Photoconductivity method for measuring effects of radiation induced defects in n-type silico

Corelli, J. C.

Kalma, A. H.

English

NASA Technical Reports Server

r." 
i 
BIATIO$?-INDUCED YKOTOCGNDUCTIVITU OF 
I 
+ A .  H. Kalma & J. C.  C o r e l l i  
%Nuclear Engineering and Science Department . ,  
Rensselaer  Polytechnic I n s t i t u t e  
Troy, New YorkI, 
ABSTRACT 
' The photoconduct ivi ty  induced i n  zone-refined s i l i c o n  doped 
wi th  A s ,  I?, and Sb t o  r e s i s t i v i t i e s  i n  t h e  range of 1 t o  100 
ohm-cm was s tud ied  a f t e r  bombardment wi th  1 . 5  MeV and 45-50 MeV 
e l e c t r o n s .  The dominant de fec t  photoconduct ivi ty  i n  a l l  samples 
i r r a d i a t e d  a t  4.5-50 MeV i s  found t o  a r i s e  from an  "energy band" 
extending from %1. t o  ~ 4 . 0 ~  . A poss ib l e  explana t ion  of 
- t h i s  energy band i s  given. S ing le  wel l -def ined d e f e c t  energy 
l e v e l s  dominate t h e  photoconduct ivi ty  of 1 . 5  M e V  i r r a d i a t e d  
samples, Using s t ress - induced  atomic and e l e c t r o n i c  r e o r i e n t a t i o n  
toge ther  w i th  anneal ing s t u d i e s  and c o r r e l a t i n g  the  r e s u l t s  
with previous e l e c t r o n  paramagnetic resonance and i n f r a r e d  
absorp t ion  experiments,  l e v e l s  l oca t ed  a t  Ec-O.54eV and E,-0.39eV 
a r e  i n d e n t i f i e d  a s  a r i s i n g  from t h e  divacancy de fec t .  
1, INTRODUCTION 
~ 
I I 
I Photoconductivity (PC) o f f e r s  an e x c e l l e n t  method f o r  
measuring effects of rad ia t ion- induced  d e f e c t s  i n  semiconductors 
It can determine t h e  energy l e v e l s  of d e f e c t s  without  hea t ing  
t h e  sample and anneal ing t h e  d e f e c t s  i n  the  process.  Recent 
experiments (1) have shown t h a t  one can a l s o  determine the  
atomic and e l e c t r o n i c  conf igu ra t ions  of t h e  de fec t s  by using 
u n i a x i a l  stress and po la r i zed  l i g h t .  This i s  accomplished by 1 $* 1 c o r r e l a t i n g  these  PC stress r e s u l t s  wi th  s i m i l a r  experiments , 
This paper i s  concerned wi th  t h e  s tudy of th.e divacancy i n  1 
Therefore,  
.'" 
. 
employing e l e c t r o n  paramagnetic resonance (EPR). 
n-type s i l i c o n  us ing  t h e  above mentioned techniques.  
*Supported by NASA under .grant  N s  G-29d and ,a UASA -Traineeship 1 ' G O ~ ~ ~ J A N I ~ '  
t o  A .  H. Kalma 
https://ntrs.nasa.gov/search.jsp?R=19680017160 2020-03-12T10:48:14+00:00Z
he shaLl c o r r e l a t e  our r e s u l t s  a s  they a r e  presented wi th  pre- 
* vious ly  published EPR work ( 2 , 3 )  and i n f r a r e d  absorp t ion  ( I R )  
work (4)  on the  divacancy. A l l  samples used i n  t h i s  s tudy were 
zone - re f ined .  
2, EXPERIMENTAL RESULTS 
A .  45 MeV v s .  1 . 5  MeV Elec t ron  I r r a d i a t i o n  
The dominant d e f e c t  photoconduct ivi ty  observed i n  1 t o  100 
ohm-cm sarrples i r r a d i a t e d  wi th  2345 MeV e l e c t r o n s  a r i s e s  n o t  from 
s i n g l e ,  wel l -def ined  energy l e v e l s ,  bu t  from an energy band" 
extending from ~ 1 . 5 ~  t o  ~ ,4 .0 ,& !  . 
gradual  f a l l  o f f  of photoconduct ivi ty  with inc reas ing  wavelength 
and i s  found i n  a11 samples independent of chemical spec ie s  
( A s ,  I), and Sb) or  concen t r a t ion  of dopant. The recovery of 
t h e  "energy band" extends over a broad temperature range ( lOO°C 
t o  above 34OoC) and c l o s e l y  resembles what w e  have observed (4.)  
i n  rad ia t ion- induced  I R  (which we  have c a l l e d  near-edge absorp- 
t i o n )  f o r  45 MeV e l e c t r o n - i r r a d i a t e d  s i l i c o n .  High temperature 
( 16006) stress experiments designed s p e c i f i c a l l y  t o  observe 
divacancies  revea led  no dichroism ( > 10%) of t h i s  band. 
I f  
This i s  cha rac t e r i zed  by a 
One explana t ion  f o r  t h e  observed "energy band" is  t h a t  
de fec t  complexes a r e  formed which c o n s i s t  of c l u s t e r s  of vacancies ,  
i n t e r s t i t i a l s ,  and impuri ty  atoms which des t roy  some of t h e  
p e r i o d i c i t y  of t h e  l a t t i c e  c r e a t i n g  an allowed continuum of 
energy s t a t e s  i n  t h e  forbidden gap thereby e f f e c t i v e l y  decreasing 
t h e  gap energy. This explana t ion  of t h e  decrease in  t h e  forbidden 
gap energy i n  i r r a d i a t e d  s i l i c o n  was f i r s t  suggested i n  a schematic 
fash ion  i n  a paper by Vavilov ( 5 ) .  
Single  d e f e c t  energy l e v e l s  can be observed i n  5 4 5  MeV 
i r r a d i a t e d  samples, b u t  i t  is  d i f f i c u l t  t o  p o s i t i o n  them e x a c t l y  
o r  t o  s tudy them due t o  t h e  ove r r id ing  e f f e c t  of t he  "energy 
band". By c o n s t r a s t ,  i n  1 .5  MeV e l e c t r o n - i r r a d i a t e d  s i l i c o n ,  
very l i t t l e  of t h i s  I 1  energy band" i s  observed and s i n g l e  l e v e l s  
)i 
predominate. 
e a s i l y .  For t h e  remainder of  t h i s  paper, we  s h a l l  concern 
ourse lves  wi th  1 . 5  MeV i r r a d i a t i o n s .  
These s i n g l e  l e v e l s  can thus  be s t u d i e d  much more 
B. Annealing Behavior 
Figure 1 presen t s  t h e  i sochronal  anneal ing behavior of a 
phosphorus-doped, 1 ohm-cm sample. A l l  curves a r e  normalized t o  
t h e  1.24 e V  po in t .  
a r s e n i c  and antimony-doped samples. The temperature a t  which 
t h e  change from curve. ( a )  t o  curve (b)  takes  p lace  (168OC here)  
is  n o t  a s  meaningful a s  t h e  Fermi l e v e l  p o s i t i o n .  The tempera- 
t u r e  depends on t h e  amount of damage i n  t h e  sample i n i t i a l l y  and 
v a r i e s  from 
The Fermi l e v e l  p o s i t i o n  where t h i s  change occurs  appears to 
be  s Ec-0.22 e V .  
S imi la r  r e s u l t s  have been obtained f o r  
< 1 3 8 O C  t o  > 2OO0C f o r  i n i t i a l l y  s i m i l a r  samples. 
This p o s i t i o n  i s  c l o s e  t o  t h e  l e v e l  below 
which the  1.8p-1 band i s  seen and above which t h e  3 . 3 ~  bands 
a r e  seen i n  I R  with monochromatic l i g h t  (6 ) .  These bands 
have been i n d e n t i f i e d  wi th  t h e  divacancy ( 4 ) .  On a l l  samples, 
when an anneal ing temperature of 328OoC was reached, t h e  PC 
dropped down t o  curve ( c ) .  This i s  i n  t h e  v i c i n i t y  of anneal ing 
temperature of t h e  divacancy (3 ) .  
When t h e  Fermi l e v e l  is  below approximately E,-0.22 e V ,  t h e  
only l e v e l  observed i s  a t  E,-0.54 e V .  When, t h e  Fermi- level  i s  
above E,-0.22 e V ,  l e v e l s  a t  E,-0.36 e V ,  and E,-0.39 e V  a r e  
observed. 
g iv ing  r i s e  t o  a small  amount of photoconduct ivi ty .  This e f f e c t  
i s  seen i n  F ig .  1. The l e v e l  a t  Ec-0.54 e V  disappears  a s  t h e  
Fermi l e v e l  moves s l i g h t l y  up from Ec-0.22 e V ,  a l s o ,  a gradual  
decrease i n  photoconduct ivi ty  extending Erom t h e  abso rp t ion  edge 
t o  ~ 0 . 8  e V  i s  observed i n  both  cond i t ions .  This could be due 
t o  t h e  small  amount of d i so rde r  caused by 1 . 5  MeV e l e c t r o n s .  
I n  a f e w  cases ,  t h e  E,-0.54 e V  l e v e l  can be seen 
C. High Temperature S t r e s s  S tudies  
A t  e l e v a t e d  temperatures ( 160OC) t h e r e  i s  enough thermal 
4 
energy a v a i l a b l e  f o r  t he  atomic conf igu ra t ion  of t h e  divacancy 
t o  r e o r i e n t  (3 ) .  S t r e s s  a t  160°C followed by cool ing  wi th  t h e  
stress on and removal of t h e  stress when t h e  sample reaches 
room temperature f r eezes  i n  the  atomic r e o r i e n t a t i o n  b u t  a l lows 
the e l e c t r o n i c  conf igu ra t ion  t o  r e d i s t r i b u t e  t o  i t s  uns t r e s sed  
s t a t e .  F ig .  2 shows t h e  r e s u l t s  of such a stress on the  Ec-0.39 
eV l e v e l .  Curves ( a )  and (b)  a r e  obtained by s u b t r a c t i n g  the 
photoconductivity due t o  t h e  Ec-0.36 and E,-0.22 e V  l eve l s  from 
t h e  t o t a l  curve.  This g ives  t h e  photoconduct ivi ty  due t o  t h e  
Ec-0.39 e V  l e v e l  a lone.  Fur ther  r e s u l t s  of h igh  temperature 
stress a r e  given i n  Table I. 
TABLE I 
Dichroism Resu l t s  on 1 Ohm-cm S i l i c o n  S t r e s sed  a t  S16OoC. 
Stress S t r e s s  I R  
Doping ( kg / cm2 ) d i r e c t i o n  d i r e c t i o n  Ec-0.39eV Ec-0.54eV 
As 1540 Dl 1.23 
A s  1540 6al 1.13 
Sb 1450 col 1.16 
P 1770 1 . 1 7  1 . 2 5  
P 1600 bo I. 00 1.01 
The dichroisms quoted a r e  in  a l l  ca ses  - +.05. These a l l  agree 
i n  sense and w i t h i n  experimental  e r r o r  i n  magnitude with t h e  ones 
found on t h e  divacancy i n  EPR (3)  and on t h e  1.8 
bands i d e n t i f i e d  wi th  t h e  divacancy i n  IR ( 4 ) .  
noted t h a t  stress i n  t h e  [loo] d i r e c t i o n  produces no s i g n i f i c a n t  
dichroism. This agrees  wi th  the  divacancy model where t h e  atomic 
symmetry i s  around a (111) a x i s .  
angles  wi th  [loo] and a r e  a l l  equiva len t  f o r  stress along t h a t  
d i r e c t  ion.  
and 3*3P 
It should be  
These four  axes a l l  make equal  
D. Low Temperature Stress S tudies  
I f  t h e  sample i s  s t r e s s e d  a t  l i q u i d  n i t r o g e n  temperature 
and t h e  stress l e f t  on during measurement, t he  atomic config-  
u r a t i o n  cannot r e o r i e n t  bu t  t h e  e l e c t r o n i c  one can. Table I1 
presen t s  t h e  r e s u l t s  of such l o w  temperature stress. 
TABLE I1 
Dichroism Resu l t s  on 1 Ohm-cm S i l i c o n  Samples S t r e s sed  a t  78OK. 
Doping 
P 
P 
P 
A s  
A s  
A s  
A sJ; 
S t r e s s  S t r e s s  I R  
(kg/cm2) d i r e c t i o n  d i r e c t i o n  
1230 g00J Lo1J 
1200 bil G 0 
1230 Do &oQl 
1130 Lo1 I i O  
1130 coz @T 
1390 Eo1 I30 
1390 E21 Eo 
*Measured w i t h  white  l i g h t  on t h e  sample. 
E,-0.39 e V  
.91  
1 . 1 7  
.90 
1 . 2 1  
1.14 
1.14 
Ec-0.54eV 
__I 
1.17 
Again, t hese  agree  i n  sense wi th  what has  been found previous ly  
i n  EPR (3) and I R  (4) a l though the  magnitude i n  most cases  i s  
smal le r  here .  A s  soon a s  the stress i s  r e l e a s e d ,  t h i s  dichroism 
disappears  a s  it should s i n c e  i t  i s  only f rozen  i n  below H20°K. 
E .  Annealing of t h e  High Temperature Stress-Induced Dichroism 
Fig .  3 shows t h e  i sochronal  anneal ing of t h e  h igh  temper-  
a t u r e  stress dichroism found in  EPR (3 ) ,  I R  ( 4 ) ,  and t h i s  PC 
s tudy.  The dichroism of the  two PC l e v e l s  disappears  a t  t h e  
same temperature a s  t h a t  of t h e  divacancy (3 ,4 ) .  This adds 
f u t h e r  evidence t h a t  the Ec-0.39eV and Ec-0.54eV l e v e l s  a r e  
a s s o c i a t e d  wi th  t h e  divacancy. 
1.10 
1.10 
6 
F. E f f e c t  of White Light  on the  Photoconductivity 
As was mentioned previous ly ,  t h e  1 . 8 ~  and 3 . 3 ~  bands 
a r e  n o t  seen a t  t h e  same t i m e  i n  I R  where monochromatic l i g h t  
s t r i k e s  t h e  sample (6) .  It has been observed t h a t  i f  t h e  Fermi 
l e v e l  i s  below Ec-0.2leV p o s i t i o n  where only t h e  1.8 
observed and t h e  spectrometer i s  s e t  up so t h a t  white  l i g h t  
str ikes t h e  sample, bo th  bands a r e  seen ( 7 ) .  F igure  4 shows 
band i s  r 
t h e  e f f e c t  on t h e  photoconduct ivi ty  of white  l i g h t  on a sample 
where t h e  Fermi l e v e l  was deeper than  E,-0.22eV. Both t h e  
Ec-0.39eV and E,-0.54eV l e v e l s  a r e  seen. The Ec-0.54eV l e v e l  
causes  much more photoconduct ivi ty  i n  t h i s  cond i t ion  than when 
t h e  Ec-0.39eV l e v e l  i s  observed wi th  no l i g h t  on t h e  sample 
(Fermi l e v e l  above Ec-0.22eV). 
3. SUMMARY AND CONCLUSIONS 
Based on t h e  s i m i l a r i t y  i n  anneal ing and s t ress - induced  
r e o r i e n t a t i o n ,  both atomic and e l e c t r o n i c ,  of t h e  Ec-0.54eV 
and Ec-0.39eV l e v e l s  observed i n  PC and t h e  divacancy previous ly  
s t u d i e d  by EPR and I R ,  we  would argue t h a t  t h e s e  two l e v e l s  
a r i s e  from t h e  divacancy. An i d e n t i f i c a t i o n  of t he  n a t u r e  of 
t he  t r a n s i t i o n s  i s  n o t  poss ib l e  a s  y e t .  Nor can we  b u i l d  
upon o r  change t h e  specu la t ive  suggest ions put  forward by 
Cheng e t  a l .  ( 4 ) .  Both l e v e l s  a r e  t r a n s i t i o n s  from l o c a l i z e d  
s t a t e s  i n  the  forbidden gap t o  t h e  conduction band ( t h i s  was 
implied a l r eady  by i d e n t i f y i n g  them a s  Ec-0.54eV and Ec-.39eV) 
a s  opposed t o  being t o  a l o c a l i z e d  "molecular" s t a t e .  
A f i n a l  no te  i s  t h a t  t h e  l e v e l  l oca t ed  a t  Ec-0.36eV occurs 
very c l o s e  t o  t h e  Ec-0.39eV l e v e l .  A s  such, i t  i s  almost 
impossible t o  study it  f o r  dichroism. The o t h e r  l e v e l s  observed 
in  t h i s  s tudy d id  n o t  e x h i b i t  dichroism c h a r a c t e r i s t i c  of t h e  
divacancy and t h e r e f o r e  can be r u l e d  out  a s  a r , i s ing  from t h e  
de fec t  . 
7 
R~FERE.NCE s
(1) L. J. Cheng, Phys. Letters,  B, 729 (1967) 
(2)  J.  W. Corbe t t  and G. D. Watkins, Phys. Rev. Let ters ,  2, 
314 (1961) 
(3)  G .  D. Watkirrs and J. W. Corbe t t ,  Phys. Rev., 138A; 543'(1965) 
(4) L. J. Cheng, J. C .  C o r e l l i ,  J. W. Corbet t ,  and G. D. Watkins, 
Phys. Rev., 152, 761 (1966) 
(5) V. S .  Vavilov, Radia t ion  Damage i n  Semiconductors, Dunod C i e ,  
P a r i s ,  1965,p;LE5 
(6) H. Y. Fan and A .  K. Ramdas, J. Appl. Phys., 3, 1127  (1959) 
(7)  L. J. Cheng, Ph. D. Thesis ,  Reiisselaer Polytechnic I n s t i t u t e ,  
January 1966 (unpublished) 
8 
FIGURE CAPTIONS 
Figure 1. Isochronal  anneal ing of P-doped, 1 ohm-cm s i l i c o n  
i r r a d i a t e d  wi th  4 x 
(a )  no anneal-l38OC, (b)  168OC - 24ZoC, ( c )  279OC. 
15 minutes a t  each temperature.  
elec./cm2 a t  1.5 MeV. 
Figure 2.  Dichroism of t h e  Ec-0.39eV l e v e l  a f t e r  high temperature 
stress. Arsenic doped, 1 ohm-cm s i l i c o n  i r r a d i a t e d  
wi th  2 x elec./cm2 a t  1 .5  MeV. 1540 kg/cm2 stress 
app l i ed  a t  157OC f o r  15 minutes.  
I sochronal  anneal ing of [OII]. s t ress - induced  dichroisms Figure 3 .  
of- photoconduct ivi ty  and i n f r a r e d .  Also shown i s  
t h e  anneal  of t h e  s t ress - induced  divacancy alignment 
a s  s t u d i e d  by EPR ( 3 ) .  15 minutes a t  each Ta. 
Figure 4 .  E f f e c t  of l i g h t  on h ighly  i r r a d i a t e d ,  n-type s i l i c o n .  
1 7  Arsenic-doped, 1 ohm-cm s i l i c o n  i r r a d i a t e d  wi th  10 
elec/cm2 a t  1.5 MeV. 
I 
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Radiation-induced photoconductivity of silicon

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