- Email: [email protected]
Deposition of high Tc YBaCuO and BiSrCaCuO superconducting thin films by pulsed excimer laser evaporation
0038-1098/88 $3.00 + .00 Pergamon Press plc
~Solid State Communications, Vol. 67, No. i0, pp. 975-979, 1988. %~w@ Printed in Great Britain.
DEPOSITION OF HIGH T c Y B a C u O a n d BiSrCaCuO SUPERCONDUCTING THIN FILMS BY P U L S E D EXCIMER LASER EVAPORATION E. Fogarassy, C. Fuchs, P. Siffert Centre de Recherches Nucl6aires (IN2P3), Laboratoire PHASE (UA du CNRS n°292) 23, rue du LOESS, F-67037 STRASBOURG CEDEX (France] J. Perri6re, X,Z. Wang, F. Rochet Groupc de Physique des Solides de l'Ecole Normale Sup6rieure, Unlversit6 Paris VII, Tour 23 2, Place J u s s i e u , F-75251 PARIS CEDEX 05 (France] (Received 30 June
1988 by
M. Balkanski)
The deposition of YBaCuO a n d BiSICaCuO t h i n films by laser evaporation in a clean e n v i r o n m e n t h a s been performed u s i n g a pulsed ArF excimer laser. The as-deposited t h i n films were s u c c e s s f u l l y c o n v e r t e d into t h e s u p e r c o n d u c t i n g p h a s e by a s u b s e q u e n t anneal in oxygen in the 850-900°C t e m p e r a t u r e range. The onset critical t e m p e r a t u r e s were respectively 85 a n d 92 K with a zero r e s i s t a n c e at 83 K for BiSrCaCuO.
1. Int:rcxluetlcm
always f o u n d n o r m a l to t h e target a n d a p p e a r to b t strongly d e p e n d e n t on t h e irradiation conditions a n d on t h e g e o m e t r y of t h e e x p e r i m e n t a l s e t - u p . In t h e configuration of Fig. 1, the m a x i m u m t h i c k n e s s ,which is obtained at O = 0, decreases rapidly to reach only 10 % of t h e initial value w h e n 0 = 60 °. The c o m p o s i t i o n of t h e b u l k target a n d of the a s - d e p o s i t e d a n d a n n e a l e d films were d e d u c e d from R u t h e r f o r d B a c k s c a t t e r i n g S p e c t r o m e t r y (RBS) performed in r a n d o m configuration. In order to compare t h e relative s t o i c h i o m e t r y of the v a r i o u s BiSrCaCuO s p e c i m e n s , the Bi concentration w a s normalized to 2 for each sample.
In t h e p a s t few m o n t h s , a very i n t e n s e activity s u r r o u n d e d t h e p r e p a r a t i o n a n d c h a r a c t e r i z a t i o n of bulk oxide superconductors, presenting superconductivity above 30 K in the LaBaCuO s y s t e m [11, a r o u n d 90 K in YBaCuO oxides [2] a n d at 85 and 110 K for t h e BiSrCaCuO c o m p o u n d s [3]. Preparing t h i n films of t h e s e m a t e r i a l s , which is of considerable interest from b o t h scientific a n d technological viewpoints, especially for micro a n d opto-electronics applications, h a s t u m e d to be a difficult t a s k . A m o n g t h e different m e t h o d s already investigated in order to deposit s u p e r c o n d u c t i n g YBaCuO t h i n films, laser evaporation a p p e a r s to be one of the m o s t promising [4-11]. We c o m p a r e in t h i s s t u d y t h e c o n d i t i o n s of p r e p a r a t i o n b y l a s e r e v a p o r a t i o n of Y B a C u O a n d BiSrCaCuO t h i n films, u s i n g a pulsed exclmer laser. The different p a r a m e t e r s responsible on t h e deposition will be optimized a n d a special a t t e n t i o n will be devoted to t h e s u b s e q u e n t t h e r m a l a n n e a l i n g in flowing oxygen w h i c h a p p e a r s v e r y c r i t i c a l to p r o d u c e t h e s u p e r c o n d u c t i n g phase.
10
,
,
SUBSTRATE
TARGET
t 011
'
~,rF(193nm)~ L .... 3 cm
8
•
/
2. Experiment
%6
B u l k s u p e r c o n d u c t i n g (onset T e = 85 K} targets of YBaCuO a n d BiSrCaCuO, prepared by s t a n d a r d ceramics techniques, were irradiated u n d e r v a c u u m (P ~ 10-5 ton-), t h r o u g h a s u p r a s i l q u a r t z window, with a p u l s e d ArF e x c i m e r l a s e r (Lambda P h y s i k EMG 201 MSC) which provides at 193 n m , 3 0 0 m J p u l s e s of 20 n s d u r a t i o n {FWHM), at a repetition rate of 60 Hz. The laser b e a m was focused onto t h e target, u n d e r n o r m a l incidence or at oblique angle in order to give energy d e n s i t i e s r a n g i n g between 0.1 a n d 2.5 m J / c m 2. Emitted m a t e r i a l s were d e p o s i t e d respectively onto silicon, yttria stabilized zirconia (YSZ) a n d MgO single crystal (100 oriented) s u b s t r a t e s s e t parallel to t h e target a n d located at a distance which could be varied between 1 a n d 3 cm. The s u b s t r a t e t e m p e r a t u r e w a s either held at 400°C or at r o o m t e m p e r a t u r e d u r i n g t h e d e p o s i t i o n of t h e compounds. W h a t e v e r the angle of incidence of t h e laser b e a m relative to the target normal, a p l u m e of i n t e n s e white light e m i s s i o n d u r i n g e a c h p u l s e could be observed n o r m a l to t h e pellet s u r f a c e . In c o n s e q u e n c e , t h e m a x i m u m deposited t h i c k n e s s a n d deposition rate are
/ ~~0088
X
0.066 ~:
/
~J F[Z:
/° / ~
Z ©
~A Z
0044
~--
I kd
/
/
oZ. 05
v
l
;,%Cc%uo o" 1
0.022
I
d 15 2 LASER ENERGYDENSITY(J/cm 2 )
25
Fig. I : Deposited t h i c k n e s s a n d deposition rate for YBaCuO (o) a n d BiSrCaCuO (o) a s a f u n c t i o n of the excimer laser energy density. 975
976
HIGH T
YBaCuO AND BiSrCaCuO
SUPERCONDUCTING
THIN FILMS
Vol. 67, No. i0
c
Due to the s t r o n g a b s o r p t i o n of the high T c c o m p o u n d s at
the target [<300 A). At the power densities (- 5 x 107 W / c m , 2 } involved in o u r e x p e r i m e n t s , we expect t h e s u r f a c e t e m p e r a t u r e of t h e i r r a d i a t e d t a r g e t s to significantly exceed t h e m e l t t e m p e r a t u r e of Y B a C u O (TM ~ 1300°C} [12] a n d B i S r C a C u O (TM ~ 865°C) [13] c o m p o u n d s w i t h i n a few t e n s of n s w h e r e b y the material c o u l d e v a p o r a t e at t h e s u r f a c e . T h i s a s s u m p t i o n is c o n f i r m e d b y a m i c r o s c o p i c o b s e r v a t i o n of t h e l a s e r i m p a c t e d a r e a w h i c h clearly indicate the existence of a m e l t i n g resolidffication p r o c e s s . By e x t r a p o l a t i n g t h e e x p e r i m e n t a l c u r v e s of Fig. 1, giving the variation of the
193 nm, (for YBaCuO, c t > 3 x 105 cm-1 at ~ < 3 0 0 n m ) the l a s e r energy is deposited into the n e a r s u r f a c e region of
d e p o s i t e d t h i c k n e s s ( m e a s u r e d at 0 = 10 ° b y a m e c h a n i c a l stylus) a s a f u n c t i o n of the i n c i d e n t l a s e r
Finally, t h e s u p e r c o n d u c t i n g p r o p e r t i e s of t h e a s - d e p o s i t e d films, r e s u l t i n g f r o m a p o s t - l a s e r t h e r m a l t r e a t m e n t u n d e r flowing oxygen, in t h e 8 5 0 - 9 0 0 ° C temperature range, were controlled by resistivity m e a s u r e m e n t s carried o u t u s i n g t h e s t a n d a r d dc f o u r p r o b e m e t h o d after d e p o s i t i n g gold c o n t a c t s on t h e samples. 3. R e s u l t s a n d d i s c u s s i o n
ENERGIE (MeV) 0.5
120
I
1.0
1.5
I
I
Simulation RUMP 100
0 o
Bi2Sr1.85Ca.gCu3.esOy
80
60 © Z
40
20
0 200
I
!
I
400
600
800
1000
CANAL F i g . 2 : R a n d o m RBS s p e c t r u m of a s u p e r c o n d u c t i n g YBaCuO film o n a YSZ s u b s t r a t e after p o s t laser t h e r m a l t r e a t m e n t . (solid line : RUMP simulation).
80
60 0 o c~
~.. 0.6
ENERGIE (MeV) 0.8
1.0
1.2
1.4
1.6
1.8
2.0
I
[
I
I
I
i
I
t-
"'" ..... ""
"
'
".... ' "
40
Y1Ba2Cu30, 20
I
I
I
100
200
300
CANAL
I
I
400
500
Fig. 3 : R a n d o m RIBS s p e c t r u m of a a s - d e p o s i t e d B i S r C a C u O o n a Si s u b s t r a t e . (solid line : RUMP simulation).
977
YBaCuO AND BiSrCaCuO SUPERCONDUCTING THIN FILMS
HIGH T
Vol. 67, No. i0
C
880J L)
I
:
'
'
YBaCuO
r--------~2N3h
860 " 750
r--------~2
LLI r~
i a/ bJ (3. ~E b3 t--
BiSrCaCuO
....
N
3
h
~ A t
i 3 : 1 - 3 h ! ( SLOW- COOLING)
400
o ~
O ANNEALING
TIME
4 : S c h e m e s of the thermal cycles (O2} applied respectively laser evaporated YBaCuO and BiSrCaCuO films. Fig.
energy density, we found the energy t h r e s h o l d s for the deposition of YBaCuO and BiSrCaCuO to be respectively 0.7 a n d 0.4 J / c m 2. M e a s u r e d film t h i c k n e s s e s are ranging typically between 0. I and I ~.rn, with maximum deposition rates in the order of 0.1 A/pulse at 2.5 J/cm,~, in the geometrical configuration of Fig. 1. In addition to the l a s e r energy density, we observed also a strong influence of the distance (d) separating the target from the s u b s t r a t e on the deposition rate which increases up to 0.25 ~Jpulse when d = 2 a n . The s t o i c h i o m e t r y : 1, 2, 3 of the YbACuO superconducting target is closely reproduced both in the a s - d e p o s l t e d film a n d a f t e r p o s t - l a s e r t h e r m a l annealing in 0 2 atmosphere, as s h o w n on the random RBS s p e c t r u m of~Fig. 2 simulated u s i n g the RUMP p r o g r a m [14]. For b i s m u t h c o m p o u n d s , we found a difference between the composition of the bulk target ( B i 2 S r l . 8 C a l . 2 C u 2 . 2 1 and the a s - d e p o s i t e d film (Bi2Sr1.85Ca0.9Cu3.85), which shows a n excess of Cu not yet well understood (Fig. 3). However, it is important to notice that, after s u b s e q u e n t 02 thermal treatment, these specimens exhibited again a stoichiometry close to : 2, 2, I, 2 which is characteristics of the 85 K superconduclng phase of the BiSrCaCuO system. A range of 02 annealing conditions in temperature a n d time h a s b e e n investigated and correlated with r e s i s t a n c e m e a s u r e m e n t s in o r d e r to achieve the superconducting p h a s e with a right amount of oxygen to be i n c o r p o r a t e d into t h e r i g h t c r y s t a l l o g r a p h i c s t r u c t u r e . The typical annealing conditions applied respectively to the two c o m p o u n d s are schematically r e p r e s e n t e d on Fig. 4. For YBaCuO, the t h e r m a l treatment involve rapid heating of the film up to 880°C where it is maintained for a time not exceeding 30 mn. The film is then slowly (3 hours) cooled to 400°C where it is held for 1-3 hours, before to be cooled down to room temperature. In t h e s e conditions, the superconducting phase is obtained with a critical temperature of 85 K (Fig. 5) a n d a r e l a t i v e l y b r o a d t r a n s i t i o n (=40K) c h a r a c t e r i s t i c s of a n h e t e r o g e n e o u s material. This behaviour is not yet clearly u n d e r s t o o d . It s e e m s however partly related to the high reactivity of this compound to water vapor. The use of YSZ s u b s t r a t e s for d e p o s i t i n g YBaCuO films c o u l d be also p a r t l y responsible on this behaviour. This point n e e d s to be carefully analyzed in the future. BiSrCaCuO oxide films require a m u c h simpler and faster thermal cycle t h a n for YBaCuO compounds, involving rapid heating of the film to 850-870°C, where it is maintained for a few tens of m i n u t e s before to be rapidly quenched to find again
the room temperature. The BiSrCaCuO superconducting transition, with a Tc onset at 92 K and zero resistance being r e a c h e d at 83 K (Fig. 5) is only achieved after heating the film J u s t above its melting point {TM = 865°C) [13]. This is confirmed by the SEM surface e x a m i n a t i o n of the BiSrCaCuO t h i n films h e a t e d respectively below {Fig. 6a) and above (Fig. 6b) T M showing a surface morphology which is characteristics of melting-resolidification p h e n o m e n a . The superconducting properties of the BiSrCaCuO oxide films on MgO s u b s t r a t e s , are obtained for very thin layers (4 150 nm) and do not seem to be significantly influenced by the presence of water vapor during the different s t e p s of the fabrication, by contrast to the YBaCuO samples. In addition, the substrate temperature (20 or 400°C) during the laser deposition process does not seem to be a critical parameter for the superconducting quality of the BiSrCaCuO films. Finally, it is interesting to notice that similar s u p e r c o n d u c t i n g properties are achieved in the Bi c o m p o u n d s w h e n deposited on YSZ substrates. ]
1.5
I
/ / Y BaCuO
/
-
[
--.
l o i i
rr"
t
# E 0.5
I l
i I
,"
-- Bi SrCaCuO
I I
I
0
, I 100
I 200
TEMPERATURE
300
(K)
Fig. 5 : Temperature dependence of the resistivity for YBaCuO and BiSrCaCuO films deposited respectively on YSZ and MgO substrates.
978
HIGH T
YBaCuO AND BiSrCaCuO SUPERCONDUCTING THIN FILMS
Vol. 67, No. i0
c
(a)
(b)
Fig. 6 : S c a n n i n g e l e c t r o n m i c r o g r a p h s of a n o n - s u p e r c o n d u c t i n g (a) a n d a s u p e r c o n d u c t i n g (b) BiSrCaCuO film on a MgO substrate. transition at 92 K, with a zero resistance at 83 K on MgO s u b s t r a t e . In t h e f u t u r e , the p o s t - l a s e r t h e r m a l treatments need to bc optimized in order to achieve the 110 K p h a s e t r a n s i t i o n , recently observed in the BiSrCaCuO bulk materials.
In this study, we have compared the conditions of p r e p a r a t i o n by l a s e r e v a p o r a t i o n of YBaCuO a n d BiSrCaCuO high T c thin films, using a pulsed excimer laser. This technique s e e m s especially suitable for the b i s m u t h c o m p o u n d s which p r e s e n t a superconducting
References
[1] [2] [3] [4]
J.G. Bednorz, K.A. Muller Z. Phys. B64, 189 (1986). C.W. Chu, P.H. Hor. R.L. Mong, L. Gao, Z.J. Huang, Y.O. Wang Phys. Rev. Lett 38, 405 (1987). H. Maeda, Y. Tanaka, M. Fukutomi and T. Asano Jpn. J. of Appl. Phys. Left. 27 (2) (1988). D. DiJkkamp, T. V e n k a t e s a n , X.D. Wu, S.A. Shaheen, N. Jisrawi, Y.H. Min-Lee, W.L. Mc Lean,
[5]
[6]
M. Croft Appl. Phys. Lett. 51 (8), 619 (1987). K. Moorjani, O. Bohandy, F.J. Adrian, B.F. Klm, R.D. Shull, C.F. Chiang, L.J. Swartzendruber, L.H. Bennett Phys. 1~'v. B, V__~.2~,N°7.4036 (1987). J, Narayan, N. Biunno, R. Singh, O.W. Holland and O. Auciello Appl. Phys. Left. 51 (22), 1845 (1987).
Vol. 67, No. i0
HIGH Tc YBaCuO AND BiSrCaCuO SUPERCONDUCTING THIN FILMS
T. Kawai, M. Kanai, M. Kawai Boston MRS Syrup. Proc., V. 99 "High Temperature Superconductors" 1987. [8] B. Dam, T.S. Bailer, G.N.A. Van Veen, H.A.M. Van Hal, J.W.C. de Vries, G.M. Stollman Boston MRS Syrup. Proc., V, 99 "High Temperature Superconductors" 1987. [9] D.N. M a s b u r n , D.B. Geohegan, D. Eres, D.H. Lowndes, L.A. Boatner, B.C. Sales, S.J. Pennycook, R.J. Culbertson, E. Sonder, D.L. Kristen Boston MRS Symp. Proc., V. 99 "High Temperature Superconductors" 1987. [IOI L. Lynds, B.R. Weinberger, G.G.T. Peterson and H.~ Krasklnski Appl. Phys. Lett. 52 (4), 320 {1988).
[7]
979
[11] S. Mlura, T. Yoshitake, T. 8atoh, Y. Miyasaka, N. Shohata Appl. Phys. Lett. 52 {12}, 1008 (1988). [12] S. Lang, N. Narita, K. Higashida and H. Mazaki Jpn. J. Appl. Phys. 26, L1394 {1987). [13] J. Perriere, E. Fogarassy, G. H a u c h e c o m e , X.Z. Wang, C. Fuchs, F. Rochet, I. Rosenman, C. Simon, R.M. D e f o u m e a u , F. Kerherve, J.P. Enard, A. Laurent To be published in Sol. Stat. Com. [14] L.R. Doolittle Nucl. Inst. Meth. B9, 344 (1985).
~Solid State Communications, Vol. 67, No. i0, pp. 975-979, 1988. %~w@ Printed in Great Britain.
DEPOSITION OF HIGH T c Y B a C u O a n d BiSrCaCuO SUPERCONDUCTING THIN FILMS BY P U L S E D EXCIMER LASER EVAPORATION E. Fogarassy, C. Fuchs, P. Siffert Centre de Recherches Nucl6aires (IN2P3), Laboratoire PHASE (UA du CNRS n°292) 23, rue du LOESS, F-67037 STRASBOURG CEDEX (France] J. Perri6re, X,Z. Wang, F. Rochet Groupc de Physique des Solides de l'Ecole Normale Sup6rieure, Unlversit6 Paris VII, Tour 23 2, Place J u s s i e u , F-75251 PARIS CEDEX 05 (France] (Received 30 June
1988 by
M. Balkanski)
The deposition of YBaCuO a n d BiSICaCuO t h i n films by laser evaporation in a clean e n v i r o n m e n t h a s been performed u s i n g a pulsed ArF excimer laser. The as-deposited t h i n films were s u c c e s s f u l l y c o n v e r t e d into t h e s u p e r c o n d u c t i n g p h a s e by a s u b s e q u e n t anneal in oxygen in the 850-900°C t e m p e r a t u r e range. The onset critical t e m p e r a t u r e s were respectively 85 a n d 92 K with a zero r e s i s t a n c e at 83 K for BiSrCaCuO.
1. Int:rcxluetlcm
always f o u n d n o r m a l to t h e target a n d a p p e a r to b t strongly d e p e n d e n t on t h e irradiation conditions a n d on t h e g e o m e t r y of t h e e x p e r i m e n t a l s e t - u p . In t h e configuration of Fig. 1, the m a x i m u m t h i c k n e s s ,which is obtained at O = 0, decreases rapidly to reach only 10 % of t h e initial value w h e n 0 = 60 °. The c o m p o s i t i o n of t h e b u l k target a n d of the a s - d e p o s i t e d a n d a n n e a l e d films were d e d u c e d from R u t h e r f o r d B a c k s c a t t e r i n g S p e c t r o m e t r y (RBS) performed in r a n d o m configuration. In order to compare t h e relative s t o i c h i o m e t r y of the v a r i o u s BiSrCaCuO s p e c i m e n s , the Bi concentration w a s normalized to 2 for each sample.
In t h e p a s t few m o n t h s , a very i n t e n s e activity s u r r o u n d e d t h e p r e p a r a t i o n a n d c h a r a c t e r i z a t i o n of bulk oxide superconductors, presenting superconductivity above 30 K in the LaBaCuO s y s t e m [11, a r o u n d 90 K in YBaCuO oxides [2] a n d at 85 and 110 K for t h e BiSrCaCuO c o m p o u n d s [3]. Preparing t h i n films of t h e s e m a t e r i a l s , which is of considerable interest from b o t h scientific a n d technological viewpoints, especially for micro a n d opto-electronics applications, h a s t u m e d to be a difficult t a s k . A m o n g t h e different m e t h o d s already investigated in order to deposit s u p e r c o n d u c t i n g YBaCuO t h i n films, laser evaporation a p p e a r s to be one of the m o s t promising [4-11]. We c o m p a r e in t h i s s t u d y t h e c o n d i t i o n s of p r e p a r a t i o n b y l a s e r e v a p o r a t i o n of Y B a C u O a n d BiSrCaCuO t h i n films, u s i n g a pulsed exclmer laser. The different p a r a m e t e r s responsible on t h e deposition will be optimized a n d a special a t t e n t i o n will be devoted to t h e s u b s e q u e n t t h e r m a l a n n e a l i n g in flowing oxygen w h i c h a p p e a r s v e r y c r i t i c a l to p r o d u c e t h e s u p e r c o n d u c t i n g phase.
10
,
,
SUBSTRATE
TARGET
t 011
'
~,rF(193nm)~ L .... 3 cm
8
•
/
2. Experiment
%6
B u l k s u p e r c o n d u c t i n g (onset T e = 85 K} targets of YBaCuO a n d BiSrCaCuO, prepared by s t a n d a r d ceramics techniques, were irradiated u n d e r v a c u u m (P ~ 10-5 ton-), t h r o u g h a s u p r a s i l q u a r t z window, with a p u l s e d ArF e x c i m e r l a s e r (Lambda P h y s i k EMG 201 MSC) which provides at 193 n m , 3 0 0 m J p u l s e s of 20 n s d u r a t i o n {FWHM), at a repetition rate of 60 Hz. The laser b e a m was focused onto t h e target, u n d e r n o r m a l incidence or at oblique angle in order to give energy d e n s i t i e s r a n g i n g between 0.1 a n d 2.5 m J / c m 2. Emitted m a t e r i a l s were d e p o s i t e d respectively onto silicon, yttria stabilized zirconia (YSZ) a n d MgO single crystal (100 oriented) s u b s t r a t e s s e t parallel to t h e target a n d located at a distance which could be varied between 1 a n d 3 cm. The s u b s t r a t e t e m p e r a t u r e w a s either held at 400°C or at r o o m t e m p e r a t u r e d u r i n g t h e d e p o s i t i o n of t h e compounds. W h a t e v e r the angle of incidence of t h e laser b e a m relative to the target normal, a p l u m e of i n t e n s e white light e m i s s i o n d u r i n g e a c h p u l s e could be observed n o r m a l to t h e pellet s u r f a c e . In c o n s e q u e n c e , t h e m a x i m u m deposited t h i c k n e s s a n d deposition rate are
/ ~~0088
X
0.066 ~:
/
~J F[Z:
/° / ~
Z ©
~A Z
0044
~--
I kd
/
/
oZ. 05
v
l
;,%Cc%uo o" 1
0.022
I
d 15 2 LASER ENERGYDENSITY(J/cm 2 )
25
Fig. I : Deposited t h i c k n e s s a n d deposition rate for YBaCuO (o) a n d BiSrCaCuO (o) a s a f u n c t i o n of the excimer laser energy density. 975
976
HIGH T
YBaCuO AND BiSrCaCuO
SUPERCONDUCTING
THIN FILMS
Vol. 67, No. i0
c
Due to the s t r o n g a b s o r p t i o n of the high T c c o m p o u n d s at
the target [<300 A). At the power densities (- 5 x 107 W / c m , 2 } involved in o u r e x p e r i m e n t s , we expect t h e s u r f a c e t e m p e r a t u r e of t h e i r r a d i a t e d t a r g e t s to significantly exceed t h e m e l t t e m p e r a t u r e of Y B a C u O (TM ~ 1300°C} [12] a n d B i S r C a C u O (TM ~ 865°C) [13] c o m p o u n d s w i t h i n a few t e n s of n s w h e r e b y the material c o u l d e v a p o r a t e at t h e s u r f a c e . T h i s a s s u m p t i o n is c o n f i r m e d b y a m i c r o s c o p i c o b s e r v a t i o n of t h e l a s e r i m p a c t e d a r e a w h i c h clearly indicate the existence of a m e l t i n g resolidffication p r o c e s s . By e x t r a p o l a t i n g t h e e x p e r i m e n t a l c u r v e s of Fig. 1, giving the variation of the
193 nm, (for YBaCuO, c t > 3 x 105 cm-1 at ~ < 3 0 0 n m ) the l a s e r energy is deposited into the n e a r s u r f a c e region of
d e p o s i t e d t h i c k n e s s ( m e a s u r e d at 0 = 10 ° b y a m e c h a n i c a l stylus) a s a f u n c t i o n of the i n c i d e n t l a s e r
Finally, t h e s u p e r c o n d u c t i n g p r o p e r t i e s of t h e a s - d e p o s i t e d films, r e s u l t i n g f r o m a p o s t - l a s e r t h e r m a l t r e a t m e n t u n d e r flowing oxygen, in t h e 8 5 0 - 9 0 0 ° C temperature range, were controlled by resistivity m e a s u r e m e n t s carried o u t u s i n g t h e s t a n d a r d dc f o u r p r o b e m e t h o d after d e p o s i t i n g gold c o n t a c t s on t h e samples. 3. R e s u l t s a n d d i s c u s s i o n
ENERGIE (MeV) 0.5
120
I
1.0
1.5
I
I
Simulation RUMP 100
0 o
Bi2Sr1.85Ca.gCu3.esOy
80
60 © Z
40
20
0 200
I
!
I
400
600
800
1000
CANAL F i g . 2 : R a n d o m RBS s p e c t r u m of a s u p e r c o n d u c t i n g YBaCuO film o n a YSZ s u b s t r a t e after p o s t laser t h e r m a l t r e a t m e n t . (solid line : RUMP simulation).
80
60 0 o c~
~.. 0.6
ENERGIE (MeV) 0.8
1.0
1.2
1.4
1.6
1.8
2.0
I
[
I
I
I
i
I
t-
"'" ..... ""
"
'
".... ' "
40
Y1Ba2Cu30, 20
I
I
I
100
200
300
CANAL
I
I
400
500
Fig. 3 : R a n d o m RIBS s p e c t r u m of a a s - d e p o s i t e d B i S r C a C u O o n a Si s u b s t r a t e . (solid line : RUMP simulation).
977
YBaCuO AND BiSrCaCuO SUPERCONDUCTING THIN FILMS
HIGH T
Vol. 67, No. i0
C
880J L)
I
:
'
'
YBaCuO
r--------~2N3h
860 " 750
r--------~2
LLI r~
i a/ bJ (3. ~E b3 t--
BiSrCaCuO
....
N
3
h
~ A t
i 3 : 1 - 3 h ! ( SLOW- COOLING)
400
o ~
O ANNEALING
TIME
4 : S c h e m e s of the thermal cycles (O2} applied respectively laser evaporated YBaCuO and BiSrCaCuO films. Fig.
energy density, we found the energy t h r e s h o l d s for the deposition of YBaCuO and BiSrCaCuO to be respectively 0.7 a n d 0.4 J / c m 2. M e a s u r e d film t h i c k n e s s e s are ranging typically between 0. I and I ~.rn, with maximum deposition rates in the order of 0.1 A/pulse at 2.5 J/cm,~, in the geometrical configuration of Fig. 1. In addition to the l a s e r energy density, we observed also a strong influence of the distance (d) separating the target from the s u b s t r a t e on the deposition rate which increases up to 0.25 ~Jpulse when d = 2 a n . The s t o i c h i o m e t r y : 1, 2, 3 of the YbACuO superconducting target is closely reproduced both in the a s - d e p o s l t e d film a n d a f t e r p o s t - l a s e r t h e r m a l annealing in 0 2 atmosphere, as s h o w n on the random RBS s p e c t r u m of~Fig. 2 simulated u s i n g the RUMP p r o g r a m [14]. For b i s m u t h c o m p o u n d s , we found a difference between the composition of the bulk target ( B i 2 S r l . 8 C a l . 2 C u 2 . 2 1 and the a s - d e p o s i t e d film (Bi2Sr1.85Ca0.9Cu3.85), which shows a n excess of Cu not yet well understood (Fig. 3). However, it is important to notice that, after s u b s e q u e n t 02 thermal treatment, these specimens exhibited again a stoichiometry close to : 2, 2, I, 2 which is characteristics of the 85 K superconduclng phase of the BiSrCaCuO system. A range of 02 annealing conditions in temperature a n d time h a s b e e n investigated and correlated with r e s i s t a n c e m e a s u r e m e n t s in o r d e r to achieve the superconducting p h a s e with a right amount of oxygen to be i n c o r p o r a t e d into t h e r i g h t c r y s t a l l o g r a p h i c s t r u c t u r e . The typical annealing conditions applied respectively to the two c o m p o u n d s are schematically r e p r e s e n t e d on Fig. 4. For YBaCuO, the t h e r m a l treatment involve rapid heating of the film up to 880°C where it is maintained for a time not exceeding 30 mn. The film is then slowly (3 hours) cooled to 400°C where it is held for 1-3 hours, before to be cooled down to room temperature. In t h e s e conditions, the superconducting phase is obtained with a critical temperature of 85 K (Fig. 5) a n d a r e l a t i v e l y b r o a d t r a n s i t i o n (=40K) c h a r a c t e r i s t i c s of a n h e t e r o g e n e o u s material. This behaviour is not yet clearly u n d e r s t o o d . It s e e m s however partly related to the high reactivity of this compound to water vapor. The use of YSZ s u b s t r a t e s for d e p o s i t i n g YBaCuO films c o u l d be also p a r t l y responsible on this behaviour. This point n e e d s to be carefully analyzed in the future. BiSrCaCuO oxide films require a m u c h simpler and faster thermal cycle t h a n for YBaCuO compounds, involving rapid heating of the film to 850-870°C, where it is maintained for a few tens of m i n u t e s before to be rapidly quenched to find again
the room temperature. The BiSrCaCuO superconducting transition, with a Tc onset at 92 K and zero resistance being r e a c h e d at 83 K (Fig. 5) is only achieved after heating the film J u s t above its melting point {TM = 865°C) [13]. This is confirmed by the SEM surface e x a m i n a t i o n of the BiSrCaCuO t h i n films h e a t e d respectively below {Fig. 6a) and above (Fig. 6b) T M showing a surface morphology which is characteristics of melting-resolidification p h e n o m e n a . The superconducting properties of the BiSrCaCuO oxide films on MgO s u b s t r a t e s , are obtained for very thin layers (4 150 nm) and do not seem to be significantly influenced by the presence of water vapor during the different s t e p s of the fabrication, by contrast to the YBaCuO samples. In addition, the substrate temperature (20 or 400°C) during the laser deposition process does not seem to be a critical parameter for the superconducting quality of the BiSrCaCuO films. Finally, it is interesting to notice that similar s u p e r c o n d u c t i n g properties are achieved in the Bi c o m p o u n d s w h e n deposited on YSZ substrates. ]
1.5
I
/ / Y BaCuO
/
-
[
--.
l o i i
rr"
t
# E 0.5
I l
i I
,"
-- Bi SrCaCuO
I I
I
0
, I 100
I 200
TEMPERATURE
300
(K)
Fig. 5 : Temperature dependence of the resistivity for YBaCuO and BiSrCaCuO films deposited respectively on YSZ and MgO substrates.
978
HIGH T
YBaCuO AND BiSrCaCuO SUPERCONDUCTING THIN FILMS
Vol. 67, No. i0
c
(a)
(b)
Fig. 6 : S c a n n i n g e l e c t r o n m i c r o g r a p h s of a n o n - s u p e r c o n d u c t i n g (a) a n d a s u p e r c o n d u c t i n g (b) BiSrCaCuO film on a MgO substrate. transition at 92 K, with a zero resistance at 83 K on MgO s u b s t r a t e . In t h e f u t u r e , the p o s t - l a s e r t h e r m a l treatments need to bc optimized in order to achieve the 110 K p h a s e t r a n s i t i o n , recently observed in the BiSrCaCuO bulk materials.
In this study, we have compared the conditions of p r e p a r a t i o n by l a s e r e v a p o r a t i o n of YBaCuO a n d BiSrCaCuO high T c thin films, using a pulsed excimer laser. This technique s e e m s especially suitable for the b i s m u t h c o m p o u n d s which p r e s e n t a superconducting
References
[1] [2] [3] [4]
J.G. Bednorz, K.A. Muller Z. Phys. B64, 189 (1986). C.W. Chu, P.H. Hor. R.L. Mong, L. Gao, Z.J. Huang, Y.O. Wang Phys. Rev. Lett 38, 405 (1987). H. Maeda, Y. Tanaka, M. Fukutomi and T. Asano Jpn. J. of Appl. Phys. Left. 27 (2) (1988). D. DiJkkamp, T. V e n k a t e s a n , X.D. Wu, S.A. Shaheen, N. Jisrawi, Y.H. Min-Lee, W.L. Mc Lean,
[5]
[6]
M. Croft Appl. Phys. Lett. 51 (8), 619 (1987). K. Moorjani, O. Bohandy, F.J. Adrian, B.F. Klm, R.D. Shull, C.F. Chiang, L.J. Swartzendruber, L.H. Bennett Phys. 1~'v. B, V__~.2~,N°7.4036 (1987). J, Narayan, N. Biunno, R. Singh, O.W. Holland and O. Auciello Appl. Phys. Left. 51 (22), 1845 (1987).
Vol. 67, No. i0
HIGH Tc YBaCuO AND BiSrCaCuO SUPERCONDUCTING THIN FILMS
T. Kawai, M. Kanai, M. Kawai Boston MRS Syrup. Proc., V. 99 "High Temperature Superconductors" 1987. [8] B. Dam, T.S. Bailer, G.N.A. Van Veen, H.A.M. Van Hal, J.W.C. de Vries, G.M. Stollman Boston MRS Syrup. Proc., V, 99 "High Temperature Superconductors" 1987. [9] D.N. M a s b u r n , D.B. Geohegan, D. Eres, D.H. Lowndes, L.A. Boatner, B.C. Sales, S.J. Pennycook, R.J. Culbertson, E. Sonder, D.L. Kristen Boston MRS Symp. Proc., V. 99 "High Temperature Superconductors" 1987. [IOI L. Lynds, B.R. Weinberger, G.G.T. Peterson and H.~ Krasklnski Appl. Phys. Lett. 52 (4), 320 {1988).
[7]
979
[11] S. Mlura, T. Yoshitake, T. 8atoh, Y. Miyasaka, N. Shohata Appl. Phys. Lett. 52 {12}, 1008 (1988). [12] S. Lang, N. Narita, K. Higashida and H. Mazaki Jpn. J. Appl. Phys. 26, L1394 {1987). [13] J. Perriere, E. Fogarassy, G. H a u c h e c o m e , X.Z. Wang, C. Fuchs, F. Rochet, I. Rosenman, C. Simon, R.M. D e f o u m e a u , F. Kerherve, J.P. Enard, A. Laurent To be published in Sol. Stat. Com. [14] L.R. Doolittle Nucl. Inst. Meth. B9, 344 (1985).