Optimum superconducting thin film tunneling devices applied to extremely high frequencie

Johnson, P. B.

English

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THEORETICAL AND EXPERIMENTAL INVESTIGATIONS OF 
OPTIMUM SUPERCONDUCTING THIN-FILM TUNNELING 
DEVICES AND THE APPLICATION TO EHF 
r ' b  
t 
F i n a l  Report 
June 1968 
" 
School of Engineering 
Old Dominion College 
Norfolk,  Virginia  23508 
u u  
Prepared under NASA gran t  No. NGR 47-003-009. 
Report Per iod:  1 February 1968 - 30 June 1968. 
P r i n c i p a l  I n v e s t i g a t o r :  P. B. Johnson, Assoc ia te  P r o f e s s o r ,  
Old Dominion College 
c 
https://ntrs.nasa.gov/search.jsp?R=19680019953 2020-03-12T09:48:37+00:00Z
ABSTRACT 
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Y 
Theore t ica l  I-V c h a r a c t e r i s t i c s  f o r  s ece ra l  superconductor ' junct ions 
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a r e  pred ic ted  with the  a id  of the  d i g i t a l  computer and a method is e s t ab l i shed  
which allows one t o  choose an optimum opera t ing  temperature. Tbo junc t ions ,  
Ta-TaZOs-Nb and Nb-NbzOs-V, were se lec ted  f o r  experimental work because 
each has  a pred ic ted  optimum operat ing temperature above t h a t  of b o i l i n g  
helium a t  atmospheric pressure,  
soon although it has  not y e t  been obtained due t o  f a b r i c a t i o n  d i f f i c u l t i e s .  
Recommendations f o r  f u t u r e  work are  out l ined .  
Experimental v e r i f i c a t i o n  i s  expected 
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' TABLE OF CONTENTS 
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  3 
Prediction of the Optimum Tunneling Junction . . . . . . . . . . . . .  4 
Experimental Results . . . . . . . . . . . . . . . . . . . . . . . .  .12 
Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . .  ..13 
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Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  .14 
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INTRODUCTION 
. The purpose of the work reported here i s  t d  extend the  inves t iga t ion  
of the p o s s i b i l i t y  of developing a p r a c t i c a l  amplifying device based on the  negat ive 
r e s i s t ance  property exhib i ted  by two superconducting metals separated by a 
t h i n  i n s u l a t i n g  f i lm.  Preliminary s tudies  have . indicated t h a t  such a device 
I 
is  s u i t a b l e  €or  operat ion a t  r f  and microwave frequencies .  m e  main 
advantage of an ampl i f ie r  of t h i s  nature i s  the  inherent  low noise  opera t ion  
made poss ib l e  by operat ing a t  temperatures c lose  t o  absolu te  zero.  For deep 
space communications and guidance systems low no i se  operat ion i s  of t he  utmost 
importance. 
Work i n  t h i s  a rea  was begun by the author during the  summer of- 1967 a t  
the Langley Research Center i n  Hampton, Virginia  under the  NASA-ASEE Summer 
Facul ty  Fellowship Program. The chief r e s u l t  of the  i n i t i a l  i nves t iga t ion  was 
the development of a d i g i t a l  computer program which predic ted  the  1-V 
c h a r a c t e r i s t i c s  f o r  a given superconducting junc t ion  based on the  BCS theory. 
Many combinations of superconducting metals were s tudied  and a few of the  most 
promising were recommended for. fu r the r  
cons ide ra t ion  were c e r t a i n  refinements 
between the physical  p r o p e r t i e s  of the 
through the junct ion.  
, 
study. Also recommended f o r  f u r t h e r  
i n  the computer program and a c o r r e l a t i o n  
junc t ion  and the  a c t u a l  cu r ren t  
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PREDICTION OF THE OPTIMUM TUNNELING JUNCTION 
I n  order  t o  p r e d i c t  the I - V  c h a r a c t e r i s t i c s  f o r  a superconducting I 
4 
junc t ion ,  the  following equat ion given by the  BCS theory had t o  be solved: 
Y 
where e i s  the e l e c t r o n i c  charge,  V is the  appl ied p o t e n t i a l ,  E i s  the  
e l e c t r o n  energy, p is the  r a t i o  of the  superconducting t o  the  normal dens i ty  
! 
of states given by 
and F is the  Fermi func t ion  given by 
1 
ZSexp (E /kT) 
F(E) = 
In  the  above equat ions e i s  one half  of the energy gap, k i s  Boltzmann's 
cons t an t ,  and T i s  the absolute  temperature. The gap energy v a r i e s  with 
temperature as shown i n  Fig.  1. 
a i d  of t he  d i g i t a l  computer f o r  d i f f e r e n t  combinations of superconductors. 
r e s u l t s  were checked aga ins t  experimentally ava i l ab le  d a t a  f o r  an Al-Al203-Pb 
junc t ion  and agreed very w e l l . '  The predic ted  curves a r e  shown i n  Fig. 2 
The above equat ions were evaluated with the 
The 
and the  reported experimental  curves i n  Fig.  3. Other types of metals s tudied 
were Al, I n ,  Nb, Pb, Sn, T a ,  and V. 
Based on the  computer s t u d i e s ,  two junc t ions  t h a t  looked p a r t i c u l a r l y  
promising were the  Ta-Ta205-Nb and the Nb-Nb2O5-V junc t ions .  The I - V  
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Nor mal ized temperature, I 
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gure 1.- Energy gap as a function of temperature. F 
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z 0 
oy 
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Voltage (mv) 
Figure 2 .  - Theoretical curves for  an Al-Al203-Pb structure.  For 
aluminum Tc= 1.2OK, and f o r  lead Tc= 7.19OK 
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L 
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--. . . .  
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1 2 3 
VOLTAGE (UV) 
-Experimentally obscrvcd current-voltage chnractcristics of 
mi AI-AI~03-PI~ siiiidwirh for dilTcrciit tcnipcratures. \Vhcn 
ricithcr of the riictnlr is siipcrcoiiductiiig (T>7.2%) the ehnrac- 
tcristic is liricnr, when oiily Pb is suprcoiiductivc (7.2OK> T 
> 1 . 2 O K )  thc characteristic is rioriliricnr arid, whet1 both metals 
arc sirprcoiidiictivc (T< 1.2'K), a iicgntive resistatice range also 
nppcnr3. 
' 
, 
Figure 3 - I - V  Characterist ics  f o r  an A l - A 1  0 -Pb sandwich. 
(Taken from Fiske and Giaever6.) 2 3  
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c h a r a c t e r i s t i c s  f o r  the  Ta-Ta205-Nb junction a re  shown i n  Fig.  4 and those f o r  
the Nb-Nb2O5-V junct ion i n  Fig.  5. Since both of these junc t ions  e x h i b i t  the 
negat ive r e s i s t ance  property above 4.2'K ( the  bo i l ing  po in t  of helium a t  
atmospheric pressure) ,  they would both be i d e a l  f o r  using i n  a reasonably low 
cos t  cryogenic system. 
The problem of determining t h e  optimum opera t ing  temperature should now 
be given some considerat ion.  A quant i ty  t h a t  p l a y s  an important r o l e  i n  t h i s  
dec is ion  i s  the  r a t i o  of the  peak cur ren t  a t  the onset  of the negat ive r e s i s t a n c e  
* 
. region t o  the va l l ey  cu r ren t  a t  the minimum poin t  i n  the negat ive r e s i s t a n c e  
U 
region. The computer was programmed t o  give these values  automatical ly  and a 
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p l o t  of the peak-to-valley cur ren t  r a t i o  i s  shown i n  Fig.  6 .  As seen from 
these  p l o t s ,  the peak-to-valley r a t i o  increases  with decreasing temperature. 
The impl ica t ion  here  i s  t h a t  one would s t i l l  be b e t t e r  of f  operat ing as c lose  
t o  abso lu t e  zero a s  poss ib ie .  However, another inspec t ion  of Figs .  4 and 5 
r evea l s  t h a t  a t  the  low temperature,  even though the peak-to-valley r a t i o  is  
l a r g e ,  t he re  i s  very l i t t l e  p r a c t i c a l  operat ing region. I n  order  t o  obta in  
a compromise between the  la rge  peak-to-valley r a t i o  and small operat ing cu r ren t  
a t  low temperatures and the s m a l l  peak-to-valley r a t i o  and la rge  operat ing 
c u r r e n t  a t  higher  temperatures,  the qua l i t y  f a c t o r  'l) i s  introduced '. One 
a r r i v e s  a t  the  value f o r  
average cu r ren t  i n  the negat ive r e s i s t ance  region,  i . e . ,  
7 by multiplying the  peak-to-valley r a t i o  by the  
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1 .o 
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0 
b 
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I a I 8 
1 .o 2.0 - 8  3.0 
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K 3 
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Mi I I ivol ts 
Figure4.6 Current-voltage characteristic of a Ta - Ta2 O5 - Nb junction. 
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1. 
m 
0 -
t 
I I .d 
I .o 
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0 I .o 2.0 
1.0' K 
3.0 
Mi I I ivol ts 
Figure 5.- Current-voltage characteristic of a Nb-Nb205 -V junction. 
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A- *0 IqN-qN 
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Q) 
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3 
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Q) 
i2 
.Q) 
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.. , 
3 
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e 
where Ip and I a r e  the peak and va l ley  c u r r e n t s ,  r e spec t ive ly .  
a l s o  shows a p l o t  of 7 as a funct ion of temperature f o r  both junc t ions .  The 
curves are observed t o  peak i n  the v i c i n i t y  of 4.4"K f o r  the  Ta-Ta205-Nb 
junc t ion  and 5.1"K f o r  the Nb-Nb2O5-V junc t ion . .  
Fig.  6 
V 
An important problem t h a t  s t i l l  remains i s  t h a t  of determining the a c t u a l  
cur ren t  which w i l l  flow based on the  physical  p r o p e r t i e s  of the  junc t ion  
(e .g . ,  the  thickness  and d i e l e c t r i c  constant of the  i n s u l a t i n g  l aye r  and the 
a rea  of the junct ion) .  This problem has been s tudied  i n  considerable  d e t a i l  
and i t  i s  hoped t h a t  a s o l u t i o n  w i l l - b e  obtained dur ing  the  summer.while the 
author  i s  p a r t i c i p a t i n g  i n  the NASA-ASEE Summer Facul ty  Fellowship Program. 
2 
EXPERIMENTAL RESULTS 
E f f o r t s  t o  v e r i f y  the  pred ic ted  behavior f o r  bo th  the  Ta-Ta205-Nb and 
I n  the  case of the  the  Nb-Nb205-V junc t ions  have no t  y e t  been successfu l .  
Ta-TazOs-Nb junc t ion  the problem has been the i n a b i l i t y  t o  achieve super- 
conduc t iv i ty  i n  the  deposi ted tantalum f i lm.  The reason f o r  t h i s  i s  bel ieved 
t o  be the  small  s i z e  of the c r y s t a l l i t e s  whose many boundaries p roh ib i t  the 
proper condi t ions  f o r  superconductivity.  This t roub le  could probably be el iminated 
by inc reas ing  the depos i t i on  r a t e  while using a heated subs t r a t e .  
equipment required f o r  t h i s  operat ion was n o t  a v a i l a b l e ,  i t  was decided t o  
temporar i ly  h a l t  work on t h i s  junct ion i n  favor  of the  Nb-Nb2O5-V junc t ion ,  
Since the  
Severa l  niobium f i lms  have been deposited which r e a d i l y  became super- 
conductive when immersed i n  l i qu id  helium. 
the superconducting p r o p e r t i e s  were lo s t .  
behavior  i s  caused by the  anneal ing of the  niobium a t  the high uemperatures 
However, a f t e r  the oxid iz ing  process ,  
It i s  s t rong ly  suspected t h a t  t h i s  
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t h a t  a r e  required during the  oxidat ion process.  
may be t o  f ind  a d i f f e r e n t  oxidat ion process.  
The so lu t ion  t o  t h i s  problem 
CONCLUDING REMARKS , 
Since the  work reported here  i s  t o  be continued under the NASA-ASEE 
Summer Facul ty  Fellowship Program, a few comments regarding f u t u r e  work a re  i n  
order. I n  regard t o  f l r r ther  t h e o r e t i c a l  development, the  major e q h a s i s  should 
be placed on completing the study r e l a t i n g  the  a c t u a l . c u r r e n t  t o  the physical  
p rope r t i e s  of the junct ion.  After  t h i s  problem i s  solved i t  w i l l  be a r e l a t i v e l y  
simple matter t o  program the  computer t o  f ind  the  numerical value of the  negat ive 
r e s i s t a n c e  a t  a des i red  operat ing poin t .  
i t  can be  used i n  the design of the  associated c i r c u i t r y .  Looking a t  i t  
Once the  negat ive r e s i s t a n c e  i s  known, 
another  way, i f  the  operat ing condi t ions of the  c i r c u i t  a r e  spec i f i ed  so t h a t  
the  des i r ed  value of the negat ive r e s i s t ance  i s  known, then a junc t ion  with 
the proper c h a r a c t e r i s t i c s  can be designed. 
With regard t o  the  experimental  phase, spec ia l  e f f o r t  m u s t  be given t o  
developing processes and techniques f o r  f a b r i c a t i n g  the  junct ion.  R e l i a b i l i t y  
and r e p r o d u c i b i l i t y  a r e  of g r e a t  importance i f  the  junc t ion  is  t o  be used as 
a p r a c t i c a l  c i r c u i t  element. 
. 
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BIBLIOGRAPHY 
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1 
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j 1. Shapiro,  S. and E .  J.  McNiff, Jr.,"Superconductive E f f e c t s  i n  Thin Films," 
Technical Documentary Report ,  A i r  Force Avionics Labs, Research and 
Technology Divis ion ,  A i r  Force Systems Command, Wright-Patterson A i r  Force 
Base, Ohio, 
I 
2, Giaever,  I. and K. Megerle, "The Superconducting Tunnel Junc t ion  as an 
Active Device," IRE Transact ions on Elec t ron  Devices,  pp. 459-461, 
/ 
I 
a November, 1962. 
> 
3. Miles, J. L . ,  P. H. Smith, and W;-Schonbein, - "Osci l la t ion  and Amplif icat ion 
a t  VHF U t i l i z i n g  Elec t ron  Tunneling between Superconducting Metals," 
Proceedings of the  IEEE Correspondence, pp. 937-938, June, 1963. 
2 
4. Bardeen, J . ,  L. N. Cooper, and J. R. S c h r i e f f e r ,  "Theory of Superconductivity," 
Phys ica l  Review, Vol. 108, No. 5 ,  pp. 1175-1204, December, 1958. 
5. F i s k e ,  M. D. and I. Giaever,  "Superconductive Tunneling," Proceedings of 
, 
t he  IEEE,  pp. 1155-1163, October, 1964. 
6. B l u m e ,  H. J. C . ,  and P. B. Johnson, "Proposed Superconducting Thin-Film 
Tunneling Junct ion  with Negative Resis tance Above Temperatures of Boi l ing  
Helium," Proceedings of the IEEE ,(Correspondence), accepted f o r  publ ica t ion .  
I. . 
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Theoretical and experimental investigations of optimum superconducting thin-film tunneling devices and the application to EHF  Final report, 1 Feb. - 30 Jun. 1968

Theoretical and experimental investigations of optimum superconducting thin-film tunneling devices and the application to EHF Final report, 1 Feb. - 30 Jun. 1968

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