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Growth, structure, and morphology of TiO2 films deposited by molecular beam epitaxy in pure ozone ambients
ARTICLE IN PRESS
Microelectronics Journal 37 (2006) 1493–1497 www.elsevier.com/locate/mejo
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, 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 2006
Abstract TiO2 films were grown using a reactive molecular beam epitaxy system equipped with high-temperature effusion cells as sources for Ti and 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 and atomic 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 that smoother surfaces were observed for those grown at higher ozone fluxes. Moreover, while RHEED patterns indicated that anatase films grown 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.Nz Keywords: Titanium dioxide; MBE; Thin film epitaxy
There is a wide technological interest in TiO2 because it has interesting physical and chemical properties, which make it appealing for optical, dielectric, electrochemical, and photocatalytic applications [1–3]. TiO2 is also of scientific interest since it has polymorphic structures (anatase, rutile, and brookite) that exhibit different stabilities and properties [4,5]. Certain applications, such as integrated dielectrics or photoelectrochemical cells, require thin films of TiO2 that exhibit a specific crystal structure, orientation, and/or morphology. Thus, there are a number of reports on the growth of TiO2 films on various substrates, including Al2O3 [5–7], LaAlO3 [8], (La, Sr)(Al, Ta)O3 (LSAT) [9], MgO [5,10], SrTiO3 [11], BaTiO3 [12], (Zr, Y)O2 [9], and GaN [13]. Depending on the choices of substrate and deposition conditions, both rutile and/or anatase can be grown. Corresponding author. Tel.: +1 412 268 2702; fax: +1 412 268 7596.
E-mail address: [email protected] (P.A. Salvador). 0026-2692/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2006.05.010
Most previously reported films were grown by pulsed laser deposition [14,15], metal organic chemical vapor deposition [6,7,11,16,17], or sputtering [5,10,18]. Molecular beam epitaxy (MBE), a technique that differs in thermodynamics and kinetics from the other approaches and that can produce high quality films, has been limited to growth of TiO2 on LaAlO3 [8], SrTiO3 [8], and GaN [13] substrates. MBE also allows the integration of charge neutral TiO2 monolayers with other chemical and structural layers, such as SrO in SrTiO3 [19]. Importantly, differences in sources used in MBE can vary the thermodynamic/kinetic aspects of the growth and, therefore, impact film properties. Oxide MBE requires a special oxygen source to completely oxidize deposited metal, to prevent source oxidation, and to allow the use of in-situ electron probes. For this purpose, RF or microwave plasma sources are often used instead of molecular oxygen (O2) [8,20]. However, plasma sources produce highly energetic species
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and have limited flux control [21]. An alternative approach is to use pure ozone (O3) or an O3/O2 gas mixture. Ozone flux is easily controlled with a mass flow controller and can be directed towards the heated substrate surface where it is thermally cracked into O and O2. There have only been a limited number of reports describing the influence of ozone flux on the growth of oxide films, particularly for the cases with no obvious oxidation problem, such as TiO2. In this study, we have grown TiO2 films on various single crystal substrates using a MBE system equipped with a high-temperature effusion cell as a source of Ti and an ozone distillation system to provide pure ozone. This paper describes the effects of substrate, growth temperature, and ozone flux on crystal structure, orientation, phase assemblage, and morphology of 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 solution for 2–3 min [22], rinsed in deionized water, and then degreased prior to the growth. Quarters of 2-inch wafers were mounted into an Inconel sample holder and introduced into the growth chamber (SVT Associates). Substrates were heated to 750 1C using a resistive boron nitride heater (temperature is measured with a thermocouple located in close proximity to the substrate) and annealed at 750 1C for 1 h under the deposition ozone flux. Ozone was generated with a commercial unit (Ozone Solutions) capable of producing 6% O3 in O2. It was distilled by passing the O2/O3 mixture through the liquidnitrogen cooled dewar filled with silica gel; the O3 was adsorbed while the remnant O2 was pumped away. After storing sufficient amount, the pure ozone stream was generated by warming the dewar. The high-concentration O3 flux was introduced into the chamber (base pressure of 1010 Torr) using a mass flow controller at a rate of 0.25–2 sccm, which corresponded to a pressure of 6 106– 2.5 105 Torr. The Ti flux was produced with a hightemperature effusion cell operated at a fixed temperature between 1500 and 1600 1C. During each growth run, the process was monitored using a differentially pumped RHEED system (Staib Instruments) operated at 12.0 kV with an incident angle of 31. X-ray diffraction (XRD) measurements [23] were made in y–2y, o, and f-scans modes. Atomic force microscopy (AFM) was carried out in contact mode [23]. X-ray reflectance was performed to 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 temperature of 1550 1C and an ozone flux of 1 sccm on (a) SrTiO3(1 0 0), (b) LSAT(1 0 0), and (c) LaAlO3(1 0 0). Fig. 1(d) is a similar RHEED image to Fig. 1(c) but for a film grown on LaAlO3(1 0 0) using a Ti-cell temperature of 1525 1C (all other conditions were the same). All RHEED patterns were consistent with anatase (0 0 1) growth and exhibited a characteristic four-fold reconstruction [8,20]. Fig. 1 shows that the RHEED patterns for the films grown on LaAlO3 are sharper and Kikuchi lines are
Fig. 1. RHEED images taken along the [1 0 0] azimuth at the end of growth for TiO2 films deposited at 750 1C using an ozone flux of 1 sccm and a Ti source temperature of 1550 1C on (a) SrTiO3(1 0 0), (b) 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.
more clearly evident than those on SrTiO3 or LSAT, indicating better surface morphology and higher crystallinity for the films on LaAlO3. This is likely a result of the excellent lattice match between anatase and LaAlO3(1 0 0) (mismatch ¼ f ¼ 0.3%). A comparison of the RHEED patterns given in Figs. 1(c and d) suggests that film quality can be improved by a growth rate reduction (as was observed in [8]). However, the growth rate for the film in Fig. 1(d) was about 0.017 A˚/s, and for most applications a higher rate would be preferred.
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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 on LaAlO3(1 0 0), using an ozone flux of 1 sccm and a Ti source temperature of 1550 1C (taken along the [1 0 0] azimuth). (b) X-ray reflectivity pattern for the film whose RHEED oscillations were shown in (a).
To study if the growth could be optimized without sacrificing growth rate, the effects that other parameters had on the growth of TiO2 on LaAlO3(1 0 0) substrates (which exhibited the sharpest 2D RHEED images in Fig. 1) were investigated. RHEED intensity oscillations, one set of which is given in Fig. 2(a) for the film whose RHEED pattern was given in Fig. 1(c), were observed consistently for the films grown on LaAlO3 substrates. The thickness of this film was determined using X-ray reflectance and the scan for this film is given in Fig. 2(b). This data was refined using Philips’ WinGixa software and the film was determined to be 180 A˚ thick. Combining this with the RHEED oscillations, we calculated that one RHEED intensity oscillation corresponded to the growth of E4.5 A˚ of anatase; that value is very close to a bilayer of anatase (E4.75 A˚), in agreement with a previous report [8]. Similar behavior (meaning growth via bilayer units) was obtained for the films grown with different Ti cell temperatures (i.e., growth rates), substrate temperatures, and ozone flux values. The absence of RHEED oscillations for films grown on SrTiO3 and LSAT is reflective of the more diffuse nature and increased spottiness of the RHEED patterns, indicating that those films surfaces are rougher than surfaces of films on LaAlO3. Our goal was to understand what effects other growth parameters had on structural/surface quality. In Fig. 3, we
present RHEED images taken along the [1 0 0] azimuth for 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 the film deposited at 0.25 sccm O3. The RHEED pattern of a film deposited at the increased O3 flux of 1.0 sccm is given in Fig. 3(b). In this higher ozone flux case, the RHEED pattern is more streaky than that in Fig. 3(a). On raising the temperature from 550 to 750 1C, one obtains the RHEED pattern given in Fig. 1(c), wherein all character of a transmission spot pattern is lost, and the pattern is dominated by spots on a circle and minor streakiness, indicating an atomically smooth surface. Furthermore, the four-fold reconstruction becomes evident for the hightemperature and high-ozone flux-grown film. To better understand surface morphologies, films were studied ex-situ with AFM. In Figs. 4(a)–(c), we show AFM images of the TiO2 films whose RHEED patterns were given in Figs. 1(a)–(c), which were grown on SrTiO3, LSAT, and LaAlO3, respectively. Two types of features are observed in the AFM images: a uniform grayish contrast that represents the matrix phase (and the major surface feature) and significantly brighter spots that stand out from the uniform contrast and represent inclusions in the films. When comparing the Figs. 4(a)–(c), the grayscale changes in the uniform background decrease on going from SrTiO3 to LSAT to LaAlO3. The overall grayscale is similar, however, between all the images because both LSAT and LAO exhibit bright white features indicative of large protruding objects from the film. Electron microscopy experiments (not shown here) carried out by us on films
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Fig. 5. y–2y scans of TiO2 films on SrTiO3(1 0 0), LSAT(1 0 0), and LaAlO3(1 0 0).
Fig. 4. (a–c) are AFM images of TiO2 films grown at 750 1C using a Ti source temperature of 1550 1C and an ozone flux of 1 sccm on (a) SrTiO3(1 0 0); (b) LSAT(1 0 0); and on LaAlO3(1 0 0); (d–f) are AFM images of TiO2 films grown on LaAlO3(1 0 0) using a Ti source temperature of 1550 1C at (d) 750 1C, 0.25 sccm O3; (e) 550 1C, 1.00 sccm O3; and (f) 550 1C, 0.25 sccm O3.
deposited on LaAlO3 agreed with Chambers et al’s [8] observations that these objects are rutile inclusions that have coherent interfaces with, and protrude out from the flat surface of, the anatase matrix. Pronounced relationships between both substrate temperature and ozone flux were observed, as illustrated in Figs. 4(c–f). Figs. 4(d–f) are AFM images of TiO2 films grown on LaAlO3(1 0 0) using a Ti source temperature of 1550 1C at (d) 750 1C, 0.25 sccm O3; (e) 550 1C, 1.00 sccm O3; and (f) 550 1C, 0.25 sccm O3. At higher temperatures (650 1C and above), large inclusions became increasingly common, as observed in Figs. 4(c and d). At higher temperatures, the stable rutile phase is more likely to nucleate during growth and the metastable anatase is more likely to transform irreversibly to the stable rutile. Based on our experimental evidence from TEM and the above given thermodynamic arguments, it seems likely that the large inclusions observed in the AFM images for the hightemperature films are rutile. The inclusions greatly increase the overall root-mean-square (rms) roughness values, even
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) correspond to the RHEED images in Figs. 3(b and a), respectively. The change in film surface with ozone pressure that we previously observed by RHEED corresponded to a major change in surface roughness, going from a rms roughness of 62 A˚ at 0.25 sccm O3 to an rms roughness of 8 A˚ at 1.00 sccm. In general for the films deposited using 1.00 sccm O3, the rms roughnesses were much lower than for films grown at 0.25 sccm, increasing from 8 A˚ at T ¼ 550 1C (Fig. 4(e)) to 22 A˚ at T ¼ 650 1C (not shown) to 52 A˚ at T ¼ 750 1C (Fig. 4(c)). Note again that the increase in rms roughnesses with increasing temperature appears to arise from the increase in the large inclusions. Over the regions of the surface where there are no inclusions, the rms roughnesses were very low: 8 A˚ at T ¼ 550 1C, 4 A˚ at T ¼ 650 1C, and 7 A˚ at T ¼ 750 1C. Clearly the inclusions play a major role in the overall surface roughness. The XRD spectra of TiO2 films deposited on all 3 substrates at 750 1C using a Ti-cell temperature of 1550 1C and an ozone flux of 1 sccm are shown in Fig. 5. These XRD patterns reveal that highly (0 0 1)-oriented anatase films grew on SrTiO3 (1 0 0), LSAT (1 0 0), and LaAlO3 (1 0 0) substrates. In fact, similar XRD results were observed by us for anatase films deposited over a wide range of conditions (4501oTdo750 1C and 0.25 o flux O3 o 2.00 sccm). f-scans showed that these films were all epitaxial and all had the same relationship:
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(0 0 1)TiO2 ||(1 0 0)Subs; [0 1 0]TiO2 ||[0 1 0]Subs. The full-widths at half-maximum (FWHM) of the rocking curve on the anatase (0 0 4) peaks were largest for the films on SrTiO3(1 0 0) (E0.71), intermediate on LSAT (0.11), and lowest (E0.031) on LaAlO3(1 0 0). The FWHM for the substrates themselves were all E0.021 under our diffraction conditions, indicating that the films on LaAlO3 substrates were 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 those observed in other reports using other deposition methods [11,16] and using MBE deposition [8], although no other reports exist on MBE growth of TiO2 films on the LSAT(1 0 0) surface. It should be pointed out that under the growth conditions investigated here, temperature seemed to play the largest role on inclusion formation; substrate choice, ozone flux, and Ti-cell temperature played secondary roles, if any. Interestingly, the inclusions did not appear to affect either the RHEED pattern or the XRD pattern in an obvious manner. Nevertheless, electron microscopy experiments carried out by us on films grown on LaAlO3 (not shown here) agreed with observations given in Ref. [8] demonstrating that rutile inclusions protrude out of the matrix anatase phase in films grown at high temperatures. It should be noted that no rutile inclusions are observed in pure TiO2 films deposited using pulsed laser deposition on LaAlO3(1 0 0) substrates at T ¼ 750 1C in elevated pressures of pure oxygen (PE200 m Torr O2). Further work is required to understand the thermodynamics and kinetics of the nucleation and growth of these rutile inclusions during thin film deposition. In conclusion, TiO2 thin films were grown on SrTiO3 (1 0 0), LSAT (1 0 0), and LaAlO3 (1 0 0) substrates by reactive MBE and characterized in-situ by RHEED and exsitu by XRD and AFM. Films grown on SrTiO3 (1 0 0), LSAT (1 0 0), and LaAlO3 (1 0 0) grew as (0 0 1) anatase, which was found to grow over a range of temperatures (450–750 1C). It was observed that the films’ structural quality and morphology highly depended on substrate temperature and ozone flux. Films were found to grow with a smoother surface at higher ozone fluxes, and films grown at higher substrate temperatures exhibited large inclusions. It is interesting that these results indicate that the values for ozone flux, Ti flux, and temperature have not been optimized for the growth of high-quality anatase TiO2 exhibiting very narrow rocking curves. To realize such films, a better understanding is required of the thermodynamic and kinetic processes that occur in MBE and that govern rutile nucleation and growth on perovskites surfaces. Acknowledgements This work was supported by the Office of Naval Research under grants N0001-05-1-0238, N00014-05-1-0152, and
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N00014-03-1-0665. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the Office of Naval Research. It also made use of the facilities maintained through the MRSEC program of the National Science Foundation under Award no. DMR-0520425.
References [1] J.G.E. Jellison, F.A. Modine, L.A. Boatner, Opt. Lett. 22 (1997) 1808–1810. [2] M.R. Hoffman, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69–96. [3] A.L. Linsebigler, G. Lu, J. John, T. Yates, Chem. Rev. 95 (1995) 735–758. [4] M. Koelsch, S. Cassaignon, J.F. Guillemoles, J.P. Olivet, Thin Solid Films 403–404 (2002) 312–319. [5] P.A.M. Hotsenspiller, G.A. Wilson, A. Roshko, J.B. Rothman, G.S. Rohrer, J. Cryst. Growth 166 (1996) 779–785. [6] D.R. Burgess, P.A.M. Hotsenspiller, T.J. Anderson, J.L. Hohman, J. Cryst. Growth 166 (1996) 763–768. [7] H.L.M. Chang, H. You, Y. Gao, J. Guo, C.M. Foster, R.P. Chiarello, T.J. Zhang, D.J. Lam, J. Mater. Res. 7 (1992) 2495–2506. [8] S.A. Chambers, C.M. Wang, S. Thevuthasan, T. Droubay, D.E. McCready, A.S. Lea, V. Shutthanandan, J.C.F. Windisch, Thin Solid Films 418 (2002) 197–210. [9] S. Yamamoto, T. Sumita, T. Yamaki, A. Miyashita, H. Naramoto, J. Cryst. Growth 237–239 (2002) 569–573. [10] T. Aoki, K. Maki, Q. Tang, Y. Kumagai, S. Matsumoto, J. Vacuum Sci. Technol. A 15 (1997) 2485–2488. [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, P.I. Cohen, J.F. Evans, J. Vacuum Sci. Technol. A 11 (1993) 2419–2429. [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. [13] P.J. Hansen, V. Vaithyanathan, Y. Wu, T. Mates, S. Heikman, U.K. Mishra, R.A. York, D.G. Schlom, J.S. Speck, J. Vacuum Sci. Technol. B 23 (2005) 499–506. [14] X. Liu, X.Y. Chen, J. Yin, Z.G. Liu, J.M. Liu, X.B. Yin, G.X. Chen, J. Vacuum Sci. Technol. A 19 (2001) 391–393. [15] Sugiharto, S. Yamamoto, T. Sumita, A. Miyashita, J. Phys. Condens. Matter 13 (2001) 2875–2881. [16] Y. Gao, S. Thevuthasan, D.E. McCready, M. Engelhard, J. Cryst. Growth 212 (2000) 178–190. [17] G.S. Herman, Y. Gao, T.T. Tran, J. Osterwalder, Surf. Sci. 447 (2000) 201–211. [18] M. Kamei, T. Mitsuhashi, Surf. Sci. 463 (2000) L609–L612. [19] J.H. Haeni, C.D. Theis, D.G. Schlom, W. Tian, X.Q. Pan, H. Chang, I. Takeuchi, X.D. Xiang, Appl. Phys. Lett. 78 (2001) 3292–3294. [20] W. Gao, C.M. Wang, H.Q. Wang, V.E. Henrich, E.I. Altman, Surf. 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. [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. [23] A. Asthagiri, C. Niederberger, A.J. Francis, L.M. Porter, P.A. Salvador, D.S. Sholl, Surf. Sci. 537 (2003) 134–152.
Microelectronics Journal 37 (2006) 1493–1497 www.elsevier.com/locate/mejo
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, 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 2006
Abstract TiO2 films were grown using a reactive molecular beam epitaxy system equipped with high-temperature effusion cells as sources for Ti and 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 and atomic 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 that smoother surfaces were observed for those grown at higher ozone fluxes. Moreover, while RHEED patterns indicated that anatase films grown 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.Nz Keywords: Titanium dioxide; MBE; Thin film epitaxy
There is a wide technological interest in TiO2 because it has interesting physical and chemical properties, which make it appealing for optical, dielectric, electrochemical, and photocatalytic applications [1–3]. TiO2 is also of scientific interest since it has polymorphic structures (anatase, rutile, and brookite) that exhibit different stabilities and properties [4,5]. Certain applications, such as integrated dielectrics or photoelectrochemical cells, require thin films of TiO2 that exhibit a specific crystal structure, orientation, and/or morphology. Thus, there are a number of reports on the growth of TiO2 films on various substrates, including Al2O3 [5–7], LaAlO3 [8], (La, Sr)(Al, Ta)O3 (LSAT) [9], MgO [5,10], SrTiO3 [11], BaTiO3 [12], (Zr, Y)O2 [9], and GaN [13]. Depending on the choices of substrate and deposition conditions, both rutile and/or anatase can be grown. Corresponding author. Tel.: +1 412 268 2702; fax: +1 412 268 7596.
E-mail address: [email protected] (P.A. Salvador). 0026-2692/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2006.05.010
Most previously reported films were grown by pulsed laser deposition [14,15], metal organic chemical vapor deposition [6,7,11,16,17], or sputtering [5,10,18]. Molecular beam epitaxy (MBE), a technique that differs in thermodynamics and kinetics from the other approaches and that can produce high quality films, has been limited to growth of TiO2 on LaAlO3 [8], SrTiO3 [8], and GaN [13] substrates. MBE also allows the integration of charge neutral TiO2 monolayers with other chemical and structural layers, such as SrO in SrTiO3 [19]. Importantly, differences in sources used in MBE can vary the thermodynamic/kinetic aspects of the growth and, therefore, impact film properties. Oxide MBE requires a special oxygen source to completely oxidize deposited metal, to prevent source oxidation, and to allow the use of in-situ electron probes. For this purpose, RF or microwave plasma sources are often used instead of molecular oxygen (O2) [8,20]. However, plasma sources produce highly energetic species
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and have limited flux control [21]. An alternative approach is to use pure ozone (O3) or an O3/O2 gas mixture. Ozone flux is easily controlled with a mass flow controller and can be directed towards the heated substrate surface where it is thermally cracked into O and O2. There have only been a limited number of reports describing the influence of ozone flux on the growth of oxide films, particularly for the cases with no obvious oxidation problem, such as TiO2. In this study, we have grown TiO2 films on various single crystal substrates using a MBE system equipped with a high-temperature effusion cell as a source of Ti and an ozone distillation system to provide pure ozone. This paper describes the effects of substrate, growth temperature, and ozone flux on crystal structure, orientation, phase assemblage, and morphology of 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 solution for 2–3 min [22], rinsed in deionized water, and then degreased prior to the growth. Quarters of 2-inch wafers were mounted into an Inconel sample holder and introduced into the growth chamber (SVT Associates). Substrates were heated to 750 1C using a resistive boron nitride heater (temperature is measured with a thermocouple located in close proximity to the substrate) and annealed at 750 1C for 1 h under the deposition ozone flux. Ozone was generated with a commercial unit (Ozone Solutions) capable of producing 6% O3 in O2. It was distilled by passing the O2/O3 mixture through the liquidnitrogen cooled dewar filled with silica gel; the O3 was adsorbed while the remnant O2 was pumped away. After storing sufficient amount, the pure ozone stream was generated by warming the dewar. The high-concentration O3 flux was introduced into the chamber (base pressure of 1010 Torr) using a mass flow controller at a rate of 0.25–2 sccm, which corresponded to a pressure of 6 106– 2.5 105 Torr. The Ti flux was produced with a hightemperature effusion cell operated at a fixed temperature between 1500 and 1600 1C. During each growth run, the process was monitored using a differentially pumped RHEED system (Staib Instruments) operated at 12.0 kV with an incident angle of 31. X-ray diffraction (XRD) measurements [23] were made in y–2y, o, and f-scans modes. Atomic force microscopy (AFM) was carried out in contact mode [23]. X-ray reflectance was performed to 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 temperature of 1550 1C and an ozone flux of 1 sccm on (a) SrTiO3(1 0 0), (b) LSAT(1 0 0), and (c) LaAlO3(1 0 0). Fig. 1(d) is a similar RHEED image to Fig. 1(c) but for a film grown on LaAlO3(1 0 0) using a Ti-cell temperature of 1525 1C (all other conditions were the same). All RHEED patterns were consistent with anatase (0 0 1) growth and exhibited a characteristic four-fold reconstruction [8,20]. Fig. 1 shows that the RHEED patterns for the films grown on LaAlO3 are sharper and Kikuchi lines are
Fig. 1. RHEED images taken along the [1 0 0] azimuth at the end of growth for TiO2 films deposited at 750 1C using an ozone flux of 1 sccm and a Ti source temperature of 1550 1C on (a) SrTiO3(1 0 0), (b) 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.
more clearly evident than those on SrTiO3 or LSAT, indicating better surface morphology and higher crystallinity for the films on LaAlO3. This is likely a result of the excellent lattice match between anatase and LaAlO3(1 0 0) (mismatch ¼ f ¼ 0.3%). A comparison of the RHEED patterns given in Figs. 1(c and d) suggests that film quality can be improved by a growth rate reduction (as was observed in [8]). However, the growth rate for the film in Fig. 1(d) was about 0.017 A˚/s, and for most applications a higher rate would be preferred.
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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 on LaAlO3(1 0 0), using an ozone flux of 1 sccm and a Ti source temperature of 1550 1C (taken along the [1 0 0] azimuth). (b) X-ray reflectivity pattern for the film whose RHEED oscillations were shown in (a).
To study if the growth could be optimized without sacrificing growth rate, the effects that other parameters had on the growth of TiO2 on LaAlO3(1 0 0) substrates (which exhibited the sharpest 2D RHEED images in Fig. 1) were investigated. RHEED intensity oscillations, one set of which is given in Fig. 2(a) for the film whose RHEED pattern was given in Fig. 1(c), were observed consistently for the films grown on LaAlO3 substrates. The thickness of this film was determined using X-ray reflectance and the scan for this film is given in Fig. 2(b). This data was refined using Philips’ WinGixa software and the film was determined to be 180 A˚ thick. Combining this with the RHEED oscillations, we calculated that one RHEED intensity oscillation corresponded to the growth of E4.5 A˚ of anatase; that value is very close to a bilayer of anatase (E4.75 A˚), in agreement with a previous report [8]. Similar behavior (meaning growth via bilayer units) was obtained for the films grown with different Ti cell temperatures (i.e., growth rates), substrate temperatures, and ozone flux values. The absence of RHEED oscillations for films grown on SrTiO3 and LSAT is reflective of the more diffuse nature and increased spottiness of the RHEED patterns, indicating that those films surfaces are rougher than surfaces of films on LaAlO3. Our goal was to understand what effects other growth parameters had on structural/surface quality. In Fig. 3, we
present RHEED images taken along the [1 0 0] azimuth for 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 the film deposited at 0.25 sccm O3. The RHEED pattern of a film deposited at the increased O3 flux of 1.0 sccm is given in Fig. 3(b). In this higher ozone flux case, the RHEED pattern is more streaky than that in Fig. 3(a). On raising the temperature from 550 to 750 1C, one obtains the RHEED pattern given in Fig. 1(c), wherein all character of a transmission spot pattern is lost, and the pattern is dominated by spots on a circle and minor streakiness, indicating an atomically smooth surface. Furthermore, the four-fold reconstruction becomes evident for the hightemperature and high-ozone flux-grown film. To better understand surface morphologies, films were studied ex-situ with AFM. In Figs. 4(a)–(c), we show AFM images of the TiO2 films whose RHEED patterns were given in Figs. 1(a)–(c), which were grown on SrTiO3, LSAT, and LaAlO3, respectively. Two types of features are observed in the AFM images: a uniform grayish contrast that represents the matrix phase (and the major surface feature) and significantly brighter spots that stand out from the uniform contrast and represent inclusions in the films. When comparing the Figs. 4(a)–(c), the grayscale changes in the uniform background decrease on going from SrTiO3 to LSAT to LaAlO3. The overall grayscale is similar, however, between all the images because both LSAT and LAO exhibit bright white features indicative of large protruding objects from the film. Electron microscopy experiments (not shown here) carried out by us on films
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Fig. 5. y–2y scans of TiO2 films on SrTiO3(1 0 0), LSAT(1 0 0), and LaAlO3(1 0 0).
Fig. 4. (a–c) are AFM images of TiO2 films grown at 750 1C using a Ti source temperature of 1550 1C and an ozone flux of 1 sccm on (a) SrTiO3(1 0 0); (b) LSAT(1 0 0); and on LaAlO3(1 0 0); (d–f) are AFM images of TiO2 films grown on LaAlO3(1 0 0) using a Ti source temperature of 1550 1C at (d) 750 1C, 0.25 sccm O3; (e) 550 1C, 1.00 sccm O3; and (f) 550 1C, 0.25 sccm O3.
deposited on LaAlO3 agreed with Chambers et al’s [8] observations that these objects are rutile inclusions that have coherent interfaces with, and protrude out from the flat surface of, the anatase matrix. Pronounced relationships between both substrate temperature and ozone flux were observed, as illustrated in Figs. 4(c–f). Figs. 4(d–f) are AFM images of TiO2 films grown on LaAlO3(1 0 0) using a Ti source temperature of 1550 1C at (d) 750 1C, 0.25 sccm O3; (e) 550 1C, 1.00 sccm O3; and (f) 550 1C, 0.25 sccm O3. At higher temperatures (650 1C and above), large inclusions became increasingly common, as observed in Figs. 4(c and d). At higher temperatures, the stable rutile phase is more likely to nucleate during growth and the metastable anatase is more likely to transform irreversibly to the stable rutile. Based on our experimental evidence from TEM and the above given thermodynamic arguments, it seems likely that the large inclusions observed in the AFM images for the hightemperature films are rutile. The inclusions greatly increase the overall root-mean-square (rms) roughness values, even
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) correspond to the RHEED images in Figs. 3(b and a), respectively. The change in film surface with ozone pressure that we previously observed by RHEED corresponded to a major change in surface roughness, going from a rms roughness of 62 A˚ at 0.25 sccm O3 to an rms roughness of 8 A˚ at 1.00 sccm. In general for the films deposited using 1.00 sccm O3, the rms roughnesses were much lower than for films grown at 0.25 sccm, increasing from 8 A˚ at T ¼ 550 1C (Fig. 4(e)) to 22 A˚ at T ¼ 650 1C (not shown) to 52 A˚ at T ¼ 750 1C (Fig. 4(c)). Note again that the increase in rms roughnesses with increasing temperature appears to arise from the increase in the large inclusions. Over the regions of the surface where there are no inclusions, the rms roughnesses were very low: 8 A˚ at T ¼ 550 1C, 4 A˚ at T ¼ 650 1C, and 7 A˚ at T ¼ 750 1C. Clearly the inclusions play a major role in the overall surface roughness. The XRD spectra of TiO2 films deposited on all 3 substrates at 750 1C using a Ti-cell temperature of 1550 1C and an ozone flux of 1 sccm are shown in Fig. 5. These XRD patterns reveal that highly (0 0 1)-oriented anatase films grew on SrTiO3 (1 0 0), LSAT (1 0 0), and LaAlO3 (1 0 0) substrates. In fact, similar XRD results were observed by us for anatase films deposited over a wide range of conditions (4501oTdo750 1C and 0.25 o flux O3 o 2.00 sccm). f-scans showed that these films were all epitaxial and all had the same relationship:
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(0 0 1)TiO2 ||(1 0 0)Subs; [0 1 0]TiO2 ||[0 1 0]Subs. The full-widths at half-maximum (FWHM) of the rocking curve on the anatase (0 0 4) peaks were largest for the films on SrTiO3(1 0 0) (E0.71), intermediate on LSAT (0.11), and lowest (E0.031) on LaAlO3(1 0 0). The FWHM for the substrates themselves were all E0.021 under our diffraction conditions, indicating that the films on LaAlO3 substrates were 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 those observed in other reports using other deposition methods [11,16] and using MBE deposition [8], although no other reports exist on MBE growth of TiO2 films on the LSAT(1 0 0) surface. It should be pointed out that under the growth conditions investigated here, temperature seemed to play the largest role on inclusion formation; substrate choice, ozone flux, and Ti-cell temperature played secondary roles, if any. Interestingly, the inclusions did not appear to affect either the RHEED pattern or the XRD pattern in an obvious manner. Nevertheless, electron microscopy experiments carried out by us on films grown on LaAlO3 (not shown here) agreed with observations given in Ref. [8] demonstrating that rutile inclusions protrude out of the matrix anatase phase in films grown at high temperatures. It should be noted that no rutile inclusions are observed in pure TiO2 films deposited using pulsed laser deposition on LaAlO3(1 0 0) substrates at T ¼ 750 1C in elevated pressures of pure oxygen (PE200 m Torr O2). Further work is required to understand the thermodynamics and kinetics of the nucleation and growth of these rutile inclusions during thin film deposition. In conclusion, TiO2 thin films were grown on SrTiO3 (1 0 0), LSAT (1 0 0), and LaAlO3 (1 0 0) substrates by reactive MBE and characterized in-situ by RHEED and exsitu by XRD and AFM. Films grown on SrTiO3 (1 0 0), LSAT (1 0 0), and LaAlO3 (1 0 0) grew as (0 0 1) anatase, which was found to grow over a range of temperatures (450–750 1C). It was observed that the films’ structural quality and morphology highly depended on substrate temperature and ozone flux. Films were found to grow with a smoother surface at higher ozone fluxes, and films grown at higher substrate temperatures exhibited large inclusions. It is interesting that these results indicate that the values for ozone flux, Ti flux, and temperature have not been optimized for the growth of high-quality anatase TiO2 exhibiting very narrow rocking curves. To realize such films, a better understanding is required of the thermodynamic and kinetic processes that occur in MBE and that govern rutile nucleation and growth on perovskites surfaces. Acknowledgements This work was supported by the Office of Naval Research under grants N0001-05-1-0238, N00014-05-1-0152, and
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N00014-03-1-0665. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the Office of Naval Research. It also made use of the facilities maintained through the MRSEC program of the National Science Foundation under Award no. DMR-0520425.
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