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Oxygen potential control in YBa2Cu3O7−δ thin films
PhysicaC 213 (1993) 1-13 North-Holland
Oxygen potential control in YBa2fu307_ 6 thin films Keikichi Nakamura, Jinhua Ye and Akira Ishii National Research Institute.for Metals, 1-2-1Sengen, Tsukuba-shi, Ibaraki 304, Japan Received 1 March 1993 Revised manuscript received 31 May 1993
The oxygenpotential of YBa2Cu307_6 ( YBCO) films has been controlled by equilibrating the films with bulk YBCO in a quartz reaction system. The samples were heated up to 600-650 °C and the oxygencontent in bulk YBCO was controlled by evacuating a calculated amount of oxygen gas from the reaction system. The temperature was then gradually decreased to 400°C where the final equilibrium state was attained. The oxygen deficient films thus controlled have the same oxygen potential as the oxygen deficient bulk. The plots of c-axis lengths of the films against oxygen deficiency~ are almost parallel to those of the bulk in spite of having different c-axislengths in the as-grownstate. Thus the oxygendeficiencyof as-grownYBCO films can be determined by comparing the c-axis length in the as-grown and fully oxygenated states. It is shown that YBCO as-grown films prepared under conventional deposition conditions are, in most cases, fully oxygenated except those cooled at lower oxygen partial pressure ( < 10-2 Torr) after deposition. The T¢versus J relation of the oxygendeficient film with better as-grown superconducting transition showed a two-plateau behavior almost the same as that observed in the polycrystallineand single crystal samples with well defined ~.
I. Introduction The importance of oxygen deficiency in the superconducting properties in YBCO has been recognized immediately after the discovery of this material [ 1 ] and extensive works have been done on the correlation between oxygen deficiency and phase relation, crystallographic a n d superconducting properties [ 2 - 6 ]. However, most of these investigations were done on bulk YBCO and not so m a n y works have been systematically done on thin films. For example, m a n y investigators do not confirm whether their as-grown films are fully oxygenated or oxygen deficient a n d some investigators ascribed their lower T¢ to the oxygen deficiency without confirming the accurate oxygen concentration in the films. One of the reasons of these problems is that no systematic method to determine oxygen deficiency, such as that from lattice parameters employed in bulk YBCO, has been established on thin YBCO films. The c-axis length of as-grown films is not 11.6711.69 ,~. as observed in bulk YBCO but varies widely from 1 1.7 to 12 A d e p e n d i n g on the deposition conditions a n d the kind of substrate [ 7 - 1 2 ]. Oxygen de/
.
ficiency has been thought to be partly responsible for these longer c d i m e n s i o n observed in thin films. The Tc of these films, however, do not so strongly depend on the c-axis length as observed in YBCO bulk [7,8]. Some investigators insist on the fact that the disordered arrangement of cationic ions such as substitution of Ba for Y [ 11,12], as well as the epitaxy c o m b i n e d with the lattice mismatch must be responsible for the observed c-axis length distribution. The oxygen deficiency problem, however, has not been solved so far. In order to obtain systematic knowledge of the oxygen deficiency of thin YBCO films, it is necessary to establish a reliable oxygen potential (concentrat i o n ) control method on YBCO films. For bulk YBCO, various oxygen potential controlling methods, for example, oxygen gettering with Zr [ 3 ], heating in controlled oxygen partial pressure for a long time [ 13 ], addition of a known a m o u n t of oxygen to tetragonal YBCO [ 14,15 ] have been proposed. Apart from bulk YBCO, there are m a n y problems in thin YBCO films, which are caused by the relatively thin layer ranging from several tenths to hundreds n m and by small quantities of maximal several lag.
0921-4534/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
2
K. Nakamuraet al. / Oxygenpotential of YBCOfilms
Because of these restrictions, various oxygen potential controlling methods used in bulk YBCO are not applicable to thin films. In spite of the various problems, the advantage of thin films is that the diffusion rate does not control the oxygen absorption and, consequently, homogeneously oxygenated samples can be easily obtained compared with bulk. On the other hand, a recent study on controlled oxygen deficient films [ 16 ] showed different behavior of the c-axis length and Tc versus oxygen deficiency from those observed in bulk. These results were also different from our results at that time. Thus it was necessary to confirm whether these discrepancies are due to the intrinsic nature of the films or to problems in the sample preparation. To establish a reliable and reproducible oxygen potential control method, it is necessary to study oxygen absorptiondesorption kinetics of thin YBCO films systematically. In this study a short, preliminary experimental result on the kinetics of oxygen absorption and desorption in YBCO films is described. The main purpose of this paper, however, is to establish a reliable method of controlling oxygen potential and to determine oxygen deficiency by measuring the c-lattice parameter. The study of T¢ versus oxygen deficiency of YBCO films prepared by different methods is also an important purpose in this paper.
2. Experimental procedure 2.1. Preparation o f YBCO films
YBCO thin films were synthesized by two different methods. One is 90 ° off-axis magnetron sputtering from a single target with 1:2:3.5 to 1:2:4.2 composition. The sputtering gas was A r - 10%02 and the oxygen partial pressure during sputtering was 0.0015 to 0.0075 Torr. The substrates were MgO (100) and SrTiO3 (100) single crystals and the substrate temperatures were between 500 and 670°C. After deposition, about 5 Torr oxygen gas was introduced and the substrate was allowed to cool to room temperature. In some cases, however, the substrate was cooled in 0.0015 Torr oxygen pressure in order to determine the effect of oxygen partial pressure during cooling on the oxygen content in the asgrown films. The highest zero resistivity critical tem-
perature, To, thus obtained was 82.6 K in spite of the excellent crystallinity of the films. Negative ion bombardment may have a detrimental effect on the Tc observed in the present study. This is in spite of the off-axis substrate position and higher sputtering pressure conditions which have been reported to prevent such negative effects [ 17,18 ]. A vacuum evaporation system equipped with twoelectron beam evaporator for Ba and Y sources and one A1203 coated tungsten basket for Cu evaporation were used to produce films with higher T¢. O26.2%03 gas generated by a commercially available ozonizer was blown onto substrate kept at a substate temperature, Ts, between 560 and 650°C. The ozone concentration monitored with a Horiba ozone meter was kept at 6.2%. The background pressure was reduced from 3.4× 10 -4 to 1.2× 10 -4 Tort during evaporation due to the gettering effect of Ba and Y evaporation. The deposition rate was 0.15 n m / s and the deposition time was 700-800 s. After evaporation, the substrate was allowed to cool to room temperature with the gas flow rate kept constant and the pressure gradually increasing to 3.4 X 10 -4 Torr due to interruption of the Ba, Y gettering effect. The best T~ thus obtained was 89.6 K with A T = 1.5 K and R3oo/Rloo = 3.12. In this paper, the samples were named after the method of preparation (S: sputter, M: vacuum deposition), the date of preparation and batch number (with hyphen). All samples were prepared without post-annealing except for $20817-2a and $20817-STa as shown in fig. 8, which were post-annealed at 850°C for 30 min. The reason for this naming is that the lattice parameters and T~ were differed from batch to batch. For M20611-1 and M20611-2 the c-axis lengths are 11.74, 11.72 A and T¢'s are 81.3 and 89.6 K, respectively. The above differences might be caused by the difference in flux of Cu and Y, and substrate temperature, Ts, during the deposition. 2.2. Oxygen potential control
To control the oxygen potential in YBCO films, we applied a volumetric method using Sievert's apparatus which is essentially the same as that used to equilibrate hydrogen concentration in amorphous films [ 19 ]. The apparatus consists of a quartz reaction tube (hereafter denoted as C2 with volume
K. Nakamura et al. / Oxygen potential of YBCO films
44.4 cm 3) and a m a i n 12 m m O D pyrex c h a m b e r ( d e n o t e d as C1 with a v o l u m e o f 194.2 cm 3) e q u i p p e d with three pressure gauges, a capacitance m a n o m e t e r ( 1 0 - 2 - 1 0 T o r r ) , a strain gauge ( 10-760 T o r r ) a n d an ion gauge ( < 10 -3 T o r r ) . The schematic d i a g r a m is shown in fig. 1. The reaction process is as follows. YBCO bulk with 6 = 0 . 0 7 ( a b o u t 0.3-0.5 g) a n d several pieces o f Y B C O as-grown films were placed in a quartz reaction tube. The system was then evacuated to < 10- 5 Torr a n d t h e ' t e m p e r a t u r e o f the tube was increased to 150°C and kept for 30 m i n in o r d e r to evacuate a d s o r b e d gases such as water vapor. The temperature was then increased to 6 0 0 - 6 5 0 °C while continuously evacuating the system until an a b r u p t rise in the pressure was observed a r o u n d 370 to 400°C. Valve V 1 is then closed to stop evacuation. The pressure in the system increased with t e m p e r a t u r e a n d then reached an e q u i l i b r i u m value within 10 min after the t e m p e r a t u r e reached 6 0 0 - 6 5 0 ° C. The highest t e m p e r a t u r e 650°C was chosen because it is high enough to reach e q u i l i b r i u m within several m i n u t e s a n d it is low enough to prevent change in the c-axis length o f Y B C O films due to the relaxation o f lattice defects or recrystallization. Such a change, if it were to occur, would m a k e it impossible to d e t e r m i n e oxygen deficiency from the change in the c-axis length. The e q u i l i b r i u m pressure at a given t e m p e r a t u r e
in the present system is d e t e r m i n e d by the system v o l u m e a n d Y B C O bulk weight, i.e. by the balance o f oxygen loss from the bulk a n d increase in the pressure in the system. I f the oxygen deficiency in the bulk estimated from the equilibrium pressure is lower than the i n t e n d e d value, the oxygen gas must be evacuated by opening valve V I while closing V2. After such a process, V1 is again closed a n d V2 is o p e n e d and a new equilibrium pressure corresponding to a higher 6 value is attained. Such a process is repeated, if necessary, several times until a target value is attained. It should be noted that the final e q u i l i b r i u m process was done a r o u n d 4 0 0 - 4 5 0 ° C . The oxygen in the gas phase is a d s o r b e d during cooling to 4 0 0 - 4 5 0 ° C and the oxygen content must be increased during this process. F o r this cooling process we a p p l i e d two different valve operations, i.e. m e t h o d s A and B as schematically shown in fig. 2. In method A, valve V2 is closed during the cooling to 4 0 0 - 4 5 0 ° C a n d kept at that t e m p e r a t u r e for about 1 h a n d then V2 is again o p e n e d to measure final equilibrium oxygen pres-
1000 Vl close
pump M
I.G.
A
--~-- ~ T~LmP"
0
IXi-
1'
V2
V2 close Vl open ~.J
o
yw c1
~
800
600
S.G, C,M.
3
400
iii n::3 200 I--
0 rr gi I000 Q.
VI close
~.~_ V2close V1open W
C2
I
iii
i
IAI 800
600
w rr
m~x l?p.
400
200
film
Fig. 1. A schematic diagram for the system used to control oxygen potential of YBCO bulk and films. |.G.=ion gauge, S.G. = strain gauge, C.M. -- capacitance manometer, V3 = oxygen inlet valve.
close. "rime
(arb.)
Fig. 2. A schematic diagram showing two typical valve operations (method A and B) during evacuating, heating and cooling processes. V1, V2 are opened and V3 is closed at t=0.
4
K. Nakamura et al. / Oxygenpotential of YBCOfihns
sure, and in most cases, it was found that the equilibrium was attained. In method B, valve V2 was opened during the cooling and the pressure change can be detected during this process. In this case the final oxygen pressure was, in most cases, higher than the equilibrium value indicating that the equilibrium was not attained. The reason for this will be discussed in the next section. Thus, in this experiment, we applied method A in most cases. The in situ resistivity measurements were performed in the same system as shown in fig. 1 using two-point DC transport due to the geometrical restriction. Electrical contacts were made by pressing gold sheets to gold overlayer evaporated onto YBCO film. Although the resistivity thus measured includes contact resistance between the gold sheets and gold overlayer. We assumed that, however, such additional resistivity does not affect the kinetic measurement. The composition and thickness of the as-grown films were determined by RBS using 2.275 to 3.05 MeV He 2+ irradiation. The lattice constants of the films and bulk were determined using a JEOL diffractometer with Cu Kc~ radiation. The lattice parameters of the bulk were obtained by applying Rietveld refinement. As for the films, only c- or a,cparameters were obtained by extraporating lattice parameters from (00/) or (/00) against cos20/sin 0. Resistivity data were obtained using a conventional four-probe method.
3. Results and discussion
3.1. NecessiO, of activation of YBCO fihns When YBCO films are exposed to air (even stored in a desiccator) for a long time, the surface of the films may become contaminated with various kind of impurities to form an insulating layer such as carbonate and hydroxide. According to an X-ray photoelectron spectroscopy (XPS) study [20], about one unit layer ( = 11.7 A) of insulating layer covers the YBCO surface but the insulating layer thickness never exceeds one unit layer. For the reaction of oxygen on the surface to dissociate into oxygen ions and positive holes, it is necessary to accept electrons from Cu + or 0 2 - according to
O 2 + 2 C u + ( 2 0 2 - ) - ~ O 2 - + 2 C u 2+ ( 2 0 - ) .
(1)
In the above equation O - in parentheses means that holes locate on the oxygen site. To promote the reaction with less activation energy, the surface of the film must be catalytically active. The insulating layer observed by XPS may act as a barrier against such a reaction and should be removed prior to oxygen treatment. To investigate how such an insulating layer can be removed and under what conditions t'he surface of the films become active, we measured the surface structure change by reflection high-energy electron diffraction (RHEED, incident beam 2.2 kV, 55 p.A). Figure 3 shows the change in the RHEED pattern during heating o f a YBCO film under blowing of oxygen-6.2% ozone, which is just the same condition as employed in the YBCO film deposition. The film used was sputter-deposited and stored In a desiccator for 3 months. Before heating, the RHEED showed a halo pattern but with increasing temperature streaks became clear and at around 540°C the clear streak pattern of a (001) surface of YBCO was observed. At the same time, the spotty patterns showing threedimensional grains epitaxially grown on the surface of YBCO are overlapped by the streaks. The period of the overlapped crystals seems to be double ( 3 . 8 5 X 2 = 7 . 7 A) that of the YBCO but the spotty patterns completely disappeared with an increase of temperature to 600°C. Although RHEED is not as sensitive to the surface layer structure change as lowenergy electron diffraction (LEED), our observation is consistent with the surface studies of YBCO by LEED [ 21,22 ] except that our annealing was carried out in an active oxygen atmosphere, instead of vacuum annealing as carried out in the LEED observation. It will be shown in table 1 that 6.2% ozone blowing in a vacuum evaporation system is equivalent at least to 5 Torr pure oxygen and this fact supports our results that heating in a quartz reaction tube up to 600-650°C under several Torr oxygen atmosphere activates YBCO films.
3.2. Oxygen absorption and desorption kinetics of YBCO bulk" and fihns The oxygen diffusion constants in YBCO strongly depend on the direction. At 600°C, for example, they
K. Nakamura et aL / Oxygen potential of YBCO fihns
(a)
5
(b)
(O)
Fig. 3. RHEED diffraction patterns ofa YBCO film ($20515-1 ) during heating in a flowing 6.2% ozone-oxygen atmosphere ( 3.4 × 10-4 Torr). The film was stored in a desiccator for 3 months before the heating process. The electron beam is incident along the [ 100 ] azimuth of the substrate. (a) 370°C, (b)545°C, (c) 610°C.
Table 1 c-axis length before and after the oxygentreatment for as-grownfilms Sample Name
$20415 $20418 $20519- l S10702 M20611-2 M20611-1
Method
Sputter Sputter Sputter Sputter MBE MBE
Deposition conditions
c-axis length (,~)
Ts (°C)
As-grown
Oxygentreated
11.87 b) 11.80 ¢) I 1.77 I 1.74 11.718 11.740
11.73 I 1.75 11.77 I 1.74 11.715 11.740
620 570 680 620 670 655
Po2 Deposition
Cooling
0.0015 a) 0.0015 a) 0.0075 0.0015 0.00034 d) 0.00034 d)
0.0015 0.0015 5.0 5.0 0.00034 0.00034
") Plasma on during sputtering and plasma offduring cooling. b) Estimated 5 is 0.80. c) Estimated 5 is 0.30. d) Ozonizer on throughout the deposition and cooling.
are about 10 -14 and 10 - 9 cm2/s for directions parallel and perpendicular to the c-axis, respectively [23-25]. If oxygen molecules adsorb only on the surface of the ab-plane of the YBCO film and migrate along the c-axis, the time necessary to absorb about half of its equilibrium value should be of the order of L2/Dc, where L is the thickness of the film and Dc is the diffusion coefficient along the c-axis. For a 100 nm thick film, this time has the order of 104 s. This is different from the experimental results that films deposited at an oxygen pressure less than
0.0015 Torr, at Ts=620°C and cooled down immediately to room temperature in 5 Torr oxygen showed a fully oxygenated state as will be shown in table 1. The in situ resistivity measurement of c-axis oriented films under the presence of oxygen plasma generated by electron cyclotron resonance (ECR) [26] showed a two orders of magnitude faster approach to equilibrium. It also showed a very small difference in the diffusion rate between the c-axis and ( a + c ) - m i x e d oriented films [26] compared to the difference in diffusion coefficient. These results sug-
6
K. Nakamura et aL / Oxygenpotentialof YBCOfilms
gest that the oxygen absorption rate for c-oriented films must be higher than that calculated from the diffusion rate along the c-axis. To study these problems in detail, we have measured oxygen absorption and desorption rates in the present system.
600
For the in situ resistivity measurement, the temperature increase sequence including valve operation was almost the same as described in the previous section. It was found, however, that while increasing the temperature, an extraordinary increase in the film resistivity corresponding to an oxygen deficiency 6 larger than 0.7 (corresponding to an equilibrium oxygen pressure less than 10 -6 Torr) was found around 400-450°C. During this process, the oxygen pressure increased due to oxygen desorption from the bulk but the oxygen deficiency corresponding to this pressure increase is only 0.1 or less. This abnormal resistivity increase was found in all YBCO films studied here when the samples were exposed to air before evacuating and heating. With further increase in the temperature and in the accompanied 02 pressure, the film resistivity again decreased to a value corresponding to the equilibrium oxygen concentration. The abnormal resistivity increase was never observed after the first run unless the samples were again exposed to air even for a short time. Thus it seems that this abnormal increase in the resistivity is due to the oxygen desorption catalytically assisted by impure species which were adsorbed when the films were exposed to air. However, at present it is not clear what kind of impurity is responsible for this behavior. This resistivity increase was never observed after the first run.
3.2. 2. High temperature resistivity vs. oxygen partial pressure In order to acquire oxygen absorption and desorption data of YBCO films from resistivity changes, it is important to know the relation between the oxygen deficiency 6 (or the equilibrium pressure) and the resistivity by measuring the resistivity of YBCO film as a function of equilibrium oxygen partial pressure. The results are shown in fig. 4. The oxygen deficiency 6 in the figure was calculated from the published 8-partial pressure relation [6 ]. Although the
,
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,
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500
I
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3.2.1. Abnormal resistivity increase during increasing temperature
•
400
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200 "~®
~
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8
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4
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0.5
0.6
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2 0 0.8
5
Fig. 4. Relation between the oxygendeficiency6 and resistivity of YBCO film ($20205-1). Correspondingequilibrium oxygen pressures at various temperatures from which 6 values are obtained are also shown. temperatures changed stepwise by 50°C, the resistivity curves show a continuous increase with oxygen deficiency, i.e. upward curvature with oxygen deficiency. This observed change is somewhat similar to the room temperature resistivity [ 3 ] except that the present change shows no resistivity minimum around 6 = 0.4 but monotonically increases with & Although the curvature (OR/06) increases with 8, the change in the curvature at a narrow 6 range is small and can be used to monitor the change in 6 during oxygen absorption and desorption.
3.2.3. Oxygen absorption and desorption rates of YBCO bulk and films Figure 5 shows a typical recorder trace showing the change in oxygen pressure and resistivity of a YBCO film ($20515-1, ~ 130 nm thick) immediately after an intended abrupt change in the oxygen pressure. The recorder trace was recorded as follows. At a given temperature, valve V2 was closed after an equilibrium pressure P,e was attained and oxygen gas in CI was allowed to decrease at a certain value P2 by operating valve VI, and then V2 was again opened. Immediately after the V2 operation, the pressure increased to P3, P3 = (/°2 Vc, +Pie Vc2)/( Vcl + Vc2) ,
(2)
K. Nakamura et al. / Oxygen potential of YBCOfilms
V2 c l o s e --------- ~/"V1 ooen
0 4~
t ~J
I
> I
60 sec
2.8 t0rr
P3=1.87 t o r r
Vl
(~IO S % I ~
P2=O. 94 t o r r - - - ~
7
imate values of the diffusion coefficient. Figure 6 shows Arrhenius plots for the reaction rate (tw2)- 1 of YBCO films. Using a simple relation between tl/2 and diffusion coefficient D/L2.~ (q/2)-: and L 2= 1.7 × 10-~o cm 2 for a 130 nm thick film, the oxygen absorption rate for the film can be converted into the diffusion coefficient provided that the rate is diffusion-controlled. In the same figure ( t i / 2 ) - t calculated from the oxygen diffusion coefficients of a single crystal along the c-axis, Dc, and in the abplain, Da,b, are also shown for comparison. From the figure, it is easily recognized that in spite of the coriented epitaxial growth of the films, the observed oxygen absorption rate is unexpectedly higher than the value expected from Do and rather closer to the value from Da.b. These facts suggest that a short-circuit diffusion process may exist in the oxygen reaction process. At higher temperatures, (tl/2) - l plots are nearly linear and the gradient seems to be parallel to those calculated from Dc and Da,b, indicating that the rate is diffusion-controlled in this region. However, with decreasing temperature the rate seems to deviate appreciably from the line. The deviation from the diffusion-controlled rate is conventionally observed in
Fig. 5. A typical recorder trace showing change in the oxygen pressure and resistivity immediately after the abrupt decrease in the oxygen pressure. The temperature was kept at 550°C.
10
2
. . . .
,
. . . .
,
. . . .
,
. . . .
,
. . . .
,
. . . .
101
where Vc: and Vc2 are the volumes of C1 and C2, respectively./'3 is lower than the equilibrium pressure Pte and the oxygen in the bulk was desorped until the pressfire reached a new equilibrium value corresponding to a lower oxygen concentration (higher 6 value). At the same time an increase in the resistivity of the YBCO film was observed. From the resistivity, the oxygen pressure and the 6 relation in fig. 4, we see that the change in the equilibrium pressure from 5.99 to 2.80 Torr at 550°C, for example, corresponds to 6=0.35-0.41. In this region, the resistivity of the film almost increases linearly with 6 as shown in fig. 4. Thus both the pressure and resistivity curves in the figure show the change in the oxygen concentration with time. It is interesting to see the time necessary to absorb or desorp half of its equilibrium value, tl/2, after the abrupt change in the pressure and estimate approx-
u3
/L 2
"¢:N 1 0 °
"~'oo~.O
10" 1 10.2 :,~
¢,
10
-3
10-4
C
~
lo.s
0 10-6 .............. 1 .I 1.1
1.2
, .... 1.3
, ......... 1.4 1.5
1.6
10 3 / T Fi& 6. Arrhenius plots of the reaction rates for YBCO films, (open circles) $20515-1, (closed circles) $20515-3. Solid lines are D ~ / L 2 and Dc/L 2, where L is 130 nm.
8
K. Nakamuraet al. / Oxygenpotential of YBCOfilms
the hydrogen absorption rate in metals [27 ] where the surface process becomes the rate controlling step at lower temperatures. From the observed gradient, it is assumed that at higher temperatures oxygen absorption is almost diffusion-controlled and then the surface process is taken into account with decreasing temperature. 3.3. Lattice parameters o f as-grown films and oxygen content
It has been pointed out that the lattice parameters of as-grown films of YBCO depend strongly on the kind of substrates and deposition conditions such as substrate temperature, composition, total and partial oxygen pressures during deposition [ 7-12 ]. For example c-lattice parameters of our films prepared by sputter-deposition ranged from 11.7 to 11.8 ~ and those by reactive vacuum evaporation using ozone were 11.72 to 11.8 A. Among the various deposition conditions, substrate temperature seems to affect most strongly the c-lattice parameter. Our question is whether the oxygen content in the as-grown films is affected by the deposition conditions and in turn affects the c-lattice parameter of the as-grown films. From the oxygen absorption rate measured under oxygen partial pressure of I to l0 Torr and temperature around at 600-650 ° C, it becomes clear that the rate is extremely high as to reach equilibrium within several seconds (/l/E= 1.5 s at 650°C). In our films, however, the oxygen potential during deposition or during the cooling process is somewhat different from the above conditions. In the vacuum evaporation, it was about 3.4× 10 -4 Torr 02-6.2%03 (in background pressure, the pressure at the substrate must be one or two orders of magnitude larger) throughout the deposition and cooling processes. In sputterdeposition, it was 1.5 × 10-3 and 5 Tort oxygen partial pressure during sputter-deposition and cooling, respectively. Thus it is the problem to know whether in such a lower oxygen partial pressure, oxygen incorporates into the films during the cooling process. To confirm this phenomenon, the as-grown films were oxygen-treated at 650°C for 30 min in a flowing Ar-20% oxygen atmosphere for 30 min and furnace-cooled to room temperature. We assumed that the films thus treated are fully oxygenated ( ~ = 0.07 according to refs. [ 6,13 ] ), and the c dimension of
the oxygen-treated films are compared with the asgrown films. Figure 7 shows X-ray diffraction patterns of as-grown and oxygen-treated films which were sputter-deposited under the same conditions but cooled down under different oxygen pressure. The film cooled in 5 Torr oxygen pressure shows almost the same diffraction pattern as the film oxygentreated afterwards both in the relative intensity and diffraction angles. This shows that both the c-axis length and the structure are the same. On the other hand, the film cooled under a constant oxygen pressure of 1.5 × 10 -3 shows a completely different pattern from that of the film oxygen-treated afterwards, both in the intensity and diffraction angles. This result strongly suggests that not only the c-lattice parameter but also the z positions of heavier elements such as Ba and Cu were changed due to the oxygen deficiency in the C u - O chain as analyzed in detail in bulk YBCO [13 ]. From the patterns in fig. 7, it is shown that not only the c dimension but also the relative intensity are a good measure of oxygen deficiency, and we will discuss this in detail in a separate paper [ 28 ]. Table 1 summarizes the c-axis lengths of YBCO films before and after oxygen treatment together with the deposition and cooling conditions. From the table it is easily recognized that only the films sputterdeposited and cooled in 1.5 mTorr pure oxygen atmosphere show oxygen deficient c-axis length and the oxygen deficiencies calculated from the c dimension by the method which will be described in the next section are 0.80 and 0.30 for the films deposited at Ts=620°C ($20415) and 570°C ($20418), respectively. It is also interesting that the films cooled in 3.4X 10 -4 (background pressure) Torr of 6.2% ozone show fully oxygenated c dimension in contrast to the films cooled in 1.5 × 10 -3 Torr of pure oxygen atmosphere. This difference can be reasonably explained if we take into account the fact that ozone possesses a higher potential than pure oxygen [29,30] and ozone is very active in oxidizing Ag and Cu films at room temperature. 3.4. Oxygen potential control in YBCO films
As described in the previous section (see fig. 5), YBCO films and bulk placed in a quartz reaction tube behave almost similarly against oxygen absorption
K. Nakamura et al. /Oxygenpotential of YBCO films
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i
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10
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( B- I )
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8=0.07
I 20
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30
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40
50
20
60 0
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20
30
40
50
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20
Fig. 7. XRD diffraction patterns of YBCOas~grownfilms cooleddown in different oxygenpartial pressures after deposition (A-I, A-2) $20415, (B-l, B-2) $20519-1. (See also Table l ). The oxygentreated films (A-2, B-2), 650°C for 30 rain in 0.2 atm 02, are also shown for comparison.
and desorption and reach equilibrium within a short time. The time necessary to attain equilibrium, however, depends strongly on the surface state (such as impurities adsorped from the gas phase) as well as the temperature. In the present study, films having a different c dimension in the as-grown state were heat-treated with bulk YBCO powder and the oxygen potential was controlled as described in the experimental section. After the treatment, lattice parameters (including a, b-axis lengths in the case of powder and a,c-oriented films) were measured and plotted as a function of 5 which were directly obtained from the amount of oxygen extracted from the bulk as described in the experimental section. It is noted that the total amount of 5 pieces of YBCO films with thickness about 100 nm and an area of typically 0.3 cm 2, for example is 150 ~g, which is about three orders of magnitude smaller than the bulk sample (typically 350-500 mg). Thus the ~ values calculated from the volumetric method are due mostly to the loss of oxygen from the bulk. If oxygen concentrations in the films and bulk are in equilibrium with the potential of oxygen gas in the quartz reaction tube, the 5 value of the films should be equal to that of the bulk and also equal to that obtained from the
volumetric method. As described in the previous section, if we applied method A to equilibrate the oxygen potential, the 5 values in the films and bulk became equal, but became different when we applied method B. The reason is not clear at present. However, if we take into account the fact that the quartz reaction tube C2 was well evacuated and was baked out at 350-400°C, but the main pyrex tube chamber C1 was evacuated only around room temperature, impure components adsorbed at the wall in the main pyrex chamber may cause a poisoning effect against oxygen absorption when cooled while keeping V2 open. Figure 8 plots c-axis lengths against 5 values for films with various initial (as-grown) c-axis length and bulk YBCO equilibrated with the films. The plots for bulk YBCO show similar behavior as reported by other investigators [ 3,13 ]. It should be emphasized that the c-axis length versus 5 value for the same series was repeatedly taken from the same portion of a sample (not divided into several pieces from one sheet of film) except for sample M20611-2 from which resistivity data shown in fig. 12 were taken. The attached numbers with arrows (1-6) shown in the upper part of fig. 8 exhibit the sequence of the
10
K. Nakamura et al. / Oxygen potential of YBCO films
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8 Fig. 8. c-axis lengths of YBCO films with various initial c-axis lengths are plotted against 5 values, c values for YBCObulk equilibrated with the films are also shown,c-axislength for bulk from Jorgensen et al. [ 13] are shown by ( + ). treatment. From this figure it can be easily shown that the c-axis length versus oxygen deficiency curves are almost parallel to that of the bulk YBCO curve in spite of their different starting ( = as-grown) c dimension, i.e. the curves shift upward depending on the difference in the starting c-axis length. This result strongly suggests that oxygen concentration of asgrown YBCO films can be determined by measuring the difference in the c-axis length of as-grown and fully oxygenated films. Figure 9 shows a-axis lengths of oxygen controlled, (a + c)-mixed oriented YBCO films and a- and b-axis lengths of bulk samples equilibrated with the film. The film was sputter-deposited on a SrTiO3 substrate and the film was repeatedly oxygen-treated as described above. From fig. 9 it is apparent that the a-axis length behaves almost similar to that in bulk YBCO, suggesting that the films have almost the same orthorhombic-tetragonal phase relation against 5. It is also interesting to see that the c-axis length of the a,c-oriented YBCO film is a little bit larger than the bulk whereas the a-axis length is a little bit smaller or equal to that of the bulk.
3.5. Effects of heat treatment In order to get better oxygen absorption and desorption rates and attain complete equilibrium, the films were heated up to 650°C for about l0 min. The
3.82 u.O
0.2
0.4
0.6
0.8
.0
5 Fig. 9. a-axis lengths versus 5 values for an (a+c)-mixed oriented YBCO film ($20817-STa) and bulk (a- and b-axis lengths) equilibrated with the film. Open and closed triangles are referred from ref. [ 13 ].
procedure was repeated a number of times to get different ~ values. Thus it is important to determine whether the heat treatment affects (in addition to the oxygen concentration effects) the c-axis length by, for example, rearrangement of cations or recrystallization. The following three experimental results strongly suggest that such heat treatment only changes the oxygen concentration: (1) After repeated oxygen treatments up to 650°C, the c-lattice parameter of the films recovered their starting value in all caaes when the films were finally treated under 0.2 atmosphere oxygen at 650°C. (2) As shown in fig. 8, all data points from the same film lie on a single line indicating that the change due to the heat treatment up to 650°C affects the oxygen concentration but not the structure. (3) The rocking curve widths from a particular film are unchanged by the repeated heat treatments for all cases as shown in fig. 10.
3.6. Resistivity vs. temperature curves of the oxygen potential controlled YBCO films Table 2 shows deposition conditions, composition, and as-grown c-axis length for the two films on which resistivity vs. temperature was measured as shown in figs. 11 and 12. The film in fig. 11 was sputter-deposited on a MgO ((100) 10× 10 mm
K. Nakamura et al. / Oxygen potential of YBCOfilms .
.
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Fig. 10. Rocking curves for YBCO films before ( A-1, B-1) and after ( A-2, 13-2) the repeated heat treatments up to 650 °C. A-1,2 = M2061 i l, B-1,2=$20817-1.
Table 2 Characteristics of YBCO films for the resistivity measurements Sample
S 10829-2 M20611-2
Method
Sputter MBE
Ts ( oC) 630 670
Po2
0.015 0.00034
square) substrate. It showed Tc o f 78 K b u t has good crystallinity. The film in fig. 12 was v a c u u m - d e p o s ited onto MgO ( ( 1 0 0 ) 20 X 15 m m square) a n d has T~o o f 89.6 K a n d Raoo/R~oo = 3.12. The samples were cut into 5 ( 10X 10 m m s a m p l e ) to 13 ( 2 0 × 15 m m s a m p l e ) pieces o f a b o u t 2 X ( 7 - 1 0 ) m m 2 square a n d the oxygen deficiency was controlled in the m a n n e r
c-lattice parameter ( A ) 11.73 11.72
Composition (%) Y
Ba
Cu
21 17.8
28 31.8
51 50.4
described in the previous section. A n e x a m i n a t i o n o f the two figures show that the film with better resistivity in its as-grown state shows better resistivity transition even at higher c~. Moreover, the b o u n d a r y oxygen deficiency value at which Tc decreases to zero lies between ~ = 0 . 6 3 a n d 0.67 in fig. 12 a n d between 0.48 a n d 0.59 in fig. 11. This observation suggests
12
K. N a k a m u r a et al. / O x y g e n potential o f Y B C O f i l m s
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Fig. 11. R e s i s t i v i t y v e r s u s t e m p e r a t u r e c u r v e s o f o x y g e n c o n t r o l l e d Y B C O f i l m p r e p a r e d b y s p u t t e r i n g ( S 1 0 8 2 9 - 2 ).
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Fig. 12. Resistivity versus temperature curves of oxygen controlled YBCO film prepared by vacuum evaporation (M206112). that the oxygen deficiency ~ vs. Tc relation also depends on the superconducting properties o f the asgrown films. Figure 13 plots Tc of the M20611-2 film shown in fig. 12 as a function of oxygen deficiency. For comparison, Tc values from the sputter-deposited films (S10829-2 and from ref. [ 16] ) are also shown. From the figure, two-plateau behavior seems to be clearly observed and also an excellent agreement with the data from bulk YBCO [ 3,13 ] is observed for both the T¢ values and their ~ dependency. In contrast to this, the films from the sputter-deposition, as well as those from ref. [ 16 ] show different behavior. Since the ~ values o f the films in fig. 11 were also determined in the same way as those in fig. 12, the different Tc vs. ~ behavior must be attributed to the su-
0 0.0
0.2
0.4
5
0.6
0.8
Fig. 13. Superconducting transition temperature T~ versus oxygen deficiency &in YBCO films shown in figs. I 1 and 12. Dotted line shows Tc of oxygen deficient films from ref. [ 16]. perconducting properties in the as-grown state. On the other hand, the films in ref. [ 16 ] which have better superconducting transition temperature than S10829-2 in their as-grown state showed also different behavior. Thus, it seems likely that the ~ values were different from their estimated value because their & dependency o f c-axis length is also different from the bulk and from our films. The excellent agreement of M20611 film with the bulk behavior shown in fig. 13, strongly suggests that such thin films properly prepared should have essentially the similar superconducting behavior against oxygen deficiency. This also confirms that the d values thus determined from the three different ways, volumetric, c-axis lengths o f equilibrated bulk and films, are accurate and can be conveniently used to determine 8.
4. Conclusion From the study of oxygen potential control of YBCO films, several conclusions can be drawn. First, the oxygen absorption and desorption rates of epitaxial films are very fast to reach equilibrium within
K. Nakamura et al. / Oxygen potential of YBCO films ten seconds above 600 ° C. Thus as-grown films cooled u n d e r appropriate oxygen partial pressure are, in most cases, fully oxygenated (c~=0.07) a n d the difference in the c-axis length and T~ of as-grown YBCO films can not be ascribed to the difference in the oxygen contents. The difference in the c-axis length a n d Tc in the as-grown state might be due to disordering of cations, radiation damage during sputter-deposition or weak intergrain couplings. The c-axis length in YBCO films varies with 6 in a m a n n e r similar to the bulk in spite o f the difference in the c-axis length in the as-grown or fully oxygenated state. However, Tc versus oxygen deficiency of the films showed different behavior depending on the superconducting transition in the as-grown state. The films with better superconducting transition in the as-grown state showed almost the same 8 dependency as bulk a n d single crystal Y B C O with homogeneously controlled oxygen deficiency. T h u s YBCO thin films should have intrinsically the same superconducting transition against oxygen deficiency.
References [1 ] D.O. Welch, V.J. Emery and D.E. Cox, Nature 327 (1987) 278. [2] J.D. Jorgensen, M.A. Beno, D.G. Hinks, L.S. Soderholm, K.J. Volin, L. Hittermann, J.D. Grace, I.K. Schuller and C.U. Segre, Phys. Rev. B 36 (1987) 3068. [3]R.J. Cava, B. Batlogg, C.H. Chert, E.A. Rietman, S.M. Zahurak and D. Werder, Phys. Rev. B 36 (1987) 5719. [4 ] Y. Kubo, T. Yoshitake, J. Tabuchi,Y. Nakabayashi,O. Ochi, IC Utsumi, H. Igarashiand M. Umezawa,Jpn. J. Appl.Phys. 26 (1977) L768. [5 ] K. Nakamura, T. Hatafio, A. Matsushita, T. Oguchi, T. Matsumoto and K. Ogawa, Jpn. J. Appl. Phys. 26 (1977) L869. [6] K. Kishio, J. Shimoyama, T. Hasegawa, K. Kitazawa and K. Fueki, Jpn. J. Appl. Phys. 26 ( 1987) L 1228. [7] O. Michikami and M. Asahi, Jpn. J. Appl. Phys. 30 ( 1991 ) 466, 939. [8]C.B. Eom, J.Z. Sun, B.M. Lairson, S.K. Striffer, A.F. Marshall, K. Yamamoto, S.M. Anlage, J.C. Bravman and T.H. Geballe, Physica C 171 (1990) 354. [9] J.A. Kittel, C.W. Nieh, D.S. Lee and W.L. Johnson, Appl. Phys. Lett. 56 (1990) 2468.
13
[ 10] H.J. Chang, Y. Watanabe, Y. Yutaka Doshida, K. Shimizu, Y. Okamoto, R. Akihara and J.T. Song, Jpn. J. Appl. Phys. 29 (1990) L2207. [ 11 ] J. Matijasevic, P. Rosenthal, K. Shinohara, A.F. Marshall, R.H. Hammond and M.R. Beasley,J. Mater. Res. 6 ( 1991 ) 682. [ 12] K. Shinohara,V. Matijascvic,P.A. Rosenthal,A.F. Marshall, R.H. Hammond and M.R. Beasley, Appl. Phys. Lett. 58 (1991) 756. [ 13] J.D. Jorgensen,B.W.Veal,A.P. Paulikas,L.J. Nowicki,G.W. Crabtree, H. Claus and W.K. Kwok, Phys. Rev. B 41 (1990) 1863. [ i 4 ] P. Meuffels,B. Rupp and E. Porschke, Physica 156C (1988) 441.
[ 15! K. Nakamura, T. Hatano, S. Ikeda and K. Ogawa, Mater. Res. Soc. Symp. Proc. 99 (1988) 579; Jpn. J. Appl. Phys. 27 (1988) 577. [ 16 ] E. Osquiguil,M. Maenhoudt,B. Wuytsand Y. Bruynseraede, Appl. Phys. Lett. 60 (1992) 1627. [17] R.L. Sandstrom, W.J. GaUagher and T.R. Dinger, Appl. Phys. Lett. 53 (1988) 44. [ 18] H.C. Li, G. Linken,F. Ratzel, R. Smitheyand J. Greek,Appl. Phys. Lett. 52 (1988) 1098. [ 19] K. Nakamura, Ber. Bunsenges.Phys. Chem. 89 (1985) 191. [20] C.C. Chang, M.S. Hegde, X.D. Wu, B. Dutta, A. Inam, T. Venkatesan, B.J. Wilkens and J.B. Wachtman Jr., J. Appl. Phys. 67 (1990) 7483. [21 ] T. Ohara, K. Sakuta, M. Kamishiro and T. Kobayashi, Jpn. J. Appl. Phys. 30 ( 1991 ) L2085. [22] S. Tanaka, T. Nakamura, M. Iiyama, N. Yoshida and S. Takano, Jpn. J. Appi. Phys. 30 ( 1991 ) L1485. [23] S.I. Bredikhin, G.A. Emel'chenko, V.Sh. Shechtman, A.A. Zhokhov, S. Cartyer, R.J. Chater, J.A. Kiloner and B.C.H. Steele, Physica C 179 ( 1991) 286. [ 24 ] S.J. Rothman, J.L. Routbort, U. Welpand J.E. Baker, Phys. Rev. B44 (1991) 2326. [251 S. Tsukii,T. Yamamoto,M. Adachi,Y. Shono:K. Kawabata, N. Fukuoka, A. Yanase and Y. Yoshioka, Physiea C 185189 (1991) 929. [26] K. Yamamoto, B.M. Lairson, C.B. Eom, R.H. Hammond, J.C. Bravmanand T.H. GebaUe,Appl.Phys. Lett. 57 (1990) 1936. [27] K. Nakamura, Z. Physik. Chem. N.F. 116 (1979) 163. [28] J. Ye and K. Nakamura, submitted to Phys. Rev. B. [29] D.D. Berkley, B.R. Jonson, N. Anand, K.M. Beauchamp, L.E. Conroy, A.M. Goldman, J. Maps, K. Mauersherger, M.L. Mecartney,J. Morton, M. Tuominen and Y.-J. Zhang, Appl. Phys. Lett. 53 (1988) 1973. [30 ] D.G. Schlom,A.F. Marshall, J.T. Sizemore,Z.J. Chert, J.N. Eckstein, I. Bozovic, ICE. Von Dessonneck, J.S. Harris Jr. and J.C. Bravman, J. Cryst. Growth 102 (1990) 361.
Oxygen potential control in YBa2fu307_ 6 thin films Keikichi Nakamura, Jinhua Ye and Akira Ishii National Research Institute.for Metals, 1-2-1Sengen, Tsukuba-shi, Ibaraki 304, Japan Received 1 March 1993 Revised manuscript received 31 May 1993
The oxygenpotential of YBa2Cu307_6 ( YBCO) films has been controlled by equilibrating the films with bulk YBCO in a quartz reaction system. The samples were heated up to 600-650 °C and the oxygencontent in bulk YBCO was controlled by evacuating a calculated amount of oxygen gas from the reaction system. The temperature was then gradually decreased to 400°C where the final equilibrium state was attained. The oxygen deficient films thus controlled have the same oxygen potential as the oxygen deficient bulk. The plots of c-axis lengths of the films against oxygen deficiency~ are almost parallel to those of the bulk in spite of having different c-axislengths in the as-grownstate. Thus the oxygendeficiencyof as-grownYBCO films can be determined by comparing the c-axis length in the as-grown and fully oxygenated states. It is shown that YBCO as-grown films prepared under conventional deposition conditions are, in most cases, fully oxygenated except those cooled at lower oxygen partial pressure ( < 10-2 Torr) after deposition. The T¢versus J relation of the oxygendeficient film with better as-grown superconducting transition showed a two-plateau behavior almost the same as that observed in the polycrystallineand single crystal samples with well defined ~.
I. Introduction The importance of oxygen deficiency in the superconducting properties in YBCO has been recognized immediately after the discovery of this material [ 1 ] and extensive works have been done on the correlation between oxygen deficiency and phase relation, crystallographic a n d superconducting properties [ 2 - 6 ]. However, most of these investigations were done on bulk YBCO and not so m a n y works have been systematically done on thin films. For example, m a n y investigators do not confirm whether their as-grown films are fully oxygenated or oxygen deficient a n d some investigators ascribed their lower T¢ to the oxygen deficiency without confirming the accurate oxygen concentration in the films. One of the reasons of these problems is that no systematic method to determine oxygen deficiency, such as that from lattice parameters employed in bulk YBCO, has been established on thin YBCO films. The c-axis length of as-grown films is not 11.6711.69 ,~. as observed in bulk YBCO but varies widely from 1 1.7 to 12 A d e p e n d i n g on the deposition conditions a n d the kind of substrate [ 7 - 1 2 ]. Oxygen de/
.
ficiency has been thought to be partly responsible for these longer c d i m e n s i o n observed in thin films. The Tc of these films, however, do not so strongly depend on the c-axis length as observed in YBCO bulk [7,8]. Some investigators insist on the fact that the disordered arrangement of cationic ions such as substitution of Ba for Y [ 11,12], as well as the epitaxy c o m b i n e d with the lattice mismatch must be responsible for the observed c-axis length distribution. The oxygen deficiency problem, however, has not been solved so far. In order to obtain systematic knowledge of the oxygen deficiency of thin YBCO films, it is necessary to establish a reliable oxygen potential (concentrat i o n ) control method on YBCO films. For bulk YBCO, various oxygen potential controlling methods, for example, oxygen gettering with Zr [ 3 ], heating in controlled oxygen partial pressure for a long time [ 13 ], addition of a known a m o u n t of oxygen to tetragonal YBCO [ 14,15 ] have been proposed. Apart from bulk YBCO, there are m a n y problems in thin YBCO films, which are caused by the relatively thin layer ranging from several tenths to hundreds n m and by small quantities of maximal several lag.
0921-4534/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
2
K. Nakamuraet al. / Oxygenpotential of YBCOfilms
Because of these restrictions, various oxygen potential controlling methods used in bulk YBCO are not applicable to thin films. In spite of the various problems, the advantage of thin films is that the diffusion rate does not control the oxygen absorption and, consequently, homogeneously oxygenated samples can be easily obtained compared with bulk. On the other hand, a recent study on controlled oxygen deficient films [ 16 ] showed different behavior of the c-axis length and Tc versus oxygen deficiency from those observed in bulk. These results were also different from our results at that time. Thus it was necessary to confirm whether these discrepancies are due to the intrinsic nature of the films or to problems in the sample preparation. To establish a reliable and reproducible oxygen potential control method, it is necessary to study oxygen absorptiondesorption kinetics of thin YBCO films systematically. In this study a short, preliminary experimental result on the kinetics of oxygen absorption and desorption in YBCO films is described. The main purpose of this paper, however, is to establish a reliable method of controlling oxygen potential and to determine oxygen deficiency by measuring the c-lattice parameter. The study of T¢ versus oxygen deficiency of YBCO films prepared by different methods is also an important purpose in this paper.
2. Experimental procedure 2.1. Preparation o f YBCO films
YBCO thin films were synthesized by two different methods. One is 90 ° off-axis magnetron sputtering from a single target with 1:2:3.5 to 1:2:4.2 composition. The sputtering gas was A r - 10%02 and the oxygen partial pressure during sputtering was 0.0015 to 0.0075 Torr. The substrates were MgO (100) and SrTiO3 (100) single crystals and the substrate temperatures were between 500 and 670°C. After deposition, about 5 Torr oxygen gas was introduced and the substrate was allowed to cool to room temperature. In some cases, however, the substrate was cooled in 0.0015 Torr oxygen pressure in order to determine the effect of oxygen partial pressure during cooling on the oxygen content in the asgrown films. The highest zero resistivity critical tem-
perature, To, thus obtained was 82.6 K in spite of the excellent crystallinity of the films. Negative ion bombardment may have a detrimental effect on the Tc observed in the present study. This is in spite of the off-axis substrate position and higher sputtering pressure conditions which have been reported to prevent such negative effects [ 17,18 ]. A vacuum evaporation system equipped with twoelectron beam evaporator for Ba and Y sources and one A1203 coated tungsten basket for Cu evaporation were used to produce films with higher T¢. O26.2%03 gas generated by a commercially available ozonizer was blown onto substrate kept at a substate temperature, Ts, between 560 and 650°C. The ozone concentration monitored with a Horiba ozone meter was kept at 6.2%. The background pressure was reduced from 3.4× 10 -4 to 1.2× 10 -4 Tort during evaporation due to the gettering effect of Ba and Y evaporation. The deposition rate was 0.15 n m / s and the deposition time was 700-800 s. After evaporation, the substrate was allowed to cool to room temperature with the gas flow rate kept constant and the pressure gradually increasing to 3.4 X 10 -4 Torr due to interruption of the Ba, Y gettering effect. The best T~ thus obtained was 89.6 K with A T = 1.5 K and R3oo/Rloo = 3.12. In this paper, the samples were named after the method of preparation (S: sputter, M: vacuum deposition), the date of preparation and batch number (with hyphen). All samples were prepared without post-annealing except for $20817-2a and $20817-STa as shown in fig. 8, which were post-annealed at 850°C for 30 min. The reason for this naming is that the lattice parameters and T~ were differed from batch to batch. For M20611-1 and M20611-2 the c-axis lengths are 11.74, 11.72 A and T¢'s are 81.3 and 89.6 K, respectively. The above differences might be caused by the difference in flux of Cu and Y, and substrate temperature, Ts, during the deposition. 2.2. Oxygen potential control
To control the oxygen potential in YBCO films, we applied a volumetric method using Sievert's apparatus which is essentially the same as that used to equilibrate hydrogen concentration in amorphous films [ 19 ]. The apparatus consists of a quartz reaction tube (hereafter denoted as C2 with volume
K. Nakamura et al. / Oxygen potential of YBCO films
44.4 cm 3) and a m a i n 12 m m O D pyrex c h a m b e r ( d e n o t e d as C1 with a v o l u m e o f 194.2 cm 3) e q u i p p e d with three pressure gauges, a capacitance m a n o m e t e r ( 1 0 - 2 - 1 0 T o r r ) , a strain gauge ( 10-760 T o r r ) a n d an ion gauge ( < 10 -3 T o r r ) . The schematic d i a g r a m is shown in fig. 1. The reaction process is as follows. YBCO bulk with 6 = 0 . 0 7 ( a b o u t 0.3-0.5 g) a n d several pieces o f Y B C O as-grown films were placed in a quartz reaction tube. The system was then evacuated to < 10- 5 Torr a n d t h e ' t e m p e r a t u r e o f the tube was increased to 150°C and kept for 30 m i n in o r d e r to evacuate a d s o r b e d gases such as water vapor. The temperature was then increased to 6 0 0 - 6 5 0 °C while continuously evacuating the system until an a b r u p t rise in the pressure was observed a r o u n d 370 to 400°C. Valve V 1 is then closed to stop evacuation. The pressure in the system increased with t e m p e r a t u r e a n d then reached an e q u i l i b r i u m value within 10 min after the t e m p e r a t u r e reached 6 0 0 - 6 5 0 ° C. The highest t e m p e r a t u r e 650°C was chosen because it is high enough to reach e q u i l i b r i u m within several m i n u t e s a n d it is low enough to prevent change in the c-axis length o f Y B C O films due to the relaxation o f lattice defects or recrystallization. Such a change, if it were to occur, would m a k e it impossible to d e t e r m i n e oxygen deficiency from the change in the c-axis length. The e q u i l i b r i u m pressure at a given t e m p e r a t u r e
in the present system is d e t e r m i n e d by the system v o l u m e a n d Y B C O bulk weight, i.e. by the balance o f oxygen loss from the bulk a n d increase in the pressure in the system. I f the oxygen deficiency in the bulk estimated from the equilibrium pressure is lower than the i n t e n d e d value, the oxygen gas must be evacuated by opening valve V I while closing V2. After such a process, V1 is again closed a n d V2 is o p e n e d and a new equilibrium pressure corresponding to a higher 6 value is attained. Such a process is repeated, if necessary, several times until a target value is attained. It should be noted that the final e q u i l i b r i u m process was done a r o u n d 4 0 0 - 4 5 0 ° C . The oxygen in the gas phase is a d s o r b e d during cooling to 4 0 0 - 4 5 0 ° C and the oxygen content must be increased during this process. F o r this cooling process we a p p l i e d two different valve operations, i.e. m e t h o d s A and B as schematically shown in fig. 2. In method A, valve V2 is closed during the cooling to 4 0 0 - 4 5 0 ° C a n d kept at that t e m p e r a t u r e for about 1 h a n d then V2 is again o p e n e d to measure final equilibrium oxygen pres-
1000 Vl close
pump M
I.G.
A
--~-- ~ T~LmP"
0
IXi-
1'
V2
V2 close Vl open ~.J
o
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~
800
600
S.G, C,M.
3
400
iii n::3 200 I--
0 rr gi I000 Q.
VI close
~.~_ V2close V1open W
C2
I
iii
i
IAI 800
600
w rr
m~x l?p.
400
200
film
Fig. 1. A schematic diagram for the system used to control oxygen potential of YBCO bulk and films. |.G.=ion gauge, S.G. = strain gauge, C.M. -- capacitance manometer, V3 = oxygen inlet valve.
close. "rime
(arb.)
Fig. 2. A schematic diagram showing two typical valve operations (method A and B) during evacuating, heating and cooling processes. V1, V2 are opened and V3 is closed at t=0.
4
K. Nakamura et al. / Oxygenpotential of YBCOfihns
sure, and in most cases, it was found that the equilibrium was attained. In method B, valve V2 was opened during the cooling and the pressure change can be detected during this process. In this case the final oxygen pressure was, in most cases, higher than the equilibrium value indicating that the equilibrium was not attained. The reason for this will be discussed in the next section. Thus, in this experiment, we applied method A in most cases. The in situ resistivity measurements were performed in the same system as shown in fig. 1 using two-point DC transport due to the geometrical restriction. Electrical contacts were made by pressing gold sheets to gold overlayer evaporated onto YBCO film. Although the resistivity thus measured includes contact resistance between the gold sheets and gold overlayer. We assumed that, however, such additional resistivity does not affect the kinetic measurement. The composition and thickness of the as-grown films were determined by RBS using 2.275 to 3.05 MeV He 2+ irradiation. The lattice constants of the films and bulk were determined using a JEOL diffractometer with Cu Kc~ radiation. The lattice parameters of the bulk were obtained by applying Rietveld refinement. As for the films, only c- or a,cparameters were obtained by extraporating lattice parameters from (00/) or (/00) against cos20/sin 0. Resistivity data were obtained using a conventional four-probe method.
3. Results and discussion
3.1. NecessiO, of activation of YBCO fihns When YBCO films are exposed to air (even stored in a desiccator) for a long time, the surface of the films may become contaminated with various kind of impurities to form an insulating layer such as carbonate and hydroxide. According to an X-ray photoelectron spectroscopy (XPS) study [20], about one unit layer ( = 11.7 A) of insulating layer covers the YBCO surface but the insulating layer thickness never exceeds one unit layer. For the reaction of oxygen on the surface to dissociate into oxygen ions and positive holes, it is necessary to accept electrons from Cu + or 0 2 - according to
O 2 + 2 C u + ( 2 0 2 - ) - ~ O 2 - + 2 C u 2+ ( 2 0 - ) .
(1)
In the above equation O - in parentheses means that holes locate on the oxygen site. To promote the reaction with less activation energy, the surface of the film must be catalytically active. The insulating layer observed by XPS may act as a barrier against such a reaction and should be removed prior to oxygen treatment. To investigate how such an insulating layer can be removed and under what conditions t'he surface of the films become active, we measured the surface structure change by reflection high-energy electron diffraction (RHEED, incident beam 2.2 kV, 55 p.A). Figure 3 shows the change in the RHEED pattern during heating o f a YBCO film under blowing of oxygen-6.2% ozone, which is just the same condition as employed in the YBCO film deposition. The film used was sputter-deposited and stored In a desiccator for 3 months. Before heating, the RHEED showed a halo pattern but with increasing temperature streaks became clear and at around 540°C the clear streak pattern of a (001) surface of YBCO was observed. At the same time, the spotty patterns showing threedimensional grains epitaxially grown on the surface of YBCO are overlapped by the streaks. The period of the overlapped crystals seems to be double ( 3 . 8 5 X 2 = 7 . 7 A) that of the YBCO but the spotty patterns completely disappeared with an increase of temperature to 600°C. Although RHEED is not as sensitive to the surface layer structure change as lowenergy electron diffraction (LEED), our observation is consistent with the surface studies of YBCO by LEED [ 21,22 ] except that our annealing was carried out in an active oxygen atmosphere, instead of vacuum annealing as carried out in the LEED observation. It will be shown in table 1 that 6.2% ozone blowing in a vacuum evaporation system is equivalent at least to 5 Torr pure oxygen and this fact supports our results that heating in a quartz reaction tube up to 600-650°C under several Torr oxygen atmosphere activates YBCO films.
3.2. Oxygen absorption and desorption kinetics of YBCO bulk" and fihns The oxygen diffusion constants in YBCO strongly depend on the direction. At 600°C, for example, they
K. Nakamura et aL / Oxygen potential of YBCO fihns
(a)
5
(b)
(O)
Fig. 3. RHEED diffraction patterns ofa YBCO film ($20515-1 ) during heating in a flowing 6.2% ozone-oxygen atmosphere ( 3.4 × 10-4 Torr). The film was stored in a desiccator for 3 months before the heating process. The electron beam is incident along the [ 100 ] azimuth of the substrate. (a) 370°C, (b)545°C, (c) 610°C.
Table 1 c-axis length before and after the oxygentreatment for as-grownfilms Sample Name
$20415 $20418 $20519- l S10702 M20611-2 M20611-1
Method
Sputter Sputter Sputter Sputter MBE MBE
Deposition conditions
c-axis length (,~)
Ts (°C)
As-grown
Oxygentreated
11.87 b) 11.80 ¢) I 1.77 I 1.74 11.718 11.740
11.73 I 1.75 11.77 I 1.74 11.715 11.740
620 570 680 620 670 655
Po2 Deposition
Cooling
0.0015 a) 0.0015 a) 0.0075 0.0015 0.00034 d) 0.00034 d)
0.0015 0.0015 5.0 5.0 0.00034 0.00034
") Plasma on during sputtering and plasma offduring cooling. b) Estimated 5 is 0.80. c) Estimated 5 is 0.30. d) Ozonizer on throughout the deposition and cooling.
are about 10 -14 and 10 - 9 cm2/s for directions parallel and perpendicular to the c-axis, respectively [23-25]. If oxygen molecules adsorb only on the surface of the ab-plane of the YBCO film and migrate along the c-axis, the time necessary to absorb about half of its equilibrium value should be of the order of L2/Dc, where L is the thickness of the film and Dc is the diffusion coefficient along the c-axis. For a 100 nm thick film, this time has the order of 104 s. This is different from the experimental results that films deposited at an oxygen pressure less than
0.0015 Torr, at Ts=620°C and cooled down immediately to room temperature in 5 Torr oxygen showed a fully oxygenated state as will be shown in table 1. The in situ resistivity measurement of c-axis oriented films under the presence of oxygen plasma generated by electron cyclotron resonance (ECR) [26] showed a two orders of magnitude faster approach to equilibrium. It also showed a very small difference in the diffusion rate between the c-axis and ( a + c ) - m i x e d oriented films [26] compared to the difference in diffusion coefficient. These results sug-
6
K. Nakamura et aL / Oxygenpotentialof YBCOfilms
gest that the oxygen absorption rate for c-oriented films must be higher than that calculated from the diffusion rate along the c-axis. To study these problems in detail, we have measured oxygen absorption and desorption rates in the present system.
600
For the in situ resistivity measurement, the temperature increase sequence including valve operation was almost the same as described in the previous section. It was found, however, that while increasing the temperature, an extraordinary increase in the film resistivity corresponding to an oxygen deficiency 6 larger than 0.7 (corresponding to an equilibrium oxygen pressure less than 10 -6 Torr) was found around 400-450°C. During this process, the oxygen pressure increased due to oxygen desorption from the bulk but the oxygen deficiency corresponding to this pressure increase is only 0.1 or less. This abnormal resistivity increase was found in all YBCO films studied here when the samples were exposed to air before evacuating and heating. With further increase in the temperature and in the accompanied 02 pressure, the film resistivity again decreased to a value corresponding to the equilibrium oxygen concentration. The abnormal resistivity increase was never observed after the first run unless the samples were again exposed to air even for a short time. Thus it seems that this abnormal increase in the resistivity is due to the oxygen desorption catalytically assisted by impure species which were adsorbed when the films were exposed to air. However, at present it is not clear what kind of impurity is responsible for this behavior. This resistivity increase was never observed after the first run.
3.2. 2. High temperature resistivity vs. oxygen partial pressure In order to acquire oxygen absorption and desorption data of YBCO films from resistivity changes, it is important to know the relation between the oxygen deficiency 6 (or the equilibrium pressure) and the resistivity by measuring the resistivity of YBCO film as a function of equilibrium oxygen partial pressure. The results are shown in fig. 4. The oxygen deficiency 6 in the figure was calculated from the published 8-partial pressure relation [6 ]. Although the
,
•
,
.
,
•
,
•
,
.
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10
500
I
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3.2.1. Abnormal resistivity increase during increasing temperature
•
400
-
200 "~®
~
/6s°°c /
8
°*c .5oo°c
4
.~.~
•
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lOO -
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i
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0.4
0.5
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2 0 0.8
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Fig. 4. Relation between the oxygendeficiency6 and resistivity of YBCO film ($20205-1). Correspondingequilibrium oxygen pressures at various temperatures from which 6 values are obtained are also shown. temperatures changed stepwise by 50°C, the resistivity curves show a continuous increase with oxygen deficiency, i.e. upward curvature with oxygen deficiency. This observed change is somewhat similar to the room temperature resistivity [ 3 ] except that the present change shows no resistivity minimum around 6 = 0.4 but monotonically increases with & Although the curvature (OR/06) increases with 8, the change in the curvature at a narrow 6 range is small and can be used to monitor the change in 6 during oxygen absorption and desorption.
3.2.3. Oxygen absorption and desorption rates of YBCO bulk and films Figure 5 shows a typical recorder trace showing the change in oxygen pressure and resistivity of a YBCO film ($20515-1, ~ 130 nm thick) immediately after an intended abrupt change in the oxygen pressure. The recorder trace was recorded as follows. At a given temperature, valve V2 was closed after an equilibrium pressure P,e was attained and oxygen gas in CI was allowed to decrease at a certain value P2 by operating valve VI, and then V2 was again opened. Immediately after the V2 operation, the pressure increased to P3, P3 = (/°2 Vc, +Pie Vc2)/( Vcl + Vc2) ,
(2)
K. Nakamura et al. / Oxygen potential of YBCOfilms
V2 c l o s e --------- ~/"V1 ooen
0 4~
t ~J
I
> I
60 sec
2.8 t0rr
P3=1.87 t o r r
Vl
(~IO S % I ~
P2=O. 94 t o r r - - - ~
7
imate values of the diffusion coefficient. Figure 6 shows Arrhenius plots for the reaction rate (tw2)- 1 of YBCO films. Using a simple relation between tl/2 and diffusion coefficient D/L2.~ (q/2)-: and L 2= 1.7 × 10-~o cm 2 for a 130 nm thick film, the oxygen absorption rate for the film can be converted into the diffusion coefficient provided that the rate is diffusion-controlled. In the same figure ( t i / 2 ) - t calculated from the oxygen diffusion coefficients of a single crystal along the c-axis, Dc, and in the abplain, Da,b, are also shown for comparison. From the figure, it is easily recognized that in spite of the coriented epitaxial growth of the films, the observed oxygen absorption rate is unexpectedly higher than the value expected from Do and rather closer to the value from Da.b. These facts suggest that a short-circuit diffusion process may exist in the oxygen reaction process. At higher temperatures, (tl/2) - l plots are nearly linear and the gradient seems to be parallel to those calculated from Dc and Da,b, indicating that the rate is diffusion-controlled in this region. However, with decreasing temperature the rate seems to deviate appreciably from the line. The deviation from the diffusion-controlled rate is conventionally observed in
Fig. 5. A typical recorder trace showing change in the oxygen pressure and resistivity immediately after the abrupt decrease in the oxygen pressure. The temperature was kept at 550°C.
10
2
. . . .
,
. . . .
,
. . . .
,
. . . .
,
. . . .
,
. . . .
101
where Vc: and Vc2 are the volumes of C1 and C2, respectively./'3 is lower than the equilibrium pressure Pte and the oxygen in the bulk was desorped until the pressfire reached a new equilibrium value corresponding to a lower oxygen concentration (higher 6 value). At the same time an increase in the resistivity of the YBCO film was observed. From the resistivity, the oxygen pressure and the 6 relation in fig. 4, we see that the change in the equilibrium pressure from 5.99 to 2.80 Torr at 550°C, for example, corresponds to 6=0.35-0.41. In this region, the resistivity of the film almost increases linearly with 6 as shown in fig. 4. Thus both the pressure and resistivity curves in the figure show the change in the oxygen concentration with time. It is interesting to see the time necessary to absorb or desorp half of its equilibrium value, tl/2, after the abrupt change in the pressure and estimate approx-
u3
/L 2
"¢:N 1 0 °
"~'oo~.O
10" 1 10.2 :,~
¢,
10
-3
10-4
C
~
lo.s
0 10-6 .............. 1 .I 1.1
1.2
, .... 1.3
, ......... 1.4 1.5
1.6
10 3 / T Fi& 6. Arrhenius plots of the reaction rates for YBCO films, (open circles) $20515-1, (closed circles) $20515-3. Solid lines are D ~ / L 2 and Dc/L 2, where L is 130 nm.
8
K. Nakamuraet al. / Oxygenpotential of YBCOfilms
the hydrogen absorption rate in metals [27 ] where the surface process becomes the rate controlling step at lower temperatures. From the observed gradient, it is assumed that at higher temperatures oxygen absorption is almost diffusion-controlled and then the surface process is taken into account with decreasing temperature. 3.3. Lattice parameters o f as-grown films and oxygen content
It has been pointed out that the lattice parameters of as-grown films of YBCO depend strongly on the kind of substrates and deposition conditions such as substrate temperature, composition, total and partial oxygen pressures during deposition [ 7-12 ]. For example c-lattice parameters of our films prepared by sputter-deposition ranged from 11.7 to 11.8 ~ and those by reactive vacuum evaporation using ozone were 11.72 to 11.8 A. Among the various deposition conditions, substrate temperature seems to affect most strongly the c-lattice parameter. Our question is whether the oxygen content in the as-grown films is affected by the deposition conditions and in turn affects the c-lattice parameter of the as-grown films. From the oxygen absorption rate measured under oxygen partial pressure of I to l0 Torr and temperature around at 600-650 ° C, it becomes clear that the rate is extremely high as to reach equilibrium within several seconds (/l/E= 1.5 s at 650°C). In our films, however, the oxygen potential during deposition or during the cooling process is somewhat different from the above conditions. In the vacuum evaporation, it was about 3.4× 10 -4 Torr 02-6.2%03 (in background pressure, the pressure at the substrate must be one or two orders of magnitude larger) throughout the deposition and cooling processes. In sputterdeposition, it was 1.5 × 10-3 and 5 Tort oxygen partial pressure during sputter-deposition and cooling, respectively. Thus it is the problem to know whether in such a lower oxygen partial pressure, oxygen incorporates into the films during the cooling process. To confirm this phenomenon, the as-grown films were oxygen-treated at 650°C for 30 min in a flowing Ar-20% oxygen atmosphere for 30 min and furnace-cooled to room temperature. We assumed that the films thus treated are fully oxygenated ( ~ = 0.07 according to refs. [ 6,13 ] ), and the c dimension of
the oxygen-treated films are compared with the asgrown films. Figure 7 shows X-ray diffraction patterns of as-grown and oxygen-treated films which were sputter-deposited under the same conditions but cooled down under different oxygen pressure. The film cooled in 5 Torr oxygen pressure shows almost the same diffraction pattern as the film oxygentreated afterwards both in the relative intensity and diffraction angles. This shows that both the c-axis length and the structure are the same. On the other hand, the film cooled under a constant oxygen pressure of 1.5 × 10 -3 shows a completely different pattern from that of the film oxygen-treated afterwards, both in the intensity and diffraction angles. This result strongly suggests that not only the c-lattice parameter but also the z positions of heavier elements such as Ba and Cu were changed due to the oxygen deficiency in the C u - O chain as analyzed in detail in bulk YBCO [13 ]. From the patterns in fig. 7, it is shown that not only the c dimension but also the relative intensity are a good measure of oxygen deficiency, and we will discuss this in detail in a separate paper [ 28 ]. Table 1 summarizes the c-axis lengths of YBCO films before and after oxygen treatment together with the deposition and cooling conditions. From the table it is easily recognized that only the films sputterdeposited and cooled in 1.5 mTorr pure oxygen atmosphere show oxygen deficient c-axis length and the oxygen deficiencies calculated from the c dimension by the method which will be described in the next section are 0.80 and 0.30 for the films deposited at Ts=620°C ($20415) and 570°C ($20418), respectively. It is also interesting that the films cooled in 3.4X 10 -4 (background pressure) Torr of 6.2% ozone show fully oxygenated c dimension in contrast to the films cooled in 1.5 × 10 -3 Torr of pure oxygen atmosphere. This difference can be reasonably explained if we take into account the fact that ozone possesses a higher potential than pure oxygen [29,30] and ozone is very active in oxidizing Ag and Cu films at room temperature. 3.4. Oxygen potential control in YBCO films
As described in the previous section (see fig. 5), YBCO films and bulk placed in a quartz reaction tube behave almost similarly against oxygen absorption
K. Nakamura et al. /Oxygenpotential of YBCO films
....
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,[ ,.,t,,,,,,
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Fig. 7. XRD diffraction patterns of YBCOas~grownfilms cooleddown in different oxygenpartial pressures after deposition (A-I, A-2) $20415, (B-l, B-2) $20519-1. (See also Table l ). The oxygentreated films (A-2, B-2), 650°C for 30 rain in 0.2 atm 02, are also shown for comparison.
and desorption and reach equilibrium within a short time. The time necessary to attain equilibrium, however, depends strongly on the surface state (such as impurities adsorped from the gas phase) as well as the temperature. In the present study, films having a different c dimension in the as-grown state were heat-treated with bulk YBCO powder and the oxygen potential was controlled as described in the experimental section. After the treatment, lattice parameters (including a, b-axis lengths in the case of powder and a,c-oriented films) were measured and plotted as a function of 5 which were directly obtained from the amount of oxygen extracted from the bulk as described in the experimental section. It is noted that the total amount of 5 pieces of YBCO films with thickness about 100 nm and an area of typically 0.3 cm 2, for example is 150 ~g, which is about three orders of magnitude smaller than the bulk sample (typically 350-500 mg). Thus the ~ values calculated from the volumetric method are due mostly to the loss of oxygen from the bulk. If oxygen concentrations in the films and bulk are in equilibrium with the potential of oxygen gas in the quartz reaction tube, the 5 value of the films should be equal to that of the bulk and also equal to that obtained from the
volumetric method. As described in the previous section, if we applied method A to equilibrate the oxygen potential, the 5 values in the films and bulk became equal, but became different when we applied method B. The reason is not clear at present. However, if we take into account the fact that the quartz reaction tube C2 was well evacuated and was baked out at 350-400°C, but the main pyrex tube chamber C1 was evacuated only around room temperature, impure components adsorbed at the wall in the main pyrex chamber may cause a poisoning effect against oxygen absorption when cooled while keeping V2 open. Figure 8 plots c-axis lengths against 5 values for films with various initial (as-grown) c-axis length and bulk YBCO equilibrated with the films. The plots for bulk YBCO show similar behavior as reported by other investigators [ 3,13 ]. It should be emphasized that the c-axis length versus 5 value for the same series was repeatedly taken from the same portion of a sample (not divided into several pieces from one sheet of film) except for sample M20611-2 from which resistivity data shown in fig. 12 were taken. The attached numbers with arrows (1-6) shown in the upper part of fig. 8 exhibit the sequence of the
10
K. Nakamura et al. / Oxygen potential of YBCO films
11.85
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8 Fig. 8. c-axis lengths of YBCO films with various initial c-axis lengths are plotted against 5 values, c values for YBCObulk equilibrated with the films are also shown,c-axislength for bulk from Jorgensen et al. [ 13] are shown by ( + ). treatment. From this figure it can be easily shown that the c-axis length versus oxygen deficiency curves are almost parallel to that of the bulk YBCO curve in spite of their different starting ( = as-grown) c dimension, i.e. the curves shift upward depending on the difference in the starting c-axis length. This result strongly suggests that oxygen concentration of asgrown YBCO films can be determined by measuring the difference in the c-axis length of as-grown and fully oxygenated films. Figure 9 shows a-axis lengths of oxygen controlled, (a + c)-mixed oriented YBCO films and a- and b-axis lengths of bulk samples equilibrated with the film. The film was sputter-deposited on a SrTiO3 substrate and the film was repeatedly oxygen-treated as described above. From fig. 9 it is apparent that the a-axis length behaves almost similar to that in bulk YBCO, suggesting that the films have almost the same orthorhombic-tetragonal phase relation against 5. It is also interesting to see that the c-axis length of the a,c-oriented YBCO film is a little bit larger than the bulk whereas the a-axis length is a little bit smaller or equal to that of the bulk.
3.5. Effects of heat treatment In order to get better oxygen absorption and desorption rates and attain complete equilibrium, the films were heated up to 650°C for about l0 min. The
3.82 u.O
0.2
0.4
0.6
0.8
.0
5 Fig. 9. a-axis lengths versus 5 values for an (a+c)-mixed oriented YBCO film ($20817-STa) and bulk (a- and b-axis lengths) equilibrated with the film. Open and closed triangles are referred from ref. [ 13 ].
procedure was repeated a number of times to get different ~ values. Thus it is important to determine whether the heat treatment affects (in addition to the oxygen concentration effects) the c-axis length by, for example, rearrangement of cations or recrystallization. The following three experimental results strongly suggest that such heat treatment only changes the oxygen concentration: (1) After repeated oxygen treatments up to 650°C, the c-lattice parameter of the films recovered their starting value in all caaes when the films were finally treated under 0.2 atmosphere oxygen at 650°C. (2) As shown in fig. 8, all data points from the same film lie on a single line indicating that the change due to the heat treatment up to 650°C affects the oxygen concentration but not the structure. (3) The rocking curve widths from a particular film are unchanged by the repeated heat treatments for all cases as shown in fig. 10.
3.6. Resistivity vs. temperature curves of the oxygen potential controlled YBCO films Table 2 shows deposition conditions, composition, and as-grown c-axis length for the two films on which resistivity vs. temperature was measured as shown in figs. 11 and 12. The film in fig. 11 was sputter-deposited on a MgO ((100) 10× 10 mm
K. Nakamura et al. / Oxygen potential of YBCOfilms .
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.
(B-2)
(A-2)
3
.
3
5 2
4
0
e
Fig. 10. Rocking curves for YBCO films before ( A-1, B-1) and after ( A-2, 13-2) the repeated heat treatments up to 650 °C. A-1,2 = M2061 i l, B-1,2=$20817-1.
Table 2 Characteristics of YBCO films for the resistivity measurements Sample
S 10829-2 M20611-2
Method
Sputter MBE
Ts ( oC) 630 670
Po2
0.015 0.00034
square) substrate. It showed Tc o f 78 K b u t has good crystallinity. The film in fig. 12 was v a c u u m - d e p o s ited onto MgO ( ( 1 0 0 ) 20 X 15 m m square) a n d has T~o o f 89.6 K a n d Raoo/R~oo = 3.12. The samples were cut into 5 ( 10X 10 m m s a m p l e ) to 13 ( 2 0 × 15 m m s a m p l e ) pieces o f a b o u t 2 X ( 7 - 1 0 ) m m 2 square a n d the oxygen deficiency was controlled in the m a n n e r
c-lattice parameter ( A ) 11.73 11.72
Composition (%) Y
Ba
Cu
21 17.8
28 31.8
51 50.4
described in the previous section. A n e x a m i n a t i o n o f the two figures show that the film with better resistivity in its as-grown state shows better resistivity transition even at higher c~. Moreover, the b o u n d a r y oxygen deficiency value at which Tc decreases to zero lies between ~ = 0 . 6 3 a n d 0.67 in fig. 12 a n d between 0.48 a n d 0.59 in fig. 11. This observation suggests
12
K. N a k a m u r a et al. / O x y g e n potential o f Y B C O f i l m s
1.o I
0.8 t ' E
100
e//-
,
...'I"'I "j'"
0.6~ :.
80
•...................
/
~=0.48
I
l~J]
\ A
0.2
,:~"f: 8=0.18 .:" ~ •" : 5=0.29 I ~=0.07 ,/i ........ i i
ol 0
S0
i
100 TEMPERATURE
i
:
150
200
Fig. 11. R e s i s t i v i t y v e r s u s t e m p e r a t u r e c u r v e s o f o x y g e n c o n t r o l l e d Y B C O f i l m p r e p a r e d b y s p u t t e r i n g ( S 1 0 8 2 9 - 2 ).
J , '~=° 63° ~ " ~ ,,,/"
1.2
,/"
~9 0.6
0 t ; 0
'
f'/"
. /
. •
/
,~ 100
v
"',i *~"'I\[
I-
\
40
]
/
~'"~
I
"" .... ,,-"
i 5=0 500 J I
8=0570
"' " '
~=0,265 /
~..~.-c.:~ "J
7 . 2 ~ " 200
TEMPERATURE
20
,-, ,
e~'
g
60
(K)
1.5
1310829-2
5-
'~-2~.'4;!..
................ 300
(K)
Fig. 12. Resistivity versus temperature curves of oxygen controlled YBCO film prepared by vacuum evaporation (M206112). that the oxygen deficiency ~ vs. Tc relation also depends on the superconducting properties o f the asgrown films. Figure 13 plots Tc of the M20611-2 film shown in fig. 12 as a function of oxygen deficiency. For comparison, Tc values from the sputter-deposited films (S10829-2 and from ref. [ 16] ) are also shown. From the figure, two-plateau behavior seems to be clearly observed and also an excellent agreement with the data from bulk YBCO [ 3,13 ] is observed for both the T¢ values and their ~ dependency. In contrast to this, the films from the sputter-deposition, as well as those from ref. [ 16 ] show different behavior. Since the ~ values o f the films in fig. 11 were also determined in the same way as those in fig. 12, the different Tc vs. ~ behavior must be attributed to the su-
0 0.0
0.2
0.4
5
0.6
0.8
Fig. 13. Superconducting transition temperature T~ versus oxygen deficiency &in YBCO films shown in figs. I 1 and 12. Dotted line shows Tc of oxygen deficient films from ref. [ 16]. perconducting properties in the as-grown state. On the other hand, the films in ref. [ 16 ] which have better superconducting transition temperature than S10829-2 in their as-grown state showed also different behavior. Thus, it seems likely that the ~ values were different from their estimated value because their & dependency o f c-axis length is also different from the bulk and from our films. The excellent agreement of M20611 film with the bulk behavior shown in fig. 13, strongly suggests that such thin films properly prepared should have essentially the similar superconducting behavior against oxygen deficiency. This also confirms that the d values thus determined from the three different ways, volumetric, c-axis lengths o f equilibrated bulk and films, are accurate and can be conveniently used to determine 8.
4. Conclusion From the study of oxygen potential control of YBCO films, several conclusions can be drawn. First, the oxygen absorption and desorption rates of epitaxial films are very fast to reach equilibrium within
K. Nakamura et al. / Oxygen potential of YBCO films ten seconds above 600 ° C. Thus as-grown films cooled u n d e r appropriate oxygen partial pressure are, in most cases, fully oxygenated (c~=0.07) a n d the difference in the c-axis length and T~ of as-grown YBCO films can not be ascribed to the difference in the oxygen contents. The difference in the c-axis length a n d Tc in the as-grown state might be due to disordering of cations, radiation damage during sputter-deposition or weak intergrain couplings. The c-axis length in YBCO films varies with 6 in a m a n n e r similar to the bulk in spite o f the difference in the c-axis length in the as-grown or fully oxygenated state. However, Tc versus oxygen deficiency of the films showed different behavior depending on the superconducting transition in the as-grown state. The films with better superconducting transition in the as-grown state showed almost the same 8 dependency as bulk a n d single crystal Y B C O with homogeneously controlled oxygen deficiency. T h u s YBCO thin films should have intrinsically the same superconducting transition against oxygen deficiency.
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