Self-Assembled Single-Phase Perovskite Nanocomposite Thin Films

Dec 29, 2009 - Nano Fab Center 335, Daejeon, 305-806, Republic of Korea, and. ⊥. Lincoln Laboratory, Massachusetts Institute of. Technology, Lexingt...
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Self-Assembled Single-Phase Perovskite Nanocomposite Thin Films Hyun-Suk Kim,† Lei Bi,† Hanjong Paik,‡,# Dae-Jin Yang,§ Yun Chang Park,| Gerald F. Dionne,†,⊥ and Caroline A. Ross*,† †

Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, ‡ Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon, 305-701, Republic of Korea, § Center for Energy Materials Research, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul, 130-650, Republic of Korea, | National Nano Fab Center 335, Daejeon, 305-806, Republic of Korea, and ⊥ Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, Massachusetts 02420 ABSTRACT Thin films of perovskite-structured oxides with general formula ABO3 have great potential in electronic devices because of their unique properties, which include the high dielectric constant of titanates,1 high-TC superconductivity in cuprates,2 and colossal magnetoresistance in manganites.3 These properties are intimately dependent on, and can therefore be tailored by, the microstructure, orientation, and strain state of the film. Here, we demonstrate the growth of cubic Sr(Ti,Fe)O3 (STF) films with an unusual self-assembled nanocomposite microstructure consisting of (100) and (110)-oriented crystals, both of which grow epitaxially with respect to the Si substrate and which are therefore homoepitaxial with each other. These structures differ from previously reported self-assembled oxide nanocomposites, which consist either of two different materials4-7 or of single-phase distorted-cubic materials that exhibit two or more variants.8-12 Moreover, an epitaxial nanocomposite SrTiO3 overlayer can be grown on the STF, extending the range of compositions over which this microstructure can be formed. This offers the potential for the implementation of self-organized optical/ ferromagnetic or ferromagnetic/ferroelectric hybrid nanostructures integrated on technologically important Si substrates with applications in magnetooptical or spintronic devices. KEYWORDS Pulsed laser deposition, oxide nanocomposite, strontium titanate, self-assembly, Fe-doping

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therefore plays an essential role in the integration of a wide range of oxide devices onto a Si platform. The SrTi1-xFexO3 (x ) 0-0.5) films in this work were grown by pulsed laser deposition at typically 750 °C on (100) Si substrates coated with single-crystal CeO2/YSZ buffer layers to promote epitaxy between the STF and the Si. Figure 1 shows X-ray diffraction (XRD) data for films of composition x ) 0, 0.1, 0.2, 0.4, and 0.5. The CeO2/YSZ buffer layers grow with excellent epitaxial quality, and only (200) CeO2 and YSZ peaks are observed with rocking curve full-widths at half-maximum of 0.6-0.8° and 1.2-1.4°, respectively, and lattice parameters close to bulk values.27 This implies that the lattice mismatch strain with the Si substrate (-5 and +4.8% for YSZ and CeO2, respectively) is released. The STF grows as single-phase perovskite over the entire composition range. The out-of-plane lattice parameters are larger than bulk values and increase with Fe content, faster for the (100)-oriented regions. This lattice parameter increase is attributed to the presence of Fe3+ in films grown under reducing conditions,28 which is confirmed by X-ray photoelectron spectroscopy (not shown). In contrast, the bulk lattice parameter of STF, measured from the sintered target material, decreases with increasing x, which is due to the presence of smaller Fe4+ ions as compared to Ti4+ ions.

erovskites exhibit a rich magnetic, optical, ferroelectric, and multiferroic behavior. Technologically important properties of perovskites such as ferroelectricity and ferromagnetism are highly anisotropic,13,14 so control over the microstructure is essential to take full advantage of the desirable properties of these materials. The growth of epitaxial single-crystal perovskite films is already well established, but a wider range of behavior may be anticipated in films containing two or more epitaxial crystal orientations, in which one might obtain not only the individual functionalities of each constituent crystal orientation, but also enhanced, or in some cases unique, properties. We demonstrate this concept of microstructural control using SrTiO3 (STO) and SrTi1-xFexO3 (STF) as a model perovskite system. STO is particularly important from the viewpoints of both fundamental solid-state physics15 and applications.16,17 It is also useful as a single-crystal substrate for epitaxial growth of a range of functional oxides,18,19 and can itself be grown epitaxially onto Si substrates by the use of buffer layers such as Sr,20 TiN,21 Ce-doped ZrO2,22 TiN/ YSZ (yttria-stabilized zirconia),23,24 and CeO2/YSZ.23-26 STO * To whom correspondence should be addressed. E-mail: [email protected]. # Current address: Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853. Received for review: 10/28/2009 Published on Web: 12/29/2009

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The in-plane epitaxial relationships in the STF (x ) 0.4)/ CeO2/YSZ/Si stack were revealed by XRD φ-scans (Figure S1 in the Supporting Information). The CeO2/YSZ buffer layers have a cube-on-cube relation to the Si(100) substrate, as expected.23-26 However, the STF simultaneously shows a 45°-rotated cube-on-cube epitaxy of the STF(100) regions (i.e., STF[011]|CeO2[001]) and a rectangle-on-cube epitaxy of the STF(110) regions (i.e., STF[1¯10]|CeO2[010] and STF[001]|CeO2[001]). Additionally, the (100) φ-scan and the (110) pole figure indicate an alternative epitaxial relation for the (110) crystals with STF[1¯11]|CeO2[011], corresponding to epitaxial alignment between the diagonal of the square CeO2 surface net and the diagonal of the rectangular STF surface net.23-30 These three STF orientations are shown in Figure 2. The STF films with x g 0.2 therefore contain both (110) and (100)-oriented regions which are epitaxial to the substrate and to each other. Transmission electron microscopy (TEM), Figure 3, reveals their spatial distribution. Lowmagnification images illustrate self-assembled tapered nanopillars with approximately circular cross-section growing perpendicular to the substrate, embedded in a single crystal matrix. High-magnification images and electron diffraction resolve these embedded nanostructures to be STF(110) grains in an STF(100) matrix such that {110}STF(100)| {110}STF(110), that is, the (100) and (110) crystals are epitaxially oriented with respect to each other as well as to the substrate. The interfaces between the (100) and (110) regions are sharp and faceted (Figure 3d), and the proportion of the different facets defines the wedge angle of the (110) pillars. The pillars do not grow directly on the CeO2 substrate, but instead, there is a (100)-oriented region between the pillars and the substrate, suggesting a growth mode in which the self-assembled double epitaxial structure forms by the nucleation of (110) pillars in a strained epitaxial (100) STF layer. Atomic force microscopy (Figure 4) shows an increase in the average size and spacing of the pillars as x increases from 0.2 to 0.4. The STO (x ) 0) film has a flat surface with an rms (root-mean-square) roughness of 0.7 nm. For the STF films, the lateral size of the surface topography increases with x up to 0.4, that is, 20-40, 40-80, and 100-400 nm for x ) 0.1, x ) 0.2, and x ) 0.4, respectively. The roughness also increases from 0.9 (x ) 0.1) to 4.8 (x ) 0.2) and 9.5 (x ) 0.4). At x ) 0.5, the film forms an interconnected morphology with a roughness of 1.3 nm. The XRD and TEM data therefore unambiguously show the formation of a self-assembled “double-epitaxial” microstructure consisting of STF(110) pillars in a STF(100) matrix. There have been a number of reports of the spontaneous formation of nanodots, nanopillars, or nanowires of one material embedded in a matrix of another material during two-phase thin-film growth.4-7 For single-phase orthorhombic or tetragonal films, there are also reports of the formation of multiple epitaxial orientations, for example, in orthor-

FIGURE 1. Structure of epitaxial STF films. (a) XRD θ-2θ patterns for STF films with x ) 0 and 0.2 grown on CeO2/YSZ/Si substrates. The right inset shows the XRD scans for STF of various compositions (x ) 0-0.5) in the angular range of 30-38° in 2θ. The (200) peaks of the CeO2/YSZ buffer layers for all the films overlap. The left inset shows the out-of-plane cubic lattice parameter calculated from the (100) and (110) out-of-plane STF lattice constants as a function of Fe content. (b) Pole figures of STF films. The fixed 2θ angle used to record the (111) pole figures was 39.640° corresponding to the STF(111) planes. For the (110) pole figure it is 32.4°. Maximum relative intensities of the pole figures are shown in parentheses. (i) x ) 0 (111) pole figure, with (110) preferred orientation; (ii) x ) 0.1 (111) pole figure, with (110) preferred orientation; (iii) x ) 0.2 (111) pole figure, with epitaxial (110) and (100) peaks; (iv) x ) 0.4 (111) pole figure, with epitaxial (110) and (100) peaks; (iv) x ) 0.4 (110) pole figure, with epitaxial (110) and (100) peaks; (vi) x ) 0.5 (111) pole figure, with epitaxial (110) and (100) peaks with an increased crystal misorientation.

The pure STO (x ) 0) film and the STF (x ) 0.1) film are polycrystalline with a preferred (110) fiber texture and small (100) and (111) peaks, as seen elsewhere.25,26 However, films with x ) 0.2 and above show a qualitatively different microstructure. Strong (100) and (110) diffraction peaks from the perovskite structure are present, without any other perovskite peaks or detectable impurity phases. The discrete peaks in the pole figures (Figure 1b) indicate that both the (100) and (110) regions of the STF films are epitaxially grown on the CeO2/YSZ/Si substrates, with the (100) orientation becoming more dominant at x ) 0.4. At x ) 0.5 we observe an increase in the distribution of crystal orientations. © 2010 American Chemical Society

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FIGURE 2. Schematic atomic configurations in epitaxial STF/CeO2/YSZ/Si. The STF(110) regions have either (i) a rectangle-on-cube epitaxy on CeO2/YSZ/Si with STF[1¯10]|CeO2[010]|YSZ[010]|Si[010], or (ii) two variants of a rotated orientation with STF[1¯11]|CeO2[011]|YSZ[011]|Si[011], while the STF(100) regions show (iii) a 45°-rotated cube-on-cube epitaxy in which STF[011]|CeO2[001]|YSZ[001]|Si[001].

hombic LaFeO3,8 SrRuO3,9 and tetragonal PbTiO3 films,10 due to the very similar interplanar spacings of the (110) and (001), and the (100) and (001) planes, respectively, while orthorhombic YBa2Cu3O7 on MgO11 or YSZ12 grew with two in-plane orientations (the c-axis out of plane and [100] or [110] parallel to substrate [100]) forming a mosaic structure. However, to our knowledge there have been no reports of the formation of a self-assembled epitaxial nanocomposite with multiple crystal orientations in a single-phase cubic material. We now consider the origin and growth mechanism of the coexisting (100) and (110) crystals. For the STO film, strain effects would promote the formation of 45°-rotated STO(100), which has a smaller lattice mismatch (+1.9%) with CeO2(100) than that between STO(110) and CeO2(100) (-4%, based on the structural alignment of three unit cells of CeO2 with four unit cells of STO along the STO[001] direction).25,26 However, surface charge considerations can instead promote STO(110) growth. The bare CeO2(100) surface is highly polar since the CeO2 crystal consists of alternating planes of Ce4+ cations and O2- anions along the [100] direction. This favors growth of the perovskite oxide such that its polar plane is parallel to the polar CeO2(100) surface.22 A stoichimetric STO crystal is a sequence of charge neutral (SrO)0 and (TiO2)0 sheets in the [100] direction, but it is composed of alternately stacked oppositely charged (SrTiO)4+ and (O2)4- atomic layers in the [110] direction.31 Therefore, even though in STO the (100) surface has the lowest surface energy,32 the (110) growth of stoichiometric STO on CeO2(100) is expected.25,26 However, oxygen vacancies lower the (110) surface polarity and can even lead to charge neutrality of the intrinsically polar STO(110) and © 2010 American Chemical Society

(111) surfaces,31,33 again favoring STO(100) growth on CeO2/ YSZ/Si under highly reducing conditions.23,24 In our STO films, both STO(100) and STO(110) form on CeO2, as shown in Figure 1a. In STF, the substitution of Fe3+ or Fe2+on the Ti4+ sites leads to the formation of additional oxygen vacancies, and the polarity of the STF(110) surface decreases with increasing x. This is expected to favor the growth of epitaxial STF(100), despite its compressive strain, which increases with x. The formation of (110) nanopillars partly reduces this strain, evident from the different stress states of the (100) and (110) regions shown in Figure 1a inset. The (100)oriented regions are under in-plane compression (+2%) compared to bulk STF, which is expected from the mismatch with the CeO2 substrate (a ) 0.541 nm). The compression is partly relaxed by nucleating (110)-oriented pillars (+1.5%), which grow with a misfit of -6.2% with the (100) matrix, assuming a structural alignment of four unit cells of STF(110) with three unit cells of STF(100) along the CeO2[001] direction,25,26 or -14% for the (110)STF[1¯11]|(100)STF[010] alignment. This is consistent with the greater broadening of the STF(110) rocking curve peaks compared to the STF(200) peaks. The microstructure and associated strain state of the film have profound effects on its magnetic and ferroelectric behavior. This is evident by comparing the properties of the double-epitaxial film with x ) 0.4 to those of a single crystal (100) film of the same composition, grown at a higher substrate temperature of 800 °C. These results will be reported in detail elsewhere,34 but briefly, in the double epitaxial film the material exhibits intrinsic ferromagnetism and the (110) pillars behave as single magnetic domains with 599

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FIGURE 3. TEM images of a double-epitaxial STF (x ) 0.4) film. (a) Low-magnification cross-sectional image of STF on CeO2/YSZ/Si. The incident electron beam is parallel to the [110] direction of the Si substrate. The edges of the (110) pyramids are outlined. (b) High-magnification cross-sectional image of interfaces between STF(100) and STF(110) regions. The insets show the fast Fourier transform (FFT) patterns taken from the (110)-oriented (upper-left) and (100)-oriented regions (upper-right) of the STF films and the (110)/(100) interface (lower-middle). (c) High-magnification cross-sectional image of the STF film close to the CeO2 buffer layer. (d) High-resolution cross-sectional image of the interface between STF(100) and STF(110) grains. (e) A plan-view electron diffraction pattern of the STF film, looking down the film normal direction. (f) High-magnification plan-view TEM image of the STF film. Two (110) pillars show the STF[1¯11]|CeO2[011] epitaxial relation; their FFTs are rotated in plane by ∼20°. (g) Schematic of the self-assembled structure with dimensions indicated for the x ) 0.4 sample.

FIGURE 4. Atomic force microscopy (AFM) showing the surface morphology of STF films as a function of Fe content x. (a) x ) 0, (b) x ) 0.1, (c) x ) 0.2, (d) x ) 0.4, and (e) x ) 0.5. The scan area was 3 µm × 3 µm. The upper left insets in (a-c) display details for 0.5 µm × 0.5 µm areas. The height scale is shown in the lower-right corner of each image.

out-of-plane magnetization, while the single-crystal film shows a maze-like domain pattern and spin-glass behavior. The double-epitaxial film also has a lower electrical leakage current and the (110) regions behave as single ferroelectric domains. Finally, we show that the double-epitaxial STF films can be used as a substrate to control the crystal orientation of a © 2010 American Chemical Society

thin film perovskite overlayer. Figure 5 shows XRD data from a 200 nm thick STO film grown on a double-epitaxial STF (x ) 0.4) film. The STO overlayer shows both (110) and (100) orientations, and the 2D XRD data shows that these crystals are epitaxial with respect to the STF (110) and (100), respectively. Therefore, a double-epitaxial STO film was obtained by using the double-epitaxial STF as a seed layer. 600

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without breaking the vacuum. The base pressure in the chamber was below 1 × 10-6 Torr before raising the substrate temperature. p-type Si(100) wafers with a resistivity of 1-5 Ωcm were used as substrates. An epitaxial YSZ thin film with a thickness of 50 nm was first grown on the Si(100) at a substrate temperature of 800 °C with a flowing O2 pressure of 0.5 mTorr. Details on the epitaxial growth of YSZ on Si can be found elsewhere.35 An epitaxial 100 nm thick CeO2 film was then deposited at the same substrate temperature in 5 mTorr O2. Finally, SrTi1-xFexO3 films with a thickness of 200 nm were grown onto the CeO2/YSZ/Si in vacuum (2 × 10-6 Torr) with a substrate temperature of 750-800 °C. To form the STO/STF/CeO2/YSZ/Si samples, STO with a thickness of 200 nm was grown by PLD on a double-epitaxial STF (x ) 0.4) film at a substrate temperature of 800 °C and oxygen partial pressure of 5 mTorr. The crystal structures of the films were examined by X-ray diffraction analysis including θ-2θ scan, φ-scan, and rocking curve as well as pole-figure measurements using Cu KR radiation (RIGAKU, D/MAX-2500). Twodimensional X-ray diffraction methods (2D XRD, Bruker D8 with General Area Detector Diffraction System)36 were also used to detect the spatial distribution of the diffraction peaks. The two frames in Figure 5 were collected at an X-ray incident angle of θ ) 16.5 and 23° and the frame center of 2θ ) 33 and 46°, respectively. The sample was rotated by 360° about the axis normal to the sample surface during the measurement. The microstructure and morphology of the films were investigated by transmission electron microscopy (TEM) and atomic force microscopy (AFM). TEM investigations were carried out in a FEI Tecnai F30 S-Twin electron microscope operating at 300 kV equipped with an energy dispersive X-ray spectroscopy (EDS). Fast Fourier transform patterns were obtained through GATAN Digital Micrograph software. The composition of constituent elements and compositional homogeneity of the film were investigated using EDS. X-ray photoelectron spectroscopy was carried out to determine the Fe valence state in SrTi1-xFexO3 films using a Kratos AXIS Ultra imaging X-ray photoelectron spectrometer.

FIGURE 5. XRD patterns for the STO film deposited on the double epitaxial STF (x ) 0.4) film. The upper inset shows the enlarged XRD θ-2θ pattern around the STO(110) peak. The lower insets show the two-dimensional X-ray diffraction (2D XRD) patterns collected by an X-ray area detector near the STO(110) and STO(200) diffraction peaks. The spot-like diffraction patterns of the STO(110) and STO(200) peaks indicate that both the (110) and (100) crystals of the STO film are epitaxially grown on the STF seed layer.

This extends the composition range over which doubleepitaxial microstructures can be obtained, enabling nanocomposite films with multiple crystal orientations to be fabricated. In summary, we describe an unusual double-epitaxial oxide microstructure formed in a cubic perovskite SrTi1-xFexO3 film grown on a CeO2/YSZ-buffered Si substrate. The STF films consist of self-assembled epitaxial nanocomposites of (110) nanopillars in a (100) matrix with a characteristic spacing that is controlled via the Fe content. The ferromagnetic and ferroelectric properties of the double-epitaxial films differ qualitatively from that of their single-epitaxial counterparts. Perovskite-based oxides form the basis of a large number of technological applications, which is due in part to the versatility of the structure in accommodating a broad range of substituents. We expect that self-assembled nanocomposites can be formed in other perovskite systems, either spontaneously or by growth on a double-epitaxial seed layer as demonstrated for STO, and we have indeed been able to create similar structures in Co-doped SrTiO3. This ability to precisely control the nanoscale structure of these singlephase perovskite systems may enable a range of optical, ferroelectric, ferromagnetic or hybrid devices integrated on Si.

Acknowledgment. The authors are grateful to Dr. D. Navas Otero for his assistance in AFM and MFM analyses. This work was supported by a Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (Grant KRF-2006-352-D00094) and the National Science Foundation, Division of Materials Research and by Lincoln Laboratories.

METHODS

Heterostructures of SrTi1-xFexO3 (x ) 0, 0.1, 0.2, 0.4, and 0.5)/CeO2/YSZ/Si were grown by pulsed laser deposition (PLD) using ceramic targets. The targets for YSZ, CeO2, and SrTi1-xFexO3 were prepared using standard solid-state reaction techniques. The sintered targets were ablated by a KrF excimer laser (Lambda Physik LPX200, λ ) 248 nm) operating at a repetition rate of 10 Hz with an energy density of 550 mJ/pulse. All layers were deposited in the same chamber © 2010 American Chemical Society

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