Negative photoconductivity in doped germanium semiconductor device - solid state physic

Davis, E. A.

English

NASA Technical Reports Server

Negat ive Photoconduct iv i ty  A s s o c i a t e d  w i t h  Impur i ty  
Conduction i n  Germanium* 
b 
t E .  A .  Davis 
Department of  Phys ics ,  U n i v e r s i t y  of I l l i n o i s  
Urbana, I l l i n o i s  
- -  * a  0 %  
N LI 1 s  
(THRUI 2 00 5 w 
- I  
~ 6 6 -  153 31 
(ACCESSION NUMBER1 
g ;  
c *,z&3f/ * : E  " G ,  
I ,@ CD 
= a  2 e :  
0 -  m o 3  
r Z ;  
4 
v) 
t 
(CATEGORY) 
z 
(NASA CR OR TMX OR AD NUM ER) 
P h o t o c o n d u c t i v i t y  i n  germanium h a s  been 0 0  
s t u d i e d  a t  tempera tures  and doping levels  a t  which 
impur i ty  conduct ion  processes  are  i m p o r t a n t .  Under 
c e r t a i n  c o n d i t i o n s  n e g a t i v e  p h o t o c o n d u c t i v i t y  is  
observed.  R e s u l t s  as a f u n c t i o n  of i n t e n s i t y  and 
wavelength are e x p l a i n e d  by a s imple  model which 
is c o n s i s t e n t  w i t h  accepted views of  i m p u r i t y  
conduct ion .  
* T h i s  work suppor ted  i n  p a r t  by a g r a n t  from t h e  N a t i o n a l  Aeronaut ics  
and Space A d m i n i s t r a t i o n ,  Grant No. NSG-228-62. 
t P r e s e n t  a d d r e s s :  Research Department, Xerox C o r p o r a t i o n ,  Roches te r ,  
New York. 
https://ntrs.nasa.gov/search.jsp?R=19660006042 2020-03-16T21:49:49+00:00Z
2 .  
A t  low tempera tures  h e a v i l y  doped germanium e x h i b i t s  i m p u r i t y  con- 
d u c t i o n  p r o c e s s e s  which depend s e n s i t i v e l y  on t h e  c o n c e n t r a t i o n  of  donors  
( o r  a c c e p t o r s )  and on t h e  compensation." Fr i tzsche" h a s  shown t h a t  
t h e s e  p r o c e s s e s  are c h a r a c t e r i z e d  by a c t i v a t i o n  e n e r g i e s  which a r e  
c l e a r l y  observed i n  l o g  r e s i s t i v i t y  v s .  r e c i p r o c a l  tempera ture  p l o t s .  
F i g u r e  1 i l l u s t r a t e s  t h e  b e h a v i o r  of n - type  germaniumwith  low compensat ion 
as t h e  donor c o n c e n t r a t i o n  is  increased .  The a c t i v a t i o n  energy  E i s  t h e  
f a m i l i a r  donor i o n i z a t i o n  energy which decreases w i t h  i n c r e a s i n g  N . I n  
t h e  low c o n c e n t r a t i o n  range  (N < 10 
o t h e r  a c t i v a t i o n  energy  E i s  apparent ;  it has  been s u c c e s s f u l l y  i n t e r -  
preted- a s  t h e  energy  a s s o c i a t e d  w i t h  t h e  t r a n s i t i o n  by t u n n e l i n g  of a n  
e l e c t r o n  from a n  occupied t o  an  unoccupied s i t e .  The e x i s t e n c e  o f  t h e  
l a t t e r  presupposes  some compensation and t h e  a c t i v a t i o n  energy  arises 
because  of  t h e  need t o  overcome t h e  Coulomb p o t e n t i a l  a s s o c i a t e d  w i t h  
t h e  compensat ing a c c e p t o r .  A s  N i s  i n c r e a s e d  E a t  f i r s t  i n c r e a s e s  and 
t h e n  d e c r e a s e s .  I n  t h e  i n t e r m e d i a t e  c o n c e n t r a t i o n  range  (2x10 
2x10 Sb atoms c m  ),  a n o t h e r  a c t i v a t i o n  energy  E appears  i n  t h e  c u r v e s .  
N o  s i n g l e  model has  been agreed upon f o r  t h e  p r o c e s s  c h a r a c t e r i z e d  by 
E 2  b u t  i t s  r a p i d  d e c r e a s e  w i t h  i n c r e a s i n g  N 
predominant ly  by t h e  o v e r l a p  between wave f u n c t i o n s  on n e i g h b o r i n g  donor  
s i tes .  F r i t z s c h e  h a s  sugges ted  i t  is  a s s o c i a t e d  w i t h  e x c i t a t i o n  t o  a 
band formed by i n t e r a c t i o n  between n e g a t i v e l y  charged donor s i t e s .  
S u p p o r t  f o r  t h i s  model h a s  been obta ined  from s t u d i e s  of E 2  as a f u n c t i o n  
of  compensation- and as a f u n c t i o n  of stress.- T h e o r i e s  based on 
1 
D 
16 antimony atoms ~ m - ~ )  o n l y  one D 
3 
31 
D 3 
17 > ND > 
16 - 3  
2 
s u g g e s t s  t h a t  i t  i s  c o n t r o l l e d  D 
41 5 6 1  
.. 
c 3 .  
t h e  model have been developed by Mikoshiba2’ and Nishimura.- 7 1  
i s  > 2 x 10 Sb atoms c m  , E = 0 and t h e  r e s i s t i v i t y  becomes tempera ture  
independent  below l O o K  cor responding  t o  t h e  format ion  of  a n  i m p u r i t y  band. 
When ND 
17 -3  
A s  t h e  compensation K = N /N i s  i n c r e a s e d ,  E p a s s e s  through a A D  3 
minimum v a l u e  a t  K = 0 . 5  i n  t h e  low c o n c e n t r a t i o n  range8’ 
by theory- and a t  K < 0 . 5  i n  t h e  i n t e r m e d i a t e  c o n c e n t r a t i o n  range.- 
The a c t i v a t i o n  energy  e2  i n c r e a s e s  w i t h  i n c r e a s i n g  K f o r  a l l  donor concen- 
t r a t i o n s . -  
as p r e d i c t e d  
3 1  41 
41 
I n  making measurements of the type  d e s c r i b e d  above i t  i s  normally 
e s s e n t i a l  t o  p r e v e n t  any r a d i a t i o n  f r o m k l l i n g  on t h e  specimen. This  is 
because  of  p r o b a b l e  e x c i t a t i o n  o f  e l e c t r o n s  i n t o  t h e  c o n d u c t i o n  band, where 
they  would have a s u f f i c i e n t l y  h i g h  m o b i l i t y  t o  mask comple te ly  t h e  i m p u r i t y  
conduct  i o n  p r o c e s s e s .  
n e g a t i v e  p h o t o c o n d u c t i v i t y  i n  h e a v i l y  doped samples a t  low tempera tures  
which s u g g e s t s  r a p i d  t r a p p i n g  of the p h o t o - e x c i t e d  c a r r i e r s .  A t  h i g h e r  
l i g h t  levels a p o s i t i v e  p h o t o c o n d u c t i v i t y  r e s u l t e d  and e v i d e n t l y ,  under  
t h e s e  c o n d i t i o n s ,  s u f f i c i e n t  f r e e  c a r r i e r s  were e x c i t e d  t o  t h e  c o n d u c t i o n  
band t o  c o n t r o l  t h e  c o n d u c t i v i t y .  
Dobrego and Ryvkin2’ have r e p o r t e d  , however, a 
The i n t e n s i t y  dependence of p h o t o c o n d u c t i v i t y  i n  germanium samples 
a t  4 . 2 O K  has  been measured i n  t h i s  l a b o r a t o r y .  
wavelength  1.51.1 a r e  shown i n  F i g .  2 .  I n  sample N 1  c o n t a i n i n g  
16 -3 15 - 3  1.8 x 1 0  Sb atoms c m  and P1 c o n t a i n i n g  3 . 7  x 10 Ga atoms c m  , n e g a t i v e  
p h o t o c o n d u c t i v i t y  was observed a t  low i n t e n s i t i e s  of i l l u m i n a t i o n .  P o s i t i v e  
p h o t o c o n d u c t i v i t y  o n l y  w a s  observed i n  s a m p l e  N2 c o n t a i n i n g  7 x 10 Sb atoms c m  
R e s u l t s  u s i n g  e x c i t a t i o n  of 
16 -3 
4 .  
F i g u r e  3 shows t h e  wavelength dependence o f  t h e  n e g a t i v e  photo- 
c o n d u c t i v i t y  i n t h e  f i r s t  n- type sample f o r  two a r b i t r a r y  v a l u e s  o f  t h e  
i n t e n s i t y .  It i s  c lear  t h a t  t h e  e f f e c t  a r i s e s  o n l y  f o r  e x c i t a t i o n  by 
l i g h t  of  energy  g r e a t e r  t h a n  t h e  band gap .  The d e c r e a s e  of  t h e  e f f e c t  
a t  s h o r t  wavelengths  cor responds  t o  a d r a s t i c  d e c r e a s e  i n  t h e  l i f e t i m e  
of t h e  e x c i t e d  car r ie rs  a s  t h e  e x c i t a t i o n  t a k e s  p l a c e  predominant ly  n e a r  
t h e  s u r f a c e .  
0 
For wavelengths  less  t h a n  about 3000 A ,  a n  i n c r e a s e  i n  c o n d u c t i v i t y  
which p e r s i s t e d  a f t e r  removal of  t h e  l i g h t  was observed i n  a l l  samples .  
No decay of  t h i s  induced conductance w a s  observed o v e r  many hours  w h i l e  
t h e  sample w a s  main ta ined  a t  a low tempera ture .  R a i s i n g  t h e  tempera ture  t o  
t h a t  o f  l i q u i d  n i t r o g e n  and subsequent ly  r e c o o l i n g  r e s t o r e d  t h e  conductance 
t o  i t s  o r i g i n a l  d a r k  v a l u e .  S u r f a c e  t r a p p i n g  i s  thought  t o  b e  r e s p o n s i b l e  
f o r  t h i s  e f f e c t .  
F i g u r e  4 i l l u s t r a t e s  a model f o r  t h e  n e g a t i v e  p h o t o c o n d u c t i v i t y  observed 
i n  samples  N 1  and P1. These specimen are i n  t h e  low c o n c e n t r a t i o n  range where 
t h e  e x i s t e n c e  of v a c a n t  donor s i t e s  is n e c e s s a r y  f o r  i m p u r i t y  conduct ion .  
F i g u r e  4a  , shows t h e  s i t u a t i o n  i n  t h e  d a r k  a t  low tempera tures ;  t h e  number 
of v a c a n t  donor s i tes  i s  e q u a l  t o  t h e  number of  a c c e p t o r s .  I f ,  a f t e r  
e x c i t a t i o n  of  e l e c t r o n - h o l e  p a i r s  (Fig. 4b) ,  some of  b o t h  k i n d s  of  carr iers  
become t r a p p e d  i n  t h e  manner shown then  t h e r e  w i l l  be an  e f f e c t i v e  decrease 
i n  t h e  compensation; t h e  number o f  vacant  donor s i t e s  is  d e c r e a s e d  and t h e  
a c c e p t o r s  which t r a p  h o l e s  become n e u t r a l .  Impur i ty  conduct ion  i n  t h e  
donor  l e v e l s  i s  t h e r e b y  reduced. A t  h i g h  l i g h t  i n t e n s i t i e s  t h e  conduct ion  
by unt rapped  car r ie rs  predominates  and normal p o s i t i v e  p h o t o c o n d u c t i v i t y  
i s  observed .  
. .  
b 5. 
Evidence f o r  t h e  above i n t e r p r e t a t i o n  was ob ta ined  by obse rv ing  
t h a t  t h e  n e g a t i v e  pho toconduc t iv i ty  was a s s o c i a t e d  w i t h  an  i n c r e a s e  i n  
t h e  a c t i v a t i o n  energy  E f o r  a d e c r e a s e  o f  compensation 
i n  t h i s  range.  F igu re  5 shows resu l t s  i n  specimen P2 which had a chem- 
i c a l  compensation of 0.4.  
as p r e d i c t e d f /  3 
Sample N2 (F ig .  2 )  i s  i n  t h e  i n t e r m e d i a t e  c o n c e n t r a t i o n  range .  
The e f f e c t  of d e c r e a s i n g  t h e  compensation i n  such  samples  i s  t o  d e c r e a s e  
E2" w i t h  an a s s o c i a t e d  i n c r e a s e  i n  t h e  c o n d u c t i v i t y  a s  observed .  
6.  
REFERENCES 
1. See review by N .  F .  Mott and W .  D .  Twose, Advances &I P h y s i c s ,  
(Taylor  and F r a n c i s ,  L td . ,  London) ed .  by N .  F .  Mott, l0, 107 (1961). 
2 .  H.  F r i t z s c h e ,  J .  Phys. Chem. So l ids .&,  69 (1958). 
3. A .  Miller and E. Abrahams, Phys. Rev. 120, 745 (1960). 
4. E .  A .  Davis and W .  D .  Compton, t o  be  p u b l i s h e d .  
5. F. H.  P o l l a k ,  Phys. Rev. 138, A618 (1965). 
6 .  H .  F r i t z s c h e ,  Phys. Rev.125, 1552 (1962) .  
7 .  H. Nishimura,  Phys.  Rev. t o  be p u b l i s h e d .  
8. H. F r i t z s c h e  and M. Cuevas, P roceed ings  of t h e  I n t e r n a t i o n a l  
Conference  on Semiconductor P h y s i c s ,  Prauge (1960). 
9.  V .  P .  Dobrego and S .  M. Ryvkin, S o v i e t  Phys ic s  - S o l i d  S t a t e  A, 
402 (1962). 
. 
.. . .  
7 .  
FIGURE CAPTIONS 
F i g u r e  1 V a r i a t i o n  of r e s i s t i v i t y  w i t h  tempera tures  f o r  Sb-doped 
G e  (from Ref. 4 ) .  The dashed l i n e s  d i v i d e  t h e  r e g i o n s  
3‘  c h a r a c t e r i z e d  by a c t i v a t i o n  e n e r g i e s  E E and E 1’ 2’ 
F i g u r e  2 P h o t o c o n d u c t i v i t y  a s s o c i a t e d  w i t h  i m p u r i t y  conduct ion  a s  
a f u n c t i o n  o f  l i g h t  i n t e n s i t y .  Sample N 1  c o n t a i n e d  
1.8 x 10 Sb atoms c m  ; sample P l y  3.7 x 1 0  Ga atoms cm , 
and sample N2, 7 x 10 Sb atoms c m  . 
16 -3 15 -3. 
16 -3  
F i g u r e  3 Wavelength dependence of n e g a t i v e  p h o t o c o n d u c t i v i t y  i n  
sample N 1  f o r  two values o f  i n t e n s i t y .  
F i g u r e  4 .  Model f o r  n e g a t i v e  p h o t o c o n d u c t i v i t y  i l l u s t r a t i n g  t h e  
e f f e c t i v e  r e d u c t i o n  i n  compensation by car r ie r  t r a p p i n g .  
3 F i g u r e  5 V a r i a t i o n  of  impur i ty  conduct ion  a c t i v a t i o n  energy  E 
w i t h  i l l u m i n a t i o n  i n  sample P2(N = 10 I n  atoms c m  
-3 ND = 4 x 1015 Sb atoms c m  ) .  
16 -3 
A 
. . .  
IO8 
lo7 
IO6 
lo5 
4 -g IO 
L 
0 
r 
0 
Y 
lo3 
3 - 
I- 
fn 
fn 
- 
z IO' 
IO 
I 
lo-' 
I I I I I 
15 
ND=6.7 x IO 
2.5 x 
J 9 \ \ 
He 
I I I lo-' 
I TEMPER ATU RE(OK-') 
d 
. 
0 
W 
b 
U 
b 
1.5 p 
N2 4.Z°K 
INTENSITY (arbitrary scale) 
FIGURE 2 
I I 1 1 I I 
6, - 0 (9 Fc 00 
0 0 0 0 0 
4 
I 
4 
h n 
Y 
n 
0 
Y 
h 
0 
. -  
. 
r 
IO2 
IO' 
I 
He 
0.1 
0 0.1 0.2 0.3 0.4 
1/T ( O K )  
FIGURE 5 


Negative photoconductivity associated with impurity conduction in germanium

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