Lokhande, C D

Mane, R S

Uplane, M D

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..idian J. Phys. 73A (2), 169-174 (1999)U P  A— an internatioiiAl journalPreparation and characterization of spray deposited CoS thin films iR S M ane, M  D  Uplane, C D  Lokhande and C H B iosale  Department of Physics, Shivaji University, Kolhapur>416 004» lidiaReceived 3 September 1997, accepted 4 September 1998Abstract : Thin films of CoS have been deposited onto glass substrates by a spray pyrolysis technique using thiourea as a sulphur source and cobalt sulphate as a metal ion source. The structural, optical and electrical properties of these films have been studied. It has been found that films are microcrystalline. The optical absorption studies show that CoS is a direct bandgap material having bandgap energy to be 0.97 eV and electrical resistivity is of the order of 10^ Q-cm at room temperature (300 K). Thermoelectric power measurements (TEP) reveals that the CoS IS a p*type semiconductor.Keywords : CoS thin films, optical absorption, electrical resistivity PACS Nds. : 78.66.-tu. 73.61.Lc1. IntroductionM etal chalcogenide thin films have bewi studied over last 25 years in view o f their potential applications. CoS is one o f them having bandgty> energy equal to 0.90 eV. It has potential applications in IR detector, optoelectronic devices, decorative coatings, solar selective coatings and as a  stw age electrode in pbotoelectrochemical storage devices [1,2]. CoS film s have been deposited by Lokhande [3] using chem ical bath deposition technique in acidic m edium and reported that the films are polycrystalline with b a n d g ^  energy o f 0.9 eV. Basu and Pram anik [4] prepared CoS thin film s by using chem ical dqiosition  (CBD) technique from alkaline thioacetam ide (TAM) bath and film s w ere polycrystalline. N o attem pt has been taken so far to prepare CoS film using spray pyrolysis method. The spray pyrolysis is a sinqile and econom ical m ethod for deposition o f chalcogenide film s [5] and growth rate can be controlled through the param eters like spray rate, substrate tem perature, nozzle frequaicy  etc.In the present study, CoS films have been prepared using spray pyrolysis technique. Preparative param eters like spray rate, deposition temperature, nozzle fiequency, etc are optimized.© 1999 lACS170 R S  Mane, MDUplane,CDLokhande end CHBhosaler f i j^ i^ /’tpri^arinn of the films bas been carried out using X*ray diffraction (XRD), optical absorption, d.c. resistivity and thennoelectric power measurements.2. Expcrimenlal detailsThe AR grade cobalt sulphate (C0SO4) and thiourea [CS(NH2>2] are used for the preparadon of CoS thin films. Double distilled water was used as a solvent The CoS thin films were prepared by talcing 0.1 M solutions of cobalt sulphate and thiourea in a C o : S ratio as 1 : 1 .  The mixed solution was then sprayed onto the glass substrates maintained at the optimized substrate temperature of ZSO’C. The spray rate was maintained to be 10 cc/min, the nozzle to substrate distance was 28 cm, and nozzle was moved back and forth over the substrates with a frequency of 0.30 Hz.Characterization:Tlie thirimftss of the prepared thin film was determined using a relationp , = ml(A.t), (1)where m is the mass of the thin film deposited onto the substrate, A is the area of the deposition film, t is the film thickness and p, is the density of deposited material which is assumed to be the same as that of bulk material (p, = 4.88 g/cm^ for CoS). The structural characterization of CoS thin films was carried out by analyzing X-ray diffraction patterns, obtained using Philips X-ray diffractometer model PW-1710 (A = 1.5405 A for Cu-K a radiation). Optical absorption studies were carried out using a UV-VIS-NIR spectrophotometer (Hitachi model 330, J^an ) in the wavelength range 900-2000 nm. To study electrical properties of CoS thin films, dark resistivity measurements were taken using a two point probe method in the temperature range 300 to 500 K. Silver paste was applied to provide ohmic contact with the CoS film [6j. ThCTioelectric power measurement was carried out in the temperature range of 300 to 420 K.3. Results and discussionIn the spray method, the starting materials required to form the desired compound are in the form of solutions, which are sprayed on the preheated substrates, resulting in the formation of thin films on the substrates. When the droplets of the above sprayed solutiem readi to the hot substrates, owing to pyrolytic decomposition of the solution, well adherent and red colored films of CoS are formed onto the substrates. The thickness of films was measured by a weight difference method and was found to be 0.83 lun. The films are deposited at different substrate temperatures from 200 to 3 0 0 ^ . It is found that the films prepared at 250°C are uniform and well adherent to the substrate.Ih e  XRD patterns of films w oe receded in the range of diffracting angle 0-80°, as shoum in Figure 1. Gose inspection of the XRD pattern (by considering peak count andTPreparation and characterization of spray deposited CoS thin films 171badcground count) indicates the presence o f (100), (211) planes o f CoS material. F u rd ia ' it is seen that the CoS films are m icrociysialline. Tlie *d’ valued o f  planes are calculated by 500,— -----------------------------------—3afigait 1. XRD pAttern of CoS thin film.Angle 2d (degree)assuming hexagonal structure and are com pared with standard [7] values o f CoS taken from ASTM  diffraction data file and are presented in Table 1.Table 1. Comparison of observed 'd* values with standard *</' values ofCoS films.Calculated'd' values(1) 2.9190(2) 1.084Standard d* values2.931.083Reflection planes100211The observed values are in good agreem ent with standard *d' values confirming that the material o f  the films deposited is CoS.Flfarc 2. (a) Variation of the relative absorbance a t with wavelength X for CoS thin films.O ptical absorption  study o f  CoS film s is carried out in  the w avelength range 9(X)-2000 nm  Figure 2(a) show s optical absorbance (a  t) versus wavelength (nm). From calculation, absorption coefficient is found to be 10* cm~^ suj^porting the presence o f  directbandgap [8]. Also the nature o f transition (direct o r indirect) involved can further be analyzed by considering the dependence of a  on hvaoc (hv-Eg)^f^, (2)where Eg is the separation between the conduction and valence bands.172 RS Mane, M D Uplane, C D Lokhande andCH BhosaleFtgarc 2. (b) Plot of {ahv~f against A vfor CoS film.Using the above data, the plot o f ( a  hvf' vs hv  is plotted. It is found that the p lo t is nonlinear indicating absence o f indirect transition. In the present case, the plot o f ( a  hv)^ vs /tv is linear (Figure 2(b)), indicating that the transition is a diredt cnte. The e x t r ^ la t io n  o f  the linear portion o f such a  plot to a  = 0 gives the bandgap energy, Eg to be 0.97 eV. The presence o f a direct bandgap in CoS films has been reported with Eg equal to 0.90 eV [3]. The difference in the present case may be attributed to the different m echanism  and  deposition technique of film formation.The variation  o f  log p  w ith l / T  is depicted  in F igure 3, ind ica ting  the semiconducting nature o f the prepared films. Non linear nature o f the graph shows o f  m any defect levels in the films. The dark resistivity is o f the order o f 10* f2-cm. The electrical resistivity at room tem perature is found to be 0.40 x  10* X3-cm. T he activation  was estimated using the relation,p  = Po exp (EaJKT)  (3)The activation eneigy o f CoS films is found to be 0.034 eV.The therm oelectric pow er (TEP) m easurem ents w as carried ou t fo r CoS film s in the tem perature range o f  300 to 420 K. Variation o f thermoelectric pow er with rise inPreparation and characterization of spray deposited CoS thin films 1732i2 -I 2 83{01^1 ) (K)Figure 3. Variation log p with reciprocal of temperature for CoS film.Figure 4. Variation of thermo emf with temperature for CoS film.temperature is shown in Figure 4. TEP increases with rise in temperature. It was found that the polarity o f  tbeim ally geoerated voltage at the hot end is negative indicating that CoS films are o f p*type conductivity.4. C onclusionsCoS film s dqx>sition is possible using spray pyrolysis technique. The film s prepared at 2S0°C are uniform  as well as ad h o en t to the substrate. The films are microcrystalline. The electrical resistivity o f the film s is found to be 0.40 x  10* fl-can at room temperature. The films deposited by spray pyrolysis show p-type conductivity.174 R S Mane, M D Uplane, CDLokhande andCH BhosakReferences[11 V V Kiliedar, C D Lokhande and C H Bhosale Mattr. Chem. Phys. 47104 (1997)(2) S S Dhumure and C D Loichandc Mater. Chem. Phys. 27 321 (1991)[3] C D Lokhande Indm J. Pure Appl. Phys. 30 245 (1992)|4J P K Basu and P Pramanik Mater. Sci. Lett. 5 1216 (1986)|5) V V Kjlledar, C D Lokhande and C H Bhosale Jhin Sobd Films 28914 (1996)[6] J W Moller A Comprehensive Treatise on Inorganic and Theoreticed Chemistry Vd. 9 (London Longmans, Green & Co) p 685 (1964)[71 ASTM Data File Caidfio.2S-mi(8| J D Desai and C D lx>khande Mater. Chem. Phys. 34 313 (1993)

Preparation and Characterization of Spray Deposited CoS Thin Films

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Preparation and Characterization of Spray Deposited CoS Thin Films

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