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Nb3In and Mo3Sn thin films with A-15 type structure
Mat. Res. Bull. Vol. 13, pp. 469-472, 1978. P e r g a m o n P r e s s , Inc. Printed in the United States.
Nb31nAND Mo3Sn THIN FILMS WITH A-15 TYPE STRUCTURE
Y. T a r u t a n i and U. Kawabe C e n t r a l Research L a b o r a t o r y , H i t a c h i L t d . , Kokubunji, Tokyo, Japan
(Received March 7, 1978; Communicated by B. T. Mattbias)
ABSTRACT Nb3In and Mo3Sn thin films are prepared by a co-evaporation technique. Both thin films are synthesized in the A-15 type crystal structure.
Introduction There i s no i n t e r m e t a l l i c compound phase in the e q u i l i b r i u m phase diagram of Nb-In and Mo-Sn systems. These systems were, however, proved to be s y n t h e s i z e d in A-15 type phase under h i g h p r e s s u r e by Banus (1) and Killpatrick (2,3). These A-15 type compounds are s t a b l e or m e t a s t a b l e a t room t e m p e r a t u r e under the a t m o s p h e r i c p r e s s u r e . On the o t h e r hand, a v a p o r - p h a s e - q u e n c h i n g method, such as s p u t t e r i n g , chemical vapor d e p o s i t i o n or e v a p o r a t i o n , is u s e f u l f o r r e a l i z i n g a low t e m p e r a t u r e phase. Regarding t h e A-15 type s t r u c t u r e , W3Re (4), Mo3Re (5), V3A1 (6), and Nb3Si (7) have been s y n t h e s i z e d in t h e A-15 type phase o n l y by u s i n g t h e s e v a p o r - p h a s e - q u e n c h i n g methods. In t h e same way, the s y n t h e s i s of Nb-In and Mo-Sn systems i n t h e A-15 t y p e phase seems p o s s i b l e by the v a p o r - p h a s e - q u e n c h i n g method.
469
470
Y. TARUTANI, et al.
Vot. 13, No. 5
Results
The N b - I n and Mo-Sn s y s t e m s were p r e p a r e d by a c o - e v a p o r a t i o n t e c h n i q u e . Component e l e m e n t s were e v a p o r a t e d from s e p a r a t e s o u r c e s . Evaporation rates were m o n i t o r e d and c o n t r o l l e d f o r s p e c i m e n s t o have a u n i f o r m c o m p o s i t i o n distribution in the f i l m t h i c k n e s s d i r e c t i o n . Details of the preparation method have p r e v i o u s l y been r e p o r t e d ( 8 ) . The f i l m d e p o s i t i o n r a t e was a b o u t 10 ~ / s . The s u b s t r a t e m a t e r i a l was s a p p h i r e . R e s i d u a l gas p r e s s u r e was 1 × 10 - 5 T e r r d u r i n g e v a p o r a t i o n . Specimens o f I n o r Sn c o n c e n t r a t i o n s from 20 t o 30 a t . % were p r e p a r e d f o r e a c h d e p o s i t i o n . The p r e p a r e d N b - I n t h i n f i l m s had A-15 t y p e p h a s e a t s u b s t r a t e t e m p e r a t u r e s l o w e r t h a n 8OO°C. At t h e s u b s t r a t e t e m p e r a t u r e o f 9OO°C, t h e A-I~ t y p e p h a s e was n o t f o r m e d . This may be due t o t h e r e - e v a p o r a t i o n o f I n . The l a t t i c e p a r a m e t e r s o f t h e A-15 t y p e compound a r e shown i n F i g . 1. The l a t t i c e p a r a m e t e r s d e c r e a s e as t h e s u b s t r a t e t e m p e r a t u r e i n c r e a s e s . The a t o m i c r a d i u s o f I n i s l a r g e r t h a n t h a t o f Nb i n t h e A - I ~ t y p e s t r u c t u r e (9). T h e r e f o r e t h e e n l a r g e m e n t o f t h e l a t t i c e p a r a m e t e r i s a c c o m p a n i e d by an i n c r e a s e i n I n c o n c e n t r a t i o n . The r e a s o n f o r l a t t i c e p a r a m e t e r v a r i a t i o n is not clear.
535.
. . . . . . . . . . . . . . . . . . A-1 5
10
r~'=-" b . c . c . -
A
A
V
~
5.30
o, o I--
5.25 400
' 600 Substrate
' 800 Temperature FIG.
0 1000
(°c)
I
Lattice parameter and transition temperature as a function of substrate temperature for Nb-ln thin films. The full circles indicate lattice parameter. The vertical bars indicate transition width and empty circles indicate the temperature where specimen resistance falls to one half of its normal value.
Vol. 13, No. 5
Nb31n AND MosSn
471
The lattice parameter was almost constant for the different film compositions although specimens were prepared in the composition ranging from 20 to 30 at.% In for each substrate temperature. The superconducting transition temperature of Nb-In thin films were measured by the four probe potential method. The results are shown in Fig. 1. The transition temperature were about 7 K and they hardly depended on the substrate temperature in spite of lattice parameter changes. The A-15 type structure of Mo-Sn thin films was formed at substrate temperatures lower than 6OO°C. The A-15 type phase was not formed but both b.c.c. Mo and ~ - S n structures appeared, at substrate temperatures higher than 7OO°C. The lattice parameter of the A-15 type structure was constant either for different film compositions or for different substrate temperatures. The lattice parameters were within a range of 5.075 to 5.080 ~. The transition temperatures of Mo-Sn thin films were lower than 4.2 K. According to Killpatrick's pressure-temperature phase diagram of the Mo-Sn and Nb-In systems, the boundary line of the pressure for A-15 type phase formation has a positive inclination with respect to temperature. The A-15 type phase of the Mo-Sn system is stable at temperatures lower than 300°C, as seen from extrapolating the b.c.c, to A-15 boundary line to the atmospheric pressure. In the same manner, the A-15 type phase of the Nb-In system is stable at lower than room temperature. However, the A-15 type phase of Mo-Sn and Nb-In thin films have been synthesized at substrate temperatures even higher than the stable temperature regions. This discrepancy between the stable temperature and the formation temperature regions of A-15 type phases may be explained as follows. If deposited elements are uniformly mixed on the film surface, then atoms need to diffuse an interatomic distance to form the A-15 type structure. However the atom diffusion of the grain size order is necessary for separation into two phases. If the substrate temperature is sufficiently low that atoms cannot diffuse a long range distance, the metastable A-15 type phase may be formed even if the A-15 type phase has higher free energy than the two phase stat S . The grain size of the Mo-Sn thin film deposited at 6OO°C was about 200 A from the electron transmission observation. The diffusion and rearrangement of Mo and Sn atoms to a distance of about 200 ~ are necessary for the formation of b.c.c. Mo and 6 - S n phases. For the same reason, the Nb-Bi system, whose A-15 type phase is not stable even at absolute zero in the pressure-temperature phase diagram (2), may be synthesized in the A-15 type phase by the vapor-phase-quenching method.
Acknowledgement The authors would like to thank Dr. K. Sato for his encouragement of this work.
472
Y. T A R U T A N L
et ~.
Vol. 13, No. 5
References I.
M. D. Banus, T. B. Reed, H. C. Gatos, M. C. Lavine and J. A. Kafalas, J. Phys. Chem. Solids, 23, 971 (1962).
2.
D. H. Killpatrick, J. Phys. Chem. Solids, 25, 1213 (1964).
3.
D. H. Killpatrick9 J. Phys. Chem. Solids, 25, 1499 (1954).
4.
J. H. Federer and R. M. Steele, Nature, 205 , 587 (1965).
5.
J. R. Gavaler, M. A. Janocko and C. K. Jones, Appl. Phys. Left., 21, 179 (1972).
6.
L. D. Hartsough and R. H. Hammond, Solid State Comm., ~, 885 (1971).
7.
R. H. Hammond, IEEE Trans. Mag., MAG-I!, 201 (1975).
8.
Y. Tarutani and M. Kudo, Jpn. J. Appl. Phys., 16, 509 (1977).
9.
Z. Tarutani and M. Kudo, J. Less Common Met., 5_~5, 221 (1977).
Nb31nAND Mo3Sn THIN FILMS WITH A-15 TYPE STRUCTURE
Y. T a r u t a n i and U. Kawabe C e n t r a l Research L a b o r a t o r y , H i t a c h i L t d . , Kokubunji, Tokyo, Japan
(Received March 7, 1978; Communicated by B. T. Mattbias)
ABSTRACT Nb3In and Mo3Sn thin films are prepared by a co-evaporation technique. Both thin films are synthesized in the A-15 type crystal structure.
Introduction There i s no i n t e r m e t a l l i c compound phase in the e q u i l i b r i u m phase diagram of Nb-In and Mo-Sn systems. These systems were, however, proved to be s y n t h e s i z e d in A-15 type phase under h i g h p r e s s u r e by Banus (1) and Killpatrick (2,3). These A-15 type compounds are s t a b l e or m e t a s t a b l e a t room t e m p e r a t u r e under the a t m o s p h e r i c p r e s s u r e . On the o t h e r hand, a v a p o r - p h a s e - q u e n c h i n g method, such as s p u t t e r i n g , chemical vapor d e p o s i t i o n or e v a p o r a t i o n , is u s e f u l f o r r e a l i z i n g a low t e m p e r a t u r e phase. Regarding t h e A-15 type s t r u c t u r e , W3Re (4), Mo3Re (5), V3A1 (6), and Nb3Si (7) have been s y n t h e s i z e d in t h e A-15 type phase o n l y by u s i n g t h e s e v a p o r - p h a s e - q u e n c h i n g methods. In t h e same way, the s y n t h e s i s of Nb-In and Mo-Sn systems i n t h e A-15 t y p e phase seems p o s s i b l e by the v a p o r - p h a s e - q u e n c h i n g method.
469
470
Y. TARUTANI, et al.
Vot. 13, No. 5
Results
The N b - I n and Mo-Sn s y s t e m s were p r e p a r e d by a c o - e v a p o r a t i o n t e c h n i q u e . Component e l e m e n t s were e v a p o r a t e d from s e p a r a t e s o u r c e s . Evaporation rates were m o n i t o r e d and c o n t r o l l e d f o r s p e c i m e n s t o have a u n i f o r m c o m p o s i t i o n distribution in the f i l m t h i c k n e s s d i r e c t i o n . Details of the preparation method have p r e v i o u s l y been r e p o r t e d ( 8 ) . The f i l m d e p o s i t i o n r a t e was a b o u t 10 ~ / s . The s u b s t r a t e m a t e r i a l was s a p p h i r e . R e s i d u a l gas p r e s s u r e was 1 × 10 - 5 T e r r d u r i n g e v a p o r a t i o n . Specimens o f I n o r Sn c o n c e n t r a t i o n s from 20 t o 30 a t . % were p r e p a r e d f o r e a c h d e p o s i t i o n . The p r e p a r e d N b - I n t h i n f i l m s had A-15 t y p e p h a s e a t s u b s t r a t e t e m p e r a t u r e s l o w e r t h a n 8OO°C. At t h e s u b s t r a t e t e m p e r a t u r e o f 9OO°C, t h e A-I~ t y p e p h a s e was n o t f o r m e d . This may be due t o t h e r e - e v a p o r a t i o n o f I n . The l a t t i c e p a r a m e t e r s o f t h e A-15 t y p e compound a r e shown i n F i g . 1. The l a t t i c e p a r a m e t e r s d e c r e a s e as t h e s u b s t r a t e t e m p e r a t u r e i n c r e a s e s . The a t o m i c r a d i u s o f I n i s l a r g e r t h a n t h a t o f Nb i n t h e A - I ~ t y p e s t r u c t u r e (9). T h e r e f o r e t h e e n l a r g e m e n t o f t h e l a t t i c e p a r a m e t e r i s a c c o m p a n i e d by an i n c r e a s e i n I n c o n c e n t r a t i o n . The r e a s o n f o r l a t t i c e p a r a m e t e r v a r i a t i o n is not clear.
535.
. . . . . . . . . . . . . . . . . . A-1 5
10
r~'=-" b . c . c . -
A
A
V
~
5.30
o, o I--
5.25 400
' 600 Substrate
' 800 Temperature FIG.
0 1000
(°c)
I
Lattice parameter and transition temperature as a function of substrate temperature for Nb-ln thin films. The full circles indicate lattice parameter. The vertical bars indicate transition width and empty circles indicate the temperature where specimen resistance falls to one half of its normal value.
Vol. 13, No. 5
Nb31n AND MosSn
471
The lattice parameter was almost constant for the different film compositions although specimens were prepared in the composition ranging from 20 to 30 at.% In for each substrate temperature. The superconducting transition temperature of Nb-In thin films were measured by the four probe potential method. The results are shown in Fig. 1. The transition temperature were about 7 K and they hardly depended on the substrate temperature in spite of lattice parameter changes. The A-15 type structure of Mo-Sn thin films was formed at substrate temperatures lower than 6OO°C. The A-15 type phase was not formed but both b.c.c. Mo and ~ - S n structures appeared, at substrate temperatures higher than 7OO°C. The lattice parameter of the A-15 type structure was constant either for different film compositions or for different substrate temperatures. The lattice parameters were within a range of 5.075 to 5.080 ~. The transition temperatures of Mo-Sn thin films were lower than 4.2 K. According to Killpatrick's pressure-temperature phase diagram of the Mo-Sn and Nb-In systems, the boundary line of the pressure for A-15 type phase formation has a positive inclination with respect to temperature. The A-15 type phase of the Mo-Sn system is stable at temperatures lower than 300°C, as seen from extrapolating the b.c.c, to A-15 boundary line to the atmospheric pressure. In the same manner, the A-15 type phase of the Nb-In system is stable at lower than room temperature. However, the A-15 type phase of Mo-Sn and Nb-In thin films have been synthesized at substrate temperatures even higher than the stable temperature regions. This discrepancy between the stable temperature and the formation temperature regions of A-15 type phases may be explained as follows. If deposited elements are uniformly mixed on the film surface, then atoms need to diffuse an interatomic distance to form the A-15 type structure. However the atom diffusion of the grain size order is necessary for separation into two phases. If the substrate temperature is sufficiently low that atoms cannot diffuse a long range distance, the metastable A-15 type phase may be formed even if the A-15 type phase has higher free energy than the two phase stat S . The grain size of the Mo-Sn thin film deposited at 6OO°C was about 200 A from the electron transmission observation. The diffusion and rearrangement of Mo and Sn atoms to a distance of about 200 ~ are necessary for the formation of b.c.c. Mo and 6 - S n phases. For the same reason, the Nb-Bi system, whose A-15 type phase is not stable even at absolute zero in the pressure-temperature phase diagram (2), may be synthesized in the A-15 type phase by the vapor-phase-quenching method.
Acknowledgement The authors would like to thank Dr. K. Sato for his encouragement of this work.
472
Y. T A R U T A N L
et ~.
Vol. 13, No. 5
References I.
M. D. Banus, T. B. Reed, H. C. Gatos, M. C. Lavine and J. A. Kafalas, J. Phys. Chem. Solids, 23, 971 (1962).
2.
D. H. Killpatrick, J. Phys. Chem. Solids, 25, 1213 (1964).
3.
D. H. Killpatrick9 J. Phys. Chem. Solids, 25, 1499 (1954).
4.
J. H. Federer and R. M. Steele, Nature, 205 , 587 (1965).
5.
J. R. Gavaler, M. A. Janocko and C. K. Jones, Appl. Phys. Left., 21, 179 (1972).
6.
L. D. Hartsough and R. H. Hammond, Solid State Comm., ~, 885 (1971).
7.
R. H. Hammond, IEEE Trans. Mag., MAG-I!, 201 (1975).
8.
Y. Tarutani and M. Kudo, Jpn. J. Appl. Phys., 16, 509 (1977).
9.
Z. Tarutani and M. Kudo, J. Less Common Met., 5_~5, 221 (1977).