Cu Thin Films Deposited by Spray Pyrolysis

Sensors & Transducers Volume 134 Issue 11 November 2011

www.sensorsportal.com

ISSN 1726-5479

Editors-in-Chief: professor Sergey Y. Yurish, tel.: +34 696067716, e-mail: [email protected] Editors for Western Europe Meijer, Gerard C.M., Delft University of Technology, The Netherlands Ferrari, Vittorio, Universitá di Brescia, Italy Editor for Eastern Europe Sachenko, Anatoly, Ternopil State Economic University, Ukraine

Editor South America Costa-Felix, Rodrigo, Inmetro, Brazil Editor for Africa Maki K.Habib, American University in Cairo, Egypt Editor for Asia Ohyama, Shinji, Tokyo Institute of Technology, Japan

Editors for North America Datskos, Panos G., Oak Ridge National Laboratory, USA Fabien, J. Josse, Marquette University, USA Katz, Evgeny, Clarkson University, USA

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Kotulska, Malgorzata, Wroclaw University of Technology, Poland Kockar, Hakan, Balikesir University, Turkey Kong, Ing, RMIT University, Australia Kratz, Henrik, Uppsala University, Sweden Krishnamoorthy, Ganesh, University of Texas at Austin, USA Kumar, Arun, University of South Florida, USA Kumar, Subodh, National Physical Laboratory, India Kung, Chih-Hsien, Chang-Jung Christian University, Taiwan Lacnjevac, Caslav, University of Belgrade, Serbia Lay-Ekuakille, Aime, University of Lecce, Italy Lee, Jang Myung, Pusan National University, Korea South Lee, Jun Su, Amkor Technology, Inc. South Korea Lei, Hua, National Starch and Chemical Company, USA Li, Fengyuan (Thomas), Purdue University, USA Li, Genxi, Nanjing University, China Li, Hui, Shanghai Jiaotong University, China Li, Xian-Fang, Central South University, China Li, Yuefa, Wayne State University, USA Liang, Yuanchang, University of Washington, USA Liawruangrath, Saisunee, Chiang Mai University, Thailand Liew, Kim Meow, City University of Hong Kong, Hong Kong Lin, Hermann, National Kaohsiung University, Taiwan Lin, Paul, Cleveland State University, USA Linderholm, Pontus, EPFL - Microsystems Laboratory, Switzerland Liu, Aihua, University of Oklahoma, USA Liu Changgeng, Louisiana State University, USA Liu, Cheng-Hsien, National Tsing Hua University, Taiwan Liu, Songqin, Southeast University, China Lodeiro, Carlos, University of Vigo, Spain Lorenzo, Maria Encarnacio, Universidad Autonoma de Madrid, Spain Lukaszewicz, Jerzy Pawel, Nicholas Copernicus University, Poland Ma, Zhanfang, Northeast Normal University, China Majstorovic, Vidosav, University of Belgrade, Serbia Malyshev, V.V., National Research Centre ‘Kurchatov Institute’, Russia Marquez, Alfredo, Centro de Investigacion en Materiales Avanzados, Mexico Matay, Ladislav, Slovak Academy of Sciences, Slovakia Mathur, Prafull, National Physical Laboratory, India Maurya, D.K., Institute of Materials Research and Engineering, Singapore Mekid, Samir, University of Manchester, UK Melnyk, Ivan, Photon Control Inc., Canada Mendes, Paulo, University of Minho, Portugal Mennell, Julie, Northumbria University, UK Mi, Bin, Boston Scientific Corporation, USA Minas, Graca, University of Minho, Portugal Moghavvemi, Mahmoud, University of Malaya, Malaysia Mohammadi, Mohammad-Reza, University of Cambridge, UK Molina Flores, Esteban, Benemérita Universidad Autónoma de Puebla, Mexico Moradi, Majid, University of Kerman, Iran Morello, Rosario, University "Mediterranea" of Reggio Calabria, Italy Mounir, Ben Ali, University of Sousse, Tunisia Mrad, Nezih, Defence R&D, Canada Mulla, Imtiaz Sirajuddin, National Chemical Laboratory, Pune, India Nabok, Aleksey, Sheffield Hallam University, UK Neelamegam, Periasamy, Sastra Deemed University, India Neshkova, Milka, Bulgarian Academy of Sciences, Bulgaria Oberhammer, Joachim, Royal Institute of Technology, Sweden Ould Lahoucine, Cherif, University of Guelma, Algeria Pamidighanta, Sayanu, Bharat Electronics Limited (BEL), India Pan, Jisheng, Institute of Materials Research & Engineering, Singapore Park, Joon-Shik, Korea Electronics Technology Institute, Korea South Penza, Michele, ENEA C.R., Italy Pereira, Jose Miguel, Instituto Politecnico de Setebal, Portugal Petsev, Dimiter, University of New Mexico, USA Pogacnik, Lea, University of Ljubljana, Slovenia Post, Michael, National Research Council, Canada Prance, Robert, University of Sussex, UK Prasad, Ambika, Gulbarga University, India Prateepasen, Asa, Kingmoungut's University of Technology, Thailand Pullini, Daniele, Centro Ricerche FIAT, Italy Pumera, Martin, National Institute for Materials Science, Japan Radhakrishnan, S. National Chemical Laboratory, Pune, India Rajanna, K., Indian Institute of Science, India Ramadan, Qasem, Institute of Microelectronics, Singapore Rao, Basuthkar, Tata Inst. of Fundamental Research, India Raoof, Kosai, Joseph Fourier University of Grenoble, France Rastogi Shiva, K. University of Idaho, USA Reig, Candid, University of Valencia, Spain Restivo, Maria Teresa, University of Porto, Portugal Robert, Michel, University Henri Poincare, France Rezazadeh, Ghader, Urmia University, Iran Royo, Santiago, Universitat Politecnica de Catalunya, Spain Rodriguez, Angel, Universidad Politecnica de Cataluna, Spain Rothberg, Steve, Loughborough University, UK Sadana, Ajit, University of Mississippi, USA Sadeghian Marnani, Hamed, TU Delft, The Netherlands Sapozhnikova, Ksenia, D.I.Mendeleyev Institute for Metrology, Russia

Saxena, Vibha, Bhbha Atomic Research Centre, Mumbai, India Schneider, John K., Ultra-Scan Corporation, USA Sengupta, Deepak, Advance Bio-Photonics, India Seif, Selemani, Alabama A & M University, USA Seifter, Achim, Los Alamos National Laboratory, USA Shah, Kriyang, La Trobe University, Australia Sankarraj, Anand, Detector Electronics Corp., USA Silva Girao, Pedro, Technical University of Lisbon, Portugal Singh, V. R., National Physical Laboratory, India Slomovitz, Daniel, UTE, Uruguay Smith, Martin, Open University, UK Soleymanpour, Ahmad, Damghan Basic Science University, Iran Somani, Prakash R., Centre for Materials for Electronics Technol., India Srinivas, Talabattula, Indian Institute of Science, Bangalore, India Srivastava, Arvind K., NanoSonix Inc., USA Stefan-van Staden, Raluca-Ioana, University of Pretoria, South Africa Stefanescu, Dan Mihai, Romanian Measurement Society, Romania Sumriddetchka, Sarun, National Electronics and Computer Technology Center, Thailand Sun, Chengliang, Polytechnic University, Hong-Kong Sun, Dongming, Jilin University, China Sun, Junhua, Beijing University of Aeronautics and Astronautics, China Sun, Zhiqiang, Central South University, China Suri, C. Raman, Institute of Microbial Technology, India Sysoev, Victor, Saratov State Technical University, Russia Szewczyk, Roman, Industrial Research Inst. for Automation and Measurement, Poland Tan, Ooi Kiang, Nanyang Technological University, Singapore, Tang, Dianping, Southwest University, China Tang, Jaw-Luen, National Chung Cheng University, Taiwan Teker, Kasif, Frostburg State University, USA Thirunavukkarasu, I., Manipal University Karnataka, India Thumbavanam Pad, Kartik, Carnegie Mellon University, USA Tian, Gui Yun, University of Newcastle, UK Tsiantos, Vassilios, Technological Educational Institute of Kaval, Greece Tsigara, Anna, National Hellenic Research Foundation, Greece Twomey, Karen, University College Cork, Ireland Valente, Antonio, University, Vila Real, - U.T.A.D., Portugal Vanga, Raghav Rao, Summit Technology Services, Inc., USA Vaseashta, Ashok, Marshall University, USA Vazquez, Carmen, Carlos III University in Madrid, Spain Vieira, Manuela, Instituto Superior de Engenharia de Lisboa, Portugal Vigna, Benedetto, STMicroelectronics, Italy Vrba, Radimir, Brno University of Technology, Czech Republic Wandelt, Barbara, Technical University of Lodz, Poland Wang, Jiangping, Xi'an Shiyou University, China Wang, Kedong, Beihang University, China Wang, Liang, Pacific Northwest National Laboratory, USA Wang, Mi, University of Leeds, UK Wang, Shinn-Fwu, Ching Yun University, Taiwan Wang, Wei-Chih, University of Washington, USA Wang, Wensheng, University of Pennsylvania, USA Watson, Steven, Center for NanoSpace Technologies Inc., USA Weiping, Yan, Dalian University of Technology, China Wells, Stephen, Southern Company Services, USA Wolkenberg, Andrzej, Institute of Electron Technology, Poland Woods, R. Clive, Louisiana State University, USA Wu, DerHo, National Pingtung Univ. of Science and Technology, Taiwan Wu, Zhaoyang, Hunan University, China Xiu Tao, Ge, Chuzhou University, China Xu, Lisheng, The Chinese University of Hong Kong, Hong Kong Xu, Sen, Drexel University, USA Xu, Tao, University of California, Irvine, USA Yang, Dongfang, National Research Council, Canada Yang, Shuang-Hua, Loughborough University, UK Yang, Wuqiang, The University of Manchester, UK Yang, Xiaoling, University of Georgia, Athens, GA, USA Yaping Dan, Harvard University, USA Ymeti, Aurel, University of Twente, Netherland Yong Zhao, Northeastern University, China Yu, Haihu, Wuhan University of Technology, China Yuan, Yong, Massey University, New Zealand Yufera Garcia, Alberto, Seville University, Spain Zakaria, Zulkarnay, University Malaysia Perlis, Malaysia Zagnoni, Michele, University of Southampton, UK Zamani, Cyrus, Universitat de Barcelona, Spain Zeni, Luigi, Second University of Naples, Italy Zhang, Minglong, Shanghai University, China Zhang, Qintao, University of California at Berkeley, USA Zhang, Weiping, Shanghai Jiao Tong University, China Zhang, Wenming, Shanghai Jiao Tong University, China Zhang, Xueji, World Precision Instruments, Inc., USA Zhong, Haoxiang, Henan Normal University, China Zhu, Qing, Fujifilm Dimatix, Inc., USA Zorzano, Luis, Universidad de La Rioja, Spain Zourob, Mohammed, University of Cambridge, UK

Sensors & Transducers Journal (ISSN 1726-5479) is a peer review international journal published monthly online by International Frequency Sensor Association (IFSA). Available in electronic and on CD. Copyright © 2011 by International Frequency Sensor Association. All rights reserved.

Sensors & Transducers Journal

Contents Volume 134 Issue 11 November 2011

www.sensorsportal.com

ISSN 1726-5479

Research Articles Nanomaterials and Chemical Sensors Sukumar Basu and Palash Kumar Basu ............................................................................................

1

Fabrication of Phenyl-Hydrazine Chemical Sensor Based on Al- doped ZnO Nanoparticles Mohammed M. Rahman, Sher Bahadar Khan, A. Jamal, M. Faisal, Abdullah M. Asiri .....................

32

Nanostructured Ferrite Based Electronic nose Sensitive to Ammonia at room temperature U. B. Gawas, V. M. S. Verenkar, D. R. Patil.......................................................................................

45

A Humidity Sensor Based on Nb-doped Nanoporous TiO2 Thin Film Mansoor Anbia, S. E. Moosavi Fard...................................................................................................

56

A Resistive Humidity Sensor Based on Nanostructured WO3-ZnO Composites Karunesh Tiwari, Anupam Tripathi, N. K. Pandey ..............................................................................

65

Highly Sensitive Cadmium Concentration Sensor Using Long Period Grating A. S. Lalasangi, J. F. Akki, K. G. Manohar, T. Srinivas, Prasad Raikar, Sanjay Kher and U. S. Raikar .................................................................................................................................

76

MEMS Based Ethanol Sensor Using ZnO Nanoblocks, Nanocombs and Nanoflakes as Sensing Layer H. J. Pandya, Sudhir Chandra and A. L. Vyas ...................................................................................

85

Simple Synthesis of ZnCo2O4 Nanoparticles as Gas-sensing Materials S. V. Bangale,S. M. Khetre D. R. Patil and S. R. Bamane.................................................................

95

Nanostructured Spinel ZnFe2O4 for the Detection of Chlorine Gas S. V. Bangale, D. R. Patil and S. R. Bamane.....................................................................................

107

Fabrication of Polyaniline-ZnO Nanocomposite Gas Sensor S. L. Patil, M. A. Chougule, S. G. Pawar, Shashwati Sen, A. V. Moholkar, J. H. Kim and V. B. Patil

120

Synthesis and Characterization of Nano-Crystalline Cu and Pb0.5-Cu0.5- ferrites by Mechanochemical Method and Their Electrical and Gas Sensing Properties V. B. Gaikwad ,S. S. Gaikwad, A. V. Borhade and R. D. Nikam........................................................

132

Surface Modification of MWCNTs: Preparation, Characterization and Electrical Percolation Studies of MWCNTs/PVP Composite Films for Realization of Ammonia Gas Sensor Operable at Room Temperature Sakshi Sharma, K. Sengupta, S. S. Islam..........................................................................................

143

An Investigation of Structural and Electrical Properties of Nano Crystalline SnO2: Cu Thin Films Deposited by Spray Pyrolysis J. Podder and S. S. Roy .....................................................................................................................

155

Nanocrystalline Cobalt-doped SnO2 Thin Film: A Sensitive Cigarette Smoke Sensor Patil Shriram B., More Mahendra A., Patil Arun V..............................................................................

163

Effect of Annealing on the Structural and Optical Properties of Nano Fiber ZnO Films Deposited by Spray Pyrolysis M. R. Islam, J. Podder, S. F. U. Farhad and D. K. Saha....................................................................

170

Amperometric Acetylcholinesterase Biosensor Based on Multilayer Multiwall Carbon Nanotubes-chitosan Composite Xia Sun, Chen Zhai, Xiangyou Wang.................................................................................................

177

Fabrication of Biosensors Based on Nanostructured Conducting Polyaniline (NSPANI) Deepshikha Saini, Ruchika Chauhan and Tinku Basu.......................................................................

187

Authors are encouraged to submit article in MS Word (doc) and Acrobat (pdf) formats by e-mail: [email protected] Please visit journal’s webpage with preparation instructions: http://www.sensorsportal.com/HTML/DIGEST/Submition.htm International Frequency Sensor Association (IFSA).

Sensors & Transducers Journal, Vol. 134, Issue 11, November 2011, pp. 155-162

Sensors & Transducers ISSN 1726-5479 © 2011 by IFSA http://www.sensorsportal.com

An Investigation of Structural and Electrical Properties of Nano Crystalline SnO2: Cu Thin Films Deposited by Spray Pyrolysis 1 1

J. Podder and 2 S. S. Roy

Department of Physics, Bangladesh University of Engineering and Technology, Dhaka-1000, Bangladesh Tel: +880-2-9665613 2 Department of Physics, Dwarika Paul Mohila Degree College, Sreemongal-3210, Bangladesh E-mail: [email protected]

Received: 5 November 2011 /Accepted: 21 November 2011 /Published: 29 November 2011 Abstract: Pure tin oxide (SnO2) and Cu doped SnO2 thin films have been deposited onto glass substrates by a simple spray pyrolysis technique under atmospheric pressure at temperature 350 C. The doping concentration of Cu was varied from 1 to 8 wt. % while all other deposition parameters such as spray rate, carrier air gas pressure, deposition time, and distance between spray nozzle to substrate were kept constant. Surface morphology of the as-deposited thin films has been studied by Scanning Electron Microscopy (SEM). The SEM micrograph of the films shows uniform deposition. The structural properties of the as-deposited and annealed thin films have been studied by XRD and the electrical characterization was performed by Van-der Pauw method. The as-deposited films are found polycrystalline in nature with tetragonal crystal structure. Average grain sizes of pure and Cu doped SnO2 thin film have been obtained in the range of 7.2445 Å to 6.0699 Å, which indicates the nanometric size of SnO2 grains developed in the film. The resistivity of SnO2 films was found to decrease initially from 4.5095×10−4 Ωm to 1.1395× 10−4 Ωm for concentration of Cu up to 4 % but it was increased further with increasing of Cu concentrations. The experimental results depict the suitability of this material for using as transparent and conducting window materials in solar cells and gas sensors. Copyright © 2011 IFSA. Keywords: Spray pyrolysis, SnO2, Gas sensor materials, XRD, Nanocrystalline.

155


1. Introduction In recent years transparent conducting tin oxide (SnO2) thin films have given much interest due to its variety of applications such as window layers, heat reflectors in solar cells, flat panel display, electrochromic devices, LEDS, liquid crystal displays, invisible security circuits, various gas sensors etc. SnO2 is an IV-VI compound and n-type semiconductor with a direct optical band gap of about 3.87– 4.3 eV [1]. SnO2 has some advantages over other oxide materials such as ZnO, In2O3, Cd2SnO4 or CdO due to its unique attractive properties like high optical transmittance, uniformity, non-toxicity, good electrical, low resistivity and piezoelectric behavior and its low cost [2-4]. The structure of SnO2 material in its bulk form is tetragonal rutile with lattice parameters a = b = 4.737 Å and c = 3.816 Å [5-6]. There are various methods used for the preparation of pure or doped thin films [7-13]. Among them spray pyrolysis deposition (SPD) technique provides a simple route of synthesizing thin films because of its simplicity, low cost experimental set up from an economical point of view. In addition, this technique could be used for the production of large-area thin film deposition without any high vacuum system. Generally, SnCl4 is used as precursor for Sn, and very few work had been reported on the preparation of this material using SnCl2 instead of SnCl4 [14-15]. SnCl2 is being cheaper than SnCl4 and to be a cost effective precursor for preparing low-cost SnO2 thin films. It is expected that various concentration of Cu in SnO2 may affect the structural, optical and electrical properties of the films. From bang gap engineering point of view, suitable band gap is essential for the fabrication of optical devices. So far our knowledge is concerned; there are very few reports available on the deposition of Cu doped SnO2 thin films by vapour deposition and spray pyrolysis method [16-17]. In considering the importance of these materials in the field of optoelectronic devices, particularly for solar cells and gas sensors, we have synthesized both undoped and Cu doped SnO2 films from SnCl2 and cupric nitrate precursor using a simple and locally fabricated spray pyrolysis system relatively at low temperature 350 °C and report the results of structural and electrical properties of as-deposited pure and copper doped SnO2 thin films.

2. Experimental Details Pure and Cu-doped SnO2 films were deposited onto glass substrates by a spray pyrolysis deposition (SPD) technique. The deposition set up includes the precursor solution, carrier air gas assembly connected to a spray nozzle and the schematic diagram of the spray pyrolysis system is described elsewhere [4]. AR grade SnCl2·2H2O and cupric nitrate [Cu(NO3)2.3H2O] were used as precursors for the source of Sn and Cu. Spray solutions was prepared by mixing 0.2M aqueous solution of SnCl2.2H2O in 50 ml of CH3CH2OH and 50 ml of H2O for pure SnO2 films. The pH of the solution was measured to be 6.5. Ethanol was used as the reducing agent in the present work. The amount of Cu (NO3)2.3H2O to be added depends on the desired doping concentration. The doping concentration of Cu was varied from 1 to 8 wt. %. To enhance the solubility of tin chloride salt, a few drops of HCl were added. Pressure of the carrier air gas was kept at 1 bar. The normalized distance between the spray nozzle and substrate was fixed at 30 cm. The spray rate of solution to the hot substrate was maintained at 5 mL/min throughout the experiment and the spray time was kept constant by 5 minutes. After deposition, films were allowed to cool at an ambient temperature slowly. The deposited films were found well adhered to the substrate and stable at room temperature. The as-deposited pure SnO2 films were observed light grayish-white in color and dark grayish and blackening for Cu doped films. The possible chemical reaction that takes place on the heated surface to produce SnO2 and SnO2: Cu thin film may be as follows: when the droplets of the aqueous solution containing 0.2M of SnCl2.2H2O reached the heated substrate, chemical reaction of tin chloride with water vapor stimulated by the temperature takes place providing the formation of SnO2 films and also SnO2 : Cu with the formation of oxides of nitrogen, steam through some intermediate products as follows: 156


SnCl2.2H2O → SnO2  + 2HCl  + steam  SnCl2.2H2O + Cu (NO3)2.3H2O → SnO2: Cu  + xNyOz + wO2 + HCl + Steam  Characterizations: The surface properties of the films were examined by using HITACHI S-3400N model Scanning Electron Microscope (SEM). A Philips PW3040 X’Pert PRO X-ray diffractometer was used to determine the lattice parameters. The monochromatic (using Ni filter) CuKα radiation was used whose primary beam power was 40 kV and 30 mA. All the data of the samples were analyzed by using computer software “X’PERT HIGHSCORE” from which structural parameters was determined.

3. Results and Discussion 3.1. Surface Morphology Fig.1 (a and b) shows the surface morphology of as-deposited pure and Cu doped SnO2 thin film having thickness of about 200 nm under 10.00 K magnifications. The SEM micrograph shows small crystallites and agglomeration of the grain particles for pure SnO2 samples but 4 wt. % Cu doped SnO2 films exhibit dense packed and homogeneous growth in the entire surface.

(b)

(a)

(a)

(b)

Fig. 1. SEM images (a) for pure, and (b) for 4 % Cu doped SnO2 thin film.

3.2 Structural Analysis The X-ray diffractogram of as-deposited and annealed SnO2 samples with different concentrations of Cu are shown in Fig. 2 (a to h). Using the dhkl (interplanar distance) values, their corresponding values have been identified as (110), (101), (200), (210), (211), and (220) for pure SnO2 films which indicate the tetragonal structure of SnO2. The calculated lattice parameters, a and c, and grain size of these as deposited and annealed samples are tabulated in Table 1. Grain size of the prepared SnO2 thin film was determined from the stronger peaks of (110) from each XRD patterns using Scherrer formula, Dg 

K ,  cos 

where Dg is the average grain size, λ is the wavelength of CuKα (λ=1.5417 Å), θ is the angle of incidence in degree (Braggs’ angle) and δω is the full width at half maximum (FWHM) of the peak in 157


radian, which was determined experimentally after correction of instrumental broadening (in the present case it is 0.05°). The value of lattice parameter ‘a’ was estimated 4.7522 Å for pure SnO2 and it was shifted to 4.7482 Å after 2 wt. % Cu doping. Similarly, c was found 3.1804 Å for pure SnO2 films and it was shifted to 3.1927 Å for 2 wt. % Cu doping. Both values of a and c changes with increasing of Cu doping. This shifting in the lattice parameters is mainly due to occupying of Cu atom into Sn interstitial sites. Average grain sizes of pure and Cu doped SnO2 thin film have been obtained in the range of 7.2445 Å to 6.0699 Å, which indicates the nanometric size of SnO2 grains developed in the film. The grain size is reduced after annealing the samples. The intensity of the reflections planes are also becoming sharp for annealed sample in compared to the as deposited raw samples. XRD patterns of SnO2 films was found to have a better polycrystalline nature in the annealed samples oriented along the (110), (101), and (211) planes at 2θ = 26.52º, 33.88º, 51.67º, respectively with single phase SnO2. This is in good agreement with the reported values [18-19]. The (110) surface of SnO2 is energetically the most stable and the predominant face found in polycrystalline samples. Hence from the X-ray diffraction patterns, it is clear that the Cu+2 ions are acting as dopants in the SnO2 structure. Table 1. X-ray diffraction data of pure and Cu doped SnO2 thin films. As Deposited Sample

Pure SnO2

2 % Cu doped SnO2

4 % Cu doped SnO2

5 % Cu doped SnO2

I/I0 [%]

(hkl)

100 29.76 8.86 22.72 5.77 100 22.83 17.39 21.54 4.83 100 20.13 30.05 19.08 7.34 100 30.09 21.53 11.57 26.67

(110) (101) (200) (211) (220) (110) (101) (200) (211) (220) (110) (101) (200) (211) (220) (110) (101) (200) (211) (220)

Lattice Parameters in Å (as deposited )

Grain Size in Å

Lattice Parameters in Å (annealed film)

Grain Size in Å

a = b=4.7522 c = 3.1804

7.2445

a=b= 4.7464 c = 3.1860

7.0411

a = b=4.7482 c = 3.1927

6.5852

a=b= 4.7466 c = 3.1894

6.2435

a=b= 4.7518 c = 3.1874

7.2440

a=b= 4.7430 c = 3.1968

7.0510

a=b= 4.7578 c = 3.1972

6.0699

a=b= 4.7586

6.0310

3.3. Electrical Properties D.C electrical resistivity measurements were made in air for freshly deposited films from room temperature 305 K to 475 K by Van der Pauw (four prove) method. At room temperature, the electrical resistivity and sheet resistance of pure SnO2 films was found to be 5.1065 ×10− 4Ω-m, and 2.5532 ×103 Ω/□ respectively (Table 2). Conductivity of pure films was varied from 1.9582 ×103 (ohm-m)-1 to 2.4065 ×103 (ohm-m)-1 in the range of 305- 475 K which indicates that these films are semiconductor in nature, shown in Fig. 3.

158


(a)

(b)

(c)

(e)

(f)

Annealed at 400 º C


(g) Annealed at 400 º C

(d)

(h)


Fig. 2. XRD spectra for pure and Cu doped SnO2 thin film, (a-d) and annealed samples (e-h). Table 2. Data for resistivity, conductivity and sheet resistance of (1 to 8 wt. %) Cu doped SnO2.

159


Doping of Copper (Cu) in SnO2 0% 1% 2% 3% 4% 5% 8%

Resistivity, ρ× 10-4 in ohm-m 5.10 4.50 2.32 1.81 1.13 3.44 3.88

Sheet Resistance, Rs× 103 in ohm/□ 2.55 2.25 1.16 0.91 0.57 1.72 1.94

Conductivity, σ×103 in (ohm-m)-1 1.96 2.22 4.30 5.52 8.78 2.90 2.57

Pure 1% Cu 2% Cu 3% Cu 4% Cu 5% Cu 8% Cu

5 4 3 2 1 0 300

350

400 Temperature in K

450

500

Fig. 3. Variation of resistivity with respect to temperature for pure and Cu (1%-8%) doped SnO2 thin films.

σ×103 in (mho-m)-1

R esistivity,  × 10 -4 in ohm -m

6

10

ρ×10-4 in ohm-m,

On Cu (1 – 4 wt. %) doping the resistivity becomes 4.50×10− 4 Ω-m to 1.13×10− 4 Ω-m. The resistivity and sheet resistance of Cu doped (till 4 wt. %) samples have higher than undoped films. Sn forms an interstitial bond with oxygen and exists either as SnO or SnO2; thus it has a valency of +2 or +4, respectively. This valence state has a direct demeanor on the ultimate conductivity of tin oxide. The incorporation of Cu in SnO2 films increases carrier concentration might be due to the substitution of Sn+4 with dopants at higher oxidation state. This can be attributed as the reason for decreasing resistivity with increasing carrier concentrations of Cu doping. The increase in the value of resistivity and sheet resistance (Rsh) beyond a certain doping concentration of Cu probably represents a solubility limit of Cu in the SnO2 lattice. The excess of Cu atoms do not occupy to the popper lattice positions to contribute to the free carrier concentration while, at the same time, enhance the disorder of the structure leading to an increase in sheet resistance. This was observed in our films above 4 wt. % of Cu shown in Fig. 4. It is seen that the activation energy for the pure and Cu doped samples are fairly low and it is in the range of 0.022 to 0.058 eV, shown in Fig. 5. The low value of activation energy may be associated with the localized levels hopping due to the excitation of carriers from donor band to the conduction band.

4

8

Resistivity Conductivity

6

2 0 0%

2%

4% 6% Doping of Copper (Cu)

8%

10%

Fig. 4.Variation of electrical resistivity and conductivity for Cu-doped SnO2 thin films.

160


Activation Energy in eV

0.08 0.06 0.04 0.02 0 0%

2%

4%

6%

8%

10%

Doping of Copper (Cu)

Fig. 5. Activation energy vs. doping concentration of Cu.

4. Conclusions The structural and electrical properties of pure and Cu-doped SnO2 films are discussed in the present work. Our results showed that Cu doping (1-8 wt. %) into the films promote film structure and resistivity of the films to a certain limit. SEM studies of Cu doped SnO2 films exhibit a smooth and homogeneous growth in the entire surface. X-ray diffraction studies show that spray pyrolysed thin films are polycrystalline in nature and average grain size of the film is about 7.2446 Å. The shifting in the lattice parameter is mainly due to the presence of dopants into the interstitial sites of the lattice. The electrical resistivity of the Cu doped film is lower than that of the pure sample and directly depends on the temperature and Cu doping. At room temperature electrical resistivity and sheet resistance of pure SnO2 films is found to be 5.10 ×10−4 Ω-m, and 2.55 ×103 Ω/□ respectively. The minimum resistivity is found to be 1.13×10−4 Ω-m for 4% Cu doped SnO2 films. This reduction in resistivity in Cu doped samples is due to the increase of carrier concentrations of Cu ions. The electrical resistivity increases further with increase of Cu ˃ 4% doping. This is resulted due to the scattering of excess conduction electrons. The best optically transparent and relatively good conductivity of SnO2 film is found to be at 4 wt % doped with Cu. The obtained experimental results infer the suitability of this material as transparent and conducting window materials in thin film solar cells and gas sensor devices.

Acknowledgement The authors are thankful to Dr. D. K. Saha, Chief Scientific Officer, Atomic Energy Center, Dhaka, Bangladesh for his kind help in taking X-ray diffractograms.

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