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STABILIZATION OF HIGH-Tc Nb3Ge
J. Gavaler, M. Ashkin, A. Braginski, A. Santhanam
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C6, suppliment uu no 8, Tome 39, uotit 1978, page
C6-400
STABILIZATION
OF
HIGH-Tc Nb3Ge*
J.R. Gavaler, M. Ashkin
,
A.I. Braginski and A.T. SanthanamWestinghouse Research
and
DeveZopment Center, Pittsburgh, PennsyZvama , 1 5 2 3 5 , U.S. A.~6sumd.- Les r6sultats d'analyses physiques nous font croire que la croissance de Nb3Ge stoechi- odtrique se fait p ~ r homodpitaxie sur une phase A15 de Nb-Ge qui se forme prZs de l'interface avec le substrat et a une constante de rdseau augment6e grzce 2 la prdsence des impuretds.
Abstract.- From analytical results we hypothesize that stoichiometric Nb3Ge is formed through a homo- epitaxial process form an A15 Nb-Ge phase having an impurity expanded lattice formed near the inter- face.
INTRODUCTION.- High-Tc Nb3Ge (Tc % 22 K) can now be
formed by low-energy sputtering
111
chemical vapor deposition (CVD) 121 and coevaporation /3/. These high-T films which crystallize in the A15 struc- ture are close to or have the ideal 311 composition,0
and have a lattice parameter Q, 5.14 A. Substantial
evidence indicates that impurities enter in a bene- ficial way in forming high-Tc Nb,Ge 14-61. However, with no deliberate introduction of impurities, epi-
taxial growth on Nb31r also produced high-Tc Nb Ge films 171. Thus the role of impurities in forming high-T Nb,Ge remains unresolved.
To try to resolve this question we have been investigating the initially nucleated material in high-Tc sputtered and CVD Nb-Ge films. Based on these results we present a hypothesis which we be- lieve explains the importance of impurities to the .'ormation of the high-T Nb Ge phase.
C 3
EXPERIMENTAL PROCEDURE.- The Nb-Ge films for this study wer'made by sputtering and CVD according to methods previously described /1,2/. Substrates used were sapphire, Hastelloy B, tantalum and vanadium. In some cases, the tantalum and vanadium substrates were first oxidized by heating in air for several minutes at 600'~. Superconducting T 's of the films were determined by a standard four-point re- sistance technique using germanium thermometry. Composition analysa were done by Auger and Secon- dary Ion Mass Spectroscopy (SINS). Structural ana-
lyses was by transmission electron microscopy (TEM)
.
RESULTS.- We have found that in sputtered Nb Ge,*
Supported in part by AFOSR Contract No. F49620- 78-C-003 1.0
none of the films of 1000 A or less had the maximum critical temperatures of between 22 and 23
K
measu- red in thicker films. Tarutani and Kudo I81 have reported similar results for their coevaporatedNb Ge films. Chemical analysis of sputtered film has previously shown the presence of oxygen throu- ghout these films with the concentration near the interface being very high (of the order of one oxy- gen atom per Nb Ge unit) and thereafter decreasing with increasing distance from the interface down to a constant level of about 1 at. % in the body of the film. Analyses of CVD films on the other hand showed that there was no oxygen in the body of these films at least down to the 0.1 at. % detection limit of the Auger apparatus used for the analysis. However the analysis did show a sharp oxygen peak at the interface similar to that observed in the sputtered films. To try to determine the signifi- cance of this oxygen peak, a series of Nb Ge films were sputtered onto alumina, tantalum, and vanadium substrates under similar conditions. Since tantalum and vanadium both have a strong affinity to oxygen, it was reasoned that any oxygen that would be pre- sent on these substrates would tend to react with the metal therefore decreasing the possibility of its being incorporated into the initially nucleated films. The results of these experiments showed that whereas the Nb-Ge film deposited onto the alumina
substrates had typically high-Tc onsets of % 22 K,
and widths ATcls of % 1 K, the films on
V
and Tahad onsets of only about 13 K and ATcls of 3 to 4 K. The deposition experiments were then repeated using vanadium and tantalum substrates which were first oxidized. With these substrates high-Tcls could be obtained similar to those on alumina. An oxygen
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19786179
p r o f i l e from SIMS f o r a high-Tc (% 21.5 K) f i l m d e p o s i t e d on an o x i d i z e d tantalum s u b s t r a t e is s i m i l a r t o a l l of t h e o t h e r high-Tc f i l m s t h a t we have analysed, i . e . , t h e l e v e l i s h i g h e s t a t t h e i n t e r f a c e and then drops o f f i n t h e body of t h e f i l m . This type of p r o f i l e i s not observed i n t h e lower-T f i l m d e p o s i t e d on t h e tantalum s u b s t r a t e ;
i n s t e a d t h e oxygen i s low a t t h e i n t e r f a c e and in- c r e a s e s i n t h e body of t h e f i l m . These r e s u l t s a g a i n p o i n t toward t h e i n i t i a l l y n u c l e a t e d m a t e r i a l a s being c r u c i a l toward a c h i e v i n g high-Tc1s.
To i n v e s t i g a t e t h i s i n i t i a l l y n u c l e a t e d mate- r i a l t r a n s m i s s i o n e l e c t r o n a n a l y s e s were made on
0
t h r e e very t h i n (< 1000 A) s p u t t e r e d f i l m s on sap- p h i r e s u b s t r a t e s . These f i l m s had T o n s e t s of 14, 16, and 17 K which were t h e h i g h e s t v a l u e s obser- ved f o r t h e p a r t i c u l a r f i l m t h i c k n e s s e s . t h e ATc were % 2 K. The s t r u c t u r e of a r e p r e s e n t a t i v e f i l m 0 w i t h a Tc o n s e t of 14 K and thickness of % 700 A i s c h a r a c t e r i z e d by extremely f i n e g r a i n s with t h e g r a i n s b e i n g c l e a n w i t h no s u b s t r u c t u r e . From t h e s e l e c t e d a r e a d i f f r a c t i o n (SAD) p a t t e r n d-spacings were c a l c u l a t e d . Most of t h e observed r e f l e c t i o n s could be i d e n t i f i e d a s belonging t o t h e A15 Nb Ge
phase. An unexpectedly l a r g e l a t t i c e parameter, a
0
of 5.25 A was c a l c u l a t e d f o r t h i s phase. S i m i l a r l y l a r g e a 's were a l s o found f o r the o t h e r two f i l m s t u d i e d .
DISCUSSION.- From t h e s e o b s e r v a t i o n s , we propose t h e f o l l o w i n g h y p o t h e s i s : d u r i n g t h e i n i t i a l %
0
1000 A of growth a Nb Ge phase i s formed which i s a t o r c l o s e t o 311 s t o i c h i o m e t r y b u t which has a much l a r g e r l a t t i c e parameter than i s usual f o r
0
:toichiometric Nb Ge (% 5.25 A compared t o % 5.14
A). We b e l i e v e t h a t because of t h i s l a r g e l a t t i c e parameter, t h i s phase having 3/1 s t o i c h i o m e t r y i s s t a b l e and c o n t i n u e s t o grow w i t h o u t decomposing i n t o a two-phase mixture a s would o t h e r w i s e occur. This concept i s c o n s i s t e n t w i t h o t h e r d a t a on A15 s t r u c t u r e superconductors having t h e g e n e r a l f o r - mula Nb X(where X = Sn, A l , Ga, o r Ge) wherein t h e A15 l a t t i c e becomes p r o g r e s s i v e l y more u n s t a b l e a s
t h e c e l l s i z e d e c r e a s e s . The boundary between a s t a b l e and u n s t a b l e l a t t i c e appears t o be when a
0
i s % 5.17 A. Continuing w i t h t h e hypothesis, we be-
l i e v e t h a t a s t h e f i l m growth proceeds by "homo- epitaxy", t h e A15 l a t t i c e g r a d u a l l y becomes s m a l l e r u n t i l i t a c h i e v e s a c e l l edge commensurate w i t h i t s
3/1 composition and w i t h t h e s i z e of t h e c o n s t i -
0
t u e n t Nb and Ge atoms, i . e . , a. %, 5.14 A. Because of i t s now small l a t t i c e parameter (small Nb-Nb d i s - t a n c e s ) i t s Tc reaches t h e very high value of %
22 t o 23 K. Accepting t h i s p i c t u r e one can now s e e t h a t t h e f u n c t i o n of t h e i m p u r i t i e s during t h e
0
f i r s t 1000 A of growth must b e , i n some way, t o ex- pand t h e l a t t i c e of t h e s t o i c h i o m e t r i c Nb Ge phase s u f f i c i e n t l y t o allow it t o be s t a b l e . One may spe- c u l a t e t h a t t h i s i s accomplishedby one of t h e f o l l o - wing ways : The presence of a l a r g e number of i n - t e r s t i t i a l i m p u r i t i e s i n t h e A15 l a t t i c e could pro- duce t h e n e c e s s a r y expansion, o r p o s s i b l y t h e pre- sence of i m p u r i t i e s d u r i n g n u c l e a t i o n could cause t h e formation of very f i n e A15 g r a i n s . I f t h e s e g r a i n s were s u f f i c i e n t l y s m a l l , t h e l a t t i c e could conceivably r e l a x s u f f i c i e n t l y t o produce t h e ne- c e s s a r y expansion. I n e i t h e r c a s e , a s t h e i m p u r i t y l e v e l d e c r e a s e s , t h e l a t t i c e g r a d u a l l y would be a l - lowed t o s h r i n k t o i t s normal dimensions. I n t h e c a s e where one u s e s an A15 s u b s t r a t e such a s Nb Ir,
3 t h e n e c e s s i t y of f i r s t forming a s t a b l e stoichiome- t r i c A15 l a y e r would no longer b e r e q u i r e d s i n c e t h e e p i t a x i a l growth p r o c e s s would commence immedia- t e l y on t h e o r i g i n a l s u b s t r a t e m a t e r i a l . Although a t p r e s e n t a l l of our r e s u l t s appear t o s u p p o r t t h i s h y p o t h e s i s a more complete a n a l y s i s of a s i g n i f i c a n t number of Nb Ge f i l m s w i l l be r e q u i r e d b e f o r e
3
one can make a f i n a l judgment r e g a r d i n g i t s v a l i d i - t y .
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