versión On-line ISSN 0717-9707
J. Chil. Chem. Soc. v.54 n.4 Concepción dic. 2009
J. Chil. Chem. Soc., 54, N° 4 (2009), págs. 345-348.
INFLUENCE OF BATH TEMPERATURE AND PH VALUE ON PROPERTIES OF CHEMICALLY DEPOSITED CU4SNS4 THIN FILMS
ANUAR KASSIM1*, TAN WEE TEE1, ATAN MOHD. SHARIF1, DZULKEFLY KUANG ABDULLAH1, MD. JELAS HARON1, HO SOON MIN1 AND NAGALINGAM SARAVANAN2
1Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia. e-mail: firstname.lastname@example.org
2Department of Bioscience and Chemistry, Faculty of Engineering and Science, Universiti Tunku Abdul Rahman, 53300 Kuala Lumpur, Malaysia.
Thin films of Cu4SnS4 semiconductors were prepared by chemical bath deposition technique in aqueous solutions. The effects of various bath temperatures (40, 50 and 60 °C) and pH values (pH 0.5, pH 1.0 and pH 1.5) on growth of films were reported. The structure and morphology characteristics of thin films of Cu4SnS4 grown on indium tin oxide glass substrates were investigated by X-ray diffraction and atomic force microscopy techniques. The optical properties were measured to determine the transition type and band gap value. The thin films produced were found to be polycrystalline with orthorhombic structure. The X-ray diffraction data showed that the most intense peak at 2θ = 30.2° which belongs to (221) plane of Cu4SnS4. The films deposited at 50 °C were found to have the best photoresponse activity and smaller crystal size. At pH 1.5, the film showed well-covered entire substrate surface and the highest absorption values in AFM and optical study, respectively. The best condition to prepare good quality thin films can be carried out at 50 °C with pH 1.5. The bandgap value was found to be 1.4 eV with direct transition.
Key words: Semiconductor, thin films, band gap, chemical bath deposition.
Interest on the preparation and study of physical properties of ternary chalcogenide compounds for their possible applications in optoelectronic devices, solar cells, infrared detectors and light emitting diodes has been increasing in the recent years. There are many techniques for preparing thin films such as plasma-enhanced chemical vapor deposition,1 metal organic chemical vapor deposition,2 thermal evaporation,3 chemical bath deposition," close spaced sublimation,5 vacuum evaporation,6 electrodeposition,7 molecular beam epitaxy,8 spray pyrolysis9 and sputter deposition10. Amongst these deposition techniques, chemical bath deposition is most commonly used because it is a simple, cost effective and convenient for larger area deposition of thin films. The chemical bath deposition method has been proved as a suitable method of preparing binary compounds like MnS,11 SnS,12 SnSe,13 CdSe,14 Sb2S3,1! PbS16 and ternary semiconductors such as CdSSe,17 CuInSe2,18 Cd0JZn0JSe19 and CuBiS220.
Here, we report the preparation and characterization of Cu4SnS4 thin films by chemical bath deposition. The effects of pH and bath temperatures on the properties of these films are investigated. The structure of the film was studied by X-ray diffraction. The morphology and optical absorption properties were determined by using atomic force microscope and UV-Visible Spectrophotometer, respectively.
All the chemicals used for the deposition were analytical grade. It includes copper sulfate (CuS04), tin chloride (SnCl2), sodium thiosulfate (Na2S203), disodium ethylenediaminetetraacetic acid (Na2EDTA) and hydrochloric acid (HC1). All the solutions were prepared in deionised water (Alpha-Q Millipore). 10 ml of CuS04 (0.05M) solution was added into 10 ml of SnCl2 (0.05M) solution in 100 ml beaker. To it, 10 ml of Na2EDTA (0.1M) solution was added and then solution was continuously stirred. 10 ml of Na2S203 (0.05M) solution was then added into a beaker slowly. The resultant solution was stirred for few minutes. The indium doped tin oxide (ITO) glass was used as the substrate. The ultrasonically cleaned glass substrates were immersed vertically into acidic bath. In order to determine the optimum condition for the deposition process, the films were deposited at different bath temperatures (40 -60 °C) and pH values (pH 0.5 to pH 1.5). During deposition period the beaker was kept undisturbed. The substrates were removed from the baths after 2 hours. The deposited films were tested for adhesion by subjecting it to a steady stream of distilled water.
X-ray diffraction analysis was carried out, using a Philips PM 11730 diffractometer for the 29 ranging from 20° to 60° with CuKa (^=1.5418 A) radiation. Topography was measured by using an atomic force microscopy (Quesant Instrument Corporation, Q-Scope 250) operating in contact mode, with Si3N4 cantilever. Photoelectrochemical (PEC) experiments were performed using a [Fe(CN)6]3/[Fe(CN)6]4- redox system, by performing linear sweep voltammetry between -500 to -1000 mV. The sequence of constant illumination and dark period were performed on the PEC cell to study the effect on photoactivity behavior. A halogen lamp (300 W, 120 V) was used for illuminating the electrode. Optical absorption study was carried out using the Perkin Elmer UV/Vis Lambda 20 Spectrophotometer. The film-coated indium doped tin oxide glass was placed across the sample radiation pathway while the uncoated ITO glass was put across the reference path. The absorption data were manipulated for the determination of the band gap energy, Eg.
RESULTS AND DISCUSSION
Fig. 1 shows the XRD patterns of the films deposited at different chemical bath temperatures. All the samples showed a polycrystalline in nature. There
Fig. 2 shows the three-dimensional representation of 20 μm X 20 μm area of the Cu4SnS4 films deposited at different chemical bath temperatures. The films deposited at 40°C and 60 °C (Fig. 2a and 2c) revealed an incomplete coverage of the substrate surface and the grains are not distributed uniformly over the substrate. The growth of grains was focused at certain nucleation centers on the surface of substrate. However, the surface morphology of the Cu4SnS4 films deposited at 50 °C showed uniform grain size as seen in Fig. 2b. The indium tin oxide substrate was covered completely indicating more nucleation sites have formed and the numbers of grains have increased.
Fig. 3 shows the absorption spectra of Cu4SnS4 films at various bath temperatures. The films show a gradually increasing absorbance throughout the visible region, which makes it possible for this material to be used in a photoelectrochemical cell. The film deposited at 50 °C showed gradual absorption at 450 nm downward. The spectrum reveals that this film has higher absorbance characteristic as compared with the films prepared at 40 °C and 60 °C. Thus, this bath temperature is more preferable in the preparation of Cu4SnS4 films of better quality on ITO substrate. The optical absorption values are in line with AFM results.
Fig. 4 shows the different between photocurrent (Ip) and darkcurrent (Id) for the films deposited at different chemical bath temperatures. The film deposited at 50 °C showed the highest photoresponse activity if compared with other deposition temperatures. This could be due to sufficient semiconducting material deposited onto the surface of substrate. The photocurrent occurs on the negative potential indicates the films are p-type and they can be deployed as a photocathode in a photoelectrochemical cell for reduction reactions.
Fig. 5 shows the XRD patterns of Cu4SnS4 thin films deposited at different pH ranging from 0.5 to 1.5. The XRD patterns are found to be poly crystalline with orthorhombic structure. There are two peaks can be observed at diffraction angles of 30.3° and 50.6° on the XRD pattern obtained on the films prepared at pH 0.5. These two peaks are assigned to interplanar distances of 2.95 and 1.81 Â which corresponding to (221) and (711) planes respectively. When the pH value was increased from 1 to 1.5, the number of peaks related with Cu4SnS4 formation increased. There are three additional Cu4SnS4 peaks could be detected at 20 = 28.6°, 33.5° and 47. Io which attributed to the (102), (321) and (040) planes.
Fig. 6 shows the three-dimensional representation of a 20 urn X 20 urn area of the Cu4SnS4 thin films deposited at different pH of the chemical bath. The irregular surface of film and discontinuous distribution of grains was detected for the film grown atpH 0.5 (Fig. 6a) and pH 1 (Fig. 6b) respectively. However, the film shows uniform, dense and well-covered entire substrate surface (Fig. 6c) when the solution pH is increased from 1 to 1.5.
Fig. 7 shows the absorption spectra of Cu4SnS4 films at different pH values. The films show a gradually increasing absorbance throughout the visible region. The films grown at pH 1.5 are thicker and have higher absorption characteristics.
This response also associated with the fact that more polycrystalline Cu4SnS4 materials were formed at this pH value. Thus, pH 1.5 is more preferable in the preparation of Cu4SnS4 films of better quality on ITO substrate.
Fig. 8 indicates the different between photocurrent (I) and darkcurrent (Id) for the films deposited at different pH of the chemical bath. The photoresponse for the films deposited at pH 0.5 was the lowest because of least Cu4SnS4 crystallite formation. It is observed that the samples prepared at higher pH (pH 1.5) values have the highest photoresponse activity.
Band gap energy and transition type can be derived from mathematical treatment of data obtained from optical absorbance versus wavelength with Stern relationship21 of near-edge absorption:
Cu4SnS4 thin films have been chemically deposited on indium tin oxide substrates from aqueous solution containing CuS04, SnCl2, Na2S203 and Na2EDTA. The thin films produced were found to be polycrystalline with orthorhombic structure. The X-ray diffraction pattern showed that the most intense peak at 20 = 30.2° which belongs to (221) plane of Cu4SnS4. The films deposited at 50 °C were found to have the best photoresponse activity and smaller crystal size. At pH 1.5, the film showed well-covered entire substrate surface and the highest absorption values in AFM and optical study, respectively. Deposition at 50 °C with pH 1.5 is the optimum condition to prepare good quality thin films under the current condition. The bandgap value was found to be 1.4 eV with direct transition. The photoresponse in the cathodic region indicate the p-type semiconductor.
The authors would like to thank the Department of Chemistry, Universiti Putra Malaysia for the provision of laboratory facilities and MOSTI for the National Science Fellowship (NSF).
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(Received: October 8, 2008 - Accepted: August 22, 2009).