Structure, optical, and magnetic properties of facing-target reactive sputtered Ti 1 − xFe x O 2 − δ filmsY. B. Jiang, W. B. Mi, E. Y. Jiang, and H. L. Bai Citation: Journal of Vacuum Science & Technology A 27, 1172 (2009); doi: 10.1116/1.3168557 View online: http://dx.doi.org/10.1116/1.3168557 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/27/5?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Structural, optical, vibrational, and magnetic properties of sol-gel derived Ni doped ZnO nanoparticles J. Appl. Phys. 114, 033912 (2013); 10.1063/1.4813868 Enhancement of optical absorption by modulation of the oxygen flow of TiO2 films deposited by reactivesputtering J. Appl. Phys. 111, 113513 (2012); 10.1063/1.4724334 Influence of Mn doping on structural, optical, and magnetic properties of Zn 1 − x Mn x O nanorods J. Appl. Phys. 108, 044910 (2010); 10.1063/1.3478709 Microstructure, magnetic, and optical properties of sputtered Mn-doped ZnO films with high-temperatureferromagnetism J. Appl. Phys. 101, 023904 (2007); 10.1063/1.2426377 Optical and electrical transport properties of facing-target sputtered Al doped ZnO transparent film J. Appl. Phys. 99, 124906 (2006); 10.1063/1.2204827

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Structure, optical, and magnetic properties of facing-target reactivesputtered Ti1−xFexO2−� films

Y. B. Jiang, W. B. Mi, E. Y. Jiang, and H. L. Baia�

Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparing Technology,Institute of Advanced Materials Physics, Faculty of Science, Tianjin University, Tianjin 300072,People’s Republic of China

�Received 19 March 2009; accepted 8 June 2009; published 10 August 2009�

Structure, optical, and magnetic properties of the anatase Ti1−xFexO2−� films fabricated byfacing-target reactive sputtering were investigated. Structural analyses indicate that there are noimpurity phases in the films and the grain size reaches maximum when Fe content x is 0.017. Theoptical band gap of the Ti1−xFexO2−� films decreases with the increasing x. Photoluminescence �PL�spectra show that direct and indirect band-to-band transitions coexist, and the indirect radiativerecombination can be regarded as a one photon and two phonon coupling courses. The PL intensitydue to oxygen vacancies enhances and the refractive index of the fims increases with the increasingx. The Ti1−xFexO2−� films exhibit room-temperature ferromagnetism due to the oxygen vacancies.© 2009 American Vacuum Society. �DOI: 10.1116/1.3168557�

I. INTRODUCTION

Wide-band-gap semiconductor titanium dioxide �TiO2�has been extensively studied for decades due to its high re-fractive index, excellent optical transmittance in the visiblelight and near-infrared region, and high photocatalytic per-formance for water cleavage.1 Most of the research workreport that anatase TiO2 shows high photocatalytic reactivityunder ultraviolet light corresponding to its band gap whichmake up a very small proportion of the solar spectrum. Inorder to extend the absorption region of TiO2 to visible light,several attempts have been made to decrease the band gapsuch as composite semiconductors, ion doping, etc.2–4 Asahiet al.2 suggested that doping of TiO2 with nitrogen shifts itsoptical absorption into the visible region below 500 nm andenhances the photoactivity of TiO2. Visible light sensitiveTiO2 can be prepared by doping with transitional metal �TM�ions.3 Additionally, the photocatalytic activity of TiO2 is in-tensely dependent on the competition between the productionand recombination of the excited electrons in conductionband and the holes in valence band. The addition of transi-tion metal ions to TiO2 has been suggested to be an effectiveway to reduce the photogenerated electron-hole recombina-tion rate.4 Moreover, the introduction of metal ions can beused to tune the refractive index of TiO2 which is very im-portant for its applications in the field of optical coatings.5

Forthermore, the ferromagnetic TM-doped TiO2 have at-tracted lots of interests thanks to their potential applicationsin spintronic devices since the discovery of room-temperature �RT� ferromagnetism in Co-doped anataseTiO2.6 In this article, the anatase Ti1−xFexO2−� films werefabricated using dc facing-target reactive sputtering withoutsubstrate heating. The influences of Fe-dopant concentration

on the microstructure, optical band gap, refractive index,photoluminescence, and magnetic properties were studied indetail.

II. EXPERIMENTAL DETAILS

Anatase Ti1−xFexO2−� films with different Fe atomic frac-tions were fabricated using facing-target reactive sputteringin Ar �99.999%� and O2 �99.999%� gas mixture on Si, quartz,and Kapton™ at ambient temperature. The target was tita-nium �99.99%� on which iron �99.99%� pieces of differentsizes were fixed. The base pressure of the vacuum chamberwas better than 5�10−4 Pa and the total pressure of the Arand O2 gas mixture with 1:1 ratio of Ar to O2 flow rate waskept at 1.71 Pa with a fixed sputtering power of 350 W. Thefilm thickness was �300 nm determined by a Dektak 6msurface profiler. The morphology of the films was character-ized by scanning electron microscopy �SEM� in a JEOL ap-paratus �model JSM–6700F�. The composition and chemicalstates of the films were investigated by x-ray fluorescenceand x-ray photoelectron spectroscopy �XPS� using Mg K�radiation �1253.6 eV�. The structural investigation was per-formed by x-ray diffraction �XRD� with Cu K� radiation.Fourier transform infrared spectroscopy �FTIR� has alsobeen used for the assessment of the crystalline structure inthe frequency range of 400–4000 cm−1 and the signal fromSi substrates was subtracted. The optical transmittance of thefilms was measured using a UV-vis spectrophotometer in thewavelength range of 200–900 nm. The optical constants ofthe Ti1−xFexO2−� films were determined by spectroscopic el-lipsometry �SE� and the photoluminescence �PL� spectrawere obtained by the photoluminescence spectrometer with a150 W high-pressure xenon lamp at room temperature. Themagnetic properties were measured using a vibrating samplemagnetometer equipped on a Quantum Design physicalproperty measurement system �PPMS–9� at temperaturesranging from 5 to 340 K.

a�Author to whom all correspondence should be addressed; electronic mail:baihaili@tju.edu.cn

1172 1172J. Vac. Sci. Technol. A 27„5…, Sep/Oct 2009 0734-2101/2009/27„5…/1172/6/$25.00 ©2009 American Vacuum Society

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III. RESULTS AND DISCUSSION

A. Chemical composition and structure

Figure 1 shows the typical Ti 2p and Fe 2p XPS spectra.The core levels of Ti 2p1/2 and 2p3/2 located at 463.98 and458.24 eV �after C 1s 284.6 eV correction�, respectively, areassigned to the Ti4+. The 0.96 eV binding energy shift of theTi 2p3/2 relative to the position reported at 459.2 eV can beattributed to the strain in the films.7 No shoulders associatedwith Ti3+ or Ti2+ are observed in the Ti 2p XPS spectrum.The spin-orbital splitting between the Ti 2p1/2 and 2p3/2peaks is 5.74 eV in accordance with the value of anataseTiO2.8 The Fe 2p3/2 peak located at a binding energy of710.7 eV corresponds to the reported value of Fe3+, which isdifferent from those of Fe3O4, FeO, and metallic Fe situatedat 710.4, 709.5, and 707 eV, respectively.9,10 A shake-up sat-ellite peak appears at the binding energy of 719.2 eV, whichconfirms the trivalent state of Fe ions in the films.9 The XPSresults indicate that the chemical valence of Fe is in +3 andexcludes the formation of the Fe3O4, FeO, and metallic Feclusters.

Figure 2 presents the XRD patterns of the Ti1−xFexO2−�

films. All the detectable diffraction peaks of the undoped filmare from anatase TiO2. The diffraction peak of �101� planepossesses higher intensity than any other diffraction peaksdue to the difference in surface formation energy of variousTiO2 planes. The surface formation energy of anatase �101�plane is 0.44 J /m2, which is smaller than those of otherplanes, such as 0.53 J /m2 for �200� plane.11 Upon dopingFe, the intensity of the anatase �101� peak first increases as x

is below 0.017, and then decreases with further increasing x.Meanwhile a small peak at 27.4° corresponding to rutile�110� is also observed in the pattern of the sample with x=0.017, which indicates that the introduction of Fe promotesthe formation of rutile. No hints of Fe-containing secondaryphase such as metallic Fe and Fe oxides are observed in theXRD patterns. The lattice parameters a, c and lattice volumeV were calculated based on the least-squares method, and theresults are listed in Table I. V reaches its maximum valuewhen Fe content is 0.017. In general, the lattice expansion ofTiO2 originates from three aspects: �1� the formation ofMagnéli shear planes,12 �2� the substitution of Fe2+ ions intothe Ti4+ site,13 and �3� the grain size effect.14 However, theMagneli phase is common in rutile TiO2, and the chemicalvalence of Fe in the films was determined to be +3. Asshown in Table I, the lattice volume increases monotonicallywith the grain size which is the result of competition be-tween the positive and negative pressures. As the particlesize decreases, the former leads to a lattice contraction whichstems from surface stresses, while the latter results in a lat-

FIG. 1. Typical Ti 2p and Fe 2p XPS spectra of Ti1−xFexO2−� films.

FIG. 2. X-ray diffraction patterns of Ti1−xFexO2−� films; the diffraction dataof powder anatase from standard file are also included for comparison, R inthe figure stands for rutile phase.

TABLE I. The lattice constants and grain size of Ti1−xFexO2−� films.

xa=b��

c��

V�Å3�

D�nm�

0 3.789 9.500 136.37 29.710.007 3.787 9.531 136.71 35.860.014 3.785 9.564 137.05 37.350.017 3.790 9.598 137.86 42.840.038 3.785 9.514 136.31 24.97

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tice expansion due to the interaction of surface defect di-poles. Anatase TiO2 has been demonstrated to possess asmall polarization field,14 therefore the negative pressure isweaker than the positive one. As a result of net positivepressure, the lattice volume decreases with the decreasinggrain size.

Additional structural information of Ti1−xFexO2−� filmswas obtained using FTIR spectra, as shown in Fig. 3. Theabsorbance centered at 2360 cm−1 �the inset in Fig. 3� origi-nates from CO2 in air.15 The main bands at 400–700 cm−1

are from the vibration modes of �TiO6� octahedral.16 Thestrong absorption peak at 433 cm−1 is attributed to transver-sal optical phonons of anatase �Eu modes�.4 The peak at478 cm−1 is assigned to the Ti–O stretching modes ofanatase.17 When x is less than 0.017, the 433 cm−1 band forEu modes sharpens and increases in intensity with the in-creasing x. While x is higher than 0.038, the broaden peakaround 433 cm−1 indicates the poor crystallization of thefilms which is in agreement with the XRD results. SEM im-ages presented in Fig. 4 show that Ti1−xFexO2−� films displaysmooth surface with particle size of �40–50 nm. At x�0.038, the particle boundaries are getting indistinct, lead-ing to several plate-shaped particles.

B. Optical properties

Figure 5 presents the UV-vis transmittance spectra of theTi1−xFexO2−� films on quartz substrates with different x. TiO2

films exhibit high transmittance �about 90%� in the region of

400–900 nm and the intense absorption in the region of 340–400 nm is observed in all the spectra. It is apparent that theabsorption edge shifts to the visible light region with increas-ing x. According to the energy band structure, semiconduc-tors are divided into those with direct and those with indirectallowed transitions. For direct transitions, the minimum ofthe lowest conduction band in k space is directly above themaximum of the highest valence band, and the absorptioncoefficient � above the threshold of the fundamental absorp-tion follows the relation18

� = Bd�h� − Eg�1/2/h� , �1�

where Bd is the absorption constant for a direct transition,h�=E the energy of the incident light, and Eg the estimatedoptical band gap. While for indirect semiconductors, theminimum of the lowest conduction band and the maximumof the highest valence band are located at different points ink spaces, and the absorption coefficient � follows therelation18

FIG. 3. FTIR spectra of Ti1−xFexO2−� films.

FIG. 4. SEM images of Ti1−xFexO2−� films with different x: �a� 0, �b� 0.017,�c� 0.038, and �d� 0.144.

FIG. 5. �Color online� Transmittance spectra of Ti1−xFexO2−� films with dif-ferent x.

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� = Bi�h� − Eg�2/h� , �2�

where Bi is the absorption constant for an indirect transition.The absorption coefficient � can also be derived from theapproximation relation �=−ln T /d, where d is the film thick-ness. Commonly, anatase TiO2 is considered as an indirectsemiconductor.19 Nevertheless, several reports assign theanatase TiO2 to a direct transition semiconductor.18,20 Theplots ��h��1/2 and ��h��2 versus �h�� are shown in Fig. 6,and then the indirect and direct optical band gaps are ob-tained by extrapolating the linear region of the curves. Theestimated indirect optical band gap of the undoped anataseTiO2 is 3.39 eV, which is larger than 3.20 eV of bulk anataseTiO2. The main reason for the larger Eg is the axial straineffect due to lattice distortion between the film andsubstrate.21 The indirect and direct band gaps of the undopedanatase, 3.39 and 3.73 eV, correspond to the indirect transi-tion �3→�1b and direct transition �2b→�1b of the elec-tronic band structure reported by Daude,22 respectively. Asshown in the inset of Fig. 6, the optical band gap of theTi1−xFexO2−� films decreases with increasing x, which is in-dependent of the assumed transition nature. It was reportedthat the band edge of TiO2 undergoes a blueshift of 50 meVdue to a 2% lattice volume contraction.20 From the XRDanalysis, we know that the lattice volume increases graduallywhen x is lower than 0.017. There might be contribution ofthe lattice volume expansion to the band gap reduction whenx is below 0.017. However, when x is above 0.038, the latticevolume is getting smaller while the band gap reducessteadily. Therefore, there are other reasons for the band gapreduction. The most possible one may be the effect of Fedoping into the anatase lattice. The Fe 3d state electrons in

the TiO2 generate a dopant level between the conduction andvalence band.23

In order to understand the effect of Fe incorporation onthe band structure, we measured the PL spectra of theTi1−xFexO2−� films as presented in Fig. 7. The PL spectra ofthe undoped TiO2 excited by 260, 280, 300, and 320 nmlights �not shown here� indicate that the direct band-to-bandradiative recombination peak at �3.62 eV vanishes whenthe light energy is below 3.88 eV �320 nm�. The PL peak at�3.32 eV can be attributed to the indirect transition. Aftermultipeak fitting, the indirect band-to-band emission is com-posed of seven Gaussian peaks, as shown in Fig. 8. Theenergy interval between each two neighboring peaks is�0.04 eV which is close to the energy sum of two Ramanphonons Eg�v5� and Eg�v6�. As we know, phonons are neededto maintain momentum balance during indirect transitionprocess. Therefore, the indirect radiative recombination is a

FIG. 6. �Color online� Plots of ��E�1/2 and ��E�2 vs photon energy E. Theinset shows the dependence of direct and indirect optical gap on x.

FIG. 7. Photoluminescence spectra of Ti1−xFexO2−� films.

FIG. 8. Indirect band-to-band PL peak and its Gaussian fitting.

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coupling emission process of one photon and two phonons.24

The broad PL bands ranging from 3.10 to 2.50 eV are gen-erally attributed to self-trapped excitons �STEs�, oxygen va-cancies, and surface states.25–27 The 3.10–2.80 eV emissionband is assigned to STE in accordance with anatase nanowirearrays.25 Nadica et al. ascribed the broad emission in the PLspectra range from 2.75 to 2.16 eV to oxygen vacancies.26 Inaddition, the PL peak at 2.70 eV is considered to originatefrom VO in the Co-doped anatase system.27 Therefore, theweak PL peak at �2.65 eV is attributed to VO. The PL spec-tra of the Ti1−xFexO2−� films indicate that the band-to-bandPL peaks shift to low energy side with Fe doping, similar tooptical band gap deduced from the transmittance spectra. Itis worthy to note that the energy position of the emissionascribed to oxygen vacancies does not shift with Fe content,while its intensity enhances. This illustrates that the numberof oxygen vacancy increase with Fe content.

Figure 9 shows the refractive index �n� and extinctioncoefficient �k� of the Ti1−xFexO2−� films as a function ofwavelength ��� with different x. The extinction coefficientsof all the films are close to zero in the visible light region,which is consistent with the transparent property of TiO2

films. Additionally, the wavelength at which the value of k isnear zero undergoes considerable redshift with increasing x,in accordance with the shift of absorption edge in Fig. 5. It isnot difficult to understand the consistency due to the relation�=4k /�. The refractive index at �=550 nm increasesfrom 2.21 at x=0 to 2.53 at x=0.092, as exhibited in the insetof Fig. 9�a�. As we know, the refractive index is dependenton the density �� and polarizability ���, given by28

n2 − 1

n2 + 2=

4

3·

NA

m��r� , �3�

where NA is Avogadro’s constant and m the molecularweight. The polarizability and the ionicity �r��� are directlyrelated to each other by approximation relation,29

��r� � � r�� + 1

r��� . �4�

According to Pauling theory, the ionicity of the chemicalbond can be obtained from

r�� = 1 − exp�− �xanion − xcation�2/4� , �5�

where xanion and xcation are the electronegativities of the anionand cation, respectively. The electronegativities of Ti, Fe,and O are 1.54, 1.83, and 3.44 from Pauling’s definition.Thus, the ionicity of Fe–O bonds is smaller than that of Ti–Obonds, leading to a higher polarizability which results in theincrease in the refractive index with Fe doping.

C. Magnetic properties

Figure 10 presents the room-temperature M-H curves ofthe Ti1−xFexO2−� films. One can see that the films are ferro-magnetic at room temperature with magnetic hysteresisloops. The saturation magnetization increases from 0.91 to2.36 emu /cm3 with x increasing from 0.007 to 0.017, andthen reduces to 1.03 emu /cm3 with x=0.092, which is notfrom the secondary phases in the films by the XPS and XRDanalyses. The top-left inset shows the Ms-x curve. With theincreasing x, Ms increases with x first and decrease withfurther increasing x. The increase in Ms in the small x rangecan ascribed to the facts that the Fe ions are ferromagneticcoupling at small x range because the distance between theFe ions is large enough, so with the increasing x the Ms

increases. The decrease with further increasing x can be ex-plained that the distance between adjacent Fe ions decreases

FIG. 9. �Color online� Refractive index �a� and extinction coefficient �b� ofTi1−xFexO2−� films. The inset shows the dependence of the refractive indexat 550 nm on x.

FIG. 10. �Color online� Room-temperature M-H curves of Ti1−xFexO2−�

films. The top-left inset is the Ms-x curve. The bottom-right inset presentsthe ZFC and FC curves of Ti0.983Fe0.017O2−� films.

1176 Jiang et al.: Structure, optical, and magnetic properties of facing-target reactive sputtered Ti1−xFexO2−� films 1176

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as x increases, so that more moments of Fe arrange antifer-romagnetically because the energy of antiferromagnetic stateis lower than that of ferromagnetic one. The variation of Ms

is also consistent with the change in the lattice constant ofthe films listed in Table I. For analyzing the magnetic behav-iors, the zero-field-cooling �ZFC� and field-cooling �FC�curves of the film with x=0.017 at 500 Oe field were mea-sured, as shown in the bottom-right inset of Fig. 10 withoutsubtracting the signal of the substrate. In the temperaturerange of 5–340 K, the ZFC and FC curves are of bifurcateshape, eliminating paramagnetic and diamagneticcontributions.30 No discriminable peak related to blockingtemperature in ferromagnetic granular systems is observedamong the entire temperature range, suggesting that there areno ferromagnetic clusters in the films. The M-H curves ofTi0.983Fe0.017O2−� �not shown here� at different temperatures�305, 200, 150, 100, 50, and 5 K� indicate that no apparentexchange bias exists and the coercive fields increase withdeceasing temperature. To summarize, the room-temperatureferromagnetism does not originate from ferromagnetic clus-ters.

As for the origin of the ferromagnetism, the mechanism ofcarrier mediated ferromagnetism does not work in thepresent anatase Ti1−xFexO2−� films because the RT resistanceof all the films with thickness of �300 nm is beyond theextreme of commercial multimeter �200 M �. From the PLresults, the oxygen vacancies of the films increase with in-creasing x. The ferromagnetism can be apprehended usingF-center model which is based on oxygen vacancies.6 Theo-retically, in order to maintain the charge neutrality, one oxy-gen vacancy would be created when two Fe3+ ions substitutefor two Ti4+ ions in the lattice. When x is below 0.017, theenhancement of the magnetization with increasing x resultsfrom the increase in the Fe3+–VO–Fe3+ ferromagnetic cou-pling groups. With further increasing x, the antiferromagneticFe3+–O2−–Fe3+ superexchange interaction strengthens,which in turn decreases the saturation magnetization. Similarresults of ferromagnetism related to the PL intensity werereported in Co-anatase and rutile TiO2.27,31

IV. CONCLUSION

Anatase Ti1−xFexO2−� films were deposited by facing-target reactive sputtering. Structural analyses indicate thatthe grain size attains maximum at x=0.017. Fe doping inTiO2 leads to a reduction in the optical band gap derivedfrom the transmittance spectra. The PL results show that di-rect and indirect band-to-band transitions coexist in the filmsand the PL intensity due to the oxygen vacancies enhanceswith Fe doping. The SE studies demonstrate that the refrac-tive index increases with x. In addition, Ti1−xFexO2 filmsshow room-temperature ferromagnetism in which the oxygenvacancies play an important role.

ACKNOWLEDGMENTSThis work was supported by National Science Foundation

of China �Grant Nos. 50672064 and 50701033�, the RFDP�Grant No. 20070056047�, Natural Science Foundation ofTianjin City �Grant Nos. 06TXTJJC13900 and 08JCY-BJC09400�, and YFF of Tianjin University �Grant Nos. TJU-YFF-08B52 and TJU-YFF-08A05�.

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