- Email: [email protected]
Deposition and characterization of pure and Cd doped SnO2 thin films by the nebulizer spray pyrolysis (NSP) technique
Materials Science in Semiconductor Processing 16 (2013) 825–832
Contents lists available at SciVerse ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Deposition and characterization of pure and Cd doped SnO2 thin films by the nebulizer spray pyrolysis (NSP) technique R. Mariappan a, V. Ponnuswamy a,n, P. Suresh b, R. Suresh a, M. Ragavendar c, C. Sankar a a b c
Department of Physics, Sri Ramakrishna Mission Vidyalaya College of Arts and Science, Coimbatore 641020, India Materials Research Centre, Indian Institute of Science, Bangalore 560012, Karnataka, India Department of Physics, RVS college of Engineering and Technology, Coimbatore 641042, Tamil Nadu, India
a r t i c l e i n f o
abstract
Available online 8 February 2013
Pure and cadmium doped tin oxide thin films were deposited on glass substrates from aqueous solution of cadmium acetate, tin (IV) chloride and sodium hydroxide by the nebulizer spray pyrolysis (NSP) technique. X-ray diffraction reveals that all films have tetragonal crystalline structure with preferential orientation along (200) plane. On application of the Scherrer formula, it is found that the maximum size of grains is 67 nm. Scanning electron microscopy shows that the grains are of rod and spherical in shape. Energy dispersive X-ray analysis reveals the average ratio of the atomic percentage of pure and Cd doped SnO2 films. The electrical resistivity is found to be 102 O cm at higher temperature (170 1C) and 103 O cm at lower temperature (30 1C). Optical band gap energy was determined from transmittance and absorbance data obtained from UV–vis spectra. Optical studies reveal that the band gap energy decreases from 3.90 eV to 3.52 eV due to the addition of Cd as dopant with different concentrations. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Nebulizer spray pyrolysis XRD SEM Electrical Optical properties
1. Introduction Tin oxide (SnO2) is an n-type semiconductor material with direct band gap energy (Eg) 3.9 eV [1–3] finds many applications in high temperature electron devices, transparent electron devices and so on. This is due to its good optical, electrical properties and excellent chemical and thermal stability [4–6]. It has also been widely used in devices like sensors for detection of gases [7,8], solar cells [9], and flat panel collectors with spectral selective windows [10]. Researchers investigated a variety of techniques to prepare SnO2 thin films, such as chemical vapor deposition [11,12], co-precipitation method [12,13], dip coating [14,15], spray pyrolysis [16–19] and sol–gel method [20,21]. The technique used in this work is known as the NSP technique which is one of the most widely
n Corresponding author. Tel.: þ91 4 222 692 461; fax: þ 91 4 222 693 812. E-mail address: [email protected] (V. Ponnuswamy).
1369-8001/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2013.01.006
used techniques. It is a very easy, low cost, safe, and vacuum less system of the deposition technique for preparing transparent conducting oxides compared with other techniques. The other advantage of this technique is that it can be easily adapted for production of large area uniform film coatings. The aim this work is to study the structural, optical and surface properties of Cd doped ZnO films deposited by the NSP technique at substrate temperature 450 1C with different Cd doping concentrations reported. The main objective is to employ the nebulizer spray pyrolysis technique to obtain Cd Doped SnO2 which provides the appropriate characteristics necessary for the fabrication of optoelectronic devices. 2. Experimental technique Analytical grade cadmium chloride, tin (IV) chloride and sodium hydroxide pellet were used for the film preparation.
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Fig. 1. Schematic diagram of the nebulizer spray pyrolysis (NSP) setup.
Pure and Cd doped SnO2 films deposited at substrate temperature 450 1C with film thickness of 222 nm, 227 nm, 232 nm and 239 nm through the nebulizer spray pyrolysis (NSP) technique. Taking the ionic radius ˚ is lower than that into account, the radius of Sn (0.69 A) ˚ and the doping of Cd leads to the increase of Cd (0.97 A) of the film thickness and crystallite size. Schematic diagram of the NSP setup is shown in Fig. 1. The spray solution was prepared by dissolving 0.1 M tin (IV) chloride dissolved in 25 ml of de-ionized water and the solution was stirred for 10 min using magnetic stirrer. NaOH solution was added slowly with tin (IV) chloride solution from a buret held vertically until pH value reached to 7. Similarly 0.1 M cadmium chloride solution was also prepared. Cadmium chloride solution was added to the above tin (IV) chloride solution with (Cd/Sn) nominal volume proportions 1%, 3% and 5%. The stirring was continued for 30 min to get clear and homogeneous spray solution. The prepared solution was sprayed onto ultrasonically cleaned glass substrates kept at 450 1C. Films prepared by this method have uniform thickness and well adherent with the substrate. The optimized preparative parameters for pure and Cd doped SnO2 thin films are listed in Table 1. X-ray diffraction data of the nebulizer sprayed pure and Cd doped SnO2 films were recorded with the help of a Philips Model PW 1710 diffractometer with Cu Ka radiation (l ¼0.1542 nm). Surface morphological studies and compositional analysis were carried out using a scanning electron microscope and energy dispersive X-ray analysis setup attached with scanning electron microscopy
Table 1 Optimized preparative parameters for nebulizer sprayed pure and Cd doped SnO2 thin films. Deposition spray rate Cd concentrations Substrate temperature pH of the solution Deposition time Nozzle to substrate distance Carrier gas pressure
0.5 ml/min 1%, 3% and 5% 450 1C 7 10 min 5 cm 30 psi
(Philips Model XL 30), respectively. The electrical resistivity of the films was studied using a four probe setup. Optical absorption spectrum was recorded using a JASCOV-570 spectrophotometer. 3. Results and discussion 3.1. Structural analysis The X-ray diffraction patterns of the pure and Cd doped SnO2 thin films deposited at 450 1C with different Cd concentrations are shown in Fig. 2a–d. X-ray diffraction spectra reveal the polycrystalline nature of the films with tetragonal structure. The most intense peak was observed at (200) plane and additional peaks were along (110), (101), (200), (211), (002), (310) and (112) planes. The peak intensity observed at (200) plane was found to increase gradually with the increase of Cd concentration from 1% to 5%. The different peaks in the diffractogram were indexed and the corresponding values of interplanar spacing ‘‘d’’ were calculated and compared with standard
R. Mariappan et al. / Materials Science in Semiconductor Processing 16 (2013) 825–832
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Fig. 2. X-ray diffraction patterns of Cd doped SnO2 thin films: (a) pure SnO2, (b) 1% Cd sample, (c) 3% Cd sample and (d) 5% Cd sample.
Fig.. 3. X-ray diffraction peak position of the pure and Cd doped SnO2 thin films: (a) pure SnO2, (b) 1% Cd sample (c) 3% Cd sample, and (d) 5% Cd sample.
values of Joint Committee on the Powder Diffraction Spectra data (JCPDS 88-0287). It is found that higher Cd concentration leads to the formation of well-crystallized films. It is also observed from Fig. 2d that higher Cd concentration gives in good quality films with improved crystallinity as evidenced by intense diffraction peaks. The Cd doped SnO2 thin films deposited at 450 1C with Cd concentration (5%) are found to be well crystalline in nature. The height of (200) peak in X-ray diffraction pattern for Cd doped SnO2 thin films deposited at higher Cd concentration (5%) has shown sharper peaks and small FWHM data as indicated in Fig. 2d. The peak at (200) plane is found to be shifted for pure and Cd doped SnO2 films as shown in Fig. 3. A peak developed by a well
Fig. 4. Variation of lattice constant (a,c) with Cd concentration of Cd doped SnO2 thin films, (b) variation of grain size and microstrain with Cd concentration of pure and Cd doped SnO2 thin films and (c) variation of dislocation density and stacking fault with Cd concentration of pure and Cd doped SnO2 thin films.
prepared Cd doped film was used as a reference to compare the peak position shift due to different reflections and hence to evaluate the microstructural parameters.
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The lattice constant (a,c) was calculated using Eq. (1) for the pure and Cd doped SnO2 films [22]: 1 d
2
2
¼
2
2
ðh þ k Þ l þ 2 a2 c
ð1Þ
The variation of lattice constant (a,c) with different Cd concentrations for Cd doped SnO2 films is shown in Fig. 4a. It is observed that (Fig. 4a) the lattice constant (a,c) value of SnO2 thin film is slightly decreased with the increased Cd concentration as compared with Joint Committee on the Powder Diffraction Spectra data (JCPDS 88-0287). The dislocation density, microstrain and stacking fault probability of the pure and Cd doped films were calculated using the following equations [22-24]:
e¼ d¼
a¼
l D siny 1 D2 "
b
ð2Þ
tany
lines=m2
ð3Þ #
2p2
45ð3 tanyÞ1=2
b
ð4Þ
The crystallite sizes of pure and Cd doped SnO2 thin films were evaluated using the Scherer formula [25]: D¼
variation of crystallite size and microstrain with Cd concentration of Cd doped SnO2 films is shown in Fig. 4b. In Fig. 4b, it is observed that the crystallite size increases along (002) plane with Cd concentration increase and attains a maximum 67 nm at 450 1C. It is observed that (Fig. 4b), a sharp increase in crystallite size and decrease in microstrain with the increased Cd concentration. The variation of dislocation density and stacking fault probability with Cd concentration of the films is shown in Fig. 4c. For Cd concentration (5%), the minimum values for dislocation density and stacking probability of the film (Fig. 4b) are obtained. Pure and Cd doped SnO2 films with lower strain, dislocation density and stacking fault probability improve the crystallinity of the films which in turn increase the volumetric expansion of the films. The lattice constant (a,c), crystallite size, microstrain, dislocation density and stacking fault probability results of the films are given in Table 2. It is concluded from the structural analysis that the addition of Cd has a strong effect on the structural properties of the films.
0:9l bcos y
ð5Þ
where D is the mean crystallite size, b is the full width at half maximum of the diffraction line, y is the diffraction angle, and l is the wavelength of the X-radiation. The
3.2. Surface morphology analysis Pure and Cd doped SnO2 films were deposited at 450 1C with different Cd concentrations from 1% to 5%. Scanning electron micrographs of the deposited films with 1500 and 10,000 magnifications are shown in Fig. 5. The morphology changes of the films with respect to Cd concentrations and the corresponding grain size estimations are depicted in Fig. 5. The pure tin oxide (SnO2) film demonstrates many pellets like grains as
Table 2 To calculate the structural parameters of pure and Cd doped SnO2 thin films. dSpacing ˚ [A]
FWHM [2y]
(hkl) Crystal system
26.592 33.823 37.844 51.687
3.352 2.650 2.377 1.767
0.299 0.149 0.224 0.456
110 101 200 211
26.490 33.792 37.852 51.629 57.817 61.775 65.872
3.364 2.652 2.376 1.770 1.594 1.501 1.416
0.187 0.093 0.224 0.187 0.448 0.897 0.547
110 101 200 211 002 310 112
26.553 33.822 37.896 51.661 61.816 65.898
3.357 2.650 2.374 1.769 1.500 1.416
0.187 0.187 0.187 0.224 0.299 0.410
110 101 200 211 310 112
26.464 33.763 37.901 51.623 61.871 65.829
3.368 2.655 2.374 1.771 1.499 1.417
0.149 0.448 0.131 0.187 0.299 0.274
110 101 200 211 310 112
2y
Pure SnO2
1% Cd
3% Cd
5% Cd
Lattice constant ˚ a, (A)
4.7472
4.7455
4.7507
4.7486
Lattice constant ˚ c, (A)
3.1893
3.1583
3.1307
3.1352
Crystallite size (nm)
d (10 14 lin/m2)
Microstrain e Stacking fault, (10 4 lin 2 m 4) a (10 4 J/m2)
28.497 57.955 39.077 20.212
12.31 02.98 06.55 24.48
14.10 05.48 07.29 10.49
27.20 11.99 16.94 28.96
45.571 92.721 39.078 49.277 21.118 10.771 18.062
04.82 01.16 06.55 04.12 22.42 08.61 03.06
08.85 03.43 07.29 04.31 09.05 16.71 09.41
18.04 07.50 16.94 11.88 26.69 51.29 30.05
45.577 46.364 46.900 41.069 32.324 24.086
04.81 04.65 04.55 05.96 09.57 17.24
08.83 06.85 06.07 05.16 05.57 07.05
17.02 14.99 14.11 14.26 17.09 22.53
56.961 19.319 67.002 49.275 32.333 36.115
03.08 26.79 02.23 04.12 09.57 07.67
07.09 16.47 04.25 04.31 05.56 04.71
13.69 36.00 09.87 11.89 17.08 15.03
Dislocation density,
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observed in the image of scanning electron microscopy. The estimated grain sizes form the SEM images are in the range 350–4500 nm. As the Cd concentration increases from 1% to 3%, the surface formed is found to have rods and spherical structures with average grain size are 450 nm and 500 nm. The grain size transformation from rod to spherical is observed for the Cd doped SnO2 film deposited 450 1C with 5% Cd concentration and the average aggregate size lies between 500 and 600 nm.
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3.3. Compositional analysis The chemical composition of sprayed pure and Cd doped SnO2 thin films deposited at substrate temperature 450 1C with different Cd concentrations from 1% to 5% is shown in Fig. 6a–d. The spectra depicted in Fig. 6a–d are for pure and Cd doped SnO2 thin films which has peak of 1.8 keV due to the presence of silicon in the substrate used for the film preparation [26]. The peaks corresponding to
Fig. 5. Scanning electron microscopy images of pure Cd doped SnO2 thin films: (a) pure SnO2, (b) 1% Cd sample, (c) 3% Cd sample and (d) 5% Cd sample.
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Fig. 6. Energy dispersive X-ray analysis spectra of pure and Cd doped SnO2 thin films: (a) pure SnO2, (b) 1% Cd sample, (c) 3% Cd sample and (d) 5% Cd sample.
Si, Cd, Sn and O are observed in the EDAX indicating the formation of Cd doped SnO2 product and the atomic percentages of Cd, Sn and O and their values are listed in the inset (table) of Fig 6. 3.4. Electrical properties Resistivity measurement of pure and Cd doped SnO2 films is very important from the application point of view. The electrical resistivity of the films was measured using a four probe setup in the temperature range 30–170 1C as shown in Fig. 7. The measurement is performed by making four electrical contacts with the sample surface. Two of the probes are used to measure the current and the other two probes are used to measure the corresponding voltage. The electrical resistivity of the pure and Cd doped films was calculated using the following relation [27]: Fig. 7. Change of resistivity (r) with temperature of pure and Cd doped SnO2 thin films: (a) pure SnO2, (b) 1% Cd sample, (c) 3% Cd sample and (d) 5% Cd sample.
r¼
V 2pS I
ð6Þ
where r is the electrical resistivity, V is the voltage, I is the
R. Mariappan et al. / Materials Science in Semiconductor Processing 16 (2013) 825–832
current and S is the distance between interpair of probes. The resistivity of pure and Cd doped SnO2 films decreases with increase in temperature indicating the development of semiconducting nature. It is observed (Fig. 7a–d) that the resistivity decreases non-linearly with the temperature increase. It is well known that electrical properties of polycrystalline films are strongly influenced by their structural characteristics and nature of purity. From Fig. 7a, the electrical resistivity of the film is found to be 8.79 103 O cm at lower temperature and 1.99 103 O cm at higher temperature. When Cd concentration increases upto 5%, the electrical resistivities of the films are in the range between 5.43 103 O cm and 4.53 102 O cm. The decrease may be due to the crystallite size increase of the films. The temperature dependence of the activation energy can be accounted using the Arrhenius relation [28]: Ea ð7Þ s ¼ s0 exp RT
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following relation [30]:
a¼
2:303xlogðTÞ d
ð8Þ
where d is the film thickness and T is the transmission. The absorption coefficient (a), energy gap (Eg) and photon energy (hu) are related as [24]: ðahuÞ ¼ AðhuEg Þn
ð9Þ
where u is the frequency, A is a constant and n assumes values 1/2, 2, 3/2, and 3 depending on the mode of
where s and s0 are the electrical conductivities, R is the ideal gas constant and T is the temperature. The electrical conductivities of the pure and Cd doped SnO2 films deposited at substrate temperature (450 1C) with different concentrations are listed in Table 3. It can be seen (Table 3) that the electrical conductivity increases with the increase of Cd concentration. The activation energies of pure and Cd doped SnO2 films were found to be in the range between 0.40 eV and 0.41 eV in the lower temperature region (30 1C) and 0.55 eV to 0.49 eV in the higher temperature region (170 1C). The calculated values of electrical resistivity, conductivity and activation energy of pure and Cd doped SnO2 films are listed in Table 3. The above results confirm the stability of the pure and Cd doped SnO2 films and their employability in optoelectronics devices.
Fig. 8. Transmittance spectra of Cd doped SnO2 films.
3.5. Optical properties The transmittance spectra of pure and Cd doped SnO2 thin films were studied by UV–vis–NIR double beam spectrophotometer in the range 250–2000 nm. The presence of transmittance maxima at wavelength region 345 nm is observed (Fig. 8) and the shift toward higher wavelength region is found with Cd concentration increase in the pure and Cd doped SnO2 films. The higher-energy electronic transitions from valence to conduction bands confirm the direct type semiconducting nature of the material [29]. The absorption coefficient (a) was calculated from the transmission data using the
Fig. 9. Plot of (ahn)2 versus hu for Cd doped SnO2 thin films.
Table 3 Electrical resistivity, electrical conductivity and activation energy of the pure Cd doped SnO2 thin films. Resistivity r (O cm)
Pure SnO2 1% Cd 3% Cd 5% Cd
Conductivity s (O cm) 1
Activation energy Ea (eV)
30 1C
170 1C
30 1C
170 1C
30 1C
170 1C
8.79 103 8.79 103 5.71 103 5.43 103
1.99 103 1.35 103 9.06 102 4.53 102
1.14 10 4 1.41 10 4 1.75 10 4 1.84 10 4
5.01 10 4 7.36 10 4 1.10 10 3 2.21 10 3
0.41 0.41 0.40 0.40
0.55 0.53 0.52 0.49
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R. Mariappan et al. / Materials Science in Semiconductor Processing 16 (2013) 825–832
interband transition, i.e., direct, allowed indirect, direct forbidden and indirect forbidden transition, respectively. For allowed direct type of transitions ðahuÞ ¼ AðhuEg Þ1=2
ð10Þ
A typical plot of (ahu)2 versus hu for pure and Cd doped SnO2 films are shown in Fig. 9. Extrapolating the linear portion of the curve to touch the energy axis which gives the value of energy gap for pure and Cd doped SnO2 films. The band gap value is estimated as 3.9 eV (pure SnO2) and is in agreement with the band gap energy of pure SnO2 [31–33]. The band gap of films is found to be decreased with increasing Cd concentration upto 5%, correspondingly the direct band gap energy decreases from 3.90 to 3.52 eV. This reduction in band gap energy may be due to the decrease of hole concentration with the increase of Cd doping in the doped thin films. 4. Conclusions The nebulizer sprayed cadmium doped tin oxide (Cd:SnO2) thin films were deposited on glass substrate at substrate temperature 450 1C with different Cd concentration. X-ray diffraction confirms the tetragonal structure with preferred orientation along (200) plane. Crystallite size is found to be increased with Cd concentration. The scanning electron microscopy studies reveal that the pure tin oxide (SnO2) film demonstrates many pellets like grains. The average grain sizes of the grains are in the range of 350–400 nm. The presence of elemental constituents was confirmed from energy dispersive X-ray analysis. The electrical resistivity of pure and Cd doped SnO2 films was estimated and found to be in between 5.43 103 O cm at lower temperature (30 1C) and 4.53 102 O cm at higher temperature (170 1C). The maximum optical transmittance 80% was obtained for pure and 5% Cd doped SnO2 film deposited at substrate temperature (450 1C). From the optical studies it is observed that the band gap energy decreases from 3.90 to 3.52 eV with increase of Cd concentration. The investigation results of the pure and Cd doped SnO2 films prepared by the NSP technique ensure the stability of the films and their employability in optoelectronics device applications.
Acknowledgments The authors are very much grateful to Sophisticated Test and Instrumentation Centre, Cochin, and Alagappa University, Karaikudi, for providing instrument facilities.
References [1] M.-M. Bagheri-Mohagheghi, N. Shahtahmasebi, M.R. Alinejad, A. Youssefi, M. Shokooh-Saremi, Solid State Sciences 11 (2009) 233–239. [2] D. Davazoglou, Thin Solid Films 302 (1997) 204–213. [3] K. Reimann, M. Steube, Solid State Communications 105 (1998) 649–652. [4] Y.P. Yadava, G. Denicolo, A.C. Arias, L.S. Roman, L.A. Hummelgen, Materials Chemistry and Physics 48 (1997) 263–267. [5] S. Raj, K.L. Choy, Materials Chemistry and Physics 82 (2003) 489–492. [6] M. Rami, E. Benamar, C. Messaoudi, D. Sayah, A. Ennaori, European Journal of Solid State and Inorganic Chemistry 35 (1998) 211–219. [7] Wang Yude, Solid State Electronics 48 (2004) 627–632. [8] G. Korotcenkov, V. Brinzari, Y. Boris, M. Ivanov, J. Schwank, J. Morante, Thin Solid Films 436 (2003) 119–126. [9] G. Frank, E. Kauer, H. Kostlin, F.J. Schmitte, Solar Energy Materials 8 (1983) 387–398. [10] A.L. Dawar, J.C. Joshi, Journal of Materials Science 19 (1984) 1–23. [11] M. Zdenek Remes, H.M. Vanecek, P. Yates, D.W. Evans, Thin Solid Films 517 (2009) 6287–6289. [12] Guoui Dai, Xilan Jiang, Yushu Zhang, Thin Solid Films 320 (1998) 216–219. [13] T. Zhang, Y. Shen, Journal of Materials Science Letters 13 (1994) 1647–1649. [14] Z. Jie, H. Li-Hua, G. Shan, Z. Hui, Z. Jing-Gui, Sensors and Actuators B: Chemical 130 (2008) 400–404. [15] M.A. Batal, H.J. Fares, Energy Procedia 6 (2011) 1–10. [16] M. Kenji, N. Kiyofumi, K. Shoji, Thin Solid Films 515 (2007) 8632–8636. [17] G. Korotcenkov, M. DiBattista, J. Schwank, V. Brinzari, Materials Science and Engineering B 77 (2000) 33–39. [18] G. Korotcenkov, V. Brinzari, J. Schwank, M. DiBattista, A. Vasiliev, Sensors and Actuators B 77 (2001) 244–252. [19] V. Brinzari, G. Korotcenkov, V. Golovanov, Thin Solid Films 391 (2001) 167–175. [20] S. Gnanam, V. Rajendran, Journal of Sol–Gel Science and Technology 56 (2010) 128–133. [21] T. Zhang, P. Hing, L. Yang, J. Zhang, Sensors and Actuators B: Chemical 60 (1999) 208–215. [22] R.R. Kasar, N.G. Deshpande, Y.G. Gudage, J.C. Vyas, R. Sharma, Physica B 403 (2008) 3724–3729. [23] R. Mariappan, M. Ragavendar, V. Ponnuswamy, Journal of Alloys and Compounds 509 (2011) 7337–7343. [24] T. Mahalingam, V.S. John, L.S. Hsu, Journal of New Materials for Electrochemical Systems 10 (2007) 9–14. [25] A.R. Babar, P.R. Deshamukh, R.J. Deokate, D. Haranath, C.H. Bhosale, K.Y. Rajpure, Journal of Physics D: Applied Physics 41 (2008) 135404–135408. [26] B. Ergin, E. Ketenci, F. Atay, International Journal of Hydrogen Energy 34 (2009) 5249–5254. [27] S. Eart, H. Metin, M. Ari, Chemistry and Physics 111 (2008) 114–120. [28] A.N. Seif, Thin Solid Films 342 (1999) 47–51. [29] B. Pejova, I. Grozdanov, Thin Solid Films 515 (2007) 5203–5211. [30] X. Zhang, X.M. Li, T.L. Chen, J.M. Bian, C.Y. Zhang, Thin Solid Films 492 (2005) 248–252. [31] M. Boshta, F.A. Mahmoud, M.H. Sayed, Journal of Ovonic Research 6 (2010) 93–98. [32] S. Belgacem, R. Bennaceur, Revue de Physique Appliquee 25 (1990) 1245–1258. [33] Manal Madhat Abdullah, Mahdi Hassan Suhail, Sabah Ibrahim Abbas, Archives of Applied Science Research 4 (2012) 1279–1288.
Contents lists available at SciVerse ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Deposition and characterization of pure and Cd doped SnO2 thin films by the nebulizer spray pyrolysis (NSP) technique R. Mariappan a, V. Ponnuswamy a,n, P. Suresh b, R. Suresh a, M. Ragavendar c, C. Sankar a a b c
Department of Physics, Sri Ramakrishna Mission Vidyalaya College of Arts and Science, Coimbatore 641020, India Materials Research Centre, Indian Institute of Science, Bangalore 560012, Karnataka, India Department of Physics, RVS college of Engineering and Technology, Coimbatore 641042, Tamil Nadu, India
a r t i c l e i n f o
abstract
Available online 8 February 2013
Pure and cadmium doped tin oxide thin films were deposited on glass substrates from aqueous solution of cadmium acetate, tin (IV) chloride and sodium hydroxide by the nebulizer spray pyrolysis (NSP) technique. X-ray diffraction reveals that all films have tetragonal crystalline structure with preferential orientation along (200) plane. On application of the Scherrer formula, it is found that the maximum size of grains is 67 nm. Scanning electron microscopy shows that the grains are of rod and spherical in shape. Energy dispersive X-ray analysis reveals the average ratio of the atomic percentage of pure and Cd doped SnO2 films. The electrical resistivity is found to be 102 O cm at higher temperature (170 1C) and 103 O cm at lower temperature (30 1C). Optical band gap energy was determined from transmittance and absorbance data obtained from UV–vis spectra. Optical studies reveal that the band gap energy decreases from 3.90 eV to 3.52 eV due to the addition of Cd as dopant with different concentrations. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Nebulizer spray pyrolysis XRD SEM Electrical Optical properties
1. Introduction Tin oxide (SnO2) is an n-type semiconductor material with direct band gap energy (Eg) 3.9 eV [1–3] finds many applications in high temperature electron devices, transparent electron devices and so on. This is due to its good optical, electrical properties and excellent chemical and thermal stability [4–6]. It has also been widely used in devices like sensors for detection of gases [7,8], solar cells [9], and flat panel collectors with spectral selective windows [10]. Researchers investigated a variety of techniques to prepare SnO2 thin films, such as chemical vapor deposition [11,12], co-precipitation method [12,13], dip coating [14,15], spray pyrolysis [16–19] and sol–gel method [20,21]. The technique used in this work is known as the NSP technique which is one of the most widely
n Corresponding author. Tel.: þ91 4 222 692 461; fax: þ 91 4 222 693 812. E-mail address: [email protected] (V. Ponnuswamy).
1369-8001/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2013.01.006
used techniques. It is a very easy, low cost, safe, and vacuum less system of the deposition technique for preparing transparent conducting oxides compared with other techniques. The other advantage of this technique is that it can be easily adapted for production of large area uniform film coatings. The aim this work is to study the structural, optical and surface properties of Cd doped ZnO films deposited by the NSP technique at substrate temperature 450 1C with different Cd doping concentrations reported. The main objective is to employ the nebulizer spray pyrolysis technique to obtain Cd Doped SnO2 which provides the appropriate characteristics necessary for the fabrication of optoelectronic devices. 2. Experimental technique Analytical grade cadmium chloride, tin (IV) chloride and sodium hydroxide pellet were used for the film preparation.
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R. Mariappan et al. / Materials Science in Semiconductor Processing 16 (2013) 825–832
Fig. 1. Schematic diagram of the nebulizer spray pyrolysis (NSP) setup.
Pure and Cd doped SnO2 films deposited at substrate temperature 450 1C with film thickness of 222 nm, 227 nm, 232 nm and 239 nm through the nebulizer spray pyrolysis (NSP) technique. Taking the ionic radius ˚ is lower than that into account, the radius of Sn (0.69 A) ˚ and the doping of Cd leads to the increase of Cd (0.97 A) of the film thickness and crystallite size. Schematic diagram of the NSP setup is shown in Fig. 1. The spray solution was prepared by dissolving 0.1 M tin (IV) chloride dissolved in 25 ml of de-ionized water and the solution was stirred for 10 min using magnetic stirrer. NaOH solution was added slowly with tin (IV) chloride solution from a buret held vertically until pH value reached to 7. Similarly 0.1 M cadmium chloride solution was also prepared. Cadmium chloride solution was added to the above tin (IV) chloride solution with (Cd/Sn) nominal volume proportions 1%, 3% and 5%. The stirring was continued for 30 min to get clear and homogeneous spray solution. The prepared solution was sprayed onto ultrasonically cleaned glass substrates kept at 450 1C. Films prepared by this method have uniform thickness and well adherent with the substrate. The optimized preparative parameters for pure and Cd doped SnO2 thin films are listed in Table 1. X-ray diffraction data of the nebulizer sprayed pure and Cd doped SnO2 films were recorded with the help of a Philips Model PW 1710 diffractometer with Cu Ka radiation (l ¼0.1542 nm). Surface morphological studies and compositional analysis were carried out using a scanning electron microscope and energy dispersive X-ray analysis setup attached with scanning electron microscopy
Table 1 Optimized preparative parameters for nebulizer sprayed pure and Cd doped SnO2 thin films. Deposition spray rate Cd concentrations Substrate temperature pH of the solution Deposition time Nozzle to substrate distance Carrier gas pressure
0.5 ml/min 1%, 3% and 5% 450 1C 7 10 min 5 cm 30 psi
(Philips Model XL 30), respectively. The electrical resistivity of the films was studied using a four probe setup. Optical absorption spectrum was recorded using a JASCOV-570 spectrophotometer. 3. Results and discussion 3.1. Structural analysis The X-ray diffraction patterns of the pure and Cd doped SnO2 thin films deposited at 450 1C with different Cd concentrations are shown in Fig. 2a–d. X-ray diffraction spectra reveal the polycrystalline nature of the films with tetragonal structure. The most intense peak was observed at (200) plane and additional peaks were along (110), (101), (200), (211), (002), (310) and (112) planes. The peak intensity observed at (200) plane was found to increase gradually with the increase of Cd concentration from 1% to 5%. The different peaks in the diffractogram were indexed and the corresponding values of interplanar spacing ‘‘d’’ were calculated and compared with standard
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Fig. 2. X-ray diffraction patterns of Cd doped SnO2 thin films: (a) pure SnO2, (b) 1% Cd sample, (c) 3% Cd sample and (d) 5% Cd sample.
Fig.. 3. X-ray diffraction peak position of the pure and Cd doped SnO2 thin films: (a) pure SnO2, (b) 1% Cd sample (c) 3% Cd sample, and (d) 5% Cd sample.
values of Joint Committee on the Powder Diffraction Spectra data (JCPDS 88-0287). It is found that higher Cd concentration leads to the formation of well-crystallized films. It is also observed from Fig. 2d that higher Cd concentration gives in good quality films with improved crystallinity as evidenced by intense diffraction peaks. The Cd doped SnO2 thin films deposited at 450 1C with Cd concentration (5%) are found to be well crystalline in nature. The height of (200) peak in X-ray diffraction pattern for Cd doped SnO2 thin films deposited at higher Cd concentration (5%) has shown sharper peaks and small FWHM data as indicated in Fig. 2d. The peak at (200) plane is found to be shifted for pure and Cd doped SnO2 films as shown in Fig. 3. A peak developed by a well
Fig. 4. Variation of lattice constant (a,c) with Cd concentration of Cd doped SnO2 thin films, (b) variation of grain size and microstrain with Cd concentration of pure and Cd doped SnO2 thin films and (c) variation of dislocation density and stacking fault with Cd concentration of pure and Cd doped SnO2 thin films.
prepared Cd doped film was used as a reference to compare the peak position shift due to different reflections and hence to evaluate the microstructural parameters.
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The lattice constant (a,c) was calculated using Eq. (1) for the pure and Cd doped SnO2 films [22]: 1 d
2
2
¼
2
2
ðh þ k Þ l þ 2 a2 c
ð1Þ
The variation of lattice constant (a,c) with different Cd concentrations for Cd doped SnO2 films is shown in Fig. 4a. It is observed that (Fig. 4a) the lattice constant (a,c) value of SnO2 thin film is slightly decreased with the increased Cd concentration as compared with Joint Committee on the Powder Diffraction Spectra data (JCPDS 88-0287). The dislocation density, microstrain and stacking fault probability of the pure and Cd doped films were calculated using the following equations [22-24]:
e¼ d¼
a¼
l D siny 1 D2 "
b
ð2Þ
tany
lines=m2
ð3Þ #
2p2
45ð3 tanyÞ1=2
b
ð4Þ
The crystallite sizes of pure and Cd doped SnO2 thin films were evaluated using the Scherer formula [25]: D¼
variation of crystallite size and microstrain with Cd concentration of Cd doped SnO2 films is shown in Fig. 4b. In Fig. 4b, it is observed that the crystallite size increases along (002) plane with Cd concentration increase and attains a maximum 67 nm at 450 1C. It is observed that (Fig. 4b), a sharp increase in crystallite size and decrease in microstrain with the increased Cd concentration. The variation of dislocation density and stacking fault probability with Cd concentration of the films is shown in Fig. 4c. For Cd concentration (5%), the minimum values for dislocation density and stacking probability of the film (Fig. 4b) are obtained. Pure and Cd doped SnO2 films with lower strain, dislocation density and stacking fault probability improve the crystallinity of the films which in turn increase the volumetric expansion of the films. The lattice constant (a,c), crystallite size, microstrain, dislocation density and stacking fault probability results of the films are given in Table 2. It is concluded from the structural analysis that the addition of Cd has a strong effect on the structural properties of the films.
0:9l bcos y
ð5Þ
where D is the mean crystallite size, b is the full width at half maximum of the diffraction line, y is the diffraction angle, and l is the wavelength of the X-radiation. The
3.2. Surface morphology analysis Pure and Cd doped SnO2 films were deposited at 450 1C with different Cd concentrations from 1% to 5%. Scanning electron micrographs of the deposited films with 1500 and 10,000 magnifications are shown in Fig. 5. The morphology changes of the films with respect to Cd concentrations and the corresponding grain size estimations are depicted in Fig. 5. The pure tin oxide (SnO2) film demonstrates many pellets like grains as
Table 2 To calculate the structural parameters of pure and Cd doped SnO2 thin films. dSpacing ˚ [A]
FWHM [2y]
(hkl) Crystal system
26.592 33.823 37.844 51.687
3.352 2.650 2.377 1.767
0.299 0.149 0.224 0.456
110 101 200 211
26.490 33.792 37.852 51.629 57.817 61.775 65.872
3.364 2.652 2.376 1.770 1.594 1.501 1.416
0.187 0.093 0.224 0.187 0.448 0.897 0.547
110 101 200 211 002 310 112
26.553 33.822 37.896 51.661 61.816 65.898
3.357 2.650 2.374 1.769 1.500 1.416
0.187 0.187 0.187 0.224 0.299 0.410
110 101 200 211 310 112
26.464 33.763 37.901 51.623 61.871 65.829
3.368 2.655 2.374 1.771 1.499 1.417
0.149 0.448 0.131 0.187 0.299 0.274
110 101 200 211 310 112
2y
Pure SnO2
1% Cd
3% Cd
5% Cd
Lattice constant ˚ a, (A)
4.7472
4.7455
4.7507
4.7486
Lattice constant ˚ c, (A)
3.1893
3.1583
3.1307
3.1352
Crystallite size (nm)
d (10 14 lin/m2)
Microstrain e Stacking fault, (10 4 lin 2 m 4) a (10 4 J/m2)
28.497 57.955 39.077 20.212
12.31 02.98 06.55 24.48
14.10 05.48 07.29 10.49
27.20 11.99 16.94 28.96
45.571 92.721 39.078 49.277 21.118 10.771 18.062
04.82 01.16 06.55 04.12 22.42 08.61 03.06
08.85 03.43 07.29 04.31 09.05 16.71 09.41
18.04 07.50 16.94 11.88 26.69 51.29 30.05
45.577 46.364 46.900 41.069 32.324 24.086
04.81 04.65 04.55 05.96 09.57 17.24
08.83 06.85 06.07 05.16 05.57 07.05
17.02 14.99 14.11 14.26 17.09 22.53
56.961 19.319 67.002 49.275 32.333 36.115
03.08 26.79 02.23 04.12 09.57 07.67
07.09 16.47 04.25 04.31 05.56 04.71
13.69 36.00 09.87 11.89 17.08 15.03
Dislocation density,
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observed in the image of scanning electron microscopy. The estimated grain sizes form the SEM images are in the range 350–4500 nm. As the Cd concentration increases from 1% to 3%, the surface formed is found to have rods and spherical structures with average grain size are 450 nm and 500 nm. The grain size transformation from rod to spherical is observed for the Cd doped SnO2 film deposited 450 1C with 5% Cd concentration and the average aggregate size lies between 500 and 600 nm.
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3.3. Compositional analysis The chemical composition of sprayed pure and Cd doped SnO2 thin films deposited at substrate temperature 450 1C with different Cd concentrations from 1% to 5% is shown in Fig. 6a–d. The spectra depicted in Fig. 6a–d are for pure and Cd doped SnO2 thin films which has peak of 1.8 keV due to the presence of silicon in the substrate used for the film preparation [26]. The peaks corresponding to
Fig. 5. Scanning electron microscopy images of pure Cd doped SnO2 thin films: (a) pure SnO2, (b) 1% Cd sample, (c) 3% Cd sample and (d) 5% Cd sample.
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Fig. 6. Energy dispersive X-ray analysis spectra of pure and Cd doped SnO2 thin films: (a) pure SnO2, (b) 1% Cd sample, (c) 3% Cd sample and (d) 5% Cd sample.
Si, Cd, Sn and O are observed in the EDAX indicating the formation of Cd doped SnO2 product and the atomic percentages of Cd, Sn and O and their values are listed in the inset (table) of Fig 6. 3.4. Electrical properties Resistivity measurement of pure and Cd doped SnO2 films is very important from the application point of view. The electrical resistivity of the films was measured using a four probe setup in the temperature range 30–170 1C as shown in Fig. 7. The measurement is performed by making four electrical contacts with the sample surface. Two of the probes are used to measure the current and the other two probes are used to measure the corresponding voltage. The electrical resistivity of the pure and Cd doped films was calculated using the following relation [27]: Fig. 7. Change of resistivity (r) with temperature of pure and Cd doped SnO2 thin films: (a) pure SnO2, (b) 1% Cd sample, (c) 3% Cd sample and (d) 5% Cd sample.
r¼
V 2pS I
ð6Þ
where r is the electrical resistivity, V is the voltage, I is the
R. Mariappan et al. / Materials Science in Semiconductor Processing 16 (2013) 825–832
current and S is the distance between interpair of probes. The resistivity of pure and Cd doped SnO2 films decreases with increase in temperature indicating the development of semiconducting nature. It is observed (Fig. 7a–d) that the resistivity decreases non-linearly with the temperature increase. It is well known that electrical properties of polycrystalline films are strongly influenced by their structural characteristics and nature of purity. From Fig. 7a, the electrical resistivity of the film is found to be 8.79 103 O cm at lower temperature and 1.99 103 O cm at higher temperature. When Cd concentration increases upto 5%, the electrical resistivities of the films are in the range between 5.43 103 O cm and 4.53 102 O cm. The decrease may be due to the crystallite size increase of the films. The temperature dependence of the activation energy can be accounted using the Arrhenius relation [28]: Ea ð7Þ s ¼ s0 exp RT
831
following relation [30]:
a¼
2:303xlogðTÞ d
ð8Þ
where d is the film thickness and T is the transmission. The absorption coefficient (a), energy gap (Eg) and photon energy (hu) are related as [24]: ðahuÞ ¼ AðhuEg Þn
ð9Þ
where u is the frequency, A is a constant and n assumes values 1/2, 2, 3/2, and 3 depending on the mode of
where s and s0 are the electrical conductivities, R is the ideal gas constant and T is the temperature. The electrical conductivities of the pure and Cd doped SnO2 films deposited at substrate temperature (450 1C) with different concentrations are listed in Table 3. It can be seen (Table 3) that the electrical conductivity increases with the increase of Cd concentration. The activation energies of pure and Cd doped SnO2 films were found to be in the range between 0.40 eV and 0.41 eV in the lower temperature region (30 1C) and 0.55 eV to 0.49 eV in the higher temperature region (170 1C). The calculated values of electrical resistivity, conductivity and activation energy of pure and Cd doped SnO2 films are listed in Table 3. The above results confirm the stability of the pure and Cd doped SnO2 films and their employability in optoelectronics devices.
Fig. 8. Transmittance spectra of Cd doped SnO2 films.
3.5. Optical properties The transmittance spectra of pure and Cd doped SnO2 thin films were studied by UV–vis–NIR double beam spectrophotometer in the range 250–2000 nm. The presence of transmittance maxima at wavelength region 345 nm is observed (Fig. 8) and the shift toward higher wavelength region is found with Cd concentration increase in the pure and Cd doped SnO2 films. The higher-energy electronic transitions from valence to conduction bands confirm the direct type semiconducting nature of the material [29]. The absorption coefficient (a) was calculated from the transmission data using the
Fig. 9. Plot of (ahn)2 versus hu for Cd doped SnO2 thin films.
Table 3 Electrical resistivity, electrical conductivity and activation energy of the pure Cd doped SnO2 thin films. Resistivity r (O cm)
Pure SnO2 1% Cd 3% Cd 5% Cd
Conductivity s (O cm) 1
Activation energy Ea (eV)
30 1C
170 1C
30 1C
170 1C
30 1C
170 1C
8.79 103 8.79 103 5.71 103 5.43 103
1.99 103 1.35 103 9.06 102 4.53 102
1.14 10 4 1.41 10 4 1.75 10 4 1.84 10 4
5.01 10 4 7.36 10 4 1.10 10 3 2.21 10 3
0.41 0.41 0.40 0.40
0.55 0.53 0.52 0.49
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R. Mariappan et al. / Materials Science in Semiconductor Processing 16 (2013) 825–832
interband transition, i.e., direct, allowed indirect, direct forbidden and indirect forbidden transition, respectively. For allowed direct type of transitions ðahuÞ ¼ AðhuEg Þ1=2
ð10Þ
A typical plot of (ahu)2 versus hu for pure and Cd doped SnO2 films are shown in Fig. 9. Extrapolating the linear portion of the curve to touch the energy axis which gives the value of energy gap for pure and Cd doped SnO2 films. The band gap value is estimated as 3.9 eV (pure SnO2) and is in agreement with the band gap energy of pure SnO2 [31–33]. The band gap of films is found to be decreased with increasing Cd concentration upto 5%, correspondingly the direct band gap energy decreases from 3.90 to 3.52 eV. This reduction in band gap energy may be due to the decrease of hole concentration with the increase of Cd doping in the doped thin films. 4. Conclusions The nebulizer sprayed cadmium doped tin oxide (Cd:SnO2) thin films were deposited on glass substrate at substrate temperature 450 1C with different Cd concentration. X-ray diffraction confirms the tetragonal structure with preferred orientation along (200) plane. Crystallite size is found to be increased with Cd concentration. The scanning electron microscopy studies reveal that the pure tin oxide (SnO2) film demonstrates many pellets like grains. The average grain sizes of the grains are in the range of 350–400 nm. The presence of elemental constituents was confirmed from energy dispersive X-ray analysis. The electrical resistivity of pure and Cd doped SnO2 films was estimated and found to be in between 5.43 103 O cm at lower temperature (30 1C) and 4.53 102 O cm at higher temperature (170 1C). The maximum optical transmittance 80% was obtained for pure and 5% Cd doped SnO2 film deposited at substrate temperature (450 1C). From the optical studies it is observed that the band gap energy decreases from 3.90 to 3.52 eV with increase of Cd concentration. The investigation results of the pure and Cd doped SnO2 films prepared by the NSP technique ensure the stability of the films and their employability in optoelectronics device applications.
Acknowledgments The authors are very much grateful to Sophisticated Test and Instrumentation Centre, Cochin, and Alagappa University, Karaikudi, for providing instrument facilities.
References [1] M.-M. Bagheri-Mohagheghi, N. Shahtahmasebi, M.R. Alinejad, A. Youssefi, M. Shokooh-Saremi, Solid State Sciences 11 (2009) 233–239. [2] D. Davazoglou, Thin Solid Films 302 (1997) 204–213. [3] K. Reimann, M. Steube, Solid State Communications 105 (1998) 649–652. [4] Y.P. Yadava, G. Denicolo, A.C. Arias, L.S. Roman, L.A. Hummelgen, Materials Chemistry and Physics 48 (1997) 263–267. [5] S. Raj, K.L. Choy, Materials Chemistry and Physics 82 (2003) 489–492. [6] M. Rami, E. Benamar, C. Messaoudi, D. Sayah, A. Ennaori, European Journal of Solid State and Inorganic Chemistry 35 (1998) 211–219. [7] Wang Yude, Solid State Electronics 48 (2004) 627–632. [8] G. Korotcenkov, V. Brinzari, Y. Boris, M. Ivanov, J. Schwank, J. Morante, Thin Solid Films 436 (2003) 119–126. [9] G. Frank, E. Kauer, H. Kostlin, F.J. Schmitte, Solar Energy Materials 8 (1983) 387–398. [10] A.L. Dawar, J.C. Joshi, Journal of Materials Science 19 (1984) 1–23. [11] M. Zdenek Remes, H.M. Vanecek, P. Yates, D.W. Evans, Thin Solid Films 517 (2009) 6287–6289. [12] Guoui Dai, Xilan Jiang, Yushu Zhang, Thin Solid Films 320 (1998) 216–219. [13] T. Zhang, Y. Shen, Journal of Materials Science Letters 13 (1994) 1647–1649. [14] Z. Jie, H. Li-Hua, G. Shan, Z. Hui, Z. Jing-Gui, Sensors and Actuators B: Chemical 130 (2008) 400–404. [15] M.A. Batal, H.J. Fares, Energy Procedia 6 (2011) 1–10. [16] M. Kenji, N. Kiyofumi, K. Shoji, Thin Solid Films 515 (2007) 8632–8636. [17] G. Korotcenkov, M. DiBattista, J. Schwank, V. Brinzari, Materials Science and Engineering B 77 (2000) 33–39. [18] G. Korotcenkov, V. Brinzari, J. Schwank, M. DiBattista, A. Vasiliev, Sensors and Actuators B 77 (2001) 244–252. [19] V. Brinzari, G. Korotcenkov, V. Golovanov, Thin Solid Films 391 (2001) 167–175. [20] S. Gnanam, V. Rajendran, Journal of Sol–Gel Science and Technology 56 (2010) 128–133. [21] T. Zhang, P. Hing, L. Yang, J. Zhang, Sensors and Actuators B: Chemical 60 (1999) 208–215. [22] R.R. Kasar, N.G. Deshpande, Y.G. Gudage, J.C. Vyas, R. Sharma, Physica B 403 (2008) 3724–3729. [23] R. Mariappan, M. Ragavendar, V. Ponnuswamy, Journal of Alloys and Compounds 509 (2011) 7337–7343. [24] T. Mahalingam, V.S. John, L.S. Hsu, Journal of New Materials for Electrochemical Systems 10 (2007) 9–14. [25] A.R. Babar, P.R. Deshamukh, R.J. Deokate, D. Haranath, C.H. Bhosale, K.Y. Rajpure, Journal of Physics D: Applied Physics 41 (2008) 135404–135408. [26] B. Ergin, E. Ketenci, F. Atay, International Journal of Hydrogen Energy 34 (2009) 5249–5254. [27] S. Eart, H. Metin, M. Ari, Chemistry and Physics 111 (2008) 114–120. [28] A.N. Seif, Thin Solid Films 342 (1999) 47–51. [29] B. Pejova, I. Grozdanov, Thin Solid Films 515 (2007) 5203–5211. [30] X. Zhang, X.M. Li, T.L. Chen, J.M. Bian, C.Y. Zhang, Thin Solid Films 492 (2005) 248–252. [31] M. Boshta, F.A. Mahmoud, M.H. Sayed, Journal of Ovonic Research 6 (2010) 93–98. [32] S. Belgacem, R. Bennaceur, Revue de Physique Appliquee 25 (1990) 1245–1258. [33] Manal Madhat Abdullah, Mahdi Hassan Suhail, Sabah Ibrahim Abbas, Archives of Applied Science Research 4 (2012) 1279–1288.