Effects of fast electron irradiation on silicon semiconductor

Compton, W. D.

English

NASA Technical Reports Server

i .’+ , 
b -  ’ 
I 
E 
:: 14: 5 .- 6; i . n) ..‘ Semi-Annual Status Report I ’. - .,+ July 1, 1964 - December 31, 1964 
An Experimental Investigation of 
4 
L 
Radiation Effects in Semiconductors 
W. Dale Compton 
Fr inc ipal Investigator 
Department of Physics 
University of Illinois 
Urbana, Illinois 
Research Grant No. NsG-228-62 
National Aeronautics and Space Administration 
Washington, D.C. 
https://ntrs.nasa.gov/search.jsp?R=19650010266 2020-03-17T00:37:57+00:00Z
t 
1 
INTRODUCTION 
Many of the  p r o p e r t i e s  of semiconductors are extremely s e n s i t i v e  t o  
the concen t r a t ion  and type of po in t  imperfect ions t h a t  a r e  p re sen t .  These 
d e f e c t s  include i n t e n t i o n a l  impur i t i e s  such as donors o r  accep to r s ,  a c c i d e n t a l  
impur i t i e s  such as oxygen, o r  i n t r i n s i c  l a r t i c e  d e f e c t s  such as vacancies  
and i n t e r s t i t i a l s .  I r r a d i a t i o n  wi th  high energy p a r t i c l e s  genera tes  l a t t i c e  
defec ts -vacancies  and i n t e r s t i t i a l s - t h a t  may then i n t e r a c t  wi th  o t h e r  d e f e c t s  
and produce s t i l l  o t h e r  d e f e c t s  of a nore complex t y p e .  The microscopic 
n a t u r e  of many of t hese  d e f e c t s  i s  unknown and the  inf luence  of t h e s e  d e f e c t s  
upon the  p r o p e r t i e s  of t h e  m a t e r i a l  i s  poorly understood. 'The experiments 
descr ibed  below are intended t o  provide information about t h e  na tu re  of t he  
d e f e c t s  and about the  l o c a t i o n  of the  e l e c t r o n i c  energy l e v e l s  t h a t  a r e  
produced by t h e s e  d e f e c t s .  These s t u d i e s  a r e  c u r r e n t l y  dea l ing  wi th  the  
e f f e c t s  of i r r a d i a t i o n  upon the minori ty  c a r r i e r  l i f e t i m e s  of  s i l i c o n ,  the 
in f luence  of i r r a d i a t i o n  upon the  recombination luminescence of s i l i c o n  and 
gemanium, and the  inf luence  of i r r a d i a t i o n  upon the  conduction process  a t  
IQW temperature  when the  impuri ty  concent ra t ion  i s  s u f f i c i e n t l y  high t h a t  
impari ty  band conduction predominates. 
CURRENT RESEARCH 
Models of several of the  prominent d e f e c t s  t h a t  a r e  generated i n  s i l i c o n  
by i r r a d i a t i Q n  a t  room temperature with f a s t  e l e c t r o n s  have been e s t a b l i s h e d  
by e l e c t r o n  s p i n  resonance s tudies . -  O f  these  d e f e c t s ,  t h r e e  are of 
p a r t i c u l a r  in te res t  t o  us. They have been c a l l e d  the Si-A, Si-E and the  
I 
11 Ji 
S i - J  c e n t e r s .  The s i l i c o n  A c e n t e r  c o n s i s t s  of a s u b s t i t u t i o n a l  oxygen atoc.. 
, 
2 
The e l e c t r o n  s p i n  resonance i s  seen only when the  Fermi l e v e l  i s  c l o s e r  than  
0.16 eV t o  t he  conduction band. E l e c t r i c a l  conduct iv i ty  measurements have 
ind ica t ed  t h a t  t h e  de fec t  i s  a donor wi th  an energy l e v e l  0.16 t o  0.17 eV below 
t h e  conduction band. The s i l i c o n  E c e n t e r  c o n s i s t s  of a s i l i c o n  vacancy 
t rapped next  t o  a s u b s t i t u t i o n a l  phosphorous impurity.  The e l e c t r o n  s p i n  
resonance i s  only found when the  Fermi l e v e l  i s  c l o s e r  than about 0.4 eV 
t o  the  conduction band. 
doped s i l i c o n  i r r a d i a t e d  wi th  Co gamma rays  i n d i c a t e s  t h a t  t h i s  de fec t  
in t roduces  an acceptor  s ta te  0.47 eV below the  conduction band.- 
s i l i c o n  the  prominent d e f e c t  appears t o  be a divacancy, c a l l e d  the  S i - J  cen te r .  ~ 
E l e c t r i c a l  conduct iv i ty  measurements on phosphorous 
60 
2 /  I n  p-type 
Two s ta tes  of  t h i s  d e f e c t  are found depending upon whether the  Fermi l e v e l  
i s  n e a r e r  than 0.3 eV t o  the  valence band o r  nea re r  than  0.47 e V  t o  the  con- 
duc t ion  band. Since a l l  t he  de fec t s  descr ibed above r e s u l t  from an a s s o c i a t i o n  
of a l a t t i c e  vacancy w i t h  an  i n t e r s t i t i a l  oxygen atom, wi th  a donor o r  wi th  
another  vacancy, i t  is reasonable  t o  expect  t h a t  t h e  r e l a t i v e  concent ra t ion  of 
each of the  complex d e f e c t s  w i l l  depend upon the r e l a t i v e  concent ra t ion  o f  the  
impur i t i e s  and of vacancies .  
MINORITY CARRIER LIFETIME. The l i f e t i m e  of minor i ty  carriers t h a t  
are r e l eased  i n  the c r y s t a l  by a s h o r t  pu l se  of l i g h t  depend upon the  m n -  
c e n t r a t i o n  of recombination centers .  Each of the  d e f e c t s  mentioqed above can, 
under appropr i a t e  cond i t ions ,  a c t  as  recombination cen te r s .  Hal l  and Shockley 
and Read,2/ have shown t h a t  f o r  a s i n g l e  recombination c e n t e r  with a small  
concen t r a t ion  of excess  c a r r i e r s  t h a t  t he  minor i ty  c a r r i e r  l i f e t i m e  T i s  given by 
4 
I n  o rde r  t o  determine the  inf luence of dopant and oxygen content  upon 
the  minor i ty  c a r r i e r  l i f e t i m e ,  a s e r i e s  of n and p-type samples of var ious  
r e s i s t i v i t i e s  w a s  measured p r i o r  t o  i r r a d i a t i o n .  Oxygen content  was va r i ed  
by us ing  zone-ref ined and Czochralski  grown crys ta l s .  C r y s t a l s  grown by t h e  
l a t t e r  technique normally con ta in  10 t o  10 more oxygen than do the  former. 
I r r a d i a t i o n s  wi th  Co gamma rays  are underway and f u r t h e r  measurements of T 
are being made. These r e s u l t s  a r e  summarized i n  Tables 1 and 11. Previous 
d a t a  taken a t  h igher  r a d i a t i o n  doses a r e  not  repor ted  i n  these  tables.-  
The p resen t  d a t a  r ep resen t  t h e  i n i t i a l  s t a g e s  of a s y s t e n a t i c  s tudy of 
i r r a d i a t i o n  and thermal anneal ing.  
of the  excess  c a r r i e r s  t o  decay. No value of the  a c t i v a t i o n  energy E i s  
l i s t e d  un le s s  a s i n g l e  unique value was observed. The following observa t ions  
can be made  from these  pre l iminary  data:  
2 3 
60 
41 
i s  the  t i m e  requi red  f o r  one-half  T1/2  
R 
1) Float-zone grown s i l i c o n  i s  more s e n s i t i v e  t o  i r r a d i a t i o n  
than  i s  Czochralski  grown s i l i c o n .  
2) Heat t reatment  p r i o r  t o  i r r a d i a t i o n  can s i g n i f i c a n t l y  a l t e r  
t h e  minor i ty  c a r r i e r  l i f e t i m e .  A comparison of samples 
L and L2 sugges ts  t h a t  t h e  change i n  l / T  introduced 
by i r r a d i a t i o n  i s  not  a f f ec t ed  by the  anneal .  
With the  except ion  of  sample E 
between minor i ty  c a r r i e r  l i f e t i m e  and room temperature 
r e s i s t i v i t y  f o r  the  f loat-zone grown ma te r i a l .  No such 
c o r r e l a t i o n  e x i s t s  f o r  Czochralski  grown ma te r i a l .  
1 112  
3)  a genera l  c o r r e l a t i o n  e x i s t s  4’ 
Fur the r  gamma r a y  i r r a d i a t i o n s  a r e  underway on these  samples. H a l l  
- c u e f f i c i e n t  and r e s i s t i v i t y  measurements have been made on specimens c u t  
3 
from the  same s l i c e  a s  t h e  l i f e t i m e  samples. Complete temperature measurements 
have been made i n  several  cases  and w i l l  be repea ted  fol lowing i r r a d i a t i o n s  
t h a t  measureably a l t e r  t he  l i f e t i m e .  
RECOMBINATION LUMINESCENCE. Study of  t h e  recombination luminescence 
i n  s i l i c o n  has been unexpectedly d i f f i c u l t .  A f t e r  many unsuccessfu l  a t tempts  
t o  f i n d  t h i s  luminescence i n  un i r r ad ia t ed  s i l i c o n ,  i t  became r a t h e r  obvious 
t h a t  t he  su r face  t rea tment  w a s  n o t  s u f f i c i e n t  t o  produce a long su r face  l i f e -  
t i m e .  ' S ince  the  o p t i c a l  e x c i t a t i o n  of the  luminescence i s  confined t o  s h o r t  
wavelenFths, f o r  which abso rp t ion  c o e f f i c i e n t s  a r e  h igh ,  i t  i s  i n p e r a t i v e  
t h a t  t h e  excess  c a r r i e r s  have a s u f f i c i e n t  l i f e t i m e  t o  enable  them t o  d i f f u s e  
away from t h e  s u r f a c e  i n  o rde r  t o  recombine i n  the  bulk  sample. Impur i t i e s  
on t h e  su r face ,  t h e  n a t u r e  o f  t he  oxide l a y e r ,  t he  e x t e n t  of the  damaged 
layer r e s u l t i n g  from c u t t i n g  and pol i sh ing ,  and many o t h e r  €ac to r s  can r e s u l t  
i n  n o i - r a d i a t i v e  recombination of excess c a r r i e r s .  This problem has  been 
solved by u t i l i z i n g  a t rea tment  of the samples t h a t  c l o s e l y  p a r a l l e l s  t h a t  
used by Haynes.5' 
p-type s i l i c o n  is  shown i n  the  at tached f igu re .  The h a l f  width a t  t he  l o w  
temperature  i s  n o t  a s  narrow as t h a t  reported by Haynes f o r  77'K. 
i s  a s e n s i t i v e  func t ion  of temperature,  i t  simply means t h a t  b e t t e r  con tac t  
must be made between the  sample and the coo lan t ,  i n  order  t o  conduct away the  
l a r g e  amount of h e a t  generated by the  e x c i t a t i o n  l i g h t .  This p re sen t s  no 
d i f f i c u l t y .  
The recombination luminescence of a sample o f  m e  ohm-cm ' 
Since  t h i s  
A cooled PbSe d e t e c t o r  has  been purchased from Infra-Red I n d u s t r i e s .  
C a l i b r a t i o n  of t h i s ,  us ing  a r a d i a t i o n  thermocouple as a r e fe renceJ  i n d i c a t e s  
t h a t  t h i s  d e t e c t o r  i s  s e n s i t i v e  t o  7.4 microns and t h a t  i t s  response p e r  
1 
6 
absorbed photon i s  cons tan t  t o  .+ 10% between 1 and 6 mic rons .  Although t h i s  
d e t e c t s r  i s  inhe ren t ly  less s e n s i t i v e  than PbS, i t  h a s  a much g r e a t e r  range 
of s p e c t r a l  s e n s i t i v i t y .  The response of the  PbS d e t e c r o r  e';ciws a zo re  LI- 
less gradLal dec l ine  frcm 1 micron t o  3.4 m i c r a ,  a t  w h f ~ h  po in t  I t s  sen- 
s i t i v i t y  i s  S O  low as to be of L i t t l e  va'lJe. These two decec ta r s  w i l l  now 
b~ used t o  scan the  e n t i r e  s p e c t r a l  region ( o f  i n t e r e s t  sf- both s,lieon and 
germanium before  and a f t e r  i r r a d i a t i c n .  It nbw appears  t h a t  a l l  majcr ai€- 
f i c u d t i e s  have been resolved and t h a t  measurements on i r r a d i a t e d  ma te r i a l  
can progress  r ap id ly .  
STUDIES OF IMPURITY BAND CONDUCTION. Cnanges ia t h e  ; a p U r i t y  band 
ccnduct ion process  as a f f e c t e d  by i r r a d i a t i o n  w i l l  not  be descr ibed in de- 
t a i l  he re  s i n c e  a manuscript  i s  i n  t h e  p rocess  cy€ k i n g  coTplersed. A b ~ m -  
mary o f  the  work i s  as follows: Samples havins  a donor eozcen t r a t ion  
1 7  -3 
between 6.7 x and 1 .7  x 10 cm have been 5 r . i d i e d .  Rearstance and 
H a l l  c o e f f i c i e n t  measurements have been Made between ~ O O M  temperature and 
1 . 8 " K .  
I r r a d i a t i o n  wi th  f a s t  neGtrons introduces deep l y i n g  a e : x p t w s  which con- 
pensa te  a p o r t i o n  of the  donors. The cumpawation has  been increased t 2  about 
'These samples have a norrial compensation of abcrtA+ L'b whe2 recei-h,tJ .  
85% in most samples. 
the  room temperature H a l l  c o e f f i c i e n t .  The changes i n  z o ~ d u c t i o n  
near  t he  l i q u i d  h e l i u n  y i e l d s  information t h a t  i s  u s e f u l  in  exaz.fnfng the  
va r ious  Todels of impurity conduction t h a t  have b e e n  proposed. ,) Fcr the  
lower donor concent ra t ions ,  the model t h a t  an e l e c t r o n  hops €PIX an occupied 
donor t o  a neighbcring ionized donor, ha5 been well e s t a b l i s h e d  experimental ly  
and theoret ical ly . -L.  Data obtained i n  t h i s  s tudy  are i n  agreernent w i th  
t h i s  model. A t  the  h ighes t  donor concent ra t ions  a band is fo r r ed  from the  
The ranpensat ion w a s  de t emined  f r m  changes i n  
6 7 /  
7 
ground s ta tes  of the impur i t i e s  and conduction takes  p l ace  without  an a c t i v a t i o n  
energy. A s  the  compensation of such a sample i s  increased ,  t he  conduction 
decreases ,  e x h i b i t s  an a c t i v a t i o n  energy, and assumes t h e  form of the  con- 
ducti.on of a less doped sample. The mechanism of  conduction in t h i s  i n t e r -  
mediate reg ion  is  less w e l l  understood. It i s  be l ieved  t o  a r f s e  from a pro- 
c e s s  t h a t  involves  t h e  exchange in t e rac t ion  of the donor atom e l e c t r o n s  i n  
exc i t ed  s t a t e s .  The r e s u l t s  of these  s t u d i e s  support  t h i s  view. A p o r t i o n  
of t h i s  work w i l l  be presented  a t  t he  American Physical  Soc ie ty  meeting i n  
Kansas C i t y  i n  March. The a b s t r a c t  of t h i s  paper is a t tached  t o  t h i s  r e p o r t .  
Measurements of  the  changes i n  the impurity Conduction r e s u l t i n g  from 
gamma r a y  i r r a d i a t i o n  have been undertaken i n  order  t o  compare wi th  r e s u l t s  
obtained on neutron i r r a d i a t e d  samples. 
Prel iminary d a t a  on the  thermal anneal ing of d e f e c t s  produced by the  
neut ron  i r r a d i a t i o n  of t hese  h ighly  doped samples i n d i c a t e s  t h a t  the ma jo r i ty  
of the  annea l ing  occurs  a t  temperatures above 250°C. There i s  v i r t u a l l y  none 
of t h e  150°C anneal ing behavior present  which has  been observed i n  h igher  
r e s i s t i v i t y  material." 
the  a c t i v a t i o n  energy f o r  annea l ing  and c o r r e l a t e  t h i s  wi th  the  disappearance 
of a s p e c i f i c  de fec t .  
These d a t a  w i l l  be extended i n  an e f f o r t  t o  e s t a b l i s h  
PE RS 0"EL 
D r .  E.A. Davis l e f t  t h e  Universi ty  i n  November t o  take a p o s i t i o n  a t  
the  Xerox Corporat ion i n  Rochester,  New York. D r .  E.L. Wolf completed h i s  
Ph.D. degree a t  Corne l l  Univers i ty  i n  August and began work on t h e  impurity 
conduct ion problem i n  September. 
M r .  Robert Spry and M r .  Ralph Hewes are cont inuing work on the  re- 
combination luminescence and t h e  minori ty  c a r r i e r  l i f e t i m e s  f o r  t h e i r  Ph.D. 
theses  e 
8 
EWERENCE S 
1. 
2. 
3. 
4. 
5. 
6. 
7. 
8. 
G.D. Watkins and J . W .  Corbe t t ,  Trans. Faraday SOC. No. 31, 86 (1961). 
E.Sonder and L.C. Templeton, J .  Appl. Phys. 34, 3295 (1963). 
R.N. H a l l ,  Phys. Rev. 87, 387 (1952); W .  Shockley and W.T. Read, Phys. 
Rev. 87, 835 (1952). 
See NASA TND-2364, An Experiment I n v e s t i g a t i o n  of Radia t ion  E f f e c t s  i n  
Semiconductors by W. Dale Compton. 
D r .  J .R .  Haynes k indly  provided us w i t h  a d e t a i l e d  d e s c r i p t i o n  of h i s  
technique. 
H. F r i t z s c h e ,  Phys. Rev. 125, 1552 and 1560 (1962); 99, 406 (1955). 
A. Miller and E. Abrahams, Phys. Rev. m9 745 (1960). 
S .  Ishimo, F. Nakazawa, and R. Hasigut i ,  J. Phys. SOC. Japan 2, 1033 
(1963). 
..' 
9 
Table I 
60 E f f e c t  of Co I r r a d i a t i o n  Upon Czochralski  Grown S i l i c o n  
6 
Sample p (ohm- cm) P r i o r  t o  I r r a d i a t i o n  I r r a d i a t i o n  
A f t e r  1 . 4 ~ 1 0  R 
(Room Temperature) ( ~ s e c )  ER(eV) T 1 / 2  (wet) ER(eV) 
Ql 
R1 
I3 
O1 
P 1  
p-112 
p-250 
n-220 
p-20 
p-45 
-ii 38 
80 -- 
365 .09 
2 40 
70 
-- 
-- 
~ 
unchanged 
unchanged 
110 .10 
10 
Table 11 
60 
E f f e c t  o f  Co I r r a d i a t i o n  Upon Float-Zone Grown S i l i c o n  
5 
Sample p (ohm- cm) P r i o r  t o  I r r a d i a t i o n  I r r a d i a t i o n  
A f t e r  2 . 2 7 ~ 1 0  R 
T (psec) ER(eV) T l / 2 ( ~ s e c )  ER(eV) 1 / 2  
(Room Temperature) 
B1 
c4 
F3 
L2 
L1 
Ll+ 
M1 
M2 
M2+ 
E4 
Note : 
n-20 
n-200 
p-200 
n-67 
h-67 
n-65 
p-65 
p-65 
p-65 
p-24 
28 
250 
.07 5.8 .085 
.134 16.5 .50 
140 
63 
68 
5.6 
10 7 
a2 
1.96 
250 
.085 110 .054 
.28  7 .2  .46 
.18 3.5 - 
0 1 2  75 . PO5 
.145 
.065 2.12 .036 
.06 
L1+ and M2+ were sample L 
a t  500°C. 
and M r e s p e c t i v e l y  following an anneal  i n  a i r  1 2 
11 
Abs t r ac t  ~f paper t o  be presented a t  Kansas C i t y  meeting i n  March, 1965 
* t Compensation Dependence of Impuri ty  Conduction i n  Germanium , E .A. Davis 
( introduced by W.D. Compton), Univers i ty  of I l l i no i s - -The  e f f e c t  of 
compensation on the  r e s i s t i v i t y  and Hal l  e f f e c t  a s soc ia t ed  wi th  impuri ty  
conduction processes  i n  antimony doped germanium has  been s tud ied  as a 
func t ion  of temperature down t o  1 . 5 " K .  The compensation was va r i ed  i n  
a c o n t r o l l e d  manner by i r r a d i a t i n g  with f a s t  neutrons,  t h e  r e s u l t i n g  
i n t e r s t i t i a l - v a c a n c y  type damage producing the  requi red  acceptor  levels. 
For donor concent ra t ions  less than  5 x 10 cm e x i s t i n g  t h e o r i e s  of  
impuri ty  conduction by tunnel ing  c o r r e c t l y  p r e d i c t  the a c t i v a t i o n  energy 
E assoc ia t ed  wi th  the  temperature  dependence of t h e  r e s i s t i v i t y .  With 
h igher  donor concent ra t ions ,  t he  r e s i s t i v i t y  and H a l l  c o e f f i c i e n t  e x h i b i t  
more complicated behavior  a s soc ia t ed  wi th  another  conduction mechanisrn 
cha rac t e r i zed  by an energy E The compensation dependence of E is 
d iscussed  wi th  re ference  t o  proposed models f o r  t h i s  conduction process.  
15 -3  
3 
2'  2 
* Supported i n  p a r t  by a g ran t  from t h e  National. Aeronaut ics  and 
Space Adminis t ra t ion.  
t Presen t  address:  Xerox Corporation, Rochester,  New York 
. I . , &  * 
3 
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An experimental investigation of radiation effects in semiconductors semiannual status report, 1 jul. - 31 dec. 1964

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