- Email: [email protected]
Josephson junctions fabricated by focused ion beam from ex situ grown MgB2 thin films
Physica C 405 (2004) 84–88 www.elsevier.com/locate/physc
Josephson junctions fabricated by focused ion beam from ex situ grown MgB2 thin films A. Malisa a
a,*
, M. Valkeap€ a€ a b, L.-G. Johansson b, Zdravko Ivanov
a
Department of Microtechnology and Nanoscience, Quantum Device Physics Laboratory, Chalmers University of Technology, SE-412 96 G€oteborg, Sweden b Department of Inorganic Chemistry, G€oteborg University, SE-412 96 G€oteborg, Sweden Received 10 October 2003; received in revised form 8 January 2004; accepted 15 January 2004
Abstract 1 0 2) substrates by e-beam evaporation of MgB2 pellet. We prepared MgB2 thin films on SrTiO3 (1 0 0) and Al2 O3 (1 The films were deposited at room temperature and post-annealed at 900 °C in Mg vapour for 5–30 min. Superconducting transition temperatures were observed between 22 and 30 K. Structure and surface morphology of the films were investigated by X-ray diffraction (XRD) and atomic force microscopy (AFM). The films grown on Al2 O3 substrates are c-axis oriented while a film grown on SrTiO3 substrate is aligned with the (1 0 1) direction normal to the substrate planes. The films have grain sizes of about 70 nm. The films were patterned into 4 and 8 lm wide microbridges. The microbridges were observed to carry large critical current densities of approximately 1 MA/cm2 at 6.7 K. Focused ion beam (FIB) was used on the bridges in order to fabricate Josephson junctions. A cut 50 nm in width was made across the microbridges followed by an in situ platinum (Pt) deposition into the cut made. SNS-like weak-link junctions were formed in the process. Ó 2004 Elsevier B.V. All rights reserved. PACS: 74.50.+r; 74.76.Db; 85.25.Cp Keywords: Superconducting; MgB2 ; Thin films; Ex situ; E-beam evaporation
1. Introduction The binary metallic MgB2 superconductor with a transition temperature (Tc ) of 39 K has attracted great interest since its discovery in 2001 [1]. Much effort has been put in the growth of thin films of this superconductor and device fabrication. This effort has been slowed down by one main problem, *
Corresponding author. Fax: +46-31-772-3471. E-mail address: [email protected] (A. Malisa).
which is high sensitivity of magnesium to oxygen. Magnesium oxidizes readily during thin film growth processes. Attempts to grow epitaxial thin films of MgB2 are increasing because MgB2 is considered to be an important material for superconducting electronic applications [2–7]. MgB2 is a suitable material for junctions because it has several advantages over the high-Tc cuprates: a simple crystal structure, low anisotropy, few interface problems, only two elements: B and Mg, and a long coherence length nc ð0Þ 2:5 nm and nab ð0Þ 5 nm.
0921-4534/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2004.01.017
A. Malisa et al. / Physica C 405 (2004) 84–88
We report in this work results on superconducting thin films of MgB2 grown on SrTiO3 (1 0 0) and Al2 O3 (1 1 0 2) substrates using e-beam evaporation technique. Four samples were prepared; three samples were grown on Al2 O3 (1 1 0 2) and one on SrTiO3 (1 0 0) substrates. The films showed Tc between 22 and 30 K. The sample with the highest Tc was grown on Al2 O3 (1 1 0 2) substrate. This was one of the films patterned into 4 and 8 lm wide microbridges and its electrical measurements are included in this work. Use of focused ion beam (FIB) to trim and make a cut across the microbridges and a subsequent in situ deposition of platinum in the cut enabled us to fabricate SNSlike Josephson junctions. A similar technique was used by Brinkman et al. [7] to make MgB2 nanobridges which were used as weak-link junctions in a superconducting quantum interference device (SQUID).
2. Experimental Commercial MgB2 powder with 99.8% purity was pressed into a pellet which was used as a source for e-beam evaporation. The substrate was kept at room temperature during evaporation. The pressure in the deposition chamber during evaporation was between 106 and 108 mbar. As Mg has a much higher vapour pressure than B. it is evaporated first before B and also is partly reevaporated by the impinging B atoms. So a bilayer of Mg and B is deposited on the substrate as a precursor film. Since the deposition is done at room temperature, Mg sticks much better onto the substrate than when deposited at elevated temperatures. Therefore, a post-anneal stage in Mg vapour was required to obtain a uniform and stoichiometric superconducting MgB2 phase. Each sample was separately annealed in a Tantalum (Ta) tube in which small Mg pieces were included. The Ta tube was then sealed in an evacuated quartz tube partially filled with Argon gas. Annealing was done at 900 °C for a duration of 5– 30 min. The thicknesses of the samples prepared are 350, 260, 220, and 160 nm. The surface morphology was investigated by atomic force microscopy (AFM). An AFM image
85
of the surface morphology of one of the films is shown in Fig. 1(a). The film has a surface rootmean-square (RMS) roughness value of about 8.5 nm. A cross-section through the same sample shown in Fig. 1(b) indicates that the peak-to-valley distance is 27 nm. The X-ray diffraction (XRD) h–2h spectra taken by Philips XÕPert diffractometer indicated small intensity peaks from the film and large intensity peaks from the substrates. The films grown on Al2 O3 (1 1 0 2) substrates showed (0 0 1) and (0 0 2) peaks as reported by other groups [8,9]. On the other hand, the film deposited on SrTiO3 (1 0 0) indicated a preferred growth orientation in the (1 0 1) plane only [2,8]. The XRD spectrum shown in Fig. 2(a) indicates that films grown on Al2 O3 (1 1 0 2) are aligned with the c-axis while Fig. 2(b) shows that the film grown on SrTiO3 (1 0 0) is well aligned with the (1 0 1) direction normal to the substrate plane. The films were patterned into microbridges 4 and 8 lm wide using conventional photolithography and Ar-ion beam milling. By using four-point
Fig. 1. An AFM image of MgB2 thin film grown on an Al2 O3 (1 1 0 2) substrate (a) the film surface is characterized by little material outgrowth (b) a cross-section through the film shows a maximum peak-to-valley distance of 27 nm and film grain sizes of 70 nm. The surface (RMS) roughness value is 8.5 nm.
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A. Malisa et al. / Physica C 405 (2004) 84–88
10
2
2
0
20
0
20
40
2θ
(a)
60
80
100
80
10 0
6
10
(b)
4
s
2
10
MgB (101)
Intensity (a.u)
s
s
10
s
MgB (002)
s
s 4
2
10
6
MgB (001)
Intensity (a.u)
10
2
40
2θ
60
Fig. 2. The h–2h X-ray diffraction spectra for MgB2 thin films grown on Al2 O3 (1 1 0 2) and SrTiO3 (1 0 0) substrates. The MgB2 films grown on (a) Al2 O3 (1 1 0 2) indicate a c-axis oriented growth whereas the film grown on (b) SrTiO3 (1 0 0) indicates a preferred growth orientation in (1 0 1) plane. Substrate peaks are denoted by S.
the film thickness and platinum metal was deposited in situ in the cut so made. In this case, due to a proximity effect and the platinum deposited which was not pure, SNS-like junctions were made. The I–V characteristics show that the junctions fabricated in this process carry excess current which may be explained by Andreev reflections occurring at the normal conductor–superconductor interface. Fig. 3 shows the temperature dependence of resistance of one of the junctions with an onset transition temperature at 30 K and a small ‘‘foot’’ structure at approximately 24 K. The onset transition temperature of 30 K shows the transition to the superconducting state of the electrodes while 24 K is the Tc of the junction. Fig. 4(a) is the I–V curve of the junction at 4.2 K. There are features appearing at 2.8 and 6.5 mV on the I–V curve of the junction. These occur in all our measurements and we interpret them as due to DS and DL , that is the small and large MgB2 superconducting energy gaps, respectively [10,11]. The temperature dependence of the I–V characteristics of the junction was investigated by performing I–V measurements from 4.3 K up to a temperature close to the Tc of the junction (Fig. 4(b)). In Fig. 5 we plot the temperature dependence of the critical current
1400 1200 1000
R(Ω)
probe measurement techniques we performed resistance–temperature (R–T ), as well as current– voltage (I–V ) measurements. From the I–V measurements we could observe that the bridges carry large critical current densities of the order of 1 MA/cm2 at 6.7 K. We observed large supercurrent in almost all microbridges. The 8 lm wide bridge seemed to carry too large supercurrents such that our measurement equipment could not supply enough biasing current. Using a focused ion beam (FIB) instrument (FEI, FIB 200 THP) with Ga and Pt ion sources, the widths of the microbridges were reduced from 4 and 8 lm to 1 or 2 lm. Further application of FIB enabled us to make a 50 nm wide cut across the bridges. The cut was made to remove part of
800 600 400 200 0 0
50
100
150
200
250
300
T(K) Fig. 3. Resistance vs temperature (R–T ) for a junction fabricated from a trimmed and cut MgB2 microbridge by FIB and in situ Pt deposition. The MgB2 film used was grown on Al2 O3 (1 1 0 2). The curve shows two superconducting transitions at 30 and 24 K. The second transition observed as a ‘‘foot’’ at 24 K indicates the critical temperature of the junction while 30 K is the critical temperature of the electrodes.
A. Malisa et al. / Physica C 405 (2004) 84–88
150
87
50
100 40
0
Ic(µA)
I (µA)
50
-50
30
20
-100 10
-150 -15 (a)
-10
-5
0 V (mV)
5
10
15 0 5
10
100
I (µA)
50
0
4.3K 4.8K 6.3K 9K 10.9K 12.7K 16.8K 17.3K 19.5K 21K
20
25
Fig. 5. Temperature dependence of the critical current of a Josephson junction whose I–V curve is shown in Fig. 4(a) above. The graph depicts an SNS-like weak-link type of junction.
-50
-100 -15 (b)
15
T (K)
-10
-5
0
5
10
15
V (mV)
Fig. 4. (a) An I–V curve of the junction at 4.2 K. The features appearing at 2.8 and 6.5 mV indicate DS and DL , the small and large MgB2 superconducting energy gaps, respectively. (b) Temperature dependence of I–V characteristics of the junction whose I–V curve is depicted in Fig. 4(a) measured from 4.3 K to a temperature close to Tc of the junction.
of one of the junctions. The observed variation of the critical current with temperature for the MgB2 –platinum proximity-effect weak-link junction fabricated is SNS-like. The argument here is that at a temperature T Tcj where Tcj is the transition temperature of the junction, the critical current (Ic ) scaled as ðTcj T Þ1:0768 . This is a sign of an SINS Josephson junction.
3. Summary We have reported a method for ex situ growth of MgB2 films using e-beam evaporation tech-
nique. The films were deposited at room temperature with a post-annealing stage at 900 °C in Mg vapour for 5–30 min. Structural studies done on the films using XRD show that films grown on Al2 O3 (1 1 0 2) are c-axis oriented while the film grown on SrTiO3 (1 0 0) has a preferred growth orientation in the (1 0 1) plane only. A surface morphology study performed using AFM shows a peak-to-valley distance of 27 nm and a surface (RMS) roughness value of 8.5 nm. We have also managed to make Josephson junctions using focused ion beam (FIB) to trim and cut microbridges. The cut was filled by Pt. The I–V characteristics show that the junctions had excess current. Future work will focus on preparation of in situ and epitaxial MgB2 thin films which are a requirement for fabricating devices and multilayers.
Acknowledgements The help of Staffan Pehrson in setting up the e-beam evaporation system is highly appreciated. The work is supported by VR (The Swedish Research Council) and the Swedish Foundation for Strategic Research within the OXIDE program.
88
A. Malisa et al. / Physica C 405 (2004) 84–88
References [1] J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani, J. Akimitsu, Nature (London) 410 (2001) 63. [2] S.R. Shinde, S.B. Ogale, R.L. Greene, T. Venkatesan, P.C. Canfield, S.L. BudÕko, G. Lapertot, C. Petrovic, Appl. Phys. Lett. 79 (2001) 227. [3] D.H.A. Blank, H. Hilgenkamp, A. Brinkman, D. Mijatovic, G. Rijinders, H. Rogalla, Appl. Phys. Lett. 79 (2001) 394. [4] X.H. Zeng, A.V. Pogrebnyakov, A. Kotcharov, J.E. Jones, X.X. Xi, E.M. Lysczek, J.M. Redwing, S. Xu, Q. Li, J. Lettieri, D.G. Schlom, W. Tian, X. Pan, Z.-K. Liu, Nature Mater. 1 (2002) 1. [5] A.J.M. van Erven, T.H. Kim, M. Muenzenberg, J.S. Moodera, Appl. Phys. Lett. 81 (2002) 4982. [6] X.H. Zeng, A.V. Pogrebnyakov, M.H. Zhu, J.E. Jones, X.X. Xi, S.Y. Xu, E. Wertz, Q. Li, J.M. Redwing, J. Lettieri, V. Vaithyanathan, D.G. Schlom, Z.-K. Liu, O.
[7]
[8] [9] [10]
[11]
Trithaveesak, J. Schubert, Appl. Phys. Lett. 82 (2003) 2097. A. Brinkman, D. Veldhuis, D. Mijatovic, G. Rijinders, D.H.A. Blank, H. Hilgenkamp, H. Rogalla, Appl. Phys. Lett. 79 (2001) 2420. W.N. Kang, H.-J. Kim, E.-M. Choi, C.U. Jurig, S.-I. Lee, Science 292 (2001) 1521. H.-J. Kim, W.N. Kang, E.-M. Choi, K.H.P. Kim, S.-I. Lee, J. Korean Phys. Soc. 40 (2002) 416. R.S. Gonnelli, D. Daghero, G.A. Ummarino, V.A. Stepanov, J. Jun, S.M. Kazakov, J. Karpinski, Phys. Rev. Lett. 89 (2002) 2470041. Ya.G. Ponomarev, S.A. Kuzmichev, M.G. Mikheev, M.V. Sudakova, S.N. Tchesnokov, N.Z. Timergaleev, A.V. Yarigin, E.G. Maksimov, S.I. Krasnosvobodtsev, A.V. Varlashkin, M.A. Hein, G. M€ uller, H. Piel, L.G. Sevastyanova, O.V. Kravchenko, K.P. Burdina, B.M. Bulychev, Solid State Commun. 129 (2004) 85.
Josephson junctions fabricated by focused ion beam from ex situ grown MgB2 thin films A. Malisa a
a,*
, M. Valkeap€ a€ a b, L.-G. Johansson b, Zdravko Ivanov
a
Department of Microtechnology and Nanoscience, Quantum Device Physics Laboratory, Chalmers University of Technology, SE-412 96 G€oteborg, Sweden b Department of Inorganic Chemistry, G€oteborg University, SE-412 96 G€oteborg, Sweden Received 10 October 2003; received in revised form 8 January 2004; accepted 15 January 2004
Abstract 1 0 2) substrates by e-beam evaporation of MgB2 pellet. We prepared MgB2 thin films on SrTiO3 (1 0 0) and Al2 O3 (1 The films were deposited at room temperature and post-annealed at 900 °C in Mg vapour for 5–30 min. Superconducting transition temperatures were observed between 22 and 30 K. Structure and surface morphology of the films were investigated by X-ray diffraction (XRD) and atomic force microscopy (AFM). The films grown on Al2 O3 substrates are c-axis oriented while a film grown on SrTiO3 substrate is aligned with the (1 0 1) direction normal to the substrate planes. The films have grain sizes of about 70 nm. The films were patterned into 4 and 8 lm wide microbridges. The microbridges were observed to carry large critical current densities of approximately 1 MA/cm2 at 6.7 K. Focused ion beam (FIB) was used on the bridges in order to fabricate Josephson junctions. A cut 50 nm in width was made across the microbridges followed by an in situ platinum (Pt) deposition into the cut made. SNS-like weak-link junctions were formed in the process. Ó 2004 Elsevier B.V. All rights reserved. PACS: 74.50.+r; 74.76.Db; 85.25.Cp Keywords: Superconducting; MgB2 ; Thin films; Ex situ; E-beam evaporation
1. Introduction The binary metallic MgB2 superconductor with a transition temperature (Tc ) of 39 K has attracted great interest since its discovery in 2001 [1]. Much effort has been put in the growth of thin films of this superconductor and device fabrication. This effort has been slowed down by one main problem, *
Corresponding author. Fax: +46-31-772-3471. E-mail address: [email protected] (A. Malisa).
which is high sensitivity of magnesium to oxygen. Magnesium oxidizes readily during thin film growth processes. Attempts to grow epitaxial thin films of MgB2 are increasing because MgB2 is considered to be an important material for superconducting electronic applications [2–7]. MgB2 is a suitable material for junctions because it has several advantages over the high-Tc cuprates: a simple crystal structure, low anisotropy, few interface problems, only two elements: B and Mg, and a long coherence length nc ð0Þ 2:5 nm and nab ð0Þ 5 nm.
0921-4534/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2004.01.017
A. Malisa et al. / Physica C 405 (2004) 84–88
We report in this work results on superconducting thin films of MgB2 grown on SrTiO3 (1 0 0) and Al2 O3 (1 1 0 2) substrates using e-beam evaporation technique. Four samples were prepared; three samples were grown on Al2 O3 (1 1 0 2) and one on SrTiO3 (1 0 0) substrates. The films showed Tc between 22 and 30 K. The sample with the highest Tc was grown on Al2 O3 (1 1 0 2) substrate. This was one of the films patterned into 4 and 8 lm wide microbridges and its electrical measurements are included in this work. Use of focused ion beam (FIB) to trim and make a cut across the microbridges and a subsequent in situ deposition of platinum in the cut enabled us to fabricate SNSlike Josephson junctions. A similar technique was used by Brinkman et al. [7] to make MgB2 nanobridges which were used as weak-link junctions in a superconducting quantum interference device (SQUID).
2. Experimental Commercial MgB2 powder with 99.8% purity was pressed into a pellet which was used as a source for e-beam evaporation. The substrate was kept at room temperature during evaporation. The pressure in the deposition chamber during evaporation was between 106 and 108 mbar. As Mg has a much higher vapour pressure than B. it is evaporated first before B and also is partly reevaporated by the impinging B atoms. So a bilayer of Mg and B is deposited on the substrate as a precursor film. Since the deposition is done at room temperature, Mg sticks much better onto the substrate than when deposited at elevated temperatures. Therefore, a post-anneal stage in Mg vapour was required to obtain a uniform and stoichiometric superconducting MgB2 phase. Each sample was separately annealed in a Tantalum (Ta) tube in which small Mg pieces were included. The Ta tube was then sealed in an evacuated quartz tube partially filled with Argon gas. Annealing was done at 900 °C for a duration of 5– 30 min. The thicknesses of the samples prepared are 350, 260, 220, and 160 nm. The surface morphology was investigated by atomic force microscopy (AFM). An AFM image
85
of the surface morphology of one of the films is shown in Fig. 1(a). The film has a surface rootmean-square (RMS) roughness value of about 8.5 nm. A cross-section through the same sample shown in Fig. 1(b) indicates that the peak-to-valley distance is 27 nm. The X-ray diffraction (XRD) h–2h spectra taken by Philips XÕPert diffractometer indicated small intensity peaks from the film and large intensity peaks from the substrates. The films grown on Al2 O3 (1 1 0 2) substrates showed (0 0 1) and (0 0 2) peaks as reported by other groups [8,9]. On the other hand, the film deposited on SrTiO3 (1 0 0) indicated a preferred growth orientation in the (1 0 1) plane only [2,8]. The XRD spectrum shown in Fig. 2(a) indicates that films grown on Al2 O3 (1 1 0 2) are aligned with the c-axis while Fig. 2(b) shows that the film grown on SrTiO3 (1 0 0) is well aligned with the (1 0 1) direction normal to the substrate plane. The films were patterned into microbridges 4 and 8 lm wide using conventional photolithography and Ar-ion beam milling. By using four-point
Fig. 1. An AFM image of MgB2 thin film grown on an Al2 O3 (1 1 0 2) substrate (a) the film surface is characterized by little material outgrowth (b) a cross-section through the film shows a maximum peak-to-valley distance of 27 nm and film grain sizes of 70 nm. The surface (RMS) roughness value is 8.5 nm.
86
A. Malisa et al. / Physica C 405 (2004) 84–88
10
2
2
0
20
0
20
40
2θ
(a)
60
80
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80
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6
10
(b)
4
s
2
10
MgB (101)
Intensity (a.u)
s
s
10
s
MgB (002)
s
s 4
2
10
6
MgB (001)
Intensity (a.u)
10
2
40
2θ
60
Fig. 2. The h–2h X-ray diffraction spectra for MgB2 thin films grown on Al2 O3 (1 1 0 2) and SrTiO3 (1 0 0) substrates. The MgB2 films grown on (a) Al2 O3 (1 1 0 2) indicate a c-axis oriented growth whereas the film grown on (b) SrTiO3 (1 0 0) indicates a preferred growth orientation in (1 0 1) plane. Substrate peaks are denoted by S.
the film thickness and platinum metal was deposited in situ in the cut so made. In this case, due to a proximity effect and the platinum deposited which was not pure, SNS-like junctions were made. The I–V characteristics show that the junctions fabricated in this process carry excess current which may be explained by Andreev reflections occurring at the normal conductor–superconductor interface. Fig. 3 shows the temperature dependence of resistance of one of the junctions with an onset transition temperature at 30 K and a small ‘‘foot’’ structure at approximately 24 K. The onset transition temperature of 30 K shows the transition to the superconducting state of the electrodes while 24 K is the Tc of the junction. Fig. 4(a) is the I–V curve of the junction at 4.2 K. There are features appearing at 2.8 and 6.5 mV on the I–V curve of the junction. These occur in all our measurements and we interpret them as due to DS and DL , that is the small and large MgB2 superconducting energy gaps, respectively [10,11]. The temperature dependence of the I–V characteristics of the junction was investigated by performing I–V measurements from 4.3 K up to a temperature close to the Tc of the junction (Fig. 4(b)). In Fig. 5 we plot the temperature dependence of the critical current
1400 1200 1000
R(Ω)
probe measurement techniques we performed resistance–temperature (R–T ), as well as current– voltage (I–V ) measurements. From the I–V measurements we could observe that the bridges carry large critical current densities of the order of 1 MA/cm2 at 6.7 K. We observed large supercurrent in almost all microbridges. The 8 lm wide bridge seemed to carry too large supercurrents such that our measurement equipment could not supply enough biasing current. Using a focused ion beam (FIB) instrument (FEI, FIB 200 THP) with Ga and Pt ion sources, the widths of the microbridges were reduced from 4 and 8 lm to 1 or 2 lm. Further application of FIB enabled us to make a 50 nm wide cut across the bridges. The cut was made to remove part of
800 600 400 200 0 0
50
100
150
200
250
300
T(K) Fig. 3. Resistance vs temperature (R–T ) for a junction fabricated from a trimmed and cut MgB2 microbridge by FIB and in situ Pt deposition. The MgB2 film used was grown on Al2 O3 (1 1 0 2). The curve shows two superconducting transitions at 30 and 24 K. The second transition observed as a ‘‘foot’’ at 24 K indicates the critical temperature of the junction while 30 K is the critical temperature of the electrodes.
A. Malisa et al. / Physica C 405 (2004) 84–88
150
87
50
100 40
0
Ic(µA)
I (µA)
50
-50
30
20
-100 10
-150 -15 (a)
-10
-5
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5
10
15 0 5
10
100
I (µA)
50
0
4.3K 4.8K 6.3K 9K 10.9K 12.7K 16.8K 17.3K 19.5K 21K
20
25
Fig. 5. Temperature dependence of the critical current of a Josephson junction whose I–V curve is shown in Fig. 4(a) above. The graph depicts an SNS-like weak-link type of junction.
-50
-100 -15 (b)
15
T (K)
-10
-5
0
5
10
15
V (mV)
Fig. 4. (a) An I–V curve of the junction at 4.2 K. The features appearing at 2.8 and 6.5 mV indicate DS and DL , the small and large MgB2 superconducting energy gaps, respectively. (b) Temperature dependence of I–V characteristics of the junction whose I–V curve is depicted in Fig. 4(a) measured from 4.3 K to a temperature close to Tc of the junction.
of one of the junctions. The observed variation of the critical current with temperature for the MgB2 –platinum proximity-effect weak-link junction fabricated is SNS-like. The argument here is that at a temperature T Tcj where Tcj is the transition temperature of the junction, the critical current (Ic ) scaled as ðTcj T Þ1:0768 . This is a sign of an SINS Josephson junction.
3. Summary We have reported a method for ex situ growth of MgB2 films using e-beam evaporation tech-
nique. The films were deposited at room temperature with a post-annealing stage at 900 °C in Mg vapour for 5–30 min. Structural studies done on the films using XRD show that films grown on Al2 O3 (1 1 0 2) are c-axis oriented while the film grown on SrTiO3 (1 0 0) has a preferred growth orientation in the (1 0 1) plane only. A surface morphology study performed using AFM shows a peak-to-valley distance of 27 nm and a surface (RMS) roughness value of 8.5 nm. We have also managed to make Josephson junctions using focused ion beam (FIB) to trim and cut microbridges. The cut was filled by Pt. The I–V characteristics show that the junctions had excess current. Future work will focus on preparation of in situ and epitaxial MgB2 thin films which are a requirement for fabricating devices and multilayers.
Acknowledgements The help of Staffan Pehrson in setting up the e-beam evaporation system is highly appreciated. The work is supported by VR (The Swedish Research Council) and the Swedish Foundation for Strategic Research within the OXIDE program.
88
A. Malisa et al. / Physica C 405 (2004) 84–88
References [1] J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani, J. Akimitsu, Nature (London) 410 (2001) 63. [2] S.R. Shinde, S.B. Ogale, R.L. Greene, T. Venkatesan, P.C. Canfield, S.L. BudÕko, G. Lapertot, C. Petrovic, Appl. Phys. Lett. 79 (2001) 227. [3] D.H.A. Blank, H. Hilgenkamp, A. Brinkman, D. Mijatovic, G. Rijinders, H. Rogalla, Appl. Phys. Lett. 79 (2001) 394. [4] X.H. Zeng, A.V. Pogrebnyakov, A. Kotcharov, J.E. Jones, X.X. Xi, E.M. Lysczek, J.M. Redwing, S. Xu, Q. Li, J. Lettieri, D.G. Schlom, W. Tian, X. Pan, Z.-K. Liu, Nature Mater. 1 (2002) 1. [5] A.J.M. van Erven, T.H. Kim, M. Muenzenberg, J.S. Moodera, Appl. Phys. Lett. 81 (2002) 4982. [6] X.H. Zeng, A.V. Pogrebnyakov, M.H. Zhu, J.E. Jones, X.X. Xi, S.Y. Xu, E. Wertz, Q. Li, J.M. Redwing, J. Lettieri, V. Vaithyanathan, D.G. Schlom, Z.-K. Liu, O.
[7]
[8] [9] [10]
[11]
Trithaveesak, J. Schubert, Appl. Phys. Lett. 82 (2003) 2097. A. Brinkman, D. Veldhuis, D. Mijatovic, G. Rijinders, D.H.A. Blank, H. Hilgenkamp, H. Rogalla, Appl. Phys. Lett. 79 (2001) 2420. W.N. Kang, H.-J. Kim, E.-M. Choi, C.U. Jurig, S.-I. Lee, Science 292 (2001) 1521. H.-J. Kim, W.N. Kang, E.-M. Choi, K.H.P. Kim, S.-I. Lee, J. Korean Phys. Soc. 40 (2002) 416. R.S. Gonnelli, D. Daghero, G.A. Ummarino, V.A. Stepanov, J. Jun, S.M. Kazakov, J. Karpinski, Phys. Rev. Lett. 89 (2002) 2470041. Ya.G. Ponomarev, S.A. Kuzmichev, M.G. Mikheev, M.V. Sudakova, S.N. Tchesnokov, N.Z. Timergaleev, A.V. Yarigin, E.G. Maksimov, S.I. Krasnosvobodtsev, A.V. Varlashkin, M.A. Hein, G. M€ uller, H. Piel, L.G. Sevastyanova, O.V. Kravchenko, K.P. Burdina, B.M. Bulychev, Solid State Commun. 129 (2004) 85.