argon or oxygen atmosphere of I0 -~ Tort and substrate temperature about 300~. Powder samples of ITO were prepared by heating the mixed In203 and SnOz ...
Hyperfine Interactions 42 0988) 1207-1210
1'207
CHARACTERIZATION AND ESTIMATION OF ITO (INDIUM-TIN-OXIDE) BY MOSSBAUER SPECTROMETRY K. NOMURA, Y. UJIHIRA, S. TANAKA" and K. MATSUMOTO" Faculty of Engineering~ The University of Tokyo~ Hongo, Bunkyo-ku, Tokyt~ Japan 9 Asahi Glass Ct~Ltd., Hazawa-cho, Kanagawa-ku, Yokohama, Kanagawa, Japan ITO films and powders, prepared by several methods, were characterized by conversion electron and transmission M 6 s s b a u e r s p e c t r o m e t r y (CEMS and TMS). The slight change of FWHM of Sn(IV), observed in ITO films, was attributed to the presence of the acid-insoluble segregated SnO2 and the acidsoluble Sn(IV) that is incorporated in the InzOa lattice. A good c o r r e l a t i o n was found between the area intensities of a doublet and a singlet and the ratios of acid-soluble Sn(IV) to acid-insoluble Sn(IV). The peaks of acid-soluble Sn(IV) in ITO were d e c o n v o l u t e d to two doublets corresponding to different sites. The results suggested that Sn(IV) atoms were preferentially substituted to the centers of trigonally distorted octahedra (site b) rather than to the centers of more d i s t o r t e d octahedra (site d). 1.Introduction ITO coated glass is one of the most prevailing advanced industrial materials with transparent and semiconductive properties and is used as electrodes of solar cells, heat reflectors for solar collections and transparent electrode for display devices. The optical and electrical properties are quite sensitive to the preparation condition and methods /I/. Kostlin et al./2/ applied the Mbssbauer S p e c t r o m e t r y to the analysis of ITO films but did not get valid p a r a m e t e r s . Mercader et el./3/ reported the hyperfine interactions of In203:Sn at In and Sn sites determined by timedifferential perturbed angular correlatlons (TDPAC) and Mbssbauer spectroscopy. Although two static quadrupole interactions were expected from the TDPAC spectra and the point-charge calculations with substitution of Sn for In, they could not attribute the M~ssbauer parameters of ITO to two tin species, dissolved in ITO. We tried to analyze the site d i s t r i b u t i o n of tin in ITO by preparing ITO powders with different c o n c e n t r a t i o n of tin atoms, and showed the results of analysis of the segregated SnOa and the acid-soluble Sn(VI). 2. E x p e r i m e n t a l Thin films of ITO were d e p o s i t e d on glass by electron beam evaporation from hot pressed In203 containing several wtg of SnO2 in argon or oxygen atmosphere of I0 -~ Tort and substrate temperature about 300~. Powder samples of ITO were prepared by heating the mixed In203 and SnOz powders at the temperature from I000~ to 1 5 0 0 ~ for several hours. Because S~(IV) incorporated in In203 lattice is soluble and Sn(IV) remained as SnO2 is insoluble in 6N HCl, the weight ratio of a c i d - s o l u b l e Sn(IV) to acid-insoluble SnO2 was obtalned by chemical analysis.
9 J.C. Baltzer A.G., Scientific Publishing Company
1208
K. Nomura et aL, Characterization and estimation of 17i0
CEHS and TMS of ITO films and p o w d e r s w e r e o b t a i n e d by u s i n g a gas-flow p r o p o r t i o n a l c o u n t e r and a NaI(TI) s c i n t i l l a t i o n counter, respectively. ImCi B a 1 1 0 m S n O a was u s e d as y - r a y source. The i s o m e r shifts (I.S.) w e r e r e f e r r e d to B a S n O 3 at room temperature. The velocity was c a l i b r a t e d by u s i n g a 10 mCi s?Co(Rh) s o u r c e and an e n r i c h e d S?Fe foil as an a b s o r b e r . M ~ s s b a u e r s p e c t r a w e r e f i t t e d as a sum of L o r e n t z i a n peaks. 3. R e s u l t s a n d d i s c u s s i o n 3.1 S e g r e g a t i o n of SnOz in ITO. CEH s p e c t r a of ITO films were s h o w n in Fig. I. ITO film, d e p o s i t e d in 10 -s T o r r H2 a t m o s p h e r e at room temperature, has d a r k c o l o r due to the f o r m a t i o n of Sn(II) species (I.S.= 2.67 m m / s and Q . S = 1.93 mm/s) in a d d i t i o n to an u n r e s o l v e d d o u b l e t due to Sn(IV) w i t h I.S.= 0 m m / s (Fig. la). The Q.S. of the Sn(II) s p e c i e s in this f i l m was g r e a t e r t h a n that of c o m m e r c i a l l y a v a i l a b l e b l a c k SnO p o w d e r ( I . S . = 2.54 mm/s, Q . S . = 1.35 mm/s), s u g g e s t i n g the l o c a t i o n of Sn(II) a t o m s in the s t r e s s e d sites formed in the c o u r s e of ITO f i l m p r e p a r a t i o n . CEM s p e c t r a of ITO films, prepared by electron beam evaporation of ITO pellet containing 7.5 w t % SnOz in 2 X 1 0 -4 T o r t O~ a t m o s p h e r e at 300~ of substrate temperature, with and without the supply of radio frequency bias, w e r e s h o w n in Fig. 1 c) and b), r e s p e c t i v e l y . The narrower Sn(IV) p e a k was o b s e r v e d in Fig. 1 b) than Fig.1 c). Segregation of SnO2 in the films was considered to contribute largely to the n a r r o w i n g of Sn(IV) p e a k b e c a u s e the f i l m thickness and the c o n t e n t s of SnO~ in ITO w e r e k e p t c o n s t a n t .
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. Ill
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R e l a t i o n s h i p between the peak d o u b l e t and t h e r a t i o of acids o l u b l e S n ( I V ) to total S n ( i V ) in ITO. P e a k was d e c o m p o s e d to a s i n g l e t and a doublet. ratio
,: '"::"i
of
. u
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Fig. 1, CEM s p e c t r a o f ITO f i l m s p r e p a r e d a) by e l e c t r o n beam evaporation (EB) in 10 -~ t o r r H= a t room t e m p . , b ) EB in 2xlO - 4 t o r r O= a t 3 0 0 ~ , and c ) by EB*RF In 2xlO - 4 t o r t O= a t 3 0 0 ~ .
K. Nomura et aL, Characterization and estimation o f / T O
Different
Sn/In
ratios between the top surface and
1209
the
bulk,
observed by Auger electron spectrometry, also suggested the s e g r e g a t i o n of a large a m o u n t of SnO= in the s u r f a c e layer. In o r d e r to evaluate the e f f e c t of the c o n t e n t of a c i d - i n s o l u b l e Sn(IV) in ITO to the p r o f i l e of M S s s b a u e r peaks, we p r e p a r e d p o w d e r samples of ITO containing 5 wt% SnO= w i t h v a r i e d ratios of acid-soluble to a c i d - i n s o l u b l e Sn(IV). A s s u m i n g that M S s s b a u e r peaks of Sn(IV) c o u l d be d e c o n v o l u t e d to a d o u b l e t due to Sn(IV) s p e c i e s i n c o r p o r a t e d in ITO l a t t i c e and a s i n g l e t due to s e g r e g a t e d SnO2, we a n a l y z e d the M o s s b a u e r d a t a o b t a i n e d and got the r e s u l t s as shown in Fig. 2. It was f o u n d that the p e a k r a t i o s of the d o u b l e t w e r e c o n s i s t e n t with the c o n t e n t r a t i o s of the a c i d - s o l u b l e Sn(IV) in ITO in the region of higher concentration of a c i d - s o l u b l e Sn(IV). But a definite deviation f r o m the l i n e a r i t y of this r e l a t i o n s h i p was o b s e r v e d for the s a m p l e s with lower c o n c e n t r a t i o n of a c i d - s o l u b l e Sn(IV). This inconvenience was derived from the assumption adopted in the spectrum analysis since the M ~ s s b a u e r s p e c t r a of p u r e SnO= gave occasionally the slightly split peaks. For the ITO samples c o n t a i n i n g a h i g h e r c o n c e n t r a t i o n of a c i d - s o l u b l e Sn(IV), no p r o b l e m a r i s e d in the a n a l y s i s . 3.2 ITO structure. In=Os c r y s t a l l i n e has the cubic bixbyite structure, w h i c h i n c l u d e s two n o n - e q u i v a l e n t sites g e n e r a t e d in the unit c r y s t a l as s h o w n in Fig.3. 4/16 of i n d i u m s o c c u p y the c e n t e r of trigonally distorted o x y g e n o c t a h e d r a (site b) and the remaining 12/16 locate at the c e n t e r of more d i s t o r t e d o c t a h e d r a of lowest s y m m e t r y (site d)/5/. P e a k s o b t a i n e d in M o s s b a u e r s p e c t r a s h o u l d be r e s o l v e d to two d o u b l e t s .
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Fig.3, Schematic structure of indium oxide (In=O3) Population ; d site: 12/16 , b site: 4/16
Tabl e 1 Sample O.l~Sn02 0.5];Sn02 I~ S n 0 2 2~ S n 0 2 3~ Sn02 16% Sn02 26~; Sn02
Mossbauer parameters of t i n in Indium-Tin-Oxides (ITO) I.S. Q.S. Area I .S. Q,S. Area FWHH (mmls) (mm/s) (~) (mmls) (mmls) (~) (mmls) 0.14 0.85 36 7 0.17 0 37 62.2 0.63 • 0.16 0.16 0.16 0.II O.ll 0.11
0.81 0.81 1.01 0.88 1.08 1.12
23 55 59 69 45 63
5 9 5 4 6 6
0.16 0.18 0.16 0.14 O.12 0.09
0 42 041 0 27 001 0 72 0 44
76,4 44.1 40.4 30.5 54.4 36.3
0.84 1.02 1.13 1.10 1.17 1.09
-+0.03 • • • • +0.02
2
2.06 I .59 l .78 1.91 l .05 2.22 1.76
K. Nomura et al., Characterization and estimation of 17'0
1210
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1~~ M ~ s s b a u e r s p e c t r a o~ ITO d o p e d w i t h a ) O . 5 w X , d ) 3 w X , e)16wX and f ) 2 6 w X SnOa.
b)lwX,
We p r e p a r e d a s e r i e s of solid solution ITO p o w d e r s containing 0 . 5 wt%, 1 wt%, 2 wt%, 3 wt%, 16 wt% a n d 26 wt% SnO=, w i t h m o r e t h a n 95% a c i d - s o l u b l e Sn(IV). X-ray d i f f r a c t o m e t r y of these samples containing less than 3 wt% SnOs gave the diffraction patterns c o r r e s p o n d i n g to InsOs structure, but the ITO sample c o n t a i n i n g 16 wtZ SnOs gave the weak u n k n o w n peaks besides those belonging to InsOs. M6ssbauer peaks obtained for these samples were fit with two doublets as shown in Fig. 4 and Table 1. The doublets with large Q.S.(=0.8 to 1.0 mm/s) and with small Q.S.(=0.3 to 0.5 mm/s) were considered to be due to Sn(IV) species s u b s t i t u t e d to d site and b site of In atoms, respectively. When doping amount of SnO= increased to 3 wt%, the peak area ratios of b site to d site became near the a n t i c i p a t e d value, 1/3. It was e s t i m a t e d from the peak area ratios that Sn(IV) atoms were s u b s t i t u t e d to the centers of site b rather than site d. In ITO powders doped with 16w~ and 26w~ SnOz, the ratios of b and d sites were c o n s i d e r e d to be d e v i a t e d b e c a u s e two doublets c o n t a i n e d the unknown product or the interstitial Sn(IV) occupying in the InsO3 lattice. The unknown species could h a r d l y be distinguished from s u b s t i t u t i o n a l Sn(IV) at In(III) sites, because In(III) atoms are o r i g i n a l l y located at the centers of highly d i s t o r t e d octahedra. Reference / 1 / S. N a s e e m , T. J . C o u t , J.Apple. Phys., 58 ( 1 9 8 5 ) 4 4 8 3 . /2/ H. Kostlin, R. Jost, W. Lems, Phy. Star. Solo, (a)29 (1975) 87. /3/ R. C. Mercader, et al., Hyperfine Interactions, 23 (1985) 211. /4/ K. Nomura, Y. Ujihira, Bunseki Kagaku, 33 (1984) T81. [5/ M. Marezio, Acta Cryst., 20(1966)723.
