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SILVER-DOPED AMORPHOUS As2Se3 FILMS STUDIED BY XPS
T. Ueno, A. Odajima
To cite this version:
T. Ueno, A. Odajima. SILVER-DOPED AMORPHOUS As2Se3 FILMS STUDIED BY XPS. Journal
de Physique Colloques, 1981, 42 (C4), pp.C4-899-C4-902. �10.1051/jphyscol:19814195�. �jpa-00220822�
CoZZoque suppldment au n020, Tome 42, octobre 2981 page C4-899
SILVER-DOPED AMORPHOUS As2Se3 FILMS STUDIED BY XPS
T. Ueno and A. Odaj ima
Hokkaido University, Sapporo, Japan
Abstract
-
X-ray photoelectron spectra were measured to determine the binding energies of the core electrons in Ag films and amorphous films of As, Se As Se and Ag-photo-doped-As2Se3. Auger spectra were measured to determine the2 3
kinetic energies of Ag and Ag-doped amorphous As2Se3 films. Chemical shifts by doping can be interpreted in terms of the electron transfer from Ag to As and Se; charged defects are induced to As-Se bonds. The mechanism of the migration of Ag ions in amorphous As2Se3 is discussed.
Introduction.- The migration of elements, such as Cu and Ag, in amorphous chalcogen- ides is much interested in p h ~ t o - d o p i n ~ ~ ' ~ ) e l e c t r o d e suitability:)photoelectric ef- fect during photo-dopingt-6)etc. For the migration of atoms in solids there are many important studies in its fundamental aspects. However, most of them are not concerned with amorphous solids but crystalline ones.
Willert and his collaborators7)have investigated the electromigration of Ag in As Se glass at various temperatures between 120 and 180°C by means of radio active
2 3
&.
Lrzcers;
Az
atooins are found to be positively charzed durinq iumoing. However, they have not shown where the lost electron of each Ag atom remains. On the other hand, it has been suggested in photovoltaic studies for Ag-photo-doping that electrons are not liberated from Ag atoms in ~tatics?'~)~hus, the mechanism of the migration of these additive atoms in amorphous chalcogenides seemd complicated and not yet fully under- stood.The electron transfer from one constituent element to the others in a compound is expected to be reflected upon the spectral shift in X-ray photoelectron spectro- scopy(XPS). In this report Ag-doped amorphous As2Seq is studied by XPS. Spectral shifts induced by Ag-doping are interpreted as a change in chemical environments for the constituent elements. The mechanism of the migration of Ag atoms in amorphous As2Se3 is discussed.
Experimentah.- XPS and Auger measurements were performed using a Vacuum Generators ESCA III spectrometer equipped with a Wg Y a X-ray source (1253.6 eV). Binding energ- ies of the core electrons were determined with Ag films and amorphous films of As, Se, As2Se3 and Ag-doped As2Se3. The kinetic energies of the Ag Auger electrons were determined with Ag and Ag-doped amorphous As2Se3 films. Contamination judged by the 0 Is and C 1s lines was negligible. For energy calibration the small spot of gold was flashly evaporated onto the specimen surface and the Au 4f7,2 was measured equal
-.
to 84.0 eV. A chemical reaction of the gold standard with the specimen8' was not de- tected by the full-width at half maximum (1.3 eV) and the C Is line (284.4 eV) as a secondary standard.
Amorphous As2Se3 films about 1000 A in thickness were prepared by evaporation of powdered ingots onto a sample holder of nickel in the pressure of Pa.; the
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814195
C4-900 JOURNAL DE PHYSIQUE
i n g o t s were prepared by melt and quench of h i g h p u r i t y A s and Se which were s e a l e d i n an evacuated q u a r t z ampoule i n t h e s t o i c h i o m e t r i c r a t i o and annealed a t 7 0 0 ' ~ f o r 1 0 h i n a r o c k i n g f u r n a c e . The e v a p o r a t i o n r a t e was about 20 A/sec. Amorphous A s and Se f i l m s were prepared by e v a p o r a t i o n of h i g h p u r i t y lumps and p e l l e t s on t h e n i c k e l s u b s t r a t e h e l d a t room temperature, and Ag f i l m s were o b t a i n e d by e v a p o r a t i o n of 99.99% pure powders. The amorphous s t a t e of prepared f i l m s were c e r t i f i e d by e l e c - t r o n d i f f r a c t i o n , though amorphous r i n g p a t t e r n s were sharpened by Ag-doping.
Specimens evaporated on t h e sample h o l d e r a r e drawn s c h e m a t i c a l l y i n F i g . 1.
Ag-photo-doping was accomplished by i r r a d i a t i o n f o r 1 0 min upon amorphous As2Se3 f i l m s , which were evaporated onto t h e Ag f i l m p r e v i o u s l y d e p o s i t e d , u s i n g a 250 W Hg a r c lamp w i t h an i n f r a r e d - c u t f i l t e r i n t h e atmosphere of A r ; Ag f i l m s 50, 100 and 200 A i n t h i c k n e s s were consumed. A few specimens were doped w i t h Ag by a n n e a l i n g a t 200°c f o r 1 h i n t h e atmosphere of A r .
Photo-
%
i r r a d i a t i o nF i g . 1: Specimens evaporated on t h e sample h o l d e r .
A!?, ( a n n e a l i n g ) R e s u l t s . - The Ag 3d
5 / 2 , 3 / 2 s p e c t r a a r e shown i n F i g . 2 ( a ) ; i n Ag-photo-doped f i l m s t h e binding e n e r g i e s a r e lowered by 0 . 3 eV w i t h o u t a p p r e c i a b l e l i n e broadening. The
350 360
KINETIC ENERGY (eV) BINDING ENERGY ( e V )
Fig. 2 : (a)The Ag 3d5/2 ,3/2 and (b)Ag MNN Auger s p e c t r a f o r Ag and Ag-photo-doped amorphous As2Se3 f i l m s .
Ag M5N4,5N4,5 and M4N4,5N4,5 s p e c t r a a r e shown i n F i g . 2 ( b ) ; t h e i r k i n e t i c e n e r g i e s a r e s h i f t e d t o t h e lower energy s i d e by Ag-photo-doping. These v a l u e s of t h e spec- t r a l s h i f t were independent of t h e t h i c k n e s s of consumed Ag f i l m s and were induced a l s o by thermal doping.
S p e c t r a l s h i f t s f o r t h e A s 3d and Se 3d e l e c t r o n s a r e shown i n Fig. 3. The As 3d and Se 3d s p e c t r a f o r amorphous As2Se3 a r e l o c a t e d a t t h e h i g h e r and lower ener- gy s i d e , r e s p e c t i v e l y , of t h e e l e m e n t a l A s and Se, a s expected from t h e e l e c t r o - n e g a t i v i t y s c a l e . While t h e y s h i f t s l i g h t l y by p h o t o - i r r a d i a t i o n o r a n n e a l i n g , con- s i d e r a b l e s h i f t s of t h e i r b i n d i n g e n e r g i e s a r e induced by Ag-photo-doping (dot=
dashed l i n e s ) . Lowering i n b i n d i n g energy was enhanced when t h e t h i c k n e s s of t h e
F i g . 3: As 3d and Se 3d s p e c t r a f o r amorphous f i l m s of As, Se, As2Se3 and Ag-photo-doped A s Se
2 3'
consumed Ag f i l m was i n c r e a s e d . S i m i l a r t r e n d s of t h e s p e c t r a l s h i f t were o b s e r v e d by t h e r m a l d o p i n g a s w e l l . The 3p312,112 s p e c t r a of As and Se s h i f t e d a l o n g w i t h t h e 3d s p e c t r a .
The r e s u l t s o b t a i n e d , . f r o m s p e c t r a l measurements a r e l i s t e d i n T a b l e I.
T a b l e I. Binding e n e r g i e s , Auger k i n e t i c e n e r g i e s and chemical s h i f t s * f o r Ag, A s and Se.
* I n p a r e n t h e s i s .
Ag (eV) A~ (eV)
se
( e v )Specimen 3d5/2 3d3/2 M5N4,5N4,5 3d 3P3/2 3P1/2 3d 3p3/2 3P1/2
Ag 368.0 374.0 352.2
- -
--
--
a-As
-
- - 41.5 140.5 145.3 - - -a-Se
- -
- - --
55.5 1 6 1 . 5 167.2a-As Se 2 3
- -
- 42.9 141.8 146.6 54.9 1 6 1 . 0 166.7Ag-a-As2Se3 (1.4) ( 1 . 3 ) ( 1 . 3 ) (-0.6) (-0.5) (-0.5)
D i s c u s s i o n . - The n e g a t i v e c h e m i c a l s h i f t i n b i n d i n g e n e r g y i s u n u s u a l i n view of t h e e l e c t r o n e g a t i v i t y s c a l e f o r Ag. However, i t h a s been observed f o r c h a l c o g e n i d e s and o x i d e s of Ag a n d Cu by Romand e t al?! I n T a b l e ll c h e m i c a l s h i f t s l i s t e d by them a r e summarized t o g e t h e r w i t h Auger c h e m i c a l s h i f t dMI, where El i s d e f i n e d a s t h e d i f - f e r e n c e i n k i n e t i c e n e r g y between t h e Auger e l e c t r o n s and t h e p h o t o e l e c t r o n s . It i s found t h a t t h e c h e m i c a l s h i f t v a r i e s a c c o r d i n g t o t h e i o n i c c h a r a c t e r of bonds i n t h e s e compounds. On t h e o t h e r hand, t h e s e c h a l c o g e n i d e s and o x i d e s a r e w e l l known a s t h e h i g h c o n d u c t i v i t y s o l i d e l e c t r o l y t e o r t h e i o n i c compound; m e t a l atoms a r e p o s i -
42.6 141.5 146.2 54.4 1 6 0 . 4 1 6 6 . 0 (1.1) ( 1 . 0 ) (0.9) (-1.1) (-1.1) (-1.2) 367.7 373.7 351.3 42.4 1 4 1 . 3 1 4 6 . 0 54.3 1 6 0 . 3 166.0
(-0.3) (-0.3) (-0.9) (0.9) ( 0 . 8 ) ( 0 . 7 ) (-1.2) (-1.1) (-1.2) 42.2 141.2 1 4 5 . 8 5 4 . 1 160.2 165.9 (0.7) (0.7) ( 0 . 5 ) (-1.4) (-1.3) (-1.3)
'Oped Ag
(A)
t i v e l y i o n i z e d . B e s i d e s , a number of i n v e s t i g a t o r s 3'10-12) have p o i n t e d o u t t h a t Ag Se(S) i s formed i n Ag doped c h a l c o g e n i d e g l a s s . Compared w i t h our r e s u l t s , Ag 2
50 1 0 0 200
atoms doped i n t o amorphous As2Se3 a r e r e g a r d e d a s p o s i t i v e l y i o n i z e d h i g h e r t h a n Ag2Se(S).
C4-902 JOURNAL DE PHYSIQUE
T a b l e IL Cu 2p3/2 a n d Ag 3dgj2 c h e m i c a l s h i f t s , a n d Auger c h e m i c a l s h i f t s o b t a i n e d 9 )
a n d q u o t e d * by Romand e t a l . .
-
- --Compound Cu 2p3,2 (eV) ~ l a / ( e v ) Compound Ag 3d5,* (eV) A la
1
(eV)Cu c u o c u s CuSe c u 2 o Cu2S Cu S e
2 CuAg Se
(-0. I ) , ( 0 ) , (+O.l) ( 2 . 2 ) , (2 - 1 ) Ag20 +0.1 1 . 4
( 1 - 7 ) , ( 1 . 3 ) Ag2 S -0.2 0 . 9
-0.7 1 . 3 Ag2Se -0.5 1 . 0
-0.7 1 . 3 AgCuSe -0.5 1.1
Ag-a-As2Se3** -0.3 1 . 2
* I n p a r e n t h e s i s ,
*;:Our r e s u l t s .
The l o w e r i n g o f As a n d S e c o r e b i n d i n g e n e r g i e s upon Ag-doping i n d i c a t e s t h a t e l e c t r o n s l i b e r a t e d f r o m Ag t r a n s f e r t o As a n d Se. I t h a s b e e n p o i n t e d o u t t h a t n e g a t i v e l y c h a r g e d As a n d Se a t o m s a r e u n d e r c o o r d i n a t e d i n amorphous c h a l c o g e n i d e s ? ) c h a r g e d d e f e c t s a r e i n d u c e d i n As-Se b o n d s .
It i s s u p p o s e d t h a t Ag a t o m s a r e n o t c o r p o r a t e d i n t h e n e t w o r k o f amorphous As Se a n d m i g r a t e w i t h As-Se b o n d s i n t h e c h a r g e d d e f e c t s t a t e s .
2 3 R e f e r e n c e s .
-
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(1971) 1 1 7 3 .
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10
(1971) 653.4 ) K o l o m i e t s , B. T . , L e b e d e v , E. A . , Kasakova, L. P. and Rogachev, N. A . , F i z . Tech.
P o l u p r o v . 7 (1973) 187.
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(1976) 225.
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(1975) 461.8 ) B e t t e r i d g e , D., C a r v e r , J . C . and H e r c u l e s , D. EI., J . E l e c t r o n S p e c t r o s c .
2
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