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Research ArticleReceived: 12 March 2009 Accepted: 24 July 2009 Published online in Wiley Interscience: 25 September 2009

(www.interscience.wiley.com) DOI 10.1002/jrs.2465

TiO2 thin films for spintronics application:a Raman studyF. Rossella,a∗ P. Galinetto,a M. C. Mozzati,a L. Malavasi,b Y. Diaz Fernandez,b

G. Drerac and L. Sangalettic

We present the results of a systematic study performed by micro-Raman spectroscopy on pure anatase, pure rutile andmixed anatase–rutile TiO2 thin films, deposited by radio frequency magnetron sputtering on quartz substrates, withdifferent thicknesses. The crystal structures of the as-deposited films were unambiguously determined and a good crystallinehomogeneity was revealed by a systematic mapping of the samples. In the mixed-phase films, the relative amount of the twophases was monitored by a simple analysis of the components of the multi-Lorentzian fitting curves. For the single-phasefilms, the influence of the thickness and the effect of different thermal treatments, carried out to obtain series of thin filmsdiffering only for oxygen content, are discussed. The analysis of the scattered light has provided indication about the presenceof an interface layer between the substrate and the film, which can play a role in driving the interesting magnetic propertiesexhibited by our samples, which are of potential usefulness for spintronics application. The results obtained from othertechniques are briefly reported and discussed in relation to our systematic Raman characterization. This study points outhow Raman investigation can provide suggestions toward the understanding of the complex physical phenomena leading toroom-temperature ferromagnetism in TiO2 thin films. Copyright c© 2009 John Wiley & Sons, Ltd.

Keywords: Raman spectroscopy; titanium dioxide; diluted magnetic semiconductors; thin films; oxygen vacancies

Introduction

The enormous interest of the actual research on oxide semicon-ductor materials is due to the possibility of finding an intriguingmixture of real application perspectives and basic physical phe-nomena in these systems. Titanium dioxide (TiO2) represents, inthis sense, an excellent example. The possibility of tailoring itsproperties by controlling the nanostructure in thin films (in ad-dition to the control of the crystalline phase) has opened newperspectives for its technological application as a versatile high-k oxide (metal oxide with wide bandgap and high dielectricconstant).[1] This kind of oxide has attracted great attention in theelectronic and optical industry,[2 – 5] for catalytic[6] or sensor[7,8] ap-plications and environmental technologies.[9] Furthermore, TiO2

represents a promising candidate also for applications in the im-portant field of spintronics based on magnetically ordered oxidesemiconductor materials.[10] Generally, these materials have re-ceived great attention due to the possibility of integrating theirmagnetic and electronic properties in spintronic devices,[11] andthe condensed matter community has devoted much effort toinvestigate their functional properties.[12 – 16] Significant attemptswere addressed, at first, to the growth of thin films of these ox-ides with techniques based on nonequilibrium processes: severalsystems, in particular ZnO, TiO2 and SnO2 matrices doped with Coand Mn, have been extensively and successfully grown.

Nevertheless, the results were somewhere contradictory, andthe interpretations proposed to explain the observed ferromag-netism (FM) were also different and controversial, ranging fromthe intrinsic mechanisms to the less interesting case of magneticion clusterization (especially in case of Co doping). Moreover,unexpected FM at room temperature (r.t.)[15,17,18] observed in un-doped HfO2 films prepared by pulsed laser deposition on sapphiresubstrates (bulk HfO2 is intrinsically nonmagnetic and electrically

insulating) provided arguments to the thesis that impurities in-troduced during sample handling and present in the substrateswere responsible for the development of FM.[19] However, r.t. FMin the absence of transition-metal ions has also been observedin vacuum-annealed TiO2 and ZnO samples and more recently inundoped laser-ablated TiO2 films and in single crystals.[20 – 23] Inparticular, the concentration of oxygen defects has been controlledby varying the oxygen pressure during the growth and a corre-lation has then been established between conductivity, inducedmagnetism and oxygen pressure, suggesting that magnetism inTiO2 thin films derives from the controlled introduction of aniondefects from both the film–substrate interface and by processingthe samples in an oxygen-deficient atmosphere.[22] Nevertheless,although an intrinsic mechanism leading to FM close to or abover.t. in diluted oxide magnetic semiconductors seems to exist, a clearpicture of the origin of the magnetic behavior has not yet beenobtained and the possible mechanisms involved are still underdebate.[24 – 26] Anyway, in all cases a crucial role seems to be playedby the defects and disorder arising from different reasons, e.g.deviation from stoichiometry, structural phase nonhomogeneityand, in case of thin films or nanocomposites, stresses and strains.

∗ Correspondence to: F. Rossella, CNISM - Dipartimento di Fisica ‘A. Volta’,Universita di Pavia, Pavia, Italy. E-mail: rossella@fisicavolta.unipv.it

a CNISM - Dipartimento di Fisica ‘A. Volta’, Universita di Pavia, Pavia, Italy

b Dipartimento di Chimica Fisica ‘M. Rolla’, INSTM and IENI-CNR, Universita diPavia, Pavia, Italy

c Dipartimento di Matematica e Fisica, Universita Cattolica del Sacro Cuore diBrescia, Brescia, Italy

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Table 1. Samples of TiO2 films deposited on quartz substrates, and related sputtering conditions. The expected phase, anatase (A) or rutile (R) ormixed (M), is indicated

Sample labelExpected

crystalline phaseSubstrate

temperature (◦C) Power (W) % Ar % N2 % O2 Pressure (mbar) Time (h) Thickness (nm)

A1 A 650 145 91 0 9 5.0 × 10−3 0.75 27.4

A2 A 650 145 91 0 9 3.7 × 10−3 2.25 83

A3 A 750 210 95 5 0 9.7 × 10−3 2 105.5

M1 M 650 210 100 0 0 4.1 × 10−3 2.25 226

M2 M 650 185 100 0 0 1.8 × 10−3 2.25 105

M3 M 650 145 100 0 0 3.8 × 10−3 2.25 83

R1 R 750 210 100 0 0 4.0 × 10−3 2 226

R2 R 750 210 100 0 0 4.0 × 10−3 0.5 56.7

These problems clearly occur in TiO2. Several growth techniques(magnetron sputtering, atomic layer deposition, sol–gel methods,laser ablation, in situ pulsed laser deposition, epitaxial depositionby laser molecular beam epitaxy) and many different substrates(quartz, (100)-LaAlO3, (111)-SrTiO3) have been employed toproduce single-phase TiO2 thin films.[28 – 34] As a result, theanatase form of TiO2, which is difficult to stabilize in bulk,can be more easily grown in the thin film form by the choiceof an appropriate technique and deposition conditions and/or(lattice-matched) substrate. However, the anatase phase stronglycompetes with the rutile phase during heat treatments.[35] Inaddition, the TiO2 structure can support marked oxygen sub-stoichiometry with a charge compensation acting on the valencestate of titanium. These facts may introduce different defects,having different electronic configurations, with possible influenceon the long-range magnetic ordering in the whole matrices whichare otherwise simply diamagnetic. This points out the role ofcompositional and structural disorder in the macroscopic behaviorand in particular in the electronic and magnetic properties, i.e. inthe functional parameters.

It is evident that a complete characterization of structural andelectronic properties of TiO2 single crystals, nanostructures orthin films requires the matching of different techniques, but ourattention in this work is addressed to Raman spectroscopy, whichis widely used to study titanium dioxide properties.[36 – 40] Indeed,Raman scattering, even though not exhaustive, is able to giveinformation on different above-mentioned phenomena, beingextremely sensitive to different TiO2 crystalline phases (anatase,rutile and brookite),[41] allowing the investigation of amorphous-to-crystalline phase transitions, oxygen defects, stress states andsize effects. The latter fact derives from the possibility to managethe TiO2 structure at the nanoscale, and this can play a key rolealso on the nature of magnetic order. A detailed analysis of Ramanspectral features leading to the derivation of band parameterssuch as energy peak, linewidth, asymmetrical shape and relativeintensity ratio allows gaining important information useful tounderstand the functional behavior of the material.

In this work, we present the results of a Raman study on theanatase/rutile phase formation in TiO2 thin films deposited byradio frequency (RF) magnetron sputtering on quartz substrates.The set of films investigated in this study consists of pure anatase,pure rutile and mixed anatase–rutile thin films, with differentthickness, all of them showing intriguing magnetic properties.[42]

We also point out how the Raman investigation can provideimportant suggestions toward the understanding of very complexphysical phenomena leading to r.t. FM in TiO2 thin films. Just

for this purpose, the results from other techniques are brieflyreported and discussed in relation to our systematic Ramancharacterization.

Experimental

The set of TiO2 films investigated in this work consists of samplesdeposited on quartz substrates by RF magnetron sputtering,labeled with letters and numbers (A1, A2, A3, M1, M2, M3, R1, R2)according to the growth conditions described in Table 1. Powdersused in the deposition are TiO2 rutile phase (Aldrich 99.99%) withmicrometer-sized grains. According to the deposition parameters,one should expect that the as-deposited samples A1, A2 andnitrogen-doped A3 were pure anatase, samples R1 and R2 werepure rutile and samples M1, M2, M3 were mixed anatase and rutilephases. Film thickness values of samples A1 and R2 were obtainedwithin the total X-ray reflectivity method, which were then used tocalculate the thickness values of the remaining samples, assumingnegligible dependence on pressure and substrate temperatureand linear scaling of thickness with deposition time and power ofthe plasma.

At 650 ◦C in 91% Ar + 9% O2 atmosphere (750 ◦C in 100% Aratmosphere), two anatase (rutile) films were deposited, namelysamples A1 and A2 (R1 and R2), with thicknesses of ∼27 and 83 nm(226 and ∼57 nm) obtained with deposition times of 0.75 and2.25 h (2 and 0.5 h). Selected samples (A2 and R1) were subjectedto post-growth annealing treatments, under pure oxygen at 400 ◦Cfor 4 h and under high vacuum (10−6 bar) at 400 ◦C for 4 h.

Micro-Raman measurements were carried out using a LabramDilor Raman H10 spectrometer equipped with an Olympusmicroscope HS BX40 objective and with a cooled CCD cameraas photodetector. The 632.8-nm light from an He–Ne laser wasused as the excitation radiation. The nominal spatial resolution isgiven by the beam diameter at the focal plane, and depends on thediameter of the main diffraction spot (1.22 λ/NA), where λ is thelaser wavelength and NA is the numerical aperture of the objective.A 100× objective (NA = 0.99) was normally used, with a spatialresolution of about 1 µm. A quasiconfocal geometry was achievedwith slit and confocal hole apertures kept at 200 µm, leading toa depth resolution of 2 µm. A grating with 1800 grooves/mmwas used. The sample was mounted on the motorized x –y stageof the microscope, and all the measurements were made at r.t.The nanoscale thickness of the films investigated in this studyimplies weak Raman signals easily masked or completely hiddenby the signals from substrates or spurious light scattering. Thus,it is important to optimize all the measurement parameters.

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Figure 1. Examples of data treatment to extract micro-Raman spectra fromTiO2 thin films: sample R2 (a) and sample A3 (b). The background spectrumis rescaled and then subtracted from the spectrum containing the signalof the film.

Typical integration times of the order of hundreds of secondswere used. Normally, each sample was measured with in-planegeometry, i.e. it was kept normal to the excitation laser beam.Measurements were also taken keeping the film parallel to theexcitation laser beam and moving the latter across the sample.In all cases, the Raman spectrum is the result of the overlappingof Raman signals from thin film and bulk substrate. We used thesignal from substrate as a background spectrum; all the spectrareported in the next section are obtained after subtraction ofthe background signal (except where indicated otherwise). As anexample, in Fig. 1 it is shown how the micro-Raman signals fromfilm samples R2 and A3 have been obtained. In case of sample R2,the bands from the quartz substrate are evident; in case of sampleA3, intense background, increasing at higher wavenumbers, isobserved.

X-ray diffraction (XRD), Rutherford backscattering spectroscopy(RBS), X-ray photoemission spectroscopy (XPS) and static magneti-zation measurements techniques, which were used in addition tothe Raman spectroscopy for exhaustive sample characterization,have been described elsewhere.[23,42]

Figure 2. Grazing incidence XRD patterns for as-deposited TiO2 thin films,namely rutile R1, mixed M1 and anatase A2. Vertical black line refers to themost intense peak of the anatase phase, while the gray line refers to themost intense peak of the rutile phase.

Figure 3. As-recorded Raman spectra of rutile TiO2 powders used as atarget during the RF sputtering deposition and of rutile single crystal(s.c.) measured at different angles with respect to the crystalline axis.Wavenumbers (in cm−1) of the observed active Raman modes are indicatedin brackets.

Results and Discussion

All the samples have been preliminarily measured by XRD, bothafter the deposition and after each thermal treatment. Thediffraction patterns confirm that samples A1, A2 and A3 havethe anatase structure; samples R1 and R2 have the rutile structure;and samples M2, M3 and M1 exhibit mixed phases. As an example,in Fig. 2 we report the XRD patterns typical for samples grown inthe conditions for anatase (A2), mixed (M1) or rutile (R1) phase.

An accurate detection of impurity phases can be convenientlysupported by micro-Raman spectroscopy. Indeed, TiO2 anatasephase, which is tetragonal with two formula units per unit cell(space group D19

4h(I41/amd), exhibits six Raman active modes(1A1g at 519 cm−1, 2B1g at 399 and 519 cm−1 and 3Eg at 144,197 and 639 cm−1), while the rutile structure, which is tetragonalwith two TiO2 units per cell and space group D14

4h(P42/mnm), hasfour Raman active modes (A1g at 612 cm−1, B1g at 143 cm−1, B2g

at 826 cm−1 and Eg at 447 cm−1).[41,43,44] Fig. 3 shows the Ramanspectra of the starting TiO2 powders used as a target during thesputtering procedure and of bulk TiO2 single crystals crystallizedin the rutile phase. These results, which were used to obtain the

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Figure 4. Raman spectra of all the as-deposited TiO2 thin films, showingthe evolution of the active Raman modes from pure anatase to pure rutileaccording to the sputtering conditions reported in Table 1.

full width at half-maximum (FWHM) values of powders and singlecrystals, provide a useful comparison to the results obtained forour thin film samples.

The micro-Raman spectra of the as-deposited samples arereported in Fig. 4, which shows the evolution of the films (rangingfrom anatase to rutile phase) with the sputtering conditionsreported in Table 1. Substrate temperature, power, pressure anddeposition time were changed by exploring the phase diagramof TiO2, with the aim to find optimum parameters to obtain pureanatase and pure rutile films with comparable thickness depositedon quartz substrates (samples A2 and R2). This result was obtainedby increasing the substrate temperature from 650 to 750 ◦C, thepower from 145 to 210 W, and the Ar content from 91 to 100%,as well as by reducing the O2 content from 9 to 0%, changing thepressure from 3.7 × 10−3 to 4 × 10−3 mbar and the depositiontime from 2.25 to 0.5 h.

Moreover, at 750 ◦C, i.e. the typical growth temperature of therutile phase, the film sample A3 was deposited by pumping5% nitrogen into the sputtering chamber. In the resultingdeposition atmosphere (95% Ar + 5% N2), the film grew withthe anatase crystalline phase, as shown in Fig. 4. Due to thedifferent thicknesses, it is inappropriate to directly compare theRaman band intensities between the samples. On the contrary, wenotice that in all the cases where a single phase is detected thepeak positions are completely consistent with the values derivedfrom the analysis of the spectra reported in Fig. 3.

The micrometer size of the laser spot was used to check thecrystalline homogeneity of each anatase, rutile and mixed as-deposited film by scanning the sample in different regions of thefilm. Three square patterns (of 16 points each one), settled atfixed distances on an imaginary line crossing the sample, wereused as the spatial reference for mapping the film with Ramanspectroscopy. For instance, in Fig. 5 the results of scanning theas-deposited samples A2 and R2 are reported. The E1g anatasebands of the 16 experimental curves (scattered points) and theaverage curve (white line) were fitted by Lorentzian curves and theratios (pi − paver)/paver with p = position (in wavenumbers) werecalculated for each experimental curves (i = 1–16). The value100 × (pi − paver)/paver, which represents the deviation percentof the E1g mode position of the spectrum i with respect to theaverage spectrum, was found to remain lower than 1% for eachdifferent point of the film, suggesting an excellent crystalline phase

Figure 5. Micro-Raman mapping of crystalline homogeneity of the films.The spatial pattern is illustrated in (a). The results related to the as-depositedsamples A2 and R2 are reported in (b) and (c), respectively.

homogeneity of the anatase film (the ratio (pmax − pmin)/pmin, withpmax and pmin respectively the maximum and minimum positionvalues obtained for the set of experimental curves, was also lowerthan 1%). Similarly, the A1g rutile bands of 16 experimental curves(scattered points) and the average curve (white line) were alsofitted by Lorentzian curves, resulting in a variation percent of thespectral position of this mode lower than 2%, reflecting as well thevery good crystalline phase homogeneity of the rutile film.

Using intermediate growth conditions with respect to thoseused to deposit pure anatase or pure rutile film depositions,we obtained films that exhibited coexistence of the two phases(samples named M2, M3 and M1; Table 1). In particular, these filmswere obtained by using the substrate temperature typical for theanatase phase formation i.e. 650 ◦C and the growth atmospheretypical for the rutile phase formation i.e. 100% Ar. In Fig. 6 theresults of the Raman spectroscopy investigation of these mixed

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Figure 6. (a) Raman spectra of the mixed phase films (M1, M2 and M3);(b) Raman spectra of a rutile sample (top curve), anatase sample (middlecurve) and mixed sample M3 (bottom curve). The dominant first-orderRaman peaks of rutile (R) and anatase (A) are marked. The numbers inthe boxes correspond to the peak positions (Raman shifts) obtainedin a multipeak fit for the anatase–rutile (A + R) Raman spectrum onsample M3.

phase films are reported. The spectra exhibit Raman fingerprintstypical of both anatase and rutile, so that a multipeak Lorentzianfitting procedure was used to assess the contribution of the twophases in each spectrum providing the wavenumber, linewidthand area of each Raman band. The strong B1g band at 143 cm−1,the Raman modes B1g (399 and 519 cm−1), Eg (144, 197 and639 cm−1) and A1g (519 cm−1) typical of anatase structure replacethe rutile modes Eg (447 cm−1), A1g (612 cm−1) and the weak B2g

(826 cm−1), thus evidencing the important role of the substratetemperature in determining the prevailing phase of the film.

In particular, we focused on the area of each Lorentziancomponent describing the anatase (LA) or rutile (LB) bands andassumed the ratio

∑A ∫ LA/(

∑A ∫ LA + ∑

B ∫ LB) as an indicatorof the anatase weight in the mixed phases. This allowed us tomonitor the evolution of the phase mixture in the samples, withthe above-mentioned ratio assuming the values of 0.52, 0.47 and0.43 in sample M1, M2 and M3, respectively.

At this point we bring back our attention to the Raman spectraof the pure phase samples, both anatase and rutile, in order toconsider the influence of the film thickness and discuss the effectof the thermal treatments. We focused our attention on the threeanatase bands: B1g (399 cm−1), B1g (519 cm−1) and Eg (639 cm−1)of pure anatase as-deposited samples (A3, A2 and A1). The mainband at 144 cm−1 can be discussed only qualitatively becauseof the fact that the shape of this Raman feature is affected bythe optical cut-off of the notch filter, which prevents a detailedanalysis of band parameters at least till 200 cm−1. Nevertheless,no appreciable changes are observed in the peak position of the144 cm−1 band and the same can be said as a general rule forthe Raman bands observed from all the considered samples thatwere analyzed by a best fitting procedure using Lorentzian curves.The values of FWHM are reported in Table 2. Samples A3 and A2,although grown in different atmospheres, with similar thickness(105.50 and 83 nm, respectively), also exhibit very similar FWHM;on the contrary, sample A1, having a thickness of ∼27 nm, exhibitsa considerably higher bandwidth. Likewise, the Lorentzian fits ofthe two rutile bands Eg (447 cm−1) and A1g (612 cm−1) of purerutile as-deposited samples R1 and R2 provide the values of FWHM,which are also reported in Table 2. In the two rutile samples havingvery different thickness (226 nm and 57 nm, respectively), the bandat ∼610 cm−1 has about the same width; on the contrary, the bandat ∼445 cm−1 is considerably wider in the thinner sample R2.

If one excludes the A1g mode of the rutile structure, in all thesingle-phase as-deposited samples a decrease of the film thicknessyields wider Raman bands, resulting from the fact that in thinnersamples there is a higher degree of structural disorder. The useof techniques that are able to inspect the local order will benecessary to study the behavior of the mode at 612 cm−1 in rutilefilms.

Let us now consider the effect of the thermal treatment. Theas-deposited thin films show an oxygen stoichiometry of ∼1.90and the oxygen annealed films give a value of ∼2.0, while vacuum-annelaed samples have an oxygen content of ∼1.90, as deducedfrom RBS.[42] In Fig. 7 we report the Raman spectra for anatase andrutile films (A2 and R1) which were thermally treated followingthe procedure reported in the experimental section. The bandsof the obtained Raman spectra were fitted by Lorenzian curves,which show opposite effects of the two annealing processes asin the FWHM. In particular, the FWHM of the A1g/B1g anatase

Table 2. FWHM values of the anatase bands B1g and Eg of pure anatase samples A3, A2 and A1, and of the rutile bands Eg and A1g of pure rutilesamples R1 and R2

FWHM (cm−1)

Sample Thickness (nm) B1g/399 cm−1 Eg/447 cm−1 B1g/519 cm−1 A1g/612 cm−1 Eg/639 cm−1

A3 105.5 26.495 25.75 25.237

A2 83 26.664 25.901 32.821

A1 27.4 83.796 47.244 37.228

R1 226 35.024 41.511

R2 56.7 56.589 42.308

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Figure 7. Raman spectra of samples A2 (a) and R1 (b) as deposited, thenannealed in oxygen and afterwards in vacuum; the dominant first-orderRaman modes of anatase and rutile are marked. Lorentzian fitting curvesof Eg and A1g bands and their linewidths are indicated by lines and arrows.FWHM values obtained from fitting procedure on the same spectra arereported in (c) for the indicated Raman modes.

mode at 519 cm−1 decreased upon annealing in oxygen from28.2 ± 0.8 cm−1 (in A2 as-deposited) to 25.7 ± 0.6 cm−1 andthen increased after the vacuum annealing to 27.5 ± 0.6 cm−1.Similarly, the rutile modes Eg (447 cm−1) and A1g (612 cm−1)exhibit FWHM values of 46.0 ± 0.9, 43.9 ± 1.2, 46.7 ± 0.6 cm−1

and 46.3 ± 0.4, 43.6 ± 0.5, 46.3 ± 0.3 cm−1 for the sample R1as-deposited, R1 oxidized, R1 reduced, respectively. Therefore,the annealing in vacuum ‘resets’ the previous heat treatments inoxygen and restores the crystallinity present in the as-depositedfilm. Thus Raman spectroscopy allowed us to observe the changeof stoichiometry induced by the annealing processes, as the FWHMof each active Raman mode decreases, weakly but systematically,as the oxygen stoichiometry increases (Fig. 7.c). In this framework,we can use these results to obtain an approximate method toestimate the oxygen stoichiometry from the broadening of Raman

Figure 8. Hysteresis cycles at room temperature extracted for the A2 TiO2thin film. Open squares, open circles and open triangles refer to theas-deposited, oxygen-annealed and vacuum-annealed films, respectively.

modes with respect to a reference sample. Our data indicatethat the effect of deviation from ideal stoichiometry on Ramanlinewidth is equal to 0.25 cm−1 of broadening for 1% oxygenvacancies in the unit formula.

We underline that the FWHM values obtained for the oxy-genated films are close to those obtained for the starting powdersand even for single crystals,[23,44] thus confirming the good crys-talline quality of the TiO2 films.

In addition, the dependence of FWHM on the thermaltreatment finds a complementary counterpart in the core-levelphotoemission results obtained on the same samples. In particular,in the case of the R2 sample, high-resolution XPS data werecollected with an Al Kα X-ray source after the annealing treatmentin both high vacuum and oxygen atmosphere. For the vacuumannealed sample, the Ti 2p emission showed a Ti 2p3/2 mainline, ascribed to the emission from the closed shell Ti4+ ion,with a binding energy BE = 458.1 eV. This main line presented ashoulder on the low binding energy side (BE = 456.4 eV), ascribedto Ti3+ contribution. This feature is known to be related to theoxygen substoichiometry in TiO2 samples.[45] The ratio betweenthe Ti3+ and Ti4+ contributions was 0.05. For the oxygen-annealedsample, the shoulder on the low BE side of the main line was notdetected, indicating that oxygen annealing restored the nominalstoichiometry of oxygen in the sample. Furthermore, the FWHM ofthe Ti4+ main line decreased with respect to the vacuum-annealedsample form 1.46 to 1.41 eV. The FWHM reduction of the oxygen-annealed sample also indicates an increase of the local crystallineordering around the photoemitting Ti ion. This is in agreementwith the expected recovery of the oxygen stoichiometry and theconsequent decrease, around the Ti ions, of the oxygen vacancies,which can induce local disorder or distortions of the crystallineenvironment.

Concerning the magnetic properties, our TiO2 thin films exhibitweak r.t. FM superimposed on a diamagnetic or paramagneticbehavior.[42] As a representative case, we report in Fig. 8 thehysteresis loops extracted from the experimental data aftersubtraction of the linear contribution for the A2 film, beforeand after the different thermal treatments.

Saturation magnetization Ms (emu/cm3) values of ∼5.5 forA2 as-deposited, ∼13.5 for A2 oxygenated and ∼47.5 for A2vacuum-annealed are obtained. These values indicate that theFM ordering, already present in the as-deposited sample, isa little enhanced after oxidation and markedly increased aftervacuum annealing. The Ms increase observed also after the

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Figure 9. (a) Raman spectra of the as-deposited sample A2 measured indifferent regions, keeping the film surface parallel to the laser beam.(b) Scheme of the different irradiation positions (top view).

oxygen annealing suggests that the magnetic behavior is notsimply determined by the different oxygen stoichiometry andconsequently by the evolution of Ti3+ population and by thecrystalline order degree, but other microscopic factors can play arole in this frame.

This hypothesis is supported by the experimental result frommicro-Raman measurements, which can act as an importantsuggestion to explain the magnetic behavior. By measuring oursamples keeping the film surface parallel to the laser beam,we observed different Raman signals corresponding to differentpositions of the laser spot on the sample, by moving the samplenormally relative to the beam. In particular, three distinct spectrawere detectable, as shown in Fig. 9. When the laser spot is set toexcite the substrate (position 1 in Fig. 9b), the Raman spectrumessentially consists of the signal from quartz. When the spot isfocused in position 2 (or 3), in the Raman spectrum the bandsof TiO2 are clearly evident over the background. But when thespot is in position 4, the Raman signal consists of a broadenedspectrum that can be ascribed neither to the quartz substrate norto the TiO2 film. This unusual Raman spectrum, which is observedin all the investigated samples, suggests the existence of a verythin interface layer between the film and the substrate. The factthat this featureless spectrum is observed for a large scanned path(∼1 µm) is obviously due to the laser spot size and convolutioneffects. Further investigations are required to unambiguously

clarify the nature of this interface layer; nevertheless, the presenceof unstructured scattered light at the interface can support thehypothesis of a large amount of free carriers. Thus, in this regionthe driving mechanism of the magnetic ordering should be theRuderman-Kittel-Kasuya-Yosida (RKKY) coupling, whereas in thewell-structured film the magnetic polarons could play a moreeffective role.

Conclusion

Micro-Raman spectroscopy has been used for the investigation ofa set of polycrystalline TiO2 thin films deposited by radio frequencymagnetron sputtering on quartz substrates. The crystalline phaseof the as-deposited samples has been unambiguously determined,as results from the assignment of the active Raman modes for thepure rutile and pure anatase phases. The systematic mapping ofthe samples (pure anatase or rutile and mixed anatase + rutilephases) reveals good local and global crystalline homogeneityof the deposited films. In the mixed-phase films, the weightsof the two phases in different samples have been monitoredby a simple analysis of the components of the multi-Lorentzianfitting curves. For the pure-phase films, the effect of thermaltreatments in oxygen and in vacuum on the lineshapes ofthe Raman bands has been highlighted; narrowed Raman linesare observed after oxidation with respect to the as-depositedsample; heating in vacuum restores the situation to that beforeoxidation. These results allow us to indirectly derive the oxygenstoichiometry in TiO2 thin films, which is seen to be consistent withX-ray photoemission spectroscopy results. Finally, experimentalevidence of the presence of an interface layer between thesubstrate and the film has been given, whose Raman signalcan be ascribed neither to the TiO2 phase nor to the quartzsubstrate, which could play a role in determining the complexmagnetic behavior of the thinner films. Further work is needed toinvestigate the nature and role of this interface layer.

Acknowledgement

This work was supported by the Italian Grant-Fondazione Cariplo(2006-0686) entitled ‘Diluted magneticoxides thin films: Towardstransparent spintronics’.

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