Growth, structure, and morphology of TiO2 films deposited by molecular beam epitaxy in pure ozone ambients


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Microelectronics Journal 37 (2006) 1493–1497

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Growth, structure, and morphology of TiO2 films deposited by molecular beam epitaxy in pure ozone ambients

  1. 1. ARTICLE IN PRESS Microelectronics Journal 37 (2006) 1493–1497 Growth, structure, and morphology of TiO2 films deposited by molecular beam epitaxy in pure ozone ambients Patrick Fishera, Oleg Maksimovb, Hui Dua, Volker D. Heydemannb, Marek Skowronskia, Paul A. Salvadora,Ã a Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA b Electro-Optics Center, Pennsylvania State University, Freeport, PA 16229, USA Available online 14 August 2006Abstract TiO2 films were grown using a reactive molecular beam epitaxy system equipped with high-temperature effusion cells as sources for Tiand an ozone distillation system as a source for O. The growth mode, characterized in-situ by reflection high-energy electron diffraction(RHEED), as well as the phase assemblage, structural quality, and surface morphology, characterized ex-situ by X-ray diffraction andatomic force microscopy (AFM), depended on the choice of substrate, growth temperature, and ozone flux. Films deposited on (1 0 0)surfaces of SrTiO3, (La0.27Sr0.73)(Al0.65Ta0.35)O3, and LaAlO3 grew as (0 0 1)-oriented anatase. Both RHEED and AFM indicated thatsmoother surfaces were observed for those grown at higher ozone fluxes. Moreover, while RHEED patterns indicated that anatase filmsgrown at higher temperatures were smoother, AFM images showed presence of large inclusions in these films.r 2006 Elsevier Ltd. All rights reserved.PACS: 81.15.Hi; 61.14.Hg; 61.10.NzKeywords: Titanium dioxide; MBE; Thin film epitaxy There is a wide technological interest in TiO2 because it Most previously reported films were grown by pulsedhas interesting physical and chemical properties, which laser deposition [14,15], metal organic chemical vapormake it appealing for optical, dielectric, electrochemical, deposition [6,7,11,16,17], or sputtering [5,10,18]. Molecularand photocatalytic applications [1–3]. TiO2 is also of beam epitaxy (MBE), a technique that differs in thermo-scientific interest since it has polymorphic structures dynamics and kinetics from the other approaches and that(anatase, rutile, and brookite) that exhibit different can produce high quality films, has been limited to growthstabilities and properties [4,5]. Certain applications, such of TiO2 on LaAlO3 [8], SrTiO3 [8], and GaN [13]as integrated dielectrics or photoelectrochemical cells, substrates. MBE also allows the integration of chargerequire thin films of TiO2 that exhibit a specific crystal neutral TiO2 monolayers with other chemical and structur-structure, orientation, and/or morphology. Thus, there are al layers, such as SrO in SrTiO3 [19]. Importantly,a number of reports on the growth of TiO2 films on various differences in sources used in MBE can vary the thermo-substrates, including Al2O3 [5–7], LaAlO3 [8], (La, Sr)(Al, dynamic/kinetic aspects of the growth and, therefore,Ta)O3 (LSAT) [9], MgO [5,10], SrTiO3 [11], BaTiO3 [12], impact film properties.(Zr, Y)O2 [9], and GaN [13]. Depending on the choices of Oxide MBE requires a special oxygen source tosubstrate and deposition conditions, both rutile and/or completely oxidize deposited metal, to prevent sourceanatase can be grown. oxidation, and to allow the use of in-situ electron probes. For this purpose, RF or microwave plasma sources are ÃCorresponding author. Tel.: +1 412 268 2702; fax: +1 412 268 7596. often used instead of molecular oxygen (O2) [8,20]. E-mail address: (P.A. Salvador). However, plasma sources produce highly energetic species0026-2692/$ - see front matter r 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.mejo.2006.05.010
  2. 2. ARTICLE IN PRESS1494 P. Fisher et al. / Microelectronics Journal 37 (2006) 1493–1497and have limited flux control [21]. An alternative approachis to use pure ozone (O3) or an O3/O2 gas mixture. Ozoneflux is easily controlled with a mass flow controller and canbe directed towards the heated substrate surface where it isthermally cracked into O and O2. There have only been a limited number of reportsdescribing the influence of ozone flux on the growth ofoxide films, particularly for the cases with no obviousoxidation problem, such as TiO2. In this study, we havegrown TiO2 films on various single crystal substrates usinga MBE system equipped with a high-temperature effusioncell as a source of Ti and an ozone distillation system toprovide pure ozone. This paper describes the effects ofsubstrate, growth temperature, and ozone flux on crystalstructure, orientation, phase assemblage, and morphologyof the films. Commercial SrTiO3(1 0 0), LSAT(1 0 0), and LaAlO3(1 0 0) substrates were etched in a 3:1 HCl:HNO3 solutionfor 2–3 min [22], rinsed in deionized water, and thendegreased prior to the growth. Quarters of 2-inch waferswere mounted into an Inconel sample holder andintroduced into the growth chamber (SVT Associates).Substrates were heated to 750 1C using a resistive boronnitride heater (temperature is measured with a thermo-couple located in close proximity to the substrate) andannealed at 750 1C for 1 h under the deposition ozone flux. Ozone was generated with a commercial unit (OzoneSolutions) capable of producing 6% O3 in O2. It wasdistilled by passing the O2/O3 mixture through the liquid-nitrogen cooled dewar filled with silica gel; the O3 wasadsorbed while the remnant O2 was pumped away. Afterstoring sufficient amount, the pure ozone stream wasgenerated by warming the dewar. The high-concentrationO3 flux was introduced into the chamber (base pressure of10À10 Torr) using a mass flow controller at a rate of0.25–2 sccm, which corresponded to a pressure of 6 Â 10À6–2.5 Â 10À5 Torr. The Ti flux was produced with a high-temperature effusion cell operated at a fixed temperaturebetween 1500 and 1600 1C. During each growth run, theprocess was monitored using a differentially pumpedRHEED system (Staib Instruments) operated at 12.0 kVwith an incident angle of 31. X-ray diffraction (XRD) Fig. 1. RHEED images taken along the [1 0 0] azimuth at the end ofmeasurements [23] were made in y–2y, o, and f-scans growth for TiO2 films deposited at 750 1C using an ozone flux of 1 sccmmodes. Atomic force microscopy (AFM) was carried out in and a Ti source temperature of 1550 1C on (a) SrTiO3(1 0 0), (b)contact mode [23]. X-ray reflectance was performed to LSAT(1 0 0), (c) and LaAlO3(1 0 0). The film in (d) is similar to (c) but was deposited on LaAlO3(1 0 0) using a Ti source temperature of 1525 1C.determine film thickness. Fig. 1 shows RHEED images, taken along the [1 0 0]azimuth, of anatase films grown at 750 1C using a Ti-cell more clearly evident than those on SrTiO3 or LSAT,temperature of 1550 1C and an ozone flux of 1 sccm on (a) indicating better surface morphology and higher crystal-SrTiO3(1 0 0), (b) LSAT(1 0 0), and (c) LaAlO3(1 0 0). linity for the films on LaAlO3. This is likely a result of theFig. 1(d) is a similar RHEED image to Fig. 1(c) but for excellent lattice match between anatase and LaAlO3(1 0 0)a film grown on LaAlO3(1 0 0) using a Ti-cell temperature (mismatch ¼ f ¼ 0.3%).of 1525 1C (all other conditions were the same). All A comparison of the RHEED patterns given in Figs. 1(cRHEED patterns were consistent with anatase (0 0 1) and d) suggests that film quality can be improved by agrowth and exhibited a characteristic four-fold reconstruc- growth rate reduction (as was observed in [8]). However,tion [8,20]. Fig. 1 shows that the RHEED patterns for the ˚ the growth rate for the film in Fig. 1(d) was about 0.017 A/s,films grown on LaAlO3 are sharper and Kikuchi lines are and for most applications a higher rate would be preferred.
  3. 3. ARTICLE IN PRESS P. Fisher et al. / Microelectronics Journal 37 (2006) 1493–1497 1495 Fig. 3. RHEED images taken along the [1 0 0] azimuth at the end of film growth for TiO2 deposited on LaAlO3 at 550 1C using a Ti source temperature of 1550 1C and an ozone flux of (a) 0.25 sccm O3 and (b) 1.00 sccm O3.Fig. 2. (a) RHEED intensity oscillations during TiO2 film growth onLaAlO3(1 0 0), using an ozone flux of 1 sccm and a Ti source temperature present RHEED images taken along the [1 0 0] azimuth forof 1550 1C (taken along the [1 0 0] azimuth). (b) X-ray reflectivity patternfor the film whose RHEED oscillations were shown in (a). anatase films grown on LaAlO3 under different ozone fluxes (at T ¼ 550 1C and at a Ti cell temperature ¼ 1550 1C). In Fig. 3(a), a spotty pattern is observed for theTo study if the growth could be optimized without film deposited at 0.25 sccm O3. The RHEED pattern of asacrificing growth rate, the effects that other parameters film deposited at the increased O3 flux of 1.0 sccm is givenhad on the growth of TiO2 on LaAlO3(1 0 0) substrates in Fig. 3(b). In this higher ozone flux case, the RHEED(which exhibited the sharpest 2D RHEED images in Fig. 1) pattern is more streaky than that in Fig. 3(a). On raisingwere investigated. the temperature from 550 to 750 1C, one obtains the RHEED intensity oscillations, one set of which is given RHEED pattern given in Fig. 1(c), wherein all character ofin Fig. 2(a) for the film whose RHEED pattern was given a transmission spot pattern is lost, and the pattern isin Fig. 1(c), were observed consistently for the films grown dominated by spots on a circle and minor streakiness,on LaAlO3 substrates. The thickness of this film was indicating an atomically smooth surface. Furthermore, thedetermined using X-ray reflectance and the scan for this four-fold reconstruction becomes evident for the high-film is given in Fig. 2(b). This data was refined using temperature and high-ozone flux-grown film.Philips’ WinGixa software and the film was determined to To better understand surface morphologies, films were ˚be 180 A thick. Combining this with the RHEED oscilla- studied ex-situ with AFM. In Figs. 4(a)–(c), we show AFMtions, we calculated that one RHEED intensity oscillation images of the TiO2 films whose RHEED patterns were ˚corresponded to the growth of E4.5 A of anatase; that given in Figs. 1(a)–(c), which were grown on SrTiO3, ˚value is very close to a bilayer of anatase (E4.75 A), in LSAT, and LaAlO3, respectively. Two types of features areagreement with a previous report [8]. Similar behavior observed in the AFM images: a uniform grayish contrast(meaning growth via bilayer units) was obtained for the that represents the matrix phase (and the major surfacefilms grown with different Ti cell temperatures (i.e., growth feature) and significantly brighter spots that stand out fromrates), substrate temperatures, and ozone flux values. The the uniform contrast and represent inclusions in the films.absence of RHEED oscillations for films grown on SrTiO3 When comparing the Figs. 4(a)–(c), the grayscale changesand LSAT is reflective of the more diffuse nature and in the uniform background decrease on going from SrTiO3increased spottiness of the RHEED patterns, indicating to LSAT to LaAlO3. The overall grayscale is similar,that those films surfaces are rougher than surfaces of films however, between all the images because both LSAT andon LaAlO3. LAO exhibit bright white features indicative of large Our goal was to understand what effects other growth protruding objects from the film. Electron microscopyparameters had on structural/surface quality. In Fig. 3, we experiments (not shown here) carried out by us on films
  4. 4. ARTICLE IN PRESS1496 P. Fisher et al. / Microelectronics Journal 37 (2006) 1493–1497 Fig. 5. y–2y scans of TiO2 films on SrTiO3(1 0 0), LSAT(1 0 0), and LaAlO3(1 0 0). though the surface of the anatase matrix is tending to flatten out at higher temperatures. For films deposited on LaAlO3 using the lower ozone flux of 0.25 sccm (Figs. 4(d and f)), the rms roughness increased somewhat with ˚ substrate temperature, going from 62 A at T ¼ 550 1C (Fig. 4(f)) to 102 A ˚ at T ¼ 750 1C (Fig. 4(d)). Importantly, films grown under higher ozone fluxes were smoother at all substrate temperatures. Figs. 4(e and f)Fig. 4. (a–c) are AFM images of TiO2 films grown at 750 1C using a Ti correspond to the RHEED images in Figs. 3(b and a),source temperature of 1550 1C and an ozone flux of 1 sccm on (a) respectively. The change in film surface with ozone pressureSrTiO3(1 0 0); (b) LSAT(1 0 0); and on LaAlO3(1 0 0); (d–f) are AFMimages of TiO2 films grown on LaAlO3(1 0 0) using a Ti source that we previously observed by RHEED corresponded to atemperature of 1550 1C at (d) 750 1C, 0.25 sccm O3; (e) 550 1C, 1.00 sccm major change in surface roughness, going from a rmsO3; and (f) 550 1C, 0.25 sccm O3. ˚ roughness of 62 A at 0.25 sccm O3 to an rms roughness of 8A ˚ at 1.00 sccm. In general for the films deposited using 1.00 sccm O3, the rms roughnesses were much lower thandeposited on LaAlO3 agreed with Chambers et al’s [8] for films grown at 0.25 sccm, increasing from 8 A at ˚observations that these objects are rutile inclusions that T ¼ 550 1C (Fig. 4(e)) to 22 A ˚ at T ¼ 650 1C (not shown)have coherent interfaces with, and protrude out from the ˚ to 52 A at T ¼ 750 1C (Fig. 4(c)). Note again that theflat surface of, the anatase matrix. increase in rms roughnesses with increasing temperature Pronounced relationships between both substrate tem- appears to arise from the increase in the large inclusions.perature and ozone flux were observed, as illustrated in Over the regions of the surface where there are noFigs. 4(c–f). Figs. 4(d–f) are AFM images of TiO2 films inclusions, the rms roughnesses were very low: 8 A at ˚grown on LaAlO3(1 0 0) using a Ti source temperature of ˚ ˚ T ¼ 550 1C, 4 A at T ¼ 650 1C, and 7 A at T ¼ 750 1C.1550 1C at (d) 750 1C, 0.25 sccm O3; (e) 550 1C, 1.00 sccm Clearly the inclusions play a major role in the overallO3; and (f) 550 1C, 0.25 sccm O3. At higher temperatures surface roughness.(650 1C and above), large inclusions became increasingly The XRD spectra of TiO2 films deposited on all 3common, as observed in Figs. 4(c and d). At higher substrates at 750 1C using a Ti-cell temperature of 1550 1Ctemperatures, the stable rutile phase is more likely to and an ozone flux of 1 sccm are shown in Fig. 5. Thesenucleate during growth and the metastable anatase is more XRD patterns reveal that highly (0 0 1)-oriented anataselikely to transform irreversibly to the stable rutile. Based films grew on SrTiO3 (1 0 0), LSAT (1 0 0), and LaAlO3on our experimental evidence from TEM and the above (1 0 0) substrates. In fact, similar XRD results weregiven thermodynamic arguments, it seems likely that the observed by us for anatase films deposited over a widelarge inclusions observed in the AFM images for the high- range of conditions (4501oTdo750 1C and 0.25 o flux O3temperature films are rutile. The inclusions greatly increase o 2.00 sccm). f-scans showed that these films werethe overall root-mean-square (rms) roughness values, even all epitaxial and all had the same relationship:
  5. 5. ARTICLE IN PRESS P. Fisher et al. / Microelectronics Journal 37 (2006) 1493–1497 1497(0 0 1)TiO2 ||(1 0 0)Subs; [0 1 0]TiO2 ||[0 1 0]Subs. The full-widths N00014-03-1-0665. Any opinions, findings, and conclusionsat half-maximum (FWHM) of the rocking curve on the or recommendations expressed in this material are those ofanatase (0 0 4) peaks were largest for the films on the authors and do not necessarily reflect the views of theSrTiO3(1 0 0) (E0.71), intermediate on LSAT (0.11), and Office of Naval Research. It also made use of the facilitieslowest (E0.031) on LaAlO3(1 0 0). The FWHM for the maintained through the MRSEC program of the Nationalsubstrates themselves were all E0.021 under our diffraction Science Foundation under Award no. DMR-0520425.conditions, indicating that the films on LaAlO3 substrateswere of superior crystalline quality to the other two films,in spite of the inherent twinning in the LaAlO3 substrate.These diffraction observations are similar in nature to Referencesthose observed in other reports using other depositionmethods [11,16] and using MBE deposition [8], although [1] J.G.E. Jellison, F.A. Modine, L.A. Boatner, Opt. Lett. 22 (1997)no other reports exist on MBE growth of TiO2 films on the 1808–1810. [2] M.R. Hoffman, S.T. Martin, W. Choi, D.W. Bahnemann, Chem.LSAT(1 0 0) surface. Rev. 95 (1995) 69–96. It should be pointed out that under the growth [3] A.L. Linsebigler, G. Lu, J. John, T. Yates, Chem. Rev. 95 (1995)conditions investigated here, temperature seemed to play 735–758.the largest role on inclusion formation; substrate choice, [4] M. Koelsch, S. Cassaignon, J.F. Guillemoles, J.P. Olivet, Thin Solidozone flux, and Ti-cell temperature played secondary roles, Films 403–404 (2002) 312–319.if any. Interestingly, the inclusions did not appear to affect [5] P.A.M. Hotsenspiller, G.A. Wilson, A. Roshko, J.B. Rothman, G.S.either the RHEED pattern or the XRD pattern in an Rohrer, J. Cryst. Growth 166 (1996) 779–785. [6] D.R. Burgess, P.A.M. Hotsenspiller, T.J. Anderson, J.L. Hohman,obvious manner. Nevertheless, electron microscopy experi- J. Cryst. Growth 166 (1996) 763–768.ments carried out by us on films grown on LaAlO3 (not [7] H.L.M. Chang, H. You, Y. Gao, J. Guo, C.M. Foster,shown here) agreed with observations given in Ref. [8] R.P. Chiarello, T.J. Zhang, D.J. Lam, J. Mater. Res. 7 (1992)demonstrating that rutile inclusions protrude out of the 2495–2506.matrix anatase phase in films grown at high temperatures. [8] S.A. Chambers, C.M. Wang, S. Thevuthasan, T. Droubay, D.E.It should be noted that no rutile inclusions are observed in McCready, A.S. Lea, V. Shutthanandan, J.C.F. Windisch, Thin Solidpure TiO2 films deposited using pulsed laser deposition on Films 418 (2002) 197–210.LaAlO3(1 0 0) substrates at T ¼ 750 1C in elevated pres- [9] S. Yamamoto, T. Sumita, T. Yamaki, A. Miyashita, H. Naramoto, J. Cryst. Growth 237–239 (2002) 569–573.sures of pure oxygen (PE200 m Torr O2). Further work is [10] T. Aoki, K. Maki, Q. Tang, Y. Kumagai, S. Matsumoto, J. Vacuumrequired to understand the thermodynamics and kinetics of Sci. Technol. A 15 (1997) 2485–2488.the nucleation and growth of these rutile inclusions during [11] S. Chen, M.G. Mason, H.J. Gysling, G.R. Paz-Pujalt, T.N. Blanton,thin film deposition. T. Castro, K.M. Chen, C.P. Fictorie, W.L. Gladfelter, A. Franciosi, In conclusion, TiO2 thin films were grown on SrTiO3 P.I. Cohen, J.F. Evans, J. Vacuum Sci. Technol. A 11 (1993)(1 0 0), LSAT (1 0 0), and LaAlO3 (1 0 0) substrates by 2419–2429.reactive MBE and characterized in-situ by RHEED and ex- [12] V.A. Burbure, P.A. Salvador, G.S. Rohrer, J. Am. Ceram. Soc. (2006), in press, Doi:10.1111/j.1551-2916.2006.01168.x.situ by XRD and AFM. Films grown on SrTiO3 (1 0 0), [13] P.J. Hansen, V. Vaithyanathan, Y. Wu, T. Mates, S. Heikman, U.K.LSAT (1 0 0), and LaAlO3 (1 0 0) grew as (0 0 1) anatase, Mishra, R.A. York, D.G. Schlom, J.S. Speck, J. Vacuum Sci.which was found to grow over a range of temperatures Technol. B 23 (2005) 499–506.(450–750 1C). It was observed that the films’ structural [14] X. Liu, X.Y. Chen, J. Yin, Z.G. Liu, J.M. Liu, X.B. Yin, G.X. Chen,quality and morphology highly depended on substrate J. Vacuum Sci. Technol. A 19 (2001) 391–393.temperature and ozone flux. Films were found to grow with [15] Sugiharto, S. Yamamoto, T. Sumita, A. Miyashita, J. Phys. Condens.a smoother surface at higher ozone fluxes, and films grown Matter 13 (2001) 2875–2881. [16] Y. Gao, S. Thevuthasan, D.E. McCready, M. Engelhard, J. higher substrate temperatures exhibited large inclusions. Growth 212 (2000) 178–190.It is interesting that these results indicate that the values for [17] G.S. Herman, Y. Gao, T.T. Tran, J. Osterwalder, Surf. Sci. 447ozone flux, Ti flux, and temperature have not been (2000) 201–211.optimized for the growth of high-quality anatase TiO2 [18] M. Kamei, T. Mitsuhashi, Surf. Sci. 463 (2000) L609–L612.exhibiting very narrow rocking curves. To realize such films, [19] J.H. Haeni, C.D. Theis, D.G. Schlom, W. Tian, X.Q. Pan, H. Chang,a better understanding is required of the thermodynamic I. Takeuchi, X.D. Xiang, Appl. Phys. Lett. 78 (2001) 3292–3294.and kinetic processes that occur in MBE and that govern [20] W. Gao, C.M. Wang, H.Q. Wang, V.E. Henrich, E.I. Altman, Surf.rutile nucleation and growth on perovskites surfaces. Sci. 559 (2004) 201–213. [21] E. Coleman, T. Siegrist, D.A. Mixon, P.L. Trevor, D.J. Trevor, J. Vacuum Sci. Technol. A 9 (1991) 2408–2409.Acknowledgements [22] V. Leca, G. Rjinders, G. Koster, D.H.A. Blank, H. Rogalla, Mater. Res. Soc. Symp. Proc. 587 (2000) O3.6.1–O3.6.4. This work was supported by the Office of Naval Research [23] A. Asthagiri, C. Niederberger, A.J. Francis, L.M. Porter, P.A.under grants N0001-05-1-0238, N00014-05-1-0152, and Salvador, D.S. Sholl, Surf. Sci. 537 (2003) 134–152.