by hydrothermal crystal growth at solid/liquid/gas interfaces

A chemical method has been used to prepare highly crystalline porous CdIn₂S₄ photocatalyst films at low temperature on a glass substrate. The conversion of cadmium and indium salts with thiourea on CdIn₂S₄ powder particles into CdIn₂S₄ has been achieved via hydrothermal reaction, which acts as a “glue” to connect the original CdIn₂S₄ particles chemically thus forming a mechanically stable photocatalyst film.     

Spectroscopic investigations on sol-gel derived organic-inorganic hybrid films for photonics from ormosils and tetrapropylorthotitanate

Titania/ormosils organic-inorganic hybrid films with a thickness of approximately 1.55-μm were prepared as planar waveguides by a single spin-coating process and low temperature heat treatment. Acid-catalyzed solutions, firstly, γ-glycidoxypropyltrimethoxysilane (GLYMO) mixed with tetrapropylorthotitanate, secondly, methyltrimethoxysilane (MTES) mixed with tetrapropylorthotitanate, and thirdly, γ-glycidoxypropyltrimethoxysilane and methyltrimethoxysilane mixed with tetrapropylorthotitanate were used as precursors. Purely inorganic, crack-free and highly transparent silica-titania films with a thickness of more than 0.5-μm were also obtained after a heat treatment temperature of 500 °C. Films were characterized by thermal gravimetric analysis, and certain optical tools such as UV-visible spectra, Fourier transform infrared spectra and Raman spectra. The obtained results indicate that the GLYMO derived films show more shrinkage and the GLYMO and MTES derived films higher refractive index during heating. The hardness and Young's modulus of the films were characterized by a nanoindenter. Hardness as high as 6.83 GPa was obtained in GLYMO and MTES derived film after being heat-treated at 800 °C. It has been proposed that the high hardness of the film could be related to the carbon and high titanium content in the film.