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978-1-4244-4826-5/09/$25.00 ©2009 IEEE 1

Third Order Non Linear Optical Properties of ZnO:Al Thin Films Prepared by Spray Pyrolysis

K. Bahedi1, M. Addou1, M. El Jouad1, Z. Sofiani1, S. Bayoud1 and M. Bouaouda1 B. Sahraoui2, Z. Essaïdi2

1 Laboratoire Optoélectronique et Physico-chimie des Matériaux Université Ibn Tofail Faculté des Sciences BP 133 Kenitra 14000, Morocco

2 Laboratoire POMA, UMR CNRS 6136, Université d’Angers 2, Bd Lavoisier, 49045, France e-mail: bahedikhadija@yahoo.fr

ABSTRACT In this study, Zinc oxide (ZnO) and Aluminium doped zinc oxide (ZnO:Al) thin films were deposited by reactive chemical pulverization spray pyrolysis technique on heated glass substrates at 450°C. X-ray diffraction (XRD), Scanning electron microscopy (SEM) and third harmonic generation (THG) are used to characterize their structure , morphology and nonlinear optical properties as a function of Al concentration (0, 3, 5, 7 at %). The intensity of third order non linear optical susceptibility increase by incorporation of aluminium. A strong value of susceptibility χ(3) was found for films with a good crystallinity. Keywords: ZnO, Al, Spray pyrolysis, THG, Thin films.

1. INTRODUCTION Study of the nonlinear optical properties of the Semiconductors oxides is of particular interest nowadays. ZnO is a transparent conducting oxide with a good third order nonlinear generation [1-5]. It is convenient to consider it as a promising material for various micro and optoelectronics applications. Optical nonlinearities of aluminium doped ZnO oxide thin films were the subject of this work. Many techniques were used for preparation: ration frequency (RF) magnetron sputtering [6,7], sol-gel [8], pulsed laser deposition [9], MOCVD [10], hydrothermal process [11] spray pyrolysis technique[12] etc. The effect on the properties including optical linear properties induced by aluminium as a dopant element has been extensively studied [6,9,11,12]. However a description concerning nonlinear optical properties has been rarely reported. In this paper, we analysed the effect of doping and the relation between the crystallinity, the morphology and the susceptibility value.

2. EXPERIMENT DETAILS The thin films of ZnO and ZnO:Al were deposited by spray pyrolysis on glass substrate. The spraying solution was 0.05 Mof zinc chloride. Aluminium doping was achieved by adding AlCl3 in a concentration of 0, 3, 5, and 7 at %. The substrate temperature was kept at 450 °C. The solution flow rate was 5 ml/min.

The crystalline structure was investigated by X-ray diffraction using Cu Kα radiation (λ = 1054 Å). The surface morphology was studied using scanning electron microscopy (SEM). Third order non linear susceptibility of the samples was performed using third harmonic generation (THG) method described elsewhere [1].

3. EXPERIMENTAL RESULTS XRD measurements of the undoped and aluminium doped ZnO deposited on glass substrates grown at 450°C with an atomic concentration of 3 %, 5% and 7% revealed that all The films are polycrystalline with a hexagonal wurtzite structure Fig. 1. For all the samples the ZnO (002) peak (i.e., 2θ = 34.48°) is only main peak obtained in films, indicating the presence of preferred orientation and no phase corresponding to other oxides was detected. We note that the doped samples are more crystalline than the undoped one. The crystallinity is improved by adding aluminium, particularly for the ZnO: 3% Al sample. The same behaviour has been observed by Z. Ben Ayadi et al. [7].

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Figure 1. X-Ray diffraction spectra of undoped and Al doped ZnO.

We confirmed our results by using SEM images. Figure 2 clearly shows that there is a change in the surface morphology of ZnO films due to a change in the Al quantity.

Figure 2. SEM micrographs of (a) undoped (b) 3% (c) 5% and (e) 7% Al doped ZnO thin film.

Scanning electron microscopy (SEM) images show that 3% Al doped samples have less void and present smooth and compact surfaces covered with mainly hexagonal grains. The increase of Al content in the layer leads to porous and rough surfaces which can predict more scattering light due to the lack of grain boundaries.

Figure 3. THG experimental results of undoped and Al doped ZnO thin films.

c)

a) b)

e)

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The experiment result of THG intensity as function of incident angle is presented in figure 3. It can be seen that the incorporation of the aluminium doping improves the non linear response. According to the work of Wang and al [13] the susceptibility values calculated is reported in Table 1. The doped samples showed the higher values than the undoped. Aluminium is believed to cause an increase of electric conductivity within the crystalline ZnO: Al films. When the electric conductivity increase the value of non linear optical properties increase as it was already explained in our earlier work [2]. The highest susceptibility value χ(3) = 13.41×10-12 (esu) is found for the ZnO:Al 3% samples. This may be explained by the better crystallinity and the roughness surface of the films. This results show that these materials exhibit large third order nonlinear susceptibility value indicates their potential application as NLO materials.

Table 1. Values of the χ(3) for undoped and Al doped ZnO films. Aluminium rate (%) 0 3 5 7

(χ(3) ± 0.1)×1012 [esu] 0.93 13.41 6.17 3.26

4. CONCLUSION In this paper, experimental studies of third order NLO properties of Al doped ZnO prepared by spray pyrolysis method were performed using THG method. The best value of susceptibility χ(3) = 13.41×10-12 (esu) was found for 3% doped samples. It has been found a deep correlation between the structures, morphology and the nonlinear optical properties.

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L. Dgoughi, Applied Surface Science 255 (2009) 4693-4695. [4] V. Narayanan, R. K. Thareja, Opt. Commun. 260, 170 (2006). [5] C. Y. Liu, B. P. Zhang, N. T. Binh, Y.Segawa, Opti.Commun. 237, 65 (2004). [6] Y.C. Lin, M.Z. Chen, C.C. Kuo, W.T. Yen, Colloids and Surfaces A: Physicochemical and Engineering

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(2008)162-166. [13] X. H. Wang, D. P. West, N. B. McKeown, and T. A. King, J. Opt. Soc. Am. B, vol. 15, (1998), 1895-1903.