Table Of ContentHybridization between the conduction band and 3d orbitals in the oxide-based diluted
magnetic semiconductor In V O
2−x x 3
M. Kobayashi,1 Y. Ishida,1,∗ J. I. Hwang,1 G. S. Song,1 M. Takizawa,1 A. Fujimori,1 Y. Takeda,2
T. Ohkochi,2 T. Okane,2 Y. Saitoh,2 H. Yamagami,2,3 Amita Gupta,4 H. T. Cao,4 and K. V. Rao4
1Department of Physics, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
2Synchrotron Radiation Research Unit, Japan Atomic Energy Agency, Sayo-gun, Hyogo 679-5148, Japan
3Department of Physics, Faculty of Science, Kyoto Sangyo University, Kyoto 603-8555, Japan
4Department of Materials Science-Tmfy-MSE, Royal Institute of Technology, Stockholm SE 10044, Sweden
8 (Dated: February 4, 2008)
0
0 The electronic structure of In2−xVxO3 (x = 0.08) has been investigated using photoemission
2 spectroscopy (PES) and x-ray absorption spectroscopy (XAS). The V 2p core-level PES and XAS
spectrarevealedtrivalentelectronicstateoftheVion,consistentwiththesubstitutionoftheVion
n
for the In site. The V 3d partial density of states obtained by the resonant PES techniqueshowed
a
J a sharp peak above the O 2p band. While the O 1s XAS spectrum of In2−xVxO3 was similar to
that of In2O3, there were differences in the In 3p and 3d XAS spectra between V-doped and pure
8
In2O3. The observations give clear evidencefor hybridization between the Inconduction band and
2
theV 3d orbitals in In2−xVxO3.
]
i PACSnumbers: 75.50.Pp,75.30.Hx,78.70.Dm,79.60.-i
c
s
-
l Diluted magnetic semiconductors (DMS’s) have been sponse,indicatingauniformdistributionofmagneticdo-
r
t studiedintensivelysincethediscoveryofferromagnetism mainsinthesematerials[12,18]. ReportsofIn2O3-based
m
in the III-V DMS Ga1−xMnxAs [1, 2] because the ferro- DMS’shavesuggestedrelationshipbetweentheelectrical
. magnetic interaction between the Mn ions mediated by conductivity andferromagnetismthroughmeasurements
t
a hole carriers enables us to manipulate both the charge of, e.g., anomalous Hall effects [16, 17, 18, 19, 20]. As
m and spin degrees of freedom of electrons [3]. Ferromag- In O -basedDMS’sarecandidatesforroom-temperature
2 3
- netic DMS’s having Curie temperature (TC) aboveroom ferromagnetic DMS’s, the knowledge of their electronic
d
temperaturehavebeenstronglydesiredforrealisticspin- structure is necessary to understand of the origin of the
n
tronic applications. Eversince the theoreticalprediction ferromagnetism.
o
c byDietletal. [4]thatMn-dopedGaNandZnOwithhigh Photoemission spectroscopy (PES) and x-ray absorp-
[ hole concentrations should show ferromagnetism above tion spectroscopy (XAS) are powerful tools to investi-
room temperature, wide-gap semiconductors have be- gateelectronicstructureofmaterials. XAS,whichmeans
1
v comepromisinghostmaterialsforhigh-TC DMS.Infact, photonabsorptionfromacore-levelelectronintounoccu-
4 therehavebeenmanyreportsonroomtemperatureferro- pied states, is an element specific technique to study the
4 magnetisminoxide-basedDMS’ssuchasCo-dopedTiO2 electronic structure. 2p → 3d resonant photoemission
2 [5], Co-doped SnO2 [6], and Mn-doped ZnO [7]. spectroscopy(RPES)enablesustoextractthe3dpartial
4 density of states (PDOS) in the valence band. In this
Recently,In O -basedDMS’s haveattractedmuchat-
. 2 3
1 tention because of the reports of room temperature fer- work, we have performed PES and XAS measurements
0 romagnetisminnotonlylighttransition-metal(TM)but onIn2−xVxO3 (IVO) thin films in orderto obtain a fun-
8 damentalunderstandingoftheelectronicstructures. The
also heavy TM doped In O [8, 9, 10, 11, 12, 13, 14,
0 2 3 valencestate ofthe dopedVionhasbeendeterminedby
15, 16, 17, 18, 19, 20] and of the potential of the host
:
v material In O for applications. Here, In O has a band core-level x-ray photoemission spectroscopy (XPS) and
2 3 2 3
i XAS. Effects of doping onthe electronic structure of the
X gap of ∼ 3.5 eV, is an n-type semiconductor, and crys-
hostmaterialhavebeenapproachedbyV2p→3dRPES
tallizes in the cubic bixbyite structure, where In atoms
r and XAS at the O and In edges.
a are coordinated by six oxygens forming octahedral (O )
h
and orthorhombic (D2h) octahedra and neighboring oc- Each of In2O3 and In2−xVxO3 (x = 0.08) thin films
was highly oriented normal to the plane of a sap-
tahedra are shared with their corners and edges [21, 22],
phire(0001)substratebythepulsedlaserdepositiontech-
and Sn-doped In O (ITO) has been famous for its high
2 3
nique. During the deposition, the substrate temperature
electrical conductivity and transparency [23, 24]. Mag-
◦
was kept at ∼ 400 C. The total thickness of the de-
neticforcemicroscopyobservationsofIn2−xCrxO3−δ and
posited layerwas ∼500 nm. X-ray diffraction confirmed
In2−xNixO3 have demonstrated that the topological un-
that the thin film had the cubic bixbyite structure and
dulations correspond to the strength of magnetic re-
no secondary phase was observed. Details of the sample
fabricationare givenin Ref. [14]. The V concentrationx
in the thin films was estimated from the intensity of the
∗Presentaddress: RIKEN,SPring-8Center,Sayo-cho,Hyogo679- V 2p core-level PES spectrum. Ferromagnetism above
5148,Japan roomtemperature wasconfirmedbymagnetizationmea-
2
surements using a SQUID magnetometer (Quantum De- andseveralvanadiumoxides. The line shape ofthe V2p
sign, Co. Ltd.). spectrum of IVO is similar to that of V O (V3+) [28]
2 3
RPES and XAS measurements were performed at the rather than that of Zn1−xVxO (V2+) [29] and of V2O5
soft x-ray beam line BL23SU of SPring-8 [25, 26]. The (V5+) [30], consistent with the observation of the V 2p
monochromator resolution was E/∆E > 10,000. XAS XPS.The results suggestthat the electronic structureof
signalsweremeasuredbythetotalelectronyieldmethod. the doped V ion in IVO is close to that of the V ion in
ThebackgroundoftheXASspectrawasassumedtobea V2O3, i.e., V is in the trivalent V3+ state octahedrally
hyperbolic tangent function. The RPES and XPS mea- coordinated by oxygens. Therefore, it is likely that the
surements were performed in a vacuum below 1.0×10−7 doped V ions are substituted for the In sites.
Pa using Gammadata Scienta SES-2000 and SES-100 In order to obtain an understanding of the electronic
hemispherical analyzers, respectively. A Mg-Kα x-ray structure of the V ion in the valence band, we measured
source(hν =1253.6eV)wasemployedfortheXPSmea- V 2p →3d resonant photoemission spectra of IVO. Fig-
surements. In the PES measurements, photoelectrons ure 2(b) shows the valence-band PES spectra of IVO
werecollectedintheangleintegratedmodeatroomtem- taken at various photon energies in the V 2p→3d core-
perature. The total resolution of the RPES and XPS excitation region. Energy difference between the top of
measurements including temperature broadening were the O 2p bands and E is ∼ 3.0 eV, comparable to the
F
∼300 and ∼800 meV, respectively. Sample surface was band gap of In O ∼3.5 eV, indicating that in the IVO
2 3
cleaned by cycles of Ar+-ion sputtering. Cleanliness of sample E is located near the bottom of the conduction
F
the sample surface was checked by the absence of a high band. We observed clear resonant enhancement in the
binding-energy shoulder in the O 1s spectrum and C 1s valenceband. ThePESintensity asafunctionofphoton
contamination by XPS. The position of the Fermi level energy (hν), i.e., constant-initial-state (CIS) spectrum,
(E )wasdeterminedbymeasuringPESspectraofevap- at E = 1.7 eV demonstrates that the enhancement is
F B
orated gold which was electrically in contact with the proportionaltotheintensityoftheV2pXASasshownin
samples. Fig.2(a). The on-andoff-resonancespectrawerechosen
First, we discuss about the valence state of the V ions as measured at hν = 516.5 eV and 513 eV, respectively.
inIVO.Figure1showstheV2pcore-levelspectraofIVO Thedifferencebetweentheon-andoff-resonancespectra
and various vanadium oxides. The binding energy (E ) yields the V 3d PDOS. Although the relative intensities
B
of a core-levelpeak position is relatedto its charge state of different spectral features change with photon energy
and, in general, becomes larger with increasing valency. due to the effects of transition matrix elements and of
In fact, it has been reported that the E of the V 2p the resonance, the structures (peak positions) of the V
B 3/2
peak increases with valency of V as shown in Fig. 1(a)
[27]. Bycomparingthepeakpositionofthe V2p core
3/2
lv(edav2lee)ln.oceFfIisgVtuaOrteew1oit(fhbV)thsinohsoIewno2sfOtth3heeisoVftoh2uepnrdvXatAnoSabdseiuptmercivtoraxaliedonefts,IVVt3hO+e y (arb. units) (VXa)A 2Sp XVA 3Sd intensity In1.92V0.08O3 V( b3)d
nsit 523.9
e
nt
I510 515 520 525 522.8
nsity (arb. units) (VXa)P 2Sp O 1Vs 2p1/2 V 2VVVp43253+ +/+ 2OOO52 (VXbA )2Sp VV2523 + +OO53 ntensity (arb. units) (cD=) iVfPVf eOO3h r2dofnefp t-n-Porr–ecenDs3es ..dOE nRSePrgEyS (eV) Intensity (arb. units) In 4d 555511126480..57
Inte 2 3 Zn V2 +O I 513 O 2p
1-x x
12 8 4 0 20 15 10 5 0
IVO Binding Energy (eV) Binding Energy (eV)
×25 IVO
FIG. 2: V 2p → 3d resonant photoemission spectra in the
5 0 -5 -10 -15 -20510 515 520 525 530
Energy relative to O 1s peak position (eV) Photon Energy (eV) valence band of of In2−xVxO3 (x = 0.08). (a) V 2p XAS
spectrumandtheintensityoftheV3dpeakatEB =1.7eVas
FIG. 1: V 2p core-level spectra of In2−xVxO3 (IVO) with functions of photon energy. (b) A series of spectra measured
x=0.08. (a)V2pXPSspectracomparedwiththoseofother athν’sdenotedbytrianglesin(a). (c)Top: On-(hν =516.5
vanadium oxides [27]. (b) V 2p XAS spectra compared with eV) and off-resonance (hν = 513 eV) spectra. Bottom: On-
those of other vanadium oxides Zn1−xVxO [29], V2O3 [28], and off-resonance difference spectrum representing the V 3d
and V2O5 [30]. partial density of states (PDOS).
3
nits)100 (a) O 1s XAS 100 (c) In 3p (a) e V3+ Oh V3+ D2h Ti3+ Oh Cr3+ Oh
Intensity (arb. u 550020V 25Pp21h5/2o IInVto2O5On33 0Ene5rIg3n5y1 .9(2eVV540)0.08O5345b. units)8600 IInV2OO3 XAS t2gg
units)18000 (b) IXnA 3Sd nsity (ar40 (b) Oh (c)eg orbitals
nsity (arb. 6400 IInV2OO3 10 Inte20 10
Inte 200 0444 446 0 0 655660665
440 445 450 455 460 465 660 680 700 720
Photon Energy (eV) Photon Energy (eV) V O
FIG. 3: X-ray absorption spectra of In2−xVxO3 (IVO) with FIG. 4: 3d electronic structure of the V ion of In2−xVxO3.
x = 0.08 measured at In and O absorption edges related to (a) A schematic representation of the electronic structure in
the host In2O3. (a) O K XAS spectra. (b), (c) In 3d and 3p the V ion. Those of the Ti3+ and Cr3+ ion in a Oh crystal
XAS spectra, respectively. The insets show enlarged plots in fieldarealsopresented. (b)OctahedralcoordinationoftheO
thepre-edge region. atoms. (c) eg orbitals.
3d PDOS are independent of photon energy, implying ofaTMatomoctahedrallycoordinatedbyoxygenssplits
thatthe Vions areina singleelectronicstate. The V3d intotwo-folddegeneratee andthree-folddegeneratet
g 2g
PDOSshowsa peakabovethe O2pband, thatis,inthe levels due to ligandcrystalfields. InIVO, the V3+ (3d2)
middle of the band gap of In2O3, as shown in Fig. 2(c). ion substituting for the In site has two electrons in the
Thenarrowwidthandstrongintensityofthepeakimply t levels (or the two lowest levels splitted from the t
2g 2g
the localized nature of the V 3d orbitals in the valence level due to the D symmetry) as shown in Fig. 4(a).
2h
band, i.e., weak V3d−O2p mixing in IVO. Because the e orbitals are directed towardthe O atoms
g
In n-type DMS’s, electronic states near the bottom as shown in Figs. 4(b) and 4(c), the O 2p band can hy-
of the conductionband areexpected to be importantfor bridizewiththeeg orbitalsstrongerthanthet2g orbitals.
carrier-inducedferromagnetismsinceE islocatedthere. Indeed, the Slater-Koster parameter (pdσ), which repre-
F
Because the valence band of the host semiconductor is sents transfer integrals between the 3d eg and ligand p
usually occupied, XAS spectra measured at the absorp- orbitals, has an absolute value about twice larger than
tion edges of the host semiconductor reflect the unoccu- (pdπ),whichrepresentstransferintegralsbetweenthe3d
pied electronic states, namely, the conduction band, and t2g and ligand p orbitals [31]. Considering the result of
areusefulfortheinvestigationoftheelectronicstructure O1sXAS,hybridizationbetweentheO2pbandandthe
ofn-type DMS’s. Figure 3shows XAS spectrameasured e orbitals is expected to be weak. It is probable that
g
at absorptionedges of Inand O,and comparesthem be- the t2g orbitals hybridize dominantly with the In band.
tweenV-dopedandpureIn O . TheO1sXASspectrum LightTM ions Ti3+ and Cr3+ under the O crystalfield
2 3 h
of IVO is nearly identical to that of In2O3 as shown in only have electron(s) in the t2g levels, too [Fig. 4(a)]. It
Fig. 3(a). It has been reported that the O 1s XAS spec- follows from those arguments that for light TM-doped
trum of Fe-doped In2O3 differs from that of pure In2O3, In2O3, hybridization between the host conduction band
andthatthedifferenceisinducedbyhybridizationofthe andthe3dt2g orbitalsshouldbetakenintoconsideration
Fe 3d orbitals with the O 2p band [15]. The observation to understand their electronic properties.
impliesweakerhybridizationbetweentheO2pbandand Considering the present observations, we shall dis-
the V 3d orbitals in IVO than that between O 2p and cuss about the mechanism of the ferromagnetism in
Fe 3d in In2−xFexO3, consistent with the result of the V In2−xVxO.The bottomofthe conductionbandofIn2O3
2p→3d RPES. In contrast,the In 3p and 3d XAS spec- are mainly composed of the In 5s bands [21]. For n-
tra were changed by V doping, i.e., there are difference type DMS’s, the electronic structure near the bottom of
betweentheXASspectraofIVOandtheseofpureIn2O3 the conduction band is important, i.e., s-d exchange in-
inboth pre-edgeregionsas showninFigs.3(b)and3(c). teraction may play an essential role rather than the p-d
The observations clearly indicate hybridization between exchangeinteraction. Itispossiblethathybridizationbe-
theIn5spconductionbandandtheV3dorbitalsinIVO. tweentheIn5sandV3dorbitalsasobservedIn3pXAS
Basedontheabovefindings,weshalldiscussaboutthe strengthens the s-d exchange interaction, as in the case
electronicstructureofIn O -basedDMS’s,inparticular, of the host conduction band − Co 3d orbital exchange
2 3
those doped with light-TM atoms. Electronic structure interaction in Ti1−xCoxO2 [32]. Generally speaking, be-
4
∗
cause in wide-gap semiconductors the effective mass m annealed in a high vacuum are ferromagnetic due to the
is large,the magnetic splitting due to the exchangeterm creationofoxygenvacancies. Afirst-principleelectronic-
in Hamiltonian (s,p-d exchange interaction) is greater structure calculation of In O has predicted that when
2 3
thanthespinsplittingofthehostvalenceandconduction electrons are localized around the oxygen vacancy, the
bands predicted by ordinarysp bandtheory, andthe ex- In-Inbond is reinforcedremarkablywhile the In-O bond
change contribution can be expressedas a largeeffective is significantlyweakened[21]. Consideringthis resultto-
g factor, which is given by getherwiththe experimentalfindings,itispossiblethat,
besides the electron carrier doping, the presence of oxy-
∗
geff = g +∆gex, gen vacancy augments the exchange interaction between
∆gex = αM/(cid:0)gTMµ2BH(cid:1), the In conduction band and the 3d orbitals. This effect
may be stronger for light TM-doped In O DMS’s than
2 3
∗
where g is the band g factor, g is the g factor of the heavy TM-doped ones because of the shrinkage of the
TM
3d ions, α is the exchange constant, and µ is the Bohr 3d orbitals with the number of 3d electrons. In order to
B
∗
magnetron, that is, |∆g | ≫ |g | for wide-gap semicon- set whether this considerationis validor not, systematic
ex
ductors [33]. Therefore, the s-d exchange interaction ac- measurementsofIn O -basedDMS’swithindependently
2 3
companied by the spin splitting of the conduction band controlled oxygen vacancies and carrier concentrations
is expected to be dominant in In2−xVxO3. In such a are desirable.
case, magnetic circular dichroism signal at the optical
In conclusion, we have performed PESand XAS stud-
absorption edge should be measurable. Additional mea-
surements which are related to the spin splitting of the ies of In2−xVxO3 thin films in order to investigate their
electronic structure and its relationship with ferromag-
conductionbandarehighlydesirable. Sincetheextentof
netism. From comparison with previous reports, the V
3d orbitals diminish with increasing number of 3d elec-
3d ions are expected to be trivalent states, indicating
trons, effects of cation-3d hybridization may weaken in
that the V ion substitutes for the In site. The V 3d
heavy TM-doped DMS’s.
PDOS in valence band has been observed using RPES
Assuming that light TM-doped In O DMS’s have
2 3 technique and shows a sharp peak within the band gap
common properties, we can discuss about the role of
of In O , implying the localized nature of the V 3d or-
oxygen vacancies in In O -based DMS’s. The oxygen 2 3
2 3 bitals in the valence band. There are differences of XAS
vacancy is a major defect in oxides and acts as a dou-
spectra measured at In 3p and 3d edges although the O
ble donor. In addition to electron doping by the va-
K XAS spectrum is independent of V doping. Based on
cancies, it has recently been proposed that the existence
the findings, it is likely that the s-d exchange interac-
of oxygenvacancies themselves affects the ferromagnetic
properties of oxide-based DMS’s through the formation tion is dominant for the ferromagnetism in In2−xVxO3.
For light TM-doped In O , the role of oxygen vacancy
of the donor-impurity band (in this case, the oxygen va- 2 3
has been discussed. The present results point to a need
canciesarethe centersofboundmagneticpolarons)[34],
for taking into account hybridization between the host
and reported that, even in non-magnetic oxide, the fer-
conduction band and 3d orbitals in In O -based DMS’s,
romagnetism can be induced by the oxygen vacancies 2 3
especially for light TM doped ones. We believe that the
[35,36]. ForlightTM-dopedIn O DMS’s,therearesev-
2 3 findingswillpromotefurthersystematicstudiesofn-type
eral works related to oxygen vacancies. Philip et al. [18]
oxide-based DMS’s.
have reported that while Cr:In O thin films deposited
2 3
at high oxygen pressure (20 mTorr) show paramagnetic This work was supported by a Grant-in-Aid for Sci-
behavior, films deposited at low oxygen pressure (0.35 entific Research in Priority Area “Spin Current: Its
mTorr) exhibits ferromagnetic properties, where the low Creation and Control” (19048012) from MEXT, Japan.
pressure growth induces in the thin films with electron Work in Sweden is supported by the Swedish Agency
carriers through the oxygenvacancies. Kharelet al. [37] VINNOVA, and the Carl Tryggers Stiftelse. MK and
have found that while air annealed Cr-doped In O film MT acknowledgesupportfromthe JapanSocietyfor the
2 3
and bulk samples do not show ferromagnetism, samples Promotion of Science for Young Scientists.
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