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Observation of the superconducting energy gap in a semiconductor (GeTe)
Volume 23, number 3
PHYSICS LETTERS
The author w i s h e s to thank R. R. H a r g r o v e f o r p r e p a r i n g the diodes studied, and M. M. Klenck f o r p e r f o r m i n g the X - r a y a n a l y s e s . References
1. N.V.Zavaritskii, Zh.Eksp. i Teor. Fiz. (USSR) 48, (1965) 837; translation: Soviet Phys. JETP 21 (1965) 557
17 October 1966
2. W.J.Tomasch, Phys. Rev. Letters 15 (1965) 672. 3. W.J.Tomasch, Phys. Rev. Letters 16 (1966) 16. 4. W.J. Tomasch and T. Wolfram, Phys. Rev. Letters 16 (1966) 352. 5. W.L. McMillan and P. W. Anderson, Phys. Rev. Letters 16 (1966) 85. 6. T . E . F a b e r and A.B.Pippard, Proc.Roy.Soc.A231 (1955) 336. 7. V.L.Ginzburg, Physics Letters 13 (1964) 101.
OBSERVATION OF THE SUPERCONDUCTING ENERGY GAP IN A SEMICONDUCTOR (GeTe) P. J. STILES and L. ESAKI* IBM Watson Research Center, Yorktown Heights, New York
J. F. SCHOOLEY ** National Bureau of Standards, Washington, D . C .
Received 5 September 1966
A report of the first observation of the superconducting energy gap in a semiconductor, GeTe, is given. The gap was observed in a tunnel junction of A1-A1203-GeTe.
We wish to r e p o r t what we b e l i e v e to be the f i r s t o b s e r v a t i o n of the s u p e r c o n d u c t i n g e n e r g y gap in a s e m i c o n d u c t o r . The s u p e r c o n d u c t i n g e n e r g y gap of GeTe has been m e a s u r e d . GeTe has been shown to be a s e m i c o n d u c t o r r e c e n t l y by a study of the tunneling c u r r e n t in a A 1 - A 1 2 0 3- GeTe junction [1]. We have p r e p a r e d t h e s e tunnel junctions by s t a n d a r d e v a p o r a t i o n t e c h n iq u e s , obtaining a p o l y c r y s t a l l i n e f i l m of GeTe about 4000 ~ thick e v a p o r a t e d on a t h e r m a l l y grown oxide on an A1 f i l m . T h e s e tunneling junctions show the e x p e c t e d n e g a t i v e r e s i s t a n c e r e g i o n c o r r e s p o n d i n g to the s e m i c o n d u c t i n g gap at high v o l t a g e s (10 -1 to 100 volt). Fig. 1 is a plot of the conductance (df/dV) as a function of v o l t a g e of one of t h e s e j u n c t io n s at t e m p e r a t u r e s r a n g i n g f r o m 2.50OK to 0.085OK. The j unct i o n s w e r e cooled in an a d i a b a t i c d e m a g n e t i z a t i o n a p p a r a t u s which has been d e s c r i b e d p r e v i o u s l y [2]. By following the s t r u c t u r e which d e v e l o p s s y m m e t r i c a l l y around V = 0 as the tern* Sponsored in part by the Army Research Office, Durham, N.C., under Contract DA-31-124-ARO-D-478. ** Sponsored in part by the Advanced Research Projects Agency. 206
p e r a t u r e is l o w e r e d , one can f i r s t s e e the b r o a d t r a n s i t i o n of the A1 f i l m develop. At t e m p e r a t u r e s below 0.5OK, the additional s t r u c t u r e c o r r e s p o n d i n g to the gap of the s u p e r c o n d u c t i n g GeTe can be seen. The s e p a r a t i o n of the two o u t si d e peaks in ~ / d V is a p p r o x i m a t e l y equal to the su m of the e n e r g y gaps, while the s e p a r a t i o n of the two inside peaks is a p p r o x i m a t e l y equal to the d i f f e r e n c e of the gaps. F r o m this m e a s u r e ment, the s u p e r c o n d u c t i n g gap of the A1 s t r i p is found to be 820 ~V and with a m e a s u r e d T c of 3.1OK ( m e a s u r e d r e s i s t i v e l y ) one obtains a r a t i o of E g to k T c of 3.1. F o r the GeTe f i l m the s u p e r conducting gap i s m e a s u r e d to be 150 ~V. B e c a u s e of p r o b l e m s which we had on this p a r t i c u l a r f i l m with high r e s i s t a n c e co n t act s, we a r e unable to m e a s u r e T c, On o t h e r f i l m s of a l o w e r hole d en si t y with wtdch we did not have c o n t a c t p r o b l e m s , T c was found to be c o m p a r a b l e with that f o r bulk GeTe [3]. The hole d en si t y of this f i l m , N = 0 . 6 / R 7 7 e , is about 1.2 × 1021/cm3. F r o m the data of Hein et al. [3] on bulk GeTe, we would e x p e c t a T c of about 0.4OK f o r this f i l m . If this w e r e the c a s e , then the r a t i o of the e n e r gy gap to k T c would be about 4~, which is not unreasonable.
Volume 23, number 3
PHYSICS LETTERS
, (oK)
O. O85
0.089 O. 124 0.250 0.31
0.36 0.46 0.67 0.84 0.97
17 October 1966
When a m a g n e t i c f i el d of 20 o e r s t e d was applied p e r p e n d i c u l a r to the plane of the junction, the four p eak s c o a l e s c e d to two whose s e p a r a tion in v o l t ag e c o r r e s p o n d to the e n e r g y gap of the A1 f i l m , i n d i c a t i n g that the GeTe was no l o n g e r superconducting. When the f i el d was inc r e a s e d f u r t h e r , the s t r u c t u r e b e c a m e l e s s s h a r p as if the t e m p e r a t u r e w e r e r a i s e d . At 1500 o e r s t e d the c u r v e had a s i m i l a r shape to the one o b s e r v e d at 1.5OK. T h e s e r e s u l t s s e e m to indicate that it will be quite s t r a i g h t f o r w a r d to m e a s u r e the e n e r g y gap f o r the s e m i c o n d u c t o r s GeTe and SnTe f o r diff e r e n t c a r r i e r d e n s i t i e s and t h e r e f o r e d i f f e r e n t Te'S as a function of t e m p e r a t u r e and m a g n e t i c field. We intend to do this in the n e a r f u t u r e as w e l l as to u s e a co u n t er e l e c t r o d e such as Mg which is a n o r m a l m e t a l to p r e v e n t the s m e a r i n g which o c c u r s when t h e r e is a v a r i a t i o n of t r a n sition t e m p e r a t u r e in the counter e l e c t r o d e (as is probably the c a s e f o r o u r A1 f i l m s) . It a l s o a p p e a r s f e a s i b l e to a t t e m p t to m e a s u r e o t h er s u p e r c o n d u c t i n g s e m i c o n d u c t o r s of which polyc r y s t a l l i n e f i l m s m a y be obtained by e v a p o r a tion. We wish to thank M. S, C h r i s t i e and J. H. Cumm i n g s f o r t e c h n i c a l a s s i s t a n c e and E. R. P f e i f f e r f o r help in c a r r y i n g out the e x p e r i m e n t s .
1.27 1.47
1.80 t97
2.5O
References Fig. 1 shows the conductance, dI/dV, of an A1-A120 3GeTe tunneling junction as a function of voltage at different temperatures. At temperatures above 0.5OK the structure is entirely due to the superconducting energy gap in the A1 film. At temperatures below 0.5OK, additional structure is observed which is due to the superconducting energy gap of GeTe.
1. p . J . S t i l e s , W.E.Howard Jr . and L.Esaki, 10th International Conference on Low Temperature Physics, Moscow, 1966 {to be published}. 2. J.F.Schooley, W.R.Hosler, E.Ambler and J.H. Becker, Phys. Rev. Letters 14 {1965) 305. 3. R.A.Hein, g.W. Gibson, R.Mazelsky, R.O.Mfller and J.K.Hulm, Phys.Rev. Letters 12 {1964) 320.
207
PHYSICS LETTERS
The author w i s h e s to thank R. R. H a r g r o v e f o r p r e p a r i n g the diodes studied, and M. M. Klenck f o r p e r f o r m i n g the X - r a y a n a l y s e s . References
1. N.V.Zavaritskii, Zh.Eksp. i Teor. Fiz. (USSR) 48, (1965) 837; translation: Soviet Phys. JETP 21 (1965) 557
17 October 1966
2. W.J.Tomasch, Phys. Rev. Letters 15 (1965) 672. 3. W.J.Tomasch, Phys. Rev. Letters 16 (1966) 16. 4. W.J. Tomasch and T. Wolfram, Phys. Rev. Letters 16 (1966) 352. 5. W.L. McMillan and P. W. Anderson, Phys. Rev. Letters 16 (1966) 85. 6. T . E . F a b e r and A.B.Pippard, Proc.Roy.Soc.A231 (1955) 336. 7. V.L.Ginzburg, Physics Letters 13 (1964) 101.
OBSERVATION OF THE SUPERCONDUCTING ENERGY GAP IN A SEMICONDUCTOR (GeTe) P. J. STILES and L. ESAKI* IBM Watson Research Center, Yorktown Heights, New York
J. F. SCHOOLEY ** National Bureau of Standards, Washington, D . C .
Received 5 September 1966
A report of the first observation of the superconducting energy gap in a semiconductor, GeTe, is given. The gap was observed in a tunnel junction of A1-A1203-GeTe.
We wish to r e p o r t what we b e l i e v e to be the f i r s t o b s e r v a t i o n of the s u p e r c o n d u c t i n g e n e r g y gap in a s e m i c o n d u c t o r . The s u p e r c o n d u c t i n g e n e r g y gap of GeTe has been m e a s u r e d . GeTe has been shown to be a s e m i c o n d u c t o r r e c e n t l y by a study of the tunneling c u r r e n t in a A 1 - A 1 2 0 3- GeTe junction [1]. We have p r e p a r e d t h e s e tunnel junctions by s t a n d a r d e v a p o r a t i o n t e c h n iq u e s , obtaining a p o l y c r y s t a l l i n e f i l m of GeTe about 4000 ~ thick e v a p o r a t e d on a t h e r m a l l y grown oxide on an A1 f i l m . T h e s e tunneling junctions show the e x p e c t e d n e g a t i v e r e s i s t a n c e r e g i o n c o r r e s p o n d i n g to the s e m i c o n d u c t i n g gap at high v o l t a g e s (10 -1 to 100 volt). Fig. 1 is a plot of the conductance (df/dV) as a function of v o l t a g e of one of t h e s e j u n c t io n s at t e m p e r a t u r e s r a n g i n g f r o m 2.50OK to 0.085OK. The j unct i o n s w e r e cooled in an a d i a b a t i c d e m a g n e t i z a t i o n a p p a r a t u s which has been d e s c r i b e d p r e v i o u s l y [2]. By following the s t r u c t u r e which d e v e l o p s s y m m e t r i c a l l y around V = 0 as the tern* Sponsored in part by the Army Research Office, Durham, N.C., under Contract DA-31-124-ARO-D-478. ** Sponsored in part by the Advanced Research Projects Agency. 206
p e r a t u r e is l o w e r e d , one can f i r s t s e e the b r o a d t r a n s i t i o n of the A1 f i l m develop. At t e m p e r a t u r e s below 0.5OK, the additional s t r u c t u r e c o r r e s p o n d i n g to the gap of the s u p e r c o n d u c t i n g GeTe can be seen. The s e p a r a t i o n of the two o u t si d e peaks in ~ / d V is a p p r o x i m a t e l y equal to the su m of the e n e r g y gaps, while the s e p a r a t i o n of the two inside peaks is a p p r o x i m a t e l y equal to the d i f f e r e n c e of the gaps. F r o m this m e a s u r e ment, the s u p e r c o n d u c t i n g gap of the A1 s t r i p is found to be 820 ~V and with a m e a s u r e d T c of 3.1OK ( m e a s u r e d r e s i s t i v e l y ) one obtains a r a t i o of E g to k T c of 3.1. F o r the GeTe f i l m the s u p e r conducting gap i s m e a s u r e d to be 150 ~V. B e c a u s e of p r o b l e m s which we had on this p a r t i c u l a r f i l m with high r e s i s t a n c e co n t act s, we a r e unable to m e a s u r e T c, On o t h e r f i l m s of a l o w e r hole d en si t y with wtdch we did not have c o n t a c t p r o b l e m s , T c was found to be c o m p a r a b l e with that f o r bulk GeTe [3]. The hole d en si t y of this f i l m , N = 0 . 6 / R 7 7 e , is about 1.2 × 1021/cm3. F r o m the data of Hein et al. [3] on bulk GeTe, we would e x p e c t a T c of about 0.4OK f o r this f i l m . If this w e r e the c a s e , then the r a t i o of the e n e r gy gap to k T c would be about 4~, which is not unreasonable.
Volume 23, number 3
PHYSICS LETTERS
, (oK)
O. O85
0.089 O. 124 0.250 0.31
0.36 0.46 0.67 0.84 0.97
17 October 1966
When a m a g n e t i c f i el d of 20 o e r s t e d was applied p e r p e n d i c u l a r to the plane of the junction, the four p eak s c o a l e s c e d to two whose s e p a r a tion in v o l t ag e c o r r e s p o n d to the e n e r g y gap of the A1 f i l m , i n d i c a t i n g that the GeTe was no l o n g e r superconducting. When the f i el d was inc r e a s e d f u r t h e r , the s t r u c t u r e b e c a m e l e s s s h a r p as if the t e m p e r a t u r e w e r e r a i s e d . At 1500 o e r s t e d the c u r v e had a s i m i l a r shape to the one o b s e r v e d at 1.5OK. T h e s e r e s u l t s s e e m to indicate that it will be quite s t r a i g h t f o r w a r d to m e a s u r e the e n e r g y gap f o r the s e m i c o n d u c t o r s GeTe and SnTe f o r diff e r e n t c a r r i e r d e n s i t i e s and t h e r e f o r e d i f f e r e n t Te'S as a function of t e m p e r a t u r e and m a g n e t i c field. We intend to do this in the n e a r f u t u r e as w e l l as to u s e a co u n t er e l e c t r o d e such as Mg which is a n o r m a l m e t a l to p r e v e n t the s m e a r i n g which o c c u r s when t h e r e is a v a r i a t i o n of t r a n sition t e m p e r a t u r e in the counter e l e c t r o d e (as is probably the c a s e f o r o u r A1 f i l m s) . It a l s o a p p e a r s f e a s i b l e to a t t e m p t to m e a s u r e o t h er s u p e r c o n d u c t i n g s e m i c o n d u c t o r s of which polyc r y s t a l l i n e f i l m s m a y be obtained by e v a p o r a tion. We wish to thank M. S, C h r i s t i e and J. H. Cumm i n g s f o r t e c h n i c a l a s s i s t a n c e and E. R. P f e i f f e r f o r help in c a r r y i n g out the e x p e r i m e n t s .
1.27 1.47
1.80 t97
2.5O
References Fig. 1 shows the conductance, dI/dV, of an A1-A120 3GeTe tunneling junction as a function of voltage at different temperatures. At temperatures above 0.5OK the structure is entirely due to the superconducting energy gap in the A1 film. At temperatures below 0.5OK, additional structure is observed which is due to the superconducting energy gap of GeTe.
1. p . J . S t i l e s , W.E.Howard Jr . and L.Esaki, 10th International Conference on Low Temperature Physics, Moscow, 1966 {to be published}. 2. J.F.Schooley, W.R.Hosler, E.Ambler and J.H. Becker, Phys. Rev. Letters 14 {1965) 305. 3. R.A.Hein, g.W. Gibson, R.Mazelsky, R.O.Mfller and J.K.Hulm, Phys.Rev. Letters 12 {1964) 320.
207