Spectroscopy of radiation defects in rutile TiO 2

Phys. Status Solidi B 250, No. 4, 843–849 (2013) / DOI 10.1002/pssb.201200917 p s sb

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basic solid state physics

Spectroscopy of radiation defectsin rutile TiO2

C. M. M. Rosario*,1, M. P. F. Graca1, M. A. Valente1, L. C. Costa1, J. Rodrigues1, T. Monteiro1,E. Alves2, and N. A. Sobolev1

1Departamento de Fısica and I3N, Universidade de Aveiro, 3810-193 Aveiro, Portugal2 Instituto Tecnologico e Nuclear/IST, Universidade Tecnica de Lisboa, 2686-953 Sacavem, Portugal

Received 31 October 2012, revised 14 December 2012, accepted 17 December 2012

Published online 20 March 2013

Keywords electron paramagnetic resonance, impedance spectroscopy, photoluminescence, radiation defects, rutile

*Corresponding author: e-mail [email protected], Phone: þ351234378117; Fax: þ351234378197

TiO2 is a prototypical memristive material, very promising for

applications in ReRAMs, a new kind of nonvolatile memories.

The electromigration of oxygen vacancies in TiO2 is commonly

accepted as a key mechanism of the memristive phenomena.

However, spectroscopic studies of defects involved in the

electromigration are rare. In this work we created defects in

rutile TiO2 single crystals by irradiationwith 50 keVArþ ions to

fluences up to 2� 1017 ion cm�2 at 1000K and investigated

them by means of Raman scattering, photoluminescence,

impedance spectroscopy and electron paramagnetic resonance.

With the latter, Ti interstitials have been detected. Against

expectations, spectra of isolated vacancies have not been

observed. Instead, a rich variety of defects appeared, indicating

a very complex character of the defect creation

and transformation processes in rutile. An interaction of the

Fe3þ and Cr3þ impurities with radiation-induced defects

has been concluded. The activation process observed in our

IS measurements is identified as the thermal ionization of

defect-related donors.

� 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Titanium dioxide (TiO2) is a veryversatile material. Recently, there has been a great interest inthe application of TiO2 in memristors [1], a fundamentalpassive circuit element predicted in 1971 [2].

The electromigration of oxygen vacancies in TiO2 andother oxide-based materials, e.g. SrTiO3 [3], is commonlyaccepted as a key mechanism of the memristive phenomena.However, themicroscopic nature of defects in thismaterial isnot yet clearly understood. Surprisingly, spectroscopicstudies of defects involved in the transport and switchingprocesses are rare. This work aims to start a detailedspectroscopic study of these defects. (110) TiO2 rutilesamples irradiated with 50 keV Arþ ions at 1000K havebeen studied by means of Raman scattering, photolumin-escence, impedance spectroscopy and electron paramagneticresonance (EPR).

2 Experimental details2.1 Sample preparation The rutile samples were

grown by the flame fusion method (SurfaceNet GmbH). Theone-side polished plates, cut along the (110) plane, had athickness of 1mm.The crystal structurewas confirmed using

X-ray diffraction. The resistivity of the as-grown sampleswas above�1012V cm. The impurity content in the sampleswas below the detection limit of the particle-induced X-rayemission technique (of the order of ppm). The EPR detectedtraces of the Cr3þ and Fe3þ ions. Two samples wereirradiated at 1000K with two different fluences of 50 keVArþ ions, F¼ 1� 1016 and 2� 1017 ion cm�2. In thefollowing, these samples will be referred to as I1 and I2,respectively. Another sample from the same batch was leftvirgin for comparison (as-grown sample V).

For the IS measurements, 75%Au–25%Pd electrodeswith a diameter of 4 or 2mmwere deposited by sputtering onthe opposite surfaces of the samples.

2.2 Experimental setup Room temperature (RT)Raman spectroscopy was performed in backscatteringconfiguration at a Jobin Yvon HR800 spectrometer using a532 nm laser line.

The excitation of the PL was implemented with the325 nm line of aHe–Cd laser. The PL emissionwas dispersedthrough a Spex 1704 monochromator and detected by aHamamatsu R928 photomultiplier. The temperature depen-


844 C. M. M. Rosario et al.: Spectroscopy of radiation defects in rutile TiO2p

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dent steady-state PL was measured using a closed-cyclehelium cryostat allowing to vary the temperatures from 14Kto RT.

The IS measurements were carried out with a PrecisionImpedance Analyser 4294A (Agilent) in the temperatureinterval from 100 to 360K and frequency range from 40Hzto 2MHz using a bath cryostat with a liquid nitrogenreservoir and helium gas as cooling agent. The data wascollected in the Cp–Rp mode, with Cp and Rp being theparallel capacitance and resistance, respectively. Theimpedance Z was then calculated through Eq. (1):

Figuthreflue

� 20

Z ¼ Rp

1þ ðvRpCpÞ2� j

vRp2Cp

1þ ðvRpCpÞ2; (1)

where the symbols have their usual meaning.The EPR measurements were made using a Bruker ESP

300E spectrometer equipped with an Oxford Instrumentshelium flux cryostat using a cylindrical cavity in the X-band( f� 9.4GHz).

3 Results The colour of the as-grown sample changesupon irradiation fromwhite transparent to blue and dark bluedue to the creation of defects which introduce energy levelsin the gap. Due to rapid diffusion of the generated radiationdefects, the colouration occurs uniformly throughout thewhole sample thickness. At the same time, the concentrationof conduction electrons grows by orders of magnitude, sothat the initially insulating material becomes electricallyconductive, mainly due to the donor action of the interstitialtitanium ions and oxygen vacancies [4–7].

3.1 Raman scattering The Raman spectra (Fig. 1)contained two sharp peaks ofEg andA1g symmetry aswell asa two-phonon band typical of rutile [8]. After irradiation,only the relative intensities of the peaks changed, thusindicating that the crystallinity of the samples was preservedafter irradiation (Fig. 1).

re 1 (online colour at: www.pss-b.com) Raman spectra of thee samples studied (as-grown and two irradiated). The irradiationnces are indicated in the graph.

13 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

3.2 Photoluminescence The PL spectra obtained forthe three samples, presented in Fig. 2(a), evidences a cleardistinction between the as-grown sampleV and the irradiatedones (I1 and I2). In sample V, the PL spectrum exhibits anemission band in the near-infrared region. On the higher-energy side of this band there are two zero-phonon lines atenergies of 1.574 and 1.579 eV. These lines become moreintense relatively to the emission band at lower temperatures.The spectra measured at 14, 30 and 50K are shown inFig. 2(b), where one can observe the thermalization of thetwo zero-phonon lines. In previous works, these lines wereattributed to the no-phonon 4T2g–

4A2g transition of the Cr3þ

impurity in TiO2 [9, 10].In the irradiated samples the intensity of the PL bands

decreases with increasing irradiation fluence. In sample I2there is no emission at any temperature down to 14K. Studiesusing Fourier transform spectroscopy employing a Gedetector corroborated the absence of PL in the near-infraredregion. The quenching of the PL intensity as a result ofirradiation can be explained by the creation of defect-relatednonradiative recombination centres competing for thecapture of photoexcited charge carriers and also by theformation of complex defects involving the Cr3þ and Fe3þ

ions. The latter effect has been corroborated by the EPRmeasurements, see Section 3.4.

3.3 Impedance spectroscopy The DC current–voltage characteristics of both irradiated samples are non-linear, indicating the formation of Schottky barriers (SB) onthe sample surface. This fact impedes a sound interpretationof the temperature dependences of the DC electricalconductivity measurements performed according to thetwo-point probe scheme. Nevertheless, the RT seriesresistance of the samples has dropped after irradiation byapproximately ten orders of magnitude, from Teraohms tohundreds of Ohms, thus revealing efficient ‘doping’ of rutileby electrically active radiation defects.

Figure 3 shows the frequency dependences of the realand imaginary part of the dielectric permittivity, e0 and e00,for the as-grown and irradiated samples, measured atdifferent temperatures. The apparent huge increase of thelow-frequency permittivity after irradiation corroboratesthe creation of space charge zones at contacts. With thedepleted zone width, W, being inversely proportional tothe square root of the net charge density due to ionizedimpurities and/or defects [11], one should expect anincrease of the barrier capacitance by several orders ofmagnitude owing to the doping with donor-like defects,which is indeed observed in the experiment. Therefore,the measured Rp and Cp values may be attributed to thespace charge layers beneath the SB. In this case we mayinterpret the IS signal as stemming from the rechargingof the defect energy levels when W is modulated by theapplied AC voltage. As the recharging occurs at a finiterate described by the (temperature-dependent) relaxationtime t, we shall measure the barrier capacitance at lowfrequencies and the geometrical capacitance of the whole

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Phys. Status Solidi B 250, No. 4 (2013) 845

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Figure 2 (online colour at: www.pss-b.com) (a) PL spectra obtained atT¼ 77K for the three samples studied(as-grown and two irradiated). The irra-diation fluences are indicated in thegraph. (b) PL spectra of the as-grownsample showing two zero-phonon linesat three different temperatures.

sample at high ones [12]. This behaviour is clearly observedin Fig. 3(c,e).

A useful check of the provenance of the observed high e0

values is the application of different types of contacts. As in

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Ref. [13], the substitution of the sputtered Au–Pd contacts bya silver paint led to much smaller permittivity values. It isanother proof that the observed IS spectra stem from thespace charge regions beneath the SB.

Figure 3 (online colour at: www.pss-b.com) Complex dielectric permittivityversus frequency of the as-grownsample V (a), (b), and of the irradiatedsamples I1 (c), (d) and I2 (e), (f) forseveral temperatures.



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Figure 4 (online colour at: www.pss-b.com) Impedance of the as-grownsample V (a), (b) and for the irradiatedsamples I1 (c), (d) and I2 (e), (f) vs.measurement frequency at various tem-peratures.

The real and imaginary parts of themeasured impedance,Z0 and Z00, respectively, are shown in Fig. 4. The temperaturedependence is different for these samples. While in theas-grown sample Z0 increases, in the irradiated samples itdecreases with increasing temperature, thus revealing thesemiconductor-type conductivity of the latter. The frequencydependence of the impedance in the irradiated samples isalso different from that in the as-grown sample. In sample V,both Z0 and Z00 decrease with increasing frequency, whereasin the irradiated samples Z0 is almost constant at lowfrequencies [especially for F¼ 2� 1017 ion cm�2, seeFig. 4(e)] and then decreases rapidly, with Z00 reaching amaximumat a certain frequency [Fig. 4(d,f)]. This behaviouris characteristic of a relaxation process.

The interdependence between the real and the imaginaryparts of the impedance is explicitly visualized in Nyquistplots like those shown in Fig. 5(a,c) for the irradiatedsamples. The plots have a shape of decentred semi circles


with their centre situated below the abscissa axis. This profileindicates that the simple exponential decay is inappropriateto describe the observed relaxation phenomena [14]. Thisconclusion appears obvious when we consider that there aremany defect-related energy levels introduced in the gap byirradiation. Besides, in analogy to Ref. [13] we may suggestthat this fact hints at variable range hopping of electronsbetween defects. Measurements with Ohmic contacts are inprogress in order to check this suggestion.

The behaviour of the impedance can be described by theCole–Cole equation, Eq. (2):

Z ¼ Z1 þ Z0 � Z1

1þ ðjvtÞ1�a; (2)

where Z1 is the real impedance at high frequencies, Z0 thestatic real impedance, t¼RpCp the relaxation time (inverseof the relaxation frequency frel) and a is a parameter

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Figure 5 (online colour at: www.pss-b.com) Nyquist plots for the irradiatedsamples I1 (a) and I2 (c) for differentmeasurement temperatures. (b), (d)Arrhenius plots of the relaxation fre-quency vs. temperature for the samesamples.

describing the width of the material property distribution[14]. We obtained an a value of ca. 0.5 that almost did notvary with temperature.

The dependence of the relaxation frequency frel¼ 1/t onthe temperature T follows an activation law, Eq. (3):

www

frel ¼ A � exp �Ea= kBTð Þð Þ; (3)

where A is a constant, Ea the activation energy and kB is theBoltzmann constant. By plotting the relaxation frequenciesin Arrhenius coordinates, Fig. 4(b,d), temperature intervalswith different activation energies are revealed. Importantly,one and the same activation energy of 75meV has beenfound for both irradiated samples. For the heavily irradiatedsample I2 two other activation energy values, 25 and42meV have been found. The activation energy of 33meV inthe less irradiated sample I1 was determined using only threeexperimental points, so that it cannot be considered reliable.

It has been formerly shown that the temperaturedependence of the electrical conductivity in reduced orirradiated rutile obeys an activation law exhibiting variousactivation energies Ea ranging from 5meV at liquid heliumtemperatures to 80meV above 200K [6, 7]. These valueshave been interpreted as ionization energies of differenttypes of defect-related donors. Importantly, the activationenergies we obtained by means of the IS in the temperatureinterval from 100 to 360K have very similar values rangingfrom 25 to 75meV, see Fig. 5(b,d). Thus, the activationprocess observed in our IS measurements is identified as thethermal ionization of defect-related donors.

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3.4 Electron paramagnetic resonance The as-grown samples exhibit EPR spectra of substitutional Cr3þ

and Fe3þ impurities [15, 16] (not shown). After irradiationwith 1� 1016 ion cm�2, the Cr3þ spectrum is still observed,but it disappears after irradiation with 2� 1017 ion cm�2.The Fe3þ spectrum is not observable anymore in bothirradiated samples. The Cr level lies 1.0 eV above thevalence band of rutile, and the Fe level lies even closer to thetop of the valence band [17]. As rutile gets increasingly n-type upon irradiation and the Fermi level rises towards thebottom of the conduction band with lowering temperature,recharging of the Cr and Fe ions cannot be the reason ofthe quenching of the respective EPR spectra. This factcorroborates the assumption made in Section 3.2 that theCr3þ and Fe3þ impurities interact with radiation defectsforming complexes.

Instead, many different paramagnetic centres of defectorigin exhibiting anisotropic spectra are observed afterirradiation, see Fig. 6. EPR spectra of defects in reducedrutile have been formerly analysed in Refs. [18–20]. Themost known are the Ti3þi interstitial ion [18] and the oxygenvacancy in two charge states, Vþ

O and V0O [19]. The Ti

interstitial introduces a shallow donor level in the gap withan ionization energy of ca. 5meV [18].

Figure 6(a) shows an angular dependence of the EPRspectrum of sample I2. The most prominent line (labelledC-line) belongs to exchange-coupled Ti3þi interstitials ratherthan to isolated ions [18]. At low temperatures, the Fermilevel is pinned to the Ti interstitial donor level, so that theC-line can be detected in the dark, together with both charge



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Figure 6 (onlinecolourat:www.pss-b.com)(a)Angulardependenceof theEPRspectrumofsampleI2uponrotationof themagneticfield inthe (110) plane.Measurement temperature T¼ 7–8K,microwave powerP¼ 6.3mW. (b) Temperature dependence of the EPR spectrumofsample I1 with the magnetic field parallel to the [110] axis. Microwave power P¼ 63mW.

states of the oxygen vacancy. In Ref. [19] theD1, D2 spectra,ascribed to the two paramagnetic charge states of the oxygenvacancy, have been detected in reduced rutile at T¼ 25Kwhen the titanium interstitials are already fully ionized.However, we did not observe the D1, D2 spectra. This meansthat the vacancies are involved in more complex defectstructures. Indeed, many new spectra are observed in bothirradiated samples that have not yet been described in theliterature. The spectra will be analysed in detail in aforthcoming publication. Let us only note that the movementof the Fermi level with temperature and the accompanyingrecharging of defects are well seen in the EPR spectra:whereas some centres disappear from the spectrum uponwarming up, other centres surge due to diamagnetic–paramagnetic transitions upon recharging, see Fig. 6(b).

With the temperature increasing above ca. 30K (thevalue is sample dependent), the EPR measurements turnimpossible due to very strong microwave absorption by freecharge carriers. When the temperature rises above ca. 100K,the non-selective microwave absorption weakens, and it isagain possible to tune the spectrometer, however, the defectspectra disappear, probably due to a strong lowering of theFermi level. Another possible reasonmay be the accelerationof the spin-lattice relaxation leading to a broadening of theresonance lines, but this reason can not be immanent to allparamagnetic centres: e.g., the Cr3þ and Fe3þ spectra arewell detectable at RT.

4 Conclusions Our spectroscopic studies of rutilecrystals subjected to Arþ ion irradiation at 1000K haveshown that many different complex defects, rather thanisolated titanium interstitials and oxygen vacancies, arecreated in the material. The samples continue single-crystalline even after irradiation to the highest fluence used,2� 1017 ion cm�2. The defect complexes determine theelectrical conductivity and give rise to electronic relaxationeffects with specific activation energies which are observedby means of the impedance measurements. The obtained


data will be used in forthcoming studies of the electro-migration of defects in rutile.

Acknowledgements The authors gratefully acknowledgethe financial support from ‘‘PEst-C/CTM/LA0025/2011 and RECI/FIS-NAN/0183/2012’’of the FCT of Portugal. J. Rodrigues thanksFCT for her PhD grant SFRH/BD/76300/2011.

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Spectroscopy of radiation defects in rutile TiO 2

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