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On the route to epitaxial growth of YBa2Cu3O7 superconducting thin films by molecular beam epitaxy.
PhysicaC 153-155 (1988)407-408 North-Holland,Amsterdam
ON THE ROUTE TO EPITAXIAL GROWTH OF YBa2Cu307 SUPERCONDUCTING TIIIN FII~IS BY MOLECULAR BEAM EPITAXY.
R. Cabanel, J.P. Hirtz~ P. Etienne, L. Fruchter (*), C. Giovannella (*~e) and G. Creuzet. THOMSON-CSF/LCR - Domaine de Corbeville~ B.P. I0 - 914010RSAY France. (~) permanent address : Physique des Solides -B~t 510 - UPS 91405 0RSAY France (~¢~) permanent address : Dipartimento di Fisica Roma II - Dia O. Raimondo 00173 ROMA - Italia.
We have successfully achieved Molecular Beam Epitaxy growth of YbaeCu~ and ErBa Cu_ metallic compounds~ producing 2000A thickness single crystal films controle~ by observat~on3of RHEED patterns during growth. We used metallic Rare Earth~ Copper and BaF 2 as source materials. After introduction of oxygen by various thermal treatments~ we obtained superconducting thin films with sharp transitions. We also report susceptibility measurements and critical current estimations. 1. INTRODUCTION The d i s c o v e r y o f t h e 90K h i g h - T c s u p e r c o n d u c t i n g o x i d e s f a m i l y YBa~Cu~O7 (1) has opened the possibility to reaIiz~ "superconducting devices operating above liquid nitrogen temperature. Due to the extremely low value of the coherence length in these materials (2), realistic devices often need the ability to produce thin films containing a low density of defects such as grain boundaries. Epitaxial growth of superconducting oxides thin films is therefore an important and stimulating challenge. Thin films preparation have been reported using a large variety of techniques. Up to now, the best results have been achieved by electron-beam coevaporation (3) or molecular beam epitaxy (4,5). These works present roughly the following common features : a) the best results are obtained on SrTiO 3 (100) substrates b) a small pressure of oxygen (10 -7 to 10 -3 ) is injected close to the substrate during deposition c) the as-grown films are mainly amorphous, as observed by the absence of RHEED patterns from the very beginning of the deposition d) a post-deposition thermal annealing under 0^, including a high temperature ( 8500C 9~0°C) stage and a slow cooling down to room temperature, is always needed to achieve both cristallinity and oxygen content e) the resulting films are polycristalline with c-axis as a preferential orientation perpendicular to the substrate f) sharp superconducting transitions (a few K) where o b t a i n e d , with Tc a r o u n ~ 85-90~ an~ critical currents in excess of i0 J to i0 A/cm at 77K. g) the films exhibit reactions to air exposure, leading probably to a poor stability.
0921 4534/88/$03.50 ©ElsevierSciencePublishersB.V. (North-Holland PhysicsPublishingDivision)
Later on, it was pointed out that in-situ rapid thermal annealing (6) can help the oxygen introduction process in the view of removing the post-deposition thermal annealing. Finally, the use of BaF~ as a starting material (7) was shown to considerably improve the films stability. 2. GROWTH TECHNIQUE In order to approach the epitaxial growth conditions, we have first investigated the 1:2:3 metallic compound growth conditions, without any oxygen flux during deposition. Such Yb single crystal 1:2:3 compound was successfully obtained using the following conditions : U~V environment ( residual pressure around 10 - ~ T o r r ) - SrTiO 2 (100) substrate, with temperature around I~OoC Use o f BaF 2 as t h e s t a r t i n g m a t e r i a l f o r Baryum D e p o s i t i o n r a t e b e t w e e n 0.1 and 0 . 3 ~ / s . -
-
-
The epitaxial growth was checked by the observation of clear RHEED pat%erns during the growth of the first 2q00A (for total thicknesses from 3000 to 7000A, depending on the film). The fluor stoechiometry was measured by microprobe before any thermal treatment to be close to 2:1 with respect to the Baryum one, so the composition of the as-deposited films was around YbBa2Cu^F. or ErBa2Cu^F.. A thermal treatment is cons~q%ently n e c e s ~ 6 to obtain the superconducting oxide compound. We performed in-situ rapid thermal annealings before ex-situ standard treatment under oxygen. No noticeable differences in the final properties of the films can prove that RTA plays a significant role in the sample preparation. After thermal treatment under "standard" conditions ( high temperature stage and
408
R. Cabanel et aL / Epitaxial growth of YBaeCu~07 superconducting thin films
slow cooling, under 1 atm 02) , the Fluor stoichiometry reveals to be considerably smaller than before ( F : Ba ratio was reduced to less than 0.3). Finally, the surface roughness, which appears as very good before annealing, (no noticeable signal neither on ta~ysurf profiles ~r SF~I observations ) ( ~ IOOA with local 400A bumps ) was subequently altered by the thermal treatment. 3. PROPERTIES OF THE FILMS. The fills prepared as described above exhibit sharp resistive transitions, as shown on figure 1 for two Yb based fills. Above Tc the behaviour is clearly metallic-type, and the resistivity just above Tc can be estimated around I00 ~ c m . The transition width (90~ to I0~ ) was measured down to 2K in the best Yb samples, while it remains higher ( around 8K ) in the Er samples. One important result is the remarkable stability of the films : the resistivity curves remain the same after two months with no special storage conditions. This observation confirms the previous work from Bell Labs group (7) but further investigations are needed to understand the microscopic process by which the use of BaF 2 can achieve this result.
R(~)
X(a.u} oo,OO°°:g~ +
oe °
oO e
•
4"
oo °
+
•
Fig. 2 :
+ ÷ +
•
+ + +
•
+ +
•
/
.
S2
Magnetic susceptibility of $2 sample for various fields ( 3 mOe ( . ) , ]00 m0e (+)
and I 0e (o)). ee
n6e
I
• I
6O
YbB,a2Cu307_x 80 T(K)
4.
CONCLUSION. We have achieved the epitaxial growth of the 1:2:3 metallic compound with Yb and Er by Molecular Beam Epitaxy. After post-deposition thermal annealing, the fills exhibit good superconducting properties as shown by resistivity and magnetic susceptibility measurements. Finally, we observed a high degree of stability of the films, were the use of BaF 2 as a starting material seems to play an important role. REFERENCES
30
(1)
M.K.
(2)
T.K.
t4
20
Fig. ] :
(3)
2 Resistivity of two
10
YbBa2Cu307 samples Sl and $2.
100
200
(4)
3C)0 T(K (5)
The susceptibility was investigated at low field as shown on figure 2. Sharp transition is observed at a field less than i00 mOe ; however we may notice it occurs at a lower temperature than the resistivity one. Magnetization versus field hysteresis measurements let us estimate the critical current to be of the order of 2.10 A/cm at 5K. These characteristics are mostly due to the granular structure of films, as seen on X-rays diffraction patterns.
(6) (7)
Wu, J.R. Ashburn, C.J. Torng, P.H. Hor, R.L. Meng, L. Gao, Z.I. Huang, Y.Q. Wang and C.W. Chu, Phys. Rev. L e t t . 58, 908 (1987)
Worthington, W.J. Oallagher and
T.R. Dinger, Phys. Rev. Lett. 59, 1160 (1987). R.B. Laibowitz, R. H. Koch, P. Chaudhari and R.J. Gambino, Phys.
Rev. B 3~5, 8821 (1987) P. Chaudhari, R.H. Koch, R.B. Laibowitz, T.R. Mc Ouire and R.J. Oambino, Phys. Rev. L e t t . 58, 2684 (1987). J. Kwo, T.C. Hsieh, R.M. Fleming, M. Hong, S.H. Liou, B.A. Davidson and L.C. Feldman, Phys. Rev. B 36, 4039 (1987). C. Webb, S.L. Weng, J.N. Eckstein, N. Missert, K. Char, D.G. Schlom, E.S. Hellman, M.R. Beasley, A. Kapitulnik and J.S. Harris Jr, Appl. Phys. Lett.
51, 1191 (1987) D.K. Lathrop, S.E. Russek and R.A. Buhrman, Appl. Phys. L e t t . 51, 1554 (1987) P.M. Mankiewich, J.H. Scofield, W.J. Skocpol, R.E. Howard, A.M. Dayem and E. Good, Appl. Phys. Lett 51,
1753 (1987).
ON THE ROUTE TO EPITAXIAL GROWTH OF YBa2Cu307 SUPERCONDUCTING TIIIN FII~IS BY MOLECULAR BEAM EPITAXY.
R. Cabanel, J.P. Hirtz~ P. Etienne, L. Fruchter (*), C. Giovannella (*~e) and G. Creuzet. THOMSON-CSF/LCR - Domaine de Corbeville~ B.P. I0 - 914010RSAY France. (~) permanent address : Physique des Solides -B~t 510 - UPS 91405 0RSAY France (~¢~) permanent address : Dipartimento di Fisica Roma II - Dia O. Raimondo 00173 ROMA - Italia.
We have successfully achieved Molecular Beam Epitaxy growth of YbaeCu~ and ErBa Cu_ metallic compounds~ producing 2000A thickness single crystal films controle~ by observat~on3of RHEED patterns during growth. We used metallic Rare Earth~ Copper and BaF 2 as source materials. After introduction of oxygen by various thermal treatments~ we obtained superconducting thin films with sharp transitions. We also report susceptibility measurements and critical current estimations. 1. INTRODUCTION The d i s c o v e r y o f t h e 90K h i g h - T c s u p e r c o n d u c t i n g o x i d e s f a m i l y YBa~Cu~O7 (1) has opened the possibility to reaIiz~ "superconducting devices operating above liquid nitrogen temperature. Due to the extremely low value of the coherence length in these materials (2), realistic devices often need the ability to produce thin films containing a low density of defects such as grain boundaries. Epitaxial growth of superconducting oxides thin films is therefore an important and stimulating challenge. Thin films preparation have been reported using a large variety of techniques. Up to now, the best results have been achieved by electron-beam coevaporation (3) or molecular beam epitaxy (4,5). These works present roughly the following common features : a) the best results are obtained on SrTiO 3 (100) substrates b) a small pressure of oxygen (10 -7 to 10 -3 ) is injected close to the substrate during deposition c) the as-grown films are mainly amorphous, as observed by the absence of RHEED patterns from the very beginning of the deposition d) a post-deposition thermal annealing under 0^, including a high temperature ( 8500C 9~0°C) stage and a slow cooling down to room temperature, is always needed to achieve both cristallinity and oxygen content e) the resulting films are polycristalline with c-axis as a preferential orientation perpendicular to the substrate f) sharp superconducting transitions (a few K) where o b t a i n e d , with Tc a r o u n ~ 85-90~ an~ critical currents in excess of i0 J to i0 A/cm at 77K. g) the films exhibit reactions to air exposure, leading probably to a poor stability.
0921 4534/88/$03.50 ©ElsevierSciencePublishersB.V. (North-Holland PhysicsPublishingDivision)
Later on, it was pointed out that in-situ rapid thermal annealing (6) can help the oxygen introduction process in the view of removing the post-deposition thermal annealing. Finally, the use of BaF~ as a starting material (7) was shown to considerably improve the films stability. 2. GROWTH TECHNIQUE In order to approach the epitaxial growth conditions, we have first investigated the 1:2:3 metallic compound growth conditions, without any oxygen flux during deposition. Such Yb single crystal 1:2:3 compound was successfully obtained using the following conditions : U~V environment ( residual pressure around 10 - ~ T o r r ) - SrTiO 2 (100) substrate, with temperature around I~OoC Use o f BaF 2 as t h e s t a r t i n g m a t e r i a l f o r Baryum D e p o s i t i o n r a t e b e t w e e n 0.1 and 0 . 3 ~ / s . -
-
-
The epitaxial growth was checked by the observation of clear RHEED pat%erns during the growth of the first 2q00A (for total thicknesses from 3000 to 7000A, depending on the film). The fluor stoechiometry was measured by microprobe before any thermal treatment to be close to 2:1 with respect to the Baryum one, so the composition of the as-deposited films was around YbBa2Cu^F. or ErBa2Cu^F.. A thermal treatment is cons~q%ently n e c e s ~ 6 to obtain the superconducting oxide compound. We performed in-situ rapid thermal annealings before ex-situ standard treatment under oxygen. No noticeable differences in the final properties of the films can prove that RTA plays a significant role in the sample preparation. After thermal treatment under "standard" conditions ( high temperature stage and
408
R. Cabanel et aL / Epitaxial growth of YBaeCu~07 superconducting thin films
slow cooling, under 1 atm 02) , the Fluor stoichiometry reveals to be considerably smaller than before ( F : Ba ratio was reduced to less than 0.3). Finally, the surface roughness, which appears as very good before annealing, (no noticeable signal neither on ta~ysurf profiles ~r SF~I observations ) ( ~ IOOA with local 400A bumps ) was subequently altered by the thermal treatment. 3. PROPERTIES OF THE FILMS. The fills prepared as described above exhibit sharp resistive transitions, as shown on figure 1 for two Yb based fills. Above Tc the behaviour is clearly metallic-type, and the resistivity just above Tc can be estimated around I00 ~ c m . The transition width (90~ to I0~ ) was measured down to 2K in the best Yb samples, while it remains higher ( around 8K ) in the Er samples. One important result is the remarkable stability of the films : the resistivity curves remain the same after two months with no special storage conditions. This observation confirms the previous work from Bell Labs group (7) but further investigations are needed to understand the microscopic process by which the use of BaF 2 can achieve this result.
R(~)
X(a.u} oo,OO°°:g~ +
oe °
oO e
•
4"
oo °
+
•
Fig. 2 :
+ ÷ +
•
+ + +
•
+ +
•
/
.
S2
Magnetic susceptibility of $2 sample for various fields ( 3 mOe ( . ) , ]00 m0e (+)
and I 0e (o)). ee
n6e
I
• I
6O
YbB,a2Cu307_x 80 T(K)
4.
CONCLUSION. We have achieved the epitaxial growth of the 1:2:3 metallic compound with Yb and Er by Molecular Beam Epitaxy. After post-deposition thermal annealing, the fills exhibit good superconducting properties as shown by resistivity and magnetic susceptibility measurements. Finally, we observed a high degree of stability of the films, were the use of BaF 2 as a starting material seems to play an important role. REFERENCES
30
(1)
M.K.
(2)
T.K.
t4
20
Fig. ] :
(3)
2 Resistivity of two
10
YbBa2Cu307 samples Sl and $2.
100
200
(4)
3C)0 T(K (5)
The susceptibility was investigated at low field as shown on figure 2. Sharp transition is observed at a field less than i00 mOe ; however we may notice it occurs at a lower temperature than the resistivity one. Magnetization versus field hysteresis measurements let us estimate the critical current to be of the order of 2.10 A/cm at 5K. These characteristics are mostly due to the granular structure of films, as seen on X-rays diffraction patterns.
(6) (7)
Wu, J.R. Ashburn, C.J. Torng, P.H. Hor, R.L. Meng, L. Gao, Z.I. Huang, Y.Q. Wang and C.W. Chu, Phys. Rev. L e t t . 58, 908 (1987)
Worthington, W.J. Oallagher and
T.R. Dinger, Phys. Rev. Lett. 59, 1160 (1987). R.B. Laibowitz, R. H. Koch, P. Chaudhari and R.J. Gambino, Phys.
Rev. B 3~5, 8821 (1987) P. Chaudhari, R.H. Koch, R.B. Laibowitz, T.R. Mc Ouire and R.J. Oambino, Phys. Rev. L e t t . 58, 2684 (1987). J. Kwo, T.C. Hsieh, R.M. Fleming, M. Hong, S.H. Liou, B.A. Davidson and L.C. Feldman, Phys. Rev. B 36, 4039 (1987). C. Webb, S.L. Weng, J.N. Eckstein, N. Missert, K. Char, D.G. Schlom, E.S. Hellman, M.R. Beasley, A. Kapitulnik and J.S. Harris Jr, Appl. Phys. Lett.
51, 1191 (1987) D.K. Lathrop, S.E. Russek and R.A. Buhrman, Appl. Phys. L e t t . 51, 1554 (1987) P.M. Mankiewich, J.H. Scofield, W.J. Skocpol, R.E. Howard, A.M. Dayem and E. Good, Appl. Phys. Lett 51,
1753 (1987).