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Preparation and physical properties of nanostructured Cu: As2S3 films by chemical bath deposition

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https://www.eduzhai.net International Journal of M aterials and Chemistry 2013, 3(2): 33-38 DOI: 10.5923/j.ijmc.20130302.03 Synthesis of Nanostructured Cu:As2S3 Thin Films by Chemical Bath Deposition Method and Their Physical Properties Ashok U. Ubale*, V. N. Mitkari, D. M. Choudhari, J. S. Kantale Nanostructured Thin film M aterials Laboratory, Departments of Physics, Govt. Vidarbha Institute Sciencesand Humanities, Amravati 444604, M aharashtra, India Abstract Metal chalcogenide thin film preparat ion by chemical bath deposition is currently attracting considerable attention as it is relatively less expensive, simple and convenient method for large area deposition. In the present work preparation of un-doped and Cu doped As2S3 thin films by chemical bath deposition method is reported. The film characterizat ion is undertaken to study structural, optical and electrical propert ies of As2S3 and Cu:As2S3 thin films. The structural characterizat ion shows mixed monoclin ic and hexagonal lattice due to As2S3 and CuS.The electrical resistivity of the As2S3 film decreases with doping as it introduces the impurity donor level in As2S3. The activation energy and optical band gap decreases from 0.26 to 0.03 eV and 3 to 2.34 eV due to doping of Cu in As2S3 film respectively. The thermo-emf measurement confirms the n-type conductivity. Keywords Che mica l Synthesis, Nanostructured Thin Films, Electrica l, Optica l , Structura l Properties 1. Introduction The industrial development of nanoscience and nanotechnology needs synthetic materials of tailor made properties that arise with decreasing size of nanomaterial having a higher packing density, higher speed performance with lower cost[1,2]. Thin films have potential applications in many devices such as solar selective coatings, as a storage electrode in photoelectrochemical storage devices, photoconductors, IR detectors and solar cells etc. With rapid technological advances in the preparation of films with controlled, reproducible and well defined structures, thin films are expected to play increasingly important role in the studies of a variety of solid state phenomena. The properties of large verity of new and interesting materials obtained by thin film techniques will undoubtedly draw considerable attention in future[3]. The advanced microelectronics based on surface engineering needs well developed deposition processes, as the most demanding approaches in the near future. New concepts and design methodologies are needed to synthesize new thin film devices and to integrate them for various operations. Arsen ic trisu lfid e (As2S3) is a tech n ically impo rtant * Corresponding author: ashokuu@yahoo.com (Ashok U. Ubale) Published online at https://www.eduzhai.net Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved material because of its good transparency in the 0.7-11-µm wavelength range and excellent resistance against diversificat ion, moisture, and corrosion. It is well known that As2S3 has variety of applications in optical imag ing, hologram recording and recently in various electronic devices, including electro-optic informat ion storage devices and optical mass memo ries[4-8]. Various methods have been employed to deposit As2S3 thin films. Pawar et al[9] have prepared As2S3 films by solution gas interface technique. Hajto et a l.[10] have reported physical properties of spin-coated As2S3 films. Lokhande[11] has reported solution growth of As2S3 films using As2O3 and Na2S2O3 as As3+and S2- ion sources, respectively, fro m co mp lexed (with EDTA) acidic and alkaline aqueous baths. Desai and Lokhande[12] have deposited As2S3 films onto glass substrates from an acidic sodium thiosulphate bath, using disodium salt o f ethylenediaminetetraacetic acid to co mplex As3+ ions. The films were amorphous, with an optical band-gap of 2.36 eV and resistivity of the order of 106 Ω-cm. Deshmukh et al[13] have prepared As2S3 thin films in a non-aqueous mediu m (methanolic solution) using simp le chemical deposition process. Mane et al[14] have deposited nanocrystalline As2S3 thin films in aqueous mediu m at low temperature by using chemical bath deposition technique. In this manuscript preparation of As2S3 and Cu:As2S3 thin films by using chemical bath deposition method are reported. These films are characterized by XRD, SEM, electrical resistivity and optical absorption measurements to investigate their physical properties. 34 Ashok U. Ubale et al.: Synthesis of Nanostructured Cu:As2S3 Thin Films by Chemical Bath Deposition M ethod and Their Physical Properties 2. Experimental Chemical bath deposition is well suited for producing nanostructured thin films suitable for solar energy related applications. The films are deposited on substrates immersed in d ilute solutions containing metal and sulfide ions. In the present work nanostructured As2S3 and Cu: As2S3 thin films were prepared by using chemical bath deposition method. For deposition of As2S3 films, 60 ml 0.5 M As2O3 solution is mixed with 60 ml 0.5 M tartaric acid. Then, 60ml 0.5 M sodium thiosulphate is added in it with constant stirring. The colour o f mixture changes from pale yello w to dark yello w after about 30 minute. The well deposited shining yellow coloured As2S3 thin films were removed fro m the solution after 6 h deposition time and used for further characterizat ion. The as-deposited As2S3 films are designated as ‘Film A’ in the further discussion. In the present work Cu doping in As2S3 is achieved by two different ways. In the first method, the as-deposited As2S3 thin films are heated in 0.5 M cupric acetate solution for 3 h at 60° C. The as-deposited As2S3 films were yellow in colour and after heating in cupric acetate bath the film colour become golden yellow. This confirms the addition or replacement of some Cu atoms in As2S3. The Cu: As2S3 films prepared by this method are designated as ‘Film B’ in the further discussion. In second method to prepare Cu: As2S3 films, 5 ml 0.5 M cupric acetate solution is directly mixed in the reaction mixture during deposition of As2S3 thin films. The solution colour becomes golden yellow after about 40 minute. The well deposited uniform Cu: As2S3 films were removed fro m the reaction beaker after 6 h deposition time. These films are designated as ‘Film C’ in the further discussion. Thickness (t) of the film is defined as the distance perpendicular to surface fro m a point on the boundary surface through the film to other boundary surface. Amongst the different methods for measuring the thickness the weight difference method is simp le and convenient. The mass of the deposited film is related to area and density of the material as, t= m (1) Ad Where, t is the thickness of the film, m is mass of the deposited sample, A is area o f the deposited sample and ρ is the density of the deposited sample. The two point dc probe method of dark e lectrical resistivity was used to study the variation o f resistivity with temperature. The film of size 1 cm2 on the glass substrate was used. Silver paste was used to make ohmic contacts. A brass block was used as sample holder cu m heater. A thermocouple with DPM was used for temperature measurement along with nanometer for current measurement. The variation of absorption density ‘αt’ with the wavelength for As2S3 and Cu:As2S3 film was carried out using double beam spectrophotometer, ELICO - SL164-SP2 in the wavelength range between 350 to 890 n m. The thermo emf measurements were carried to find the type of conductivity. The crystallographic studies of thin films were characterized by using a PANalyticalX’Pert PRO MRD X-ray diffractometer with Cu Kα radiation in the 2Ɵ range fro m 20 to 80 degree. A lso the film microstructure was studied by using JOEL’s JSM -7600F scanning electron microscope with 1 n m resolution. 3. Results and Discussion 3.1. Reaction Mechanism The As2S3 and Cu:As2S3films were prepared in the presence of complexing agenttartaric acid. The metal ions produces complex in the solutions, which dissociates slowly to release them for further process. The precipitate formation in the solution takes place when the ionic product (I.P) e xceeds the solubility product (S.P). In aqueous solution Na2S2O3 dissociates as, Na2 S2O3 → 2Na+ + S2 O 2− 3 (2) Na2S2O3is a reducing agent by virtue of half-cell reaction, 6S2 O32− → 3S4 O62− + 6e− (3) In acidic mediu m dissociation of S2O32− takes place as, 3S2 O32− + 3H+ → 3HSO3− + 3S (4) The electrons released in equation 3 react with S as, 3S + 6e− → 3S2− (5) The tartarate ion forms co mplex with arsenic, wh ich further reacts with S2- to give C OO− As2S3 thin film as, | As 3+ 3(CHOH)2 → [As(C4H4O6)3]3− (6) | C OO− 2[As(C4H4O6)3]3− + 3S2− → As2S3 ↓ +6C4H4O62− (7) In order to dope Cu in As2S3, the as deposited film is heated in 0.5 M cupric acetate solution for 3 h at 60° C. The yellow co loured As2S3 film becomes golden yellow indicating addition of Cu in As2S3. However in second method the 5ml 0.5 M cupric acetate solution is directly mixed in deposition bath to deposit Cu : As2S3 films. The thickness of un-doped As2S3 film is 217 n m. When this film is heated in cupric acetate bath, film thickness decreases to 205 n m. The fraction of As2S3 film may be dissolved in cupric acetate bath due to heating in it for 3 h. However, the Cu: As2S3 film deposited by adding cupric acetate directly in reaction bath has thickness 288 n m. 3.2.Structural Properties Fig.1 shows XRD patterns of As2S3 and Cu: As2S3 films. The diffraction peaks observed in the XRD pattern corresponds to monoclinic As2S3 and hexagonal covellite CuS lattice. The observed XRD data is in good agreement with standard data (Table 1). The (301) orientation due to As2S3 is repeated in both As2S3 and Cu: As2S3 film. The (310), (320) and (411) orientations due to monoclinic As2S3 and (207) orientation due to hexagonal CuS is repeated in both Cu: As2S3 thin films deposited by post heating of International Journal of M aterials and Chemistry 2013, 3(2): 33-38 35 As2S3film in cupric acetate source and by direct addition of cupric acetate in deposition bath. The average crystallite size was determined fro m diffraction peaks using the Scherrer formula, d= (0.9 λ)/β Cosθ, where λ is wavelength used (0.154 n m); β is angular line width at half maximu m intensity in radians; θ is Bragg’s angle. The g rain size of As2S3 is 15 nm and it becomes to 11 and 13n m for Cu: As2S3 thin films deposited by post heating of As2S3 film in cupric acetate and by direct addition of cupric acetate in deposition bath. Figure 1. XRD pattern of doped and un-doped As2S3 films: (A) As2S3, (B) Cu: As2S3 film in which Cu is added by post heating of As2S3 film in Cu source and (C) Cu: As2S3, in which Cu is added during deposition process 3.3. S EM Studies Figure 2 shows the SEM images of As2S3 and Cu: As2S3 thin films . The SEM image of as depos ited As 2S3 film s hows random distribution of spherical grains covering whole substrate surface. At some places the substrate surface is quite rough indicating porous nature of film. However, the SEM image of Cu : As2S3 obtained by heating as deposited As2S3 film in Cu source shows growth of some wafer like disc-shaped crystals of CuS on the ho mogenous surface. It should be noted that while heating some amount of As2S3 may be dissolved in the bath that makes the film surface smoother as seen in SEM. While the SEM of Cu: As2S3 film deposited by mixing cupric acetate directly in reaction bath during deposition shows grain g rowth with better mixing of Cu in As2S3 . 3.4. Electrical Properties To investigate electrical properties of the films silver paste is applied for making ohmic contacts. The existence of barrier is usually observed when contact is made between metal and semiconductor because of either imp roper matching of their work function or the presence of surface states on the semiconductor[4, 5]. The nature of contact was checked by using two probe method. The I-V characteristic of As2S3/Ag and Cu: As2S3/Ag is shown in figure 3. The I-V characteristics is found to be linear within the voltage range up to 30 V and shows that silver electrode produces ohmic contacts with As2S3 and Cu: As2S3. Also it was observed that Cu: As2S3 films deposited by direct mixing of cupric acetate in deposition bath are more conductive. Table 1. Comparison of XRD data of AsS3 and Cu: As2S3thin films Observed values Standard value Film 2θ(deg.) d (A0) 2θ (deg.) d (A0) h k l 31.800 2.811 31.785 2.813 301 42.600 2.120 42.543 2.123 002 A 53.800 55.000 1.702 1.668 53.895 55.019 1.699 611 1.667 251 56.200 1.635 56.208 1.635 630 25.120 3.542 25.050 3.551 310 29.832 2.984 29.874 2.988 320 31.121 2.862 31.231 2.861 301 33.234 2.074 33.174 2.077 006 38.823 2.316 38.858 2.315 411 B 42.511 2.120 42.543 2.123 002 48.390 1.873 48.417 1.878 110 53.434 1.714 53.458 1.712 531 57.787 1.595 57.758 1.594 202 70.798 1.342 70.746 1.330 207 25.123 3.553 25.050 3.551 310 27.921 3.190 27.948 3.189 101 29.811 2.989 29.874 2.988 320 31.125 2.863 31.231 2.861 301 C 36.555 38.788 2.451 2.314 36.601 38.858 2.453 321 2.315 411 56.601 1.629 56.523 1.626 200 58.610 1.571 58.589 1.574 109 70.734 1.333 70.746 1.330 207 The resistivity of thin films was studied by using two point dc probe method in the temperature range 303 to 438 K. Fig 4 shows the variation of resistivity (log ρ) with rec iprocal of temperature (1/T)×103 for As2S3 and Cu: As2S3 films. It was observed that the resistivity of As2S3 at 303K temperature is 2.8×106 Ω-cm and it decreases to 1.5×105 Ω-cm for Cu: As2S3 deposited by heating As2S3 in Cu source and becomes 1.8 ×102 Ω-cm for Cu : As2S3 deposited by adding Cu source directly in deposition bath. This decrease in resistivity is due to Cu-doping. The addition of Cu may increase the donor states in As2S3, which enhances the conductivity. Also the electrical resistivity decreases with temperature indicating semiconductors nature of the film.The electrical resistivity follows the relation, ???????? = ????????0 exp �E0 � KT (8) Where ???????? is the resistivity at temperature T, ????????0 is a constant, K is Bolt zmann’s constant and E0 is activation energy required for conduction. The activation energy for As2S3 decreases fro m0.26 to 0.18eV due to Cu doping in As2S3 by first method and becomes 0.03e V for Cu: As2S3 film deposited by adding Cu source directly in deposition bath. It may be due to decrease in resistivity with Cu doping. 36 Ashok U. Ubale et al.: Synthesis of Nanostructured Cu:As2S3 Thin Films by Chemical Bath Deposition M ethod and Their Physical Properties Addition Cu in As2S3 may increase the donor i.e. trap levels in the band structure. Figure 2. SEM images of doped and un-doped As2S3 films: (A) As2S3, (B) Cu: As2S3 film Cu is added by post heating of As2S3 film in Cu source and (C) Cu: As2S3, Cu is added during deposition process Figure 4. Variation of log of electrical resistivity with reciprocal of temperature of doped and un-doped As2S3 films: (A) As2S3, (B) Cu: As2S3 film in which Cu is added by post heating of As2S3 film in Cu source and (C) Cu: As2S3, in which Cu is added during deposit ion process The TEP measurement is used to determine the type of conductivity of thin film. Temperature difference between two ends of semiconductor causes a transport of carriers fro m hot to cold end creat ing an electric field which g ives rise to thermo-emf. The polarity of the thermally generated voltage at hot end was positive, indicating that the As2S3 and Cu:As2S3 films are o f n-type. Fig. 5 shows that the thermo-emf generated across the film increase with applied temperature differenceacross the film. The rise in thermo-emf in doped film is attributed to the increase in carrier concentration and mobility. Figure 3. IV characteristics of doped and un-doped As2S3 films: (A) As2S3, (B) Cu: As2S3 film in which Cu is added by post heating of As2S3 film in Cu source and (C) Cu: As2S3, in which Cu is added during deposition process Figure 5. Variation of thermo-emf with temperature difference applied across doped and un-doped As2S3 films: (A) As2S3, (B) Cu: As2S3 film in which Cu is added by post heating of As2S3 film in Cu source and (C) Cu: As2S3, in which Cu is added during deposit ion process 3.5. Optical Properties International Journal of M aterials and Chemistry 2013, 3(2): 33-38 37 The study of materials by means of optical absorption provides a simp le method for exp lain ing some features concerning band structure of materials. In the present investigation optical absorption of As2S3 and Cu:As2S3films was studied in the wavelength range of 350 to 890 n m. The variation of absorbance (????????????????) with wavelength (nm) for As2S3 and Cu: As2S3 is shown in figure 6.The absorption for As2S3 film is quite small as co mpared to Cu :As2S3 films. The nature of the transition (direct or indirect) is determined by using the relation[15], ????????ℎ???????? = ????????(ℎ???????? − ????????????????)???????? (9) Fig.7 for As2S3 and Cu: As2S3 films. The nature of the plots indicates the existence of direct transition. The band gap energy Eg is determined by extrapolation the straight portion of the plot to the energy axis. The band gap energy of as deposited As2S3 film is 3 eV and Cu: As2S3 film prepared by post heating of As2S3 film in Cu source is 2.34eV and beco mes 2.84 eV when Cu is doped during deposition . 4. Conclusions In the present work Cu doped and un-doped As2S3 thin films are prepared by using CBD method. The structural, electrical and optical properties of As2S3 and Cu As2S3 thin films are studied. The I-V-characteristics of these films is almost linear indicating oh mic contact between Ag and film. Electrical resistivity measurement indicates semiconducting nature of As2S3 and Cu: As2S3 films. The resistivity decreases with Cu doping as it increases the impurity donor levels in As2S3. The activation energy decreases from 0.26 to 0.03 eV due to addition of Cu in As2S3. It was found that the band gap energy is decreased fro m 3 eV to 2.34 eV due to doping of Cu in As2S3. ACKNOWLEDGEMENTS Figure 6. Variation of optical absorption with wavelength for doped and un-doped As2S3 films: (A) As2S3, (B) Cu: As2S3 film in which Cu is added by post heating of As2S3 film in Cu source and (C) Cu: As2S3, in which Cu is added during deposition process The authors are thankful to University Grants Co mmission, for financial support under the project (No: F.47-1695/ 10 dated 16/ 3/2011). REFERENCES [1] J. Schoonman, “Nanostructured materials in solid state ionics”,Solid State Ionics, vol. 135, pp. 5-19, 2000. [2] L. I. M aissel and R. Gilana, “Hand Book of thin film technology”, M c. Grow Hill, New York (1970). [3] K. L. Chopra in “Thin film phenomena”, M c. Graw Hill, New York, (1969). [4] H. Rawson, “Inorganic Glass Forming System, Nonmetallic Solids”, Academic Press, New York, 1967. [5] T. Kawaguchi1, S. M aruno1 and S. R. Elliott,“Effect of addition of Au on the physical, electrical and optical properties of bulk glassy As2S3”, J. Appl. Phys.,vol. 80, pp. 5625-5633, 1996. Figure 7. Variation of (αhν)2 vshν for doped and un-doped As2S3 films: [6] K. Tanaka, “Effect OfUv Exposure On Optical Properties in (A) As2S3, (B) Cu: As2S3 film in which Cu is added by post heating of As2S3 film in Cu source and (C) Cu: As2S3, in which Cu is added during deposition Intermolecular Distance in Amorphous As2S3 Network”, Appl. Phys. Lett., vol. 26, pp 243-245, 1975. process [7] K.Tanaka and Y. Ohtsuka, “Composition dependence of Where A is constant, hυ is photon energy and Eg is the photo-induced refractive index changes in amorphous AsS optical band gap. The exponent n depends on the nature of films”, Thin Solid Films, vol. 57, pp. 59-64, 1979. the transition, n=1/2, 2, 3/2 or 3 fo r allowed direct, allowed indirect, forb idden direct or forb idden indirect transitions, respectively. The plots of (αhν)2 versus hν are shown in [8] V. A.Danko, I. Z.Indutnyi, A. A.Kudryavstev and V. I. M inko, “Photodoping in As2S3:A g thin films”, Phys. Status Solidi, vol. 124, pp. 235-242, 1991. 38 Ashok U. Ubale et al.: Synthesis of Nanostructured Cu:As2S3 Thin Films by Chemical Bath Deposition M ethod and Their Physical Properties [9] S. H. Pawar, S.P.Tamhankar, P. N. Bhosale and M . D. Uplane, [13] L.P.Deshmukh, J. S.Dargad, and C.B. Rotti, , “Preparation “Growth of Sb2S3 Films by Solution-Gas Interface Technique”, Ind. J. Pure Appl. Phys., vol. 21, pp. 665, 1983. and characterization of As2S3 thin films deposited by CBD”, Ind. J. Pure Appl. Phys., vol. 33, pp. 687, 1995. [10] E. Hajto, P. J. S. Ewen, R. Belford, J. Hajto and A. E. Owen, “Optical properties of spin-coated amorphous chalcogenide thin films”, J. Non-Cryst. Solids, vol. 97-98, pp. 1191-1194, 1987. [11] C. D. Lokhande, “Solution growth of As2S3 and Sb2S3 thin films”, Ind. J. Pure Appl. Phys., vol. 29, pp. 300-302, 1991. [12] J. D. Desai and C. D. Lokhande, “Preparation and characterization of As2S3 thin films”, Ind. J. Pure Appl. Phys., vol. 33, pp. 247-247, 1995. [14] R.S.M ane, C.D. Lokhande and M . D. Uplane, “Chemical deposition and characterization of silver films”, Ind. J. Pure Appl. Phys., vol. 33, pp. 693-699, 1995 [15] A. U. Ubale, “Effect of complexing agent on growth process and properties of nanostructured Bi2S3 thin films deposited by chemical bath deposition method”, M ater. Chem. Phys., vol. 12, pp. 555–560, 2010.

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