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Microelectronics Journal 37 (2006) 1493–1497

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

Patrick Fishera, Oleg Maksimovb, Hui Dua, Volker D. Heydemannb,Marek Skowronskia, Paul A. Salvadora,�

aDepartment of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USAbElectro-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 ithas interesting physical and chemical properties, whichmake it appealing for optical, dielectric, electrochemical,and photocatalytic applications [1–3]. TiO2 is also ofscientific interest since it has polymorphic structures(anatase, rutile, and brookite) that exhibit differentstabilities and properties [4,5]. Certain applications, suchas integrated dielectrics or photoelectrochemical cells,require thin films of TiO2 that exhibit a specific crystalstructure, orientation, and/or morphology. Thus, there area number of reports on the growth of TiO2 films on varioussubstrates, 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 ofsubstrate and deposition conditions, both rutile and/oranatase can be grown.

e front matter r 2006 Elsevier Ltd. All rights reserved.

ejo.2006.05.010

ing author. Tel.: +1412 268 2702; fax: +1 412 268 7596.

ess: paul7@andrew.cmu.edu (P.A. Salvador).

Most previously reported films were grown by pulsedlaser deposition [14,15], metal organic chemical vapordeposition [6,7,11,16,17], or sputtering [5,10,18]. Molecularbeam epitaxy (MBE), a technique that differs in thermo-dynamics and kinetics from the other approaches and thatcan produce high quality films, has been limited to growthof TiO2 on LaAlO3 [8], SrTiO3 [8], and GaN [13]substrates. MBE also allows the integration of chargeneutral TiO2 monolayers with other chemical and structur-al layers, such as SrO in SrTiO3 [19]. Importantly,differences in sources used in MBE can vary the thermo-dynamic/kinetic aspects of the growth and, therefore,impact film properties.Oxide MBE requires a special oxygen source to

completely oxidize deposited metal, to prevent sourceoxidation, and to allow the use of in-situ electron probes.For this purpose, RF or microwave plasma sources areoften used instead of molecular oxygen (O2) [8,20].However, plasma sources produce highly energetic species

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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.

P. Fisher et al. / Microelectronics Journal 37 (2006) 1493–14971494

and 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 2–3min [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 of10�10 Torr) using a mass flow controller at a rate of0.25–2 sccm, which corresponded to a pressure of 6� 10�6–2.5� 10�5 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)measurements [23] were made in y–2y, o, and f-scansmodes. Atomic force microscopy (AFM) was carried out incontact mode [23]. X-ray reflectance was performed todetermine film thickness.

Fig. 1 shows RHEED images, taken along the [1 0 0]azimuth, of anatase films grown at 750 1C using a Ti-celltemperature 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 fora film grown on LaAlO3(1 0 0) using a Ti-cell temperatureof 1525 1C (all other conditions were the same). AllRHEED patterns were consistent with anatase (0 0 1)growth and exhibited a characteristic four-fold reconstruc-tion [8,20]. Fig. 1 shows that the RHEED patterns for thefilms grown on LaAlO3 are sharper and Kikuchi lines are

more clearly evident than those on SrTiO3 or LSAT,indicating better surface morphology and higher crystal-linity for the films on LaAlO3. This is likely a result of theexcellent 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 agrowth 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. 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).

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.

P. Fisher et al. / Microelectronics Journal 37 (2006) 1493–1497 1495

To study if the growth could be optimized withoutsacrificing growth rate, the effects that other parametershad 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 givenin Fig. 2(a) for the film whose RHEED pattern was givenin Fig. 1(c), were observed consistently for the films grownon LaAlO3 substrates. The thickness of this film wasdetermined using X-ray reflectance and the scan for thisfilm is given in Fig. 2(b). This data was refined usingPhilips’ WinGixa software and the film was determined tobe 180 A thick. Combining this with the RHEED oscilla-tions, we calculated that one RHEED intensity oscillationcorresponded to the growth of E4.5 A of anatase; thatvalue is very close to a bilayer of anatase (E4.75 A), inagreement with a previous report [8]. Similar behavior(meaning growth via bilayer units) was obtained for thefilms grown with different Ti cell temperatures (i.e., growthrates), substrate temperatures, and ozone flux values. Theabsence of RHEED oscillations for films grown on SrTiO3

and LSAT is reflective of the more diffuse nature andincreased spottiness of the RHEED patterns, indicatingthat those films surfaces are rougher than surfaces of filmson LaAlO3.

Our goal was to understand what effects other growthparameters had on structural/surface quality. In Fig. 3, we

present RHEED images taken along the [1 0 0] azimuth foranatase films grown on LaAlO3 under different ozonefluxes (at T ¼ 550 1C and at a Ti cell temperature ¼1550 1C). In Fig. 3(a), a spotty pattern is observed for thefilm deposited at 0.25 sccm O3. The RHEED pattern of afilm deposited at the increased O3 flux of 1.0 sccm is givenin Fig. 3(b). In this higher ozone flux case, the RHEEDpattern is more streaky than that in Fig. 3(a). On raisingthe temperature from 550 to 750 1C, one obtains theRHEED pattern given in Fig. 1(c), wherein all character ofa transmission spot pattern is lost, and the pattern isdominated by spots on a circle and minor streakiness,indicating an atomically smooth surface. Furthermore, thefour-fold reconstruction becomes evident for the high-temperature 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 AFMimages of the TiO2 films whose RHEED patterns weregiven in Figs. 1(a)–(c), which were grown on SrTiO3,LSAT, and LaAlO3, respectively. Two types of features areobserved in the AFM images: a uniform grayish contrastthat represents the matrix phase (and the major surfacefeature) and significantly brighter spots that stand out fromthe uniform contrast and represent inclusions in the films.When comparing the Figs. 4(a)–(c), the grayscale changesin 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 andLAO exhibit bright white features indicative of largeprotruding objects from the film. Electron microscopyexperiments (not shown here) carried out by us on films

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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.

Fig. 5. y–2y scans of TiO2 films on SrTiO3(1 0 0), LSAT(1 0 0), and

LaAlO3(1 0 0).

P. Fisher et al. / Microelectronics Journal 37 (2006) 1493–14971496

deposited on LaAlO3 agreed with Chambers et al’s [8]observations that these objects are rutile inclusions thathave coherent interfaces with, and protrude out from theflat surface of, the anatase matrix.

Pronounced relationships between both substrate tem-perature and ozone flux were observed, as illustrated inFigs. 4(c–f). Figs. 4(d–f) are AFM images of TiO2 filmsgrown on LaAlO3(1 0 0) using a Ti source temperature of1550 1C at (d) 750 1C, 0.25 sccm O3; (e) 550 1C, 1.00 sccmO3; and (f) 550 1C, 0.25 sccm O3. At higher temperatures(650 1C and above), large inclusions became increasinglycommon, as observed in Figs. 4(c and d). At highertemperatures, the stable rutile phase is more likely tonucleate during growth and the metastable anatase is morelikely to transform irreversibly to the stable rutile. Basedon our experimental evidence from TEM and the abovegiven thermodynamic arguments, it seems likely that thelarge inclusions observed in the AFM images for the high-temperature films are rutile. The inclusions greatly increasethe overall root-mean-square (rms) roughness values, even

though the surface of the anatase matrix is tending toflatten out at higher temperatures. For films deposited onLaAlO3 using the lower ozone flux of 0.25 sccm (Figs. 4(dand f)), the rms roughness increased somewhat withsubstrate 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 pressurethat we previously observed by RHEED corresponded to amajor change in surface roughness, going from a rmsroughness of 62 A at 0.25 sccm O3 to an rms roughness of8 A at 1.00 sccm. In general for the films deposited using1.00 sccm O3, the rms roughnesses were much lower thanfor films grown at 0.25 sccm, increasing from 8 A atT 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 theincrease in rms roughnesses with increasing temperatureappears to arise from the increase in the large inclusions.Over the regions of the surface where there are noinclusions, the rms roughnesses were very low: 8 A atT 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 overallsurface roughness.The XRD spectra of TiO2 films deposited on all 3

substrates at 750 1C using a Ti-cell temperature of 1550 1Cand an ozone flux of 1 sccm are shown in Fig. 5. TheseXRD patterns reveal that highly (0 0 1)-oriented anatasefilms grew on SrTiO3 (1 0 0), LSAT (1 0 0), and LaAlO3

(1 0 0) substrates. In fact, similar XRD results wereobserved by us for anatase films deposited over a widerange of conditions (4501oTdo750 1C and 0.25 o flux O3

o 2.00 sccm). f-scans showed that these films wereall epitaxial and all had the same relationship:

ARTICLE IN PRESSP. 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-widthsat half-maximum (FWHM) of the rocking curve on theanatase (0 0 4) peaks were largest for the films onSrTiO3(1 0 0) (E0.71), intermediate on LSAT (0.11), andlowest (E0.031) on LaAlO3(1 0 0). The FWHM for thesubstrates themselves were allE0.021 under our diffractionconditions, 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 tothose observed in other reports using other depositionmethods [11,16] and using MBE deposition [8], althoughno other reports exist on MBE growth of TiO2 films on theLSAT(1 0 0) surface.

It should be pointed out that under the growthconditions investigated here, temperature seemed to playthe 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 affecteither the RHEED pattern or the XRD pattern in anobvious manner. Nevertheless, electron microscopy experi-ments carried out by us on films grown on LaAlO3 (notshown here) agreed with observations given in Ref. [8]demonstrating that rutile inclusions protrude out of thematrix anatase phase in films grown at high temperatures.It should be noted that no rutile inclusions are observed inpure TiO2 films deposited using pulsed laser deposition onLaAlO3(1 0 0) substrates at T ¼ 750 1C in elevated pres-sures of pure oxygen (PE200mTorr O2). Further work isrequired to understand the thermodynamics and kinetics ofthe nucleation and growth of these rutile inclusions duringthin film deposition.

In conclusion, TiO2 thin films were grown on SrTiO3

(1 0 0), LSAT (1 0 0), and LaAlO3 (1 0 0) substrates byreactive MBE and characterized in-situ by RHEED and ex-situ 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’ structuralquality and morphology highly depended on substratetemperature and ozone flux. Films were found to grow witha smoother surface at higher ozone fluxes, and films grownat higher substrate temperatures exhibited large inclusions.It is interesting that these results indicate that the values forozone flux, Ti flux, and temperature have not beenoptimized for the growth of high-quality anatase TiO2

exhibiting very narrow rocking curves. To realize such films,a better understanding is required of the thermodynamicand kinetic processes that occur in MBE and that governrutile nucleation and growth on perovskites surfaces.

Acknowledgements

This work was supported by the Office of Naval Researchunder grants N0001-05-1-0238, N00014-05-1-0152, and

N00014-03-1-0665. Any opinions, findings, and conclusionsor recommendations expressed in this material are those ofthe authors and do not necessarily reflect the views of theOffice of Naval Research. It also made use of the facilitiesmaintained through the MRSEC program of the NationalScience Foundation under Award no. DMR-0520425.

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