Mo thin films

Physica C 469 (2009) 1005–1008

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Thickness dependence of superconductivity for In/Mo thin films K. Makise a,*, T. Nakamura b, B. Shinozaki b a b

High Voltage Electron Microscopy Station, National Institute for Material Science, 3-13 Sakura, Tukuba, Ibaraki 305-0003, Japan Department of Physics, Kyushu University, Ropponmatsu, Fukuoka 810-8560, Japan

a r t i c l e

i n f o

Article history: Available online 30 May 2009 PACS: 74.78.Db 74.81.Bd Keywords: Thin film Multilayer Granular Critical field

a b s t r a c t We report on the superconducting characteristics of the Indium thin films on molybdenum under-layer as a function of the In film thickness. Our molybdenum under-layer with thickness of 50 Å does not cause the occurrence of superconductivity until 1.5 K and the sheet resistance has logarithmic temperature dependence observed in the present investigation. As thickness of In increased, the oscillation phenomenon of TC was observed at early stage of deposition and the value of TC is higher than the that for bulk of In. Furthermore, it is found that with increase of the In thickness, there are large differences of the strengths of the upper critical magnetic field HC2(T), resistivity and TC between films with thickness below and above 100 Å. On the other hand, the TC decreases monotonously as sheet resistance increases, when the TC is plotted against sheet resistance. To clarify the relation of superconducting characteristics and the surface structure of the films with different thickness, we have performed surface observation by atomic force microscope. As a result, we have found that the surface changes from homogeneous structure to inhomogeneous (or percolative) structure, when the thickness of in films pass through about 100 Å. Superconductivity of In/Mo films with relatively thick-inhomogeneous films cannot be explained in terms of the simple percolation theory. Therefore, we analysis the experimental data of HC2(T) near TC, using a extended Landau–Ginzburg model. It is found out that our In/Mo films must consider some factors; such as, grain size, the distance of grain space, and the strength of couplings between grains. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction Two-dimensional granular superconductors have attracted much attention because of the relation between superconductivity and microstructure. Superconductivity-normal metal in multilayers and granular mixture films was widely studied both experimentally and theoretically. In granular films, the resistive transition has a long resistive tail and eventually become zero at any temperature. In these cases, actual superconducting transition temperature TC which becomes zero resistance is ambiguous. It is considered that a resistance of the film which is such a random arrangement of the superconducting islands is mainly due to the tunneling between islands. The junctions between islands consist of quasiparticle tunnel junction part and Josephson junction part. The resistance of the Josephson junction part in the film becomes zero, when the temperature is lowered down to the TC, and the resistance of the whole film decreases at TC. The resistance of the quasiparticle tunnel junction part increases, when the temperature

* Corresponding author. Address: High Voltage Electron Microscopy Station, National Institute for Material Science, 3-13 Sakura, Tukuba-shi., Ibaraki 305-0003, Japan. Tel.: +81 29 863 5548; fax: +81 29 863 5571. E-mail address: [email protected] (K. Makise). 0921-4534/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2009.05.182

lowers further. As a result, the resistance of the whole film increases with decrease below TC. This can explain the quasireentrant behavior of the temperature dependence of resistance. However, the cause in the real film has not been clarified, since the accurate structure of films is unknown. The study of superconductive/insulator mixture superconductor have the interest for clarifying the relationship between percolation and superconducting characteristics. Superconductivity of two-dimensional percolative superconductor, for example, structure of superconducting grains embedded in an insulating matrix, depend on grain size, intergrain distance and so on. Gerber et al. reported an anomalous behavior of upper critical field of percolating In–Ge mixture films [1]. Since percolation correlation length becomes longer than superconducting coherence length, the behavior of upper critical field changed form homogeneous to inhomogeneous And it was reported that not only the linkage between grain and intergrain affects the temperature dependence of upper critical magnetic field HC2(T) and that it has the curvature near a superconducting transition temperature. The superconductivity of other materials and Mo multilayer films were investigated for Mo/BN, Mo/Sb, Mo/Ni and Mo/Si [2–5]. Although a bulk crystal Mo is a superconductor with superconducting transition temperature TC = 0.92 K, a TC of Mo and

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non-superconducting material multilayer becomes higher than its of bulk Mo. As for this enhancement of superconductivity, there are several possibilities: lattice strain induced by the mismatch of the lattice parameter between another layer, intermixing and/or interdiffusion layers formed at interface [6]. By replacing Ge of a In/Ge film with Mo, we measured R(T,H) of a In/Mo film in order to examine the change of HC2(T) and TC. The aim of the present paper is to study the In thickness dependence of In/Mo films of the TC, HC2 and to discuss the superconducting mechanism in relation to the structure. 2. Experimental details Samples were made by using ultra high vacuum system, which was equipped with an e-gun and two K-cells. The base pressure was below 1  108 Pa. Mo and In were evaporated by using an e-gun and K-cell, respectively. The substrate was a glass whose surface was cleaned by alcohol and then it was preheated at 810 K for 1 h in the evaporation chamber. The average thickness d of the film was determined by a quartz crystal oscillator. The surface morphology of the In/Mo films have been characterized by the atomic force microscopy (AFM) in the tapping mode. All images (512 pixel wide) of the films were recorded with an AFM operating in the tapping mode in air using 512 lines resolution at 1 Hz scanning frequency. Furthermore, in order to clarify the structure of the interface of Mo and In, a cross-sectional of sample was observed by transmission electron microscope (TEM). A cross-sectional TEM sample was prepared using a focused ion beam. Resistance measurements were taken by the conventional four-probe method, using currents 1–10 lA. During the measurement, the samples were mounted on a copper block. The perpendicular magnetic field was applied from a superconducting magnet, immersed in the liquid helium bath. The TC and HC2 were defined as the point at which the resistance reached half its value.

Fig. 2. A plot of the zero-field sheet resistance Rsq against temperature, for In/Mo films having 0–120 Å of In thickness.

reduced. However, the value of the Rsq is reversed between 70 and 120 Å of In thickness. Fig. 3 shows the atomic force microscope (AFM) topography of two films. Firstly, we checked that these glass plates were found to have a very smooth surface of average roughness of Ra = 2–3 Å. However, the hole like shape was also observed on the surface. Cleary, the surface changed from homogeneous structure to inhomogeneous (or percolative) structure, when the thickness of In for films pass through between 70 and 120 Å. The reason may be that the aggregation of In occurs, when the film of In is thick. As a results, electrical conduction occurs by tunneling between grain of In and interface of In and Mo. The room-temperature resistivity q for the In/Mo films are plotted versus In thickness in Fig. 4a. Surprisingly, there is an increase in the resistivity with increasing In thickness. In addition, a slightly

3. Results and discussion The interface of In/Mo films as observed from the TEM micrographs are shown in Fig. 1. The thickness of the In/Mo films is about 15 nm. We did not observe clear interface between In and Mo. Fig. 2 shows the sheet resistance Rsq as a function of temperature for four films, with In thickness from 0 to 120 Å. Mo thin films with thickness of 5 nm does not cause the occurrence of superconductivity until 1.5 K. As thickness of In increased, Rsq at 10 K was

Fig. 1. TEM data for In/Mo films. The films as-deposited on a SiO2 at roomtemperature.

Fig. 3. Atomic force micrographs of (a) 70 Å and (b) 120 Å of In thickness on a 50 Åthick Mo under-layer.

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K. Makise et al. / Physica C 469 (2009) 1005–1008

Fig. 6. A plot of the parameter a ¼ 14

 2  t n0 l

1 4

 35



2tR S2

2 

against q. A parameter a was

decided from the gradient of plots on HC2/(1  T/TC) vs. (1  T/TC) (inset). A dotted curve is a function of a q2 (see text).

Here t is coupling length of grain, n is coherence length of grain, S is lattice constant of stack, and 2R is grain’s diameter. Using the freeelectron model of conductivity r, and coherence length in the dirty limit n(T), t2 and n(T) are given as: Fig. 4. Resistivity q (upper) and superconducting transition temperature TC (down) as a function of In-layer thickness.

t2 ¼ 4S2 rqn

ð2Þ

and jump of resistivity appear from 70 to 120 Å. In Fig. 4b, we showd that TC is higher than the bulk TC and the oscillating of TC’s. Fogel et al. reported such a oscillating behavior in Mo/Si multilayers [7]. They explain that oscillation effects are due to the variation of the Josephson coupling. The HC2 as a function of reduced temperature are shown in Fig. 5 for several In/Mo films. As In film thickness increase, HC2(T) showed d2HC2/dT2 > 0 near TC0 and it rises between In film thickness 70–120 Å at the once again. Finally, HC2(T) decreases with the In film thickness increase. It may be correspondent to the change of resistivity dependent on the In film thickness (shown in Fig. 4a). Therefore, d2HC2/dT2 > 0 may means that the coupling strength between grain is strong. Now, we analysis the experimental data of HC2(T) near TC, using a extended Landau–Ginzburg model, as shown in Fig. 6. The theory of the HC2 for granular superconductor has been developed by Deutscher [8]. According to this theory, in weak magnetic field, the HC2 is given as:

HC2 ¼

 2 (  2 "  2 #) c t h 1 t 1 3 2tR : 1þ  2e Sn 4 n 4 5 S2

ð1Þ

1 nðTÞ ¼ 0:85 n0 l 1  TTC

!1=2 :

ð3Þ

Here qn and l are the normal-state resistance and mean free path, respectively. By use of both Eqs. (2) and (3), Eq. (1) can be rewritten as follows:

HC2 /

   2 T T þa 1 1 TC TC

ð4Þ

and

a¼

 2 "  2 # 1 t 1 3 2tR :  4 n0 l 4 5 S2

ð5Þ

From Eq. (2), it is found that a is a quadratic function of q. Next, we compared the experimental results with calculation of a for Eq. (5). A parameter a was decided from the gradient of plots on HC2/(1  T/ TC) vs. (1  T/TC). Dotted curves is a function of a q2. In the model of considering the coupling strength between superconducting grains, q dependence of HC2 can qualitatively explain. 4. Conclusions We have shown the In thickness of R(T,H) for In/Mo films. From AFM image, it is found that the surface changes from homogeneous structure to inhomogeneous (or percolative) structure, when the thickness of in films pass through about 100 Å. This effects has appeared in the In thickness dependence of the resistivity. The oscillation phenomenon of TC was observed at early stage of deposition and the TC is higher than the bulk TC of In with increasing In thickness. We also analysis the experimental data of Hc2(T) near TC, using a extended Landau–Ginzburg model. It is found out that our In/Mo films must consider some factors; such as, grain size, the distance of grain space, and the strength of couplings between grains. References

Fig. 5. Upper critical fields as a function of reduced temperature TC/T for six In/Mo films.

[1] G. Deutscher, I. Grave, S. Alexander, Phy. Rev. Lett. 48 (1982) 1497.

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[2] M. Ikebe, N.S. Kazama, Y. Muto, H. Fujimori, IEEE Trans. Mag. MAG-19 (1983) 204. [3] C. Usher, R. Clarke, G.G. Zheng, I.K. Schuller, Phys. Rev. B 30 (1984) 453. [4] Y. Asada, K. Ogawa, Solid State Commun. 60 (1986) 161. [5] T. Hatano, Y. Asada, K. Nakamura, Abstract of the Fall Meeting of the Jpn. Institute of Metals, 1986, pp. 550.

[6] H. Nakajima, M. Ikebe, Y. Muto, H. Fujimori, J. Appl. Phys. 65 (1989) 1637. [7] N. Fogel, O. Turutanov, A. Sidorenko, E. Buchstab, Phys. Rev. B 56 (1997) 2372. [8] G. Deutscher, O. Entin-Wohlman, Y. Shapira, Phys. Rev. B 22 (1980) 4264.