- Email: [email protected]
Influence of substrate temperature and Cobalt concentration on structural and optical properties of ZnO thin films prepared by Ultrasonic spray technique
Superlattices and Microstructures 52 (2012) 807–815
Contents lists available at SciVerse ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Influence of substrate temperature and Cobalt concentration on structural and optical properties of ZnO thin films prepared by Ultrasonic spray technique Said Benramache a,b,⇑, Boubaker Benhaoua b a b
Material Sciences Department, Faculty of Science, University of Biskra, Biskra 07000, Algeria Physics Laboratory, Institute of Technology, University of El-oued, 39000, Algeria
a r t i c l e
i n f o
Article history: Received 9 April 2012 Received in revised form 27 May 2012 Accepted 1 June 2012 Available online 15 June 2012 Keywords: ZnO Co Thin films Ultrasonic spray technique TCO
a b s t r a c t Pure and Cobalt doped zinc oxide were deposited on glass substrate by Ultrasonic spray method. Zinc acetate dehydrate, Cobalt chloride, 4-methoxyethanol and monoethanolamine were used as a starting materials, dopant source, solvent and stabilizer, respectively. The ZnO samples and ZnO:Co with Cobalt concentration of 2 wt.% were deposited at 300, 350 and 400 °C. The effects of substrate temperature and presence of Co as doping element on the structural, electrical and optical properties were examined. Both pure and Co doped ZnO samples are (0 0 2) preferentially oriented. The X-ray diffraction results indicate that the samples have polycrystalline nature and hexagonal wurtzite structure with the maximum average crystallite size of ZnO and ZnO:Co were 33.28 and 55.46 nm. An increase in the substrate temperature and presence doping the crystallinity of the thin films increased. The optical transmittance spectra showed transmittance higher than 80% within the visible wavelength region. The band gap energy of the thin films increased after doping from 3.25 to 3.36 eV at 350 °C. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Zinc oxide (ZnO) has attracted more attentions in recent years due to its unique properties such as a direct wide band gap (3.37 eV), a large exciton binding energy (60 meV), strong emission, large saturation velocity (3.2 107 cm s 1) and a high breakdown voltage [1]. ⇑ Corresponding author at: Material Sciences Department, Faculty of Science, University of Biskra, Biskra 07000, Algeria. Tel.: +213 779276135; fax: +213 33740401. E-mail address: [email protected] (S. Benramache). 0749-6036/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.spmi.2012.06.005
808
S. Benramache, B. Benhaoua / Superlattices and Microstructures 52 (2012) 807–815
Transparent conducting oxide (TCO) films were used for applications in microelectronic devices, light emitting diodes, thin film, antireflection coatings, transparent electrodes in solar cells [2], gas sensors surface acoustic wave devices [3], varistors, spintronic devices, and lasers [4]. The conventional TCO films included tin-doped indium oxide (ITO). However, the cost of ITO films is expensive. In recent years, ZnO:Co films have been extensively studied because they present exhibit high mobility, high optical transparency, high electrical conductivity and have a lower material cost. Currently, many methods are being used to prepare TCO films, such as molecular beam epitaxy (MBE), chemical vapor deposition, electrochemical deposition [3], pulsed laser deposition (PLD), sol–gel process [4], reactive evaporation, magnetron sputtering technique and spray pyrolysis [5]. The conductivity property of the ZnO:Co films can be improved after doping and low temperature. In this work, we have deposited two series of ZnO samples and ZnO:Co samples on glass substrates by Ultrasonic spray technique. The Cobalt concentration of Cobalt is 2 wt.%, which was used to prepare ZnO:Co films. We have studied the effect of the substrate temperatures and doping of the Cobalt on the crystallinity, band gap energy and electrical conductivity of the semiconductors. 2. Experimental procedure 2.1. Preparation of spray solution ZnO solution were prepared by dissolving 0.1 M (Zn(CH3COO)2, 2H2O) in the solvent containing equal volumes absolute ethanol solution (99.995%) purity, then have added a drops of monoethanolamine solution as a stabilized, the mixture solution was stirred at 50 °C for 120 min to yield a clear and transparency solution. ZnO:Co solution were prepared by adding to the precedent solution a 0.02 M solution of Cobalt chloride, 4-methoxyethanol, such that the ratio of Co/Zn = 0.02. This Co content can also be expressed as 2 wt.%. The solution became clear and homogeneous after stirring for 120 min at 50–70 °C. The substrate was R217102 glass in a size of 1 cm 1 cm 0.1 cm, prior to pumping, the substrate (R217102 glass) was cleaned with alcohol in an ultrasonic bath and blow-dried with dry nitrogen gas. 2.2. Deposition of thin films The resulting solutions were sprayed on the heated glass substrates by ultrasonic nebulizer system (Sonics) which transforms the liquid to a stream formed with uniform and fine droplets of 35 lm average diameter (given by the manufacturer). The deposition was performed at a different substrate temperature of the substrates were 300, 350 and 400 °C with 2 min of deposition time [6,7]. 2.3. Characterization Crystallographic and phase structures of the thin films were determined by X-ray diffraction (XRD, Bruker AXS-8D) with CuKa radiation (k = 0.1541 nm) in the scanning range was between 2h = 20° and 70°. The optical properties of the deposited films was measured in the range of 250–900 nm using by an ultraviolet–visible spectrophotometer (UV, Lambda 35), and the electrical conductivity of the films was measured in a coplanar structure obtained with evaporation of four golden stripes on film surface. All spectra were measured at room temperature (RT) in air. 3. Results and discussion 3.1. Crystalline structure Fig. 1 shows the XRD patterns of ZnO thin films deposited on glass substrate at three substrate temperatures. Three diffraction peaks were observed at 2h = 31.74°, 34.52° and 36.40° which can be attributed respectively to (1 0 0), (0 0 2) and (1 0 1) planes of ZnO phase. This result showed the ZnO thin films were polycrystalline and had a hexagonal wurtzite structure. This result is consistent with
809
S. Benramache, B. Benhaoua / Superlattices and Microstructures 52 (2012) 807–815
Fig. 1. X-ray diffraction spectra of ZnO thin films at three different substrate temperatures.
Table 1 The unit cell parameters of ZnO and ZnO:Co thin films calculated from XRD patterns. Temperature (°C)
2h(0 0 2) (°) d(0 0 2) (A°) c (A°) FWHM (°) G (nm) Strain e (%)
ZnO films
ZnO:Co films
300
350
400
300
350
400
34.46 2.6005 5.2011 0.26 31.99 0.094
34.52 2.5962 5.1923 0.24 33.28 0.263
34.48 2.5991 5.1981 0.28 29.71 0.152
34.58 2.5918 5.1836 0.18 46.22 0.430
34.56 2.5932 5.1865 0.15 55.46 0.375
34.52 2.5962 5.1923 0.21 39.61 0.263
investigations given by many authors on structure of ZnO thin films [1,4,8–10], where only a (0 0 2) diffraction peak is highest one. This observation shows that the all films having preferential c-axis orientation along the (0 0 2) plane [11]. The intensity and the full-width-at-half-maximum (FWHM) of the (0 0 2) diffraction peak for the undoped ZnO are shown in Fig. 1 and Table 1. It is found that the intensity of (0 0 2) orientation is weak for the films grown at 300 °C, but it increases with increasing substrate temperature to 350 °C. However, the intensity decreases when ZnO is deposited at 400 °C. As the substrate temperature increases from 300 to 350 °C, the FWHM decreases from 0.26 to 0.24° .On a further increase in substrate temperature to 400 °C, the FWHM gradually increases to 0.28°. It seems to be that 350 °C is an optimum temperature with minimum disorder for the undoped films. Fig. 2 shows the XRD patterns of ZnO thin films doped with Cobalt at three substrate temperatures. All films show a dominant (0 0 2) peak corresponding to ZnO hexagonal wurtzite structure at 2h = 34.56° with orientation along c-axis. Other peak of (1 0 1) plane at 36.48 show the polycrystalline nature of doped ZnO thin films. The intensity of the (0 0 2) peak increased after doping ZnO:Co films and their the FWHM values decrease which means that Co doped ZnO samples are less disordered than undoped ones. However, Table 1 shows that the diffraction angle of the (0 0 2) peak was increase with ZnO:Co films has been indicates that the lattice parameter is decrease, which the strain of the films decreased (less defects) [12,13]. The effect of temperature on the Co doped ZnO is the same as in the undoped; we have obtain that the crystalline quality of thin films enhanced at a substrate temperature of 350 °C. It means Co doped ZnO films with minimum disorder too. These results are in good agreement with the calculated strain and crystallite size below.
810
S. Benramache, B. Benhaoua / Superlattices and Microstructures 52 (2012) 807–815
Fig. 2. X-ray diffraction spectra of ZnO:Co thin films at three different substrate temperatures.
The diffraction peak angles of the ZnO thin films were estimated in Table 1, and the lattice parameter c for this film was calculated from XRD patterns by using the following equation [14]: 2
dhkl ¼
2
2
4 h þ hk þ k l þ 2 3 a2 c
!1=2 ð1Þ
where a and c are the lattice parameters, h, k and l are the Miller indices of the planes and dhkl is the interplanar spacing. The variations of lattice parameters are shown in Table 1. The lattice parameters are substrate dependant. This gives rise to a mismatch between the substrate and the deposited thin films. The latter is responsible of the resulting strains and stresses. We estimated the strain e values in each thin film deposition via the formula [10]:
e¼
c c0 100% c0
ð2Þ
where e is the mean strain in ZnO thin films (Table 1), c is the lattice constant of ZnO thin films and c0 is the lattice constant of bulk (standard c0 = 0, 5206 nm). In order to calculate the crystallite size G (0 0 2) of the ZnO films from the XRD patterns, we used Scherer’s equation [15]:
G¼
0:9k bcosh
ð3Þ
where G is the crystallite size, k is the wavelength of X-ray (k = 1.5406 A°), b is the full-width-at-halfmaximum (FWHM), and h is angle of diffraction peak. The values of crystallite sizes and FWHM are illustrated in Table 1. Note that the experimental accuracy in reading the 2h angle is 0.01° of arc. In Fig. 3 we have reported the variation of the crystallite size of ZnO and ZnO:Co films deposited at three different substrate temperature of the (0 0 2) diffraction peak. As seen in Fig. 3, the crystallite sizes are increased with 350 °C and then decreased within increasing substrate temperature of 350–400 °C. The increases of the crystallite size, this indicating an improvement in the crystallinity of the films. Moreover, the decreases of the crystallite size, this confirms the deterioration in the crystallinity of the films. The increase of the crystallite size (less defects) after doping indicated the enhancement of the crystallinity and c-axis orientation of Co doped ZnO thin films. We found that the crystallite size along height direction [16].
S. Benramache, B. Benhaoua / Superlattices and Microstructures 52 (2012) 807–815
811
Fig. 3. Average crystallite size of undoped ZnO and Co doped ZnO films of the (0 0 2) diffraction peak as a function of substrate temperature.
3.2. Optical characteristics Fig. 4 shows the transmittance of ZnO films deposited at three different substrate temperatures. As can be seen, an increase in the substrate temperature from 300 to 350 °C improves the films optical transmission. It passes from 80% to 85% at the photon energy between 1.55 and 3.1 eV corresponding to (400–800 nm). The later is a region of strong transparency. However, the range occurs between 3.25 and 3.40 eV is the region of the absorption in the layers due to the transition between the valence band and the conduction band, Nian et al. [17], as shown in the inset of Fig. 4, in this region the transmittance decreased because of the onset fundamental absorption. We are noting that the temperature effect is clearly observed in the layer quality. Fig. 5 shows the optical transmission spectra of Co doped ZnO thin films at three substrate temperatures; it was also measured for comparison. As can be seen, all films exhibit an average optical transparency over 62–90%, in the visible range. The range of inter-band transition is 365–385 nm [16,18], in this region the transmittance decreased because of the onset fundamental absorption. However, compared with the absorption of undoped ZnO film, the absorption edge of Co doped ZnO film exhibit is an
Fig. 4. Transmission spectra of undoped ZnO samples for (a) 300, (b) 350 and (c) 400 °C.
812
S. Benramache, B. Benhaoua / Superlattices and Microstructures 52 (2012) 807–815
Fig. 5. Transmission spectra of Co doped ZnO samples with the substrate temperature.
Table 2 The optical properties and electrical conductivity of ZnO and ZnO:Co thin film. Temperature (°C)
Eg (eV) Eu (meV) Average T (%) r (O cm)1
ZnO films
ZnO:Co films
300
350
400
300
350
400
3.168 918 83 7.407
3.250 209 85 7.547
3.229 304 80 6.579
3.351 206 62 7.424
3.362 108 72 7.634
3.348 237 90 6.743
obvious blue shift, which shows that the optical band gap of the films are broadened after doping. We are noting that the transparency affected by the substrate temperature. The optical band gap energy Eg as shown in Table 2 was obtained from the transmission spectra using the following relations [19]:
A ¼ a d ¼ lnT
ð4Þ
ðAhmÞ2 ¼ Bðhm EÞ=A ¼ ad
ð5Þ
where A is the absorbance, d is the film thickness; T is the transmittance spectra of thin films; a is the absorption coefficient values; B is a constant, hm is the photon energy and Eg is the band gap energy of the semiconductor. Fig. 6(a) shows the graph of A vs. photon energy hm for ZnO thin films deposited at three different substrate temperatures. The inset shows plot of (Ahm)2 vs. hm of Co doped ZnO thin films deposited at 350 °C. Extrapolation of linear portion of the graph to the energy axis at A = 0 in the range between 360 and 380 nm gives band gap energy Eg is shown in Table 2 [20]. Besides, we have used the Urbach energy (Table 2), which is related to the disorder in the film network, is expressed as [21]:
hm A ¼ A0 exp Eu
ð6Þ
where A0 is a constant and Eu is the Urbach energy, the latter decreased with increasing the band gap is indicating to decrease of defects as shown in (Table 2). Fig. 6(b) shows the variation of the optical band gap energy Eg of undoped ZnO and Co doped ZnO films at three different substrate temperatures. We have obtained increase of the optical band gap energy with increasing the substrate temperature for all the films in the average range of temperature (300–350 °C), one can see that the increase in Eg is more significantly for the undoped ZnO films;
S. Benramache, B. Benhaoua / Superlattices and Microstructures 52 (2012) 807–815
813
Fig. 6(a). Plot of A vs. hv of ZnO thin films deposited at three different substrate temperatures. The inset shows plot of (Ahv)2 vs. hv of Co doped ZnO thin films deposited 350 °C.
Fig. 6(b). Variation of optical band gap of undoped ZnO and Co doped ZnO films with the substrate temperature.
beyond 350 °C sleight decrease in Eg within the substrate temperature for undoped ZnO and Co doped ZnO films. An obvious increase in Eg for the Co doped ZnO thin films for the three different substrate temperatures compared to the undoped ZnO ones. At 350 °C the increase in optical gap after Co doping from 3.25 eV (undoped ZnO) to 3.36 eV (Co doped ZnO) can be originated from the active transitions involving 3d levels in Co+2 ions and strong sp-d exchange interaction between itinerant sp ZnO orbits and the localized d of the dopant which result in narrowing the conduction band EC and the valence band EV and causes the motion of EC upwards and EV downwards hence Co doping causes the band gap broadening. The same phenomena are carried out by Talaat et al. where they have observed a blue shift of the absorption edges from 3.33 eV (undoped ZnO) to 4.13 eV (Co doped ZnO) [22]. From Table 2 one can see that a minimum Urbach energy were reached at 350 °C for undoped and Co doped ZnO thin films which means the adequate temperature for less disorder. As expressed in the literatures [21,23,24].
814
S. Benramache, B. Benhaoua / Superlattices and Microstructures 52 (2012) 807–815
Fig. 7. Variation of electrical conductivity of undoped ZnO and Co doped ZnO films with the substrate temperature.
3.3. Electrical conductivity Fig. 7 shows the variation of the electrical conductivity r of undoped ZnO and Co doped ZnO films as a function of substrate temperature. As can be seen, the electrical conductivity increases from undoped ZnO to Co doped ZnO thin films, which mean an increase in the carrier concentration. The increase in conductivity of films with increasing substrate temperature has been explain by displacement of the electrons [25] The maximum conductivity values were obtained at 350 °C of the films before and after doping are 7.54 and 7.63 (O cm)1, respectively, due to the minimum disorders retched at this temperature, The decrease of the electrical conductivity with increasing substrate temperature are explained by increasing of the disorder in the films hence the potential barriers increased [26]. 4. Conclusions In conclusion, highly transparent conductive Co doped ZnO thin films have been deposited on glass substrate by Ultrasonic spray technique. The influence of substrate temperature on structural, optical and electrical properties was investigated. The whole obtained films have a polycrystalline wurtzite structure and are mainly (0 0 2) oriented. We have observed an improvement in the films crystallinity with increasing substrate temperature up to 350 °C in both undoped and Co doped films. The average transmittance is about 62–90% for ZnO:Co samples, and 77–85% for ZnO samples in the visible region, and the band gap increased after doping due to the active transitions involving 3d levels in Co+2 ions and strong sp–d exchange interaction between sp ZnO orbits and the localized d of the doping. Acknowledgments The authors would like to thanks Prf. M.S Aida, Prf. Z. Boumerzoug, Prf. A. Attaf, Prf. A. Chala, Dr. H. Ben Temam, Dr. S. Rahmane and Mr. B. Gasmi for helpful counseling. References [1] L. Ma, X. Ai, X. Huang, S. Ma, Superlattices and Microstructures 50 (2011) 703–712. [2] Z. Zhang, C. Bao, W. Yao, S. Ma, L. Zhang, S. Hou, Superlattices and Microstructures 49 (2011) 644–653. [3] D. Vernardou, G. Kenanakis, S. Couris, A.C. Manikas, G.A. Voyiatzis, M.E. Pemble, E. Koudoumas, N. Katsarakis, Journal of Crystal Growth 308 (2007) 105–109. [4] P. Prepelita, R. Medianu, B. Sbarcea, F. Garoi, M. Filipescu, Applied Surface Science 256 (2010) 1807–1811. [5] Z.B. Bahsi, A.Y. Oral, Optical Materials 29 (2007) 672–678. [6] L.H. Van, M.H. Hong, J. Ding, Journal of Alloys and Compounds 449 (2008) 207–209.
S. Benramache, B. Benhaoua / Superlattices and Microstructures 52 (2012) 807–815 [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
815
T.V. Vimalkumar, N. Poornima, C. Sudha Kartha, K.P. Vijayakumar, Applied Surface Science 257 (2011) 8334–8337. H. Kavak, E.S. Tuzemen, L.N. Ozbayraktar, R. Esen, Vacuum 83 (2009) 540–544. Y. Wang, B. Chu, Superlattices and Microstructures 44 (2008) 54–61. M. Mekhnache, A. Drici, L.S. Hamideche, H. Benzarouk, A. Amara, L. Cattin, J.C. Bernede, M. Guerioune, Superlattices and Microstructures 49 (2011) 510–518. A. Kalaivanan, S. Perumal, N.N. Pillai, K.R. Murali, Materials Science in Semiconductor Processing 14 (2011) 94–96. B.L. Zhu, X.H. Sun, X.Z. Zhao, F.H. Su, G.H. Li, X.G. Wu, J. Wu, R. Wu, J. Liu, Vacuum 82 (2008) 495–500. J. Lv, W. Gong, K. Huang, J. Zhu, F. Meng, X. Song, Z. Sun, Superlattices and Microstructures 50 (2011) 98–106. A.K. Zak, W.H.A. Majid, M.E. Abrishami, R. Yousefi, Solid State Sciences 13 (2011) 252. S. Rani, P. Suri, P.K. Shishodia, R.M. Mehra, Solar Energy Materials and Solar Cells 92 (2008) 1639–1645. C. Zhang, Journal of Physics and Chemistry of Solids 71 (2010) 364–369. H. Nian, S.H. Hahn, K.K. Koo, J.S. Kim, S. Kim, E.W. Shin, E.J. Kim, Materials Letters 64 (2010) 157–160. Y. Li, J. Gong, M. McCune, G. He, Y. Deng, Synthetic Metals 160 (2010) 499–503. Z. Ben Ayadi, L. El Mir, K. Djessas, S. Alaya, Thin Solid Films 517 (2009) 6305–6309. H. Abdullah, M.N. Norazia, S. Shaari, J.S. Mandeep, Thin Solid Films 518 (2010) 174–180. W. Daranfed, M.S. Aida, A. Hafdallah, H. Lekiket, Thin Solid Films 518 (2009) 1082–1084. T.M. Hammad, J.K. Salem, R.G. Harriso, Applied Nanoscience 2 (2012), http://dx.doi.org/10.1007/s13204-012-0077-9. M. Subramanian, M. Tanemura, T. Hihara, T. Jimbo, Chemical Physics Letters 487 (2010) 97–101. A. Jain, P. Sagar, R.M. Mehra, Solid State Electronics 50 (2006) 1420–1424. D.H. Zhang, T.L. Yang, J. Ma, Q.P. Wang, R.W. Gao, H.L. Ma, Applied Surface Science 158 (2000) 43–48. A. Hafdallah, F. Yanineb, M.S. Aida, N. Attaf, Journal of Alloys and Compounds 509 (2011) 7267–7270.
Contents lists available at SciVerse ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Influence of substrate temperature and Cobalt concentration on structural and optical properties of ZnO thin films prepared by Ultrasonic spray technique Said Benramache a,b,⇑, Boubaker Benhaoua b a b
Material Sciences Department, Faculty of Science, University of Biskra, Biskra 07000, Algeria Physics Laboratory, Institute of Technology, University of El-oued, 39000, Algeria
a r t i c l e
i n f o
Article history: Received 9 April 2012 Received in revised form 27 May 2012 Accepted 1 June 2012 Available online 15 June 2012 Keywords: ZnO Co Thin films Ultrasonic spray technique TCO
a b s t r a c t Pure and Cobalt doped zinc oxide were deposited on glass substrate by Ultrasonic spray method. Zinc acetate dehydrate, Cobalt chloride, 4-methoxyethanol and monoethanolamine were used as a starting materials, dopant source, solvent and stabilizer, respectively. The ZnO samples and ZnO:Co with Cobalt concentration of 2 wt.% were deposited at 300, 350 and 400 °C. The effects of substrate temperature and presence of Co as doping element on the structural, electrical and optical properties were examined. Both pure and Co doped ZnO samples are (0 0 2) preferentially oriented. The X-ray diffraction results indicate that the samples have polycrystalline nature and hexagonal wurtzite structure with the maximum average crystallite size of ZnO and ZnO:Co were 33.28 and 55.46 nm. An increase in the substrate temperature and presence doping the crystallinity of the thin films increased. The optical transmittance spectra showed transmittance higher than 80% within the visible wavelength region. The band gap energy of the thin films increased after doping from 3.25 to 3.36 eV at 350 °C. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Zinc oxide (ZnO) has attracted more attentions in recent years due to its unique properties such as a direct wide band gap (3.37 eV), a large exciton binding energy (60 meV), strong emission, large saturation velocity (3.2 107 cm s 1) and a high breakdown voltage [1]. ⇑ Corresponding author at: Material Sciences Department, Faculty of Science, University of Biskra, Biskra 07000, Algeria. Tel.: +213 779276135; fax: +213 33740401. E-mail address: [email protected] (S. Benramache). 0749-6036/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.spmi.2012.06.005
808
S. Benramache, B. Benhaoua / Superlattices and Microstructures 52 (2012) 807–815
Transparent conducting oxide (TCO) films were used for applications in microelectronic devices, light emitting diodes, thin film, antireflection coatings, transparent electrodes in solar cells [2], gas sensors surface acoustic wave devices [3], varistors, spintronic devices, and lasers [4]. The conventional TCO films included tin-doped indium oxide (ITO). However, the cost of ITO films is expensive. In recent years, ZnO:Co films have been extensively studied because they present exhibit high mobility, high optical transparency, high electrical conductivity and have a lower material cost. Currently, many methods are being used to prepare TCO films, such as molecular beam epitaxy (MBE), chemical vapor deposition, electrochemical deposition [3], pulsed laser deposition (PLD), sol–gel process [4], reactive evaporation, magnetron sputtering technique and spray pyrolysis [5]. The conductivity property of the ZnO:Co films can be improved after doping and low temperature. In this work, we have deposited two series of ZnO samples and ZnO:Co samples on glass substrates by Ultrasonic spray technique. The Cobalt concentration of Cobalt is 2 wt.%, which was used to prepare ZnO:Co films. We have studied the effect of the substrate temperatures and doping of the Cobalt on the crystallinity, band gap energy and electrical conductivity of the semiconductors. 2. Experimental procedure 2.1. Preparation of spray solution ZnO solution were prepared by dissolving 0.1 M (Zn(CH3COO)2, 2H2O) in the solvent containing equal volumes absolute ethanol solution (99.995%) purity, then have added a drops of monoethanolamine solution as a stabilized, the mixture solution was stirred at 50 °C for 120 min to yield a clear and transparency solution. ZnO:Co solution were prepared by adding to the precedent solution a 0.02 M solution of Cobalt chloride, 4-methoxyethanol, such that the ratio of Co/Zn = 0.02. This Co content can also be expressed as 2 wt.%. The solution became clear and homogeneous after stirring for 120 min at 50–70 °C. The substrate was R217102 glass in a size of 1 cm 1 cm 0.1 cm, prior to pumping, the substrate (R217102 glass) was cleaned with alcohol in an ultrasonic bath and blow-dried with dry nitrogen gas. 2.2. Deposition of thin films The resulting solutions were sprayed on the heated glass substrates by ultrasonic nebulizer system (Sonics) which transforms the liquid to a stream formed with uniform and fine droplets of 35 lm average diameter (given by the manufacturer). The deposition was performed at a different substrate temperature of the substrates were 300, 350 and 400 °C with 2 min of deposition time [6,7]. 2.3. Characterization Crystallographic and phase structures of the thin films were determined by X-ray diffraction (XRD, Bruker AXS-8D) with CuKa radiation (k = 0.1541 nm) in the scanning range was between 2h = 20° and 70°. The optical properties of the deposited films was measured in the range of 250–900 nm using by an ultraviolet–visible spectrophotometer (UV, Lambda 35), and the electrical conductivity of the films was measured in a coplanar structure obtained with evaporation of four golden stripes on film surface. All spectra were measured at room temperature (RT) in air. 3. Results and discussion 3.1. Crystalline structure Fig. 1 shows the XRD patterns of ZnO thin films deposited on glass substrate at three substrate temperatures. Three diffraction peaks were observed at 2h = 31.74°, 34.52° and 36.40° which can be attributed respectively to (1 0 0), (0 0 2) and (1 0 1) planes of ZnO phase. This result showed the ZnO thin films were polycrystalline and had a hexagonal wurtzite structure. This result is consistent with
809
S. Benramache, B. Benhaoua / Superlattices and Microstructures 52 (2012) 807–815
Fig. 1. X-ray diffraction spectra of ZnO thin films at three different substrate temperatures.
Table 1 The unit cell parameters of ZnO and ZnO:Co thin films calculated from XRD patterns. Temperature (°C)
2h(0 0 2) (°) d(0 0 2) (A°) c (A°) FWHM (°) G (nm) Strain e (%)
ZnO films
ZnO:Co films
300
350
400
300
350
400
34.46 2.6005 5.2011 0.26 31.99 0.094
34.52 2.5962 5.1923 0.24 33.28 0.263
34.48 2.5991 5.1981 0.28 29.71 0.152
34.58 2.5918 5.1836 0.18 46.22 0.430
34.56 2.5932 5.1865 0.15 55.46 0.375
34.52 2.5962 5.1923 0.21 39.61 0.263
investigations given by many authors on structure of ZnO thin films [1,4,8–10], where only a (0 0 2) diffraction peak is highest one. This observation shows that the all films having preferential c-axis orientation along the (0 0 2) plane [11]. The intensity and the full-width-at-half-maximum (FWHM) of the (0 0 2) diffraction peak for the undoped ZnO are shown in Fig. 1 and Table 1. It is found that the intensity of (0 0 2) orientation is weak for the films grown at 300 °C, but it increases with increasing substrate temperature to 350 °C. However, the intensity decreases when ZnO is deposited at 400 °C. As the substrate temperature increases from 300 to 350 °C, the FWHM decreases from 0.26 to 0.24° .On a further increase in substrate temperature to 400 °C, the FWHM gradually increases to 0.28°. It seems to be that 350 °C is an optimum temperature with minimum disorder for the undoped films. Fig. 2 shows the XRD patterns of ZnO thin films doped with Cobalt at three substrate temperatures. All films show a dominant (0 0 2) peak corresponding to ZnO hexagonal wurtzite structure at 2h = 34.56° with orientation along c-axis. Other peak of (1 0 1) plane at 36.48 show the polycrystalline nature of doped ZnO thin films. The intensity of the (0 0 2) peak increased after doping ZnO:Co films and their the FWHM values decrease which means that Co doped ZnO samples are less disordered than undoped ones. However, Table 1 shows that the diffraction angle of the (0 0 2) peak was increase with ZnO:Co films has been indicates that the lattice parameter is decrease, which the strain of the films decreased (less defects) [12,13]. The effect of temperature on the Co doped ZnO is the same as in the undoped; we have obtain that the crystalline quality of thin films enhanced at a substrate temperature of 350 °C. It means Co doped ZnO films with minimum disorder too. These results are in good agreement with the calculated strain and crystallite size below.
810
S. Benramache, B. Benhaoua / Superlattices and Microstructures 52 (2012) 807–815
Fig. 2. X-ray diffraction spectra of ZnO:Co thin films at three different substrate temperatures.
The diffraction peak angles of the ZnO thin films were estimated in Table 1, and the lattice parameter c for this film was calculated from XRD patterns by using the following equation [14]: 2
dhkl ¼
2
2
4 h þ hk þ k l þ 2 3 a2 c
!1=2 ð1Þ
where a and c are the lattice parameters, h, k and l are the Miller indices of the planes and dhkl is the interplanar spacing. The variations of lattice parameters are shown in Table 1. The lattice parameters are substrate dependant. This gives rise to a mismatch between the substrate and the deposited thin films. The latter is responsible of the resulting strains and stresses. We estimated the strain e values in each thin film deposition via the formula [10]:
e¼
c c0 100% c0
ð2Þ
where e is the mean strain in ZnO thin films (Table 1), c is the lattice constant of ZnO thin films and c0 is the lattice constant of bulk (standard c0 = 0, 5206 nm). In order to calculate the crystallite size G (0 0 2) of the ZnO films from the XRD patterns, we used Scherer’s equation [15]:
G¼
0:9k bcosh
ð3Þ
where G is the crystallite size, k is the wavelength of X-ray (k = 1.5406 A°), b is the full-width-at-halfmaximum (FWHM), and h is angle of diffraction peak. The values of crystallite sizes and FWHM are illustrated in Table 1. Note that the experimental accuracy in reading the 2h angle is 0.01° of arc. In Fig. 3 we have reported the variation of the crystallite size of ZnO and ZnO:Co films deposited at three different substrate temperature of the (0 0 2) diffraction peak. As seen in Fig. 3, the crystallite sizes are increased with 350 °C and then decreased within increasing substrate temperature of 350–400 °C. The increases of the crystallite size, this indicating an improvement in the crystallinity of the films. Moreover, the decreases of the crystallite size, this confirms the deterioration in the crystallinity of the films. The increase of the crystallite size (less defects) after doping indicated the enhancement of the crystallinity and c-axis orientation of Co doped ZnO thin films. We found that the crystallite size along height direction [16].
S. Benramache, B. Benhaoua / Superlattices and Microstructures 52 (2012) 807–815
811
Fig. 3. Average crystallite size of undoped ZnO and Co doped ZnO films of the (0 0 2) diffraction peak as a function of substrate temperature.
3.2. Optical characteristics Fig. 4 shows the transmittance of ZnO films deposited at three different substrate temperatures. As can be seen, an increase in the substrate temperature from 300 to 350 °C improves the films optical transmission. It passes from 80% to 85% at the photon energy between 1.55 and 3.1 eV corresponding to (400–800 nm). The later is a region of strong transparency. However, the range occurs between 3.25 and 3.40 eV is the region of the absorption in the layers due to the transition between the valence band and the conduction band, Nian et al. [17], as shown in the inset of Fig. 4, in this region the transmittance decreased because of the onset fundamental absorption. We are noting that the temperature effect is clearly observed in the layer quality. Fig. 5 shows the optical transmission spectra of Co doped ZnO thin films at three substrate temperatures; it was also measured for comparison. As can be seen, all films exhibit an average optical transparency over 62–90%, in the visible range. The range of inter-band transition is 365–385 nm [16,18], in this region the transmittance decreased because of the onset fundamental absorption. However, compared with the absorption of undoped ZnO film, the absorption edge of Co doped ZnO film exhibit is an
Fig. 4. Transmission spectra of undoped ZnO samples for (a) 300, (b) 350 and (c) 400 °C.
812
S. Benramache, B. Benhaoua / Superlattices and Microstructures 52 (2012) 807–815
Fig. 5. Transmission spectra of Co doped ZnO samples with the substrate temperature.
Table 2 The optical properties and electrical conductivity of ZnO and ZnO:Co thin film. Temperature (°C)
Eg (eV) Eu (meV) Average T (%) r (O cm)1
ZnO films
ZnO:Co films
300
350
400
300
350
400
3.168 918 83 7.407
3.250 209 85 7.547
3.229 304 80 6.579
3.351 206 62 7.424
3.362 108 72 7.634
3.348 237 90 6.743
obvious blue shift, which shows that the optical band gap of the films are broadened after doping. We are noting that the transparency affected by the substrate temperature. The optical band gap energy Eg as shown in Table 2 was obtained from the transmission spectra using the following relations [19]:
A ¼ a d ¼ lnT
ð4Þ
ðAhmÞ2 ¼ Bðhm EÞ=A ¼ ad
ð5Þ
where A is the absorbance, d is the film thickness; T is the transmittance spectra of thin films; a is the absorption coefficient values; B is a constant, hm is the photon energy and Eg is the band gap energy of the semiconductor. Fig. 6(a) shows the graph of A vs. photon energy hm for ZnO thin films deposited at three different substrate temperatures. The inset shows plot of (Ahm)2 vs. hm of Co doped ZnO thin films deposited at 350 °C. Extrapolation of linear portion of the graph to the energy axis at A = 0 in the range between 360 and 380 nm gives band gap energy Eg is shown in Table 2 [20]. Besides, we have used the Urbach energy (Table 2), which is related to the disorder in the film network, is expressed as [21]:
hm A ¼ A0 exp Eu
ð6Þ
where A0 is a constant and Eu is the Urbach energy, the latter decreased with increasing the band gap is indicating to decrease of defects as shown in (Table 2). Fig. 6(b) shows the variation of the optical band gap energy Eg of undoped ZnO and Co doped ZnO films at three different substrate temperatures. We have obtained increase of the optical band gap energy with increasing the substrate temperature for all the films in the average range of temperature (300–350 °C), one can see that the increase in Eg is more significantly for the undoped ZnO films;
S. Benramache, B. Benhaoua / Superlattices and Microstructures 52 (2012) 807–815
813
Fig. 6(a). Plot of A vs. hv of ZnO thin films deposited at three different substrate temperatures. The inset shows plot of (Ahv)2 vs. hv of Co doped ZnO thin films deposited 350 °C.
Fig. 6(b). Variation of optical band gap of undoped ZnO and Co doped ZnO films with the substrate temperature.
beyond 350 °C sleight decrease in Eg within the substrate temperature for undoped ZnO and Co doped ZnO films. An obvious increase in Eg for the Co doped ZnO thin films for the three different substrate temperatures compared to the undoped ZnO ones. At 350 °C the increase in optical gap after Co doping from 3.25 eV (undoped ZnO) to 3.36 eV (Co doped ZnO) can be originated from the active transitions involving 3d levels in Co+2 ions and strong sp-d exchange interaction between itinerant sp ZnO orbits and the localized d of the dopant which result in narrowing the conduction band EC and the valence band EV and causes the motion of EC upwards and EV downwards hence Co doping causes the band gap broadening. The same phenomena are carried out by Talaat et al. where they have observed a blue shift of the absorption edges from 3.33 eV (undoped ZnO) to 4.13 eV (Co doped ZnO) [22]. From Table 2 one can see that a minimum Urbach energy were reached at 350 °C for undoped and Co doped ZnO thin films which means the adequate temperature for less disorder. As expressed in the literatures [21,23,24].
814
S. Benramache, B. Benhaoua / Superlattices and Microstructures 52 (2012) 807–815
Fig. 7. Variation of electrical conductivity of undoped ZnO and Co doped ZnO films with the substrate temperature.
3.3. Electrical conductivity Fig. 7 shows the variation of the electrical conductivity r of undoped ZnO and Co doped ZnO films as a function of substrate temperature. As can be seen, the electrical conductivity increases from undoped ZnO to Co doped ZnO thin films, which mean an increase in the carrier concentration. The increase in conductivity of films with increasing substrate temperature has been explain by displacement of the electrons [25] The maximum conductivity values were obtained at 350 °C of the films before and after doping are 7.54 and 7.63 (O cm)1, respectively, due to the minimum disorders retched at this temperature, The decrease of the electrical conductivity with increasing substrate temperature are explained by increasing of the disorder in the films hence the potential barriers increased [26]. 4. Conclusions In conclusion, highly transparent conductive Co doped ZnO thin films have been deposited on glass substrate by Ultrasonic spray technique. The influence of substrate temperature on structural, optical and electrical properties was investigated. The whole obtained films have a polycrystalline wurtzite structure and are mainly (0 0 2) oriented. We have observed an improvement in the films crystallinity with increasing substrate temperature up to 350 °C in both undoped and Co doped films. The average transmittance is about 62–90% for ZnO:Co samples, and 77–85% for ZnO samples in the visible region, and the band gap increased after doping due to the active transitions involving 3d levels in Co+2 ions and strong sp–d exchange interaction between sp ZnO orbits and the localized d of the doping. Acknowledgments The authors would like to thanks Prf. M.S Aida, Prf. Z. Boumerzoug, Prf. A. Attaf, Prf. A. Chala, Dr. H. Ben Temam, Dr. S. Rahmane and Mr. B. Gasmi for helpful counseling. References [1] L. Ma, X. Ai, X. Huang, S. Ma, Superlattices and Microstructures 50 (2011) 703–712. [2] Z. Zhang, C. Bao, W. Yao, S. Ma, L. Zhang, S. Hou, Superlattices and Microstructures 49 (2011) 644–653. [3] D. Vernardou, G. Kenanakis, S. Couris, A.C. Manikas, G.A. Voyiatzis, M.E. Pemble, E. Koudoumas, N. Katsarakis, Journal of Crystal Growth 308 (2007) 105–109. [4] P. Prepelita, R. Medianu, B. Sbarcea, F. Garoi, M. Filipescu, Applied Surface Science 256 (2010) 1807–1811. [5] Z.B. Bahsi, A.Y. Oral, Optical Materials 29 (2007) 672–678. [6] L.H. Van, M.H. Hong, J. Ding, Journal of Alloys and Compounds 449 (2008) 207–209.
S. Benramache, B. Benhaoua / Superlattices and Microstructures 52 (2012) 807–815 [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
815
T.V. Vimalkumar, N. Poornima, C. Sudha Kartha, K.P. Vijayakumar, Applied Surface Science 257 (2011) 8334–8337. H. Kavak, E.S. Tuzemen, L.N. Ozbayraktar, R. Esen, Vacuum 83 (2009) 540–544. Y. Wang, B. Chu, Superlattices and Microstructures 44 (2008) 54–61. M. Mekhnache, A. Drici, L.S. Hamideche, H. Benzarouk, A. Amara, L. Cattin, J.C. Bernede, M. Guerioune, Superlattices and Microstructures 49 (2011) 510–518. A. Kalaivanan, S. Perumal, N.N. Pillai, K.R. Murali, Materials Science in Semiconductor Processing 14 (2011) 94–96. B.L. Zhu, X.H. Sun, X.Z. Zhao, F.H. Su, G.H. Li, X.G. Wu, J. Wu, R. Wu, J. Liu, Vacuum 82 (2008) 495–500. J. Lv, W. Gong, K. Huang, J. Zhu, F. Meng, X. Song, Z. Sun, Superlattices and Microstructures 50 (2011) 98–106. A.K. Zak, W.H.A. Majid, M.E. Abrishami, R. Yousefi, Solid State Sciences 13 (2011) 252. S. Rani, P. Suri, P.K. Shishodia, R.M. Mehra, Solar Energy Materials and Solar Cells 92 (2008) 1639–1645. C. Zhang, Journal of Physics and Chemistry of Solids 71 (2010) 364–369. H. Nian, S.H. Hahn, K.K. Koo, J.S. Kim, S. Kim, E.W. Shin, E.J. Kim, Materials Letters 64 (2010) 157–160. Y. Li, J. Gong, M. McCune, G. He, Y. Deng, Synthetic Metals 160 (2010) 499–503. Z. Ben Ayadi, L. El Mir, K. Djessas, S. Alaya, Thin Solid Films 517 (2009) 6305–6309. H. Abdullah, M.N. Norazia, S. Shaari, J.S. Mandeep, Thin Solid Films 518 (2010) 174–180. W. Daranfed, M.S. Aida, A. Hafdallah, H. Lekiket, Thin Solid Films 518 (2009) 1082–1084. T.M. Hammad, J.K. Salem, R.G. Harriso, Applied Nanoscience 2 (2012), http://dx.doi.org/10.1007/s13204-012-0077-9. M. Subramanian, M. Tanemura, T. Hihara, T. Jimbo, Chemical Physics Letters 487 (2010) 97–101. A. Jain, P. Sagar, R.M. Mehra, Solid State Electronics 50 (2006) 1420–1424. D.H. Zhang, T.L. Yang, J. Ma, Q.P. Wang, R.W. Gao, H.L. Ma, Applied Surface Science 158 (2000) 43–48. A. Hafdallah, F. Yanineb, M.S. Aida, N. Attaf, Journal of Alloys and Compounds 509 (2011) 7267–7270.