JOURNAL OF SOLID STATE CHEMISTRY 135,140—148(1998)
ARTICLE NO.
SC977613
Structural Studies of Tin-Doped Indium Oxide (ITO)and In 4Sn 3O 12
N.Nadaud, N.Lequeux,and M.Nanot
Laboratoire Ce r amiques et Mate r iaux Mine r aux,UA —CNRS Mate r iaux Inorganiques,Ecole Supe r ieure de Physique et de Chimie Industrielles de Paris,
10Rue Vauquelin,F-75231Paris Cedex 05,France
J.Jove Institut Curie,Section de Recherche,26Rue d’Ulm,F-75231Paris Cedex 05,France
and T.Roisnel
Laboratoire Le o n Brillouin (CEA /CNRS),Centre d’Etudes de Saclay,F-99191Gif-sur-Y v ette,France
Received April 15,1997;in revid form August 25,1997;accepted September 1997
Structural changes in the indium oxide lattice due to doping
with Sn 4؉(ITO)were studied by Mo
核桃粉ssbauer spectroscopy,EXAFS,and neutron powder diffraction.There is a decrea in electrical conductivity as the tin content and oxygen partial pressure increa,which is related to a distortion in the first Sn –O shell.Doping with tin increas the oxygen /cation ratio and the lattice parameter and decreas the Sn –O distances,which disorders the host network.The low-conduction In 4Sn 3O 12pha precipitates when the tin content exceeds 6at.%.In 4Sn 3O 12is rhombohedral (space group R 3,a ؍9.4604(2)A ,and c ؍8.8584(2)A in a hexagonal basis).There is a cation ordering with octahedral sites fully occupied by tin,the tin sites being equivalent in both highly doped ITO and In 4Sn 3O 12. 1998
Academic Press
1.INTRODUCTION
Thin films of tin-doped indium oxide (ITO,for in-dium —tin oxide)exhibit high electrical conductivity (up to 10 \ )cm \ ),high transparency in the visible range,and high infrared reflectivity for wavelengths
higher than 1 m.ITO films are ud as transparent electrodes in liquid-crys-tal displays and heat reflection filters (1).ITO is a wide-gap,degenerate miconductor where electrons are the major charge carriers.The Sn atoms are considered to substitute for In,without ordering.The difference of valence between In >and Sn >results in the donation of a free electron to the lattice.For most applications,ITO films must be as
To whom correspondence should be addresd.
conducting as possible,which requires a high carrier density (N )and a high carrier mobility ( ).The electrical and optical properties of ITO films (tin content less than 10at.%)have been extensively studied (2—4).The main features are as follows:
(i)Oxidizing conditions decrea the carrier concentra-tion,whatever the tin content.The introduction of tin into In O
is considered to facilitate the migration of interstitial oxygens within the anionic sublattice.For low-doped ITO films prepared by a spray technique,Frank and Ko stlin (4)suggested the prence of neutral clusters (‘‘Sn ·O
’’),which
trap the free carriers.The clusters can be eliminated by thermal treatments in a reducing atmosphere.
(ii)The carrier concentration is at a maximum for 6—8at.%Sn (4—6)and decreas beyond this limit.The neutral-ization of the tin donor effect is found whatever the oxygen partial pressure but is enhanced by highly oxidizing condi-tions.Frank and Ko stlin (4)suggested that an increa in the tin content or the oxygen partial pressure leads to the formation of nonreducible fluorite-type Sn O
clusters.In
contrast,Parent et al .(5)showed by X-ray absorption spec-troscopy on ITO films prepared by a pyrolysis technique that the prence of tin disorganizes the host network and relaxes the oxygen environment toward a configuration which is similar to that of rhomboedral In O
(for indium
cations)and SnO
(for tin cations).Shigesato et al .(7)
showed that tin can gregate clo to the film surface or along grain boundaries to form an insulating pha.
There are numerous studies on ITO films,but few on ITO sintered ceramics.The solubility limit of tin in In O
ceram-ics is not known with precision but has been estimated to be between 6and 11%Sn (6,8—11)For the rest of the paper,
140
0022-4596/98$25.00
Copyright 1998by Academic Press
All rights of reproduction in any form rerved.
the dopant content will always be expresd as atomic ,[dopant]/([dopant]#[In]).The existence of a tin-rich compound offixed composition(In Sn O ,i.e., 2In O —3SnO )has been reported(10—12),but we lack
structural information on this compound and on the tin neutralization effect.
In the prent study,the local environment of tin in ITO ceramics(tin content from0to9%)has been investigated by Mo ssbauer spectroscopy and EXAFS.The crystal structure of In Sn O has been clarified.We have tried to establish the relationship between the tin atomic environment and the electrical properties of the considered , In Sn O and ITO,using Mo ssbauer,EXAFS,and neu-tron diffraction data.Finally,the models propod for ITO films(4,5,7)are discusd.
EXPERIMENTAL
Preparation of Ceramics
ITO materials with0.5—9%Sn were prepared from powdered mixtures of99.99wt%pure In O (Metaleurop Recherche)and99.9wt%pure SnO (Merck)calcined at 1400°C for3h in either a reducing atmosphere(p O " 2;10\ atm)or an oxidizing atmosphere(p O "1atm) (13).
In Sn O was prepared by reacting In O and SnO powders at1550°C for10h and then quenching in air.To prevent volatilization of the materials,the green bodies were compacted and then placed in a bed of prereacted In Sn O powders.The solubility limit of tin in In O and the pha purity of In Sn O were determined by X-ray powder diffraction.
Mo ssbauer Spectroscopy
The Mo ssbauer source was10mCi of Ca SnO (Amer-sham).Tofilter the Sn K X-rays,a0.05-mm Pd foil was placed in front of the samples.The spectra were recorded using a constant acceleration type spectrometer.Calib-ration velocity was accomplished using a25-mCi Co source.SnO spectra were recorded to reference the isomer shift.Absorption experiments were carried out with a con-ventional device.
Neutron and X-Ray Powder Diffraction
X-ray diffraction data were recorded at room temper-ature on a Philips PW1710diffractometer(Cu K ).Neutron diffraction experiments were performed at room temper-ature at the Orphe e reactor(Saclay,France)with the3T2 high-resolution powder diffractometer( "1.2272A).Data were analyzed by the Fullprof Rietveld-type program(14), taking into account indium neutron absorption by a pre-viously measured transmission coefficient.X-Ray Absorption Spectroscopy
EXAFS experiments were conducted at LURE(Orsay, France)using the synchrotron radiation from the DCI stor-age ring.EXAFS spectra of In O ,SnO ,ITO(6%Sn),and In Sn O were recorded at room temperature in the trans-
mission mode at the In(27,940eV)and Sn(29,200eV) K-edges using a Ge(400)double-crystal monochromator. Incident and transmitted beam intensities were measured using krypton-filled ionization chambers.When possible, the sample thickness was chon such that x"1,an edge jump of x" being ud for ITO samples studied at the Sn edge to limit the absorption by indium.The EXAFS oscillations were extracted from the whole spectrum by subtracting a spline-fitted background(which describes the atomic absorption)and normalizing to the height of the edges.The edge energies were chon at half-height of the absorption jump.The (E)line was normalized to the pre-edge absorption byfitting afirst-order polynomial extrapo-lated to the EXAFS area.The k (k)signal was Fourier transformed over the3.5-to14-A\ (10.5A\ for ITO at the Sn K-edge)k range,using a Kair—Besl window ( "3).The pha shifts and backscattering amplitudes of In and Sn were extracted from In O and SnO spectra and fitted to the back-transformed data corresponding to a par-ticular shell(In—O or Sn—O)using the program FITEX(15).
RESULTS AND DISCUSSION
In O —SnO Pha Diagram
The lattice parameters for a body cubic centered ITO solid solution were plotted versus the tin conte
nt(Fig.1). The lattice parameters were deduced from Rietveld refine-ment of X-ray and neutron powder diffraction profiles.
As
FIG.1.Lattice parameters of Sn-doped In O materials as given by X-ray and neutron powder diffraction data(ceramics sintered at1400°C for 3h in an oxygen atmosphere).
STRUCTURAL STUDIES OF ITO AND In Sn O 141
previously reported (8,11),the tin doping (up to 5—6%Sn)leads to an increa in the lattice constant.This is in contradiction with Vegard’s law,which predicts a linear decrea in the lattice parameter as the tin content in-creas [the Sn >ionic radius (0.71A )is smaller than that of In >(0.81A )].There is no change in lattice parameter for tin content greater than 6at.%and the prence of an In Sn O
pha is detected by X-ray
powder diffraction data (10—12).This indicates that the solubility limit of tin in indium oxide at 1400°C must be clo to 6%,as previously found for films (4)and ceramics (11).
Electrical Properties of ITO Ceramics
Figure 2shows a plot of carrier density versus tin content in ITO ceramics (6),single crystals (16),and thin films (3,5,17).The carrier density no longer follows the expected behavior (one electron for each tin added,as repre-nted by a dotted straight line).In fact,the carrier concen-tration shows a plateau (for tin contents depending on elaboration method)and,finally,decreas for higher tin contents.A strong correlation is found between the solubil-ity limit of tin in tho materials and the tin content corres-ponding to the carrier density maximum,which suggests that the electrical properties cloly depend on structural changes.However,the prence of low-conduction In Sn O
precipitates must also be involved in the de-crea in carrier concentration in ITO ceramics with tin contents greater than 6%.Nevertheless,the reason for the low efficiency of tin as a carrier donor in single-pha ITO remains
unclear.
FIG.2.Carrier density (N )versus tin content in ITO thin films [(᭺)(3),(᭡)(5),()(17)],single crystals [(᭝)(16)],and ceramics [(᭜)(6)].The dotted line (N "3;10 [Sn])reprents the carrier density if each tin provides one free electron.
Tin En v ironment in ITO
In O
exhibits the cubic bixbyite structure (C-type,rare-earth squioxide structure:Ia 3space group,No.206).It shows a fluorite-related superstructure (18)where one-fourth of the anions are missing.Indium cations are located in two different six-fold-coordinated sites (Fig.3).One-fourth of the cations are located in trigonally compresd octahedra (b site,symmetry 3,In —O distance:2.18A ).The remaining three-fourths are located in highly distorted oc-tahedra (d site,symmetry 2,t of three In —O distances:2.13,2.19,and 2.23A ).Each cationic site can be described as a cube where two anion sites are empty at opposite vertices for b sites and along one diagonal of a face for d sites.The structural oxygen vacancies are located along the four 11112axes. Sn Mo ssbauer spectroscopy,X-ray absorption,and neutron diffraction were ud to study the environment of tin in ITO ceramics.The aim was to correlate structural features with dopant concentration and oxygen partial pressure.
The Sn Mo ssbauer spectra of Sn-doped In O
(0.5—9%Sn,sintered in either O or N
atmospheres)and
In Sn O
are prented in Figs.4a and 4b.The best fits (i.e.,with the lowest )are obtained by assuming the prence of two doublets.The fits with one doublet are unacceptable and tho with two doublets and a singlet do not improve the fit significantly.The Mo ssbauer parameters are given in Table 1.Isomer shift ( )is nsitive to the effective number of 5s and 5p electrons prent in the bond-ing.Isomer shifts for compounds of tetravalent tin range from about !0.6mm )s \ for the esntially covalent com-pounds,to about #1.5mm )s \ for fully ionic compounds.It is obvious that only Sn '4is detected in ITO compounds,as lies from 0.1to 0.4mm )s \
.雨后小故事
FIG.3.
Cationic sites in the cubic In O
structure.
142
NADAUD ET AL.
FIG.4. Sn Mo ssbauer spectra for ITO (0.5,5,and 9%Sn)and In Sn O treated in an O (a)or N
(b)atmosphere.The quadrupole splitting ( )aris from two main com-ponents:one due to the valence electrons of the transition-metal ion,which are not spherically symmetric,and another due to the electrostatic contribution of the neighbor oxygen ions (pure ionic lattice account).The values of in ITO are nsitive to the tin content and oxygen partial pressure as shown in Fig.5.The value of that corresponds to site 1( )or site 2(
)increas as the tin content increas,
whatever the p O
.Highly oxidizing conditions lead to
higher values than reducing ones.Finally,the quadrupo-lar splitting relative to In Sn O
is similar to what is found
in highly doped ITO.
We attribute and
to sites b and d ,respectively (19),
in agreement with the results of Nomura et al .(20)but in contradiction with the results of refs 21and 22.More information is needed to correlate the data and the
TABLE 1
Mo ssbauer Parameters in In 2O 3–SnO 2Materials
Doping
content p O
(atm) $0.02(mm )s \ ) $0.02(mm )s \ )Area $0.3(%) $0.01(mm )s \ ) 0.5%Sn
10.250.110.340.5150.949.10.780.992;10\
0.180.170.290.5653.047.00.810.925%Sn
10.170.140.470.9250.449.6 1.150.722;10\
0.180.150.390.8645.854.20.750.939%Sn
10.180.180.561.0455.045.0 1.250.802;10\
0.250.180.430.8853.047.00.75 1.0845%Sn In Sn O
—
0.160.14
0.441.02
梦见洪水
25.075.0
2.20
0.66
FIG.5.Quadrupole splitting ( )versus dopant content for
In O —SnO materials sintered at 1400°C for 3h in an O or N atmo-sphere.
indium oxide crystallographic data without ambiguity.However,using the area ratio given by the Mo ssbauer data (e Table 1),we consider that the tin occupies the less distorted b site rather than the d site.Independently of the site attribution,the changes suggest major changes in the environment of tin as the dopant content and p O
increa.
蒲安臣条约The inrtion of tin leads to a progressive distortion of the first tin —oxygen shell toward a configuration clo to that of In Sn O
.EXAFS was ud to clarify the difference between the tin environment and the indium environment.The k -weighted Fourier transform and the structural parameters for the first oxygen shell around the cations in In O ,SnO
,and ITO
(6%Sn,sintered in oxygen)are given in Fig.6and Table
2,
FIG.6.k -weighted Fourier transforms for In O ,SnO
,ITO (6%Sn,
Sn K -edge),and In Sn O
(both In and Sn K -edges).
STRUCTURAL STUDIES OF ITO AND In Sn O
143
胸背肌筋膜炎
TABLE2
Parameters of the First(In/Sn)–O Contribution as Determined by EXAFS
EXAFS data Structural data
2( ) E
R (A)?N @(10\ A )(eV) R (A)N
ITO6%Sn,In edge 2.18 5.9 2.6!0.070.72In O 2.186 ITO6%Sn,Sn edge 2.00 6.1 5.40.41 3.90
In Sn O ,In edge 2.16 5.1 4.4In Sn O
2.52 1.39.2 1.920.99M1 2.066
M2 2.07—2.647 In Sn O ,Sn edge 2.05 5.8 4.1 1.32 1.11
?Shell radius.
@Coordination number.
respectively.The coordination does not em modified with the prence of Sn(R"2.18A,N"5.9in6%Sn:In O ). We did notfind a splitting of the In—O distance into two contributions,contrary to what was found by Parent et al.
(5)in highly doped ITOfilms(39%Sn).An increa in the relative Debye—Waller factor(In K-edge:2( ) "
0.0026A )suggests a more highly distorted oxygen environ-ment around indium,due to the inrtion of tin.
The Sn—O shell radius is much smaller than the In—O radius in In O (2.00instead of2.18A).This value demon-strates that the tin induces strong modifications in itsfirst oxygen shell,as suggested by Mo ssbauer spectroscopy.The shortening of the Sn—O distance previously reported in ITO layers()is confirmed in ITO bulk materials.However,an EXAFS analysis is not sufficiently accurate to detect slight changes in the tin coordination.Rietveld refinement of neu-tron diffraction patterns was conducted on ITO with0and 3%Sn(sintered in oxygen)and6%Sn(sintered in either oxygen or nitrogen).In O crystallographic features(18) were ud tofit the ITO diffraction data.The occupancy of structural oxygen vacancy[Wyckoffposition: , , (» )]was also refined.The atomic positions,B values, refinement parameters,and oxygen occupancies of» sites are given in Table3.The structural data are quite similar to tho of indium oxide,except for the lattice parameters(e Fig.1).The differences between the indium (0.41;10\ cm)and tin(0.62;10\ cm)scattering length values are too small and the tin content is too low to allow us to discriminate indium sites from tin sites.Never-theless,the overall occupancy of the» site is found to increa as the tin content increas in oxidized ITO mater-ials.In contrast,the» site occupancy does not signifi-cantly change in In O or in reduced ITO.
Structure of In Sn O
Neutron powder diffraction data were indexed in a rhom-bohedral unit cell with a"6.2071(4)A and "99.29°,which correspond to a"9.4604(2)A and c"8.8584(2)A in
a hexagonal basis(Fig.7).The space group was taken as R3
(No.148),as reported earlier(11,12).The parameters de-
duced from the refinement treatment(considering514re-
flections)were R"4.4%,R "11.1%,and R "8.4%. The Wyckoffpositions and the main interatomic distances
and angles of the structure are given in Tables3and4,
respectively.In Sn O is isostructural with a wide range of M O compounds that exhibit afluorite-related super-structure,in particular Tb O (23),Yb Zr O (24), Sc Zr O (25),UY O and ULu O (26),Pr O (27,28),Sc Ti O ,Er Zr O ,and Sc Hf O (29).Two nonequivalent cationic sites,labeled M1and M2,are found. One-venth of the cations are located in the(3a)M1site. The coordination of cations i
n the M1site is six,instead of eight in the idealfluorite structure.The configuration can be described as either a trigonally compresd octahedron or a distorted cube with empty anionic positions at opposite vertices.The values of angles O—M1—O obtained from data refinement(82.9°and97.1°;e Table4)are clor to the regular octahedron angles(90°)than to the regular cubic angles(70.5°and109.5°).In the isostructural compounds, a weakly distorted octahedron is obrved around uranium
(r >
3"0.80A)in UY O (26)and around titanium (r >
2 "0.68A)in Sc Ti O (29),whereas Yb Zr O and Sc Zr O (r >7 "0.86A,r >8 "0.79A)exhibit cubic con-figurations(24,25).The relationship between the special-site distortion and the cation radius is not clear.
The M1cation and the two structural oxygen vacancies
are located on the inversion-triad axis that coincides with
one of the11112 axes(Fig.8).The building block is an oxygen vacancy pair along the3axis,with th
e center occu-pied by an M1cation.The polyhedra M1O » (where»is the formal oxygen vacancy)are isolated from each other. They are located at the three lattice points in the hexagonal basis that are shown in Fig.8.
The remaining six-venths of the M2cations are in(18f)
general positions.They are surrounded by ven oxygen
144NADAUD ET AL.
TABLE 3
Structural Data and Refinement Parameters for In 2O 3–SnO 2Materials as Given by Rietveld Refinement
of Neutron Powder Diffraction Patterns
ITO 3%Sn
ITO 6%Sn
ITO 6%Sn
In Sn O In O
(O )(O )(N )Lattice parameter (A )10.1192(2)
10.1234(2)
10.1247(2)
10.1309(2)
a "9.4604(2)c "8.8584(2)Cation 1x 0y 0z
0B (A )
0.40(7)0.38(13)0.46(13)0.42(11)0.54(6)Sn/(In #Sn)(%)03?10(1)21(1)100(1)Cation 2x 0.4667(2)0.4673(2)0.4678(2)0.4667(2)0.2526(2)y 0.00000.00000.00000.00000.2145(2)z 0.00000.00000.00000.00000.3497(2)B (A )
0.34(4)0.31(5)0.38(6)0.35(5)0.47(4)Sn/(In #Sn)(%)03?5(1)1(1)32(1)O x 0.3909(2)0.3899(2)0.3893(
2)0.3901(2)0.1979(2)y 0.1543(2)0.1547(2)0.1545(2)0.1545(2)0.1768(2)z 0.3820(2)0.3821(2)0.3820(2)0.3820(2)0.1162(2)B (A )0.45(2)
0.50(2)
0.59(2)
0.51(2)
0.96(4)O x 0.1886(3)y 0.9745(2)z
0.3917(2)B (A )0.45?0.50?0.59?0.51?0.82(3)Occ (%)@0.2(1)
0.8(1)
3.2(1)
0.1(1)
—R
(%)18.414.316.214.511.1R (%)15.8512.212.412.08.4R
(%) 4.46 3.7 5.8 5.8
4.4
?Fixed equal to B (O
)for fitting.
@Occ "oxygen occupancy (%)of ‘‘structural’’vacancy in the In O
甘罗十二为丞相fluorite-related structure.
atoms,t at the vertices of a highly distorted cube where one corner is lacking (M 2O »
).The metal —oxygen distan-ces vary widely (from 2.07to 2.64A ,average 2.23A ),the distance M 2—O being longer than M 1—O (2.06A ).
The polyhedra M 1O » and M 2O »
can be connected
with the so-called ‘‘Bevan’’cluster (24)where the sixfold-coordinated M 1polyhedron is surrounded by six venfold-coordinated polyhedra M 2with sharing edges.M 1and M 2polyhedra are stacked along the threefold rotation axis,as shown in Fig.9.
Neutron diffraction and EXAFS were ud to determine whether cation ordering occurs in In Sn O什么人忧天
.Refinement
of neutron data suggests that M 1sites are fully occupied by tin atoms whereas M 2sites are occupied by indium and the remaining tin atoms.EXAFS data of In Sn O
at the In
and Sn K -edges are shown in Figs.6and 10.The In and Sn edge EXAFS contributions for the first metal —oxygen shell are rather different.Table 2shows the EXAFS data-fitting results of the first M—O shell analyzed.(i)At the In edge,two contributions are needed to refine properly the k (k )relative to the first oxygen shell.The first In —O shell is estimated at 2.16A (5.1oxygen atoms)and a cond In —O shell at 2.52A (1.3oxygen atoms).This configuration is similar to that of the M 2site deduced from neutron powder diffraction data.
(ii)At the Sn edge,the Sn —O shell radius is found to be shorter than the average In —O shell radius (R "2.05A ,with 5.8oxygens).This Sn —O configuration looks like an M 1site.
As a conquence,cation ordering in In Sn O
is found
when the sixfold-coordinated M 1sites are fully occupied by tin,with one-third of the venfold-coordinated M 2sites occupied by tin atoms and the remaining sites by indium atoms.
Cation ordering in M O
occurs when the cation sizes
英语花体字
are different enough to change the metal —oxygen bond strength (29).The cation with smaller size and higher charge occupies the M 1site,with a shorter M—O distance and
STRUCTURAL STUDIES OF ITO AND In Sn O
145
