Deposition of superconducting CeCoIn5 thin films by co-sputtering and evaporation

Physica C 469 (2009) 52–54

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Deposition of superconducting CeCoIn5 thin films by co-sputtering and evaporation A.G. Zaitsev a,*, A. Beck a, R. Schneider a, R. Fromknecht a, D. Fuchs a, J. Geerk a, H. v. Löhneysen a,b a b

Forschungszentrum Karlsruhe, Institut für Festkörperphysik, D-76021 Karlsruhe, Germany Physikalisches Institut, Universität Karlsruhe, D-76128 Karlsruhe, Germany

a r t i c l e

i n f o

Article history: Received 20 October 2008 Accepted 10 November 2008 Available online 18 November 2008 PACS: 74.70.Tx 74.78.Db

a b s t r a c t Thin superconducting films of CeCoIn5 were prepared in situ by simultaneous thermal evaporation of indium and dc magnetic field assisted sputtering of planar metallic Ce and Co targets. To achieve an effective sputtering of the magnetic Co target a special geometry with two facing planar targets (Ce and Co) and magnetic field perpendicular to the targets was used. The stoichiometric (0 0 1)-oriented CeCoIn5 films were grown on r-cut sapphire substrates with a high-rate of 100 nm/min. The temperature dependence of the electrical resistivity revealed the characteristic heavy-fermion behavior and a superconducting transition at about 2 K in agreement with the literature data for CeCoIn5 bulk material and thin films. Ó 2008 Elsevier B.V. All rights reserved.

Keywords: Superconducting thin films Sputtering Heavy-fermion materials

Since the discovery of the first heavy-fermion superconductor CeCu2Si2 [1] much research effort was focused on the investigation and explanation of the superconducting properties of these materials. However, several aspects including the coupling mechanism are still the subject of controversial discussion [2–4]. Research in this field may therefore benefit from the availability of high-quality thin film samples with high superconducting transition temperature, Tc, at ambient pressure. The highest Tc of a heavy-fermion superconductor reached so far at ambient pressure is about 2.3 K for the bulk CeCoIn5 [5] sintered in excess indium environment at around 800 °C. However, the preparation of thin films of CeCoIn5 faces a number of technical problems. The synthesis temperature of the bulk material suggests reasonable substrate temperatures of at least 300–500 °C which is far above the melting temperature of indium (156 °C). One may expect a poor sticking coefficient of indium at the surface of the growing film and, therefore, the need for excess supply of indium to the substrate. This makes the use of stoichiometric CeCoIn5 sources either for evaporation or for sputtering very unfavorable. In the case of sputtering, the conventional multi-target magnetron techniques are hindered by the magnetism of cobalt. In addition, the extremely fast oxidation of pure Ce requires an ambitious ultra high vacuum environment, unless a high film growth rate can be achieved. So far only the molecular beam epitaxy (MBE) from three Knudsen cells (Co, Ce and In) allowed a successful preparation of high* Corresponding author. Tel.: +49 7247 82 3284; fax: +49 7247 82 4624. E-mail address: [email protected] (A.G. Zaitsev). 0921-4534/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2008.11.001

quality superconducting CeCoIn5 films [6,7]. The films were grown on r-cut sapphire [6] and on (1 0 0) MgO buffered with an epitaxial Cr layer [7]. They exhibited (0 0 1) textured structure, a micrograin surface morphology and a temperature dependence of the electrical resisitivity typical for the heavy-fermion materials. In both cases Tc around 2 K was obtained. In this paper the successful use of sputtering in combination with thermal evaporation for the preparation of CeCoIn5 films with similarly good properties is reported. The film deposition system is sketched in Fig. 1. It was assembled in a vacuum chamber with residual gas pressure below 10 7 mbar. Cerium and cobalt were dc sputtered in argon atmosphere from commercially available planar metal targets (37 mm in diameter and 4 mm thick), while indium was thermally evaporated. The evaporation cell was made of boron nitride and heated with a tungsten wire imbedded in the cell walls. A Pt–Rh thermocouple controlling the cell temperature was also imbedded there. To achieve an effective evaporation of In, the cell should be heated to above 900 °C. The substrate, 10  20 mm2 r-cut sapphire, was fixed with Ag paste on a stainless steel heater. The substrate and the evaporation cell were located at the same height 60 mm below the target edge, see Fig. 1. Therefore, the transport of indium atoms to the substrate was possible only via diffusion through the Ar sputtering gas, and the deposition rate of In on the substrate was dependent on the gas pressure (typically 3  10 2 mbar). In high vacuum no indium reached the substrate. For simultaneous sputtering of Ce and Co we used the facingtarget magnetron system proposed for high-rate dc sputtering of

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magnetic targets by Naoe et al. [8]. Like most sputtering systems it employs a magnetic field for trapping electrons in order to increase the gas ionization and therefore the sputtering rate of the targets. In the system of Naoe et al. [8] the magnetic field is directed from one planar target to the other. The secondary electrons emitted from the targets under ion bombardment cannot escape across the magnetic field lines, but oscillate between two negatively charged targets and ionize the gas there. In such a system the magnetic field geometry and, therefore, the system efficiency do not depend upon whether the targets themselves are ferromagnetic or not. In the present work the targets were spaced 20 mm apart from each other, see Fig. 1. A magnetic field of 0.1 T between the targets was produced by two Nd–Fe–B permanent magnets. However, unlike Naoe et al. [8], we did not use two identical targets, but one of Co (ferromagnetic) and the other one of Ce (non-magnetic). The targets were fed from two separate dc power supply sources, which allowed us to apply different sputtering power to each target in order to adjust the Ce:Co ratio of the deposited film. Since the electric discharge in the described system is sustained by the continuous oscillation of electrons between two targets, one might expect that disturbing the systems symmetry, either by using different target materials (different coefficients of secondary electron emission) or by applying differing voltages to the two targets, could disturb the glowing of the discharge. However, we found that the system tolerates a rather high asymmetry. The use of two different target materials did not disturb the symmetry of the discharge at all. As long as the dc power applied to the Ce and Co targets was the same, the discharge voltages and, hence, the discharge currents, at both targets were also identical. The application of differing sputtering powers to the targets did affect the discharge, but effective sputtering was still possible. A realistic range for the variation of the dc power, the current, and the voltage is shown in Fig. 2. In that case, keeping the constant dc power of 25 W at one target, either Ce or Co, allowed the variation of the dc power at the other target from 15 W to 60 W without loss of discharge stability. Outside this range the discharge was extinguished. Remarkably, if strongly different dc powers were applied to each target, the currents on both targets had similar values, however, the voltages were strongly different, see Fig. 2. For comparison we measured the same dependences for the case of two identical Co targets and found them very similar to those shown in Fig. 2.

Fig. 2. An example for the variation range of the dc power, the current, and the voltage at two cathodes of the facing-target sputtering system. Voltages and currents on both targets are shown as a function of the dc power on the Ce target. The power on the Co target was constant at 25 W. Ar pressure was 3  10 2 mbar. Full circles: voltage at the Ce target (left axis), open circles: current at the Ce target (right axis), full triangles: voltage at the Co target (left axis), and open triangles: current at the Co target (right axis).

By varying the sputtering power at each target we were able to adjust the Ce:Co ratio at the substrate within the range between 1:2 and 2:1. The optimal ratio Ce:Co = 1:1 was obtained for 30 W at the Ce target and 25 W at the Co target. Under these sputtering conditions we optimized the In evaporation temperature to 1050 °C for obtaining the stoichiometric CeCoIn5 films on substrates heated to 550 °C, which was the optimal temperature for the formation of the proper tetragonal crystalline structure. The films were prepared in situ. No further annealing steps were performed. The film thickness was typically 400 nm. The film growth rate was 100 nm/min, i.e., 80 times higher than for the MBE-prepared CeCoIn5 films [6]. The composition of the obtained films was determined by Rutherford backscattering (RBS) of 2 MeV He+ ions and energy dispersive X-ray (EDX) analysis with a 20 kV electron beam. The film surface imaging was carried out with scanning electron microscopy (SEM). The crystalline structure of the films was examined by X-ray diffraction (XRD) analysis. The electrical resistivity of the films was measured by the four-point technique as a function

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Fig. 1. Schematic representation of the film deposition system. The cathodes consist of the two permanent magnets (1), the water-cooled iron bars (2), grounded shields (3), and the Ce and Co targets (5). The shadowed area (4) between the targets depicts plasma. The indium evaporation cell (6), is located 20 mm in front of the substrate heater with the substrate (7).

Fig. 3. XRD pattern obtained for a CeCoIn5 thin film on r-cut sapphire using Cu Ka1 radiation. The peaks of CeCoIn5 are denoted by their crystallographic indices and the letters ‘‘F”. Letters ‘‘S” denote the substrate peaks. The peaks of the cerium oxide grains are denoted by ‘‘CeO2” and their indices. The inset shows the normalized rocking curves of the (0 0 3), (0 0 4), (0 0 6), and (0 0 7) peaks.

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Fig. 4. SEM image of a CeCoIn5 thin film surface. The contrast demonstrates the grain size and the large-scale homogeneity of the sample.

of temperature between 300 K and 1.2 K using a liquid He bath cryostat. According to the results of the h–2h XRD measurements, see Fig. 3, the resultant CeCoIn5 films were predominantly (0 0 1) oriented, with minor amounts of (1 1 2), (1 1 1), (1 0 0) and (1 0 2) oriented grains. The Bragg reflections yielded the lattice parameters of c = 7.530 Å and a = 4.606 Å, which are considerably lower (2– 3%) than the bulk CeCoIn5 values [5–7]. The observed h–2h peaks of CeCoIn5 were quite sharp, e.g., the full width at half maximum (FWHM) of the (0 0 3) peak was 0.1°. This indicates that neither the lattice was nonuniformly distorted, nor the grain size was small compared to the film thickness. The films were free of CeIn3 phase, which was often present in indium deficient films. Small amounts of foreign phases were present, mostly CeO2 inclusions of various orientations (note the logarithmic intensity scale in Fig. 3). The rocking curve measurements indicated strong (0 0 1) alignment of the CeCoIn5 phase. The rocking curves of different (0 0 l) CeCoIn5 reflections exhibited an identical form, which is clearly seen in the inset of Fig. 3, were the curves are normalized to their maximum intensity as I(x)/I(x0) and to their position as (x x0). The mosaic spread of the films determined from the FWHM of these rocking curves amounts to 1°, which is considerably lower than 1.8° of the MBE-prepared CeCoIn5 films on r-cut sapphire [6]. According to the /-scan XRD measurements the (0 0 1) CeCoIn5 grains did not show any pronounced in-plane alignment. A SEM image of the CeCoIn5 film surface is shown in Fig. 4. The film consists of fine grains with size of several micrometers. This surface morphology is very similar to that of the MBE-grown CeCoIn5 films [6,7]. Fig. 5 shows the temperature dependence of the electrical resistivity. The order of magnitude and the temperature dependence, q(T), agree well with the literature data of bulk [5] and thin film [6,7] CeCoIn5. Cooling the sample down from room temperature resulted in a slow decrease of q(T) with a very broad minimum around 170 K followed by an increase of resistivity, which is com-

Fig. 5. Temperature dependence of the resistivity of a CeCoIn5 thin film. The inset provides a closer view on the superconducting transition.

monly attributed to the Kondo effect [5,7]. Further cooling leads to a broad maximum at 35 K and an almost linear decrease of the resistivity with temperature, followed by the superconducting transition between 2 K and 1.7 K. Both the transition temperature and the transition width are similar to those of the MBE-prepared CeCoIn5 films [5,7]. In summary, using a relatively simple equipment for simultaneous thermal evaporation of In and co-sputtering of Co and Ce, we successfully prepared highly (0 0 1)-oriented superconducting CeCoIn5 thin films on r-cut sapphire substrates. The quality of the films, including the crystalline structure, the surface morphology, the temperature dependence of the electrical resistivity, and the superconducting transition temperature, was similar to those of CeCoIn5 films prepared by molecular beam epitaxy. The application range of the facing-target sputtering system based on electron oscillation was expanded allowing the ’asymmetrical’ operation with two different targets biased with different voltages.This operation mode allows an effective control of the two-component film stoichiometry over a broad range. Acknowledgements The authors acknowledge valuable discussions with M. Huth, O.K. Soroka, and T. Wolf. Technical support by F. Ratzel is gratefully acknowledged. References [1] F. Steglich, J. Aarts, C.D. Bredl, W. Lieke, D. Meschede, W. Franz, H. Schäfer, Phys. Rev. Lett. 43 (1979) 1892. [2] M. Jourdan, M. Huth, H. Adrian, Nature (London) 398 (1999) 47. [3] N.K. Sato, N. Aso, K. Miyake, R. Shiina, P. Thalmeier, G. Varelogiannis, C. Geibel, F. Steglich, P. Fulde, T. Komatsubara, Nature (London) 410 (2001) 340. [4] J. Geerk, H.v. Löhneysen, Phys. Rev. Lett. 99 (2007) 257005. [5] C. Petrovic, P.G. Pagliuso, M.F. Hundley, R. Movshovich, J.L. Sarrao, J.D. Tompson, Z. Fisk, P. Monthoux, J. Phys.: Condens. Matter 13 (2001) L337. [6] O.K. Soroka, G. Blendin, M. Huth, J. Phys.: Condens. Matter 19 (2007) 056006. [7] M. Izaki, H. Shishido, T. Kato, T. Shibauchi, Y. Matsuda, T. Terashima, Appl. Phys. Lett. 91 (2007) 122507. [8] M. Naoe, S. Yamanaka, Y. Hoshi, IEEE Trans. MAG-16 646 (1980); M. Matsuoka, Y. Hoshi, M. Naoe, S. Yamanaka, IEEE Trans. MAG-16 800 (1984).