HAL Id: jpa-00220922
https://hal.archives-ouvertes.fr/jpa-00220922
Submitted on 1 Jan 1981
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
FIRST OBSERVATIONS OF PHOTOCONDUCTIVITY IN BULK DOPED GeY Se1-Y GLASSES
J. Chamberlain, A. Moseley
To cite this version:
J. Chamberlain, A. Moseley. FIRST OBSERVATIONS OF PHOTOCONDUCTIVITY IN BULK DOPED GeY Se1-Y GLASSES. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-309-C4-312.
�10.1051/jphyscol:1981465�. �jpa-00220922�
JOURNAL DE PHYSIQUE
CoZZoque C4, suppZ6ment au nOIO, Yome 42, octobre 1981 page C4-309
F I R S T OBSERVATIONS OF PHOTOCONDUCTIVITY I N BULK DOPED Gey Sel-y GLASSES
J . M . Chamberlain and A.J. Moseley
Physics Department, Nottingham University, Nottingham NG7 ZRD, Great Britain
A b s t r a c t . - IJe r e p o r t t h e f i r s t measurements of p h o t o c o n d u c t i v i t y i n b u l k doped and undoped Gey Sel-y ( ~ 3 . 4 ) g l a s s e s . S p e c t r a between 1.2 and 3.0eV a r e p r e s e n t e d i n t h e t e m p e r a t u r e r a n g e 80-300K. S t e a d y - s t a t e b i m o l e c u l a r recombin- a t i o n o c c u r s . An enhancement of t h e p h o t o c o n d u c t i v i t y t a k e s p l a c e f o r i n c l u s - i o n of s m a l l amounts of I n dopant. The r e d u c t i o n of p h o t o c u r r e n t due t o a d d i t i o n of Cu and Fe dopants i s n o t e d . A p e r s i s t e n t p h o t o c o n d u c t i v i t y i s observed which i s s t r o n g l y dependent on germanium c o n t e n t , y , b u t indepeddent of doping. A model t o e x p l a i n t h i s i s proposed.
I n t r o d u c t i o n . - The Se-Ge f a m i l y of c h a l c o g e n i d e g l a s s e s p r o v i d e s a n i d e a l system f o r examining v a r i a t i o n s o f o p t i c a l l y induced phenomena a s a f u n c t i o n of chemical com- p o s i t i o n and dopant a d d i t i o n ( 1 ) . Although t h e e x i s t e n c e of p h o t o c o n d u c t i v i t y has been r e p o r t e d i n f i l m s of Geo.i Se ( 2 ) and i n t h e pseudobinary a l l o y s Ge(Seo.13 TeO. 67) ( 3 ) no measurements have k e n r e p o r t e d of photoconduction f o r b u l k samp e s of t h e s i m p l e b i n a r y a l l o y Gey Sel-y (Y<0.4). -
It i s t h e purpose of t h i s paper t o examine t h e s p e c t r a l dependence, t h e lux- ampere c h a r a c t e r i s t i c s , t h e t e m p e r a t u r e v a r i a t i o n and t h e decay of t h e p h o t o c u r r e n t i n terms of p o s s i b l e s t r u c t u r a l and t r a p - d i s t r i b u t i o n models. Following o u r r e c e n t o b s e r v a t i o n ( 1 ) t h a t photoinduced a b s o r p t i o n i n t h e s e m a t e r i a l s i s i n s e n s i t i v e t o t h e a d d i t i o n of s m a l l amounts of m e t a l l i c i m p u r i t y , i t was a l s o of i n t e r e s t t o examine t h e e f f e c t of t h e above p a r a m e t e r s f o r a r e p r e s e n t a t i v e s e l e c t i o n of dopants ( I n , Cu, Fe) which had been shown t o s i g n i f i c a n t l y modify t h e h o l e t r a n s i t times i n w A s 2 Se3, w i t h o u t a p p a r e n t e f f e c t on o t h e r photoinduced p r o p e r t i e s .
Experimental.- Bulk samples o f Gey Sel-y were p r e p a r e d by a i r o r w a t e r quenching t e c h n i q u e s a c c o r d i n g t o composition. Low oxygen c o n t e n t s e l e n i u m was used i n a l l m a t e r i a l s s y n t h e s e s . Samples of GeOel5 Seo.15 c o n t a i n i n g 0 . 1 and 1.0 a t % of I n , Cu and Fe were a l s o p r e p a r e d . Water-quenching was n e c e s s a r y t o a c h i e v e m i c r o s c o p i c a l l y homogeneous d i s t r i b u t i o n s of Cu and Fe a s determined by SEM a n a l y s i s .
Semi-transparant f r o n t - and t h i c k r e a r - s u r f a c e e l e c t r o d e s were e v a p o r a t e d o n t o p o l i s h e d specimens lOOym t h i c k . Some samples were f a b r i c a t e d w i t h two t h i c k gold c o n t a c t s f o r s i d e - i l l u m i n a t i o n . Gold-epoxy bonded c o n n e c t i o n s were a t t a c h e d t o t h e e l e c t r o d e s . I n most c a s e s , p h o t o c u r r e n t measurements were made a t lOHz u s i n g a 6 t h o r d e r low-pass f i l t e r and s t a n d a r d l o c k - i n d e t e c t i o n . Some DC measurements were made u s i n g a K e i t h l e y 600B e l e c t r o m e t e r . E x c i t a t i o n was p r o v i d e d by a l k i i Xenon lamp and monochromator.
R e s u l t s . - F i g u r e 1 shows t h e p h o t o c u r r e n t s p e c t r a a t 295K n o r m a l i s e d t o i n c i d e n t photon f l u x f o r specimens of undoped m a t e r i a l i n t h e s i d e i l l u m i n a t i o n geometry.
The photoconductive e f f i c i e n c y a t photon energy d e c r e a s e s smoothly w i t h i n c r e a s i n g germanium c o n t e n t . A t low i n c i d e n t photon energy, E, t h e p h o t o c u r r e n t , J p h r f o l l o w s t h e a b s o r p t i o n edge shape ( 1 ) . For h i g h E, J p h b e g i n s t o r i s e w i t h increasing E .
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981465
C4-3 10 JOURNAL DE PHYSIQUE
This behaviour h a s been i n t e r p r e t e d i n terms of bulk recombination, and i s t y p i c a l of o t h e r chalcogenides. The doped m a t e r i a l s e x h i b i t s p e c t r a of s i m i l a r g e n e r a l form.
The v a r i a t i o n of Jph w i t h l i g h t i n t e n s i t y , I, depends i n a complex manner on both,E and t h e chopper frequency, w, i n t h e r e g i o n 5-500Hz. For h i g h E and low w, J p h a I z , and a t low E o r h i g h w, J p h a I . This i s t o be i n t e r p r e t e d a s bimolecular recombina- t i o n of t h e s t e a d y - s t a t e p h o t o c o n d u c t i v i t y . An E- and wdependent phase s h i f t , @,
-9 between s t i m u l u s and
p h o t o c u r r e n t i s obser-
a ved which tends t o a
-10 - c o n s t a n t w-dependent
v a l u e f o r l a r g e E. I#I a l s o i n c r e a s e s w i t h d e c r e a s i n g E a t const- a n t w. A t low E t h i s i s i n t e r p r e t e d i n terms of t h e e x c i t a t i o n of c a r r i e r s i n l o c a l i s e d s t a t e s which a r e sub-
-
s e q u e n t l y t h e r m a l l y e x c i t e d t o t h e m o b i l i t y edge w i t h a n a s s o c i a t e d-I4' I :6 ;a 2IO 2l2 2 4 I Z1b 2 ' 8 YU A time d e l a y .
photon e n e r g y eV
P i g . 1: P h o t o c u r r e n t s p e c t r a f o r Ge Sel- a t 295K. The temperature dependence of Iph f o r a ) y=-05 b) y=.15 c ) y=.25 d) y=.33 ey y=.38 undoped m a t e r i a l w i t h
y=0.15 i s shown i n f i g u r e 2 f o r E=2.6eV i n t h e temperature range 80-340K. The lower temperature i s s e t by s i g n a l : n o i s e con- s i d e r a t i o n s and t h e upper temperature l i m i t i s determined by t h e o n s e t of m a t e r i a l s o f t e n i n g . The r e p o r t e d behaviour i s r e p r e s e n t a t i v e of a l l our undoped m a t e r i a l s , and h a s a l s o been observed i n GeOe5 SeOv5 f i l m s . The c h a r a c t e r i s t i c I-dependent peak i n t h e Jph v s T graphs shown by many chalcogenides a t approximately room temp- e r a t u r e was o n l y noted f o r y=0.05 m a t e r i a l a t 310K. It was t h u s imposible t o per- form a d i s t r i b u t i o n a n a l y s i s (4) s i n c e a l l p r e s e n t d a t a i s l i m i t e d t o one photo- c u r r e n t regime (regime ' 1 1 ' ) . IJe n o t e , however, t h a t f o r p a r t of t h i s regime, i n which JphaexpE-/KT, t h a t E-=0.35eV f o r
y 4 . 1 5 m a t e r i a l f a l l i n g w i t h y t o E-4.18eV -9
a t y=0.33. b
1
F i g u r e 3 shows t h e v a r i a t i o n of sam- p l e c o n d u c t i v i t y and p h o t o c o n d u c t i v i t y a s a f u n c t i o n of c o n c e n t r a t i o n of m e t a l l i c
-
dopants. A t a g i v e n E , t h e p h o t o c u r r e n t 3 - i s enhanced by i n c r e a s i n g t h e I n concen-
g
-t r a t i o n , b u t i s depressed by i n c r e a s i n g
-,,
Fe o r Cu c o n c e n t r a t i o n s . The conductivity, g a, i n c r e a s e s w i t h i n c r e a s i n g I n concentra-
2
-t i o n w i t h o u t any change i n t h e conductiv- - i t y a c t i v a t i o n energy, measured i n t h e
range 295-340K, w h i l e both t h e s e q u a n t i t - -12 i e s a r e u n a l t e r e d by t h e a d d i t i o n of Fe
and Cu dopants.
The p a r a l l e l i n c r e a s e i n a and J
f o r i n c r e a s i n g I n c o n c e n t r a t i o n i s inekr- ienrpcraiurc ( K )
p r e t e d a s a m o d i f i c a t i o n of t h e m i c r o s c ~ p - ~ ; ~ . 2 : Temperature dependence of dark i c m o b i l i t y of t h e c a r r i e r s due t o a c u r r e n t (a) and Jph (b and c ) i n Ge 15 m o d i f i c a t i o n of t h e conducting channel by
se
85 a t two d i f f e r e n t intensities f o r t h e I n . The r e d u c t i o n i n photoconductive C=i.6eV.e f f i c i e n c y w i t h i n c r e a s e d Cu o r Pe concen- t r a t i o n w i t h o u t change i n a i s explained
by a build-up of t r a p s around mid-gap: s i m i l a r c o n c l u s i o n s have been drawn f o r As2 Se3 from h o l e t r a n s i t - t i m e measurements ( 5 ) .
- 1 4
0 1 I 1 0 II
Although long-term decays of JDopant concentrataon (at 6 ) have been observed i n a-As2Se3 ( 7 ) gkd
many c r y s t a l l i n e m a t e r i a l s ( 6 ) no P i g . 3 : I n f l u e n c e of m e t s l l i c dopants on u n i f i e d e x p l a n a t i o n of t h e e f f e c t s
c o n d u c t i v i t y and p h o t o c o n d u c t i v i t y i n e x i s t s . For t h e p r e s e n t o b s e r v a t i o n s ,
Ge.15 Se 8 5 - i t i s suggested t h a t a model might be
~ o n d u c t i k t ~ 0 - I n 8 - Cu
A -
Fe used i n v o l v i n g b a r r i e r s t o recombinat- P h o t o c o n d u c t i v i t y 0- I n 0 - CuA -
Fe i o n p r e s e n t e d by d e f e c t s w i t h s t r o n ge l e c t r o n - l a t t i c e c o u p l i n g ( 8 ) . Using 1WS terminology, t h e h o l e s a r e i n quasi- thermal e q u i l i b r i u m w i t h D-centres according t o t h e r e a c t i o n D-+~++DO. An i n i t i a l f a s t e l e c t r o n c a p t u r e t a k e s p l a c e through t h e r e a c t i o n D++e-+Do. The h o l e s t h e n slowly recombine with t h e s e trapped e l e c t r o n s by a t u n n e l l i n g process between Do c e n t r e s . The i n s e t diagram of f i g u r e 4 maps out-according t o t h e p r e s e n t i n t e r p r e t - a t i o n - t h e r a t e s of c a r r i e r recombination f o r t h e t u n n e l l i n g p r o c e s s , which i s expected t o r e f l e c t t h e e x t e n t of d i s o r d e r i n t h e m a t e r i a l and which should b e max- imal f o r y=0.15. The indium i s p o s s i b l e i n c o r p o r a t e d i n a n e u t r a l bonding c o n f i g u r - a t i o n w i t h energy s t a t e s c l o s e t o t h o s e of t h e conducting channel (5) and t h u s does n o t a f f e c t [D-1, by mass a c t i o n arguments. The i n c l u s i o n of t h i s m a t e r i a l t h u s has l i t t l e e f f e c t on t h e J r a t e decay. S i m i l a r l y , Fe and Cu w i l l have l i t t l e e f f e c t on t h e c o n c e n t r a t i o n
[g-]
s i n c e a l t h o u g h i n c o r p o r a t e d a s p o s i t i v e l y charged impurit- i e s , t h e e l e c t r o n s r e l e a s e d by t h e i m p u r i t y atoms a r e almost c e r t a i n l y taken up by neighbours bonding t o t h e i m p u r i t y . We a r e a t p r e s e n t unable t o f u l l y i n t e r p r e t t h e very s i g n i f i c a n t d i f f e r e n c e between t h e Jph c u r v e s f o r E=1.2eV and 1.8eV i n j e c t i o n .Both doped and undoped m a t e r i a l s show a f a s t (<ips) and slow ( > l s ) p h o t o c u r r e n t decay. We a r e unable a t p r e s e n t t o r e s o l v e t h e f a s t component of decay ( i n o r d e r t o determine recombination parameters) b u t
Conclusions.- P h o t o c o n d u c t i v i t y s p e c t r a f o r t h e undoped Gey SelTy g l a s s e s have a g e n e r a l form t y p i c a l of o t h e r chalcogenide g l a s s e s , although ( w l t h t h e e x c e p t i o n of y=0.05 m a t e r i a l ) t h e y do n o t d i s p l a y t h e c h a r a c t e r i s t i c J v e r s u s T peak i n t h e temperature range s t u d i e d . This may be a consequence of f k o - c a r r i e r conduction i n t h i s m a t e r i a l , a s h a s b e e n suggested f o r o t h e r germanium chalcogenides ( 9 ) . I n view of t h i s r e s t r i c t i o n t o 'regime 11' photoconduction, a t r a p - l e v e l a n a l y s i s i s n o t p o s s i b l e on t h e b a s i s of p r e s e n t d a t a . The i n c l u s i o n of small amounts of m e t a l i c dopant i s shown, f o r t h e f i r s t time i n a chalcogenide g l a s s , t o modify t h e photo-current i n a manner t o be expected from t r a n s i t - t i m e measurements i n o t h e r
-8-
- 9
-10
=
5
-
e ; - 1 1 -
2
Bz5
-
o - I 2-13
0'
-
-
%doped
,/'
A
-
.
L.
,-
undop*
-
-9
- 9 a t y p i c a l s e r i e s of long-term decays f o r w e l l - r e s t e d doped and undoped sam- p l e s of y=0.15 m a t e r i a l a r e shown i n f i g u r e 4. The r a t i o of r e s i d u a l photo- --lo c u r r e n t ( J ) t o s t e a d y s t a t e photocur-
r e n t (JO ) I S p l o t t e d v e r t i c a l l y f o r ~h room temperature e x c i t a t i o n w i t h absor- ph ption-edge l i g h t (1.8-2.0eV). The e f f e c t of i r r a d i a t i o n w i t h low-energy
- :
8 photons (E=1. 2eV) i s shown f o r one 2 specimen which was p a r t i c u l a r l y photo--i2q
r e s p o n s i v e . T h e i n s e t o f f i g u r e 4 shows t h e v a r i a t i o n i n time f o r t h e r a t i o ( J ph ph / J O ) t o reach 5% f o r v a r i o u s--,33
germanium c o n c e n t r a t i o n s of undopedL m a t e r i a l . There i s some evidence t h a t f o r I n doped m a t e r i a l J p e r s i s t s t o very l o n g times (>1000sf- a n e f f e c t
-14 p r e v i o u s l y noted i n s e v e r a l c r y s t a l l i n e semiconductors ( 6 ) .
C4-3 12 JOURNAL DE PHYSIQUE
s i m i l a r m a t e r i a l s . The long-term decay of J h i s s e e n t o b e e x p l i c a b l e w i t h a s i m p l e MDS model b a s e d o n t h e t i m e r e q u i r e d !or t u n n e l l i n g between DO c e n t r e s .
1
-
2
-J-
V
-
.c,
Q .o 1
-
I I I I I I I I
1 l o time ( s ) 100
F i g . 4: P e r s i s t e n t p h o t o c u r r e n t i n doped and undoped Ge l5 Se 8 5 .
( a j 0 . 1 A t .
Z
Cu, Z=2eV (b) 0 . 1 a t . % Fe, C=2eV ( c ) Undoped, 2=2eV (d) 1 a t . % I n , E=2eV( e ) 1 a t . % I n , E=1.2eV. I n s e t shows time t o decay t o 5% o f s t e a d y s t a t e a s a f u n c t i o n o f y .
R e f e r e n c e s
.-
(1) Eloseley, A.J. and Chamberlain, J.M., P h i l . Hag. (B), J u n e 1981 ( i n t h e p r e s s ) . (2) H a u s l e r , H. and Pusch, T., Phys. S t a t . S o l . ( a )
40
(1977) K15.( 3 ) Arnoldussen, T.C., Menezes, C.A., Nakagawa, Y . , Bube, R.H., Phys. Rev. (B)
2
(1974) 3377.
(4) Moustakas, T.D. and Weiser, K . , Phys. Rev. (B)
12
(1975) 2448.(5) P f i s t e r , G. and Morgan, TI., P h i l . Mag. (B)
5
(1980) 209.( 6 ) Eaves, L. and W i l l i a m s , P . , J. Phys. C .
12
(1979) L725.( 7 ) Oheda, H. and Namikawa, H., J a p . J . App. Phys. (1976) 1465.
( 8 ) Lang, D.V. and Logan, R.A., Phys. Rev. L e t t .
2
(1977) 635.( 9 ) Bube, R.H., Mahan, J . and S h i a h , T.S., Appl. Phys. L e t t .
