- Email: [email protected]
In-situ YBa2Cu3O7−x thin films epitaxially grown by single target DC sputtering
PhysicaC 166 (1990) 105-l 10 North-Holland
IN-SITU YBa2Cu307_x THIN FILMS EPITAXIALLY GROWN BY SINGLE TARGET DC SPUTTERING M. GUILLOUX-VIRY
‘, M.G. KARKUT,
A. PERRIN,
0. PENA, J. PADIOU
and M. SERGENT
Laboratoirede Chimie MnPrale B, URACNRS 254, Universitkde Rennes I, Avenuedu GtWral Leclerc, 35042 RENNES-CPdex, France ’ Division OCM, CNET, Centre de Lannion B, BP 40 22301 LANNION-Ckdex, France
Received 19 December 1989
We have grown in-situ thin films of YBaZCuJO,_x (YBCO) using a simple DC sputtering system equipped with a single stoichiometric target. Operating at very high sputtering pressures of greater than 1 mbar, we have been able to grow, with excellent reproducibility, primarily c-axis oriented, thin (between 250 and 1000 A) YBCO films on ( 100)MgO and ( IOO)SrTiOj substrates heated to - 700°C. These films have resistivity ratios R(290 K)/R( 100 K) between 2.2 and 2.8, resistive T, onsets between 85 and 89 K, zero resistance between 8 1and 86.5 K, and extremely narrow ( < I K) AC susceptibility transitions indicative of excellent film homogeneity.
1. Introduction The discovery of superconductivity above liquid nitrogen temperature opened the way to a number of potential applications. However the technological exploitation of these ceramic materials may be limited by their very low critical current densities at 77 K. Even microelectronic applications require current densities higher than the values reported for bulk ceramics or for randomly oriented thin films. By contrast, high-T, thin films epitaxially grown on single crystal SrTiO, and MgO substrates can have critical current densities greater than lo6 A/cm* at 77K[ l-31. Films initially deposited at room temperature are amorphous and insulating and must be annealed between 800°C and 900°C in order to form the highT, phase. At these high temperatures, film-substrate interactions can become very destructive to the superconducting properties [ 41. Moreover, in any device fabrication process, ex-situ post-treatments can be a source of contamination thereby reducing the attainable level of integration. Thus the challenge is to prepare in-situ thin films at lower temperatures so that a wider array of technologically relevant substrate materials become available. Magnetron and RF 092 l-4534/90/$03.50 ( North-Holland )
0 Elsevier Science Publishers B.V.
sputtering techniques [ 51 have so far proven to be among the most successful means of producing not only in-situ films but also artificial structures [ 61 of the 1: 2 : 3 material. However, up to now we are not aware of any group producing in-situ thin films using straightforward DC sputtering. In this paper, we report on the in-situ growth of YBaZCu307_-x (YBCO) thin films by a simple DC sputtering system using a single stoichiometric 1: 2 : 3 target. The films are sputtered at the very high pressures of greater than 1 mbar in order to avoid destructive backsputtering of the film [ 71. Using single crystal substrates of ( 100)SrTiOs and ( lOO)MgO, we have been able to routinely produce highly oriented YBCO films with zero resistances in the mid80’s and which have extremely narrow ( < 1 K) AC susceptibility transitions.
2. Deposition procedure We fixed a stoichiometric YBa2Cu307 target 1 inch in diameter and 5 mm thick on a vertically mounted DC sputtering gun. The base pressure Pb of the system is between 5 x 10e6 and 3 x 10e5 mbar. The sputtering gas is a flowing mixture of 4.5NAr and
106
hf.Guilloux- Viry et al. /In-situ
2.5N O2 pumped by a sorption pump. The ratio of Ar to O2 is maintained at typically 10 : 1 and the total sputtering pressure is at least 1 mbar. The sputtering rate is only about 300 A/h due to the high total pressure and the presence of oxygen in the plasma. The ( 100)SrTi03 or (100)MgO single crystal substrates are placed on a flat plate of stainless steel which is situated 15 mm from, and directly in front of, the target. The plate is heated to about 800°C and is monitored by a thermocouple although we estimate the substrate temperature to be about from 100’ C to 150°C below the plate temperature. Following deposition, the plate is cooled from 800 oC to 500’ C in no more than 15 min and then maintained in 1 atm of O2 at this temperature for 30 min before power is cut to the heater to allow the substrates to cool to room temperature. All samples prepared in this manner are black, highly reflecting, and show varying degrees of transparency depending on their thickness when directly observed. The samples are then analyzed with no further annealing.
3. Characterization
and discussion
Films grown on substrates of ( lOO)SrTiO, and (100)MgO have been analyzed using 0-20 X-ray diffractometry, oscillating mode Weissenberg X-ray photography, DC resistance and AC susceptibility measurements. Using the deposition conditions described above, we are able to routinely produce highly oriented films on SrTiO, which have zero resistances between 83 and 86.5 K ( Tco's) and extremely narrow (AT< 1 K) AC susceptibility transitions. Oriented films grown on MgO have only slightly less good T,,'s (between 80 and 84 K) but with susceptibility transitions widths still less than 1 K. Figure 1 shows two X-ray diffractograms of films prepared on a) (100)MgO and b) ( 100)SrTiOx. The film on MgO shows highly oriented growth with predominantly c-axis orientation, i.e. the c-axis is perpendicular to the plane of the film. There are also traces of a-axis orientation. The film on SrTi03 displays a mixed a- and c-axis orientation and this is partly due to the substrate temperature T,.At higher T,,the intensities of the (hO0) reflections decrease with respect to the (OOQreflections. At lower T,,the films start to become randomly oriented and the peak
YBa2Cu307_,
thin Jlms
intensities are about two orders of magnitude weaker than the highly oriented films. The widths of the peaks in fig. 1 are narrow - in general the full width at half maximum FWHM of the (005 ) reflection for the best films is about 0.15’ - and are at the limit of instrumental broadening. Simple estimates based on the Scherrer formula [8] indicate that the crystalline coherence of these films is very long and extends over the entire thickness of the film. Finally, the positions of the (001) reflections are shifted to slightly lower Bragg angles giving a longer c-axis ( - 11.74A) than book values for the YBazCuJO, phase. These peak shifts could imply a lack of oxygen in the films and this could be the reason why the superconducting resistive onset occurs at slightly lower values (between 87 and 89 K for films on SrTiOJ and between 84 and 87 K for films on MgO) than those reported elsewhere. However, as we shall see, the widths of the transitions, a measure of film quality, are extremely sharp. The mixed a- and c-axis orientation, the longer c-axis parameter and the lower superconductive onset temperatures could be due to the not-yet-optimal growth conditions. This should not be surprising considering the large number of parameters involved in the growth of the 1: 2 : 3 phase. Another possibility to be considered is the high base pressure of the system before sputtering resulting in the possible degradation of the films during the deposition. Unfortunately, since the lower limit of our system is 5 x 10e6 mbar, it is not possible to test this assumption thoroughly. Additional information about the crystallinity of the films can be obtained by taking crystal diffraction photographs in the oscillating mode using a conventional Weissenberg camera. Photographs of films grown on ( 100 ) MgO, in addition to confirming the c-axis orientation of the films, reveal a strict epitaxial orientation of the a- and b-axes with respect to the crystallographic axes of the substrate and on the best films no measurable misalignment was found 191. Figure 2 shows the resistively measured R( 7‘) curve for a film grown on ( 100) SrTi03. The residual resistance ratio (RRR) R(290K)/R( lOOK) is 2.8. The linear part of the R (T) curve extrapolates nearly to zero as T+O. The onset temperature of the transition is rather low 88.5 K but T,,=86.4K. We define the transition width AT as the difference be-
hf. Guilloux-Viry
et al. /In-situ
YBa2CuJ0,_,
thin films
107
a /
;rT;03 * 006
‘rT;03 003
#
43.5
005
I
IOC J
7
J I
50
i
/
(_cuo
I
004 A I
40 DIFFRACTION
30 ANGLE
(28
I
I
20
10
Cu)
Fig. I. (a) X-ray diffractogram of a YBCO film grown on ( 100) MgO. The orientation is primarily (001). (b) X-ray diffractogram of a YBCO film grown on ( 100) SrTi03. The orientation is mixed a- and c-axis. The (003) and the (006) peaks are masked by the SrTiOJ peaks.
tween 90% of the normal state resistance and zero resistance. In general the best films grown on SrTi03 have RRR’s between 2.5 and 2.8, very narrow AT’s between 1.5 and 2.5 K and T,,‘s between 84 and 86.4 K. Films grown on MgO have RRR’s between 2.2 and 2.55, AT’s between 2 and 3.5 K and To’s between 8 1 and 84 K. The films have not been photolithographically patterned so we can only put a
lower limit on the critical current density J, at 77.3 K. Since we have been able to pass as much as 400 mA through some films before a voltage appears, given the dimensions of the sample, we estimate the J, at 77.3 K to be at least 10’ A/cm’. Susceptibility measurements provide additional information about the superconducting transition and the sample quality. They can probe subtle dif-
hf. Guilloux-Viry et al. /In-situ YBa2Cu307_,
thin films
_ # 45.5 SrTl
03
I
78
J 80
82
84
Temperature
0
30
60
90
120 150 180 210 240 270 300 TEMPERATURE
88
90
92
(K)
vs. temperature
of the same film as
K
Fig. 2. Normalized resistance vs. temperature for a film grown on SrTiO,.
in the crystallization state of the film which are not necessarily taken into account by a resistance measurement. We used a mutual inductance bridge operating at a fixed 119 Hz. In phase x’ and quadrature x” components were simultaneously detected and plotted as a function of temperature using exactly the same cooling and cycling procedure for all runs. Small sized primary and secondary coils were used with the film sandwiched between them and the AC excitation field normal to the film surface. The films’ dimensions were larger than, or the same size as, the coil diameter so as to promote full shielding of the magnetic field when T-c T,. However, sample size did not seem to be a factor in the qualitative analysis of the results. The response amplitudes were not calibrated. Figure 3 shows x’ (T) and x”(T) for the same sample presented in fig. 2. What is striking here is the extreme narrowness of the transition. The narrow transition width of x’ ( T) (a lo%-90% criterium gives less than 0.5 K), the absence of step-like anomalies in x’ ( T) and the sharp (FWHM _ 0.60 K) single peak in x” ( T) indicate the absence of granular effects and the excellent homogeneity of the film over its entire surface ( 5 x 7 mm2 ). Our best films all display similar behaviour which reflects the very high ferences
Fig. 3. The AC susceptibility in fig. 2.
86
quality of the YBCO films produced in our sputtering system. We now wish to illustrate by way of fig. 4 (a) and 4(b) the utility of the susceptibility measurements by directly comparing them to their respective resistivity curves. The two films deposited on MgO both have transition widths of about 2.7 K and only slightly different T,,‘s (82.9 and 80.7 K). However, their magnetic characteristics are strikingly different in that their inductive transition widths differ by a factor of 10 (of the order of 3 K and 0.3 K, respectively). Moreover it is the film with the lower T, which has the narrower transition. The difference in the AC transition is apparently related to weak link behavior at the grain boundaries. We have noted that those films having the highest critical currents systematically have shown the narrowest peaks of the dissipation component x” ( T). Similar results have been found by Neumann et al. [ lo] who established a clear correlation between the critical current densities and the transition widths of the AC susceptibility transitions. Thus, we feel confident in presenting the susceptibility widths as a reliable measure of film quality.
4. Conclusions We have used a simple DC sputtering system to grow in-situ highly oriented YBa2Cu@_, super-
M. Guilloux-Vity
et al. /In-situ
thin films
YBa,Cu,O,_,
109
0
# 48 2 2I-
0
3-
I
/
I
,
I
I
I
/----------
# L9 5
# 48.2
78
80
82 84 86 @a 90 Temperature (K)
92
78
a0
82 84 86 88 Temperature (K)
90
92
Fig. 4. (a) R( 7’) and x( T) for YBCO sample f49.5 grown on ( 100) MgO. (b) R(T) and x( T) for YBCO sample #48.2 also grown on ( 100) MgO. Note that even though the resistive transition widths are essentially the same, the inductive transition widths differ by about a factor of 10.
conducting thin films. On substrates of ( 100)SrTi03 we routinely obtain films that have superconducting resistive onsets between 87 and 89 IS, Tco's between 84 and 86.4 K and inductive transition widths of less than 1 K. Oriented films grown on MgO have systematically lower onsets of between 84 and 87 K, Tco's between 8 1 and 84 K, and slighter broader inductive transitions. The reasons for the slightly depressed resistive onsets and the systematic differences between films grown on SrTi03 and MgO have not yet been determined but could be in part due to Mg diffusion into the latter films. The present excellent results demonstrate the effectiveness of our DC sputtering system. Neither the high base pressure nor the elevated sputtering pressure (to avoid backsputtering from the film) seems to have a very negative influence on the properties of the films. The small stoichiometric targets are an
added advantage since they are easy to make and are more robust than larger targets needed for, say magnetron sputtering. The films are homogeneous over a diameter of 1 cm as revealed by the inductive measurements and the narrow inductive transitions compare favorably with those performed on films fabricated by other deposition techniques.
Acknowledgements
We thank J.C. Jegaden for technical assistance. Work supported by Centre National d’Etudes des Telecommunications under contract no 89 8B 054 and Ministere de la Recherche et de la Technologie under contract no 89H0556. The submission for
110
M. Guilloux- Viry et al. /In-situ
publication of this work has been authorized Director of CNET Lannion B.
by the
References [ 1 ] P. Chaudhari, R.H. Koch, R.B. Laibowitz, T.R. McGuire and R.J. Gambino, Phys. Rev. Lett. 58 (1987) 2684. [2] Y. Enomoto, T. Murakami, M. Suzuki and K. Moriwaki, Jpn. J. Appl. Phys. 26 (1987) L1248. [ 31 H. Itozaki, S. Tanaka, K. Higaki and S. Yazu, Physica C 153-155(1988)1155. [4] A. Perrin, Z.Z. Li, 0. Petia, J. Padiou and M. Sergent, Rev. Phys. Appl. 23 (1988) 257; A. Perrin, Film-Substrate Interaction in High-T, Superconducting Thin films, in: Studies of High
YBa2Cu30,_
x thin films
Temperature Superconductors, ed. A.V. Narlikar (Nova Science Pub]., 1990) to be published. [ 5 ] Probably the best source of the available array of deposition procedures is the Proceedings of the M’S-HTSC, Stanford (1989),PhysicaC 162-164 (1989). [ 61 J.M. Ttiscone, M.G. Karkut, L. Antognazza. 0. Brunner and g. Fischer, Phys. Rev. Lett. 63 (1989) 1016. [7]This solution was first proposed by X.X. Xi, H.C. Li. J. Geerk, G. Linker, 0. Meyer, B. Obst, F. Ratzel, R. Smithey and F. Weschenfelder, Physica C, 153-l 55 ( 1988) 794. [ 8 ] See, for example, B.D. Cullity, Elements of X-ray diffraction (Addison-Wesley, Reading, 1967) p. 99. [9] M.G. Karkut, M. Guilloux-Viry, A. Perrin and M. Sergent. in preparation. [ IO] Ch. Neumann, P. Ziemann, J. Geerk and X.X. Xi, Proc. M’S-HTSC, Stanford (1989), Physica C 162-164 (1989) 321.
IN-SITU YBa2Cu307_x THIN FILMS EPITAXIALLY GROWN BY SINGLE TARGET DC SPUTTERING M. GUILLOUX-VIRY
‘, M.G. KARKUT,
A. PERRIN,
0. PENA, J. PADIOU
and M. SERGENT
Laboratoirede Chimie MnPrale B, URACNRS 254, Universitkde Rennes I, Avenuedu GtWral Leclerc, 35042 RENNES-CPdex, France ’ Division OCM, CNET, Centre de Lannion B, BP 40 22301 LANNION-Ckdex, France
Received 19 December 1989
We have grown in-situ thin films of YBaZCuJO,_x (YBCO) using a simple DC sputtering system equipped with a single stoichiometric target. Operating at very high sputtering pressures of greater than 1 mbar, we have been able to grow, with excellent reproducibility, primarily c-axis oriented, thin (between 250 and 1000 A) YBCO films on ( 100)MgO and ( IOO)SrTiOj substrates heated to - 700°C. These films have resistivity ratios R(290 K)/R( 100 K) between 2.2 and 2.8, resistive T, onsets between 85 and 89 K, zero resistance between 8 1and 86.5 K, and extremely narrow ( < I K) AC susceptibility transitions indicative of excellent film homogeneity.
1. Introduction The discovery of superconductivity above liquid nitrogen temperature opened the way to a number of potential applications. However the technological exploitation of these ceramic materials may be limited by their very low critical current densities at 77 K. Even microelectronic applications require current densities higher than the values reported for bulk ceramics or for randomly oriented thin films. By contrast, high-T, thin films epitaxially grown on single crystal SrTiO, and MgO substrates can have critical current densities greater than lo6 A/cm* at 77K[ l-31. Films initially deposited at room temperature are amorphous and insulating and must be annealed between 800°C and 900°C in order to form the highT, phase. At these high temperatures, film-substrate interactions can become very destructive to the superconducting properties [ 41. Moreover, in any device fabrication process, ex-situ post-treatments can be a source of contamination thereby reducing the attainable level of integration. Thus the challenge is to prepare in-situ thin films at lower temperatures so that a wider array of technologically relevant substrate materials become available. Magnetron and RF 092 l-4534/90/$03.50 ( North-Holland )
0 Elsevier Science Publishers B.V.
sputtering techniques [ 51 have so far proven to be among the most successful means of producing not only in-situ films but also artificial structures [ 61 of the 1: 2 : 3 material. However, up to now we are not aware of any group producing in-situ thin films using straightforward DC sputtering. In this paper, we report on the in-situ growth of YBaZCu307_-x (YBCO) thin films by a simple DC sputtering system using a single stoichiometric 1: 2 : 3 target. The films are sputtered at the very high pressures of greater than 1 mbar in order to avoid destructive backsputtering of the film [ 71. Using single crystal substrates of ( 100)SrTiOs and ( lOO)MgO, we have been able to routinely produce highly oriented YBCO films with zero resistances in the mid80’s and which have extremely narrow ( < 1 K) AC susceptibility transitions.
2. Deposition procedure We fixed a stoichiometric YBa2Cu307 target 1 inch in diameter and 5 mm thick on a vertically mounted DC sputtering gun. The base pressure Pb of the system is between 5 x 10e6 and 3 x 10e5 mbar. The sputtering gas is a flowing mixture of 4.5NAr and
106
hf.Guilloux- Viry et al. /In-situ
2.5N O2 pumped by a sorption pump. The ratio of Ar to O2 is maintained at typically 10 : 1 and the total sputtering pressure is at least 1 mbar. The sputtering rate is only about 300 A/h due to the high total pressure and the presence of oxygen in the plasma. The ( 100)SrTi03 or (100)MgO single crystal substrates are placed on a flat plate of stainless steel which is situated 15 mm from, and directly in front of, the target. The plate is heated to about 800°C and is monitored by a thermocouple although we estimate the substrate temperature to be about from 100’ C to 150°C below the plate temperature. Following deposition, the plate is cooled from 800 oC to 500’ C in no more than 15 min and then maintained in 1 atm of O2 at this temperature for 30 min before power is cut to the heater to allow the substrates to cool to room temperature. All samples prepared in this manner are black, highly reflecting, and show varying degrees of transparency depending on their thickness when directly observed. The samples are then analyzed with no further annealing.
3. Characterization
and discussion
Films grown on substrates of ( lOO)SrTiO, and (100)MgO have been analyzed using 0-20 X-ray diffractometry, oscillating mode Weissenberg X-ray photography, DC resistance and AC susceptibility measurements. Using the deposition conditions described above, we are able to routinely produce highly oriented films on SrTiO, which have zero resistances between 83 and 86.5 K ( Tco's) and extremely narrow (AT< 1 K) AC susceptibility transitions. Oriented films grown on MgO have only slightly less good T,,'s (between 80 and 84 K) but with susceptibility transitions widths still less than 1 K. Figure 1 shows two X-ray diffractograms of films prepared on a) (100)MgO and b) ( 100)SrTiOx. The film on MgO shows highly oriented growth with predominantly c-axis orientation, i.e. the c-axis is perpendicular to the plane of the film. There are also traces of a-axis orientation. The film on SrTi03 displays a mixed a- and c-axis orientation and this is partly due to the substrate temperature T,.At higher T,,the intensities of the (hO0) reflections decrease with respect to the (OOQreflections. At lower T,,the films start to become randomly oriented and the peak
YBa2Cu307_,
thin Jlms
intensities are about two orders of magnitude weaker than the highly oriented films. The widths of the peaks in fig. 1 are narrow - in general the full width at half maximum FWHM of the (005 ) reflection for the best films is about 0.15’ - and are at the limit of instrumental broadening. Simple estimates based on the Scherrer formula [8] indicate that the crystalline coherence of these films is very long and extends over the entire thickness of the film. Finally, the positions of the (001) reflections are shifted to slightly lower Bragg angles giving a longer c-axis ( - 11.74A) than book values for the YBazCuJO, phase. These peak shifts could imply a lack of oxygen in the films and this could be the reason why the superconducting resistive onset occurs at slightly lower values (between 87 and 89 K for films on SrTiOJ and between 84 and 87 K for films on MgO) than those reported elsewhere. However, as we shall see, the widths of the transitions, a measure of film quality, are extremely sharp. The mixed a- and c-axis orientation, the longer c-axis parameter and the lower superconductive onset temperatures could be due to the not-yet-optimal growth conditions. This should not be surprising considering the large number of parameters involved in the growth of the 1: 2 : 3 phase. Another possibility to be considered is the high base pressure of the system before sputtering resulting in the possible degradation of the films during the deposition. Unfortunately, since the lower limit of our system is 5 x 10e6 mbar, it is not possible to test this assumption thoroughly. Additional information about the crystallinity of the films can be obtained by taking crystal diffraction photographs in the oscillating mode using a conventional Weissenberg camera. Photographs of films grown on ( 100 ) MgO, in addition to confirming the c-axis orientation of the films, reveal a strict epitaxial orientation of the a- and b-axes with respect to the crystallographic axes of the substrate and on the best films no measurable misalignment was found 191. Figure 2 shows the resistively measured R( 7‘) curve for a film grown on ( 100) SrTi03. The residual resistance ratio (RRR) R(290K)/R( lOOK) is 2.8. The linear part of the R (T) curve extrapolates nearly to zero as T+O. The onset temperature of the transition is rather low 88.5 K but T,,=86.4K. We define the transition width AT as the difference be-
hf. Guilloux-Viry
et al. /In-situ
YBa2CuJ0,_,
thin films
107
a /
;rT;03 * 006
‘rT;03 003
#
43.5
005
I
IOC J
7
J I
50
i
/
(_cuo
I
004 A I
40 DIFFRACTION
30 ANGLE
(28
I
I
20
10
Cu)
Fig. I. (a) X-ray diffractogram of a YBCO film grown on ( 100) MgO. The orientation is primarily (001). (b) X-ray diffractogram of a YBCO film grown on ( 100) SrTi03. The orientation is mixed a- and c-axis. The (003) and the (006) peaks are masked by the SrTiOJ peaks.
tween 90% of the normal state resistance and zero resistance. In general the best films grown on SrTi03 have RRR’s between 2.5 and 2.8, very narrow AT’s between 1.5 and 2.5 K and T,,‘s between 84 and 86.4 K. Films grown on MgO have RRR’s between 2.2 and 2.55, AT’s between 2 and 3.5 K and To’s between 8 1 and 84 K. The films have not been photolithographically patterned so we can only put a
lower limit on the critical current density J, at 77.3 K. Since we have been able to pass as much as 400 mA through some films before a voltage appears, given the dimensions of the sample, we estimate the J, at 77.3 K to be at least 10’ A/cm’. Susceptibility measurements provide additional information about the superconducting transition and the sample quality. They can probe subtle dif-
hf. Guilloux-Viry et al. /In-situ YBa2Cu307_,
thin films
_ # 45.5 SrTl
03
I
78
J 80
82
84
Temperature
0
30
60
90
120 150 180 210 240 270 300 TEMPERATURE
88
90
92
(K)
vs. temperature
of the same film as
K
Fig. 2. Normalized resistance vs. temperature for a film grown on SrTiO,.
in the crystallization state of the film which are not necessarily taken into account by a resistance measurement. We used a mutual inductance bridge operating at a fixed 119 Hz. In phase x’ and quadrature x” components were simultaneously detected and plotted as a function of temperature using exactly the same cooling and cycling procedure for all runs. Small sized primary and secondary coils were used with the film sandwiched between them and the AC excitation field normal to the film surface. The films’ dimensions were larger than, or the same size as, the coil diameter so as to promote full shielding of the magnetic field when T-c T,. However, sample size did not seem to be a factor in the qualitative analysis of the results. The response amplitudes were not calibrated. Figure 3 shows x’ (T) and x”(T) for the same sample presented in fig. 2. What is striking here is the extreme narrowness of the transition. The narrow transition width of x’ ( T) (a lo%-90% criterium gives less than 0.5 K), the absence of step-like anomalies in x’ ( T) and the sharp (FWHM _ 0.60 K) single peak in x” ( T) indicate the absence of granular effects and the excellent homogeneity of the film over its entire surface ( 5 x 7 mm2 ). Our best films all display similar behaviour which reflects the very high ferences
Fig. 3. The AC susceptibility in fig. 2.
86
quality of the YBCO films produced in our sputtering system. We now wish to illustrate by way of fig. 4 (a) and 4(b) the utility of the susceptibility measurements by directly comparing them to their respective resistivity curves. The two films deposited on MgO both have transition widths of about 2.7 K and only slightly different T,,‘s (82.9 and 80.7 K). However, their magnetic characteristics are strikingly different in that their inductive transition widths differ by a factor of 10 (of the order of 3 K and 0.3 K, respectively). Moreover it is the film with the lower T, which has the narrower transition. The difference in the AC transition is apparently related to weak link behavior at the grain boundaries. We have noted that those films having the highest critical currents systematically have shown the narrowest peaks of the dissipation component x” ( T). Similar results have been found by Neumann et al. [ lo] who established a clear correlation between the critical current densities and the transition widths of the AC susceptibility transitions. Thus, we feel confident in presenting the susceptibility widths as a reliable measure of film quality.
4. Conclusions We have used a simple DC sputtering system to grow in-situ highly oriented YBa2Cu@_, super-
M. Guilloux-Vity
et al. /In-situ
thin films
YBa,Cu,O,_,
109
0
# 48 2 2I-
0
3-
I
/
I
,
I
I
I
/----------
# L9 5
# 48.2
78
80
82 84 86 @a 90 Temperature (K)
92
78
a0
82 84 86 88 Temperature (K)
90
92
Fig. 4. (a) R( 7’) and x( T) for YBCO sample f49.5 grown on ( 100) MgO. (b) R(T) and x( T) for YBCO sample #48.2 also grown on ( 100) MgO. Note that even though the resistive transition widths are essentially the same, the inductive transition widths differ by about a factor of 10.
conducting thin films. On substrates of ( 100)SrTi03 we routinely obtain films that have superconducting resistive onsets between 87 and 89 IS, Tco's between 84 and 86.4 K and inductive transition widths of less than 1 K. Oriented films grown on MgO have systematically lower onsets of between 84 and 87 K, Tco's between 8 1 and 84 K, and slighter broader inductive transitions. The reasons for the slightly depressed resistive onsets and the systematic differences between films grown on SrTi03 and MgO have not yet been determined but could be in part due to Mg diffusion into the latter films. The present excellent results demonstrate the effectiveness of our DC sputtering system. Neither the high base pressure nor the elevated sputtering pressure (to avoid backsputtering from the film) seems to have a very negative influence on the properties of the films. The small stoichiometric targets are an
added advantage since they are easy to make and are more robust than larger targets needed for, say magnetron sputtering. The films are homogeneous over a diameter of 1 cm as revealed by the inductive measurements and the narrow inductive transitions compare favorably with those performed on films fabricated by other deposition techniques.
Acknowledgements
We thank J.C. Jegaden for technical assistance. Work supported by Centre National d’Etudes des Telecommunications under contract no 89 8B 054 and Ministere de la Recherche et de la Technologie under contract no 89H0556. The submission for
110
M. Guilloux- Viry et al. /In-situ
publication of this work has been authorized Director of CNET Lannion B.
by the
References [ 1 ] P. Chaudhari, R.H. Koch, R.B. Laibowitz, T.R. McGuire and R.J. Gambino, Phys. Rev. Lett. 58 (1987) 2684. [2] Y. Enomoto, T. Murakami, M. Suzuki and K. Moriwaki, Jpn. J. Appl. Phys. 26 (1987) L1248. [ 31 H. Itozaki, S. Tanaka, K. Higaki and S. Yazu, Physica C 153-155(1988)1155. [4] A. Perrin, Z.Z. Li, 0. Petia, J. Padiou and M. Sergent, Rev. Phys. Appl. 23 (1988) 257; A. Perrin, Film-Substrate Interaction in High-T, Superconducting Thin films, in: Studies of High
YBa2Cu30,_
x thin films
Temperature Superconductors, ed. A.V. Narlikar (Nova Science Pub]., 1990) to be published. [ 5 ] Probably the best source of the available array of deposition procedures is the Proceedings of the M’S-HTSC, Stanford (1989),PhysicaC 162-164 (1989). [ 61 J.M. Ttiscone, M.G. Karkut, L. Antognazza. 0. Brunner and g. Fischer, Phys. Rev. Lett. 63 (1989) 1016. [7]This solution was first proposed by X.X. Xi, H.C. Li. J. Geerk, G. Linker, 0. Meyer, B. Obst, F. Ratzel, R. Smithey and F. Weschenfelder, Physica C, 153-l 55 ( 1988) 794. [ 8 ] See, for example, B.D. Cullity, Elements of X-ray diffraction (Addison-Wesley, Reading, 1967) p. 99. [9] M.G. Karkut, M. Guilloux-Viry, A. Perrin and M. Sergent. in preparation. [ IO] Ch. Neumann, P. Ziemann, J. Geerk and X.X. Xi, Proc. M’S-HTSC, Stanford (1989), Physica C 162-164 (1989) 321.