OPTICAL PROPERTIES OF ZnO THIN FILMS DEPOSITION BY SPRAY PYROLYSIS
R. Zellagui, W. Bedjaoui, H. Dehdouh, N.Ouafek, A. Bougheloute, S. Boudour
Development Unit and Thin Film Applications/ research center in industrial technologies CRTI/P.O.Box 64, Cheraga 16014 Algiers, Algeria
Abstract: The aim of our work presented in this paper is the development and characterization of zinc oxide thin film (ZnO), using spray pyrolysis process (simple and cheap method). Tow conditions of deposition are used: temperature of sample Ts = 350
°C, the concentration of solution [Zn+2] = 0.01 and 0.4 mol.l-1. The films prepared are characterized by UV-visible to study the optical properties and the X-ray diffraction (XRD) for the structural one. The films obtained are composed of Nano-crystallites with average diameter of 18 nm. ZnO films have a transmittance over 80 % in visible range and band gap about Eg = 3.6 eV.
Keywords: thin films, ZnO, characterizations, Spray Pyrolysis.
1. INTRODUCTION
The metal oxides are materials which have a dual property, on the one hand, high electrical conductivity and second hand, low absorbance in the visible, for this reason they are called transparent conductive oxides (TCO). The film of metal oxide semiconductors have been extensively studied and have attracted much interest in the last years because of their optical and electrical properties. ZnO is a very attractive material for many applications in the microelectronic and optoelectronic devices. It can be used as thin films of transparent conductive oxide (TCO), solar cells, light emitting diodes LEDs, acoustic wave transducer, liquid crystal displays and heat mirrors [1-4].
ZnO has become one of the most promising materials because of its properties: high chemical and mechanical stability, its abundance in nature (low cost material relative to transparent conducting oxides as currently used (ITO and SnO2)) [5].
ZnO has a direct band gap with Eg = 3-4 eV, having a hexagonal wurtzite structure [6]. Lot of deposition techniques are developed such as: the sputtering [7] RF Sputtering [8] Sputtering DC [9] and spray pyrolysis [10]. This last one is preferable because it is easy to control and adapt the film properties for the application of solar cells.
2. Experimental
The spray pyrolysis is a chemical process, which consists on a solution sprayed on a glass substrate. The spray solution is prepared with 0.01 and 0.4 M of zinc acetate dehydrated (Zn (CH3COOH)2·2H2O) (Sigma-Aldrich, 99.5%) dissolved in ethanol and add distilled water as a stabilizer. The solution was stirring during 2 h. It should be noted that special attention was taken for cleaning the glass substrate.
Glass plates were immersed in successive baths of ultrasonic different solutions of ethanol, acetone for 20 to 30 minutes and finally immersed in distilled water. The deposition of the film by a spray pyrolysis at temperature T = 350 °C, the thermal treatment is effected in a furnace at temperature 500 °C for 1 hours.
The crystal structure of the samples was characterized by an X-ray diffractometer Emprean Panalytical with a Cu-Kα source (λ = 1.54056 Å). The performance of the transmittance of the sample of visible light was measured using a visible-UV spectrophotometer light Optizen 3220.
3. Results and Discussion 3.1. Optical Properties
Transmission by UV-VIS spectrophotometer shows that the transmittance of these samples is about 80%.
The optical gap of ZnO films was 3.62 eV (0.4 M) and 3.43 eV (0.01M).
300 400 500 600 700 800 0
20 40 60 80 100
T(%)
Wavelength nm.
echantillon1 echantillon 2
400 600 800
0 10 20 30 40 50 60 70 80 90
T(%)
(nm) Fig.1. Transmission spectra of ZnO thin films (a) 0.01 M, (b) 0.4 M).
The measurement of the thickness (h) by the method of the interference fringes, [11] and from the Figure.1 transmittance curve, we derive the physical constants (T, λ, n, s): T is the transmission coefficient, α is the absorption coefficient of the film, λ is the wavelength of the incident light, n and s are the refractive indices of the film and substrate respectively. These are used to calculate the thickness and refractive index. To calculate the thickness of the film we use the following relationship:
(1) Where n1 and n2 are the refractive indices of the films measured at two different wavelengths λ1 and λ2, the thickness of the film thus obtained is about 300 nm, which estimated from data transmission using the method of the envelope of Swanepoel [11]. The relationship between the photon energy and the optical absorption coefficient (α) for the direct transition is expressed as:
(2) A is a constant, the optical gap Eg is [eV], and hυ is the energy of a photon. By scanning the entire energy range, are plotted (αhυ) 2 as a function of photon energy E = hυ (where: hυ (eV) = hc / λ = 12400 / λ (A
°)).
1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5
0 5000 10000 15000 20000
3.62 eV
(h)2 (cm-1.eV)
h (eV)
1,5 2,0 2,5 3,0 3,5 4,0 4,5
0,00E+000 1,00E+014 2,00E+014
3.48 eV
h)2(cm-1.eV)
h(eV)
Fig.2.Plots of (αhυ)2 as function of the photon energy of ZnO film (a) 0.4 M, (b) 0.01 M.
3.2. Structural Properties
The structural properties of ZnO films can be determined from XRD spectrum (figure 3). The peaks:
(100), (101), (110) and (112) correspond to the hexagonal (wurtzite) ZnO (ASTM JCPDS file No. 890 511) with lattice parameters: a = 3.25 Å and c = 5.21 Å. The peak (100) is the most intense. The average
(a)
(b)
(a) (b)
size of the crystallites of Nano-ZnO estimated according to the formula of Debye-Scherrer [12] is 18.1 nm.
(3)
20 30 40 50 60 70 80
0 100 200 300 400 500
(201) (110) (101)
(100)
intensité (u.a)
Fig.3 X-ray diffraction patterns of the ZnO films. 4. Conclusion
In the present study, ZnO thin films were prepared by the spray method. According to the XRD pattern the polycrystalline ZnO layer is presents the hexagonal structure. The crystallite size is of the order of 18.1nm, the average transmittance of prepared layers is 80% in the visible region. The present results show that the layer deposited by spray pyrolysis is nano-cryslline high transparency.
5. References
[1] B.J. Lokhande, M.D. Uplane, Structural, optical and electrical studies on spray deposited highly oriented ZnO films, Applied Surface Science 167 (2000) 243–246.
[2] Y. Zhang, H. Jia, P. Li, F. Yang, Z. Zheng, Influence of glucose on the structural and optical properties of ZnO thin films prepared by sol gel method, Optics Communication 284 (2011) 236–239.
[3] K.L. Chopra, S. Major, D.K. Panday, Transparent conductors – a status review, Thin Solid Films 102 (1983) 1–
46.
[4] E. Fortunato, P. Barquinha, A. Pimentel, L. Pereira, A. Goncalves, A. Marques, R. Martins, Wide-bandgap high- mobility ZnO thin-film transistors produced at room temperature, Applied Physics Letters 85 (2004) 2541–2543.
[5] Y.S. Kim, Y.J. Lee, S.B. Heo, H.M. Lee, J.H. Kim, S.K. Kim, J.H. Chae, J.I. Choi, D. Kim, Fabrication and characterization of Ag intermediate transparent and conducting TiON/Ag/TiON multilayer films, Optics Communication 284 (2011) 2303–2306.
[6] S.H. Mousavi, H. Haratizadeh, H. Minaee, The effect of morphology and doping on photoluminescence of ZnO nanostructures, Optics Communication 284 (2011) 3558–3561.
[7] C.C. Kuo, C.C. Liu, S.C. He, J.T. Chang, J. Liang He, The influences of thickness on the optical and electrical properties of dual ion beam sputtering deposited molybdenum doped zinc oxide layer.
<Journals/jnm/2011/140697>.
[8] X. Xiu, Z.Y. Pang, M. Lv, Y. Dai, L. Ye, S. Han, Transparent conducting molybdenum-doped zinc oxide films deposited by RF magnetron sputtering, Applied Surface Science 253 (2007) 3345–3348.
[9] J.L. Shi, H. Ma, G.H. Ma, H.G. Ma, J. Shen, Structure and ultrafast carrier dynamics in n-type transparent Mo:
ZnO nanocrystalline thin films, Applied Physics A 92 (2008) 357–360.
[10] J.D. Merchant, M. Cocivera, Preparation and doping of zinc oxide using spray pyrolysis, Chemistry of Materials 7 (1995) 1742–1749
[11] R. Swanepoel, Determination of the thickness and optical constants of amorphous silicon, J. Phys. E: Sci.
Instrum. 16 (1983) 1214–1222.
[12] H. Dehdouh, Propriétés physico-chimiques des couches minces del’oxyde de titane. Effet de la concentration, Université Mentouri-Constantine Faculté des Sciences Exactes Département de Physique, 20 / 10 /2009.
