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Applied Surface Science 256 (2010) 6228–6232

Contents lists available at ScienceDirect

Applied Surface Science

journa l homepage: www.e lsev ier .com/ locate /apsusc

urface segregation in Nb-doped BaTiO3 films

mmanuel Arveuxa,b,∗, Sandrine Payana, Mario Maglionea, Andreas Kleinb

ICMCB-CNRS, University of Bordeaux, 87 Avenue, Dr. A. Schweitzer, Pessac 33608, France1

Darmstadt University of Technology, Institute of Materials Science, Petersenstraße 23, D-64287 Darmstadt, Germany1

r t i c l e i n f o

rticle history:eceived 30 July 2009eceived in revised form 10 February 2010ccepted 31 March 2010vailable online 7 April 2010

a b s t r a c t

We have used in situ photoemission spectroscopy to investigate Niobium doping in polycristalline BaTiO3.The valence band maximum position progressively shifts from 2.5 eV for undoped to 2.84 eV for Nb-dopedfilms. Ceramics and single crystal have been investigated for comparison with thin films. Nb-doped BaTiO3

ceramics and Nb-doped SrTiO3 single crystal show higher Fermi level position indicating that our dopedfilms are less conducting regarding their bulk parents. This was confirmed by impedance spectroscopy

eywords:aTiO3 and titanatesilmsTCRegregation

under variable temperature. Large amount of niobium is clearly observable at surface but the amount ofdopant is drastically reduced below the near-surface region, as evidenced by depth profile. Therefore, weprovide evidence of surface segregation which would explain the contrasted resistivity values reportedin literature for such donor-doped films.

© 2010 Elsevier B.V. All rights reserved.

urfacesPS

. Introduction

Barium titanate (BTO) is a typical ferroelectric perovskite,idely used in applications such as capacitors [1]. StoichiometricaTiO3 is an insulator with a large band gap of ∼3.2 eV [2], whichan become conducting upon donor doping. The semiconductingroperty can be obtained through oxygen reduction (BaTiO3−x) orhrough heterovalent A-site (e.g. LaBa) or B-site (e.g. NbTi) substitu-ion [3,4]. The resistivity can be modified by changing the dopantoncentration [5]. The resistivity of ceramic samples exhibits a pro-ounced minimum for donor concentrations below 1% as well asositive temperature coefficient of resistivity (PTCR) [6,7] mak-

ng them suitable in all fields of engineering. In thin film form,o PTCR effect has been reported for such donor-doped BTO andhe resistivity values available in literature are strongly contrasteds compiled in Fig. 1. The graph summarizes the resistivity ver-us doping for typical ceramics and thin film samples derivedrom literature. All thin films behave strongly different from theulk, regardless of the technique of preparation. Large fluctuations

re also observed among the films themselves. Samples preparedy Laser Molecular Beam Epitaxy (L-MBE) show extremely lowesistivity ∼(10–10−4 � cm) almost comparable to pure metalliciobium also reported in Fig. 1. In contrast, sputtered films exhibit

∗ Corresponding author at: ICMCB-CNRS, University of Bordeaux, 87 Avenue, Dr.. Schweitzer, Pessac 33608, France. Tel.: +33 540 00 7392; fax: +33 540 00 2761.

E-mail address: arveux@icmcb-bordeaux.cnrs.fr (E. Arveux).1 European Multifunctional Materials Institute (EMMI).

169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.apsusc.2010.03.146

very high resistivity ∼(108–1012 � cm) similar to undoped BTO. Thechange of resistivity for films grown by chemical vapor deposition(MOCVD) is roughly comparable to ceramics doped, but the mini-mum resistivity is only observable at higher doping concentrations.Therefore, this comparison of data questions the doping efficiencyin thin film form. However, only a few studies have been focusedon it. We will demonstrate that Nb segregation can occur alteringconsequently the electrical properties.

In this work, a combination of in situ X-ray photoelectronspectroscopy (XPS) and other techniques has been applied tounderstand the high resistivity of magnetron sputtered dopedBaTiO3 films. We first determine the Fermi energy level positionwith respect to the band edges and the chemical composition. Foroxide samples, the Fermi level position observed by XPS correlateswell with electrical conductivity [8–10]. In the second part, permit-tivity and loss factor were measured using impedance spectroscopyin dependence on temperature, confirming the lack of semiconduc-tivity of our films. Finally, we will provide evidence that niobiumis not incorporated in large amount in the grains of the films butsegregates to the surface (and probably also to grain boundaries).Nb-doped BaTiO3 ceramics and single crystal SrTiO3 samples havealso been investigated since there are ideal references of semicon-ducting samples.

2. Experimental procedure

Thin film deposition and XPS analysis were performed inthe Darmstadt Integrated System for MATerials research (DAISY-MAT [9,17]), which combines a multi-technique surface analysis

E. Arveux et al. / Applied Surface Science 256 (2010) 6228–6232 6229

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tion band, as observed for Nb-doped SrTiO3 single crystals [18] alsoshowed in Fig. 4. According to the band gap of BTO (3.2 eV) [2], thiswould lead to a VBM of 3 eV at least. The VB spectra recorded fordoped BTO ceramic (VBM = 3.1 eV) is in good agreement with this

ig. 1. Resistivity of BaTiO3 vs. doping concentration dependence for thin films anderamics samples. The references used for the plotting are the following: Jonker [6],eywang [7], Nagano [11], Lemée [12], Guo [13], Yan [14], Sasaki [15] and Shao [16].

hamber (Physical Electronics PHI 5700) with several samplereparation and thin film deposition chambers. Sample transferetween sample preparation and surface analysis can be performedithout breaking vacuum, allowing for analysis of contamination-

ree surfaces. Polycrystalline BaTiO3 films have been deposited bysing radiofrequency magnetron sputtering from ceramic targetsurchased to Williams A.M. Company. The undoped target showsypical black color of reduced BTO. Doped targets are respectivelyight blue and dark blue for 0.05 wt% and 0.5 wt% niobium concen-ration. The blue color is a typical donor dopant indication. Table 1ummarizes the detailed deposition conditions. The Si/SiO2/TiO2/Ptubstrates were first annealed in the deposition chamber for 45 mint a temperature of 650 ◦C. The ceramic used as a reference, has beenntroduced in the system and heated for 40 min to 450 ◦C in an oxy-

en pressure of 0.05 Pa before measurement. Fig. 2 shows typicalPS survey spectra of platinized wafer and BaTiO3 thin-film, indi-ating that our experiments are performed free of contamination.

able 1puttering conditions.

Substrate SiO2/TiO2/Pt coated Si waferTarget BTO without dopingPower RF 1.5 W/cm2

Gaz mixture 2.5% oxygenWork pressure 5 PaDeposition rate 1.85 nm/mnTemperature 650 ◦CDistance to target 10 cm

Fig. 2. XPS survey spectra of platinized wafer (gray) and BaTiO3 (black) used in theexperiments. The dashed line indicates the carbone peak position, revealing a cleansurface.

3. Results

The Ti/Ba ratio was calculated using XPS by extraction of thepeak surface (Ti 2p and Ba 3d5/2) after background subtractionswith 3rd polynomials for both core levels. The values are 1.1, 1.04and 0.81 for the undoped film, doped ceramic (0.2 wt% Nb) anddoped film (0.5 wt% Nb), respectively. The bulk exhibits a cationicratio close to the ideal one (1:1). On the other hand, large Baexcess is observed for the 0.5 wt% Nb-doped BTO film whereas theundoped film is Ti-rich. This Ba-rich surface will be discussed inSection 4. Crystalline nature of thin-films was identified by X-raydiffraction performed with the aid of a Philips X’Pert high resolu-tion diffractometer. Fig. 3 shows the XRD pattern of the 0.5 wt%Nb-doped and the undoped BaTiO3 film. The observed diffractionangles correspond either to the Pt substrate or to the BTO film,confirming the polycristalline nature, the absence of preferred ori-entation and the lack of secondary phases.

The valence band spectra were recorded using monochromatedAl K� on clean surfaces (Fig. 4). The valence band maxima (VBM)were determined by linear extrapolation of the leading edge.The Fermi energy reference (0 eV) was calibrated using a sputtercleaned metallic Ag reference standard.

The undoped film exhibits a VBM of 2.55 eV whereas the filmcontaining the highest doping content has a VBM of 2.84 eV. TheNb-doped film has thus a higher Fermi level position, as expectedfor n-type doping. However, a typical n-type semiconductor sampleshould have its Fermi level position located close to the conduc-

Fig. 3. X-ray diffraction pattern of BTO and BTO:Nb thin film on platinized wafer.

6230 E. Arveux et al. / Applied Surface Sc

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ig. 4. X-ray valence band spectra recorded from: (a) thin film of undoped BTO,b) thin film of BTO:Nb (0.05 wt%), (c) thin film of BTO:Nb (0.5 wt%), (d) ceramicf BTO:Nb (0.2 wt%) and (e) single crystal of STO:Nb (0.05 wt%). For (b) and (c) themount of Nb is related to the sputtered target stoichiometry.

xpectation. Both doped films do not reach comparable high lev-ls (2.84 eV and 2.73 eV). It appears that the doping in the filmsoes not shift the Fermi energy significantly towards the conduc-ion band. Hence, the Nb-doped films are expected to have a higheronductivity level.

To confirm the lack of conductivity in our BaTiO3:Nb films, weave performed impedance spectroscopy experiments. Variation ofhe dielectric properties with temperature for two films is shown inig. 5. At 100 Hz and room temperature, the undoped film exhibit aermittivity value around 350, much higher than the observed per-ittivity for the Nb-doped film (∼200). Both profiles do not show

ny obvious temperature dependence. The increase of loss factornd permittivity observed at high temperature is a typical signa-ure of thermally activated conductivity. In case of doped materials,his typical enhancement should be higher than the undoped par-

nt. But no significant difference is observed at high temperature.he doped film appears to be even a better insulator (lower loss)ompared to the undoped film. However, both films exhibit resis-ances of the order of 1010 ohms as computed from the impedanceata.

ig. 5. Relative permittivity and loss factor vs. temperature dependence at 100 Hz for undor doped sample is observed at high temperature.

ience 256 (2010) 6228–6232

In bulk form, when suitably selected, a small amount of dopants(e.g. few ppm of Nb,La. . . in BaTiO3) is able to induce a macro-scopic increase of conductivity [19] because of energy levels locatedrelatively close to the conduction band. As in any semiconductor,such charged states are well known to accumulate at interfacesinducing space charges. Because of the very small thickness ofsuch depleted interfaces, the apparent capacitance can be subse-quently increased, especially at low frequency/high temperaturerange [20]. There is thus a close link between the semiconductorbehaviour of doped BaTiO3 and an artificial increase of the apparentpermittivity due to the additional space charge capacitance. In oursputtered BaTiO3:Nb films we see neither an increased conductiv-ity nor a strong apparent permittivity. This confirms the inefficientNb doping in our BaTiO3 films since we were not able to measurethe resistivity by 4-point techniques. Both Pt and InGa electrodeswere used but the observed resistivity was obviously too high tobe measurable. This agrees with the previously reported resistivityexperiments in sputtered Sb-doped BaTiO3 [15].

Evidence for the origin of the inefficient doping of mag-netron sputtered BaTiO3:Nb films can be obtained from the cationstoichiometry. As the concentration of dopant is very weak,niobium-doped single crystals of SrTiO3 have been measured withour XPS setup as reference. The surfaces were cleaned beforephotoemission measurements by in situ thermal annealing in anoxygen pressure of 0.05 Pa. Single crystals were purchased with dif-ferent niobium concentrations; 0.5 wt% and 0.05 wt% are the dopantconcentrations given by the provider (Crystec). As evident from Fig.6, a Niobium emission is clearly observed for the 0.5 wt% sample.Using the sensitivity factors for the XPS setup and the appropriatemolar mass, a Nb concentration (0.8 wt%) is derived from the peakintensities, in good agreement with the nominal Nb content. No Nbcould be detected for the 0.05 wt% Nb-doped SrTiO3 crystal. Alsono niobium was detected on the conducting ceramic BTO sample,which shows a high Fermi level position (see Fig. 4).

Evidently, the Nb content required to induce electrical conduc-tivity is below the detection limit for ceramic and single crystalsamples. As indicated in Fig. 6, the amount of Nb observed by XPS is∼3 wt% for the films prepared from either 0.05 or 0.5 wt% doped tar-gets. We are now going to show that doping enrichment is confinedto surfaces.

The substantial Nb contents measured at the surfaces by XPSmight be caused by segregation of Nb at the surface. To check ifthe dopant concentration is lower below the surface, depth profileanalysis has been performed. Fig. 6 displays the effect of ion-beam

etching with 1 keV Ar+ ions on the Nb 3d core level. The red spectrarepresent the situation prior to etching and the blue correspond tothe last etching step. The amount of niobium is drastically reducedafter sputtering. The Nb concentration observed after ion etching isless than 0.5 wt%. The binding energy shift of ∼1 eV detected after

oped BaTiO3 thin film and Nb-doped BaTiO3 thin film. No increase of conductivity

E. Arveux et al. / Applied Surface Sci

Fig. 6. X-ray photoelectron spectra of Nb 3d core levels. Left: Recorded during depthprofile. The amount of Nb is clearly reduced within the film. Right: Spectra obtainedfor Nb-doped BaTiO3 thin films (red) and single crystals (black). The Nb content isdsl

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erived from XPS spectra and reveals strong Nb enrichment for thin films regardingingle crystal samples. (For interpretation of the references to color in this figureegend, the reader is referred to the web version of the article.)

he first etching step occurs in all core levels and can thereforee explained by a charging of the surface due to small amountsf Ar+ implantation rather by chemical changes of the Nb species.he reduction of niobium concentration in the course of sputteringndicates that Nb is not incorporated in large amounts in thin filmsut segregates to the surface.

. Summary and discussions

Doping effect on electrical properties seems to be drastically dif-erent between thin-films and their bulk parents. Our results show:i) different Fermi level position for sputtered films and ceramic,ii) dielectric properties of BaTiO3 films not affected with niobiumnd finally, (iii) niobium segregation to the surface for thin films asell surface Ba enrichment in our sputtered donor-doped BTO (see

ection 3).The measurements of large grain materials (ceramic and single

rystals) shows that the dopant concentrations below the detectionimit of XPS can lead to strong shifts of the Fermi level. This indi-ates a high doping efficiency and properly incorporated dopants.he situation for sputtered thin films is strongly different. Althoughlarge amount of niobium is observed, the Fermi level positionseasured in the films are only slightly affected by Nb doping. The

neffective doping is attributed to a poor incorporation of Nb inhe grains. The available Nb largely segregates to the surface, and,

ost likely, also to grain boundaries. The dielectric properties of theaTiO3:Nb films are also in agreement with poor Nb incorporation

n the grains. Nb-oxide is a good insulator with a low dielectric per-ittivity. It can induce an increase of the resistivity and a decrease

f the effective permittivity. This is exactly what has been observedn comparing pure and Nb-doped BaTiO3 films. The lack of conduc-ivity in our sputtered films can thus be related to the segregationf Nb. The fundamental question which is thus raised concern theechanisms of surface segregation.We will provide a qualitative model for this segregation but

e have first to refer to previous studies in donor-doped BaTiO3

eramic [4,21]. When sintered in reducing atmosphere (which ishe most realistic case for us since the sputtering takes place atow oxygen partial pressure), a Ba-rich second phase is formed atrain boundaries. This extra phase is a direct consequence of ionicompensation of Nb5+ on Ti4+ site by formation of [V ′′

BA]. In our

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Nb-doped films we observed large Ba-enrichment at surface of ourdoped films as described in Section 3. The absence of extra peakin our XRD spectra suggests that the observed Ba enrichment canform an amorphous phase. This Ba-rich phase might give evidenceof ionic compensation resulting from well incorporated dopantsin the films. Similar Ba enrichment in sputtered BaTiO3 thin filmshave been reported such as Ba2TiO4 phases with Ho doping [22], Sbdoping [15] and Nb doping [23]. The formation of niobium excessat surface will result to the part non-incorporated within the filmsand thus segregated at surface. At least, the presence of impurityphases involving Nb segregation was also evidenced in Nb-dopedBTO prepared by MOCVD explaining the observed loss of epitaxy[12]. TEM experiment was used in sputtered Mg-doped BST to pro-vide segregation evidence of MgO which was not observable intheir XRD spectra [24]. For ceramics samples, non-complete Nbincorporation inducing dopant segregation in the grain boundaryregion was also successfully investigated [25,26] for heavily dopedceramics. Reducing the amount of dopant in the sputtered targetmight be a possible way to reduce the formation of doping segre-gation in order to obtain semiconducting sample. At least we notethat semiconducting bulk BaTiO3 can only be obtained for very lowdopant concentration (<0.1 wt%) in which the compensation modeis achieve by electrons and not [V ′′

BA].Therefore, it appears as very challenging to control the dopant

solubility. The surface properties are often underestimated but asdemonstrated here, they are submitted to important modificationdue to doping effect. XPS is a useful tool to investigate the surfacestoichiometry and the electronic structure in addition to standardspectroscopy such as X-ray diffraction or conventional electricalmeasurements.

Acknowledgements

This work was supported by the Conseil Régional dı̌Aquitainein France, the Sonderforschungsbereich 595 of the DeutscheForschungsgemeinschaft, the European Multifunctional MaterialsInstitute (EMMI), through collaboration between the Institute ofCondensed Matter Chemistry of Bordeaux (ICMCB) and the Instituteof Materials Science of Darmstadt.

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