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Characteristics of YBa2Cu3Ox films heat-treated in inert gases
Physica C 161 (1989) 66-70 North-Holland, Amsterdam
C H A R A C T E R I S T I C S OF YBa2Cu3Ox F I L M S H E A T - T R E A T E D I N I N E R T G A S E S Hirotoshi N A G A T A a n d Eungi M I N Sumitomo Cement Co. Ltd., CentralResearch Laboratory, Toyotomi-cho 585, Funabashi-shL Chiba 274, Japan M a s a o A I H A R A and Tadatsugu I T O H School of Science and Engineering, Waseda University, Ohkubo 3-4-1, Shinjuku-ku, Tokyo 169, Japan Hiroshi T A K A I Tokyo Denki University, Kanda Nishiki-cho 2-2, Chiyoda-ku, Tokyo 101, Japan Received 24 July 1989
Influences of gases used for heat treatment upon the crystallization and superconductivity of sputter-deposited YBa2Cu30~, films were investigated. The heat treatment process consists of three stages; heating up and holding at a certain temperature and then cooling down. On each stage, the air, 02, N2, Ar, He was flowed and exchanged step by step. The film heated up and held at 800 °C in N2 followed by cooling down in 02 exhibits the zero-resisitivity temperature (To) to be 71 K. Low content of 02 (less than 1%) in the ambient atmosphere during the stages at high temperature promotes the crystallization of the film, and 02 is incorporated into the film during the step of cooling down in 02 resulting in the superconducting phase.
1. Introduction Many methods to fabricate YBa2Cu3Ox ( Y B a C u O ) films by low t e m p e r a t u r e processes have been studied. The m e t h o d s to prepare superconducting Y B a C u O films are distinguished into two groups. One is the in-situ heating during the film deposition, a n d the other is ex-situ annealing. The form e r one has an excellent advantage that crystalline YBaCuO films can be o b t a i n e d on ( 1 0 0 ) MgO, ( 1 0 0 ) and ( 1 1 0 ) SrTiO3 at the substrate temperature higher than 4 0 0 ° C [ 1,2 ]. However, it is difficult to m a k e high quality films reproducible. On the contrary, the ex-situ annealing method, which is carried out after the d e p o s i t i o n o f an a m o r phous films, is a suitable way to form a polycrystalline Y B a C u O film because o f its controllability a n d reproducibility. In ex-situ annealing process, the t e m p e r a t u r e required to crystalline an a m o r p h o u s Y B a C u O film is generally higher than 850°C in air or in flowing 02 [3,4]. F i l m s a n n e a l e d at lower temperature result in micrograins having r a n d o m orientation and p o o r superconducting properties. 0921-4534/89/$ 03.50 © Elsevier Science Publishers B.V. ( North-Holland )
Higher t e m p e r a t u r e annealing causes troubles o f interdiffusion a n d chemical reaction between the film a n d substrate [ 5 ]. Considering an application o f polycrystalline film o f superconducting Y B a C u O to devices such as the infrared detector, which utilizes grain b o u n d a r i e s o f superconductors [6 ], a t t e m p t s to decrease the annealing t e m p e r a t u r e to fabricate high quality polycrystalline YBaCuO films should be important. In this paper, we have d e m o n s t r a t e d an ex-situ heat t r e a t m e n t process using inert gases. Influences o f oxygen content in the gas on the crystalline a n d superconducting properties o f Y B a C u O films p r e p a r e d by this process are discussed.
2. Experimental procedure F i l m s were deposited on ( 1 0 0 ) MgO substrates at a m b i e n t t e m p e r a t u r e by the rf-magnetron m e t h o d using Ar. The sputtering apparatus consisted o f four sputter-guns (two for b a r i u m - c o p p e r ( 1 : 1 ), one for y t t r i u m a n d one for c o p p e r ) m o u n t e d a r o u n d a cy-
H. Nagata et al. I Characteristics of YBa2Cu sOx films heat-treated in inert gases
lindrical chamber as shown in fig. 1. Each metal target was 100 mm in diameter and 5 mm thick. Substrates of 5 mm × 10 mm were settled on the rotatable coaxial cylindrical holder. This system enables the substrates to face the four targets one after another. The rotating speed of the substrate holder was fixed to be 60 rpm to obtain a homogeneous alloy of the sputtered elements. A distance between the substrata and the target was about 50 mm. Film composition was adjusted by the if-power supplied to each gun. In this experiment, if-powers of 36 W and 26 W were supplied to the yttrium gun and the copper gun, respectively, and 60 W to each barium-copper gun. Under these conditions, film composition determined by inductively coupled plasma emission was Y : B a : C u = 1.00-2.27:3.08. The film thickness was controlled to be 200 nm at a deposition rate of 9 n m / rain. Deposited films were heat-treated in a horizontal tube furnace. The procedure of the heat treatment was as follows: the furnace was held previously at a certain temperature (typically 800 °C), then the film inserted into it and heated up within 15 min. After holding the film at that temperature for 30 min, the film was cooled down to 400 °C with a cooling rate of a few °C/rain, and further cooled to less than 200°C in the furnace. Gasses flowing during the heating up stage, holding stage and cooling stage were changed intentionally. Oxygen, nitrogen, argon, helium and air were used for each stage. In this paper, the heat treatment process in which a film was heated up in N2, held in 02 and then cooled down in 02 is simplified as an ( N / O / O ) process. Other combi-
67
nations of gasses are simplified similarly. The flow rate of each gas was controlled at 250 cm3/min. The surface morphology of the films was observed by SEM. Crystalline orientation of the films was measured by an X-ray diffractometer ( X R D ) with a monochromated CuK~ radiation within the diffraction angle range of 20 from 5 ° to 90 °. In addition to ICP, the film composition was determined by Rutherford backscattering spectroscopy (RBS) with 3 to 5 MeV He 2÷. The elemental depth profiles were measured by Auger electron spectroscopy (AES) using Ar + sputtering. Resistivities of the heat-treated films were measured by a standard four probe method using four evaporated gold terminals at a current level of 5 ~tA.
3. Results and discussion Before taking out the film from the chamber, asdeposited films are black and smooth. But they change into hazy gradually in a few tens of seconds in the air. The oxygen content in the film exposed to the air for 50 h after the deposition was determined to be Y: Ba: Cu: O = 1 : 2: 3: 7 by RBS measurement. It is clear by AES measurement that oxygen penetrates through the film and that the barium content increases near the surface as shown in fig. 2. The surface becoming hazy is considered to be due to interactions of the film with oxygen and humidity in the air. In the deposition chamber, sputtering was carried out using metal targets with Ar, so that the deposited film should be oxygen free. Therefore the as-deposited films are very active with oxygen and
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50
100
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150 2 0 0
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Sputtering Time ( min ) Fig. 2. AES depth profiles of the as-deposited film kept for about 50 h in air. The measurement was performed with Ar + ion sputtering under 5 × 10-8 A at 5 kV.
68
H. Nagata et al. / Characteristics of YBa2Cu30x films heat-treated in inert gases
humidity in the air. AES measurements suggest that barium is chemically very active with the air, and this causes the accumulation of barium near the surface. Here, it should be noted that as-deposited films contain oxygen already before the heat treatment even though they are oxygen free during the sputterdeposition from metal targets. Fig. 3 shows SEM micrographs of the film heattreated by ( N / N / N ) , ( N / N / O ) , ( N / O / O ) and ( O / O / O ) processes. The films were inserted into the furnace within 2 min after taking them out of the chamber, and the films were held at 800°C on the second heat treatment stage. The ( O / O / O ) film shows sphere-like grains with the smallest feature size of a few submicrons. Fig. 4 shows XRD patterns of these four films. The ( N / N / N ) , ( N / N / O ) and ( N / O / O ) films show high intensities of <00l> peaks of YBa2Cu3Ox and small diffraction peaks from < 013 >, < 103> and < 110> planes. In the ( O / O / O ) film, on the other hand, < 013 >, < 103 > and < 110 > peaks are higher than < 001> peaks. According to these data,
the ( N / N / N ) , ( N / N / O ) and ( N / O / O ) films are assigned to be of orthorhombic phase, while the ( O / O / O ) film is a mixture of orthorhombic and tetragonal phases. It is very interesting, here, that the ( N / N / N ) film which was heat-treated in an almost oxygen free atmosphere was oxidized and that the orthorhombic phase is formed. The oxidation of the ( N / N / N ) film is considered to be due to the residual 02 in N2. In the case of ( N / N / O ) process, for example, the N2 content was measured to be higher than 99% and 02 to be less than 1% by gas chromatography. The oxygen content increased to 3% by inserting the films into the furnace, but it decreased to less than 1% again within 10 min. By exchanging flow gas from N2 to 02, the oxygen content increased to 61% after 5 min, 92% after 10 min, 94% after 15 min and higher than 99% after 30 min. Accordingly, as in the case of the ( N / N / N ) process, the film is held at 800°C and cooled down in > 99% N2 atmosphere. However, the equilibrium oxygen content (x) in YBa2Cu3Ox films at 800°C is
Fig. 3. SEM images of the annealed films; (a) (N/N/N), (b) (N/N/O), (e) (N/O/O) and (d) (O/O/O). Bars in the photographs show 5 ~tm.
69
H. Nagata et a L I Characteristics o f YBa2Cu30x films heat-treated in inert gases
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(N/N/O), (c) (N/O/O) and (d) (O/O/O). Peaks attributed to YBa2Cu3Oxand MgO (substrate). 1.176 A
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Fig. 5. Comparisonof c-axis length of the films annealed under different gas conditions. Open circles are for (N/N/N), (N/N/ O), (N/O/O) and (O/O/O) films. Closed circles show c-axis length of the (N/N/N) film after annealing at 500°C in flowing oxygen. calculated to be 6.2 at the oxygen partial pressure of 1.01 kPa (0.01 atm) and 6.5 at 101 kPa (l atm) [7,8]. This means that an oxygen content less than 1% is high enough to form the layered perovskite structure of YBaECu3Ox at 800°C. Taking into account the fact that the as-deposited film was oxidized before insertion into the furnace, results and calculations above suggest that the ( N / N / N ) and ( N / N / O ) films contained oxygen in it and an orthorhombic phase was formed. Fig. 5 shows the c-axis length of the ( N / N / N ) ,
( N / N / O ) , ( N / O / O ) and ( O / O / O ) films calculated from XRD patterns. The c-axis length in the ( N / N / N ) film is the longest. This is caused by lack of oxygen in the film. Oxygen contents (x) calculated from an equation concerning c-axis length are 6.55 for the ( N / N / N ) film and 6.89 for the others [9]. By annealing at 500°C in an 02 ambient atmosphere, the c-axis length in the ( N / N / N ) film is shortened as shown by closed circles in the figure. This result means that oxygen deficient YBaCuO films can be oxidized easily by annealing at relatively low temperature. Fig. 6 shows the onset and zero-resistivity temperature (To) of the films. The ( N / N / O ) and ( N / O / O ) films show metal-like temperature dependence of resistivity and achieve moderately high Tc in the range between 60 and 70 K. Low onset and low T~ of the ( N / N / N ) film is presumed to be due to the deficient of oxygen in the film which increases the c-axis length. After the oxidation at 500°C, the onset of the ( N / N / N ) film increases to 85 K, but T~ remains at as low as 33 and 40 K even after the oxidation for 60 and 300 min, respectively. The (O/ O / O ) film did not show T¢ above 4.5 K although the onset temperature of 80 K is obtained. This is probably due to poor grain growth in the ( O / O / O ) film. The films heat-treated by the ( N / N / O ) process at temperatures of 780°C and 750°C show a T¢ of 70 and 47 K, respectively. The grain growth in the film 100
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Fig. 6. Comparisonof onset temperatures (opencircles) and zeroresistivity temperature (closed circles) of the film annealed under different gas conditions; (N/N/N), (N/N/O), (N/O/O) and (O/O/O) films. The (O/O/O) film did not show zero-resistivity above 4.5 K.
70
H. Nagata et al. / Characteristics of YBa2Cu30x films heat-treated in inert gases
heat-treated at 750°C is as p o o r as that processed by ( O / O / O ) at 800°C. The heat t r e a t m e n t in inert gas is also effective for YBaCuO films sputtered from YBa2Cu3Ox targets. Figs.7 and 8 show X R D patterns and t e m p e r a t u r e dependences o f resistivity o f YiBaE.o9Cu3.25Ox films which were heated up a n d held at 800°C in air, N2, Ar and He, a n d cooled down in O2. The films heattreated in N2, A r a n d He showed growth o f plate-like shaped grains with a side length o f a few p.m, but the films heat-treated in air shows small grains less than 1 ~tm in SEM observation. This means p o o r grain growth o f Y B a C u O films in air. A i r a n d O2 heat treatments n e e d e d t e m p e r a t u r e s higher than 850°C to achieve the same film properties as that o f films heat-treated in inert gases. According to the results above, it is considered that f o r m a t i o n o f the layered perovskite structure is pro-
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m o t e d by the low oxygen content in the film, namely, in the a m b i e n t atmosphere, because the oxygen content in the film is equilibrated to the oxygen content in the a t m o s p h e r e during the high t e m p e r a t u r e treatment. In other words, it seems that the heat treatments in air or 02 interrupt the grain growth.
4. Conclusion We have investigated influences o f gases used during the heat t r e a t m e n t u p o n sputter-deposited YBaECUaOx properties. The film heated up a n d held at 800°C in N2 followed by cooling down in 02 shows a Tc o f 71 K while the film heat-treated in O2 or air shows extremely p o o r properties. It is concluded that low oxygen content in the a m b i e n t a t m o s p h e r e is necessary for the grain growth during which the film is held at high temperature, irrespective o f the initial oxygen content in the as-deposited film. However, the oxygen content required to exhibit superconducting properties can be supplied during the cooling down stage in 02.
Acknowledgements
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40
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Fig. 7. XRD patterns of the films annealed at 800°C for 60 min in flowing air, nitrogen, argon and helium followed by cooling down in flowing oxygen.
We thank H. Shimizu o f the Science a n d Engineering L a b o r a t o r y o f W a s e d a University for RBS measurements, a n d T. K a t s u r a o f E n v i r o n m e n t a l Safety o f W a s e d a U n i v e r s i t y for ICP measurements.
References --- 1.5 E
.u
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6 0 8 0 100 120 140 Temperature ( K )
Fig. 8. Relation between temperature and resistivity of the films annealed in flowing air, nitrogen, argon and helium. Zero-resistivity temperature was obtained at 45, 70, 70 and 68 K, respectively.
[ 1] S. Witanachchi, H.S. Kwok, X.W. Wang and D.T. Shaw,Appl. Phys. Lett. 53 (1988) 234. [ 2 ] H.C. Li, G. Linker, F. Ratzel, R. Smithey and J. Geerk, Appl. Phys. Lett. 52 (1988) 1098. [3] C.X. Qiu and I. Shih, Appl. Phys. Lett. 52 (1988) 587. [4] R. Feenstra, L.A. Boatner, J.D. Budai, D.K. Christen, M.D. Galloway and D.B. Poker, Appl. Phys. Lett. 54 (1989) 1063. [ 5 ] H. Koinuma, K. Fukuda, T. Hashimoto and K. Fueki, Jpn. J. Appl. Phys. 27 (1988) L1216. [6] B. H~iuser, B.B.G. Klopman, G.J. Gerritsma, J. Gao and H. Rogalla, Appl. Phys. Lett. 54 (1989) 1368. [7] K. Kishio, J. Shimojima, T. Hasegawa, K. Kitazawa and K. Fueki, Jpn. J. Appl. Phys. 26 (1987) L1228. [ 8 ] R. Bormann and J. NSlting, Appl. Phys. Lett. 54 (1989) 2148. [9] Z. Jir~ik, J. Hejtm~inek, E. Poltert, A. T[iska and P. Va~ek, Physica C 156 (1988) 750.
C H A R A C T E R I S T I C S OF YBa2Cu3Ox F I L M S H E A T - T R E A T E D I N I N E R T G A S E S Hirotoshi N A G A T A a n d Eungi M I N Sumitomo Cement Co. Ltd., CentralResearch Laboratory, Toyotomi-cho 585, Funabashi-shL Chiba 274, Japan M a s a o A I H A R A and Tadatsugu I T O H School of Science and Engineering, Waseda University, Ohkubo 3-4-1, Shinjuku-ku, Tokyo 169, Japan Hiroshi T A K A I Tokyo Denki University, Kanda Nishiki-cho 2-2, Chiyoda-ku, Tokyo 101, Japan Received 24 July 1989
Influences of gases used for heat treatment upon the crystallization and superconductivity of sputter-deposited YBa2Cu30~, films were investigated. The heat treatment process consists of three stages; heating up and holding at a certain temperature and then cooling down. On each stage, the air, 02, N2, Ar, He was flowed and exchanged step by step. The film heated up and held at 800 °C in N2 followed by cooling down in 02 exhibits the zero-resisitivity temperature (To) to be 71 K. Low content of 02 (less than 1%) in the ambient atmosphere during the stages at high temperature promotes the crystallization of the film, and 02 is incorporated into the film during the step of cooling down in 02 resulting in the superconducting phase.
1. Introduction Many methods to fabricate YBa2Cu3Ox ( Y B a C u O ) films by low t e m p e r a t u r e processes have been studied. The m e t h o d s to prepare superconducting Y B a C u O films are distinguished into two groups. One is the in-situ heating during the film deposition, a n d the other is ex-situ annealing. The form e r one has an excellent advantage that crystalline YBaCuO films can be o b t a i n e d on ( 1 0 0 ) MgO, ( 1 0 0 ) and ( 1 1 0 ) SrTiO3 at the substrate temperature higher than 4 0 0 ° C [ 1,2 ]. However, it is difficult to m a k e high quality films reproducible. On the contrary, the ex-situ annealing method, which is carried out after the d e p o s i t i o n o f an a m o r phous films, is a suitable way to form a polycrystalline Y B a C u O film because o f its controllability a n d reproducibility. In ex-situ annealing process, the t e m p e r a t u r e required to crystalline an a m o r p h o u s Y B a C u O film is generally higher than 850°C in air or in flowing 02 [3,4]. F i l m s a n n e a l e d at lower temperature result in micrograins having r a n d o m orientation and p o o r superconducting properties. 0921-4534/89/$ 03.50 © Elsevier Science Publishers B.V. ( North-Holland )
Higher t e m p e r a t u r e annealing causes troubles o f interdiffusion a n d chemical reaction between the film a n d substrate [ 5 ]. Considering an application o f polycrystalline film o f superconducting Y B a C u O to devices such as the infrared detector, which utilizes grain b o u n d a r i e s o f superconductors [6 ], a t t e m p t s to decrease the annealing t e m p e r a t u r e to fabricate high quality polycrystalline YBaCuO films should be important. In this paper, we have d e m o n s t r a t e d an ex-situ heat t r e a t m e n t process using inert gases. Influences o f oxygen content in the gas on the crystalline a n d superconducting properties o f Y B a C u O films p r e p a r e d by this process are discussed.
2. Experimental procedure F i l m s were deposited on ( 1 0 0 ) MgO substrates at a m b i e n t t e m p e r a t u r e by the rf-magnetron m e t h o d using Ar. The sputtering apparatus consisted o f four sputter-guns (two for b a r i u m - c o p p e r ( 1 : 1 ), one for y t t r i u m a n d one for c o p p e r ) m o u n t e d a r o u n d a cy-
H. Nagata et al. I Characteristics of YBa2Cu sOx films heat-treated in inert gases
lindrical chamber as shown in fig. 1. Each metal target was 100 mm in diameter and 5 mm thick. Substrates of 5 mm × 10 mm were settled on the rotatable coaxial cylindrical holder. This system enables the substrates to face the four targets one after another. The rotating speed of the substrate holder was fixed to be 60 rpm to obtain a homogeneous alloy of the sputtered elements. A distance between the substrata and the target was about 50 mm. Film composition was adjusted by the if-power supplied to each gun. In this experiment, if-powers of 36 W and 26 W were supplied to the yttrium gun and the copper gun, respectively, and 60 W to each barium-copper gun. Under these conditions, film composition determined by inductively coupled plasma emission was Y : B a : C u = 1.00-2.27:3.08. The film thickness was controlled to be 200 nm at a deposition rate of 9 n m / rain. Deposited films were heat-treated in a horizontal tube furnace. The procedure of the heat treatment was as follows: the furnace was held previously at a certain temperature (typically 800 °C), then the film inserted into it and heated up within 15 min. After holding the film at that temperature for 30 min, the film was cooled down to 400 °C with a cooling rate of a few °C/rain, and further cooled to less than 200°C in the furnace. Gasses flowing during the heating up stage, holding stage and cooling stage were changed intentionally. Oxygen, nitrogen, argon, helium and air were used for each stage. In this paper, the heat treatment process in which a film was heated up in N2, held in 02 and then cooled down in 02 is simplified as an ( N / O / O ) process. Other combi-
67
nations of gasses are simplified similarly. The flow rate of each gas was controlled at 250 cm3/min. The surface morphology of the films was observed by SEM. Crystalline orientation of the films was measured by an X-ray diffractometer ( X R D ) with a monochromated CuK~ radiation within the diffraction angle range of 20 from 5 ° to 90 °. In addition to ICP, the film composition was determined by Rutherford backscattering spectroscopy (RBS) with 3 to 5 MeV He 2÷. The elemental depth profiles were measured by Auger electron spectroscopy (AES) using Ar + sputtering. Resistivities of the heat-treated films were measured by a standard four probe method using four evaporated gold terminals at a current level of 5 ~tA.
3. Results and discussion Before taking out the film from the chamber, asdeposited films are black and smooth. But they change into hazy gradually in a few tens of seconds in the air. The oxygen content in the film exposed to the air for 50 h after the deposition was determined to be Y: Ba: Cu: O = 1 : 2: 3: 7 by RBS measurement. It is clear by AES measurement that oxygen penetrates through the film and that the barium content increases near the surface as shown in fig. 2. The surface becoming hazy is considered to be due to interactions of the film with oxygen and humidity in the air. In the deposition chamber, sputtering was carried out using metal targets with Ar, so that the deposited film should be oxygen free. Therefore the as-deposited films are very active with oxygen and
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r-
Lv
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Cu metal
Fig. 1. Schematic diagram of the sputtering apparatus with four rf-magnetron sputter guns. The substrates were mounted on the holder rotating at 60 rpm.
0
I
I
50
100
I
!
150 2 0 0
250
Sputtering Time ( min ) Fig. 2. AES depth profiles of the as-deposited film kept for about 50 h in air. The measurement was performed with Ar + ion sputtering under 5 × 10-8 A at 5 kV.
68
H. Nagata et al. / Characteristics of YBa2Cu30x films heat-treated in inert gases
humidity in the air. AES measurements suggest that barium is chemically very active with the air, and this causes the accumulation of barium near the surface. Here, it should be noted that as-deposited films contain oxygen already before the heat treatment even though they are oxygen free during the sputterdeposition from metal targets. Fig. 3 shows SEM micrographs of the film heattreated by ( N / N / N ) , ( N / N / O ) , ( N / O / O ) and ( O / O / O ) processes. The films were inserted into the furnace within 2 min after taking them out of the chamber, and the films were held at 800°C on the second heat treatment stage. The ( O / O / O ) film shows sphere-like grains with the smallest feature size of a few submicrons. Fig. 4 shows XRD patterns of these four films. The ( N / N / N ) , ( N / N / O ) and ( N / O / O ) films show high intensities of <00l> peaks of YBa2Cu3Ox and small diffraction peaks from < 013 >, < 103> and < 110> planes. In the ( O / O / O ) film, on the other hand, < 013 >, < 103 > and < 110 > peaks are higher than < 001> peaks. According to these data,
the ( N / N / N ) , ( N / N / O ) and ( N / O / O ) films are assigned to be of orthorhombic phase, while the ( O / O / O ) film is a mixture of orthorhombic and tetragonal phases. It is very interesting, here, that the ( N / N / N ) film which was heat-treated in an almost oxygen free atmosphere was oxidized and that the orthorhombic phase is formed. The oxidation of the ( N / N / N ) film is considered to be due to the residual 02 in N2. In the case of ( N / N / O ) process, for example, the N2 content was measured to be higher than 99% and 02 to be less than 1% by gas chromatography. The oxygen content increased to 3% by inserting the films into the furnace, but it decreased to less than 1% again within 10 min. By exchanging flow gas from N2 to 02, the oxygen content increased to 61% after 5 min, 92% after 10 min, 94% after 15 min and higher than 99% after 30 min. Accordingly, as in the case of the ( N / N / N ) process, the film is held at 800°C and cooled down in > 99% N2 atmosphere. However, the equilibrium oxygen content (x) in YBa2Cu3Ox films at 800°C is
Fig. 3. SEM images of the annealed films; (a) (N/N/N), (b) (N/N/O), (e) (N/O/O) and (d) (O/O/O). Bars in the photographs show 5 ~tm.
69
H. Nagata et a L I Characteristics o f YBa2Cu30x films heat-treated in inert gases
~'~ -+-
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(N/N/O), (c) (N/O/O) and (d) (O/O/O). Peaks attributed to YBa2Cu3Oxand MgO (substrate). 1.176 A
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I
heat up N2 N2 N2 02 hoLd N2 N2 02 Oa coot N2 O~ 02 02 Ftowing Gas
Fig. 5. Comparisonof c-axis length of the films annealed under different gas conditions. Open circles are for (N/N/N), (N/N/ O), (N/O/O) and (O/O/O) films. Closed circles show c-axis length of the (N/N/N) film after annealing at 500°C in flowing oxygen. calculated to be 6.2 at the oxygen partial pressure of 1.01 kPa (0.01 atm) and 6.5 at 101 kPa (l atm) [7,8]. This means that an oxygen content less than 1% is high enough to form the layered perovskite structure of YBaECu3Ox at 800°C. Taking into account the fact that the as-deposited film was oxidized before insertion into the furnace, results and calculations above suggest that the ( N / N / N ) and ( N / N / O ) films contained oxygen in it and an orthorhombic phase was formed. Fig. 5 shows the c-axis length of the ( N / N / N ) ,
( N / N / O ) , ( N / O / O ) and ( O / O / O ) films calculated from XRD patterns. The c-axis length in the ( N / N / N ) film is the longest. This is caused by lack of oxygen in the film. Oxygen contents (x) calculated from an equation concerning c-axis length are 6.55 for the ( N / N / N ) film and 6.89 for the others [9]. By annealing at 500°C in an 02 ambient atmosphere, the c-axis length in the ( N / N / N ) film is shortened as shown by closed circles in the figure. This result means that oxygen deficient YBaCuO films can be oxidized easily by annealing at relatively low temperature. Fig. 6 shows the onset and zero-resistivity temperature (To) of the films. The ( N / N / O ) and ( N / O / O ) films show metal-like temperature dependence of resistivity and achieve moderately high Tc in the range between 60 and 70 K. Low onset and low T~ of the ( N / N / N ) film is presumed to be due to the deficient of oxygen in the film which increases the c-axis length. After the oxidation at 500°C, the onset of the ( N / N / N ) film increases to 85 K, but T~ remains at as low as 33 and 40 K even after the oxidation for 60 and 300 min, respectively. The (O/ O / O ) film did not show T¢ above 4.5 K although the onset temperature of 80 K is obtained. This is probably due to poor grain growth in the ( O / O / O ) film. The films heat-treated by the ( N / N / O ) process at temperatures of 780°C and 750°C show a T¢ of 70 and 47 K, respectively. The grain growth in the film 100
8O -I
m
60
ao 0
I
I
|
!
!
heat up N2 Na Na 02 hotd N2 N2 02 02 cool. N2 O2 02 02 FLowing Gas
Fig. 6. Comparisonof onset temperatures (opencircles) and zeroresistivity temperature (closed circles) of the film annealed under different gas conditions; (N/N/N), (N/N/O), (N/O/O) and (O/O/O) films. The (O/O/O) film did not show zero-resistivity above 4.5 K.
70
H. Nagata et al. / Characteristics of YBa2Cu30x films heat-treated in inert gases
heat-treated at 750°C is as p o o r as that processed by ( O / O / O ) at 800°C. The heat t r e a t m e n t in inert gas is also effective for YBaCuO films sputtered from YBa2Cu3Ox targets. Figs.7 and 8 show X R D patterns and t e m p e r a t u r e dependences o f resistivity o f YiBaE.o9Cu3.25Ox films which were heated up a n d held at 800°C in air, N2, Ar and He, a n d cooled down in O2. The films heattreated in N2, A r a n d He showed growth o f plate-like shaped grains with a side length o f a few p.m, but the films heat-treated in air shows small grains less than 1 ~tm in SEM observation. This means p o o r grain growth o f Y B a C u O films in air. A i r a n d O2 heat treatments n e e d e d t e m p e r a t u r e s higher than 850°C to achieve the same film properties as that o f films heat-treated in inert gases. According to the results above, it is considered that f o r m a t i o n o f the layered perovskite structure is pro-
u) ¢--
JO I.. U v
m o t e d by the low oxygen content in the film, namely, in the a m b i e n t atmosphere, because the oxygen content in the film is equilibrated to the oxygen content in the a t m o s p h e r e during the high t e m p e r a t u r e treatment. In other words, it seems that the heat treatments in air or 02 interrupt the grain growth.
4. Conclusion We have investigated influences o f gases used during the heat t r e a t m e n t u p o n sputter-deposited YBaECUaOx properties. The film heated up a n d held at 800°C in N2 followed by cooling down in 02 shows a Tc o f 71 K while the film heat-treated in O2 or air shows extremely p o o r properties. It is concluded that low oxygen content in the a m b i e n t a t m o s p h e r e is necessary for the grain growth during which the film is held at high temperature, irrespective o f the initial oxygen content in the as-deposited film. However, the oxygen content required to exhibit superconducting properties can be supplied during the cooling down stage in 02.
Acknowledgements
¢-0 .I-
c
20
30 CuK=26
40
50
60
(degree)
Fig. 7. XRD patterns of the films annealed at 800°C for 60 min in flowing air, nitrogen, argon and helium followed by cooling down in flowing oxygen.
We thank H. Shimizu o f the Science a n d Engineering L a b o r a t o r y o f W a s e d a University for RBS measurements, a n d T. K a t s u r a o f E n v i r o n m e n t a l Safety o f W a s e d a U n i v e r s i t y for ICP measurements.
References --- 1.5 E
.u
E 1.0
-£
Ar
~- 0.5 0 40
N2
6 0 8 0 100 120 140 Temperature ( K )
Fig. 8. Relation between temperature and resistivity of the films annealed in flowing air, nitrogen, argon and helium. Zero-resistivity temperature was obtained at 45, 70, 70 and 68 K, respectively.
[ 1] S. Witanachchi, H.S. Kwok, X.W. Wang and D.T. Shaw,Appl. Phys. Lett. 53 (1988) 234. [ 2 ] H.C. Li, G. Linker, F. Ratzel, R. Smithey and J. Geerk, Appl. Phys. Lett. 52 (1988) 1098. [3] C.X. Qiu and I. Shih, Appl. Phys. Lett. 52 (1988) 587. [4] R. Feenstra, L.A. Boatner, J.D. Budai, D.K. Christen, M.D. Galloway and D.B. Poker, Appl. Phys. Lett. 54 (1989) 1063. [ 5 ] H. Koinuma, K. Fukuda, T. Hashimoto and K. Fueki, Jpn. J. Appl. Phys. 27 (1988) L1216. [6] B. H~iuser, B.B.G. Klopman, G.J. Gerritsma, J. Gao and H. Rogalla, Appl. Phys. Lett. 54 (1989) 1368. [7] K. Kishio, J. Shimojima, T. Hasegawa, K. Kitazawa and K. Fueki, Jpn. J. Appl. Phys. 26 (1987) L1228. [ 8 ] R. Bormann and J. NSlting, Appl. Phys. Lett. 54 (1989) 2148. [9] Z. Jir~ik, J. Hejtm~inek, E. Poltert, A. T[iska and P. Va~ek, Physica C 156 (1988) 750.