Journal of Crystal Growth 310 (2008) 545–550

1. ARTICLE IN PRESS
Journal of Crystal Growth 310 (2008) 545–550
www.elsevier.com/locate/jcrysgro
Structural 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 2007
Abstract
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). TiO2
films grew epitaxially in the anatase polymorph and exhibited the crystallographic orientation relation of
ð0 0 1Þð0 1 0ÞTiO2 jjð0 0 1Þð0 1 0Þsubstrate . High-resolution TEM and EELS studies indicated the presence of a cubic TiOx phase at the
TiO2/STO interface. Interfacial TiOx phases were eliminated and a sharp TiO2/STO interface was achieved by growing the TiO2 film on a
heteroepitaxial STO buffer layer.
r 2007 Elsevier B.V. All rights reserved.
PACS: 68.37.Lp; 61.72.Ff; 81.15.Hi
Keywords: A1. Electron Energy Loss Spectrometry; A1. Transmission Electron Microscopy; A3. Molecular Beam Epitaxy; B1. TiO2
1. 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 crystalline
dielectrics for microelectronic devices [4]. Often such defects and external interfaces may cause spin-flips in
applications 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 and
material 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 of
brookite, 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 have
photocatalytic 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 laser
addition, Co-doped anatase shows the highest Curie deposition (PLD) [15,16], and molecular beam epitaxy
temperature 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: maksimov@netzero.net (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. ARTICLE IN PRESS
546 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 discussion
the MBE growth of TiO2. To compensate for the low
activity of molecular oxygen, oxygen plasma sources are 3.1. TiO2 film on LAO
generally used to supply activated species (OPA–MBE) and
to 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/LAO
AlGaN templates) [20] and oxide (LAO, LSAT, and STO) heterostructures were previously grown using OPA-MBE
substrates [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 this
films using a combination of reflection high-energy elec- work, as well as a baseline for understanding the TiO2/STO
tron diffraction (RHEED), X-ray diffraction (XRD), films. Fig. 1 shows (a) a low-magnification cross-section
and 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 interfacial
energy loss spectrometry (EELS) investigations of the region of the TiO2/LAO heterostructures. As shown in
crystal and interfacial structure of such TiO2 films grown Fig. 1(a), the film has a domain structure. The domain size
on LAO and STO substrates. For films on STO, we is 100 nm and domain boundaries are parallel to the
observe an interfacial phase between the epitaxial anatase
layer 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 the
removal of the interfacial phase for the latter TiO2/STO
interface.
2. Experimental procedure
Films were grown as described previously [21] on
commercially available (0 0 1) LAO and (0 0 1) STO
wafers (MTI Corporation) in an MBE system equipped
with the high-temperature Ti effusion cells, a low-
temperature Sr effusion cell, and an ozone distillation
system (SVT Associates) [21]. All substrates were
etched ex-situ in a 3:1 HCl:HNO3 solution for 2–3 min
and annealed in-situ prior to the growth for 1 h at 750 1C
under an ozone/oxygen flux of 0.5 sccm. Identical condi-
tions were used for the growth of TiO2 films and STO
buffer layers, except that for the latter the Sr-source was
also operated in a fashion to yield co-deposited films of
SrTiO3 stoichiometry. The growth rates, determined using
X-ray reflectivity, were 6 and 13 nm/h for TiO2 and
STO, respectively. The growth was monitored in-situ
using a differentially pumped RHEED system. We
previously demonstrated that the RHEED patterns
confirmed epitaxial growth of high-quality TiO2 films
[21]. For TEM and EELS experiments, cross-sectional
specimens were prepared using conventional mechanical
thinning followed by an argon ion milling. TEM imaging,
electron diffraction, and EELS were carried out on a JEOL
2010F field-emission microscope equipped with a Gatan
EnfinaTM 1000 EELS system. All EEL spectra were
collected in diffraction mode (image coupling) with a
dispersion of 0.2 eV/channel and a resolution of 1.5 eV.
The electron beam convergence semiangle (a) and the
EELS collection semiangle (b) were 2.5 and 6.9 mrad, Fig. 1. (a) Low-magnification cross-section TEM image, (b) the SAD
respectively. The microscope was operated at 200 keV for pattern and (c) a high-resolution TEM image of the interface region of the
all experiments. TiO2/LAO heterostructure.

3. ARTICLE IN PRESS
X. Weng et al. / Journal of Crystal Growth 310 (2008) 545–550 547
growth direction (perpendicular to the substrate interface).
The film surface is flat, except for trenches present at the
intersection of the domain boundaries with the surface.
Fringe-like contrast modulation is also evident within the
TiO2 domains. Such contrast modulation often appears in
TiO2 thin films [19], and is likely related to the existence of
crystallographic shear planes [22].
The crystallographic relationship between the TiO2 film
and the LAO substrate is determined by SAD. Fig. 1(b)
shows a typical SAD pattern collected from a region
containing both the film and the substrate. It is obvious
that the film and the substrate have an ð0 0 1Þ ð0 1 0ÞTiO2 jj
ð0 0 1Þ ð0 1 0ÞLAO orientation relationship, consistent with
prior 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 TiO2
films grown on LAO by OPA–MBE [19].
In Fig. 1(a), a 1–2 nm thick layer of a light contrast (as
compared to the rest of the film) is evident at the TiO2/
LAO interface. Such contrast may indicate the presence of
nanometer-scale disordered regions at the interface, similar
to those observed in the OPA–MBE-grown TiO2/LAO
heterostructure [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 observed
immediately 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 jj
monolayers. At this point, intensity oscillations become ð0 0 1Þ ð0 1 0ÞSTO , despite the presence of the interfacial
evident, indicating that further growth continues in a layer- layer.
by-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 STO
structure of the interface between the TiO2 film and the substrate. Fig. 3(a) shows a high-resolution TEM image
LAO substrate. An atomically flat interface is evident in collected near the hole of the TEM specimen. A crystalline
the representative high-resolution image shown in Fig. 1(c). interface layer is present between the anatase TiO2 layer
Furthermore, there are no second phases at the interface. and the STO substrate. Note that in this image, the top
The absence of obviously disordered regions in Fig. 1(c) part of the film (denoted as the surface layer) has a crystal
may be due to their overlap with crystalline TiO2 regions. structure different from anatase. Such a surface layer was
On the other hand, this also suggests that the size and the not observed in regions located away from the hole of the
areal density of the disordered regions in the film grown TEM specimen. Therefore, the surface layer in Fig. 3(a) has
using reactive MBE are lower than those in the film grown formed during the ion-milling process, most probably
using OPA–MBE. owing to ion damage, which is severe at the thin region
near the specimen hole. Careful measurements reveal that
3.2. 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 have
patches) 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-damaged
Fig. 2(a). The TiO2 film contains small domains and has a surface layer. For the STO substrate, both the Ti–L2 and
relatively rough surface when compared with the film Ti–L3 edges exhibit a characteristic splitting and energy
grown 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 is
there 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 those
from a region consisting of both the film and the substrate. of the L-edges of STO: to a small degree for the anatase
Similar to the TiO2/LAO heterostructure described TiO2 layer and to a significantly larger degree for the
earlier, the TiO2/STO heterostructure shows a crystal- interface and surface layers.

4. ARTICLE IN PRESS
548 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 heterostructure
Fig. 3. (a) High-resolution TEM image of the TiO2/STO heterostructure
collected near the hole of the TEM specimen. A surface layer formed due
to the severe ion damage during the sample preparation. The EEL spectra A sub-stoichiometric TiOx interface layer could arise in
corresponding 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 activity
and 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 LAO
decreases as the x value decreases for TiOx (24x41), and substrate surfaces. [27,28]. During the early stages of film
cannot be resolved when x is less than a value between 1.5 growth, the arriving Ti atoms will be in contact with the
and 1.2 [24]. In addition, the Ti–L2,3 edges shift to lower reconstructed/sub-stoichiometric surface of STO and LAO
energies as x decreases. [26]. Because the splitting of the and will come to equilibrium both with the chamber
Ti–L2,3 edge is evident for the TiO2 layer, albeit slightly less oxygen activity and the substrate surface/oxygen activity.
so 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 surfaces
results suggest that the oxygen content in the anatase layer of STO and LAO substrates. These inclusions will be
is 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 to
clear splitting of the O–K edge. This further supports that ion damage than the bulk of the film, owing to the stress or
x2 for the anatase layer since it has been shown that such structural variations at the interface, these inclusions can
a splitting is indiscernible for TiOx (xo1.5) [24]. Finally, grow in size during the ion milling process resulting in a
this oxygen stoichiometry is consistent with the anatase continuous interfacial layer, as is evident for the TiO2 film

5. ARTICLE IN PRESS
X. Weng et al. / Journal of Crystal Growth 310 (2008) 545–550 549
grown on STO. Since LAO and STO should have a being ð0 0 1ÞTiO2 kð0 0 1ÞSTO kð0 0 1ÞLAO and ð0 1 0ÞTiO2 k
different 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 arcs
different for the TiO2 films grown on LAO and STO instead of sharp spots, suggesting that the STO layer
substrates 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 TEM
substrate surface and, therefore, to eliminate the interface image of the heterostructure. Sharp interfaces between
layer, we prepared a different STO surface and used it for layers are evident, suggesting high-quality epitaxial growth
the TiO2 growth. An epitaxial STO buffer layer was used of the layers. Second phases are observed neither at the
instead of a STO substrate; we deposited a 10 nm thick TiO2/STO nor at the STO/LAO interfaces, indicating that
STO 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 buffer
this 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 LAO
that of the bulk STO and LAO. Thus, the strain state of the substrate the TiO2 film grown on the STO buffer exhibits
TiO2 film is also intermediate between two previously fringe-like contrast modulation likely related to the
discussed 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 buffer
to that used for the growth on LAO and STO substrates. layer. They could form due to the lattice mismatch between
Fig. 4(a) shows a low-magnification cross-sectional TEM the LAO and STO, and may account for the formation of
image 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 from
than the film grown directly on the LAO. Domain the STO buffer layer at the interface region are shown in
boundaries are also observed, with one of them being Fig. 5. The spectrum collected from the STO buffer layer is
indicated 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–L2,3
substrate. 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/LAO
heterostructure.

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550 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
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Counts (a.u.)
O-K
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