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Superconducting thin films of MgB2 by pulsed-laser deposition
Physica C 372–376 (2002) 1258–1261 www.elsevier.com/locate/physc
Superconducting thin films of MgB2 by pulsed-laser deposition Dragana Mijatovic *, Alexander Brinkman, Guus Rijnders, Hans Hilgenkamp, Horst Rogalla, Dave H.A. Blank Department of Applied Physics, Low Temperature Division, MESA+ Research Institute, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands
Abstract Superconducting thin MgB2 films have been prepared using pulsed-laser deposition. We have studied the influences of deposition conditions such as pressure and temperature, the substrate-material, and annealing-procedures. Various approaches have been pursued to obtain the right Mg content in the film during ablation and annealing. Special care has been taken to avoid oxidation of Mg in the laser plasma and deposited film, by optimizing the background pressure of Ar gas in the deposition chamber. The annealing procedure was found to be the most critical to obtain superconducting films. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: MgB2 ; Thin films; Pulsed-laser deposition
1. Introduction The report of Akimitsu et al. [1,2] about the surprising discovery of superconductivity in MgB2 at 39 K has aroused the interest to investigate the properties of this material. The availability of the superconducting thin films is of great value for basic studies on this compound as well as for applications such as superconducting sensors and electronics. Two complicating factors for the fabrication of superconducting films of MgB2 are the high vapor pressure of magnesium at low temperatures and the high sensitivity of magnesium to oxidation, requiring very low oxygen partial pressures in the
*
Corresponding author. Fax: +31-53-489-1099. E-mail address: [email protected] (D. Mijatovic).
deposition system. To overcome both and to reach the bulk value of the transition temperature, ex situ methods have been exploited. Up to date, reports on superconducting MgB2 films with the highest zero resistance transition temperatures TC;0 of up to 39 K, are based on magnesium diffusion into a boron or Mg–B film at temperatures of 900– 950 °C and at high Mg pressures [3,4]. However, for a number of applications, such as Josephson junctions, and for various basic studies it is important to obtain smooth, well-defined, and preferably, epitaxial films in an all in situ process. Up to now there have been a few successful attempts in making in situ superconducting films by pulsedlaser deposition [5–8], but the transition temperatures of those samples were considerably below the bulk value. As magnesium-diffusion at high temperatures and pressures into pre-deposited boron films leads to films with superconducting
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 0 9 8 7 - 5
D. Mijatovic et al. / Physica C 372–376 (2002) 1258–1261
properties comparable with the bulk, it is likely that the plasma temperature promotes the formation of other Mg–B phases in addition to MgB2 , leading to films with suppressed TC [5]. Possible other explanations are very small grains of MgB2 , impurities like MgO or solid B that could be formed in the plasma and incorporated in the film [5]. This paper gives a study of the influences of the deposition conditions such as pressure and temperature, the substrate-material, and annealingprocedures of two step in situ deposited thin films by pulsed-laser deposition.
2. Experimental Two different approaches towards the growing of MgB2 thin films were pursued: by deposition from an Mg-enriched MgB2 target and by growing multilayered films of Mg and B. To study the influence of the substrates, Si, SiC, SrTiO3 , Al2 O3 , as well as MgO were used. Table 1 gives an overview of the influences of the type of the substrates, laser energy, deposition temperatures and annealing procedures on TC which will be discussed below. The films made from Mg-enriched MgB2 target are described in [5,9]. The films were deposited at temperatures ranging from room temperature to 300 °C. Initially, the films were deposited at 500 mJ
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laser energy on different substrates (Table 1). Due to the fact that the transition temperature was similar of all of the samples, it can be concluded that the type of substrate is not very significant in this method of the deposition, because the films are not epitaxially grown. Since TC has been increased on the samples made on MgO substrates by decreasing the laser energy to 400 mJ, we performed further experiments on this substrate and laser energy optimizing the other deposition parameters, such as deposition temperature, annealing process, etc. in order to improve TC . The energy density at the target was about 4 J/cm2 . The films were usually deposited for 6 min at 10 Hz, leading to a thickness of about 200 nm. A high temperature annealing step was needed to form the superconducting phase. Therefore, the films were heated up to Tann ¼ 600 °C, kept for 5 min and cooled down to room temperature in 0.22 mbar of Ar. The total annealing procedure was as short as possible to avoid Mg-evaporation out of the film. In the temperature range between 350 and 600 °C, the sample was kept in a pulsed (5 Hz) Mg plasma for the same reason [5]. The films deposited at room temperatures had the TC of 24–25 K. Higher TC;0 ’s (about 26 K) were achieved when the films were grown at a substrate temperature of 200 °C. Films prepared at 300 °C, however, were not superconducting (Table 1). Films deposited at 200 °C had a considerable lower etching rate in an
Table 1 An overview of the influences of the substrate material, laser energy, deposition temperature and annealing procedures on TC;0 for MgB2 films prepared from Mg-enriched MgB2 target (‘‘Mg þ MgB2 ’’) and from Mg–B multilayers Target
Substrate
Elaser (mJ)
Tdep (°C)
Annealing
‘‘Mg þ MgB2 ’’
MgO(1 0 0) SrTiO3 (1 0 0) Si(1 0 0) SiC (6H) MgO(1 0 0)
500 500 500 500 400
RT RT RT RT RT 200
Mg and B (multilayers)
MgO(1 0 0) SrTiO3 (1 1 1) Al2 O3 (R-plane)
400 400 400
600 600 600 600 600 600 600 600 600 600 600 600 600 600
300 RT RT RT
°C/5 min °C/5 min °C/5 min °C/5 min °C/5 min °C/5 min °C/10 min °C/10 min þ 500 °C/30 min °C/10 min þ 500 °C/60 min °C/10 min þ 500 °C/90 min °C/5 min °C/5 min °C/5 min °C/5 min
TC;0 (K) 20 22 22 22 24 26 25 26 27 27 Non-supercond. 28 23 22
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at 10 Hz and boron for 3 min at 25 Hz alternatively (10 times). The total thickness of this multilayer was 180 nm. The grain-size is presumably still small, indicated by the absence of XRD peaks. Due to the fact that the two targets are metallic, instead of a composite, it is likely that less oxygen is incorporated in the film. This could be the reason why the multilayer technique leads to higher TC even without an extra anneal step. Typically, RRR ðRð300 KÞ=Rð40 KÞÞ value for most of the samples was close to 1, except on the sample made on SrTiO3 (reported in [5]) where the RRR value was 18. Fig. 1. RT measurement of superconducting thin films made from Mg-enriched MgB2 target (TC;0 27 K) and multilayerd film (TC;0 28 K).
3. Conclusions
Ar-etching process than the films deposited at room temperature, indicating that these films contains larger crystallites. Because the anneal step is very critical it has been studied in more detail and presented in Table 1. The increase of TC;0 (TC;0 27 K, Fig. 1) was noticed in the films annealed at 600 °C for 10 min in a Mg plasma, followed by a heat treatment at 500 °C for 1 h in 0.22 mbar of Ar. That would be consistent with the consideration that the grain size is of direct influence to the transition temperature: during the extra annealing the grain size has increased. However, the absence of MgB2 peaks in the XRD spectrum of our films indicates that the films are polycrystalline with a very small grain size even after longer annealing. The highest TC;0 has been obtained on MgO substrates. Since the films were deposited at low substrate temperatures, there is no epitaxial growth and the lattice mismatch does not play a crucial role in growing the films. However, it is possible that the films made on other substrates would result in the increase of TC as well. In an alternative fabrication procedure, multilayered films were grown from Mg and B targets on several substrates at room temperature (Table 1). The preablation and short annealing procedures were the same as described in [5]. The highest TC;0 of 28 K (Fig. 1) was obtained by depositing on MgO substrate, magnesium for 1 min
In this paper the preparation and properties of superconducting MgB2 thin films deposited from Mg-enriched MgB2 target, as well as prepared from Mg–B multilayers, is presented. The influence of the laser energy, substrate temperature and annealing procedure is found to be significant in obtaining the superconducting films. The highest TC ’s are achieved in multilayer films, most probably due to less MgO formation. We note that this work does not present a completed study, but may present useful information for the further optimization of MgB2 film growth. The annealing procedure was found to strongly influence the value of TC . Annealing of the sample at 500 °C for 1 h leads to better results than just quenching from 600 °C. We suppose that in the case of extra annealing the very small superconducting grains grow during annealing, and consequently enhance TC . Due to the fact that they are in both cases undetectable for XRD, we assume that their size is still small to be observed by this method. The films show suppressed transition temperature of up to 28 K compared to the bulk value of TC of 39 K, possibly due to a small grain-size, secondary phases and MgO inclusions. Comparing the two deposition methods we can conclude that the formation of different phases in the plasma is not the limiting factor, since Mg and B are in the multilayer technique not simultaneously in the plasma, a prerequisite to form these phases.
D. Mijatovic et al. / Physica C 372–376 (2002) 1258–1261
The anneal step as well as using metal targets, both leading to higher TC ’s, emphasize the idea that the suppressed TC is due to the small grain sizes as well as MgO formation.
Acknowledgements This work was supported by the Dutch Foundation for Research on Matter (FOM) and the Royal Dutch Academy of Arts and Sciences.
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References [1] J. Akimitsu, Presentation at ‘‘Symposium on Transition Metal Oxides’’, Sendai, January 10, 2001. [2] J. Nagamatsu et al., Nature 410 (2001) 63. [3] S.H. Moon et al., Presented at the post deadline session on MgB2 , APS March meeting 2001, Seattle. [4] C.-B. Eom et al., Nature 411 (2001) 558. [5] D.H.A. Blank et al., Appl. Phys. Lett. 79 (2001) 394. [6] X.H. Zeng et al., Appl. Phys. Lett. 79 (2001) 1840. [7] H.M. Christen et al., Physica C 353 (2001) 157. [8] G. Grassano et al., Supercond. Sci. Technol. 14 (2001) 762. [9] A. Brinkman et al., Physica C 353 (2001) 1.
Superconducting thin films of MgB2 by pulsed-laser deposition Dragana Mijatovic *, Alexander Brinkman, Guus Rijnders, Hans Hilgenkamp, Horst Rogalla, Dave H.A. Blank Department of Applied Physics, Low Temperature Division, MESA+ Research Institute, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands
Abstract Superconducting thin MgB2 films have been prepared using pulsed-laser deposition. We have studied the influences of deposition conditions such as pressure and temperature, the substrate-material, and annealing-procedures. Various approaches have been pursued to obtain the right Mg content in the film during ablation and annealing. Special care has been taken to avoid oxidation of Mg in the laser plasma and deposited film, by optimizing the background pressure of Ar gas in the deposition chamber. The annealing procedure was found to be the most critical to obtain superconducting films. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: MgB2 ; Thin films; Pulsed-laser deposition
1. Introduction The report of Akimitsu et al. [1,2] about the surprising discovery of superconductivity in MgB2 at 39 K has aroused the interest to investigate the properties of this material. The availability of the superconducting thin films is of great value for basic studies on this compound as well as for applications such as superconducting sensors and electronics. Two complicating factors for the fabrication of superconducting films of MgB2 are the high vapor pressure of magnesium at low temperatures and the high sensitivity of magnesium to oxidation, requiring very low oxygen partial pressures in the
*
Corresponding author. Fax: +31-53-489-1099. E-mail address: [email protected] (D. Mijatovic).
deposition system. To overcome both and to reach the bulk value of the transition temperature, ex situ methods have been exploited. Up to date, reports on superconducting MgB2 films with the highest zero resistance transition temperatures TC;0 of up to 39 K, are based on magnesium diffusion into a boron or Mg–B film at temperatures of 900– 950 °C and at high Mg pressures [3,4]. However, for a number of applications, such as Josephson junctions, and for various basic studies it is important to obtain smooth, well-defined, and preferably, epitaxial films in an all in situ process. Up to now there have been a few successful attempts in making in situ superconducting films by pulsedlaser deposition [5–8], but the transition temperatures of those samples were considerably below the bulk value. As magnesium-diffusion at high temperatures and pressures into pre-deposited boron films leads to films with superconducting
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 0 9 8 7 - 5
D. Mijatovic et al. / Physica C 372–376 (2002) 1258–1261
properties comparable with the bulk, it is likely that the plasma temperature promotes the formation of other Mg–B phases in addition to MgB2 , leading to films with suppressed TC [5]. Possible other explanations are very small grains of MgB2 , impurities like MgO or solid B that could be formed in the plasma and incorporated in the film [5]. This paper gives a study of the influences of the deposition conditions such as pressure and temperature, the substrate-material, and annealingprocedures of two step in situ deposited thin films by pulsed-laser deposition.
2. Experimental Two different approaches towards the growing of MgB2 thin films were pursued: by deposition from an Mg-enriched MgB2 target and by growing multilayered films of Mg and B. To study the influence of the substrates, Si, SiC, SrTiO3 , Al2 O3 , as well as MgO were used. Table 1 gives an overview of the influences of the type of the substrates, laser energy, deposition temperatures and annealing procedures on TC which will be discussed below. The films made from Mg-enriched MgB2 target are described in [5,9]. The films were deposited at temperatures ranging from room temperature to 300 °C. Initially, the films were deposited at 500 mJ
1259
laser energy on different substrates (Table 1). Due to the fact that the transition temperature was similar of all of the samples, it can be concluded that the type of substrate is not very significant in this method of the deposition, because the films are not epitaxially grown. Since TC has been increased on the samples made on MgO substrates by decreasing the laser energy to 400 mJ, we performed further experiments on this substrate and laser energy optimizing the other deposition parameters, such as deposition temperature, annealing process, etc. in order to improve TC . The energy density at the target was about 4 J/cm2 . The films were usually deposited for 6 min at 10 Hz, leading to a thickness of about 200 nm. A high temperature annealing step was needed to form the superconducting phase. Therefore, the films were heated up to Tann ¼ 600 °C, kept for 5 min and cooled down to room temperature in 0.22 mbar of Ar. The total annealing procedure was as short as possible to avoid Mg-evaporation out of the film. In the temperature range between 350 and 600 °C, the sample was kept in a pulsed (5 Hz) Mg plasma for the same reason [5]. The films deposited at room temperatures had the TC of 24–25 K. Higher TC;0 ’s (about 26 K) were achieved when the films were grown at a substrate temperature of 200 °C. Films prepared at 300 °C, however, were not superconducting (Table 1). Films deposited at 200 °C had a considerable lower etching rate in an
Table 1 An overview of the influences of the substrate material, laser energy, deposition temperature and annealing procedures on TC;0 for MgB2 films prepared from Mg-enriched MgB2 target (‘‘Mg þ MgB2 ’’) and from Mg–B multilayers Target
Substrate
Elaser (mJ)
Tdep (°C)
Annealing
‘‘Mg þ MgB2 ’’
MgO(1 0 0) SrTiO3 (1 0 0) Si(1 0 0) SiC (6H) MgO(1 0 0)
500 500 500 500 400
RT RT RT RT RT 200
Mg and B (multilayers)
MgO(1 0 0) SrTiO3 (1 1 1) Al2 O3 (R-plane)
400 400 400
600 600 600 600 600 600 600 600 600 600 600 600 600 600
300 RT RT RT
°C/5 min °C/5 min °C/5 min °C/5 min °C/5 min °C/5 min °C/10 min °C/10 min þ 500 °C/30 min °C/10 min þ 500 °C/60 min °C/10 min þ 500 °C/90 min °C/5 min °C/5 min °C/5 min °C/5 min
TC;0 (K) 20 22 22 22 24 26 25 26 27 27 Non-supercond. 28 23 22
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D. Mijatovic et al. / Physica C 372–376 (2002) 1258–1261
at 10 Hz and boron for 3 min at 25 Hz alternatively (10 times). The total thickness of this multilayer was 180 nm. The grain-size is presumably still small, indicated by the absence of XRD peaks. Due to the fact that the two targets are metallic, instead of a composite, it is likely that less oxygen is incorporated in the film. This could be the reason why the multilayer technique leads to higher TC even without an extra anneal step. Typically, RRR ðRð300 KÞ=Rð40 KÞÞ value for most of the samples was close to 1, except on the sample made on SrTiO3 (reported in [5]) where the RRR value was 18. Fig. 1. RT measurement of superconducting thin films made from Mg-enriched MgB2 target (TC;0 27 K) and multilayerd film (TC;0 28 K).
3. Conclusions
Ar-etching process than the films deposited at room temperature, indicating that these films contains larger crystallites. Because the anneal step is very critical it has been studied in more detail and presented in Table 1. The increase of TC;0 (TC;0 27 K, Fig. 1) was noticed in the films annealed at 600 °C for 10 min in a Mg plasma, followed by a heat treatment at 500 °C for 1 h in 0.22 mbar of Ar. That would be consistent with the consideration that the grain size is of direct influence to the transition temperature: during the extra annealing the grain size has increased. However, the absence of MgB2 peaks in the XRD spectrum of our films indicates that the films are polycrystalline with a very small grain size even after longer annealing. The highest TC;0 has been obtained on MgO substrates. Since the films were deposited at low substrate temperatures, there is no epitaxial growth and the lattice mismatch does not play a crucial role in growing the films. However, it is possible that the films made on other substrates would result in the increase of TC as well. In an alternative fabrication procedure, multilayered films were grown from Mg and B targets on several substrates at room temperature (Table 1). The preablation and short annealing procedures were the same as described in [5]. The highest TC;0 of 28 K (Fig. 1) was obtained by depositing on MgO substrate, magnesium for 1 min
In this paper the preparation and properties of superconducting MgB2 thin films deposited from Mg-enriched MgB2 target, as well as prepared from Mg–B multilayers, is presented. The influence of the laser energy, substrate temperature and annealing procedure is found to be significant in obtaining the superconducting films. The highest TC ’s are achieved in multilayer films, most probably due to less MgO formation. We note that this work does not present a completed study, but may present useful information for the further optimization of MgB2 film growth. The annealing procedure was found to strongly influence the value of TC . Annealing of the sample at 500 °C for 1 h leads to better results than just quenching from 600 °C. We suppose that in the case of extra annealing the very small superconducting grains grow during annealing, and consequently enhance TC . Due to the fact that they are in both cases undetectable for XRD, we assume that their size is still small to be observed by this method. The films show suppressed transition temperature of up to 28 K compared to the bulk value of TC of 39 K, possibly due to a small grain-size, secondary phases and MgO inclusions. Comparing the two deposition methods we can conclude that the formation of different phases in the plasma is not the limiting factor, since Mg and B are in the multilayer technique not simultaneously in the plasma, a prerequisite to form these phases.
D. Mijatovic et al. / Physica C 372–376 (2002) 1258–1261
The anneal step as well as using metal targets, both leading to higher TC ’s, emphasize the idea that the suppressed TC is due to the small grain sizes as well as MgO formation.
Acknowledgements This work was supported by the Dutch Foundation for Research on Matter (FOM) and the Royal Dutch Academy of Arts and Sciences.
1261
References [1] J. Akimitsu, Presentation at ‘‘Symposium on Transition Metal Oxides’’, Sendai, January 10, 2001. [2] J. Nagamatsu et al., Nature 410 (2001) 63. [3] S.H. Moon et al., Presented at the post deadline session on MgB2 , APS March meeting 2001, Seattle. [4] C.-B. Eom et al., Nature 411 (2001) 558. [5] D.H.A. Blank et al., Appl. Phys. Lett. 79 (2001) 394. [6] X.H. Zeng et al., Appl. Phys. Lett. 79 (2001) 1840. [7] H.M. Christen et al., Physica C 353 (2001) 157. [8] G. Grassano et al., Supercond. Sci. Technol. 14 (2001) 762. [9] A. Brinkman et al., Physica C 353 (2001) 1.