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Superconducting and structural properties of BSCCO thin films by molecular beam epitaxy
ICEC 15 Proceedings
Superconducting and Structural Properties of BSCCO Thin Films by Molecular Beam Epitaxy. M. Salvato*, C. Attanasio, C. Coccorese, L. Maritato, and S.L. Prischepa ~ Dipartimento di Fisica, Universit& degli Studi di Salerno, Baronissi (Sa), 1-84081, Italy * Ansaldo-CRIS, Via Nuova delle Brecce 260, 1-80147, Napoli, Italy @ State University of Computer Science and RadioElectronics, P. Brovka str. 6, Minsk, Belarus
We have realized superconducting thin films of BSCCO 2212 using a molecular beam epitaxy (MBE) technique. The stoichiometry of the films is carefully controlled via EDS analysis and the repeatability of the deposition process is increased by realizing single element multilayers of Bi, Sr, Ca and Cu. The annealing of the films is accomplished during a post deposition process. Critical temperature of about 80 K has been obtained. The use of a source of activated atomic oxygen will allow in the close future the realization of "in situ" BSCCO thin films. INTRODUCTION High temperature superconductors (HTS) were produced by several techniques [1]. BSCCO thin films were mainly realized by laser ablation [2], electron beam evaporation [3], ion sputtering [4], etc., starting from pellets with right stoichiometry. Molecular Beam Epitaxy (MBE) technique, already used for semiconductors fabrication [5], has been recently used to obtain high quality BSCCO thin films [6-8]. This technique consists either in a codeposition or in a layer-by-layer deposition of single elements in an ultra high vacuum (UHV) system: the oxygen is continuously supplied to the sample during the growth and the surface is monitored "in situ" by reflection high energy electron diffraction (RHEED) [9]. In this paper we shall present results of layer-by-layer deposition of Bi2Sr2CaCu2Ox thin films by MBE technique using an "ex situ" annealing process. DESCRIPTION OF THE DEPOSITION APPARATUS Our evaporation apparatus is schematically shown in fig.1. It consists of two separated and individually pumped chambers: an evaporation chamber and an introduction chamber. A pumping system, formed by sorption, titanium sublimator and ionic pumps allows to obtain a pressure of about 10 -11 Torr in the whole environment. Four shutter controlled sources are situated inside the evaporation chamber: they consist of two electron guns and two Knudsen cells. Bismuth and calcium are evaporated by Knudsen cells, copper and strontium by electron beams. Knudsen cell temperature, which value fixes the evaporation rate, can be externally controlled in the range 201300 °C with an accuracy of 0.1 degree. The energy of each electron beam is 12 KeV and the maximum emission current is 500 mA: gun evaporation rates are measured by two quartz thickness monitors. Owing to a substrates load-lock apparatus the evaporation chamber pressure can be always kept to less than 10 -9 Torr. During the deposition the substrates are attached on a X, Y, Z, O, ~ manipulator where they can be heated up to a maximum temperature of 800 °C, in the presence of oxygen gas. Sample growth process can be observed "in situ" by a 10 KeV RHEED system in order to elucidate the growth mechanism. Source evaporation rate calibration has been made before of the realization of BSCCO thin films. We ]lave measured the thickness of single element films by interferometric technique and the atomic percentage by EDS analyses in order to obtain the right tooling factors for the thickness monitors. Further informations about the evaporation rates can also be obtained recording the manipulator pressure during the film growth.
Cryogenics 1994Vol 34 ICEC Supplement
859
ICEC 15 Proceedings
EXPERIMENTAL RESULTS We made B S C C O films by a layer-by-layer single elements deposition on MgO substrates at room temperature. Evaporation rates less than 1 /~/s were used for each source. Partial thickness of single layers are about 100 A and relative thicknesses are in such a ratio to obtain 2212 stoichiometry. The final thickness of the films is usually about 5000 _~. The "ex situ" annealing process was held in air using a muffle furnace. The granularity of the samples was confirmed by scanning microscope pictures; the structural properties have been studied by means of EDS and X - R a y s analysis. In Table 1 the stoichiometry ratios of several samples, normalized to Cu, for different values of the annealing time at the annealing temperature of 870 °C are shown. We made EDS analyses in more than one zone of each film and the showed values represent their average. The stoichiometric composition of each sample depends on the annealing time and temperature as well as on the starting stoichiometry. We didn't observe any transition when we used annealing temperature different from 870 °C. The samples, for which T¢ has not been observed, always show a strong variation of the electrical resistance at a temperature of a b o u t 100 K. It can be seen from the table that the stoichiometry of each element oscillates around the expected value when the annealing time increases. However the EDS analysis is not able by itself to supply certain information a b o u t the T¢ value. For instance, we point out, as an example, that sample MBE114 is not superconducting even though its stoichiometry is very close to the 2212. Table 1 Stoichiometry of several samples for different values of the annealing time, at the temperature of 870 °C. Also reported are the measured transition temperatures. Bi
Sr
Ca
Cu
tann.
Tann.
Tc
MBE114
1.9
2.2
1.2
2.0
24 '
870°C
---
MBE103
2.1
1.7
0.8
2.0
30 '
870 °C
40 K
MBE105
2.2
1.8
0.7
2.0
42 '
g70 *C
75 K
MBE101
2.6
3.2
1.1
2.0
1h
870 °C
---
MBE65
1.7
1.7
1.1
2.0
3h
870 °C
69 K
MBE74
1.7
1.4
1.2
2.0
6h
870 *C
56 K
More information can be obtained studying the X - R a y diffraction patterns realized for each sample by a 0 - 2 0 Bragg-Brentano diffractometer. All the spectra show peaks of the 2201 phase (T¢(R in0~g~2 7K)[10] and of the 2223 phase (T¢(R = 0) = l l 0 K ) [ l l ] as well as the 2212 phase peaks, are shown the spectra of MBE105 and M B E l l 4 samples. The first shows a higher preferential orientation of its grains with c-axis perpendicular to the plane of the substrate. Moreover the M B E l l 4 spectrum shows a lot of peaks of BiO and other compounds that suppress the superconductivity of the sample. The presence of some peaks of the 2201 phase in tile MBE105 spectrum justify its slightly low value of T¢. In fig.3 is shown the electrical resistance vs. temperature of MBE105 sample. The large drop in resistance around 110 K, and the Tc(R = 0) value about 75 K indicate the presence of both 2223 and 2201 phase, confirming the X - R a y spectrum results. CONCLUSIONS The EDS and the X-Ray analyses have been utilized to optimize the deposition process. The realization of B S C C O films with a Tc ~ 85K is tightly dependent on the annealing process. Problems connected to this technique can be avoided by an "in situ" oxidation and R H E E D
860
Cryogenics 1994 Vol 34 ICEC Supplement
ICEC 15 Proceedings
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80
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30
40
50
60
70
80
20
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Figure 2 X-Ray spectra for MBE105 (curve a) and MBEll4 (curve b) samples. O11e, two and three stars, labelling the peaks, refer, respectevely, to the 2201, 2212, and 2223 phase
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0
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,
,
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,
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150
200
T (K)
Figure 3 Resistive transition for MBE105 sample
Cryogenics 1994 Vol 34 ICEC Supplement
861
ICEC 15 Proceedings
analysis during the samples growth. Using an atomic oxygen plasma source it will be possible to deposit single atomic layers at low partial oxygen pressure. RHEED analysis, that we have already used on MgO and A1203 substrates, as well as on single layer films, will allow us to modify the samples stoichiometry and to control the epitaxy. With this procedure we should be able to obtain epitaxial films with thickness of few unit cells. REFERENCES
6 7 8 9 10 11
Maritato, L. and Falco, C.M., Mod. Phys. Left, B, (1990) 4 639 Fork, D.K. et al., Appl. Phys. Lett., (1988) 53 337 Attanasio, C. et al., Mod. Phys. Lett. B, (1991) 5 1203 Lorentz, R.D. and Sexton J.H., Appl. Phys. Lett., (1988) 53 1654 See, for example, Molecular Beam Epitaxy of Advanced Structures, Editrice Compositori, Bologna (1993) Eckstein, J.N. and al., Appl. Phys. Lett., (1990) 57 931 Nakayama, Y. et al., J. Appl. Phys., (1991) 70 4371 Kasai, Y. and Sakai, S., J. of Crystal Growth, (1991) 115 758 Sakai, S. et al., Jpn. J. Appl. Phys., (1992) 31 L399 Michel, C. et al., Z. Phys. B, (1987) 68 421 Kuroda, K. et al., Jpn. J. Appl. Phys., (1988) 27 625 Ports for sputtering sources
Transfer port
Rheed- ~ - - . ~ E ~ ~ gun Effusion cell ports CE-4 evaporator
, ~
Substrate manipulator CE-1 evaporators
k~
~
Effusion
cell ports
Ion pumping
Figure 1 Schematic lay-out of our MBE machine
862 Cryogenics 1994 Vol 34 ICEC Supplement
Superconducting and Structural Properties of BSCCO Thin Films by Molecular Beam Epitaxy. M. Salvato*, C. Attanasio, C. Coccorese, L. Maritato, and S.L. Prischepa ~ Dipartimento di Fisica, Universit& degli Studi di Salerno, Baronissi (Sa), 1-84081, Italy * Ansaldo-CRIS, Via Nuova delle Brecce 260, 1-80147, Napoli, Italy @ State University of Computer Science and RadioElectronics, P. Brovka str. 6, Minsk, Belarus
We have realized superconducting thin films of BSCCO 2212 using a molecular beam epitaxy (MBE) technique. The stoichiometry of the films is carefully controlled via EDS analysis and the repeatability of the deposition process is increased by realizing single element multilayers of Bi, Sr, Ca and Cu. The annealing of the films is accomplished during a post deposition process. Critical temperature of about 80 K has been obtained. The use of a source of activated atomic oxygen will allow in the close future the realization of "in situ" BSCCO thin films. INTRODUCTION High temperature superconductors (HTS) were produced by several techniques [1]. BSCCO thin films were mainly realized by laser ablation [2], electron beam evaporation [3], ion sputtering [4], etc., starting from pellets with right stoichiometry. Molecular Beam Epitaxy (MBE) technique, already used for semiconductors fabrication [5], has been recently used to obtain high quality BSCCO thin films [6-8]. This technique consists either in a codeposition or in a layer-by-layer deposition of single elements in an ultra high vacuum (UHV) system: the oxygen is continuously supplied to the sample during the growth and the surface is monitored "in situ" by reflection high energy electron diffraction (RHEED) [9]. In this paper we shall present results of layer-by-layer deposition of Bi2Sr2CaCu2Ox thin films by MBE technique using an "ex situ" annealing process. DESCRIPTION OF THE DEPOSITION APPARATUS Our evaporation apparatus is schematically shown in fig.1. It consists of two separated and individually pumped chambers: an evaporation chamber and an introduction chamber. A pumping system, formed by sorption, titanium sublimator and ionic pumps allows to obtain a pressure of about 10 -11 Torr in the whole environment. Four shutter controlled sources are situated inside the evaporation chamber: they consist of two electron guns and two Knudsen cells. Bismuth and calcium are evaporated by Knudsen cells, copper and strontium by electron beams. Knudsen cell temperature, which value fixes the evaporation rate, can be externally controlled in the range 201300 °C with an accuracy of 0.1 degree. The energy of each electron beam is 12 KeV and the maximum emission current is 500 mA: gun evaporation rates are measured by two quartz thickness monitors. Owing to a substrates load-lock apparatus the evaporation chamber pressure can be always kept to less than 10 -9 Torr. During the deposition the substrates are attached on a X, Y, Z, O, ~ manipulator where they can be heated up to a maximum temperature of 800 °C, in the presence of oxygen gas. Sample growth process can be observed "in situ" by a 10 KeV RHEED system in order to elucidate the growth mechanism. Source evaporation rate calibration has been made before of the realization of BSCCO thin films. We ]lave measured the thickness of single element films by interferometric technique and the atomic percentage by EDS analyses in order to obtain the right tooling factors for the thickness monitors. Further informations about the evaporation rates can also be obtained recording the manipulator pressure during the film growth.
Cryogenics 1994Vol 34 ICEC Supplement
859
ICEC 15 Proceedings
EXPERIMENTAL RESULTS We made B S C C O films by a layer-by-layer single elements deposition on MgO substrates at room temperature. Evaporation rates less than 1 /~/s were used for each source. Partial thickness of single layers are about 100 A and relative thicknesses are in such a ratio to obtain 2212 stoichiometry. The final thickness of the films is usually about 5000 _~. The "ex situ" annealing process was held in air using a muffle furnace. The granularity of the samples was confirmed by scanning microscope pictures; the structural properties have been studied by means of EDS and X - R a y s analysis. In Table 1 the stoichiometry ratios of several samples, normalized to Cu, for different values of the annealing time at the annealing temperature of 870 °C are shown. We made EDS analyses in more than one zone of each film and the showed values represent their average. The stoichiometric composition of each sample depends on the annealing time and temperature as well as on the starting stoichiometry. We didn't observe any transition when we used annealing temperature different from 870 °C. The samples, for which T¢ has not been observed, always show a strong variation of the electrical resistance at a temperature of a b o u t 100 K. It can be seen from the table that the stoichiometry of each element oscillates around the expected value when the annealing time increases. However the EDS analysis is not able by itself to supply certain information a b o u t the T¢ value. For instance, we point out, as an example, that sample MBE114 is not superconducting even though its stoichiometry is very close to the 2212. Table 1 Stoichiometry of several samples for different values of the annealing time, at the temperature of 870 °C. Also reported are the measured transition temperatures. Bi
Sr
Ca
Cu
tann.
Tann.
Tc
MBE114
1.9
2.2
1.2
2.0
24 '
870°C
---
MBE103
2.1
1.7
0.8
2.0
30 '
870 °C
40 K
MBE105
2.2
1.8
0.7
2.0
42 '
g70 *C
75 K
MBE101
2.6
3.2
1.1
2.0
1h
870 °C
---
MBE65
1.7
1.7
1.1
2.0
3h
870 °C
69 K
MBE74
1.7
1.4
1.2
2.0
6h
870 *C
56 K
More information can be obtained studying the X - R a y diffraction patterns realized for each sample by a 0 - 2 0 Bragg-Brentano diffractometer. All the spectra show peaks of the 2201 phase (T¢(R in0~g~2 7K)[10] and of the 2223 phase (T¢(R = 0) = l l 0 K ) [ l l ] as well as the 2212 phase peaks, are shown the spectra of MBE105 and M B E l l 4 samples. The first shows a higher preferential orientation of its grains with c-axis perpendicular to the plane of the substrate. Moreover the M B E l l 4 spectrum shows a lot of peaks of BiO and other compounds that suppress the superconductivity of the sample. The presence of some peaks of the 2201 phase in tile MBE105 spectrum justify its slightly low value of T¢. In fig.3 is shown the electrical resistance vs. temperature of MBE105 sample. The large drop in resistance around 110 K, and the Tc(R = 0) value about 75 K indicate the presence of both 2223 and 2201 phase, confirming the X - R a y spectrum results. CONCLUSIONS The EDS and the X-Ray analyses have been utilized to optimize the deposition process. The realization of B S C C O films with a Tc ~ 85K is tightly dependent on the annealing process. Problems connected to this technique can be avoided by an "in situ" oxidation and R H E E D
860
Cryogenics 1994 Vol 34 ICEC Supplement
ICEC 15 Proceedings
-'
' ''l
''
' 'l
' ' ~ T
~r~-'
'
....
i, r
i ....
i ....
J ....
a
f ~
I0
20
3O
L
40
A
'
.50
' ' ' '
60
70
• i ....
0
80
lid
20
30
40
50
60
70
80
20
2O
Figure 2 X-Ray spectra for MBE105 (curve a) and MBEll4 (curve b) samples. O11e, two and three stars, labelling the peaks, refer, respectevely, to the 2201, 2212, and 2223 phase
I
'
'
'
'
1
'
'
'
'
1
R (O)
0
~
0
,
i
,
I
I
50
I (X)
,
,
,
,
I
150
200
T (K)
Figure 3 Resistive transition for MBE105 sample
Cryogenics 1994 Vol 34 ICEC Supplement
861
ICEC 15 Proceedings
analysis during the samples growth. Using an atomic oxygen plasma source it will be possible to deposit single atomic layers at low partial oxygen pressure. RHEED analysis, that we have already used on MgO and A1203 substrates, as well as on single layer films, will allow us to modify the samples stoichiometry and to control the epitaxy. With this procedure we should be able to obtain epitaxial films with thickness of few unit cells. REFERENCES
6 7 8 9 10 11
Maritato, L. and Falco, C.M., Mod. Phys. Left, B, (1990) 4 639 Fork, D.K. et al., Appl. Phys. Lett., (1988) 53 337 Attanasio, C. et al., Mod. Phys. Lett. B, (1991) 5 1203 Lorentz, R.D. and Sexton J.H., Appl. Phys. Lett., (1988) 53 1654 See, for example, Molecular Beam Epitaxy of Advanced Structures, Editrice Compositori, Bologna (1993) Eckstein, J.N. and al., Appl. Phys. Lett., (1990) 57 931 Nakayama, Y. et al., J. Appl. Phys., (1991) 70 4371 Kasai, Y. and Sakai, S., J. of Crystal Growth, (1991) 115 758 Sakai, S. et al., Jpn. J. Appl. Phys., (1992) 31 L399 Michel, C. et al., Z. Phys. B, (1987) 68 421 Kuroda, K. et al., Jpn. J. Appl. Phys., (1988) 27 625 Ports for sputtering sources
Transfer port
Rheed- ~ - - . ~ E ~ ~ gun Effusion cell ports CE-4 evaporator
, ~
Substrate manipulator CE-1 evaporators
k~
~
Effusion
cell ports
Ion pumping
Figure 1 Schematic lay-out of our MBE machine
862 Cryogenics 1994 Vol 34 ICEC Supplement