ARTICLE IN PRESS                                                Journal of Crystal Growth 310 (2008) 545–550              ...
ARTICLE IN PRESS546                                X. Weng et al. / Journal of Crystal Growth 310 (2008) 545–550   LAO and...
ARTICLE IN PRESS                                     X. Weng et al. / Journal of Crystal Growth 310 (2008) 545–550        ...
ARTICLE IN PRESS548                                                         X. Weng et al. / Journal of Crystal Growth 310...
ARTICLE IN PRESS                                      X. Weng et al. / Journal of Crystal Growth 310 (2008) 545–550       ...
ARTICLE IN PRESS550                                                     X. Weng et al. / Journal of Crystal Growth 310 (20...
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Structural characterization of TiO2 films grown on LaAlO3 and SrTiO3 substrates using reactive molecular beam epitaxy


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Structural characterization of TiO2 films grown on LaAlO3 and SrTiO3 substrates using reactive molecular beam epitaxy

  1. 1. ARTICLE IN PRESS Journal of Crystal Growth 310 (2008) 545–550 characterization of TiO2 films grown on LaAlO3 and SrTiO3 substrates using reactive molecular beam epitaxy X. Wenga, P. Fisherb, M. Skowronskib, P.A. Salvadorb, O. Maksimovc,Ã a Department of Materials Science and Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA b Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA c Electro-Optics Center, The Pennsylvania State University, Freeport, PA 16229, USA Received 5 June 2007; received in revised form 22 October 2007; accepted 30 October 2007 Communicated by K.H. Ploog Available online 17 November 2007Abstract We have studied the microstructure of TiO2 films, grown by reactive molecular beam epitaxy (MBE) on LaAlO3 (LAO) and SrTiO3(STO) substrates, using a combination of transmission electron microscopy (TEM) and electron energy loss spectrometry (EELS). TiO2films grew epitaxially in the anatase polymorph and exhibited the crystallographic orientation relation ofð0 0 1Þð0 1 0ÞTiO2 jjð0 0 1Þð0 1 0Þsubstrate . High-resolution TEM and EELS studies indicated the presence of a cubic TiOx phase at theTiO2/STO interface. Interfacial TiOx phases were eliminated and a sharp TiO2/STO interface was achieved by growing the TiO2 film on aheteroepitaxial STO buffer layer.r 2007 Elsevier B.V. All rights reserved.PACS: 68.37.Lp; 61.72.Ff; 81.15.HiKeywords: A1. Electron Energy Loss Spectrometry; A1. Transmission Electron Microscopy; A3. Molecular Beam Epitaxy; B1. TiO21. Introduction ferromagnetic oxides [7]. Importantly, the properties of TiO2 thin films are greatly affected by their crystalline TiO2 has many important applications, such as in quality, which is strongly affected by the nature of the film/photocatalysts [1,2], in photovoltaic cells [3], and as substrate interface. For example, both internal crystallinedielectrics for microelectronic devices [4]. Often such defects and external interfaces may cause spin-flips inapplications use TiO2 in thin film form. Recently, room- ferromagnetic TiO2 films, which reduces the spin polariza-temperature ferromagnetism was observed in thin films of tion value [8]. Thus, to optimize the performance of TiO2-TiO2 doped with magnetic cations [5], which makes this based devices it is critical to control crystalline andmaterial promising for the spintronic device applications. interfacial quality.TiO2 has three common polymorphs rutile, anatase, and TiO2 thin films have been grown on a wide range ofbrookite, and the properties of TiO2 depend on the oxide substrates, such as Al2O3 [9–11], SrTiO3 (STO) [11],polymorph. For example, anatase has considerably higher and LaAlO3 (LAO) [12]. Several growth techniques havephotocatalytic activity for the photoelectrochemical de- been used, including metalorganic chemical vapor deposi-composition of water than the other polymorphs [6]. In tion (MOCVD) [9–11], sputtering [13,14], pulsed laseraddition, Co-doped anatase shows the highest Curie deposition (PLD) [15,16], and molecular beam epitaxytemperature and remnant magnetization among the (MBE) [17,18]. It was determined that the polymorphic form, quality, structure of the film/substrate interface, and ÃCorresponding author. Tel.: +1 724 295 6624; fax: +1 724 295 6617. subsequently the film properties were affected both by the E-mail address: (O. Maksimov). choice of substrate and the growth conditions.0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.jcrysgro.2007.10.084
  2. 2. ARTICLE IN PRESS546 X. Weng et al. / Journal of Crystal Growth 310 (2008) 545–550 LAO and STO are the most common substrates used for 3. Results and discussionthe MBE growth of TiO2. To compensate for the lowactivity of molecular oxygen, oxygen plasma sources are 3.1. TiO2 film on LAOgenerally used to supply activated species (OPA–MBE) andto ensure sufficient oxidation of metal species [18,19]. TiO2 Owing to its low lattice mismatch (0.26%) to anatase,was recently grown with reactive MBE using an ozone/ LAO is widely used as a substrate for the growth of (0 0 1)oxygen gas mixture on both nitride (using GaN and anatase TiO2 thin films. High-quality anatase TiO2/LAOAlGaN templates) [20] and oxide (LAO, LSAT, and STO) heterostructures were previously grown using OPA-MBEsubstrates [21]. In the latter work, we investigated the and were investigated with electron microscopy [19],influence of the substrate, growth temperature, and providing us a sound comparison study for the TiO2/ozone flux on the structure and morphology of the TiO2 LAO heterostructures grown by reactive MBE in thisfilms using a combination of reflection high-energy elec- work, as well as a baseline for understanding the TiO2/STOtron diffraction (RHEED), X-ray diffraction (XRD), films. Fig. 1 shows (a) a low-magnification cross-sectionand atomic force microscopy [21]. Here, we report TEM image, (b) a selected area diffraction (SAD) pattern,transmission electron microscopy (TEM) and electron and (c) a high-resolution TEM image of the interfacialenergy loss spectrometry (EELS) investigations of the region of the TiO2/LAO heterostructures. As shown incrystal and interfacial structure of such TiO2 films grown Fig. 1(a), the film has a domain structure. The domain sizeon LAO and STO substrates. For films on STO, we is 100 nm and domain boundaries are parallel to theobserve an interfacial phase between the epitaxial anataselayer and the substrate. Finally, we describe the micro-structure of a TiO2/STO/LAO heterostructure and com-pare it to that of the TiO2/STO film, demonstrating theremoval of the interfacial phase for the latter TiO2/STOinterface.2. Experimental procedure Films were grown as described previously [21] oncommercially available (0 0 1) LAO and (0 0 1) STOwafers (MTI Corporation) in an MBE system equippedwith the high-temperature Ti effusion cells, a low-temperature Sr effusion cell, and an ozone distillationsystem (SVT Associates) [21]. All substrates wereetched ex-situ in a 3:1 HCl:HNO3 solution for 2–3 minand annealed in-situ prior to the growth for 1 h at 750 1Cunder an ozone/oxygen flux of 0.5 sccm. Identical condi-tions were used for the growth of TiO2 films and STObuffer layers, except that for the latter the Sr-source wasalso operated in a fashion to yield co-deposited films ofSrTiO3 stoichiometry. The growth rates, determined usingX-ray reflectivity, were 6 and 13 nm/h for TiO2 andSTO, respectively. The growth was monitored in-situusing a differentially pumped RHEED system. Wepreviously demonstrated that the RHEED patternsconfirmed epitaxial growth of high-quality TiO2 films[21]. For TEM and EELS experiments, cross-sectionalspecimens were prepared using conventional mechanicalthinning followed by an argon ion milling. TEM imaging,electron diffraction, and EELS were carried out on a JEOL2010F field-emission microscope equipped with a GatanEnfinaTM 1000 EELS system. All EEL spectra werecollected in diffraction mode (image coupling) with adispersion of 0.2 eV/channel and a resolution of 1.5 eV.The electron beam convergence semiangle (a) and theEELS collection semiangle (b) were 2.5 and 6.9 mrad, Fig. 1. (a) Low-magnification cross-section TEM image, (b) the SADrespectively. The microscope was operated at 200 keV for pattern and (c) a high-resolution TEM image of the interface region of theall experiments. TiO2/LAO heterostructure.
  3. 3. ARTICLE IN PRESS X. Weng et al. / Journal of Crystal Growth 310 (2008) 545–550 547growth direction (perpendicular to the substrate interface).The film surface is flat, except for trenches present at theintersection of the domain boundaries with the surface.Fringe-like contrast modulation is also evident within theTiO2 domains. Such contrast modulation often appears inTiO2 thin films [19], and is likely related to the existence ofcrystallographic shear planes [22]. The crystallographic relationship between the TiO2 filmand the LAO substrate is determined by SAD. Fig. 1(b)shows a typical SAD pattern collected from a regioncontaining both the film and the substrate. It is obviousthat the film and the substrate have an ð0 0 1Þ ð0 1 0ÞTiO2 jjð0 0 1Þ ð0 1 0ÞLAO orientation relationship, consistent withprior XRD results [21]. No noticeable orientation differ-ence is observed between the domains of the TiO2 film.These results agree well with the data reported for the TiO2films grown on LAO by OPA–MBE [19]. In Fig. 1(a), a 1–2 nm thick layer of a light contrast (ascompared to the rest of the film) is evident at the TiO2/LAO interface. Such contrast may indicate the presence ofnanometer-scale disordered regions at the interface, similarto those observed in the OPA–MBE-grown TiO2/LAOheterostructure [19]. Formation of this layer agrees with Fig. 2. (a) Low-magnification cross-section TEM image and (b) the SAD pattern of the TiO2/STO heterostructure.the drastic decrease of RHEED intensity observedimmediately upon TiO2 growth initiation. RHEED in-tensity starts to increase after deposition of a few lographic orientation relationship of ð0 0 1Þ ð0 1 0ÞTiO2 jjmonolayers. At this point, intensity oscillations become ð0 0 1Þ ð0 1 0ÞSTO , despite the presence of the interfacialevident, indicating that further growth continues in a layer- mode [19,21]. We have used high-resolution TEM and EELS to further High-resolution TEM is used to further examine the investigate the interface between the TiO2 film and the STOstructure of the interface between the TiO2 film and the substrate. Fig. 3(a) shows a high-resolution TEM imageLAO substrate. An atomically flat interface is evident in collected near the hole of the TEM specimen. A crystallinethe representative high-resolution image shown in Fig. 1(c). interface layer is present between the anatase TiO2 layerFurthermore, there are no second phases at the interface. and the STO substrate. Note that in this image, the topThe absence of obviously disordered regions in Fig. 1(c) part of the film (denoted as the surface layer) has a crystalmay be due to their overlap with crystalline TiO2 regions. structure different from anatase. Such a surface layer wasOn the other hand, this also suggests that the size and the not observed in regions located away from the hole of theareal density of the disordered regions in the film grown TEM specimen. Therefore, the surface layer in Fig. 3(a) hasusing reactive MBE are lower than those in the film grown formed during the ion-milling process, most probablyusing OPA–MBE. owing to ion damage, which is severe at the thin region near the specimen hole. Careful measurements reveal that3.2. TiO2 film on STO the lattice fringe spacing in both directions parallel to (0 1 0)STO and (0 0 1)STO is 0.2 nm both for the interface A thin layer of secondary phases (such as disordered and surface layers, suggesting that these two layers havepatches) was previously observed at the film/substrate similar crystal structure.interface of the TiO2 films grown on STO by OPA–MBE Fig. 3(b) shows the EEL spectra, which reveal the[19]. A low-magnification cross-sectional TEM image of a Ti–L2,3 and O–K edges, collected from the STO substrate,TiO2 film grown on a STO using reactive MBE is shown in the interface layer, the anatase TiO2, and the ion-damagedFig. 2(a). The TiO2 film contains small domains and has a surface layer. For the STO substrate, both the Ti–L2 andrelatively rough surface when compared with the film Ti–L3 edges exhibit a characteristic splitting and energygrown on LAO, apparently owing to the larger lattice values that are similar to the results of earlier studies [23].mismatch (3.1%) between TiO2 and STO. Furthermore, However, the splitting is less distinct in the TiO2 film and isthere is a thin layer of light contrast at the film/substrate not apparent for either the interface or the surface layers.interface; Figs. 2(a) and (b) show a SAD pattern collected Furthermore, the L-edges shift to energies lower than thosefrom a region consisting of both the film and the substrate. of the L-edges of STO: to a small degree for the anataseSimilar to the TiO2/LAO heterostructure described TiO2 layer and to a significantly larger degree for theearlier, the TiO2/STO heterostructure shows a crystal- interface and surface layers.
  4. 4. ARTICLE IN PRESS548 X. Weng et al. / Journal of Crystal Growth 310 (2008) 545–550 formula unit and the fact that the film was demonstrated to have the anatase structure from SAD (Fig. 2(b)). Surface Using similar arguments concerning both the Ti–L and layer O–L edges, the EEL spectra taken from the interface and surface layers are consistent with those expected from a TiOx phase with xo1.5. It has been reported that oxygen- sub-stoichiometric TiOx compounds adopt a cubic rocksalt TiO2 structure with a lattice parameter of 0.4 nm when 0.58pxp1.33. More highly reduced phases also exist; Ti2O has a trigonal structure [24–26]. As was discussed earlier, high-resolution TEM analysis shows a fringe Interface spacing of 0.2 nm in directions parallel to (0 1 0)STO and layer (0 0 1)STO for the interface and surface layers, suggesting that they have a cubic symmetry that is consistent with a rocksalt structure but not the anatase structure. Combining STO the high-resolution TEM results with the EELS observa- 2 nm tions of reduced oxygen contents, the surface and interface layers are best identified as TiOx (0.58pxp1.33) with a rocksalt structure and a lattice parameter of 0.4 nm. It is Ti L2 likely that the oxygen content is on the higher side of the L3 O-K rocksalt stability (or possibly larger if it is in a metastable Surface layer state owing to epitaxial stabilization). It should be noted that the ion milling (procedure used for TEM sample preparation) can cause oxygen loss and/or Counts (a.u.) TiO2 structural damage in the TiO2 layers (although the oxygen loss is expected to be minor). The ion milling-induced oxygen loss/structural damage should be most severe at the Interface layer film surface near the specimen hole, and in this sample results in the formation of the rocksalt TiOx surface layer. A similar TiOx layer also exists at the substrate/film interface. Although we cannot completely exclude that this STO TiOx layer is caused by the ion damage, we suggest that it arises during initial stages of film growth owing to the 450 475 500 525 550 surface pretreatments. Energy Loss (eV) 3.3. The TiO2/STO/LAO heterostructureFig. 3. (a) High-resolution TEM image of the TiO2/STO heterostructurecollected near the hole of the TEM specimen. A surface layer formed dueto the severe ion damage during the sample preparation. The EEL spectra A sub-stoichiometric TiOx interface layer could arise incorresponding to the different layers in (a) are shown in (b). The two the film grown on STO owing to surface reconstructions/dashed lines reveal energy shift of the Ti–L2,3 edges for different layers, stoichiometry changes caused by the low oxygen activityand the arrows show the O–K splitting. (ozone/oxygen flux of 0.5 sccm) during the pre-growth annealing. The low ozone/oxygen partial pressure will lead It has been shown that the splitting of Ti–L2,3 edges to oxygen loss and reconstruction of the STO and LAOdecreases as the x value decreases for TiOx (24x41), and substrate surfaces. [27,28]. During the early stages of filmcannot be resolved when x is less than a value between 1.5 growth, the arriving Ti atoms will be in contact with theand 1.2 [24]. In addition, the Ti–L2,3 edges shift to lower reconstructed/sub-stoichiometric surface of STO and LAOenergies as x decreases. [26]. Because the splitting of the and will come to equilibrium both with the chamberTi–L2,3 edge is evident for the TiO2 layer, albeit slightly less oxygen activity and the substrate surface/oxygen than for the STO substrate, and because there is also Under such conditions, oxygen-poor TiOx rocksalt inclu-only a small shift in the edge energy values, the EELS sions may form on the reconstructed and reduced surfacesresults suggest that the oxygen content in the anatase layer of STO and LAO substrates. These inclusions will beis close to the stoichiometric value, i.e., TiOx with x2. The overgrown by the anatase film and localized at the anatase/spectrum for this layer (given in Fig. 3(b)) also reveals a substrate interface. If the interfacial layer is more prone toclear splitting of the O–K edge. This further supports that ion damage than the bulk of the film, owing to the stress orx2 for the anatase layer since it has been shown that such structural variations at the interface, these inclusions cana splitting is indiscernible for TiOx (xo1.5) [24]. Finally, grow in size during the ion milling process resulting in athis oxygen stoichiometry is consistent with the anatase continuous interfacial layer, as is evident for the TiO2 film
  5. 5. ARTICLE IN PRESS X. Weng et al. / Journal of Crystal Growth 310 (2008) 545–550 549grown on STO. Since LAO and STO should have a being ð0 0 1ÞTiO2 kð0 0 1ÞSTO kð0 0 1ÞLAO and ð0 1 0ÞTiO2 kdifferent level of reconstruction/reduction and exhibit ð0 1 0ÞSTO kð0 1 0ÞLAO . However, as indicated by the arrows,different lattice mismatch with TiO2, the effect will be the reflections from the STO layer appear as short arcsdifferent for the TiO2 films grown on LAO and STO instead of sharp spots, suggesting that the STO layersubstrates under identical conditions. contains domains with slightly different orientations. In order to eliminate the reconstructed nature of the Fig. 4(c) shows a high-resolution cross-sectional TEMsubstrate surface and, therefore, to eliminate the interface image of the heterostructure. Sharp interfaces betweenlayer, we prepared a different STO surface and used it for layers are evident, suggesting high-quality epitaxial growththe TiO2 growth. An epitaxial STO buffer layer was used of the layers. Second phases are observed neither at theinstead of a STO substrate; we deposited a 10 nm thick TiO2/STO nor at the STO/LAO interfaces, indicating thatSTO buffer layer on a LAO substrate using the same the regional oxygen deficiency and the subsequent forma-chamber and the same conditions as described above. At tion of TiO have been eliminated by using a STO bufferthis thickness the STO buffer layer is only partially relaxed layer instead of growing TiO2 film directly on the substrate.and its in-plane lattice parameter is intermediate between Similar to the TiO2 film grown directly on the LAOthat of the bulk STO and LAO. Thus, the strain state of the substrate the TiO2 film grown on the STO buffer exhibitsTiO2 film is also intermediate between two previously fringe-like contrast modulation likely related to thediscussed cases. existence of crystallographic shear planes. Defects, such Then, a TiO2 film was grown under conditions identical as the one circled, are also observed in the STO bufferto that used for the growth on LAO and STO substrates. layer. They could form due to the lattice mismatch betweenFig. 4(a) shows a low-magnification cross-sectional TEM the LAO and STO, and may account for the formation ofimage of a TiO2/STO/LAO heterostructure. The TiO2 film STO domains with various orientations.grown on the STO buffer has surface steps and is rougher The EEL spectra collected from the TiO2 film and fromthan the film grown directly on the LAO. Domain the STO buffer layer at the interface region are shown inboundaries are also observed, with one of them being Fig. 5. The spectrum collected from the STO buffer layer isindicated by arrow. nearly identical to the one collected from the STO Fig. 4(b) shows a SAD pattern collected from an area substrate, indicating the same composition. In addition,consisting of the TiO2 film, STO buffer layer, and LAO no shift of Ti–L2,3 edges is observed and splitting of Ti–L2,3substrate. It shows three sets of diffraction patterns from and O–K edges is clearly evident for the TiO2 film,these three materials. The TiO2 has an anatase structure verifying that the oxygen content is close to the stoichio-with the crystallographic correlations between the layers metric value.Fig. 4. (a) Low-magnification TEM image, (b) selective area diffraction pattern and (c) high-resolution TEM image of the TiO2/STO/LAOheterostructure.
  6. 6. ARTICLE IN PRESS550 X. Weng et al. / Journal of Crystal Growth 310 (2008) 545–550 Ti necessarily reflect the views of the Office of Naval L2 Research. The authors thank Drs. M. Olszta and G.Y. Yang at PSU for helpful discussions on EELS analysis. L3 ReferencesCounts (a.u.) O-K TiO2 [1] A. Fujishima, K. Honda, Nature 238 (1972) 37. [2] A.L. Linsebigler, G. Lu, J.J.T. Yates, Chem. Rev. 95 (1995) 735. [3] B. O’Regan, M. Gratzel, Nature 353 (1991) 737. ¨ [4] W.D. Brown, W.W. Grannemann, Solid-State Electron 21 (1978) 837. STO [5] Y. Matsumoto, M. Murakami, T. Shono, T. Hasegawa, T. Fukumura, M. Kawasaki, P. Ahmet, T. Chikyow, S-Y. Koshihara, H. Koinuma, Science 291 (2001) 854. 450 475 500 525 550 [6] L. Kavan, M. Gratzel, S.E. Gilbert, C. Klemenz, H.J. Scheel, J. Am. ¨ Energy Loss (eV) Chem. Soc. 118 (1996) 6716. [7] S.A. Chambers, R.F.C. Farrow, MRS Bull. 28 (2003) 729.Fig. 5. EEL spectra collected from the TiO2 film and from the STO buffer [8] S.A. Chambers, Y.K. Yoo, MRS Bull. 28 (2003) 706.layer at the regions near the interface. [9] H.L.M. Chang, H. You, J. Guo, D.J. Lam, Appl. Surf. Sci. 48/49 (1991) 12. [10] Y. Gao, K.L. Merkle, H.L. Chang, T.J. Zhang, D.J. Lam, Philos. Thus, by using an epitaxial STO layer as the substrate, a Mag. A 65 (1992) 1103.highly crystalline, well-controlled STO surface was pro- [11] S. Chen, M.G. Mason, H.J. Gysling, G.R. Paz-Pujalt, T.N. Blanton, T. Castro, K.M. Chen, C.P. Fictorie, W.L. Gladfelter, A. Franciosi,duced without the surface rearrangements that develop P.I. Cohen, J.F. Evans, J. Vac. Sci. Technol. A 11 (1993) 2419.during the longer-term substrate anneal. Consequently, the [12] M. Murakami, Y. Matsumoto, K. Nakajima, T. Makino, Y. Segawa,TiOx interface layer did not form and a sharp TiO2/STO T. Chikyow, P. Ahmet, M. Kawasaki, H. Koinuma, Appl. Phys. Lett.interface without secondary phases was obtained. 78 (2001) 2664. [13] P.A.M. Hotsenpiller, G.A. Wilson, A. Roshko, J.B. Rothman, G.S. Rohrer, J. Crystal Growth 166 (1996) 779.3.4. Conculsion [14] T. Aoki, K. Maki, Q. Tang, Y. Kumagai, S. Matsumoto, J. Vac. Sci. Technol. A 15 (1997) 2485. We studied the microstructure of TiO2 films grown on [15] X.H. Liu, X.Y. Chen, J. Yin, Z.G. Liu, J.M. Liu, X.B. Yin,LAO and STO substrates by reactive MBE. TiO2 films G.X. Chen, M. Wang, J. Vac. Sci. Technol. A 19 (2001).grew epitaxially in the anatase polymorph and exhibited [16] S. Yamamoto, T. Sumita, Sugiharuto, A. Miyashita, H. Naramoto, Thin Solid Films 401 (2001) 88.the crystallographic orientation relation of ð0 0 1Þ [17] Y. Gao, S.A. Chambers, Mater. Lett. 26 (1996) 217.ð0 1 0ÞTiO2 kð0 0 1Þ ð0 1 0Þsubstrate . While only disordered [18] S.A. Chambers, S. Thevuthasan, R.F.C. Farrow, R.F. Marks,patches were present at the TiO2/LAO interface, a rocksalt J.U. Thiele, L. Folks, M.G. Samant, A.J. Kellock, N. Ruzycki,TiOx (0.58pxp1.33) layer was present at the TiO2/STO D.L. Ederer, U. Diebold, Appl. Phys. Lett. 79 (2001).interface. We believe that the interface layer formed due to [19] S.A. Chambers, C.M. Wang, S. Thevuthasan, T. Droubay, D.E. McCready, A.S. Lea, V. Shutthanandan, C.F. Windisch Jr.,the substrate surface rearrangements that occurred during Thin Solid Films 418 (2002) 197.the pre-deposition annealing stage and was enhanced by [20] P.J. Hansen, V. Vaithyanathan, Y. Wu, T. Mates, S. Heikman,the oxygen loss during the ion milling process. By growing U.K. Mishra, R.A. York, D.G. Schlom, J.S. Speck, J. Vac. Sci.the TiO2 film on an epitaxial STO buffer layer, instead of Technol. B 23 (2005) 499.on a STO substrate, an abrupt TiO2/STO interface without [21] P. Fisher, O. Maksimov, H. Du, V. Heydemann, M. Skowronski, P.A. Salvador, Microelectron. J. 37 (2006) 1493.secondary phases was obtained. [22] L.A. Bursill, G.J. Wood, Philos. Mag. A 38 (1978) 673. [23] R.F. Klie, Y. Zhu, Micron 36 (2005) 219.Acknowledgments [24] C. Mitterbauer, G. Kothleitner, F. Hofer, Micros. Microanal. 9 (Suppl. 2) (2003) 834. This material is based upon work supported by ONR [25] M.D. Banus, T.B. Reed, A.J. Strauss, Phys. Rev. B 5 (1972) 2775. [26] R.W.G. Wyckoff, Crystal Structure, vol. 1, Wiley-Interscience,under Contract No. N00014-05-1-0238. Any opinions, New York, 1960.findings, and conclusions or recommendations expressed [27] C.C. Chin, T. Morishita, Physica C 245 (1995) this material are those of the authors and do not [28] Q.D. Jiang, J. Zegenhagen, Surf. Sci. 425 (1999) 343.