Ferromagnetic Co-doped ZnO thin films grown using pulsed laser deposition from Zn and Co metallic targets

Ferromagnetic Co-doped ZnO thin films grown using pulsed laser deposition from Zn and Co metallic targets

Materials Science and Engineering B 109 (2004) 192–195 Ferromagnetic Co-doped ZnO thin films grown using pulsed laser deposition from Zn and Co metal...

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Materials Science and Engineering B 109 (2004) 192–195

Ferromagnetic Co-doped ZnO thin films grown using pulsed laser deposition from Zn and Co metallic targets W. Prellier∗ , A. Fouchet, Ch. Simon, B. Mercey Laboratoire CRISMAT, CNRS UMR 6508, 6 Boulevard Marechal Juin, F-14050 Caen Cedex, France

Abstract We report the synthesis of high-quality Co-doped ZnO thin films using the pulsed laser deposition technique on (0 0 0 1)-Al2 O3 substrates under an oxidizing atmosphere, using Zn and Co metallic targets. The growth of ZnO was optimized in order to obtain the least strained film using the sin2 Ψ model. Highly crystallized Co:ZnO thin films are obtained by an deposition from alternating Zn and Co metal targets. This procedure results in homogenous incorporation of the Co in the ZnO wurtzite structure, which is confirmed by the linear dependence of the out-of-plane lattice parameter as a function of the Co dopant up to 9%. The Zn1−x Cox O films exhibit ferromagnetism with a Curie temperature close to room temperature for x = 0.08 and at 150 K for x = 0.05. © 2003 Elsevier B.V. All rights reserved. Keywords: Diluted magnetic semiconductors; ZnCoO; Thin films; Pulsed laser deposition

1. Introduction Diluted magnetic semiconductors (DMS) of the type III–V or II–VI have been obtained by doping semiconductors with magnetic impurities such as Mn [1,2]. These materials are very interesting due to their potential applications for spintronics [3], but are limited by their low Curie temperatures (TC ) has limited their interest [4]. Based on the theoretical works of Dietl et al. [5], several groups have studied the growth of Co-doped ZnO or TiO2 films [6–9] which is a good candidate having a high TC [5]. Using pulsed laser deposition (PLD), Ueda et al. reported ferromagnetism (FM) above room temperature [6], while Jin et al. found no indication of FM by utilizing laser molecular beam epitaxy [7]. This difference in results may originate in the used growth method used and/or from the growth conditions (oxygen pressure, deposition temperature, etc. . . . ). In the particular case of the PLD technique, it may also arise from the target preparation, which has not been considered up to this point. One reason to consider the target is that the control of the dopant incorporation would be difficult using a pre-doped ceramic oxide target [10]. This is a crucial point since the properties of the DMS are very sensitive to the ∗ Corresponding author. Tel.:+33-2-31-45-26-07; fax: +33-2-31-95-16-00. E-mail address: [email protected] (W. Prellier).

0921-5107/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2003.10.039

concentration of dopant [11]. The homogeneity of dopant incorporation as well as the precise control of the film growth may be responsible for the changes in the physical properties of the films obtained by the different groups. Therefore, the objective of this investigation is to develop an accurate method to grow Co:ZnO films with a precise control of the doping and to understand their properties. To achieve such a goal, Co-doped ZnO films were deposited from two pure metal targets of Zn and Co and our results are reported in this communication.

2. Experimental The Zn1−x Cox O thin films were grown using the pulsed laser deposition technique. Zinc (99.995%) and cobalt (99.995%) targets were used as purchased (NEYCO, France) without further preparation. The films were deposited using a KrF laser (λ = 248 nm) [12] on (0 0 0 1)-oriented Al2 O3 substrates. The substrates were kept at a constant temperature in the range 500–750 ◦ C during the deposition which was carried out under an atmosphere of approximately 0.1 Torr of pure oxygen. After deposition, the samples were slowly cooled to room temperature at a pressure of 300 mbar of O2 . The laser rate was 3 Hz and the energy density was close to 2 J/cm2 . The composition of the film was checked by EDS and corresponds to the nominal one to the limit of

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the accuracy. Structural studies were performed by X-ray diffraction (XRD) using a seifert XRD 3000P for the Θ–2Θ scans and an Phillips X’Pert for the in-plane measurements (Cu K␣1, λ = 1.5406 Å). To grow Zn1−x Cox O films, we fired m pulses of Co and n pulses of Zn. Various compositions of Zn1−x Cox O have been grown with different m/n ratios that correspond to a x-value. The sequence is repeated until a typical thickness of 1000 Å is obtained.

dh k l , is normal to the diffraction vector L and one can define the strain ε, along this diffraction vector dh k l ε=

dh k l − d0 d0

The resulting XRD pattern of ZnO is shown in Fig. 1. The two diffractions peaks observed around 2Θ close to 34.48 and 72.66◦ , are characteristic of the hexagonal ZnO wurtzite, the c-axis being perpendicular to the substrate plane. The out-of-plane lattice parameter is calculated to be 5.2 Å which corresponds to the theoretical bulk value [13]. The sharp and intense peaks observed indicate that the films are highly crystallized which is also confirmed by the low value of the full-width at half maximum (FWHM) of the rocking curve recorded around the (0 0 2)-reflection equal to 0.26◦ . The epitaxial relationship between the ZnO films and the Al2 O3 substrate were determined using asymmetrical XRD (see inset of Fig. 1). The peaks recorded around the {1 0 3} are separated by 60◦ , indicating a six-fold symmetry with a rotation of 30◦ of the ZnO symmetry in the plane, with respect to the sapphire substrate. The average in-plane lattice parameter of ZnO is 3.25 Å and the FWHM of the peaks in the Φ-scan of ZnO is small (0.68◦ ). This value is close to previous values reported in the literature [14]. In order to obtain additional information on the structural properties of the ZnO films, we have determined the thin film strain by measuring the distance between atomic plane of a crystalline specimen as an internal strain gauge [15]. The plane spacing,

(1)

where d0 is the unstressed d-spacing of the (h k l) planes (i.e. the bulk value). To measure the stress, the sin2 Ψ technique was used [15]. Briefly, in this model, the strain is defined as follows: ε = A sin2 Ψ + B

3. Results

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(2)

(where A and B are constants that depend on the strain along the surface direction, the Young’s modulus, Poisson’s ratio and the stress along the direction, for details see [15]). The advantage of this technique is that whenever a Bragg peak can be obtained experimentally, one can uniquely calculate the strain along the diffraction vector. Fig. 2a and b show respectively the evolution of the strain ε as a function of sin2 Ψ for ZnO films grown at different temperatures and various oxygen pressures. The strain increases with the temperature, indicating that the film is more strained along the in-plane direction, since the out-of-plane lattice parameter remains almost constant whatever the growth conditions. A similar conclusion is obtained for the dependence of the pressure (see Fig. 2b). In other words, a deposition temperature of 600 ◦ C and a pressure of 0.1 Torr show the lowest average stress for all reflections. Thus, to minimize the substrate-induced strain of the ZnO, we deposited the film at 600 ◦ C under 0.1 Torr of O2 . The latter conditions were also used to synthesize Co-doped ZnO films. In each case the film is single phase and highly crystallized (the FWHM is always around 0.25◦ ). In the XRD patterns, only the diffraction peaks corresponding to the Zn1−x Cox O phase are observed suggesting that Co clusters are not present. This is confirmed by X-ray topography recorded in with scanning transmission electron

Fig. 1. Room temperature Θ–2Θ XRD pattern of ZnO film on Al2 O3 (0 0 0 1). The inset depicts the Φ-scan of the {1 0 3} family of peaks of a typical ZnO film showing the in-plane orientation of the film.

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Fig. 3. Evolution of d0 0 2 in Zn1−x Cox O films as a function of the Co content (x). The line is a guide for the eyes. The plateau above 10% indicates that the limit of the solid solution is around 9%.

Fig. 2. ε vs. sin2 Ψ for ZnO films grown at various (a) deposition temperatures and (b) different oxygen pressure. The values of (h k l) planes corresponding to the measured sin2 Ψ are indicated. The dotted lines are only a guide for the eyes.

microscope (JEOL 2010F) of the Co:ZnO films, in which a homogenous dispersion Co is observed. The details of the structural and microstructural characterizations will be presented elsewhere. Moreover, it has been shown that a high

oxygen pressure of (0.1 Torr in the present case) reduces the Co clusters formation [16]. The out-of-plane lattice parameter of the Co:ZnO films increases almost linearly as a function of the Co substitution up to 9%, suggesting that the Co2+ replaces Zn2+ within the wurtzite structure (Fig. 3). Regarding this curve, it seems that the limit of the solution is close to 9%, since for higher Co content, the lattice parameter does not change. The magnetic properties of these thin film samples were measured using a SQUID magnetometer. Fig. 4 shows the M(T) recorded for a 5 and 8% Co-doped ZnO film. The ferromagnetic behavior is observed but the hysteresis of the magnetization is very small (about few Gauss). The M(T) curves (Fig. 4) clearly show that the film is ferromagnetic with a Curie temperature around 300 K for x = 8% and close to 150 K for x = 5%. The transition from the ferromagnetic state to the paramagnetic state is clearly seen,

Fig. 4. M(T) of the Zn1−x Cox O films (x = 0.05 and 0.08 of Co) recorded under a field of 2000G.

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suggesting that metallic Co clusters are not responsible for the effect observed at 300 K since the TC of the metal Co clusters is above 1000 K [8,9]. Moreover, the saturation moment (0.7 ␮B/mol Co for x = 8% and 1.2 ␮B/mol Co for x = 5%) are lower compared to the 1.7 ␮B of metallic Co[0] , suggesting that the Co state should be close to Co2+ . The increase of the out-of-plane lattice parameter as the cobalt content increases is also in favor of Co2+ [17]. We believe that this is due to the technique used in the study where not only the conditions of the deposition minimize the strains but also the alternately deposition from the two targets favors the homogeneity of the doped films. Moreover, it has been seen that the low temperature of deposition leads to homogenous films [8]. 4. Conclusion In conclusion, we have developed an alternative method for the growth of pulsed laser deposited ZnO thin films. This method permits an accurate control of the dopant in the matrix. To illustrate this procedure, we firstly synthesized high quality ZnO thin films on Al2 O3 (0 0 0 1) substrates and minimized the substrate-induced strain by optimizing the growth conditions. Secondly, we utilized this process to deposit high quality Zn1−x Cox O films with a Curie temperature close to room temperature for x = 8%. The growth of these ferromagnetic films opens the route for the fabrication of spin-based electronics since this original method can be used to grow various oxide thin films. Acknowledgements One of us (WP) greatly acknowledges partial support from IFPCAR/CEFIPRA (Indo-French Centre for the Promotion

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of Advanced research/Centre Franco-Indien pourla Promotion de la Recherche Avancée).

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