Annealing of neutron-irradiation-induced changes in impurity conduction in antimony-doped germaniu

Compton, W. D.

Wolf, E. L.

English

NASA Technical Reports Server

ANNEALING OF NEUTRON-IRRADIATION-INDUCED CHANGES I N  
I M P U R i i Y  ZOND'U'CTIOB IN ANTIMONY-DOPED GERMANIUM t 
E. L. Wolf and W. Dale Compton 
University of I l l i n o i s ,  Urbana, I l l i n o i s  
ABSTRACT 
3 1 Germanium doped with antimony i n  the  range of 1 t o  2 x 1017 atomslcm 
exh ib i t s  a temperature independent r e s i s t i v i t y  f o r  temperatures less than 
1OoK. 
neutrons,  i t  is  found t h a t  the low temperature r e s i s t i v i t y  exh ib i t s  an 
As acceptor s t a t e s  are introduced by i r r a d i a t i o n  with f a s t  
ac t iva t ion  energy E 2' 2 
bution and the  number of occupied donors, t h i s  parameter i s  usefu l  f o r  
Since the magnitude of E depends upon the  d i s t r i -  
studying the thermal s t a b i l i t y  of the defec ts  t h a t  are introduced by 
i r r a d i a t i o n .  It i s  found i n  these highly doped materials t h a t  t h e  motion 
of charged defec ts  t o  form neu t r a l  complexes does not seem t o  involve the  
n e u t r a l  donors. 
It i s  well-known t h a t  the  low temperature r e s i s t i v i t y  of germanium 
i s  s t rongly  dependent upon the concentration of t he  dopant. Figure 1 
presents  r e s i s t i v i t y  d a t a  on four samples of antimony doped with german- 
ium. 
cen t r a t ions ,  and the  average separation between donors i s  given i n  Table 
The room temperature r e s i s t i v i t i e s ,  H a l l  coe f f i c i en t s  , donor con- 
I f o r  these samples. For temperatures below about 10"K, t h e  r e s i s t i v i t y  
This work was supported i n  p a r t  by the  National Aeronautics and Space 
Administration under Grant NsG-228-62. 
U 
I 
D N66 (ACCESSION NUMBER) 33386 
> e (PAGES1 /5 - fk- m3r 4 
'NASA CR OR TYX OR AD NUMBER) 
2 
https://ntrs.nasa.gov/search.jsp?R=19660024096 2020-03-16T20:22:55+00:00Z
- 2 -  
of samples F1 and SM1 i s  independent of temperature. If the low temper- 
a t u r e  r e s i s t i v i t y  i s  described by the  equation p = p ( m )  exp (E2/kT), the 
a c t i v a t i o n  energy E., of samples E l  and L 1  is  0.62 and 0.08 mil l i -e lec t ron  
Sample 
SM1 
F1 
L 1  
E l  
L 
R e s i s t i v i t y  Hal 1 Coef f i c i e n t  Donor Average 
(Room (Room Conc t r a t i o n  Doqr 
d (A) e7 Temperature) Te erature) (10 a-3) 
( 0  cm) (a "s/coulomb) 
40 3 .016 28.0 2.2 
* 020 36.4 1.7 112 
.029 58.6 1.1 131 
,037 84.0 0.74 147 
. .  
v o l t s ,  respectively.  
Figure 1. Variation of r e s i s t i v i t y  with 
rec iproca l  temperature f o r  the 
specimens l i s t e d  i n  Table I. 
Table I 
Room-temperature r e s i s t i v i t i e s ,  H a l l  c o e f f i c i e n t s ,  donor 
concentrations and the  average d is tance  d etween donors 
f o r  t he  specimens measured. d = (3/4rND) 1 /!I 
A completely s a t i s f a c t o r y  theory of t h i s  thermally ac t iva ted  conduc- 
t i o n  has not  been developed. 
be associated with the  energy t o  p u t  a second e l e c t r o n  onto a n e u t r a l  
donor s i te .  Mott and Twose(2) consider t h a t  the  c r i t i c a l  parameter i s  
the  magnitude of the overlap in t eg ra l  associated with e lec t rons  on two 
d i f f e r e n t  sites. As the average separation of t he  impur i t ies  decreases,  
i t  i s  supposed t h a t  t h e  magnitude of the overlap energy increases  t o  such 
an ex ten t  t h a t  a long-range order is es tab l i shed .  
f o r  the  lowest energy state of the system t o  be nonconducting. 
Fritzsche") f i r s t  suggested t h a t  i s  t o  
It i s  poss ib le ,  then, 
Separated 
- 3 -  
from i t  by a s m a l l  energy decreasing with increasing overlap. 
cen t r a t ion  of impur i t ies  f u r t h e r  increases ,  t he  energy between the  non- 
conducting and conducting states decreases and becnmes zero. It seem 
reasonable t o  a s soc ia t e  the  state having a zero ac t iva t ion  energy with a 
system t h a t  shows no temperature dependence of t he  conductivity at l a w  
temperatures, bu t  s t i l l  e x h i b i t s  a thermal ac t iva t ion  energy a t  higher 
temperatures (Samples F1 and SM1). Consistent with t h i s  view is  the  
constancy of the  H a l l  coe f f i c i en t  a t  t h e  l o w  temperatures ( f igu re  2) .  
As the con- 
N i ~ h i m u r a ' ~ )  has recent ly  calculated the magnitude of E: f o r  a simple 2 
model t h a t  assumes t h a t  t h e  ground state of t he  donor i s  hydrogenic and 
t h a t  a band a t  higher energies r e s u l t s  from the i n t e r a c t i o n  between 
states of negatively charged donors. 
Figure 2. Variation of H a l l  coe f f i c i en t  
with r ec ip roca l  temperature 
f o r  the specimens l i s t e d  i n  
Table I. 
Davis and C~mpton '~ )  have studied the  dependence of E upon the  2 
degree of compensation of t h e  sample. An increase i n  the  compensation 
may be viewed i n  a q u a l i t a t i v e  sense as merely reducing the average elec- 
t ron  overlap of the occupied donors. 
increase  with increasing compensation as t h e  number of neu t r a l  donors is 
Thus, c2 would be expected t o  
decreased. Data ind ica t ing  t h a t  t h i s  i s  indeed the  case are given f o r  
samples F1 and SM1 i n  Figure 3. The t h e o r e t i c a l  expression derived by 
Nishimura f o r  t he  compensation dependence of E 
pred ic t  the propef va f i a t ion .  This i s  no t  a l toge ther  su rp r i s ing  since 
does no t  quan t i t a t ive ly  2 
the  f i e l d  of the ionized donors and acceptors w a s  omitted i n  the  treat- 
- 4 -  
~ -~ 
Figure 3. Variation of r e s i s t i v i t y  of 
specimens F1 and SM1 f o r  var- 
ious values of compensation. 
~~ ~~ ~~~~ ~ 
ment of the  la t t ice  po ten t i a l .  Similarly,  deviations of the  donor wave 
functibns from sphe r i ca l  symmetry w e r e  neglected. It is  believed, 
however, t h a t  t he  ideas developed by Fr i tzsche ,  Mott and Twose, and 
Nishimura co r rec t ly  describe the physical o r i g i n  of the process giving 
r i s e  t o  the  thermally ac t iva ted  process described by the  energy c2. 
Since the  parameters associated with the low temperature conduction 
and t h e  value of t he  i n t e r c e p t  of the  low temperature process,  namely E 
r e s i s t i v i t y  curve at  1 /T  = 0,  p(m), depend upon the  d i s t r i b u t i o n  and 
number of neu t r a l  donors, they can provide addi t iona l  information about 
the  s t a b i l i t y  and na tu re  of r ad ia t ion  induced defec ts .  
describes the  r e s u l t s  of the thermal annealing of f a s t  neutron induced 
changes i n  the e l e c t r i c a l  properties of samples of germanium t h a t  contain 
2.2 x 10 cm antimony donors (Sample SM1). The i r r a d i a t i o n  introduces 
acceptors (of dens i ty  NA) which have energy levels deep i n  the  forbidden 
gap, thereby reducing the  concentration of e l ec t rons  f r e e  t o  conduct t o  
n = N ( 1 4 )  electr&s which are condensed onto the  broadened donor ground 
state levels a t  temperatures below 1O0K. 
p o s s i b i l i t y  of changing the d i s t r i b u t i o n  of compensating defec ts  without 
changing t h e i r  t o t a l  number, and therefore the  p o s s i b i l i t y  of observing 
the  influence of the  d i s t r i b u t i o n  of t he  compensating defec ts  on the  
impurity conduction parameters. 
2 
This paper 
17 -3 
D 
A set of anneals of fe red  t h e  
The samples w e r e  i r r a d i a t e d  a t  ambient wi th  neutrons of energy 
, *  . 8 
J 
C' 
- 5 -  
2 exceeding 10 Kev t o  an in tegra ted  f lux  of % 4 x 10l6/cm . Thermal neu- 
t rons  were removed by enclosing the samples i n  a cadmium f o i l  sh ie ld .  
Since the i r r a d i a t i o n s  w e r e  performed i n  the "Lazy Susan" pos i t i on  of a 
Triga Mark I1 Experimental r eac to r  a t  a depth of 6" i n  the g raph i t e  
r e f l e c t o r ,  r a t h e r  than a t  the core, the  energy spectrum is somewhat 
degraded from a f i s s i o n  spectrum. 
tance of l a rge  damage c lus te rs (5)  which have been shown t o  e l e c t r i c a l l y  
This w i l l  tend t o  reduce the  impor- 
resemble in su la t ing  voids. 
concentration furthermore w i l l  lead t o  a s h o r t  Debye screening length 
of about 150 A a t  3 0 0 " K ,  thus reducing the  s i z e  of t he  space charge 
The high doping and consequent l a rge  carrier 
(5) 
0 
region surrounding l a r g e r  damage c lus t e r s .  
i r r a d i a t i o n  was determined from the measured values of t he  H a l l  coeffi-  
The compensation a f t e r  
c i e n t  a t  O°C carrier concentration t o  about 56% of i t s  i n i t i a l  value, 
corresponding t o  a value of K = 0.44. 
from 1700 cm /vo l t  sec t o  1250 cm / vo l t  sec. 
increase  i n  charged center  s ca t t e r ing ,  as given by t h e  Brooks-Herring for- 
mula,(6) of NA = K N addi t iona l  charged centers .  Although t h i s  agree- 
ment does not necessar i ly  imply the  absence of r ad ia t ion  induced scatter- 
ing  o ther  than t h a t  from charged centers ,  i t  is l i ke ly ,  i n  v i e w  of t h e  
probable unimportance of la rge  in su la t ing  voids,  t h a t  charged center  
The mobili ty a t  O°C w a s  reduced 
2 2 This is  cons is ten t  with an 
D 
s c a t t e r i n g  accounts f o r  t he  major p a r t  of t he  mobili ty change. 
Following i r r a d i a t i o n ,  t he  samples w e r e  annealed a t  temperatures 
between 100°C and 3 8 O o C  a t  in t e rva l s  of 10°C. The samples w e r e  wrapped 
i n  platinum f o i l ,  placed i n  a metal container sea led  i n  a i r ,  and immersed 
i n  an o i l  bath f o r  45 minutes a t  each temperature. Electrical contacts 
- 6 -  
were made with Cerroseal 35 solder which w a s  removed with aqua r eg ia  
before  each anneal. 
and 78OK w e r e  measured a f t e r  each anneal. 
d a t a  w e r e  taken after se l ec t ed  anneals. 
The H a l l  constant % and the  r e s i s t i v i t y  p a t  O°C 
Complete r e s i s t i v i t y  ys- 1 / T  
The annealing da ta  are presented 
i n  terms of t h e  f r a c t i o n  unannealed f 
value a f t e r  i r r a d i a t i o n  - value a f t e r  anneal 
value 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  carrier concentration ( l / s )  and the  r ec ip roca l  of the  mobili ty a t  
78'K. It i s  presumed t h a t  changes i n  the  l a t te r  quant i ty ,  l / p  = p/%, 
are proportional t o  changes i n  the  concentration of charged centers.  The 
annealing of t he  impurity conduction ( r e s i s t i v i t y  from 2' t o  1OoK) is  
represented by the  f r a c t i o n  remaining f of t h e  a c t i v a t i o n  energy E and 2 
the  r e s i s t i v i t y  i n t e r c e p t  p(- ) .  These d a t a  are shown i n  Figures 4 and 5. 
Measurements a t  O°C ind ica ted  a close s i m i l a r i t y  t o  the  78'K data.  
Figure 4. Isochronal annealing of t he  78'K 
c a r r i e r  concentration ( t h e  
squares) and the  rec iproca l  
mobility , presumed proportional 
t o  the charged s c a t t e r i n g  center  
concentration ( the  circles). 
Figure 5. Isochronal annealing of t h e  
impurity conduction parameters; 
the  ac t iva t ion  energy €2 ( t h e  
squares) and the high tempera- 
t u r e  in t e rcep t  p (-) ( t h e  c i r c l e s ) .  
The t r i ang le s  are the  f r a c t i o n  
unannealed of t he  r e s i s t i v i t y  a t  
4.2'K, which, by v i r t u e  of the  
constancy of €2,  is s i m i l a r  t o  
the anneal of p( - ) .  
, .  1 ' 
.A, 
- 7 -  
The major f ea tu res  of these  data are the lack of major annealing 
below 3OO0C of a l l  q u a n t i t i e s  except t he  s c a t t e r i n g  center  concentration, 
the  gradual annealing and then d i s t i n c t  annealing s tage  a t  l / p  a t  275OC, 
and the  f a i l u r e  of t h e  carrier concentration t o  completely anneal, The 
observations are t o  be contrasted with t h e  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 60 gamma rays. (7-9) 
Annealing of the  damage introduced by room temperature gamma ray i r r a d i a -  
t i o n  of antimony doped germanium indica tes  a s u b s t a n t i a l  anneal of t he  
c a r r i e r  concentration a t  about 157OC with a near ly  complete anneal by 
277°C.(7) 
and the  concentration of antimony following gamna i r r a d i a t i o n s  a t  77OK, (8) 
with t h e  recovery occurring a t  a lower temperature and with a more unique 
ac t iva t ion  energy f o r  c r y s t a l s  with 1.445 x 
1.074 x 1015 Sb atoms/an3. 
An inverse  r e l a t ionsh ip  e x i s t s  between the  recovery of damage 
- 
than f o r  crystals with 
These comparisons suggest t ha t  two f a c t o r s  
may be responsible f o r  the d i f fe rences  observed i n  the  present experiment. 
F i r s t ,  the nature of the  damage i s  c e r t a i n l y  d i f f e r e n t  f o r  the  neutron 
i r r a d i a t i o n  than f o r  the  gamma i r r ad ia t ion .  Second and perhaps more 
important, the very high antimony content present i n  the  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 the  in t e r -  
s t i t i a l s  with the  impur i t ies  forming i n t e r s t i t i a l  antimony o r  a subs t i tu -  
t i o n a l  antimony-intersti t ia1 germanium and vacancy-antimony pa i r s .  Low 
temperature annealing observed i n  l i g h t l y  doped samples has been in t e r -  
p re ted  as r e su l t i ng  from the motion of ind iv idua l  vacancies o r  t h e  break- 
up of impurity i n t e r s  t i t ial-vacancy complexes. (9'10) The lack of a com- 
p l e t e  anneal of t h e  c a r r i e r  concentration i n  the  present case appears 
- 8 -  
cons i s t en t  with the  observation of B e r t o l o t t i ,  e t  a l .  (I1) t h a t  remnants 
of neutron-induced damage regions remain i n  germanium a f t e r  an anneal of 
12 hours a t  460'C. 
It is  clear t h a t  the  anneal of t h e  impurity conduction parameters 
more nearly resembles that of t h e  78OK carrier concentration than of t he  
s c a t t e r i n g  center concentration. In p a r t i c u l a r ,  t he  impurity conduction 
parameters do not r e f l e c t  t he  gradual anneal of l / p  which occurs between 
110°C and 25OoC nor the i n i t i a l  drop i n  the 275'C l/p annealing stage.  
It seems l i k e l y  t h a t  t h i s  anneal of l / p  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 defec ts  and involves t h e  pa i r ing  of oppositely charged spec ies  , 
without reducing the  number of deep acceptor states axa i lab le .  
more, i t  seems unl ike ly  t h a t  t he  pa i r ing  involves a s i g n i f i c a n t  nucaber 
Purther- 
of neu t r a l  donors, s ince  an accompanying change i n  E does not occur. 2 
Although a d e t a i l e d  in t e rp re t a t ion  of the  annealing is  not poss ib le ,  
i t  is suggested t h a t  the sharp annealing s tage  at  31OoC i s  more l i k e l y  
the  breakup of some complex of simple defec ts  with the  abundant antimony 
than the breakup of disordered regions. The damage regions presumably 
have a broad d i s t r i b u t i o n  of s izes  and breakup temperatures, which should 
lead t o  a r e l a t i v e l y  f e a t u r e l e s s  anneal. Gradual breakup of t he  damage 
regions,  we specula te ,  may occur i n  t h e  range 110' t o  250°,  w i t h  charged 
decay products complexing with the  ionized antimony, thereby reducing the  
charged s c a t t e r i n g  cen te r  concentration. 
f r a c t i o n  of t he  K N 
not  a f f e c t  E and p ( - )  is  consistent with t h e  view t h a t  the  E conduction 
2 2 
process does not depend on the presence of unoccupied donor ground s ta te  
That such complexing w i t h  some 
ionized donors i n  the  range llOo t o  250'C should D 
- 9 -  
levels, and supports the  contention t h a t  the impurity conduction param- 
eters, a t  least f o r  concentrations near t he  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 fec ted  by t h e  d i s t r i b u t i o n  of compensating defec ts .  
Thus, the  complexing of the  acceptors does not  measurably a f f e c t  t he  
overlap of t h e  occupied donors. The observation t h a t  E recovers sug- 
g e s t s  t h a t  the i r r a d i a t i o n  and anneal have not  a l t e r e d  the  averagd 'd is t r i -  
bu t ion  of the antimony i n  any s u b s t a n t i a l  fashion. It should be noted 
t h a t  t h i s  is  not incons is ten t  with the  observation t h a t  the  carrier con- 
cen t r a t ion  has no t  completely recovered, s ince  b r i e f  i n i t i a l  i r r a d i a t i o n s  
d id  not increase  E from zero, although the l o w  temperature H a l l  coeffi-  
c i e n t  ind ica ted  a decrease i n  t h e  concentration of carriers i n  t h e  
impurity band. 
2 .. 
2 
The complex which decays at 31OoC could possibly be the s t rongly  
bound antimony-divacancy. The remaining deep states which do not  anneal 
by 380°C are most probably associated with l a rge r  damaged regions. 
Although the nature of t he  annealing process i n  highly doped m a t e -  
r ials t h a t  have been i r r a d i a t e d  with neutrons is  subs t an t i a l ly  d i f f e r e n t  
from the 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 ,  t he re  is l i t t l e  evidence t h a t  any r e d i s t r i b u t i o n  of donors 
occurs as a r e s u l t  of t h e  i r r ad ia t ion .  
- 10 - 
References 
1. H. Fr i tzsche ,  J. Phys. Chem. Solids 5, 69 (1958) 
2. N. F. Mott and W. D. Twose, Suppl. Phi l .  Mag. l0, 107 (1961) 
3. H. Nishimura, Phys. Rev. 138, A815 (1965) 
4. E. A. Davis and W. D. Compton, Phys. Rev. 140, A2183 (1965) 
5. B. R. Gossick, J. Applied Phys. 30, 1214 (1959); J. H. Crawford, Jr. 
and J. W. Cleland, J. Applied Phys. 30, 1204 (1959) 
6. P. P. Debye and E. M. Conwell, Phys. Rev. 93, 693 (1954) 
7. S. Ishino, F. Nakazawa, and R. R. Haseguti, J. Phys. Chem. Sol ids  24, 
1033 (1963) 
8 .  J. C. Pigg and J. H. Crawford, Jr., Phys. Rev. 135, A1141 (1964) 
9. H. Sa i to ,  J. C. Pigg, and J. H. Crawford, Jr., Phys. Rev. 3, 725 
(1966) 
10. For a review see J. H. Crawford, pp. 421-471, In t e rac t ion  of 
Radiation with So l ids ,  Edited by R. Strumane, 3. Nihoul, R. Gevers, 
and S .  Amelinckx, North-Holland Publishing Co., Amsterdam (1963) 
11. M. B e r t o l o t t i ,  T. Papa, D. Se t t e ,  and G. V i t a l i ,  J. Applied Phys. 36, 
3506 (1965) 
f L . 
f 
10 - I I I I I I 
F1 E - 0 
u)-2 
i! 
0 Q1 0.2 0.3 0.4 0.5 0.6 0-7 
. . .. 
I I I I I 
'1 a 
N2 He - -  
0 0.1 0.2 0.3 
-.  
I" 101 4 i I 1 I I 
1 / Temperature (OK'') 


Annealing of Neutron-irradiation-induced Changes in Impurity Conduction in Antimony- Doped Germanium

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