Deposited CdSe Thin Films Research Articles

Exploration of doping-dependent structural, optical, and electrical properties of thermally evaporated CdSe thin films for improved photodetection performance

This paper is devoted to studying the influence of indium doping on the structure, morphology, and electrical properties of thermally deposited CdSe thin films as a powerful approach toward optimized photodetection performance. The variation of incorporated In-level modulated crystallographic and surface morphology as obtained from the X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM), respectively. Besides, the optical properties of the prepared In:CdSe thin films were investigated spectrophotometrically in the UV-Vis-NIR regions. We have deeply evaluated not only the dispersion and absorption features but also the opto-(electronic) parameters of the prepared films. The spectral nonlinear optical refractive index, absorption coefficient, and susceptibility were estimated and analyzed. The tunability of the structural and optical properties is introduced in a systematic explainable manner in terms of the recorded elemental analysis using EDX. The dark conductivity measurements of the prepared films declared that doped CdSe film of 37.5 wt% indium achieved the highest electrical conductivity which coincident with all the structural and optical characterization findings. Furthermore, the fabricated In:CdSe films were integrated with p-type silicon for Au/In:CdSe/p-Si/Al heterojunction formation. The electrical and microelectronic parameters of the designed junctions have been assessed using the measured current-voltage relations. The charge carriers' conduction mechanism within the constructed junctions has been explained in detail. Eventually, the photodetection performance of the fabricated heterojunctions has been tested using the recorded (I-V) and (I-t) response curves. Evidently, 37.5 wt% In doping concentration in the CdSe structure achieved superior optical and electrical properties which have been reflected in its promising photodetection performance compared to not only the other pure and doped films in the present study but also other prior reports. Such a device achieved Jphoto, Rλ, EQE, D*, and tr/tf of about 34.4 mA/cm2, 3.43 A/W, 800%, 4.6 × 1010 Jones, and 3.06 ms/6.10 ms, respectively.

Structural, optical, and morphological properties of chemically deposited CdSe thin films by ammonia-free precursor solution

We present a simple route for developing high-quality CdSe thin films at low temperature by means of the chemical bath deposition technique using an ammonia-free formulation. The effect of the concentration of the precursor solutions was evaluated through four formulations. The crystalline, optical, and morphological properties of the CdSe thin films were analyzed for the four deposition conditions. As-deposited CdSe films exhibited hexagonal phase and strong crystalline orientation along the [002] direction as observed by XRD analysis. The diffraction peak corresponding to the (002) crystalline planes shifts to higher two theta values in the patterns of the films deposited with increasing precursors concentration, showing the shrinkage of the crystalline lattice of up to 1.58%, as a product of the tensile stresses through the substrate-film interface. UV-Vis-NIR analysis revealed highly reflective films and good optical quality. The refractive index showed a maximum of 2.20–2.26 around 640 nm, while the extinction coefficient was zero at wavelengths lower than the absorption edge. The SEM and AFM analysis revealed the homogeneous and compact CdSe films surface with low average roughness. The energy band gap of the CdSe thin films varied in the range from 1.78 to 1.9 eV and the variation has been associated with the stresses induced in the film-substrate interface during the film growth. We found an opposite trend between the lattice constant (c) and the energy band gap of the CdSe thin films, which enable the adjustment of their optical properties through the concentration of the precursor solutions during the deposition process.

Size-controlled synthesis of nanocrystalline CdSe thin films by inert gas condensation

Size, shape and structure are considered to have significant influence on various properties of semiconducting nanomaterials. Different properties of these materials can be tailored by controlling the size. Size-controlled CdSe crystallites ranging from ∼ 04 to 95 nm were deposited by inert gas-condensation technique (IGC). In IGC method, by controlling the inert gas pressure in the condensation chamber and the substrate temperature or both, it was possible to produce nanoparticles with desired size. Structure and crystallite size of CdSe thin films were determined from Hall–Williamson method using X-ray diffraction data. The composition of CdSe samples was estimated by X-ray microanalysis. It was confirmed that CdSe thin film with different nanometer range crystallite sizes were synthesized with this technique, depending upon the synthesis conditions. The phase of deposited CdSe thin films also depend upon deposition conditions and cubic to hexagonal phase transition was observed with increase in substrate temperature. The effect of crystallite size on optical and electrical properties of these films was also studied. The crystallite size affects the optical band gap, electrical conductivity and mobility activation of nanocrystalline CdSe thin films. Mobility activation study suggested that there is a quasi-continuous linear distribution of three different trap levels below the conduction band.

Photovoltaic performance of an alternating cold–hot method deposited CdSe thin films

Cadmium selenide (CdSe) thin films were prepared on indium tin oxide substrate by an alternating cold–hot method, in which cadmium nitrate solution was used as a cold deposition solution, while sodium selenite and potassium borohydride mixed solution was used as a hot deposition solution. The influences of the preparation conditions such as the concentration of Cd(NO3)2, the number of deposition cycle and the cycle time, on the photoelectric performance of the sample under simulated sunlight were explored. The results show that the CdSe thin film prepared under the reaction conditions of 0.06 mol/l Cd(NO3)2 at the tenth deposition cycle (30 s per cycle) reaches the highest photovoltage of 0.285 V. Under the simulated solar illumination, the open-circuit voltage and short-circuit currents are 0.419 V and 5.57 mA/cm2, respectively. X-ray diffraction indicates that the strongest diffraction peak at 42.215° of the (111) crystal plane is corresponding to 15.15 nm CdSe nanocrystals. Scanning electron microscopy observation shows that the thickness of the CdSe film is about 200 nm and the size of the spherical and uniformly dispersed nanocrystals is around 50 nm.

Invoking stoichiometric protocols for chemical synthesis of CdSe thin films

An attempt is made to establish the procedure for regulating stoichiometry and hence the materials properties of the chemically deposited CdSe thin films. Reliance of the structural, morphological, optical and electrical properties on the stoichiometry was studied to support the established protocols. To upkeep the appealed modus operandi, growth mechanism of CdSe thin films is proposed. The compositional studies revealed stoichiometric adaptations harmonizing the synthesis protocols. Crystallography remained the same regardless of the variation in deposition conditions. The as-deposited CdSe thin films are polycrystalline in nature with the hexagonal (P63/mmc space group) crystal geometry. Globule shaped grains of nanosize are observed through the scanning electron microscopy. Morphological modifications coupled with the stoichiometric deviations established control over the bandgap and electrical transport characteristics. Thus, with the ambitions to set up the standards for regulating stoichiometry and consequently the physical properties, we communicate the synthesis and preliminary studies on the CdSe thin films through this paper.

Structural, optical and electrical properties of electrodeposited cadmium selenide thin films for applications in photodetector and photoelectrochemical cell

Nanocrystalline cadmium selenide (CdSe) thin films have been deposited on indium tin oxide (ITO) coated glass substrates at room temperature (28°C) by using simple two electrode electrodeposition process. The preparative parameters such as deposition time, deposition potential, concentration of solution, pH value of electrolyte have been optimized for deposition of uniform CdSe thin films. The films were annealed at 250°C for 30min in air and characterized using X-ray diffraction, optical absorption spectrophotometer, Scanning Electron Microscope, Current–Voltage–Temperature measurements, photoconductivity and photoelectrochemical cell (PEC) measurements. XRD studies identify that the hexagonal (Wurtzite) phase present in the deposited CdSe thin films is highly oriented to [002] direction. Optical band gap is found to decrease with increase in annealing temperature. Electrical properties exhibit that the films are semiconducting. Photoresponse of the films increases with increase of exposure time. The construction of fabricated photoelectrochemical cell is CdSe|NaOH (1M)+S (1M)+Na2S (1M)|Graphite. The cell shows that the CdSe thin films have two different zones of metallic character under both dark and illumination conditions and their conversion efficiency is 1.16%.

Effect of Dip Time on the Structural and Optical Properties of Chemically Deposited CdSe Thin Films

In this study, the effect of dip time on the structural and optical properties of cadmium selenide (CdSe) thin films grown by the chemical bath deposition method is reported. The films were grown with varying dip time in the range of 4 h to 12h, and the other deposition variables (substrate temperature, source to substrate distance, pH, and concentration) were kept constant. X-ray diffractometry (XRD) and optical spectroscopy were used to characterise the layers. The results show that the crystallite size and the film thickness increased with an increase in the dip time up to a “critical value” and then decreased otherwise for the latter. The increase in the crystallite size was more pronounced at the lower dip time ( 8 h. The energy band gap was found to be direct with an optimum value of 1.2 eV obtained for films grown at a dip time of 8 h.

Effects of annealing on cadmium selenide nanocrystalline thin films prepared by chemical bath deposition

CdSe thin films were deposited on the glass substrates by chemical bath deposition method at 80 °C using cadmium acetate and sodium selenosulphate as source materials and ethylene diamine tetra acetic acid (EDTA) as a chelating agent with concentrations 0.025, 0.075 and 0.1 M. The various preparative parameters such as composition, solution temperature, pH of the solution and deposition time were optimized for depositing good quality CdSe films. The deposited CdSe thin films are annealed in the air atmosphere for 3 h at 350 °C. The structural, optical, morphological and electrical properties of as deposited and annealed samples were studied. The crystallite size of the film was estimated by Scherrer's formula and the dislocation density was evaluated using the high intense X-ray diffraction peaks. The phase change of CdSe from cubic to hexagonal after annealing at 350 °C was confirmed from the X-ray diffraction pattern. The thickness of the film is found to be 0.6 μm using interferometery technique. The optical properties were studied using UV–vis and photoluminescence spectrometer. The optical band gap of the film has been estimated to be 2.1 and 1.84 eV for as deposited and annealed films. The photoluminescence band shows red shift with increasing particle size after annealing at a higher temperature. The homogeneous crystalline grains were observed from SEM as well as AFM images. The as-deposited and annealed CdSe films exhibit negative Hall coefficient, which confirms n-type conductivity. The Hall mobility, resistivity, carrier concentration and conductivity were evaluated.

Photoelectrochemical properties of spray deposited n-CdSe thin films

Polycrystalline cadmium selenide (CdSe) thin films have been prepared by spraying a mixture of an equimolar aqueous solutions of cadmium chloride and selenourea on preheated fluorine doped tin oxide (FTO) coated glass substrates at different substrate temperatures. The cell configuration n-CdSe/1 M (NaOH + Na 2S + S)/C is used for studying the capacitance–voltage ( C– V) characteristics in dark, current–voltage ( I– V) characteristics in dark and under illumination, photovoltaic power output and spectral response characteristics of the as deposited films. Photoelectrochemical study shows that as deposited CdSe thin films exhibits n-type of conductivity. The spectral response characteristics of the films at room temperature show a prominent sharp peak at 725 nm. The measured values of efficiency ( η) and fill factor (FF) are found to be 0.50% and 0.44 respectively for film deposited at 300 °C. Electrochemical impedance spectroscopy studies show that the CdSe film deposited at 300 °C shows better performance in PEC cell.

Structural, optical and electrical characterization of chemically deposited CdSe thin films

Cadmium selenide (CdSe) thin films have been deposited on glass substrate. CdSe thin films were characterized by various techniques such as X-ray diffraction, scanning electron microscopy and UV–vis–NIR double beam spectrophotometer. The electrical and thermo-electrical properties are also studied. The X-ray diffraction analysis shows that the film samples are in cubic crystal structure. The optical band gap energy (Eg) was found to be 1.7 eV.

Mobility activation in thermally deposited CdSe thin films

Effect of illumination on mobility has been studied from the photocurrent decay characteristics of thermally evaporated CdSe thin films deposited on suitably cleaned glass substrate held at elevated substrate temperatures. The study indicates that the mobilities of the carriers of different trap levels are activated due to the energy of incident illumination, which results in the existence of two distinct trap levels. In each trap depth the energy of the trap increases linearly. It infers that there is a linear distribution of traps of different energies below the conduction band.

Influence of the annealing in nitrogen atmosphere on the XRD, EDX, SEM and electrical properties of chemical bath deposited CdSe thin films

The influence of annealing in nitrogen atmosphere on the structure, optical and electrical properties of cadmium selenide (CdSe) thin films deposited by chemical bath deposition (CBD) onto glass substrates was studied. The samples were annealed in nitrogen atmosphere at various temperatures. A transition from metastable nanocrystalline cubic to stable polycrystalline hexagonal phase has been observed after annealing. The as-deposited CdSe thin films grow in the nanocrystalline cubic phase with optical band gap 1.93 eV. The electrical resistivity of the thin films has been measured in order of 10 6 Ω cm. The activation energy of the samples has been found to be 0.26–0.19 eV at low temperature region, and 0.36–0.56 eV at high temperature region. It was also found that the activation energy and the resistivity of the films decrease with the increasing annealing temperature.

Investigations on chemical bath deposited cadmium selenide thin films

CdSe polycrystalline thin films were grown onto glass substrates by chemical bath deposition at 70 °C. The samples were characterised by structural, optical and electrical properties. XRD results revealed that the deposited CdSe thin films are polycrystalline in nature with cubic structure having (1 1 1) plane as the preferred orientation. The structural parameters such as the lattice constant, crystallite size, dislocation density, number of crystallites per unit area and strain were calculated for films and it was found that the estimated structural parameters are better compared with those of vacuum based hot wall deposited CdSe thin films. EDAX results indicated that the prepared CdSe thin films are non-stoichiometric in composition (Cd 1.3Se 0.7). The optical analysis helped to establish the type of transition as direct allowed. Various optical parameters such as absorption coefficient, extinction coefficient and band gap energy have also been evaluated. The thickness dependence of structural and optical parameters has been discussed in this paper in detail. The high temperature conductivity mechanism has been found to be due to the grain boundary scattering contribution and the evaluated high temperature electrical parameters strongly confirm the semiconducting nature of CdSe thin films.

Thermal treatment effects in the photovoltaic conversion of spray-painted TiO 2 coatings sensitized by chemically deposited CdSe thin films

We have investigated the photovoltaic conversion of spray “painted” (spray deposited) titanium dioxide coatings sensitized with chemically deposited cadmium selenide thin films. The structural, optical and photoelectrochemical characterization of the composite films indicate the importance of thermal treatments in improving the photocurrent quantum yields. Up to 400°C, the effect of air annealing is to shift the onset of absorption to longer wavelengths and to improve the photocurrent substantially. Thermal treatment above 450°C converts the heterosystem into a cadmium titanate with poor photoelectrochemical properties. Due to the thin nature of the CdSe film it is used as sensitizer, the presence of the TiO 2 matrix helps to improve the performance of the chemically deposited films. That is, higher photocurrents and a red shifted spectral response were obtained in the sensitized films, when compared to the spectra of the corresponding CdSe used as sensitizer. This behavior, along with the good photostability of the coatings, is promising for the use of the sensitized film in photocatalytic as well as photovoltaic applications.

Sensitization of screen-printed and spray-painted TiO 2 coatings by chemically deposited CdSe thin films

We have investigated the sensitization of screen-printed and spray-painted titanium dioxide coatings by chemically deposited cadmium selenide thin films. The structural, optical and photoelectrochemical characterization of the composite films indicate the strong role played by the titanium dioxide morphology during the sensitization step and during photovoltaic conversion. Air annealing at temperatures close to 400°C helps to improve the photocurrent quantum yields and the photostability of the composite coatings in broad pH media. The spectral response, I– V curves and photostability of the sensitized spray-painted and screen-printed TiO 2 coatings is superior in polysulfide media than in ferro/ferricyanide electrolytes, indicating the possibility of electroless etching and/or surface modification by ferricyanide. Overall, the sensitized films based on spray-painted TiO 2 show stronger photocurrents under visible light and withstand better the deleterious effect of the ferro/ferricyanide media.

Thickness-dependent properties of chemically deposited CdSe thin films

Thin films of cadmium selenide with different thicknesses were prepared by the chemical deposition method from an alkaline bath using sodium selenosulphate as Se −2 ion source. The effect of film thickness on the optical, structural and electrical properties was studied. The shift of 0.5 eV in the optical band gap energy, E g and a decrease in electrical resistivity from 10 4 to 10 3 Ω cm and increase in the grain size of CdSe crystallites from 40 to 80 Å were observed when the film thickness was varied from 600 to 2400 Å. These changes are attributed to the quantum size effect in the semiconducting films.

Photoelectrochemical behavior of chemically deposited CdSe and coupled CdS/CdSe semiconductor films

Photoelectrochemical effects at chemically deposited CdSe thin films (2000 Å) coupled with as-prepared and air annealed (250°C) CdS films have been investigated by monitoring open-circuit voltage ( V oc) and short-circuit current density ( I sc) at varying incident light intensities and for different heat-treatments temperatures. Two consecutive chemical baths were used in the coupled system. Each bath has been optimized in earlier studies for the deposition of highly photosensitive CdS and CdSe thin films. The photoelectrochemical behavior of single and coupled films was investigated in ferricyanide redox couples. The enhanced short-circuit photocurrent of the as-deposited CdS/CdSe system, despite their lower photosensitivity, indicated that charge separation improved in the coupled system. The role of post-deposition thermal treatments in improving the photoelectrochemical cell characteristics and stability of coupled semiconductors was investigated. Excellent I–V properties were obtained for CdSe and CdS 250/CdSe photoelectrodes annealed at 280°C. For the coupled system: V oc=960 mV; I sc=8.6 mA/cm 2; fill factor (ff)=0.53 and cell efficiency ( η)=4.2%. The linearity of V oc/ln( I L) and I sc/ I L plots supports the Schottky–Mott model for these interfaces. The stability of the coupled photoanode is superior to that of the CdSe only-film for the initial 3 h.

Simultaneous observation of strong and weak quantum confinement effect in chemically deposited CdSe thin films: A spectro-structural study

CdSe thin films deposited chemically on glass substrates for 4, 8, and 16 h, and subsequently annealed at 400 °C for 1 h, have been studied by a combination of spectroscopic (photoluminescence and Raman scattering) and structure-determining (x-ray diffraction and atomic force microscopy) techniques. Due to a size distribution of constituent grains, photoluminescence spectra of the as-deposited films show weak but broad bands at ∼2.2 eV (strongly confined band) and ∼1.73 eV (weakly confined band). On annealing, intensity of the weakly confined band, at ∼1.7 eV increases as a result of an improvement in the crystalline quality of CdSe nanoclusters. A surface-optic Raman mode at ∼250 cm−1 in as-deposited samples has been observed for the first time. The x-ray diffraction studies of annealed samples show a diffraction peak at 2θ=13° from the (001) plane. The improvement in crystallinity of the films as observed by atomic force microscopy and photoluminescence techniques, the appearance of (001) reflection in the x-ray diffraction pattern, the disappearance of surface-optic Raman mode, and the enhancement of weakly confined band—all as a consequence of annealing—have been discussed and correlated with each other. A film deposition mechanism has been described, which explains the origin of the simultaneous existence of strong and weak quantum confinement effects; the significance this observation in the development of high efficiency photovoltaic solar cells has been emphasized.

Enhancement of photosensitivity in chemically deposited CdSe thin films by air annealing

Improvement in photosensitivity, (σphoto−σdark)/σdark, of chemically deposited CdSe thin films on annealing in air is discussed. The as-prepared films of ∼0.5 μm thickness show photosensitivities of <10 under 600 W m−2 illumination. Upon annealing the films in air for 1 h each at various temperatures their photosensitivity increases depending on the temperature of annealing: ∼10 (200 °C), ∼102 (300 °C), ∼103 (400 °C), and ∼107 (450 °C). Air annealing at temperatures beyond 450 °C was found to cause degradation in the photosensitivity. The high photosensitivity is also accompanied by growth in photocurrent while maintaining a fast decay of ∼6 decades in <1 s after shutting off the illumination. Such a short decay time is unusual with chemically deposited photoconductive thin films. The results are explained on the basis of improvement in crystallinity and increase in chemisorption of oxygen upon annealing the films in air. X-ray-diffraction data and x-ray photoelectron spectroscopy depth profiling of the annealed films are presented in support of this explanation.