Thermal annealing of fast neutron induced changes in electrical properties of highly doped n-type germaniu

Compton, W. D.

Depp, S. W.

Wolf, E. L.

English

NASA Technical Reports Server

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A 
nneal ing  of Fa Neutron Damage i n  Impuri 
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N-Type Germanium 
y-Conducting 
E .  L.  Wolf, W. Dale Compton, and S.  W. Depp 
Department of Phys ics ,  Univers i ty  of I l l i n o i s  
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The n a t u r e  of t h e  damaged reg ions  produced by i r r a d i a t i o n  of 
germanium by f a s t  n e u t r o n s , l a 2  and the  r e s u l t a n t  changes i n  t h e  elec- 
t r i ca l  p rope r t i e s3  of t h e  ma te r i a l  have been previous ly  s tud ied .  I n  
a d d i t i o n ,  t h e  annea l ing  of electrical changes produced by x-ray and 
f a s t  e l e c t r o n  damage i n  l i g h t l y  doped samples has  been c a r e f u l l y  
4-6 
s t u d i e d  and has  been expla ined  i n  terms of t he  formation and sub- 
sequent  d i s s o c i a t i o n  of vacancy-impurity complexes. 
The present  work is  concerned with t h e  thermal  annea l ing  of f a s t  
neu t ron  induced changes i n  t h e  e l e c t r i c a l  p r o p e r t i e s  of h igh ly  doped 
(but  non-degenerate) n-type germanium. Two samples con ta in ing  
17 -3 2.2 x 10 cm antimony donors were s tud ied  and gave i d e n t i c a l  r e s u l t s .  
A t  temperatures  below l O o K  t h e  r e s i s t i v i t y  i n  these  samples is 
temperature-independent.  
i n  F i g .  1, r e s u l t s  from conduction i n  an antimony impurity-conduction 
band t h a t  arises from such a s t rong  overlap of t h e  donor e l e c t r o n  wave- 
This  behavior shown i n  t h e  lowest curve 
7 
f u n c t i o n s  t h a t  t hese  e l e c t r o n s  become de loca l i zed .  The concen t r a t ion  
* 
This  work w a s  supported i n  par t  by the  Nat iona l  Aeronaut ics  and Space 
Adminis t ra t ion  under Grant N s G  228-62. 
https://ntrs.nasa.gov/search.jsp?R=19660025937 2020-03-16T20:39:27+00:00Z
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is s u f f i c i e n t l y  high i n  these  samples t h a t  hopping-conduction 
processes  are unimportant.  
7 
I r r a d i a t i o n  of such a sample with f a s t  neutrons has  been shown 
t o  break up  such an  impuri ty  band g iv ing  rise t o  a temperature  depend- 
ence of t h e  low temperature  
r e s i s t i v i t y  by more than  an 
t o  i r r a d i a t i o n .  A t  t h e  low 
descr ibed  by t h e  equat ion  g 
r e s i s t i v i t y  and an inc rease  i n  t h i s  
order  of magnitude over i ts  va lue  p r i o r  
temperatures t h e  r e s i s t i v i t y  may then be 
= p(m) exp (E2/kT) where p(m) is  t h e  
i n t e r c e p t  of t he  low temperature  curve a s  1 / T  -, 0. 
germanium in t roduces  acceptors  (of d e n s i t y  NA) with l e v e l s  deep i n  t h e  
forb idden  gap and thus  reduces the  concen t r a t ion  of e l e c t r o n s  f r e e  t o  
conduct t o  n = N (1-K) where K = N /N is the  compensation. Below 
1 0 ° K  t h e  N (1-K) e l e c t r o n s  a r e  condensed (but  no t  n e c e s s a r i l y  "frozen") 
on t h e  concentration-broadened donor ground s t a t e  l e v e l s .  P r i o r  t o  
i r r a d i a t i o n  conduction occurs  i n  a ground s ta te  band; a f t e r  i r r a d i a t i o n  
conduct ion is be l ieved  t o  occur by e x c i t a t i o n  of c a r r i e r s  t o  a s p l i t -  
2 '  
o f f  impurity-conduction band lying h igher  i n  energy by the  amount E 
It is hcl ieved  t h a t  t h e  dominant e f f e c t  of i r r a d i a t i o n  on the  impuri ty  
conduct ion parameters arises from t h e  r e s u l t i n g  change i n  occupat ion of 
t h e  broadened donor ground s t a t e ,  and t h a t  t h e  inf luence  of t h e  d e t a i l e d  
n a t u r e  of t he  compensating de fec t s  o r  t h e i r  d i s t r i b u t i o n  is r e l a t i v e l y  
minor. A set of annea ls  of fe red  the p o s s i b i l i t y  of changing the  d i s t r i -  
bu t ion  of compensating d e f e c t s  without changing t h e i r  t o t a l  number, and 
t h e r e f o r e  a p o s s i b i l i t y  of observing the  poss ib l e  inf luence  of t he  d i s -  
t r i b u t i o n  of t h e  compensating de fec t s  on the  impurity conduction parameters. 
I r r a d i a t i o n  of 
D A D  
D 
- 3 -  
The samples w e r e  i r r a d i a t e d  f o r  s i x  hours a t  ambient i n  the  
Tr iga  Mark I1 Experimental r e a c t o r ,  corresponding t o  a n  i n t e g r a t e d  neu- 
t r o n  f l u x  of -4 x 10 / c m  having an  energy exceeding 10 KeV. The samples 16 2 
were sh ie lded  from thermal neutrons by a cadmium f o i l  s h i e l d  and w e r e  
placed i n  t h e  "Lazy Susan" pos i t i on  a t  a depth of 6" i n  t he  g r a p h i t e  
r e f l e c t o r ,  r a t h e r  than  a t  t h e  core .  This  is expected t o  g ive  an energy 
spectrum somewhat degraded from a f i s s i o n  spectrum, and t o  reduce t h e  
importance of l a r g e  damage which have been shown t o  e l e c -  
t r i c a l l y  resemble i n s u l a t i n g  voids .  The high doping and consequent 
l a r g e  carr ier  concen t r a t ion  n furthermore lead t o  a s h o r t  Debye screening  
s 
0 
(about 150 A a t  300°K) and t o  correspondingly 
4 m e  
1 
l eng th  
narrow space charge reg ions  surrounding damage c l u s t e r s .  The c a r r i e r  
concen t r a t ion  a t  O°C w a s  reduced t o  about 56% of i t s  value p r i o r  t o  
i r r a d i a t i o n ,  corresponding t o  a value of K = 0.44 f o r  t h e  compensation. 
The mobi l i ty  a t  0°C  w a s  reduced by t h e  i r r a d i a t i o n  from 1700 c m  / v o l t  s e c  
t o  1250 cm / v o l t  sec, a r e s u l t  c o n s i s t e n t  with an inc rease  i n  charged 
c e n t e r  s c a t t e r i n g ,  as given by t h e  Brooks-Herring formula,8 of 
N, = 0.44 N 
e s s a r i l y  imply t h e  absence of r a d i a t i o n  induced s c a t t e r i n g  o t h e r  than 
t h a t  from charged c e n t e r s .  It is l i k e l y ,  however, i n  view of the  probable 
unimportance of l a r g e  i n s u l a t i n g  voids  t h a t  charged c e n t e r  s c a t t e r i n g  
accounts  f o r  t h e  major p a r t  of the mobi l i ty  change. 
2 
2 
a d d i t i o n a l  charged cen te r s .  Such agreement does not  nec- -- n 
Following i r r a d i a t i o n  the  samples  were annealed a t  10°C i n t e r v a l s  
from 100°C t o  38OoC, 45 minutes a t  each temperature.  
wrapped i n  plat inum f o i l ,  placed i n  a metal con ta ine r  sea led  i n  a i r ,  
The samples  w e r e  
- 4 -  
and immersed i n  an  o i l  ba th .  E l e c t r i c a l  c o n t a c t s  w e r e  made with 
Cerrosea l  35 so lde r  which w a s  removed with aqua r e g i a  before  each annea l .  
The H a l l  cons tan t  F$ and t h e  r e s i s t i v i t y  p a t  0 ° C  and 78°K were measured 
a f t e r  each anneal .  Af t e r  selected annea ls  t h e  complete r e s i s t i v i t y  E 
1 / T  d a t a  were taken as shown i n  Fig. 1. The annea l ing  d a t a  are presented 
i n  terms of t h e  f r a c t i o n  unannealed f :  
va lue  a f t e r  i r r a d i a t i o n  - value  a f t e r  anneal  
va lue  a f t e r  i r r a d i a t i o n  - value  before  i r r a d i a t i o n  f =  
of t h e  carr ier  concen t r a t ion  (CCl/%) and t h e  r e c i p r o c a l  of t he  mobi l i ty  
a t  0°C and 78°K.  It is  presumed t h a t  changes i n  t h e  l a t t e r  q u a n t i t y ,  
1/11 = p/%, are p ropor t iona l  t o  changes i n  the  concent ra t ion  of charged 
c e n t e r s .  The annea l ing  of the  impurity conduction ( r e s i s t i v i t y  from 
2OK t o  1O0K) i s  represented  by t h e  f r a c t i o n  f of t he  a c t i v a t i o n  energy 
E 2  and t h e  r e s i s t i v i t y  i n t e r c e p t  p(w) remaining. These d a t a  are shown 
i n  Fig.  2 and F ig .  3 .  The 0°C da t a ,  no t  shown, a r e  very s imi l a r  t o  t h e  
f 
78°K d a t a .  
The major f e a t u r e s  of these  d a t a  a r e  t h e  l ack  of major annea l ing  
below 300'C o t  a l i  q u a n t i t i e s  excepi Lilr s~a2 te r i i i g  ceiitsr cz~cc-tz=ticn, 
t h e  gradual  annea l ing  and then d i s t i n c t  annea l  s t a g e  of 1/11 a t  275"C, and 
t h e  f a i l u r e  of t h e  c a r r i e r  concent ra t ion  t o  completely anneal .  These 
obse rva t ions  are t o  be cont ras ted  with the  r e s u l t s  obtained on the  anneal ing 
of n-type germanium t h a t  has been i r r a d i a t e d  with Co gamma rays .  
Annealing of t h e  damage introduced by room temperature  gamma ray  i r r a d i a -  
60 6 ,9 ,10  
t i o n  of antimony doped germanium i n d i c a t e s  a s u b s t a n t i a l  anneal  of t he  
- 5 -  
c a r r i e r  concent ra t ion  a t  about 157°C w i t h  a nea r ly  complete anneal  by 
277°C.  
and the  concent ra t ion  of antimony fol lowing gamma i r r a d i a t i o n s  a t  77°K. 
That is ,  t h e  recovery occurred a t  a lower temperature  and with a more 
unique a c t i v a t i o n  energy f o r  c r y s t a l s  with 1.445 x 10 than f o r  c r y s t a l s  
wi th  1.074 x 
may be r e spons ib l e  f o r  t h e  d i f f e rences  between the  present  r e su l t s  and 
t h e  previous r e s u l t s  on gamma-ray i r r a d i a t e d  c r y s t a l s .  F i r s t ,  t h e  n a t u r e  
of t he  damage is c e r t a i n l y  d i f f e r e n t  f o r  t h e  neut ron  i r r a d i a t i o n  than  f o r  
t h e  g a m  i r r a d i a t i o n .  Second, the  very high antimony content  i n  t h e  
c r y s t a l s  of t h i s  experiment would tend t o  r e s u l t  i n  a more complete 
complexing of t h e  i n t e r s t i t i a l s  with the impur i t i e s  forming i n t e r s t i t i a l  
antimony o r  a s u b s t i t u t i o n a l  a n t i m o n y - i n t e r s t i t i a l  germanium o r  vacancy- 
antimony p a i r s .  This  assumption is  c o n s i s t e n t  with t h e  absence of low 
temperature  annea l ing  t h a t  has  been i n t e r p r e t e d  as r e s u l t i n g  from motion 
of i nd iv idua l  vacancies  o r  t h e  break-up of impuri ty  i n t e r s t i t i a l - v a c a n c y  
complexes. 
appears  c o n s i s t e n t  with t h e  observa t ion  of B e r t o l o t t i  e t  a l l '  t h a t  remnants 
of neutron-induced damage reg ions  remain i n  germanium a f t e r  an annea l  
of 12 hours  a t  460°C. 
9 An inverse  r e l a t i o n s h i p  e x i s t s  between the  recovery of damage 
10 
14 
3 Sb atoms/cm . These comparisons suggest  t h a t  two f a c t o r s  
The l a c k  of a complete anneal  of t h e  c a r r i e r  concen t r a t ion  
It is  clear t h a t  t h e  anneal  of t he  impurity conduction parameters 
more nea r ly  resembles t h a t  of t h e  78' c a r r i e r  concent ra t ion  than  of t h e  
s c a t t e r i n g  c e n t e r  concent ra t ion .  I n  p a r t i c u l a r ,  the  impurity conduction 
parameters  do not  r e f l e c t  t h e  gradual  anneal  of l / p  which occurs  between 
. 
- 6 -  
110" and 250" nor t h e  i n i t i a l  drop i n  t h e  275" 1/11 anneal ing  s t age .  
It seems l i k e l y  t h a t  t h i s  annea l  of 1/11 r e s u l t s  from a r e d i s t r i b u t i o n  
of charged d e f e c t s  r e s u l t i n g  i n  pa i r ing  of oppos i t e ly  charged s p e c i e s ,  
without  reducing t h e  number of deep acceptor  states a v a i l a b l e .  
A d e t a i l e d  i n t e r p r e t a t i o n  o f  t h e  anneal ing has  not  been poss ib l e .  
We sugges t ,  however, t h a t  t he  sharp anneal ing s t a g e  a t  310°C i s  more 
l i k e l y  t h e  breakup of some complex of simple d e f e c t s  with t h e  abundant 
antimony than  the  breakup of d i sordered  reg ions .  The damage r eg ions  
presumably have a b r o a d d i s t r i b u t i o n o f  s i z e s  and breakup temperatures ,  
which should lead  t o  a re la t ively f e a t u r e l e s s  anneal .  Gradual breakup 
of t h e  damage reg ions ,  we specu la t e ,  may occur i n  t h e  range 110" t o  250°, 
w i t h  charged decay products  complexing with the  antimony and reducing 
t h e  charged s c a t t e r i n g  cen te r  concent ra t ion .  That such complexing with 
some f r a c t i o n  of t h e  K N, ionized donors i n  t h e  range 110" t o  250°C 
should n o t  a f f e c t  c2 and p(w) is c o n s i s t e n t  wi th  t h e  view t h a t  t h e  E 
conduct ion process  does n o t  depend on t h e  presence of unoccupied donor 
ground s ta te  l e v e l s ,  and supports  t h e  con ten t ion  t h a t  t h e  impuri ty  
conduct ion parameters ,  a t  least  f o r  concent ra t ions  nea r  t h e  m e t a l l i c  
t r a n s i t i o n ,  are only  s l i g h t l y  a f f ec t ed  by t h e  d i s t r i b u t i o n  of compensating 
d e f e c t s .  Thus, t h e  complexing of t h e  acceptors  does not  measurably 
a f f e c t  t h e  over lap  of t h e  occupied donors. The obse rva t ion  t h a t  c2 
completely recovers  sugges ts  t ha t  t h e  i r r a d i a t i o n  and anneal  have not  
a l t e r e d  the  average d i s t r i b u t i o n  of  t he  antimony i n  any s u b s t a n t i a l  
f a s h i o n .  It should be noted t h a t  t h i s  i s  n o t  i n c o n s i s t e n t  with t h e  
2 
- 7 -  
observa t ion  t h a t  t h e  carrier concen t r a t ion  has  not  completely recovered,  
s i n c e  b r i e f  i n i t i a l  i r r a d i a t i o n s  were found t o  decrease  n without  changing 
c2 from zero.  
donors f e l l  below some minimum did E: become non-zero. 
Only when t h e  average over lap  of e l e c t r o n s  on the  occupied 
2 
The complex which decays a t  310" could poss ib ly  be t h e  s t r o n g l y  
bound antimony-divacancy. The remaining deep states which do not  annea l  
by 380°C are most probably a s soc ia t ed  with the  more seve re ly  damaged reg ions .  
It is  c l e a r l y  seen t h a t  the n a t u r e  of t h e  anneal ing process  i n  
h i g h l y  doped materials t h a t  have been i r r a d i a t e d  wi th  neutrons is 
s u b s t a n t i a l l y  d i f f e r e n t  from t h e  processes  observed f o r  more pure material 
t h a t  has  been gamma ray  i r r a d i a t e d .  
B 
. 
- 8  - 
REFERENCES 
1. 
2 .  J. H. Crawford, Jr. and J. W .  C le land ,  J .  App. Phys. - 30,  1204 (1959). 
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B. R.  Gossick,  J .  App. Phys. - 30, 1214 (1959). 
J .  W. Cleland,  J .  H. Crawford, and J. C. Pigg,  Phys. Xev. -’ 95 1742 
(1953), and f o r  a review see J .  H. Crawford, p .  421-470 i n  t h e  I n t e r -  
a c t i o n  of I r r a d i a t i o n  with So l ids ,  e d i t e d  by R. Strumane, J.  Nihoul,  
R.  Gevers,  and S. Amelinckx, (North Holland Publ i sh ing  Co., Amsterdam, 
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J .  C .  Pigg and J .  H.  Crawford, J r . ,  Phys. Rev. - 135, A 1 1 4 1  (1964). 4.  
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8. P. P. Debye and E .  M. Conwell, Fhys. Rev. - 93, 693 (1954). 
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I .  
I 0 
4 
- 9 -  
FIGURE CAPTIONS 
Big. 1 R e s i s t i v i t y  on log  s c a l e  versus  r e c i p r o c a l  temperature f o r  
17 
16 2 
/ c m  
germanium con ta in ing  2.2 x 10 antimony, inc luding  d a t a  a f t e r  
a f a s t  neutron f l u x  of 4 x 10 
annea ls .  
and a sequence of 45 Einute  
Fig.  2 I sochronal  annea l ing  of the 78'K c a r r i e r  concent ra t ion  ( t h e  
squares)  and t h e  r e c i p r o c a l  mob i l i t y ,  presumed p ropor t iona l  
t o  t h e  charged s c a t t e r i n g  c e n t e r  concent ra t ion  ( t h e  c i r c l e s ) .  
Fig.  3 I sochrona l  annea l ing  of the  impur i ty  conduction parameters ;  
t h e  a c t i v a t i o n  energy E ( the  squares)  and t h e  high temperature  
i n t e r c e p t  p(m) ( the  c i r c l e s ) .  The t r i a n g l e s  are t h e  f r a c t i o n  
unannealed of t h e  r e s i s t i v i t y  a t  4.2OK, which, by v i r t u e  of t h e  
constancy of c2 is similar t o  t h e  annea l  of p(m). 
2 
I 
t 
I I I I I I I I 
2Ktc 
2 9 d C  
U 300% 
- 
” - - e  
A -  NmAL 0 
Ge: 2.2 x 0% 
o J I  .  -2 I ,  .3 -4 I ,  .5 -6 I -7 
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I I I I I I I 1 I I 
1 I I I I 
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Annealing of fast neutron damage in impurity-conducting n-type germanium

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