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Fabrication and properties of thin superconducting oxide films
Physica 148B (1987) 182-184 North-Holland, Amsterdam
FABRICATION AND PROPERTIES OF THIN SUPERCONDUCTING OXIDE FILMS R.B. L A I B O W I T Z , R.H. K O C H , P. C H A U D H A R I and R.J. G A M B I N O I B M Research Center, JR O. Box 218, Yorktown Heights, N Y 10598, USA
Received 20 August 1987
Thin films of YiBa2Cu3Oy have been deposited in a three source electron-beam heated deposition system. The films have superconducting transition temperatures of about 90 K after a post deposition annealing treatment. Film thicknesses range from 0.1 to 10 txm and several types of substrates have been used. Films grown on SrTiO3 single crystals generally show the sharpest transitions (AT. < 1 K). The films have been characterized in a variety of ways including chemical analysis, electron and tunneling microscopy,storage and patterning studies, critical currents and fields, SQUIDs and other physical measurements.
We have fabricated thin films of Y1Ba2Cu3Oy (1-2-3) composition in a three source deposition system [1] which uses electron-beam heating to produce the evaporants from the base starting metals. While some details of the fabrication process can be found in references [1-3], the process will be summarized here. Three separate sources are used, one for each metal and the evaporation rates are independently computer monitored and controlled during the deposition. In this way deviations in the evaporation rates can be corrected during the actual run. Typical deposition rates were in the range of 0.01 to 1 n m / s using 10 kV electron guns with very little power needed to evaporate the low melting point barium. Both Ba (and Sr) could be easily handled in hearth liners, e.g. vitreous C, which facilitated changing material. The rates and thicknesses were preset to give a particular metal ratio which was previously obtained by chemical analysis. This analysis was accomplished using an inductively coupled plasma technique [4]. This technique yielded an average composition which was very helpful in evaluating each run. More detailed compositional analysis will be reported on in a later publication. The fourth element needed to obtain good superconducting 1-2-3 films is oxygen and there are several ways of introducing oxygen into the films. In our fabrication process oxygen is admitted to the vacuum chamber at a level of about 0378-4363/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) and Yamada Science Foundation
10 3Torr during the evaporation. The three metals are condensed onto the substrates at relatively low rates in the presence of the oxygen. If O is not incorporated during the growth, the surface of the metal film will react rapidly on exposure to room ambient forming diffusive-like patterns that are difficult to redissolve. These patterns will also form when room temperature substrates are used even when O is used in the evaporator. However, these films can be successfully annealed, particularly if done immediately upon removal from the vacuum system. Substrate temperature could be controlled from - 1 0 0 ° C to about 700°C and most films were made at about 375°C. Results obtained with room temperature substrates will also be reported here. A wide variety of substrates was used in this study including sapphire (A1203) both aand c-axis oriented, MgO (100) oriented, Si and oxidized Si, SrTiO3, fused silica and SrF 2. The SrTiO 3 substrates generally gave the best results which will be shown in this paper while some results from sapphire are given in ref. [1]. G o o d superconducting films could be made also on MgO but as yet the narrowest transition widths have been obtained on the SrTiO 3 to be < 1 K [2]. Here we use the transition width as obtained from the resistive transition 90% and 10% points. The as-deposited films were generally smooth in appearance and non-superconducting and had
R.B. Laibowitz et al. / Fabrication and properties o f thin superconducting oxide films
resistivities of the order of several hundred l) cm at room temperature. A post-deposition annealing treatment mainly in oxygen was required to produce the final superconducting film. The samples were inserted into a tube type furnace set at about 920°C. Throughout the entire annealing process gas flowed through the tube at roughly atmospheric pressure. The time at the high temperature was generally about three to five minutes and then the furnace was allowed to cool to close to room temperature in about three hours in oxygen. Some samples, particularly the thicker ones, could be annealed for longer times with small improvements observed in the resistive transition. However, the annealing did seem to be somewhat furnace dependent, leaving room for future analytical work. Additional details on the annealing of these films is given in ref. [3]. Both scanning electron and transmission electron microscopy (SEM and T E M ) studies have been made on these films and show the films to have rough surfaces composed in part of crystalline inclusions. This has been seen on a variety of substrates. For example, as-deposited films on MgO show inclusions of copper oxides and B a C u O 2 in an amorphous matrix with the Y being in the amorphous material [5]. Crystallite size generally is about 5 to 20 nm. After annealing the orthorombic 1-2-3 phase is observed with crystallites size in the several micron range. In addition, inclusions of Y203 are also observed. A much more epitaxial film is obtained on SrTiO 3 [2]; however, inclusions are also observed in these films. Studying substrate film interaction are thus seen to be important in understanding the film properties, particularly in view of the high annealing temperatures. In order to begin studies of the film substrate interaction, we have attempted to grow films of varying thickness on SrTiO 3 and the results of one such series is shown in fig. 1. In this plot we see the resistive transition plotted as a function of temperature for several runs on a SrTiO 3 substrate. The as-deposited thicknesses ranged from about 0.1 to 1 Ixm, and both room temperature and 375°C were used. The 1 Ixm data is actually from two runs and demonstrates the
183
30
YBa2Cu30y on Srrio 3
J 20
thickness = 0.1 / ~ m Y
_
J
0.2 10
o
0.3
0
1O0 200 Temperoture (K)
300
Fig. 1. Resistance vs. t e m p e r a t u r e for several Y B a C u O of varying thickness on SrTiO 3 substrates. The two thicker films are made with a substrate t e m p e r a t u r e of 375°C while the thinner ones are deposited at room t e mpe ra t ure .
reproducibility obtainable for the resistive transition at that thickness. All films were superconducting above liquid nitrogen temperature. It can be seen that the room temperature resistance scales roughly with the thickness and that the resistance ratio diminishes with decreasing thickness as might be expected with some impurity scattering. The resistance ratio (room temperature resistance divided by the resistance prior to superconductivity) for the largest thickness in fig. 1 is about 2.6, while for the thinnest a value of 2.1 is obtained. The resistance versus temperature curves above the transition are quite linear as has been observed in this compound. A small decrease in the transition temperature of the thinner films is also observed. While chemical and structural analysis of these films is underway, it is interesting to note that the thickness of the 0.1 Ixm film after annealing appears to be less than the as-deposited film while the micron thick films generally show an increase in thickness after annealing, again showing the importance of the substrate film interaction. Some films tend to show some deterioration of their resistive transitions with time which is an important parameter for future applications. However, some initial results on the 0.3 I~m film as shown in fig. 2 are encouraging in this regard. It can be seen that after about 4 weeks of storage in a plastic box in air virtually no change in the transition temperature has occurred. The small
184
R.B. Laibowitz et al. / Fabrication and properties o f thin superconducting oxide films 40
i
i
d
[
i
YBa2Cu30yon SrTiO3 0.3 #m thick 30
~
"
after 28 d a y ~ f
a~
[~ initial [ measurement
10
O 0
j_
i 50
/
it ~ t 100 150 200 250 300 Temperature (K)
Fig. 2. Resistance vs. temperature for a 0.3 ~m thick sample before and after 28 days of storage.
increase in resistance is due in part to using a different area on the same chip with slightly different contacts. The deterioration of the films has also been observed in attempts to pattern the films lithographically. In general it is possible to make patterned films with reasonable edge definition in the 10 p~ line width range on sapphire and MgO but after processing a downward shift in the T c is usually observed. While this work will take more effort in the future, it is interesting to note that it is possible to use this downward shift to pattern films, a technique used to make the first thin film S Q U I D operating in the liquid nitrogen range [6, 7]. The S Q U I D patterning was accomplished using ion implantation of O or As ions to convert superconducting regions to non-superconducting regions resulting in a planarized weak link S Q U I D structure. Many physical properties have been measured on these films including critical currents and fields, energy gaps by scanning tunneling microscopy and infra-red measurements and Hall effect. Most of this is covered in the references; however, we shall summarize a few of the findings here. First observation of high critical currents in the 1-2-3 compound were made in the epitaxial films and showed values in excess of 10 5 A / c m 2 at liquid nitrogen temperature [2]. Other labs have recently reported high critical currents in thin films [8, 9]. Critical field parameters such as d H c z / d T of about 4.3 T / K [3] have been observed in these films. The superconduct-
ing energy gaps [3] have been determined from both infra-red [10] and scanning tunneling microscopy [11] studies. Interestingly the value of 2 A / k T c is about 4.5 which is larger than the BCS value of 3.5 but about the same as that observed for strong coupled superconductors such as Hg. Hall measurements are also discussed in ref. [3].
Acknowledgements The authors wish to gratefully acknowledge the assistance of J. Lacey and J. Viggiano in many phases of the film fabrication and characterization.
References [1] R.B. Laibowitz, R.H. Koch, P. Chaudhari and R.J. Gambino, Phys. Rev. B 35 (1987) 8821. [2] P. Chaudhari, R.H. Koch, R.B. Laibowitz, T.R. McGuire and R.J. Gambino, Phys. Rev. Lett. 58 (1987) 2684. [3] P. Chaudhari, R.J. Collins, P. Freitas, R.J. Gambino, J. Kirtley, R.H. Koch, R.B. Laibowitz, F. Legoues, T.R. McGuire, T. Penney, A. Schlesinger, A. Segmuller, S. Foner and E.J. McNiff, Jr., Phys Rev., submitted. [4] R.B. Laibowitz, R.H. Koch, P. Chaudhari, G.J. Clark, R.J. Gambino, M. Plechaty, J.A. Lacey, C.P. Umbach, A.D. Marwick and J.M. Viggiano, J. Electrochem. Soc., submitted. [5] S. Herd, R.B. Laibowitz and R.H. Koch, unpublished results. [6] R.H. Koch, C.P. Umbach, G.J. Clark, P. Chaudhari and R.B. Laibowitz, Appl. Phys. Lett. 51 (1987) 200. [7] G.J. Clark, A.D. Marwick, R.H. Koch and R.B. Laibowitz, Appl. Phys. Lett. 51 (1987) 139. [8] B. Oh, M. Naito, S. Arnason, P. Rosenthal, R. Barton, M.R. Beasley, T.H. Geballe, R. HY. Hammond and A. Kapitulnik, preprint. [9] Y. Enomoto, T. Murakami, M. Suzuki and K. Moriwaki, NT1~ preprint. [10] R.T. Collins, Z. Schlesinger, R.H. Koch, R.B. Laibowitz, T.S. Plaskett, P. Freitas, W.J. Gallegher, R.L. Sandstrom and T.R. Dinger, Phys. Rev. Lett. 59 (1987) 704. [11] J.R. Kirtley, R.M. Feenstra, A.E Fein, S.I. Raider, W.J. Gallegber, R. Sandstrom, T. Dinger, M.W. Shafer, R. Koch, R.B. Laibowitz and B. Bumble, Proc. STM 87, J. Vac. Sci. and Technol., March/April 1988, to appear.
FABRICATION AND PROPERTIES OF THIN SUPERCONDUCTING OXIDE FILMS R.B. L A I B O W I T Z , R.H. K O C H , P. C H A U D H A R I and R.J. G A M B I N O I B M Research Center, JR O. Box 218, Yorktown Heights, N Y 10598, USA
Received 20 August 1987
Thin films of YiBa2Cu3Oy have been deposited in a three source electron-beam heated deposition system. The films have superconducting transition temperatures of about 90 K after a post deposition annealing treatment. Film thicknesses range from 0.1 to 10 txm and several types of substrates have been used. Films grown on SrTiO3 single crystals generally show the sharpest transitions (AT. < 1 K). The films have been characterized in a variety of ways including chemical analysis, electron and tunneling microscopy,storage and patterning studies, critical currents and fields, SQUIDs and other physical measurements.
We have fabricated thin films of Y1Ba2Cu3Oy (1-2-3) composition in a three source deposition system [1] which uses electron-beam heating to produce the evaporants from the base starting metals. While some details of the fabrication process can be found in references [1-3], the process will be summarized here. Three separate sources are used, one for each metal and the evaporation rates are independently computer monitored and controlled during the deposition. In this way deviations in the evaporation rates can be corrected during the actual run. Typical deposition rates were in the range of 0.01 to 1 n m / s using 10 kV electron guns with very little power needed to evaporate the low melting point barium. Both Ba (and Sr) could be easily handled in hearth liners, e.g. vitreous C, which facilitated changing material. The rates and thicknesses were preset to give a particular metal ratio which was previously obtained by chemical analysis. This analysis was accomplished using an inductively coupled plasma technique [4]. This technique yielded an average composition which was very helpful in evaluating each run. More detailed compositional analysis will be reported on in a later publication. The fourth element needed to obtain good superconducting 1-2-3 films is oxygen and there are several ways of introducing oxygen into the films. In our fabrication process oxygen is admitted to the vacuum chamber at a level of about 0378-4363/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) and Yamada Science Foundation
10 3Torr during the evaporation. The three metals are condensed onto the substrates at relatively low rates in the presence of the oxygen. If O is not incorporated during the growth, the surface of the metal film will react rapidly on exposure to room ambient forming diffusive-like patterns that are difficult to redissolve. These patterns will also form when room temperature substrates are used even when O is used in the evaporator. However, these films can be successfully annealed, particularly if done immediately upon removal from the vacuum system. Substrate temperature could be controlled from - 1 0 0 ° C to about 700°C and most films were made at about 375°C. Results obtained with room temperature substrates will also be reported here. A wide variety of substrates was used in this study including sapphire (A1203) both aand c-axis oriented, MgO (100) oriented, Si and oxidized Si, SrTiO3, fused silica and SrF 2. The SrTiO 3 substrates generally gave the best results which will be shown in this paper while some results from sapphire are given in ref. [1]. G o o d superconducting films could be made also on MgO but as yet the narrowest transition widths have been obtained on the SrTiO 3 to be < 1 K [2]. Here we use the transition width as obtained from the resistive transition 90% and 10% points. The as-deposited films were generally smooth in appearance and non-superconducting and had
R.B. Laibowitz et al. / Fabrication and properties o f thin superconducting oxide films
resistivities of the order of several hundred l) cm at room temperature. A post-deposition annealing treatment mainly in oxygen was required to produce the final superconducting film. The samples were inserted into a tube type furnace set at about 920°C. Throughout the entire annealing process gas flowed through the tube at roughly atmospheric pressure. The time at the high temperature was generally about three to five minutes and then the furnace was allowed to cool to close to room temperature in about three hours in oxygen. Some samples, particularly the thicker ones, could be annealed for longer times with small improvements observed in the resistive transition. However, the annealing did seem to be somewhat furnace dependent, leaving room for future analytical work. Additional details on the annealing of these films is given in ref. [3]. Both scanning electron and transmission electron microscopy (SEM and T E M ) studies have been made on these films and show the films to have rough surfaces composed in part of crystalline inclusions. This has been seen on a variety of substrates. For example, as-deposited films on MgO show inclusions of copper oxides and B a C u O 2 in an amorphous matrix with the Y being in the amorphous material [5]. Crystallite size generally is about 5 to 20 nm. After annealing the orthorombic 1-2-3 phase is observed with crystallites size in the several micron range. In addition, inclusions of Y203 are also observed. A much more epitaxial film is obtained on SrTiO 3 [2]; however, inclusions are also observed in these films. Studying substrate film interaction are thus seen to be important in understanding the film properties, particularly in view of the high annealing temperatures. In order to begin studies of the film substrate interaction, we have attempted to grow films of varying thickness on SrTiO 3 and the results of one such series is shown in fig. 1. In this plot we see the resistive transition plotted as a function of temperature for several runs on a SrTiO 3 substrate. The as-deposited thicknesses ranged from about 0.1 to 1 Ixm, and both room temperature and 375°C were used. The 1 Ixm data is actually from two runs and demonstrates the
183
30
YBa2Cu30y on Srrio 3
J 20
thickness = 0.1 / ~ m Y
_
J
0.2 10
o
0.3
0
1O0 200 Temperoture (K)
300
Fig. 1. Resistance vs. t e m p e r a t u r e for several Y B a C u O of varying thickness on SrTiO 3 substrates. The two thicker films are made with a substrate t e m p e r a t u r e of 375°C while the thinner ones are deposited at room t e mpe ra t ure .
reproducibility obtainable for the resistive transition at that thickness. All films were superconducting above liquid nitrogen temperature. It can be seen that the room temperature resistance scales roughly with the thickness and that the resistance ratio diminishes with decreasing thickness as might be expected with some impurity scattering. The resistance ratio (room temperature resistance divided by the resistance prior to superconductivity) for the largest thickness in fig. 1 is about 2.6, while for the thinnest a value of 2.1 is obtained. The resistance versus temperature curves above the transition are quite linear as has been observed in this compound. A small decrease in the transition temperature of the thinner films is also observed. While chemical and structural analysis of these films is underway, it is interesting to note that the thickness of the 0.1 Ixm film after annealing appears to be less than the as-deposited film while the micron thick films generally show an increase in thickness after annealing, again showing the importance of the substrate film interaction. Some films tend to show some deterioration of their resistive transitions with time which is an important parameter for future applications. However, some initial results on the 0.3 I~m film as shown in fig. 2 are encouraging in this regard. It can be seen that after about 4 weeks of storage in a plastic box in air virtually no change in the transition temperature has occurred. The small
184
R.B. Laibowitz et al. / Fabrication and properties o f thin superconducting oxide films 40
i
i
d
[
i
YBa2Cu30yon SrTiO3 0.3 #m thick 30
~
"
after 28 d a y ~ f
a~
[~ initial [ measurement
10
O 0
j_
i 50
/
it ~ t 100 150 200 250 300 Temperature (K)
Fig. 2. Resistance vs. temperature for a 0.3 ~m thick sample before and after 28 days of storage.
increase in resistance is due in part to using a different area on the same chip with slightly different contacts. The deterioration of the films has also been observed in attempts to pattern the films lithographically. In general it is possible to make patterned films with reasonable edge definition in the 10 p~ line width range on sapphire and MgO but after processing a downward shift in the T c is usually observed. While this work will take more effort in the future, it is interesting to note that it is possible to use this downward shift to pattern films, a technique used to make the first thin film S Q U I D operating in the liquid nitrogen range [6, 7]. The S Q U I D patterning was accomplished using ion implantation of O or As ions to convert superconducting regions to non-superconducting regions resulting in a planarized weak link S Q U I D structure. Many physical properties have been measured on these films including critical currents and fields, energy gaps by scanning tunneling microscopy and infra-red measurements and Hall effect. Most of this is covered in the references; however, we shall summarize a few of the findings here. First observation of high critical currents in the 1-2-3 compound were made in the epitaxial films and showed values in excess of 10 5 A / c m 2 at liquid nitrogen temperature [2]. Other labs have recently reported high critical currents in thin films [8, 9]. Critical field parameters such as d H c z / d T of about 4.3 T / K [3] have been observed in these films. The superconduct-
ing energy gaps [3] have been determined from both infra-red [10] and scanning tunneling microscopy [11] studies. Interestingly the value of 2 A / k T c is about 4.5 which is larger than the BCS value of 3.5 but about the same as that observed for strong coupled superconductors such as Hg. Hall measurements are also discussed in ref. [3].
Acknowledgements The authors wish to gratefully acknowledge the assistance of J. Lacey and J. Viggiano in many phases of the film fabrication and characterization.
References [1] R.B. Laibowitz, R.H. Koch, P. Chaudhari and R.J. Gambino, Phys. Rev. B 35 (1987) 8821. [2] P. Chaudhari, R.H. Koch, R.B. Laibowitz, T.R. McGuire and R.J. Gambino, Phys. Rev. Lett. 58 (1987) 2684. [3] P. Chaudhari, R.J. Collins, P. Freitas, R.J. Gambino, J. Kirtley, R.H. Koch, R.B. Laibowitz, F. Legoues, T.R. McGuire, T. Penney, A. Schlesinger, A. Segmuller, S. Foner and E.J. McNiff, Jr., Phys Rev., submitted. [4] R.B. Laibowitz, R.H. Koch, P. Chaudhari, G.J. Clark, R.J. Gambino, M. Plechaty, J.A. Lacey, C.P. Umbach, A.D. Marwick and J.M. Viggiano, J. Electrochem. Soc., submitted. [5] S. Herd, R.B. Laibowitz and R.H. Koch, unpublished results. [6] R.H. Koch, C.P. Umbach, G.J. Clark, P. Chaudhari and R.B. Laibowitz, Appl. Phys. Lett. 51 (1987) 200. [7] G.J. Clark, A.D. Marwick, R.H. Koch and R.B. Laibowitz, Appl. Phys. Lett. 51 (1987) 139. [8] B. Oh, M. Naito, S. Arnason, P. Rosenthal, R. Barton, M.R. Beasley, T.H. Geballe, R. HY. Hammond and A. Kapitulnik, preprint. [9] Y. Enomoto, T. Murakami, M. Suzuki and K. Moriwaki, NT1~ preprint. [10] R.T. Collins, Z. Schlesinger, R.H. Koch, R.B. Laibowitz, T.S. Plaskett, P. Freitas, W.J. Gallegher, R.L. Sandstrom and T.R. Dinger, Phys. Rev. Lett. 59 (1987) 704. [11] J.R. Kirtley, R.M. Feenstra, A.E Fein, S.I. Raider, W.J. Gallegber, R. Sandstrom, T. Dinger, M.W. Shafer, R. Koch, R.B. Laibowitz and B. Bumble, Proc. STM 87, J. Vac. Sci. and Technol., March/April 1988, to appear.