Deposited CdS Thin Films Research Articles

Annealing Effects on the Properties of Chemically Deposited CdS Thin Films at Ambient Condition

Chemically-deposited CdS thin films have been investigated using various techniques to discuss annealing effects on the structural, morphological, optical and electrical properties of the films. It has been observed from XRD that the deposited layers are mainly consisting of CdS phase. After annealing, metastable cubic phase was transformed into stable hexagonal phase. The average grain sizes were found to increase and the lattice constant, micro-strain and dislocation density were found to decrease after annealing. Optical absorption measurements show that band gap is observed to be 2.42 eV for as deposited and be 2.27 eV upon heat treatment at 673 K for one hour in air. The conductivity of this film has been determined by I-V measurement and observed to increase with increase of temperature. The activation energy of electrical conductivity of this film is also determined.DOI: http://dx.doi.org/10.3329/dujs.v60i2.11536 Dhaka Univ. J. Sci. 60(2): 283-288, 2012 (July)

Effect of flow dynamics on the growth kinetics of CdS thin films in chemical bath deposition

During chemical bath deposition of CdS thin films, the solution is typically stirred to prevent unnecessary concentration gradient and precipitates sticking onto the film surface. Stirring rate changes the thickness of laminar boundary layer over the substrate, affecting the diffusion length of the reactants toward the substrate. It can also change hydrodynamic condition in the bath between laminar and turbulent flow. Interestingly, effect of stirring rate on the deposition kinetics of CdS thin films reported in the literature is conflicting. In this study, we showed that the diffusion-limited deposition kinetics has created a decreasing profile of film thickness along the sample as a function of boundary layer thickness when the substrate was under laminar flow and deposition temperature was between 45°C and 63°C. On the other hand, the deposition became feed-limited when the substrate was under turbulent flow and the deposition temperature was between 45°C and 63°C. While the hydrodynamic condition changed the deposition rate significantly, all the CdS films deposited showed a cubic crystal structure and the optical bandgap was slightly lower than that of single crystalline cubic CdS. The tensile stress in film probed by Raman spectroscopy can explain the low bandgap.

Structural, optical and magnetic properties of Mn diffusion-doped CdS thin films prepared by vacuum evaporation

The effect of Mn-doping on the vacuum deposited CdS thin films has been investigated by studying the changes in the structural, optical and magnetic properties of the films. A thin Mn layer evaporated on the CdS film surface served as the source layer for the diffusion doping. Doping was accomplished by annealing the CdS/Mn stack layers at the temperature range from 300 °C to 400 °C in step of 50 °C for 30 min under vacuum. The X-ray diffraction results showed that the undoped CdS film had a zinc-blende structure with a strong preferred orientation along the (1 1 1) direction. The incorporation of Mn did not cause any change in the texture but reduced the peak intensity and lead to a smaller crystallite size. Investigation of surface morphology using atomic force microscopy confirmed the decrease in the grain size with the Mn diffusion. In addition, a more uniform grain size distribution was observed in the doped films. X-ray photoelectron spectroscopy analysis showed that Mn atoms on the surface of the films were bounded to either sulphur or oxygen atoms. Auger electron spectroscopy of the diffusion-doped CdS sample at 350 °C indicated that the atomic Mn concentration was higher close to the surface region and Mn was distributed with a steadily decreasing profile through the bulk of the film with an average atomic concentration value around few percent. Band gap energy of the undoped sample decreased from 2.42 to 2.36 eV upon Mn diffusion at 400 °C. The magnetization of films as a function of magnetic field and temperature were measured. Clear ferromagnetic loops were observed for the Mn diffusion-doped CdS films prepared by annealing above 350 °C.

Synthesis, characterization and photoelectrochemical properties of n-CdS thin films

Cadmium sulfide thin films have been deposited on glass substrates by simple and cost effective chemical bath deposition technique. Triethanolamine was used as a complexing agent. The preparative parameters like ion concentration, temperature, pH, speed of substrate rotation and deposition time have been optimized for good quality thin films. The ‘as-grown’ films are characterized for structural, electrical, optical and photoelectrochemical (PEC) properties. The X-ray diffraction (XRD) studies reveal that the films are polycrystalline in nature. Energy-dispersive analysis by X-ray (EDAX) shows that films are cadmium rich. Uniform deposition of CdS thin films on glass substrate is observed from scanning electron microscopy (SEM) and atomic force microscopy (AFM) micrographs. Optical studies reveal a high absorption coefficient (10 4 cm −1) with a direct type of transition. The band gap is estimated to be 2.47 eV. The film shows n-type conduction mechanism. The photoelectrochemical (PEC) cell with CdS thin film as a photoanode and sulfide/polysulfide (1 M) solution as an electrolyte have been constructed and investigated for various cell parameters. The solar to electrical conversion efficiency ( η) and fill factor (ff) are found to be 0.049% and 0.36, respectively.

The analysis of CdS thin film at the processes of manufacturing CdS/CdTe solar cells

In sequence, the deposited CdS thin film had undergone physical and optical changes by the processes of manufacturing CdS/CdTe solar cells. CdS thin film was manufactured by the Chemical Bath Deposition (CBD) method. The aqueous solution was based on ammonia solution. The temperature of bath system was 75 °C and deposition time was 50 min. The thickness of deposited CdS thin film was about 200 nm. The substrate was the glass coated with SnO 2:F thin film. The following process was the deposition of CdTe thin film by the Closed-Space-Sublimation (CSS) method. The final process was the CdCl 2 heat treatment at N 2+O 2 atmosphere, and the contrast experiment progressed for CdCl 2–CdS thin film after CSS process at N 2 atmosphere. The phase transition of CdS thin film, stress relaxation and optical band gap narrowing were developed by each process. And so, the formation of cadmium oxide was detected after the CdCl 2 heat treatment. It influenced to increase the optical band gap of CdS thin film. The variation in the structure properties, optical properties and residual stresses of CdS thin film was analyzed by X-ray diffractometer (XRD), Raman spectroscopy and ultraviolet (UV)–visible (VIS) spectroscopy.

ANNEALING EFFECT ON THE DARK ELECTRICAL TRANSPORT AND ABSORBANCE SPECTRA OF CHEMICAL BATH DEPOSITED CdS NANOCRYSTALLITES

We report here our investigations on dark DC resistivity and optical absorption of chemical bath deposited CdS thin films prepared by chemical bath deposition. Crystallite sizes of about 30 nm are found from XRD using Scherrer formula with an added term for strain broadening. Energy-dispersive X-ray measurement shows correct stoichiometry of Cd and S atomic ratio. The grain size of the as-deposited film determined from both AFM and TEM is found to be ~180 nm. On vacuum annealing, the dark resistivity decreases from ~109 to 106 Ω-cm. Dark resistivity variation of as-deposited and vacuum-annealed films with temperature in the temperature region (95–350 K) reveals that each film exhibits two types of conduction in this temperature region. From the optical absorption study the optical band gap of as-deposited film is found to be ~2.29 eV.

Influence of substrate temperature, growth rate and TCO substrate on the properties of CSS deposited CdS thin films

The growth of CdS thin films by close space sublimation (CSS) has been systematically studied using an ultra-high vacuum system known as DAISY-SOL in order to understand the basic growth mechanisms and their impact on the film properties. Substrate temperature and deposition rate were varied, and the surface properties of the CdS layer were determined by photoelectron spectroscopy (XPS) without breaking the vacuum. To analyze the influence of the deposition conditions on the layer morphology and crystallographic structure, the films were further characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The SEM and AFM studies show a correlation between the deposition rate and the film morphology. For high deposition rates, edged grain shapes and smoother surfaces were observed than for low deposition rates. CdS films were deposited onto two different commercially available fluorine-doped tin oxide (FTO) substrates. XRD studies show that a high <200> texture of the FTO substrate prefers the CdS growth in <0001> orientation of the hexagonal crystal modification.

Characterisation of chemical bath deposited CdS thin films on different substrates using electrolyte contacts

Aqueous electrolyte contacts have been used to characterize CdS thin films grown using chemical bath deposition (CBD) on 4 different fluorine-doped tin oxide (FTO) coated conducting glass substrates (identified by the supplier Pilkington Group Limited as TEC-15, TEC-7, NSG low-E, and NSG Solar). The porosity of CdS layers prepared on different substrates was evaluated by voltammetry using the oxidation of hexacyanoferrate (II) to probe residual electrochemical activity arising from the underlying conducting substrate. Using a simple model, it can be concluded that thin films deposited on NSG substrates at 85 °C are probably nanoporous with a porosity of up to ~ 20% of the coverage of the CdS surface. The porosity appears to depend on the nature of the substrate rather than the physical parameters of the deposition. In the case of non-porous films deposited on TEC substrates, analysis of the impedance gave values of the space charge capacitance from which the doping density and flat band potential were derived. The doping densities for CdS were found to be in the order of 10 17 cm −3–10 20 cm −3 depending on the substrate. The transmission spectra, recorded in the UV–visible range, reveal a relatively high transmission coefficient in the obtained films. The transmittance data further indicate that the optical band gap is closely related to the deposition conditions. Micro-Raman analysis showed structural similarity and differences between porous and non-porous thin films.

Preparation of Large Grain and Dense CdS Thin Films Using Ultrasonic Agitation Chemical Bath Deposition

We deposited CdS thin films onto F-doped SnO2 transparent conductive glass by ultrasonic agitation chemical bath deposition (UCBD). The influence of the annealing and CdCl2-treatment on the surface morphology, crystal structure, and direct band gap of the UCBD-CdS thin films was investigated. The effect of deposition time on the grain size of the CdS aggregates and the stack denseness of the UCBD-CdS thin films was compared. The results reveal that the small grains in the CdS aggregates were melted together and the CdS aggregate size did not change in the UCBD-CdS thin films after the CdCl2-treatment procedure. It is interesting that the ratio of the horizontal to vertical deposition rate varied with deposition time over the deposition period of the UCBD-CdS thin films. The deposition time was very important to obtain large CdS aggregate grains and dense UCBD-CdS thin films. Over a deposition time of 40 min the resulting UCBD-CdS thin films were dense and had a 180 nm grain size of CdS aggregates and a 80.8 nm of thin film thickness. The large-grained and dense UCBD-CdS thin films were suitable for thin film solar cells as a window layer.

Fabrication of CdS/SnS heterostructured device using successive ionic layer adsorption and reaction deposited SnS

In the present work successive ionic layer adsorption and reaction (SILAR) method has been employed for the growth of SnS films on chemical bath deposited CdS thin films. The as-grown and post annealed CdS/SnS heterostructures were investigated under dark and illuminated conditions. It has been observed that annealing improves the quality of the device. This paper presents the attempt towards realizing SnS based heterostructured devices using SnS films grown by SILAR technique.

Effects of cadmium salts on the structure, morphology and optical properties of acidic chemical bath deposited CdS thin films

CdS thin films were deposited from different cadmium salts by using acidic chemical bath deposition (CBD) process. Quality characterizations were performed using different methods such as scanning electron microscopy (SEM), X-ray diffraction (XRD) and photoluminescence (PL). The crystal quality of CdS thin films was improved after CdCl 2 thermal annealing at 380 °C. A compact structure of CdS thin films can be obtained by using CdCl 2 in comparison with the other cadmium salts. Although a Cd/S molar ratio of ∼1 was used in our experiments, strong PL signals confirmed the high quality of CdS thin films.

Influence of the thiourea/CdCl2 concentration ratio used for the chemical bath deposition of CdS thin films, upon the CdS/CdTe interface recombination velocity in CdTe/CdS/glass structures.

In this work we study the influence of the thiourea/CdCl2 concentration ratio used for the chemical bath deposition of CdS thin films on glass substrates, upon the CdS/CdTe interface recombination velocity in CdTe/CdS/glass structures, where the CdTe layer was grown on the CdS thin film by close space vapor deposition (CSVT)-hot wall technique. The interface recombination velocity was measured by means of the photoacoustic (PA) technique in a heat transmission configuration, in which minority carriers are photoexcited at the CdTe layer after illumination through the substrate and the CdS thin film. For data processing, a theoretical model was developed for the generation of the PA signal. We show a reduction in the value of the recombination velocity respecting those obtained for samples where CdS was grown by CSVT, and we observe that a minimal value appears for a thiourea/CdCl2 ratio in the CdS deposition solution equal to 5. These results show a good correlation with those of electrical measurements performed in solar cell devices.

Characterization of chemical bath deposited CdS thin films at different deposition temperature

Abstract CdS thin films were deposited onto glass substrates by the chemical bath deposition (CBD). The influence of the deposition temperature varied from 55 °C to 85 °C in a step of 5 °C on the crystallographic structure, morphology as well as optical and electrical properties was investigated in detail. Increasing deposition temperature can promote phase transformation from cubic to hexagonal and improvement of crystallinity in CdS films. CdS film deposited at 75 °C shows compact and smooth surface, and excellent transmission in visible light range. The band gaps are found to decrease from 2.56 eV to 2.38 eV with the increase of deposition temperature, and the sub-band gap of about 2.32 eV is noticed at low deposition temperature of 55–70 °C. All CdS films are of n-type conductivity and the carrier concentration is in the order of 10 12 –10 13 cm −3 . The lowest resistivity and highest mobility obtained are in the case of 85 °C.

Effect of complexing agent on the photoelectrochemical properties of bath deposited CdS thin films

In the present paper photoelectrochemical (PEC) performance of bath deposited CdS thin films based on complexing agents i.e. ammonia and triethanolamine (TEA) has been discussed. Effect of annealing has also been analyzed. The as-deposited and annealed (at 523 K for 1 h in air) films were characterized by X-ray diffraction (XRD), ultraviolet–visible (UV–vis) absorption spectroscopy, SEM, electrochemical impedance spectroscopy (EIS), and PEC properties. XRD studies revealed that the films were nanocrystalline in nature with mixed hexagonal and cubic phases. TEA complex resulted in better crystallinity. Further improvement in the crystallinity of the films was observed after air annealing. The marigold flower-like structure, in addition to flakes morphology, was observed with TEA complex, whereas for ammonia complex only flakes morphology was observed. The UV–vis absorption studies revealed that the optical absorption edge for the films with ammonia and TEA complex was around 475 nm and 500 nm, respectively. Annealing of the films resulted in red shift in the UV–vis absorption. The PEC cell performance of CdS films was found to be strongly affected by crystallinity and morphology of the films resulted due to complexing agent and annealing. The air annealed film deposited using TEA complex showed maximum short circuit current density ( J sc) and open circuit voltage ( V oc) i.e. 99 μA/cm 2 and 376 mV respectively, under 10 mW/cm 2 of illumination. The films deposited using TEA complex showed good stability under PEC cell conditions.

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.

Chlorine Doping of Cadmium Sulfide on the Example of CBD CdS

AbstractThe role of thermal annealing and of CdCl2 as a main source of electrically active but vaporizable chlorine doping in chemical bath deposited CdS thin films is studied. The films were deposited on glass substrates from aqueous solution of either CdCl2, NH4Cl, NH4OH, and thiourea, or CdSO4, (NH4)2SO4, NH4OH, and thiourea. Films deposited in the presence of CdCl2 and annealed in H2 atmosphere at 310 and 420 °C show a resistivity lower than 10 Ω·cm, one order of magnitude less than for identically annealed films deposited in absence of CdCl2. Annealing at 420 °C in closed ampoules, where a counter pressure of CdCl2 builds up, leads to a lower resistivity on the order of 10−1 Ω·cm, confirming the key role of chlorine on the electronic properties. However, further characterization via photoluminescence raises new questions about chlorine-related defects and their role in the mechanisms that govern film resistivity.

The synthesis and structure of a cadmium complex of dimorpholinodithioacetylacetonate and its use as single source precursor for CdS thin films or nanorods

A facile method for the preparation of dimorpholides of dithioacetylacetonate is described together with a X-ray single crystal structure of the ligand and of [Cd(msacmsac)(2)(NO(3))(2)] (msacmsac = dimorpholinodithioacetylacetonate). The cadmium complex has been used as a single source precursor for the deposition of the CdS thin films by the aerosol assisted chemical vapour deposition (AACVD) method or as nanorods by thermolysis in oleylamine. The thin films and nanorods were characterized by electronic spectra (UV-Vis), photoluminescence (PL), X-ray diffraction (XRD), selected area electron diffraction (SAED), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). To the best of our knowledge [Cd(msacmsac)(2)(NO(3))(2)] is the first complex in its class to be used as a single source precursor to deposit CdS thin films or nanoparticles.

A cleaner approach towards deposition of CdS thin films by an electrochemical technique

AbstractCdS thin films have been deposited on Transparent Conducting Oxide (TCO, containing SnO2: F) coated glass substrates from aqueous sodium thiosulfate solutions, using the galvanic technique. A cadmium foil was used as a sacrificial anode and was the only source of Cd++ ions for CdS thin films, no external bias being used during film depositions. The films have been characterized using X‐ray diffraction, SEM, EDX, AFM and optical absorption studies. AAS analyses were carried out to determine the Cd content in solutions after film deposition. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Investigation of aluminium and indium in situ doping of chemical bath deposited CdS thin films

Aluminum and indium in situ doping of CdS using chemical bath deposition (CBD) is investigated. The effects of Al and In-doping on optical properties as well as on electrical properties, crystal structure, chemistry and morphology of CdS films are studied. Al doping of CdS using CBD is shown to be successful where a resistivity as low as 4.6 × 10−2 Ω cm and a carrier density as high as 1.1 × 1019 cm−3 were achieved. The bandgap of Al-doped films decreases to a minimum of 2.26 eV, then slightly increases and finally saturates at 2.30 eV as the [Al]/[Cd] ratio in solution increases from 0.018 to 0.18. X-ray diffraction studies showed Al3+ ions entering the lattice substitutionally at low concentration and interstitially at high concentration. Phase transition, due to annealing, and induced lattice damage, due to doping, were detected by micro-Raman spectroscopy. Film stoichiometry was found to be sensitive to Al concentration, while film morphology was unaffected by Al doping. Indium doping using CBD, however, was found to be highly unlikely due to the low solubility of indium sulfide. Instead, the formation of InS/In2S3 dominated the deposition process over CdS.

INFLUENCE OF CADMIUM SALTS ON A MODIFIED CHEMICAL BATH DEPOSITION OF CADMIUM SULFIDE THIN FILMS

A simply modified bath deposition technique has been successfully used to deposit uniform CdS thin films using cadmium chloride or cadmium acetate as the cadmium ion source, and thiourea as the sulfur source on glass substrates. Both the traditional magnetic agitation and the frequent substrate vibration by hand were done simultaneously during the deposition. Various properties of the deposited films such as surface morphology, crystallinity, and optical properties were investigated. The CdS films deposited from cadmium acetate show a good crystallinity and exhibit a preferential orientation along the hexagonal (002) direction. Their surface morphologies are more homogeneous with smaller grains than those from cadmium chloride. The CdS films prepared from both cadmium ion sources present a high optical transmission (more than 80%) in the visible range with the band gap value of about 2.4 eV. The substrate vibration reduces the formation and residence of gas bubbles on the glass substrate during growth and resulted in deposition of CdS thin films with high quality. XRD, SEM, and UV–Vis measurements have provided the supported data. The fundamental CBD growth mechanisms involving different cadmium salts are discussed.