We report here the influence of thickness on the photosensing properties of copper sulfide (CuS) thin films. The CuS films were deposited onto glass substrate by using a simple and cost effective chemical bath deposition method. The changes in film thickness as a function of time were monitored. The films were characterized using X-ray diffraction technique (XRD), field emission scanning electron microscopy (FE-SEM), optical measurement techniques and electrical measurement. X-ray diffraction results indicate that all the CuS thin films have an orthorhombic (covellite) structure with preferential orientation along (113) direction. The intensity of the diffraction peaks increases as thickness of the film increases. Uniform deposition having nanocrystalline granular morphology distributed over the entire glass substrate was observed through FE-SEM studies. The crystalline and surface properties of the CuS thin films improved with increase in the film thickness. Transmittance (except for 210 nm thick CuS film) together with band gap values was found to decrease with increase in thickness. I-V measurements under dark and illumination condition show that the CuS thin films give a good photoresponse.
Semiconductor metal chalcogenide thin films have a potential application in the field of the energy storage, gas, and photo-sensing mechanism [
There are several reports found on synthesis and characterization of CuS thin films deposited by different physical as well as chemical methods. For instance, Yuan et al. [
Although large amount of literature is available on the CuS thin film, but little is found on CuS as photosensors. In addition to this, the influence of thickness on the photo-sensitive CuS thin film is not properly studied and explained. Therefore, considering all the above mentioned facts, here in the present study we report about the influence of the thickness on the physical as well as photo-sensor properties of p-type CuS thin films synthesized by the simple chemical bath deposition (CBD).
The glass substrate of dimensions 75 mm × 75 mm × 1.10 mm was used for deposition of CuS thin film. Substrate were washed with double distilled water and boiled in chromic acid for 2 hours. Further they were washed with detergent and rinsed in acetone with ultrasonic treatment. This process is necessary for substrate cleaning, which creates nucleation centers required in thin film deposition.
CBD is simple and promising method used in CuS thin film preparation. It requires low processing temperature and possibility for large scale deposition. Copper sulphate (CuSO4∙5H2O) was used as a copper source and thiourea (SC(NH2)2) as a sulphur source. Reaction bath contains 10 ml 0.1 M CuSO4・5H2O, 11 ml of 25% aq. ammonia, 10 ml of 0.1 M SC(NH2)2 in 100 ml beaker and rest distilled water to make the volume 50 ml. The pH value was adjusted at 10, to get uniform thin films of CuS on glass substrates. By several trails, the preparative parameters are optimized, concentration of the reactant solutions (viz. CuSO4・5H2O and SC(NH2)2) 0.1 M, and pH 10. The deposition was allowed to proceed at RT for different time durations. Hydroxyl ions are created by ammonia when it reacts with water. These ions make a reaction with thiourea and ionized sulphur is obtained. Due to attractive forces between positive and negative ions of copper (Cu2+) and sulphur (S2−), small cluster of CuS is formed. These clusters are directly nucleated on the substrate surface and there they grow into islands of the condensed phase. Such islands start to form layers consisting of the CuS molecules and CuS thin film gets deposited on the glass substrate surface. The process of nucleation and growth is schematically shown in the
The CuS thin films were characterized for structural, morphological, optical and electrical properties. Thickness of CuS thin film is measured by Fizeau fringe technique in which, thickness of thin film is determined as a difference of measured position of interference fringe patterns. When light is incident on the substrate, reflection of light from the surface, interface of film and substrate occurs. Thereby, phase difference will be generated giving interference patterns. The obtained interference pattern can be used for thickness measurement using the following relation:
where, d―thickness, n―refractive index, φ―refractive angle, Δ―optical path difference.
X-ray diffraction (XRD) patterns of the film were recorded on a Bruker AXS, Germany (D8 Advanced) diffractometer in the scanning range 20˚ - 80˚ (2θ) using CuKα radiations with wavelength 0.15405 nm. S-48500 Type-II (HITACHI HIGH TECHNOLOGY CORPORATION Tokyo, Japan) field emission scanning electron microscope (FESEM) was used for the determination of surface morphology. Using JASCO UV-VIS spectrometer (V-630) the optical properties of CuS thin film were investigated by estimating the absorbance and transmittance relation with wavelength having range 400 nm - 800 nm. The I-V characteristics were studied using Keithley meter (Model No. 2400), over the range from ±2V.
presence of a number of peaks corresponding to CuS indicates that the films are polycrystalline with preferred orientation along (113) plane. It was also seen from the XRD pattern that the intensity of the peak increases with increase in thickness, while no significant shift in the peak position was observed. In fact there is a near-linear dependence of (113) peak intensity with respect to thickness found (see
where: λ is wavelength of X-ray; β is broadening of diffraction line measured at full width at half of the peak maxima in radians; k is a constant (0.94) and θ is Bragg’s angle. We found that the crystallite size ranges between 30 - 34 nm for all the samples, while there was no specific dependence found with respect to thickness.
Figures 4(a)-(e) show the FESEM images for different thickness of CuS thin film. It is observed from these images that all the CuS thin films have uniform distribution of grains over the surface and mostly these grains fall in nanometer regime. When thickness of the film increases, morphology turned from tiny nanoparticle-like to heavy granules. Furthermore some CuS nanorods were found at
The transmittance spectra of deposited CuS thin films having different thicknesses are shown in
where “hν” is the photon energy, “Eg” is the optical band gap, “A” is a constant.
Using the above relation, we calculated (αhν)2 and hν values, which is plotted as shown in
Study of I-V characterization is done in dark and by varying illumination intensities of light onto different thickness CuS thin films [see Figures 7(a)-(e)]. For studying the photosensor properties, an area of 1 cm2 of CuS thin film were selected and silver paste was applied (two Ag contacts separated by a distance of 1 cm) to ensure the good neutral electrical contact. Before light illumination, the I-V curve in dark was measured and then subsequently after light illumination (having different intensities) the corresponding I-V curve was measured. It was seen from all the I-V curves [see Figures 7(a)-(e)], that there is a linear dependence between I-V characteristics indicating ohmic nature of the CuS thin films. Furthermore, a general trend for all CuS samples showed that with increase in the illumination intensity, the photoresistance decreases (see
CuS thin films with different thickness have been successfully deposited by the simple and cost-effective chemical bath deposition technique. Increase in the thickness of the CuS films increased the crystallinity as well as morphological properties. Dense morphology with globular structures was confirmed for the large thickness CuS samples. Uneven changes in the transmittance helped us to understand the photo-resistance properties of the CuS thin films. Significant influence of the thickness on the photoresistance values was confirmed indicating that one could manipulate the photoresistance by simply controlling the thickness of the CuS thin films.
The authors are thankful to Head, UDCT Department, North Maharashtra University Jalgaon and Principal, Dr. P. V. Ramaiah, C. H. C. Arts, S. G. P. Commerce, and B. B. J. P. Science College, Taloda, for providing the laboratory facilities. SRG thank Chairman and Management members of Arts and Commerce College Trusts for constant encouragement for doing research work. NGD thanks Department of Science and Technology (DST) for awarding DST Inspire Faculty Award [IFA-PH-61/01/08/2013].