Optical and Electrical Properties of CdIn2S4 Thin Films for different Substrate Temperature
Semiconducting CdIn2S4 thin films have been deposited by spray pyrolysis technique onto glass substrates of varied temperatures from 340 to 420 Using van der Pauw method, the variation of the electrical resistivity was found for different substrate temperatures. The as grown layers were optically characterized in order to evaluate the optical band gaps.Optical investigations show an indirect transition )of (2.29 2.20)eV and an optical direct gap of (2.75 2.61)eV Introduction Spray pyrolysis is one of the most convenient, economical, inexpensive and simple method for preparation of large area of semiconductor thin films.Films manufactured by chemical spray pyrolysis technique, are of promising future applications. To deal with chemical spray pyrolysis technique, one has to deal with different affecting factors: substrate nozzle distance, substrate temperature, solvent used and solute's concentration, to mention some. Consequently, a study of preparation conditions of these films and their effects on the films properties is a necessity. Indium doped CdS thin films prepared by chemical spray pyrolysis have been investigated by different authors [A.Palafox et.al;1998, K.S.Ramaiah,et.al;1999] It has been found that with increasing indium concentration in the sprayed solution there is an increase tendency of the formation of such compounds as CdIn2S4 and In2O3 in the film. It Usama A. Khalid et.al. Optical and Electrical Properties 50 was suggested that the substrate temperature plays some role in the final product's composition and properties. Apparently at high temperatures, the thermal energy provided for the formation reaction is enough to promote the synthesis of such compounds. Cadmium indium sulphide (CdIn2S4)is one of the indirect ternary semiconductors chalcogenide compound of the class (where A=Zn, Cd and Hg; B=Ga, In; and C=S, Se and Te) [R.R.Sawant,et.al;2010, M.Fuentes,et.al;2000] They are potential materials for applications in solar cells, optoelectronic and photochemical devices, light emitting diodes, non linear optics, photoconductor optical sensing, etc.[R.R.Sawant,et.al;2007,G.F.Epps and R.S.Backer;1982, K.Y.Rajpure, et.al;1999].CdIn2S4 compound is indirect band gap energy which found to be 2.29eV while, the inter band gap of the direct transition has a value of 2.62ev at Room Temperature [H Nakanishi ;1980] The films of CdIn2S4 can be synthesized by multimethods as for example: hot wall epitaxy [S.H You,et.al;2004 vacuum evaporation on glass and sapphire substrate [S.Fafard and E.Fortin;1990,A.Bhirud,et.al;2011] chemical sprayed pyrolysis [R.R.Sawant,et.al;2010, R.R.Sawant,et.al;2007, G.F.Epps and R.S.Backer;1982 R.R.Sawant and C.H.Bhosale ;2006 The films show a polycrystalline structure [K.Y.Rajpure, et.al;1999 R.R.Sawant and C.H.Bhosale ;2006 Unfortunately, due to complexity of heavily Indium dopedCadmium Sulphide films formed by chemical spray pyrolysis the results published by different workers of the physical properties are not always in a good agreement [K.Y.Rajpure, et.al;1999 R.R.Sawant and C.H.Bhosale 2006 A N Georgobiani et.al 1984 S.N.Baek,et.al;2004, P.PHankare,et.al;2006, A.V.Kokate,et.al; 2006, A.V.Kokate,et.al;2006] In this article, we report the results on an investigation of the optical and electrical resistivity of chemical spray pyrolysis CdIn2S4 films under different substrate temperature. The Cd:In:S ratio used in the sprayed solution is 1:2:4. The study shows the role plays by the substrate temperature on the properties of the prepared films. Experimental Technique The sprayed is achieved by mixing 0.05M aqueous solution, of CdCl2, CS(NH2)2 and In(NO3)3. The Cd:In:S ratio in the solution was 1:2:4. The obtained solution is immediately sprayed onto temperature controlled glass substrate. The substrate is heated at the required temperature for 20min previous to the spraying. The spraying was carried out on and off periodically (10sec on and 20sec off) The sprayed rate is 1ml/min. By this procedure; the Basrah Journal of Science (A) Vol.31(3),49 58, 2013 51 substrate is kept at the required temperature. It also gives enough time for the solution droplets to evaporate and the compound to settle down. The diameter of the spray nozzle is about 0.3mm. The nozzle to substrate distance was 30cm. More details of the spray set up and the preparation technique have been described elsewhere [O.P.Agnihotri, et.al;1983] In this work, the spraying set up is kept constant except the substrate temperature has changed from 340 420 The thickness of the prepared films was in the range (0.120.26) The films obtained were clear, transparent, and yellow in color, with good adhesive to the substrate and exhibit smooth surfaces free from pinholes. The optical absorbance measurements are carried out using Pye Unicom SP 800 UV/VIS spectrophotometer. It covers the range from 200 900nm. The resistivity was measured using Van der Pauw method [J.L.van der Pauw; 1958] Ohmic contacts were prepared by evaporation of Aluminum electrodes. All measurements (optical and electrical) were carried out at room temperature. Hot probe measurements proved that the films were Ptype. Results and Discussion The absorption data was analyzed in accordance with the theory of Bardeen et al [J.Bardeen,et.al;1956] which gives for direct transition: