HAL Id: jpa-00220824
https://hal.archives-ouvertes.fr/jpa-00220824
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.
DOPING OF CHALCOGENIDE GLASSES IN THE Ge-Se AND Ge-Te SYSTEMS
P. Nagels, M. Rotti, S. Vikhrov
To cite this version:
P. Nagels, M. Rotti, S. Vikhrov. DOPING OF CHALCOGENIDE GLASSES IN THE Ge- Se AND Ge-Te SYSTEMS. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-907-C4-910.
�10.1051/jphyscol:19814197�. �jpa-00220824�
JOURNAL D E P H Y S I Q U E
CoZloque C4, suppl&ment au nO1O, Tome 4 2 , octobre 1981 page C4-907
DOPING OF CHALCOGENIDE GLASSES I N THE Ge-Se AND Ge-Te SYSTEMS
P . Nagels, M. Rotti and S . ~ i k h r o v *
Materials Science Department, S . C.K./C. E.ill., B-2400 MoZ, Belgium
*
~ y a z a n Radio Engineering I n s t i t u t e , Ryazan, U. R. S . S.A b s t r a c t . - The e l e c t r o n i c t r a n s p o r t p r o p e r t i e s o f c h a l c o g e n i d e g l a s s e s o f t h e t y p e Ge-Se and Ge-Te t o w h i c h f o r e i g n elements such as B i , As, Cu and I n a r e added, were i n v e s t i g a t e d . Glasses o f t h e (GeSega5) ~o-,Bi, system show a t r a n - s i t i o n f r o m p t o n - t y p e c o n d u c t i o n a t x = 7 a t . % 81 as e v i d e n c e d by thermo- power measurements. The s i g n o f t h e H a l l c o e f f i c i e n t i s n e g a t i v e . T h i s f e a t u r e r e v e a l s t h e p o s s i b i 1 i t y o f a1 t e r i n g t h e d e n s i t y o f charged d a n g l i n g bonds by t h e i n c o r p o r a t i o n o f f o r e i g n a d d i t i v e s . The c o n d u c t i v i t y o f GeTeg g l a s s e s i s l i t t l e a f f e c t e d by t h e a d d i t i o n o f As, Cu and I n . The thermopower i s p o s i t i v e .
I n t r o d u c t i o n . - F o r many y e a r s i t was b e l i e v e d t h a t t h e e l e c t r i c a l p r o p e r t i e s o f c h a l - cogenide g l a s s e s c a n n o t be m o d i f i e d b y d o q i n g f o r t h e r e a s o n t h a t i n t h e amorphous s t a t e each atom can have t h e number o f n e i g h b o u r s r e q u i r e d f o r a l l i t s v a l e n c e e l e c - t r o n s t o f o r m bonds. E x p e r i m e n t a l l y i t was found t h a t t h e c h a l c o g e n i d e g l a s s e s , e x c e p t a few cases, a r e n - t y p e semiconductors. The d o m i n a t i n g h o l e c o n d u c t i o n was a s c r i b e d t o a l o c a t i o n o f t h e Fermi l e v e l somewhat c l o s e r t o t h e m o b i l i t y edge of t h e v a l e n c e band t h a n mid-gap. I n o r d e r t o e x p l a i n t h e o i n n i n g o f t h e Fermi l e v e l , D a v i s and Y o t t ( 1 ) proposed t h e e x i s t e n c e o f compensating deep donors and a c c e p t o r s . The o r i g i n o f t h e s e gap s t a t e s was a t t r i b u t e d by Y o t t e t a l . ( 2 ) t o p o s i t i v e l y and n e g a t i v e l y charged d a n g l i n g bonds, D+ and D-, o f equal c o n c e n t r a t i o n . K a s t n e r e t a l . ( 3 ) p r o v i d e d a more p r e c i s e model f o r t h e s t r u c t u r e o f t h e d e f e c t s which t h e y c a l l e d v a l e n c e - a l t e r n a t i o n c e n t r e s . I n f o l l o w i n g papers, M o t t ( 4 ) , K a s t n e r ( 5 ) and
F r i t z s c h e e t a l . ( 6 , 7 ) d i s c u s s e d t h e e f f e c t o f c e r t a i n charged i m p u r i t i e s on t h e e l e c t r i c a l c o n d u c t i v i t y o f c h a l c o g e n i d e g l a s s e s . They p o i n t e d o u t t h a t , when t h e a d d i t i v e s a l t e r t h e r a t i o o f t h e c h a r ~ e d d a n g l i n g bonds, t h e c o n d u c t i v i t y may be i n c r e a s e d b y many o r d e r s o f magnitude and even r e v e r s a l o f c o n d u c t i o n t y p e may occur.
I n r e c e n t s t u d i e s , Tohge and coworkers (8,9) r e p o r t e d t h a t Gep0Se80-,Bi, and Se2 S e 7 0 - ~ T e ~ ~ B i ~ show a s i g n r e v e r s a l o f t h e thermo?ower f r o m p t o n a t a g i v e n B i con?ent. Because o f t h i s u n i q u e f e a t u r e i t seemed i n t e r e s t i n g t o us t o s t u d y t h e B i doped Ge-Se and Ge-Se-Te systems i n d e t a i l b y p e r f o r m i n g measurements o f dc e l e c t r i - c a l c o n d u c t i v i t y , thermoqower and H a l l e f f e c t . F o r comparison t h e e f f e c t o f o t h e r a d d i t i v e s , such as .4s, Cu and I n , on t h e e l e c t r o n i c : r o o e r t i e s o f Ge-Te g l a s s e s was a l s o examined.
E x p e r i m e n t a l procedures
.-
The m a t e r i a l s were p r e o a r e d i n t h e c o n v e n t i o n a l way by m e l t i n g a m i x t u r e o f t h e o u r e elements i n q u a r t z tubes a t 1050°C f o r 48 h o u r s w i t hc o n t i n u o u s r o t a t i o n . The m e l t was t h e n quenched i n i c e - w a t e r . A f i r s t group o f mate- r i a l s had t h e c o m p o s i t i o n (GeSe3.5)100-xBix, w i t h x i n t h e range 0 t o 20 a t . %. The g l a s s f o r m i n g r e g i o n as checked b y X-ray d i f f r a c t i o n extends t o 14 a t . % B i . I n a second s e r i e s o f m a t e r i a l s t h e B i c o n t e n t was k e p t c o n s t a n t , whereas s e l e n i u m was p a r t l y r e p l a c e d by t e l l u r i u m y i e l d i n g a l l o y s o f c o m p o s i t i o n ( G e S e 3 . 5 - x T e y ) 9 3 B i l ~ . A t h i r d group c o n t a i n e d a l l o y s o f t h e t y n e GeTe6 i n w h i c h some Te i s s u b s t i t u t e d by As, Cu and I n . The g l a s s f o r m i n g a b i l i t y i n t h e B i doped Ge-Se and Ge-Se-Te systems
i s r e l a t i v e l y s m a l l . The homogeneity o f t h e s e g l a s s e s was examined by m i c r o p r o b e a n a l y s i s and b y scanning e l e c t r o n microscopy. As-prepared samples k e p t a t room tem- p e r a t u r e d i d n o t show any microphase s e p a r a t i o n . A f t e r a n n e a l i n g t o a b o u t 150°C t h e m a t e r i a l showed a microheterogeneous s t r u c t u r e b u t no o o l y c r j / s t a l l i n i t y was observed.
A f t e r t h i s process o f phase s e p a r a t i o n , r e c r y s t a l l i z a t i o n s t a r t s around 250°C.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814197
C4-908 JOURNAL DE PHYSIQUE
Results and discussion.- The thernopower S o f t h e (GeSe3.5)100-xBix glasses measured a t 400 K i s p l o t t e d i n f i g u r e 1 as a f u n c t i o n o f t h e B i content. The f i g u r e shows t h a t t h e s i g n o f t h e thermopower changes from p o s i t i v e t o negative a t x approximate- l y equal t o 7 a t . % B i . I n the c r y s t a l l i n e m a t e r i a l obtained f o r x > 18 a t . % e l e c - t r o n s a r e t h e dominant charge c a r r i e r s . The composition dependence o f t h e thermo- power a t 300 K f o r c r y s t a l l i n e and amorphous (GeSe3 5-xTex)g~Bil a l l o y s i s shown i n f i g u r e 2. These m a t e r i a l s e x h i b i t n-type c o n d u c t i o n ' ~ n the amorpious s t a t e . I n t h e c r y s t a l l i n e nhase, which i s obtained f o r x
L
1 t h e thermopower i s p o s i t i v e . A g l a s s o f composition (GeSe3Teo 5)96B24 WaS found be p-type. We a l s o t r i e d t o dope amor- phous Se w i t h B i b u t t h e ' a d d i t i o n o f 3 a t . % r e s u l t e d i n c r y s t a l l i z a t i o n o f t h e mate- r i a l . Pure c r y s t a l l i n e Se i s a p-type semiconductor whereas t h e thermopower o f Seg7Bi3 becomes negative, probably due t o the presence of seggregated Bi2Se3 which i s n-type as a pure compound.The dc e l e c t r i c a l c o n d u c t i v i t y data o f f o u r glasses o f t h e (GeSeg .5) 00-~Bi, system are represented i n f i g u r e 3 as l o g o versus 1 0 3 / ~ . I t can be seen t k a t t h e c o n d u c t i v i t y markedly increases when t h e B i c o n t e n t increases from 8 t o 10 a t . %.
The a d d i t i o n o f h i g h e r amounts o f B i (10 t o 14 a t . %) s t i l l f u r t h e r a f f e c t s t h e con- d u c t i v i t y b u t t o a much l e s s e r e x t e n t . The c o n d u c t i v i t y data o f a (GeSe3Teo. )9 Bi10 glass i s a l s o represented i n f i g u r e 3. The s u b s t i t u t i o n o f t e l l u r i u m f o r par! o?
selenium has l i t t l e i n f l u e n c e on t h e c o n d u c t i v i t y . I n t h e same f i g u r e we a l s o show a chalcogenide glass i n which bismuth was replaced by antimony. This glass, having t h e composition (GeSe3.5) Sb12, e x h i b i t s a much lower c o n d u c t i v i t y than t h e correspon- d i n g B i doped glass. ?Be c o n d u c t i v i t y o f a l l compositions can be expressed by an exponential r e l a t i o n s h i p o f the form: o = a, exp ( - Eo/kT). The values o f t h e a c t i - v a t i o n energy E, and o f t h e pre-exponential c o n s t a n t a. are l i s t e d i n t a b l e I. The marked increase i n c o n d u c t i v i t y when x passes from 8 a t 10 a t . % i s m a i n l y caused by a r e d u c t i o n o f the a c t i v a t i o n energy which amounts t o about 1 / 3 . I n f i g u r e 4 t h e thekmopower o f f o u r d i f f e r e n t compositions i s p l o t t e d versus r e c i p r o c a l temperature.
The s i g n o f t h e thermopower o f t h e B i doped a l l o y s i s negative, i n c o n t r a s t w i t h t h e Sb doped g l a s s f o r which p-type conduction was observed. The S vs. 1 0 3 / ~ p l o t s d i s - p l a y a l i n e a r behaviour and can be represented by the usual formula:
S =
-
k/e ( E /kT+
A ) . The values o f the a c t i v a t i o n energies ES and o f A are a l s o given i n tab?e I . The most s t r i k i n g f e a t u r e i s t h e h i g h e r a c t i v a t i o n energy o f t h e thermopower as compared t o t h a t o f t h e c o n d u c t i v i t y . The temFerature dependence o f t h e H a l l c o e f f i c i e n t i s shown i n t h e upper p a r t o f f i g u r e 5. The s i g n o f the H a l l c o e f f i c i e n t i s negative, thus y i e l d i n g the same r e s u l t as the thermopower. The H a l lF i g . 1 : Composition dependence o f thermo- F i g . 2 : Composition dependence o f t h e r - power a t 400 K f o r c- and mopower a t 300 K f o r c- and a - ( G e S e 3 . 5 ) 1 0 0 - ~ B i ~ a1 l o y s . a-(GeSe3.5-xTex)P~Bi 10 a1 l o y s .
Fig. 3 : Temperature dependence of conduc- Fig. 4 : Thermopower versus reciprocal t i v i t y of a-(GeSe3 . 5 ) 100-xBix, temperature of three
a-(GeSegTe .5)90B~ 10 and (GeSe3 .5 ) 100-~Bi glasses and a - ( ~ e ~ e x 5 ? 8 8 ~ b 1 2 - a (GeSe3Te0 .5)906i 10 g l a s s .
Fig. 5 : Temperature dependence of Hall Fig. 6 : Thermopower versus reciprocal c o e f f i c i e n t and Hall mobility of temperature of four GexTe A, amor2hous (GeSe3.5) 100-xBix and glasses with A = As, Cu aXd In.
(GeSe3Te0.!j)90B110-
mobility i s represented i n the lower p a r t of the same f i g u r e . Corn ared t o other chal- cogenide systems, the Hail mobility i s very low (pH = 10-2 cm-1"-fs-1 a t 370 K) and e x h i b i t s , in contrast t o them, a s l i g h t decrease w i t h increasing temperature.
The appearance of n-type conduction in the Bi doped glasses must r e s u l t from a s h i f t of the Fermi level towards the mobility edge of the conduction band. As sugges- ted by Mott ( 4 ) , the e l e c t r o n i c properties of chalcogenide glasses can be d r a s t i c a l - l y influenced i f the added i n ~ p u r i t i e s a r e charged and compensate one type of the
C4-9 10 JOURNAL DE PHYSIQUE
Table I : Physical parameters of c o n d u c t i v i t y , thermopower and H a l l m o b i l i t y o f Ge-Se-Te based glasses
The t r a n s p o r t mechanism i n t h e B i doped glasses can be described i n terms o f the Davis-Mott model ( 1 ) f o r the band s t r u c t u r e of an amorphous semiconductor. The values o f t h e pre-exponentials a. (see t a b l e I ) p o i n t t o conduction i n extended s t a - t e s . The a p p a r e n t l y h i g h e r a c t i v a t ~ o n energy o f t h e thermopower can be e x p l a i n e d i f one takes i n t o account a b i p o l a r t r a n s p o r t process, a r i s i n g from t h e conduction o f e l e c t r o n s and holes i n the extended s t a t e s o f t h e conduction and valence band.
Composition
The a d d i t i o n o f o t h e r elements, such as As, Cu and I n , i n glasses o f t h e type GeTeg always y i e l d s p-type semiconductors. F i g u r e 6 shows a p l o t o f t h e thermopower versus r e c i p r o c a l temperature f o r glasses c o n t a i n i n g these t h r e e elements. T h e i r i n - c o r p o r a t i o n does n o t enhance t h e c o n d u c t i v i t y t o a l a r g e e x t e n t . As can be seen from t a b l e I, where oo, E,, ES and EuH values are l i s t e d f o r these glasses, t h e t r a n s p o r t data show the c h a r a c t e r i s t i c f e a t u r e s o f t h e chalcogenide glasses: ( i ) a discrepancy between the a c t i v a t i o n energies o f the dc c o n d u c t i v i t y and t h e thermopower
(E,
-
ES > 0 ) ; ( i i ) a t h e r m a l l y a c t i v a t e d H a l l m o b i l i t y . A TypeReferences.
-
( 1 )
E.A., Mott,N.F., P h i l . Hag. 22 (1970) 903.( 2 ) Wott, N.F., Davis, E.A., S t r e e t , R . r , P h i l . Mag. 32 (1975) 961.
( 3 ) Kastner, PI., Adler, D., F r i t z s c h e , H., Phys. Rev.
Lett. -
37 (1976) 1504.(4) Mott, N.F., P h i l . Nag. 34 (1976) 1101.
(5) Kastner, I . I . , P h i l . Mag.37 (1978) 127.
(6) F r i t z s c h e , H., Kastner,
K,
P h i l . Mag. 37 (1978) 285.( 7 ) F r i t z s c h e , H., Gaczi, P.J., Kastner, M . T P h i l . Mag. 37 (1978) 593.
( 8 ) Tohge, N., Plinani, T., Yanamoto, Y., Tanaka, ?4., J. q p l . Phys. 51 (1980) 1048.
(9) Tohge, N., tlinami, T., Tanaka, ?I., J. Non-Cryst. S o l i d s
-
37 ( 1 9 8 0 r 2 3 . (GeSe3.5)88Sb12(GeSe3.5)96Bi4 (GeSe3 .5)92Bi8 (GeSe3.5)99Bi10 (GeSe3.5)88Bi12 (GeSe3.5)86Bi14 (GeSe3Te0 .5 )96Bi4 (GeSe3Te0.5)90Bilo Ge15Te81As4 Ge16Te8~Cu4 Ge16Te801n4 Ge14Te761n10
charged dangling bonds (D+ o r D-). I n t h a t case they a c t by unpinning t h e Fermi l e - v e l , which i n t h e undoped chalcogenide g l a s s i s l o c a t e d midway t h e deep-lying l e v e l s o f t h e D+ and
D-
d e f e c t centres. I n selenium-rich Ge-Se glasses t h e b a s i c s t r u c t u r a l u n i t i s formed by a t e t r a h e d r o n c o n t a i n i n g a germanium atom surrounded by f o u r s e l e - n i u m ~ . The e x t r a selenium i n t h e a l l o y forms chains which e n t e r t h e s t r u c t u r e i n t h e form of m i c r o - i n c l u s i o n s . It i s supposed t h a t B i i s p r e f e r e n t i a l l y d i s s o l v e d i n t h e v i t r e o u s selenium chains and forms cross l i n k s by means o f two o f i t s p e l e c t r o n s . To f u l f i l a l l i t s valence requirements B i should capture an e x t r a e l e c t r o n forming a n e g a t i v e l y charged i m p u r i t y . The conpensating charge w i l l be D+ c e n t r e s which w i l l disappear and, hence, unbalance t h e r a t i o between D+ and D-.p P n n n
n
P n p p p p1.00
0.92
0.645 0.655
3.64 0.45 0.43 0.42 0.43
1.3 x
l o 3
4.4 x
l o 3
0 . 6 4 5 1 . 5 ~ 1 0 ~ 3 . 1 x 10' 1.7 x
l o 3
3.3 x 13' 1 . 6 x 1 0 3 1.4 x
l o 3
1.7 x
l o 3
1.5 x
l o 3
0.76 0.69 0.705
0.87 0.38 0.28 0.28 0.35
r 9 . 4
