Grown of ZnO: Ce Layers by Spray Pyrolysis Method

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Grown of ZnO: Ce Layers by Spray Pyrolysis Method for Nonlinear Optical Studies Z. Sofiani1,3, B. Derkowska2 P. Dalasiński2, Z. Łukasiak2, K. Bartkiewicz2, W. Bała2, M. Addou3 A. Lamrani Mehdi3, L. Dghughi3 I. V. Kityk4 and B. Sahraoui1 1 Laboratoire POMA, Equipe Photonique de Puissance, UMR CNRS 6136 Université d’Angers. 2, Boulevard Lavoisier, 49045 Angers, France 2 Institute of Physics, N. Copernicus University, Grudziądzka 5/7, PL 87-100 Toruń, Poland 3 Laboratoire Optoélectronique et Physico-Chimie des Matériaux Université d’Ibn Tofail. BP 133, 14000 Kénitra, Maroc 4 Institute of Physics, J. Dlugosz University Czestochowa Al. Armii Krajowej 13/15 Czestochowa, Poland ABSTRACT We have investigated the linear and nonlinear optical properties of high quality cerium-doped zinc oxide films (ZnO:Ce). The layers were grown by the reactive chemical pulverization spray pyrolysis technique using zinc and cerium chlorides as precursors at temperature up to 450 oC. The influence of Ce concentration on the structural, linear and nonlinear optical properties of ZnO thin films is presented. The films were characterized by X-ray diffraction, scanning electron microscope and photoluminescence measurements. The X-ray diffraction analysis indicates that all films are polycrystalline in nature and clearly shows the appropriate incorporation of the Ce atoms in the ZnO films. The third order nonlinear optical properties, which are the main subject of this investigation, were studied. For this propose, the third harmonic degeneration (THG) technique has been employed. A laser source has been used for the fundamental beam at 1064 nm so that the generated third harmonic signal is made at 355 nm. Keywords: ZnO, spray pyrolysis, photoluminescence, X-ray diffraction, THG. 1. INTRODUCTION There has been great interest in the oxide semiconductor and its ternary alloys in recent years for its potential application in optoelectronics. ZnO presents interesting electrical, optical, acoustic and chemical properties, which find wide applications in acoustic materials and short wavelength optical devices [1, 2]. Stoichiometric zinc oxide is an insulator that crystallizes from the wurtzite structure to transparent form of needle-shaped crystals. ZnO is a direct band-gap II–VI semiconductor material having the energy gap of 3.37 eV at room temperature with high exciton binding energy (60 meV) [3-5]. It exhibits good piezoelectric, photoelectric and optical properties. The structure contains large voids, which can easily accommodate interstitial atoms. Consequently, it is virtually impossible to prepare really pure crystals. Until now, ZnO thin films have been prepared by many different techniques such as chemical vapor deposition (CVD) [6], sputtering [7], laser deposition [8], sol–gel [9], as well as the spray pyrolysis deposition technique [10-12]. Spray pyrolysis is an ideally suited synthetic method for preparation of ZnO samples due to its many advantages over more conventional synthetic techniques including (1) formation of non-agglomerated particles without milling, (2) control over particle size and particle size distribution, (3) control over crystallinity (grain size), and (4) formation of homogeneous materials. These advantages are primarily due to the temperature/time history available through spray pyrolysis. In this present paper, we report the linear and nonlinear optical properties of high quality cerium-doped zinc oxide films (ZnO:Ce) grown by spray pyrolysis technique. We have shown the dependence of the influence of Ce concentration on the structural and optical properties of ZnO thin films. The films were studied using X-ray diffraction (XRD), scanning electron microscope (SEM), and photoluminescence (PL) measurements. The X-ray diffraction analysis indicates that all films are polycrystalline in nature and clearly shows the appropriate incorporation of Ce atoms into the ZnO films. The characteristics of surface morphology of the studied films deposited on glass and silicon substrates will be presented by using SEM technique. The dopants impurities, introduced into the ZnO nanostructures, can modulate the local structure and cause the dramatic change, which can lead to decrease of their optical properties and photoluminescence. The photoluminescent spectra of the films have been studied as a function of doping concentrations. Atomic force microscopy measurements indicate that the surface of the films is very flat. The chemical composition of the films as determined by energy dispersive spectroscopy is also reported. The ZnO layers display three major PL peaks: an UV near-band edge emission peak at 378 nm, a green emission around 510 nm, and a red emission around 650 nm. The green and red emissions are probably associated with the oxygen vacancies and interstitial Zn ions in the ZnO lattice. The influences of Ce concentration on the nonlinear optical properties using THG technique will be presented.

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2. EXPERIMENTAL The films of ZnO doped cerium used for the present work were grown using the spray pyrolysis technique [11]. The structural characterisation of ZnO films was analyzed by X-ray diffraction pattern obtained from a diffractometre Intel (Model XRG3000, Cu Kα radiation, λ = 105406 A). The surface morphology and uniformity of the films were measured by field emission scanning microscope. The photoluminescence was measured by SPM-2 monochromators at room temperature. The optical properties of ZnO thin films were characterized by photoluminescence with He-Cd laser (Omnichrom) as a light source using an excitation wavelength of 325 nm and a power of 25 mW. We have measured the XRD 2θ scan spectra for 200 nm thick films of ZnO with different content of Ce deposited on glass substrates. We can see that the peak intensity of the (0002) reflection is changed with increase of the Ce concentration. These results illustrate that there is a good crystalline structure with preferential orientation along the (0002) axis in the ZnO films. The peaks diffracted only from c-planes are observed, which reveals the growth of highly c-axis oriented films on the substrates. The locations of the (0002) peaks and the full width at half maximum (FWHM) are given in Table. 1. Table. 1. Influence of Ce concentration on various characteristics of ZnO films. Thin film un-doped ZnO ZnO: Ce2% ZnO: Ce5% ZnO:Ce10% ZnO: Ce15% ZnO: Ce20% Peak

34.48

34.39

34.34

34.40

34.10

34.42

FWHM

0.20o

0.20o

0.23o

0.20o

0.20o

0.20o

Fig. 1 shows the photoluminescent spectra of the 200 nm thick ZnO films as a function of the concentration of the impurities It is commonly accepted that the UV emission at about 390 nm in ZnO films originates from the near band-edge transition, namely the recombination of free excitons through an exciton-exciton collision process [13]. The emission in the blue band at 437 nm may come from the electron transition from the shallow donor level of oxygen vacancies to the valence band, and electron transition from the shallow donor level of zinc interstitials to the valence band. The emissions in the wavelength band of 450 – 500 nm may originate from the electron transition from the electron transition from the level of the ionized oxygen vacancies to the valence vacancies.

5000

Photoluminescence Zn0:Ce

ZnO:Ce2%

ZnO:Ce5%

Intensity [a. u.]

4000

ZnO:Ce10%

3000

ZnO:Ce20% ZnO:Ce15%

2000

ZnO0%

1000 0

400

600

800

Wavelength [nm]

Fig. 1. Photoluminescence of the ZnO and ZnO:Ce thin film as a function of the wave length Surface morphologies were investigated by using a scanning electron microscope (SEM). Figure 2 shows the changes of the surface morphology with the increase of Ce concentration.

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Fig. 2. SEM images of the ZnO and the ZnO:Ce grown at the temperature 450 deg. and different concentration of the cerium impurities with 2% to 20%. 1 is un-doped ZnO, 2, 3, 4, 5 and 6 are 2%, 5%, 10%, 15% and 20%, respectively. From Table 1 and Figure 2, we can see that structure of the thin films is dependent on the concentration of Ce in the ZnO layers. At low and at high concentration, the both films have a good crystalline structure. The surface morphology changes with the contamination of Ce in the layers. For ZnO:Ce2% and ZnO:Ce20%, smooth surfaces (Fig. 2.1, 2.6) were observed. However for the other concentrations, the films at 5% and 10% exhibit a rough surface with spherical and hexagonal grains (Fig. 2.3, 2.4), and at the 15% of the impurities (Fig. 2.5), some kind of rods are observed. The diameter of the rods were about 0,5 µm. 3. THIRD HARMONIC GENERATION (THG) EXPERIMENTS The THG is a third order nonlinear optical process, in which the fundamental beam at fundamental wavelength (λ) interacts with the nonlinear medium and then a beam at the wavelength λ/3 can be created [14]. Because of the high frequencies involved, the third harmonic generation can probes purely coherent, electronic nonlinearity. It also permits to explore the absorption edge of the material without causing damages to the sample with the strong fundamental beam. THG is sensitive only to ultrafast electronic mechanisms of nonlinear response, while being insensitive to slower effects, such as thermal ones. Assuming that the TH is generated in a very thin film i.e., the film thickness is much less than a confocal parameter of the laser beam b, the intensity of the generated TH is given by the following formula:

χ (3) = Cte.

A1 =

N 3(ω2) + N ω( 2) N 3(ω3) + N 3(ω2)

A3 I 3ω A1{exp[i3ωl ( N ω( 2) − N 3(ω2) ) / c] − 1}

⎛ N (1) − N 3(ω2) A 3 = 1 − ⎜ 3(1ω) ⎜ N + N ( 2) 3ω ⎝ 3ω

N ωj ,3ω = nωj ,3ω cos θ ωj ,3ω

⎞⎛ N 3(ω3) − N 3(ω2) ⎟⎜ ⎟⎜ N (3) + N ( 2) 3ω ⎠⎝ 3ω

⎞ i 6ωN ( 2 )l / c 3ω ⎟e ⎟ ⎠

(1)

j=1,2,3 : mediums

where nω and n3ω are the refractive indices of the film at the fundamental frequency and the frequency of the TH, respectively, λω is the wavelength of the fundamental beam, l is the film thickness, χ(3) is the nonlinear optical susceptibility of the film, ∆k is the wave vector mismatch between the fundamental and the TH waves in the film, and Ιω is the intensity of the incident beam. When the film thickness is much less than the coherence length, lc = 2π/∆k, the intensity of the TH becomes proportional to the square of the film thickness. To achieve the high conversion efficiency of the TH, one has to increase the incident intensity of the laser, use material with the highest possible nonlinear optical coefficient χ(3), and use a good quality of the film surface. We can explain this need to increase the intensity is that for a certain threshold level of energy will damage the material, so it is why there is need to work with wide band gap materials, which have shown a resistance up to level of >1 J/cm2 . Competing nonlinear optical effects, such as self-phase modulation, which typically lead to destruction of useful nonlinear optical mixing processes, are significantly reduced in thin films, because of a limited distance of light interaction with a nonlinear material.

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The properties of the third harmonic generation were studied for a fundamental wavelength at 1,064 µm from a Q-switch Nd:YAG laser (Model: Quantum elite) with a pulse duration of 15 ps, and a power of 1,62 mJ per pulse at the repetition rate of 10 Hz. The polarization and the power of the fundamental beam with the half wave plate were seated between two polarizers. The power of the fundamental beam was measured with a power metre (Model: Gentec). The beam was focalised on the sample passing through a lens with a focal of 20 cm. The third harmonic signal was detected by a tube photomultiplier (Model: Hamamatsu) than integrated by a box-car and processed by a Computer. The nonlinear medium (sample) was turned by the step motor (Fig. 3). Nd:YAG λω=1064nm cm LASER

BS1

Lens F=20 cm

λ/2 P

PMT

Sample

F

BS2 1ω 3ω

1ω

Phc

Phs

3ω

PC and acquisition board

Fig. 3. Experimental setup for the THG; BS1,2 are dichroic beam splitters, l/2 is a half-wave plate, F is a 355 nm filter, Phs and Phc are photodiodes of synchronisation and control respectively and PMT is the tube photomultiplier. 4. RESULTS AND DISCUSSION: Fig. 4 and Fig. 5 show that the third harmonic generation is dependent of the concentration of the impurities in the ZnO layers. We can see that the THG signal decrease when concentration of impurities increase (see Fig. 5.a.b, ZnO:Ce2%, ZnO:Ce5% and ZnO:Ce10%), until about 7% and next we can observe that the THG signal is slowly increase. We can deduce that the THG signal is dependent on the crystalline quality and concentration of dopants. In our experiments, the strongest TH signal was observed for the film at the concentration 2% of the Cerium. For this concentration the film has a poor crystalline but the surface of the film is smooth (see Fig. 2.2). The dispersion of the fundamental beam is lower than that from the other films.

TH conversion: ZnO:Ce 1 ZnOCe2% ZnOCe20% ZnOCe15% ZnOCe5% ZnOCe10%

10 susceptibility χ (3).e-13 [a.u.]

TH conversion efficient I3ω/Iω[a.u]

ZnO

9 8 7 6 5 4 3

0,28

0,3

0,32

0,34

0,36

0,38

2

Incident intensity [Gw/cm ]

Fig. 4. Experimentally measured conversion efficiency I3ω/Iω for the ZnO and ZnO doped Cerium films, where I3ω is the intensity of the third harmonic and Iω is the incident intensity for the fundamental beam.

0

5

10

15

20

25

concentration[% ]

Fig. 5. χ(3) values versus concentration for the ZnO and ZnO:Ce thin films.

We find that the efficiency of the third harmonic generation at the maximum power level is high. We can say that the energy of the generated TH depends on the position in the film, the thickness of the films and the point of interaction. We can employ the equation (1) to calculate the value of χ(3) for measured conversion efficiency. Fig. 4 show the experimental data of THG measurement for ZnO and ZnO:Ce thin films. We got the value of χ(3)=9,28x10-13 esu, for the ZnO thin film. This difference is attributed to the nanosized structure of the film. Table.2 gives the values for different concentration of the Ce impurities on the ZnO films, of the third nonlinear susceptibilities

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Table 2. values for the measured χ(3) by the THG technique. Thin film fused Silica un-doped ZnO χ(3) (esu)

4.38x10

-14

9.28x10

-13

ZnO: Ce2% ZnO: Ce5% ZnO:Ce10% 6.38x10

-13

4.61x10

-13

4.49x10

-13

ZnO: Ce15% 5.27x10

-13

ZnO: Ce20% 5.90x10-13

5. CONCLUSION Cerium doped ZnO thin films were prepared by the spray pyrolysis method. All films were oriented preferentially along the (0002) direction. Films at 2% cerium concentration had a stronger c-axis orientation perpendicular to the substrate, larger grain, more smooth surface morphology and higher photoluminescence than the others. The third harmonic generation in thin films of ZnO and ZnO:Ce obtained by spray pyrolysis method at different concentration was studied. It was found that the THG of the films was dependent on the deposited concentration of the impurities. We have shown that at higher concentrations the THG decrease until a certain threshold then begin slightly to increase. But for a special concentration was found that the THG signal was higher than other concentrations. The third harmonic signal is a high dependent of the film growth and smoothness. ACKNOWLEDGEMENT We would like to acknowledge support from, the University of Angers, France, the “Centre de la Recherche Scientifique et Technique”, Rabat, Morocco and the Nicolas Copernicus University, Torun, Poland. REFERENCES [1] C. Klingshirm, Semiconductor Optics. Berlin: Springer, 1995. [2] D.C. Look, B. Claflin, Ya. I. Alivov and S. J. Park, Physica status solidi 201, 10, pp. 2203-2212, 2004. [3] V. Srikant, D.R. Clarke, J. Appl. Phys. 83, 5447, 1998. [4] S. King, J.G.E. Gardeniers, I.W. Boyd, App. Surf. Sci. 811, pp. 96-98, 1996. [5] B. Jin, S. Bac, S. Lee, and S. Im Mater. Sci. Eng. B 71, p.p. 301, 2000 [6] Natsume Y, Sakata H, Hirayama T and Yanagita H Phys.Status Solidi (a) pp. 148-485, 1995 [7] Gupta V and Mansingh A J. Appl. Phys. 80, p. 1063, 1996. [8] Strikant V and Clarke D R J. Appl. Phys. 81, p. 6357, 1997. [9] D.P. Norton, Y.W. Heo, M.P. Ivill, K. Ip, S.J. Pearton, MF, Materials Today, 2004 [10] J. Aronovich, A. Ortiz, and R.H. Bube, J. Vac. Sci. Technol. 16, p. 994, 1979 [11] Stidenikin S A, Golego N and Cocivera M J. Appl. Phys. 84, p. 2287, 1998 [12] A. Bougrine, M. Addou, A. El Hichou, A. Kachouane, J. Ebothé, M. Lamrani and L. Dghoughi, Phys. Chem. News 13 pp. 36-39, 2003. [13] K. Vanheusden, W. Warren, C. Seager, D. Tallant, and J.A. Voigt, J. Appl. Phys., 79, p. 7983, 1996. [14] F. Kajzar, J. Messier and C. Rosilio, J. App. Phys. 60(9), pp. 3040-3044, 1986.