Ammonia-free Chemical Bath Deposition Research Articles

Cu-doped CdS thin films by chemical bath deposition and ion exchange

This work presents the synthesis and characterization of nanostructured cadmium sulfide thin films obtained by an ammonia-free chemical bath deposition process. Different levels of Cu ion doping were achieved by the ion exchange method of CdS at room temperature. The main objective was to control the energy band gap adjusting the copper concentration and to gather information about Cu-doped CdS thin films. Optical absorbance measurements were performed to obtain the band gap value through Tauc’s plot method. XPS measurements were used to obtain the copper and cadmium concentration. In addition, SEM micrographs were collected to obtain information about surface and structural arrangement on Cu-doped CdS thin films. It was also found that the exchange of cadmium by Cu-ions promoted the formation of a hexagonal crystalline structure, which was different than the initial pure cubic structure. Through optical characterization, it is possible to evaluate the impact of the metallic dopant on the band gap value, finding that it can be controlled by the percentage of added copper, which opens the possibility of applying this material to the development of electronic devices such as field effect transistors. In addition, a prototype of a MOS structure was fabricated employing both the undoped and the Cu-doped CdS thin film to corroborate its possible application as a field effect transistor.

CuS films grown by a chemical bath deposition process with amino acids as complexing agents

CuS thin films were deposited by an ammonia-free chemical bath deposition process. This approach uses amino acids (alanine, glycine and serine) as complexing agents. The conditions under which amino acids form complexes and release Cu ions are discussed. All the resulting CuS films, formed by nanoflake particles, had the hexagonal crystalline structure (covellite). Moreover, the coexistence of Cu+ and Cu2+ states in these films were confirmed by X-ray photoelectron spectroscopy. Amino acids as complexing agents were then observed to affect mainly to the growth kinetics of the CuS films. Thus, thicknesses of 42, 55.4 and 70 nm were obtained for the films processed with solution reactions containing alanine, glycine and serine, respectively. The optical band gaps of the films with moderate transmittance had values in the range from 2.25 to 2.4 eV. Finally, the resulting CuS films with electrical resistivities from 1.84 × 10−3 to 2.8 × 10−3 Ω-cm showed a decrease of photosensitivity with the film thickness.

Effects of tin-doping on cadmium sulfide (CdS:Sn) thin-films grown by light-assisted chemical bath deposition process for solar photovoltaic cell

Thin transparent films of tin-doped cadmium sulfide (CdS) on different substrates were obtained by an ammonia-free chemical bath deposition technique assisted by ultraviolet illumination with a wavelength of 265 nm. Several sets of films were deposited by varying the amount of SnCl2 in the reaction bath and analyzed by X-ray diffraction, scanning electron microscopy, optical transmission and reflection spectroscopy and current versus voltage measurements. In the doped films, the position of the main X-ray diffraction peak is shifted to larger angles showing a decrease of the crystal lattice constant; this is attributed to the substitution of tin by cadmium in the doping process. The morphological characterization of the doped films shows homogeneously distributed spherical grains with an average grain size of 58.5 nm. The grains are composed of crystallites with an average size of 23 nm; the surface of the doped films does not show conglomerates that are typical for materials grown by the chemical bath deposition technique. The electrical properties reveal a decrement in the resistivity of an order of magnitude with respect to the films without doping. The optical results exhibit variation in the band gap that we consider as the consequence of the films' porosity; the theoretical analysis presented confirms this point. The films show a reflectance at a normal incidence less than 30%, which is an indication of their good optical quality. The main effects of the tin doping on the CdS films were observed as changes in the crystal structure and electrical resistivity.

Non-toxic growth of CuxS thin films in alkaline medium by ammonia free chemical bath deposition

CuxS thin films were deposited on commercial glass substrates at different temperatures by an ammonia-free chemical bath deposition process. The deposition process is based in a copper chloride- sodium citrate- thiourea system. The pH value in the reactive solution was adjusted at 10 by addition of KOH and buffer solution. In order to investigate the effect of the deposition temperature and time, four reaction baths were prepared and set at temperatures 25, 40, 55 and 70 °C. The deposited films were studied by X-ray diffraction, Raman spectroscopy, 3D optical microscopy, energy dispersive X-ray spectroscopy, electron probe X-ray microanalysis, UV–vis spectroscopy and two-points probe electrical measurements. The chemically deposited CuxS films showed good adherence to the substrate, homogeneity, metallic luster and light reddish-brown appearance. The thickness of the films varied in the range of 40–80 nm, depending on the deposition conditions. The X-ray diffraction revealed that the films present amorphous nature, excepting those deposited at the highest temperature, which had CuS covellite crystalline structure. The chemical composition of the films exhibited the presence of the covellite phase with excess of sulphur atoms in all cases. The electrical analysis showed a strong decrease in the resistivity with increasing the deposition temperature with values in the range from 106 to 10−2 Ωcm.

Chemically deposited ammonia-free cadmium hydroxide thin films

Polycrystalline thin films of cadmium hydroxide Cd(OH)2 with high purity and good adhesion to the substrate were synthesized by an ammonia free chemical bath deposition method. The as-deposited films have a monoclinic crystalline structure with lattice constants a=6.5768Å, b=10.2704Å, c=3.4125Å, α=90.00°, β=120.16°, γ=90.00°, and crystallite size 26.1±0.9nm. The cubic phase of cadmium oxide with lattice constants a=4.7155Å and crystallite size of 10.9±0.9nm was obtained by heating the as-deposited films at 220°C. The absorption bands in the 3500–3600cm−1 range observed in the infrared spectra of the as-deposited films were used to detect the presence Cd(OH)2 compound. The possible ways to optimize the synthesis of Cd(OH)2 to be used as precursor for production of chalcogenide films is discussed.

Chemical deposition of CdS films by an ammonia-free process with amino acids as complexing agents

Highly crystalline CdS films were deposited by an ammonia-free chemical bath deposition process which uses amino acids as complexing agents. All films have shown the hexagonal (wurzite) structure with preferential orientation in the plane (002). As consequence of the difference in the stability between the Cd–glycine and Cd–alanine complexes, CdS films with different thicknesses were obtained. The band gap energy values of the resulting films were at the range of 2.43 to 2.52eV.

Ultrasound-modulated microstructure of PbS film in ammonia-free chemical bath deposition

Ultrasound effectively modulated PbS-film uniformity and microstructures associated with growth mechanism and photoelectrochemical property in ammonia-free chemical bath deposition.

Preparation of nanocrystalline Ni doped ZnS thin films by ammonia-free chemical bath deposition method and optical properties

Nanocrystalline Ni doped ZnS thin films were deposited on quartz, silicon, and glass substrates using chemical bath deposition method in a weak acidic solution containing ethylenediamine tetra acetic acid disodium salt (Na2EDTA) as a complexing agent for zinc ions and thioacetamide (TAA) as a sulfide source at 80°C. The films were characterized by energy-dispersive X-ray spectrometer (EDX), inductively coupled plasma atomic emission spectroscopy (ICP-AES), Fourier transform-infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), ultraviolet–visible spectrophotometry, and photoluminescence (PL) spectroscopy. UV–vis transmission data showed that the films were transparent in the visible region. The X-ray diffraction analysis showed a cubic zinc blend structure. FE-SEM revealed a homogeneous morphology and dense nanostructures. The PL spectra of the ZnS:Ni films showed two characteristic bands, one broad band centered at 430 and another narrow band at 523nm. Furthermore, concentration quenching effect on the photoluminescence intensity has been observed.

Cd1 − xZnxS thin films with low Zn content obtained by an ammonia-free chemical bath deposition process

Cd1−xZnxS films with low Zn content were obtained on glass substrates by an ammonia-free chemical bath deposition process. Alkaline reaction solutions were prepared using cadmium chloride, zinc chloride, sodium citrate, thiourea and potassium hydroxide. As a result of varying the mixture ratio between Cd and Zn precursors, microstructural studies from X-ray diffraction reveal that the resulting films have hexagonal, wurzite type, crystalline structure with changes in the preferential growth orientation. Important changes on the surface morphology and thickness of the Cd1−xZnxS films were also observed as effects of adding Zn to the CdS lattice. Optical studies show that Cd1−xZnxS thin films with energy band gaps in the range from 2.48 to 2.65eV were obtained.

The effects of porosity on optical properties of semiconductor chalcogenide films obtained by the chemical bath deposition

This paper is dedicated to study the thin polycrystalline films of semiconductor chalcogenide materials (CdS, CdSe, and PbS) obtained by ammonia-free chemical bath deposition. The obtained material is of polycrystalline nature with crystallite of a size that, from a general point of view, should not result in any noticeable quantum confinement. Nevertheless, we were able to observe blueshift of the fundamental absorption edge and reduced refractive index in comparison with the corresponding bulk materials. Both effects are attributed to the material porosity which is a typical feature of chemical bath deposition technique. The blueshift is caused by quantum confinement in pores, whereas the refractive index variation is the evident result of the density reduction. Quantum mechanical description of the nanopores in semiconductor is given based on the application of even mirror boundary conditions for the solution of the Schrödinger equation; the results of calculations give a reasonable explanation of the experimental data.

Synthesis of CdS nanostructures using template-assisted ammonia-free chemical bath deposition

CdS micro- and nano-structures (micro/nanotubes and nanostructured films) were obtained by ammonia-free chemical bath deposition using polymer templates (ion track-etched polycarbonate membranes and poly(styrene-hydroxyethyl methacrylate) nanosphere arrays). The semiconductor structures were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), optical absorption, photoluminescence and electrical measurements. The diameters of CdS tubes are between 300nm and few microns and the lengths are up to tens of micrometers. The SEM images prove that the CdS films are nanostructured due to the deposition on the polymer nanosphere arrays. For both CdS structures (tubes and films) the XRD patterns show a hexagonal phase. The optical studies reveal a band gap value of about 2.5−2.6eV and a red luminescence at ∼1.77eV. A higher increase of conductivity is observed for illuminating the CdS nanostructured film when compared to the simple semiconductor film. This is a consequence of the periodic patterning induced by the polymer nanosphere array.

CdS sensitized nanoporous TiO2/CuO layer prepared by dealloying of Ti–Cu amorphous alloy

A CdS sensitized nanoporous TiO2/CuO layer was fabricated by ammonia free chemical bath deposition (CBD) process on dealloyed Ti–Cu amorphous alloy. The surface morphology and the photoelectric characterization of as-prepared CdS-TiO2/CuO composite were investigated. It is shown that the nanoporous surface is extensively covered with a uniform layer of CdS nanoparticles with diameter ranging from 5 to 20nm. Both the sensitization of CdS and the annealing process could enhance photocurrent of the material. The highest photocurrent density around 2.5mA/cm2 without bias potential was observed. The CdS-TiO2/CuO structured on Ti–Cu amorphous alloy has the potential application in photoelectrodes and photocatalysis.

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 120nm. 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 600nm.

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 cm 2V −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 cm 2V −1 s −1 and the threshold voltage decreased to 8.4–12 V. I on / I off for the annealed devices was approximately 10 5–10 6.

Study of CdS-sensitized solar cells, prepared by ammonia-free chemical bath technique

In this study, cadmium sulfide (CdS)-sensitized solar cells have been fabricated, where nanoporous titanium oxide (TiO2) photoelectrode of equal thickness has been prepared on SnO2:F coated glass substrate using TiO2 paste. Different amounts of CdS have been deposited by an ammonia-free chemical bath deposition technique with various deposition times. The CdS-sensitized TiO2 photoelectrodes show polycrystalline nature. The optical measurement reveals that absorbance edge of the CdS-sensitized TiO2 photoelectrode extends up to 540nm and the amount of absorbance increases with the enhancement of CdS-deposition time. The CdS solar cell, with deposition time of 12min, shows impressive photocurrent and moderate solar cell efficiency.

Growth technology, X-ray and optical properties of CdSe thin films

An ammonia-free chemical-bath deposition was used to obtain CdSe thin films on glass substrate. The materials used in the chemical bath were cadmium chloride complexed with sodium citrate and sodium selenosulphate. The preparation conditions, especially the starting solution characteristics, such as concentration of dissolved materials, temperature, pH value as well as deposition time and immersion cycles were optimized to obtain homogeneous stoichiometric films with good adherence to the glass substrate. The films thickness was in the range of 400–500 nm with a growing time of 4 h. The material obtained was characterized by optical absorption, SEM with the energy dispersive X-ray analysis (EDS) and X-ray diffraction. The films obtained at bath temperatures of 70 and 80 °C had the hexagonal structure (of wurtzite type), with crystallite size of about 20 nm. Room temperature deposition results in films with the cubic structure and crystallite size of about 4 nm. From optical transmission data, an energy gap equal to 1.88 eV was found. The material is interesting for applications in hybrid systems for solar energy conversion.

Ammonia-free chemical bath deposition of nanocrystalline ZnS thin film buffer layer for solar cells

In this work, we prepared zinc sulfide thin films on glass substrates by ammonia-free chemical bath deposition method using thioacetamide as the sulfide source and Ethylene Diamine Tetra Acetic Acid disodium salt as the complexing agent in a solution of pH = 6.0. Thin films of ZnS with different thicknesses of 18–450 nm were prepared. The effect of film thickness and annealing temperature in atmospheric air, on optical properties, band gap energy and grain size of nanocrystals were studied. The X-ray diffraction analysis showed a cubic zinc blend structure and a diameter of about 2–5 nm for ZnS nanocrystals. The Fourier Transform Infrared spectrum of films revealed no peaks due to impurities. The as-deposited ZnS films had more than 70% transmittance in the visible region. The direct band gap of as-deposited films ranged from 3.68 to 3.78 eV and those of annealed films varied from 3.60 to 3.70 eV.

Morphological and chemical study of the initial growth of CdS thin films deposited using an ammonia-free chemical process

We study the initial growth stages of CdS thin films deposited by an ammonia-free chemical bath deposition process. This ammonia-free process is more environmentally benign because it reduces potential ammonia release to the environment due to its high volatility. Instead of ammonia, sodium citrate was used as the complexing agent. We used atomic force microscopy (AFM), Rutherford backscattering (RBS) and X-ray photoelectron spectroscopy (XPS) to investigate the morphological and chemical modifications at the substrate surface during the first initial stages of the CdS deposition process. Additionally, X-ray diffraction (XRD) and optical transmission spectroscopy measurements were carried out to compliment the study. XPS results show that the first nucleation centers are composed by Cd(OH) 2 which agglomerate in patterns of bands, as demonstrated by AFM results. It is also observed that the conversion to CdS (by anionic exchange) of the first nucleus begins before the substrate surface is completely covered by a homogenous film.

Optical properties and layer microstructure of CdS films obtained from an ammonia-free chemical bath deposition process

In this work we report the optical and structural properties of chemically deposited CdS films obtained from an ammonia-free chemical bath deposition process. The deposition process is based in the substitution of ammonia by sodium citrate as the complexing agent in the reaction solution. We applied spectroscopic ellipsometry and optical transmission and reflection spectroscopy to determine the thickness, optical constants and energy band gap of a set of CdS samples obtained by this process at different deposition times. Additionally, the spectroscopic ellipsometry analysis provided information about the microstructure and kinetic growth process of the CdS films. We found that experimental data fit quite well with a model of two different layers, corresponding to different growing stages, to simulate the CdS films. The first layer, formed at the earlier growing stages by means of the ion by ion deposition process, was assigned to a pure compact CdS layer and their optical properties were simulated by a Kramers–Kronig consistent model. The second external layer composed by a mixture of CdS and voids was modeled by the Bruggeman effective medium approximation. The results were compared to those obtained for a set of CdS samples obtained from a deposition process which employs ammonia in the reaction solution.

Ammonia-free chemical bath deposition of CdS films: tailoring the nanocrystal sizes

We report on the control of nanocrystal sizes in CdS nanocrystalline films prepared by ammonia-free chemical bath deposition technique. We studied the effect of deposition duration, bath temperature during deposition, and post-preparation heat treatment. Nanocrystals (NCs) with radii from 2.6 nm to more than 10 nm were prepared and characterised by transmission electron microscopy, X-ray diffraction, and optical photoluminescence and absorption spectroscopy. We observed cubic to hexagonal phase transition for large NCs prepared by the heat treatment.