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Yoshiki Maekawa, Koji Hirai, Kazuki Okamoto, [Takao Shimizu](https://orcid.org/0000-0001-9508-7601), Hiroshi Funakubo

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[Strain Control of (001)-Polar-Axis-Oriented Epitaxial Y-Doped HfO<sub>2</sub> Thin Films](https://mdr.nims.go.jp/datasets/562ae5fe-8ca2-4f53-b647-93ae06bfff18)

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Strain Control of (001)-Polar-Axis-Oriented Epitaxial Y-Doped HfO2 Thin FilmsStrain Control of (001)-Polar-Axis-Oriented Epitaxial Y‑Doped HfO2Thin FilmsYoshiki Maekawa, Koji Hirai, Kazuki Okamoto,* Takao Shimizu, and Hiroshi Funakubo*Cite This: ACS Appl. Electron. Mater. 2024, 6, 5525−5535 Read OnlineACCESS Metrics & More Article Recommendations *sı Supporting InformationABSTRACT: {100}-Oriented epitaxial orthorhombic (Hf0.93Y0.07)O2 (7YHO) filmswere grown at 700 °C on various substrates, with and without (Ce0.35Zr0.65)O2 (CZO)buffer layers, by using pulsed laser deposition. The crystal phase and orientation weredetermined by X-ray diffraction. The long axis [100] or short axis [010]/[001]orientations were controlled by lattice matching. Nearly relaxed epitaxial orthorhombic(100) 7YHO films on LaAlO3 and SrRuO3/SrTiO3 substrates were obtained, and theirunclamped lattice constants were experimentally observed. The in-plane latticeparameter of the (010)/(001)-oriented 7YHO films obtained by using the CZO bufferlayers was within the range of 0.518−0.523 nm. This study demonstrates selectivegrowth and lattice strain control with respect to the in-plane lattice spacing of theunderlying layers. These findings provide further possibilities for understanding thefundamental ferroelectric properties of HfO2-based materials.KEYWORDS: Ferroelectric HfO2-based film, Epitaxial film, {100} Orientation control, Polar axis orientation, Pulse laser deposition■ INTRODUCTIONThe ferroelectricity of HfO2-based materials was first reportedin 2011.1 Since then, HfO2-based thin films have been widelyand intensively investigated for ferroelectric memory applica-tions, such as ferroelectric random access memories,1,2ferroelectric field-effect transistors,3 and ferroelectric tunneljunctions.4 These films show stable ferroelectricity even inpolycrystalline thin films with thicknesses as low as 5 nm,4,5unlike conventional ferroelectric materials such as perovskite-structured ferroelectric films, including Pb(Zr,Ti)O3 andBaTiO3.6,7Prompt research is required to understand the fundamentalferroelectric properties of epitaxial HfO2-based films, such asthe spontaneous polarization (Ps), Curie temperature (Tc), andorientation-dependence or strain-dependence of the ferroelec-tricity and piezoelectricity.8−13 Choi et al. demonstrated thatapplying a large compressive strain to BaTiO3 (001) films, as aconventional ferroelectric, enhances the spontaneous polar-ization and increases the Tc through selective growth of polar-axis-oriented films on (001)pc DyScO3 and (001)pc GdScO3single-crystal substrates.14 The selective growth of polar-axis(001)-oriented HfO2 films is not simple due to the smallcrystal-anisotropy. Therefore, characterization studies generallyemploy (111)-oriented films on (111)ITO//(111) yttrium-stabilized zirconia (YSZ)15 and (100)(La,Sr)MnO3//(100)-SrTiO316,17 substrates because the possible contribution topolarization along the surface normal direction in the films isnearly the same regardless of the type of domains formed inthe films. There are several reports on {100}-oriented epitaxialfilms on (100)YSZ,18 (100)Si,19 and (100)Nb:SrTiO3.20However, controlling the orientation of {100}-oriented filmsand their strain effects is not well understood.In this study, we demonstrate control of the (100) and(010)/(001) orientation for epitaxial (Hf0.93Y0.07)O2 (7YHO)orthorhombic films by selecting an underlying layer with alattice match. Using this approach, (100)- and (010)/(001)-oriented films with different in-plane strains are successfullygrown.■ DETERMINATION OF FILM ORIENTATIONThe orthorhombic phase has three axes: a relatively long axis(aO-axis) and two relatively short axes (bO- and cO-axes), wherethe subscript o denotes the ferroelectric orthorhombic phase.Here, the polar cO-axis is slightly longer than the bO-axis, butthe length is roughly similar (cO/bO ≈ 1.004; based on thecalculated data for ferroelectric orthorhombic HfO2(Pca21)).21 Herein, the aO-axis is defined as the long axis,whereas the bO- and cO-axes are denoted as the short axes.Figure S1 illustrates the possible orientations and the XRDpatterns of the 7YHO film consisting of a ferroelectricorthorhombic phase (Pca21) for the determination of thefilm orientation in this study. The x- and y-axes are the in-planeReceived: February 27, 2024Revised: June 29, 2024Accepted: July 14, 2024Published: July 25, 2024Articlepubs.acs.org/acsaelm© 2024 The Authors. Published byAmerican Chemical Society5525https://doi.org/10.1021/acsaelm.4c00368ACS Appl. Electron. Mater. 2024, 6, 5525−5535This article is licensed under CC-BY 4.0Downloaded via NATL INST FOR MATLS SCIENCE (NIMS) on December 13, 2024 at 05:58:39 (UTC).See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Yoshiki+Maekawa"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Koji+Hirai"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Kazuki+Okamoto"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Takao+Shimizu"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Hiroshi+Funakubo"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/showCitFormats?doi=10.1021/acsaelm.4c00368&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?goto=articleMetrics&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?goto=recommendations&?ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?goto=supporting-info&ref=pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsaelm.4c00368/suppl_file/el4c00368_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=agr1&ref=pdfhttps://pubs.acs.org/toc/aaembp/6/8?ref=pdfhttps://pubs.acs.org/toc/aaembp/6/8?ref=pdfhttps://pubs.acs.org/toc/aaembp/6/8?ref=pdfhttps://pubs.acs.org/toc/aaembp/6/8?ref=pdfpubs.acs.org/acsaelm?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://doi.org/10.1021/acsaelm.4c00368?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://pubs.acs.org/acsaelm?ref=pdfhttps://pubs.acs.org/acsaelm?ref=pdfhttps://acsopenscience.org/researchers/open-access/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/directions and the z-axes are the out-of-plane direction on thesubstrate coordination (see Figure S1(a)). There are sixpossible orientations labeled as orientation I−VI shown inFigure S1(b), following the nomenclature in our previousreport.22 The crystal phase and the orientation weredetermined based on four types of XRD measurements: (1)2θ−ω scans along the z-direction, (2) 2θ−ψ scans along the y-z-direction, (3) 2θχ−φ scans along the y-direction, and (4)2θχ−φ scans along the x-y-direction. An illustration of the scandirections, relevant lattice planes, and XRD patterns for thepossible orientation of 7YHO can be found in Figure S1(c-f).The indices of Bragg diffraction and the angles correspondingto the XRD measurements are also shown. Note that the 2θangles corresponding to these XRD measurements are shownin these figures based on the calculated values for (Hf,Zr)O2films.21 The observations of 030 and 110 diffractions allowphase identification of the ferroelectric orthorhombic phasefrom the perspective of its reflection conditions22 as 0kl: l = 2n,h0l: h = 2n, h00: h = 2n, and 00l: l = 2n, since the diffractionpeaks corresponding to the 030 and 110 in the orthorhombicphase are not observed in the case of a higher symmetrictetragonal phase. The film orientation can be determined basedon the appearance of such diffractions unique to theorthorhombic phase and difference in diffraction angles, e.g.,between 200 and 020/002 as well. More detailed explanationof the measurements can be found in our previous report.23■ EXPECTED ORIENTATION IN {100}-ORIENTEDFILMSHigh-temperature XRD analysis of the {100}-oriented 7YHOfilms revealed that the Tc was approximately 500 °C.9 Notethat during cooling after depositing the films at 700 °C, thetetragonal phase (P42/nmc) is converted to the orthorhombicphase (Pca21), after which domains can form, as discussed inthe literature.23−25 Note that the long axis direction differs forspace groups P42/nmc and Pca21. Throughout this report, weFigure 1. Schematic illustration of possible orientation and domain formation resulting from phase transition at the Curie temperature, Tc, from thehigh-temperature tetragonal to ferroelectric orthorhombic phase in epitaxial HfO2 thin films. Here, domains having 180° domain relationships arenot shown for simplification. This illustration was drawn based on the literature.23,25,27ACS Applied Electronic Materials pubs.acs.org/acsaelm Articlehttps://doi.org/10.1021/acsaelm.4c00368ACS Appl. Electron. Mater. 2024, 6, 5525−55355526https://pubs.acs.org/doi/suppl/10.1021/acsaelm.4c00368/suppl_file/el4c00368_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsaelm.4c00368/suppl_file/el4c00368_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsaelm.4c00368/suppl_file/el4c00368_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig1&ref=pdfpubs.acs.org/acsaelm?ref=pdfhttps://doi.org/10.1021/acsaelm.4c00368?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asuse the description of long axis and short axis for the tetragonalphase in the same manner as for the orthorhombic phase: longaxis denotes the cT-axis and short axis denotes the aT-axes.Figure S1 shows the possible crystallographic domains and therelationship between the tetragonal and orthorhombic phases.The (1) short-axis-oriented and (2) long-axis-orientedorthorhombics result from short-axis-oriented and long-axis-oriented tetragonal phases of 7YHO at high temperature.(I) The out-of-plane long-axis-oriented orthorhombic filmsare obtained at room temperature (see Orientation I−II inFigures S1 and 1) when out-of-plane long-axis-orientedtetragonal films are grown at deposition temperatures aboveTc. The polar axis (cO-axis) lies in the in-plane direction.(II) Out-of-plane short-axis-oriented orthorhombic films areexpected to be obtained at room temperature (see OrientationIII-VI in Figures S1 and 1) when out-of-plane short-axis-oriented tetragonal films are grown at the depositiontemperature. It is known that the application of an electricfield can induce polarization-switching from the in-plane to theout-of-plane direction in out-of-plane short-axis-oriented Y-doped HfO2 films.26 In this case, the polar axis can be orientedtoward either the out-of-plane or in-plane direction, and itsdirection can be controlled by an external electric field.Orientation control of the long axis and short axis intetragonal films can be achieved by selecting the underlyinglayer based on the in-plane lattice parameters. We previouslyascertained that short-axis- and long-axis-oriented orthorhom-bic films can be grown on (100)YSZ and (100)pc (indium−tin-oxide, ITO)//(100)YSZ substrates,23 respectively. Theseobservations suggest that there is a critical in-plane latticeparameter for the growth of short-axis- and long-axis-orientedfilms. In this study, a nominal film thickness of 20 nm was usedfor estimation of the critical thickness. The critical thicknessfor strain relaxation is an important indicator for the growth ofcoherently strained films. The estimated range for latticematching with underlying layers was discussed with respect tothe critical thickness for plastic relaxation in the nextparagraph. Figure 2 shows the calculated critical thickness forthe long-axis- and short-axis-oriented 7YHO films as a functionof the lattice spacing of the underlying layer at a depositiontemperature of 700 °C based on the model reported by Peopleand Bean.28 The lattice parameters of tetragonal 7YHO andvarious single-crystal substrates at the deposition temperaturewere estimated from the reported values at room temperatureand the thermal expansion coefficient.28 Those values are listedin Table S1.23,29−37The approximate lattice matching range was estimated basedon the People and Bean theory for plastic relaxation. AsHartmann et al. reported that real critical thicknesses oftendiffer from the theoretically predicted value, the real criticalthickness for strain relaxation, for example, the difference in thecrystal structure between the perovskite structure and fluorite-like structure, can influence on the critical thickness.38−41 Theestimated ranges where strained films with thickness of belowthe critical thickness can be obtained are 0.524−0.527 nm and0.511−0.513 nm for the in-plane long axis and short axis,respectively. Therefore, the 7YHO film on YAlO3 is expectedto adopt an out-of-plane short-axis orientation owing to thesmall lattice mismatch (below 0.1%) with respect to the in-plane long axis, whereas the orientation of the films on LaAlO3and SrTiO3 cannot be estimated on this basis because of therelatively large in-plane lattice mismatch between the film andthe substrates.Figure 2. Theoretically calculated critical thickness of tetragonal 7% YO1.5-93%HfO2 (7YHO) film at 700 °C as a function of the lattice spacingalong [100]. Blue line: the critical thickness of short-axis-oriented tetragonal 7YHO film, purple line: the thickness of long-axis-oriented tetragonal7YHO film. The critical thicknesses were estimated based on the People and Bean theory.28 A sketch of unit cells for the 7YHO and its latticeparameter were described in an inset. Calculated lattice parameters at 700 °C of relevant substrates are also shown.ACS Applied Electronic Materials pubs.acs.org/acsaelm Articlehttps://doi.org/10.1021/acsaelm.4c00368ACS Appl. Electron. Mater. 2024, 6, 5525−55355527https://pubs.acs.org/doi/suppl/10.1021/acsaelm.4c00368/suppl_file/el4c00368_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsaelm.4c00368/suppl_file/el4c00368_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsaelm.4c00368/suppl_file/el4c00368_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsaelm.4c00368/suppl_file/el4c00368_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig2&ref=pdfpubs.acs.org/acsaelm?ref=pdfhttps://doi.org/10.1021/acsaelm.4c00368?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-as■ RESULTSDirect Growth on Various Substrates. Figure 3 showsthe out-of-plane XRD 2θ−ω scans near the {200} and {300}peaks of the 7YHO films grown on (100)pcSrRuO3//SrTiO3,(100)pcLaAlO3, (100)pcYAlO3, and (100)YSZ substrates. The{200} peaks were observed for all of the films, whereas the 030peak was observed only for the films on the (100)YSZsubstrates. Taking into account the peak positions of 7YHO200 at 2θ ≈ 34.4° (see Figure 3(a-1, b-1, c-1)), theorientations of the films, except that on YSZ, were identifiedas long-axis orientations (Orientations I−II). On the otherhand, the films on YSZ possibly adopt the short-axisorientations (Orientations III−IV and/or V−VI), the identi-fication of which is also supported by a fact that the 2θ peakposition of 7YHO 020/002 is 35.4°.Figure 4 shows the XRD 2θ−ψ scans for the 7YHO films.Note that the two XRD 2θ−ψ scans at different samplerotation angles φ (0° along <110>pc YHO and 45° along<100>pc YHO) are shown in the single maps. The 110 spots ofthe perovskite-structured substrates were observed in the scanareas framed by the dashed line. The observation of 110 spotsat 2θ ≈ 24.7° and ψ ≈ 45° provides an indicator of thepresence of the orthorhombic phase or monoclinic phase basedon the reflection conditions. The 110 spots were observedwhen ψ > 45°, as shown in Figure 3(a−c), supporting theidentification of the long-axis orientations (Orientations I−II)for the films, except for that on YSZ (Figure 4(a−c)). TheXRD 2θ−ψ scan for the films on YSZ (Figure 4(d)) shows a7YHO 110 spot at ψ < 45°, which is consistent with the short-axis orientations (Orientations III−IV) shown in Figure 3(d-1,2).Figure 5 displays the three kinds of in-plane XRD 2θχ−φscans for the 7YHO films: (1) near {200}, (2) {300} along thedirection of <100>pcYHO, and (3) near 110 along thedirection of <110>pcYHO. The films other than that on YSZwere also found to have long-axis orientations (Orientation Iand/or II) based on observation of the in-plane {200} and 030peaks, as displayed in Figure 5(a-1, b-1, c-1) and (a-2, b-2, c-2), respectively. The in-plane 110 peaks were not significantlyobserved in Figure 5(a-3, b-3, and c-3), indicating Orientation Iand/or II. The same in-plane XRD 2θχ−φ scans along thedirection of <010>pcYHO also showed in-plane 2 peaks; lower2θ angle of 200 peak than {020} and 030 peaks as found inFigure 3(d-1) and (d-2). These results suggest the formationof an in-plane 90° domain with both Orientation I and II.Figure 3. XRD 2θ−ω patterns near (1) {200} peaks and (2) {300}peaks for the 7YHO films grown on (a) (100)pcSrRuO3/SrTiO3, (b)(100)pcLaAlO3, (c) (100)pcYAlO3, and (d) (100) YSZ substrates.Figure 4. XRD 2θ−ψ patterns near the (1) {200} peak and (2) {300} peak for the 7YHO films grown on (a) (100)pcSrRuO3/SrTiO3, (b)(100)pcLaAlO3, (c) (100)pcYAlO3, and (d) (100) YSZ substrates.ACS Applied Electronic Materials pubs.acs.org/acsaelm Articlehttps://doi.org/10.1021/acsaelm.4c00368ACS Appl. Electron. Mater. 2024, 6, 5525−55355528https://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig4&ref=pdfpubs.acs.org/acsaelm?ref=pdfhttps://doi.org/10.1021/acsaelm.4c00368?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asHowever, the 030 and 110 peaks were of negligible intensityfor the film on (100)YSZ, as shown in Figures 5(d-2 and d-3),respectively. These results indicate that the film on (100)YSZhas a single short-axis orientation (Orientation III−IV), withthe polar axis aligned along the in-plane direction. The in-planelattice parameters of the films on YSZ were almost the same asthose on YSZ itself, suggesting that the in-plane lattice wasalmost clamped.Difference in the long-axis lattice parameter of 7YHO andthe corresponding lattice spacing of YAlO3 can be estimated tobe below 0.1%. Therefore, it was expected to grow with theout-of-plane, short-axis orientation on YAlO3. However, 7YHOfilms exhibited an out-of-plane long-axis orientation exper-imentally.Insertion of the Buffer Layer. The growth of the long-axis-oriented YHO films on YAlO3 is inconsistent with the factthat the short-axis-oriented film exhibits a better lattice matchwith that of YAlO3. Electrical matching at the interfaces isanother factor that can affect the growth orientation. Thecrystal structures of 7YHO and YAlO3 and their electricalmatching were examined. Figure S2 shows schematic fluoriteand perovskite structures drawn using VESTA.42 The fluorite-type structure consists of alternating planes A and B, which arepositively and negatively charged, respectively. The perovskitestructure is composed by alternating planes A’ and B’. Thetotal charge of the respective plane is neutral for SrTiO3, orcharged for YAlO3 and LaAlO3, but the planes consist of bothcations and anions. This suggests that the electrostaticpotentials of the layers in these two structures are differentwith respect to total charge and local charge distribution. Thus,despite matching of the ionic positions of these two structuresat the interface, the electrical matching of the ions is not asgood when the 7YHO film is grown on the YAlO3 substrate.This consideration suggests us that the insertion of the bufferlayer with a pseudocubic fluorite structure and the same latticeparameter as YAlO3 can form out-of-plane short-axis-orientedfilms by the matching the long axis of the 7YHO film along thein-plane direction.Figure S3 shows the reported lattice parameters of(AxZr1−x)O2 (A = Ce,43 Y,44 Ca,45 or Mg45) with a fluoritestructure as a function of x. Here, the lattice parameters for(AxZr1−x)O2 with higher-symmetry phases, such as tetragonaland pseudocubic phases, were plotted. CeO2 has a fluoritestructure and can form a (CexZr1−x)O2 solid-solution over awide x range, with cubic or pseudocubic symmetry.(Ce0.35Zr0.65)O2 (CZO) was selected as the buffer layerbecause its lattice parameter is close to the diagonal latticespacing of YAlO3 (0.522 nm).The structural properties of approximately 15 nm thick CZObuffer layers epitaxially grown on (100)pcLaAlO3,(100)pcYAlO3, and (100)YSZ substrates were investigated.Here, (100)pcSrRuO3//SrTiO3 was excluded because of thelarge lattice mismatch between the substrate and CZOrequired to obtain a coherently strained film, as shown inFigure S4. Figures S5 and S6 respectively show the out-of-plane XRD 2θ−ω scans and in-plane XRD 2θχ−φ scans forthe 10 nm thick CZO buffer layer on the substrates. The latticespacings of the underlying layers increased sequentially forYSZ, YAlO3, and LaAlO3. In addition, it was found that theCZO film on the YSZ substrate showed stronger peak intensitythan those on YAlO3, and LaAlO3 substrates. This might bebecause the consistency of the fluorite structure can result inbetter crystallinity and stronger intensity than the films grownon the perovskite-structured substrate. As shown in Figure S5,the out-of-plane lattice parameters of the CZO films decreasedas the lattice spacing of the underlying layers increased. The in-plane lattice parameters of the CZO films increased as thelattice spacing of the underlying layers increased (Figure S6).Figure 5. XRD 2θχ−φ patterns near (1) {200} and (2) {300} peaks along the direction of <100>pc YHO and (3) near 110 peaks along thedirection of <110>pc YHO for the 7YHO films grown on (a) (100)pcSrRuO3/SrTiO3, (b) (100)pcLaAlO3, (c) (100)pcYAlO3, and (d) (100) YSZsubstrates. Black circles show the substrate diffraction peaks.ACS Applied Electronic Materials pubs.acs.org/acsaelm Articlehttps://doi.org/10.1021/acsaelm.4c00368ACS Appl. Electron. Mater. 2024, 6, 5525−55355529https://pubs.acs.org/doi/suppl/10.1021/acsaelm.4c00368/suppl_file/el4c00368_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsaelm.4c00368/suppl_file/el4c00368_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsaelm.4c00368/suppl_file/el4c00368_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsaelm.4c00368/suppl_file/el4c00368_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsaelm.4c00368/suppl_file/el4c00368_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsaelm.4c00368/suppl_file/el4c00368_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig5&ref=pdfpubs.acs.org/acsaelm?ref=pdfhttps://doi.org/10.1021/acsaelm.4c00368?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asThe peak positions of CZO {200} and LaAlO3 {110} differedin the in-plane measurements (Figure S6(a)), suggesting strainrelaxation of the CZO film from the substrate. In contrast, thepeaks of CZO {200} and the YAlO3 and YSZ substratesoverlapped, as shown in Figure S6(b,c).Figure S7 summarizes the in-plane lattice parameters of theCZO buffer layer, estimated from the in-plane XRD 2θχ−φscans (Figure S6), as a function of the in-plane lattice spacingof the underlying substrate. The lattice parameters of the bufferlayers matched those of the YAlO3 substrate and matched fairlywell with those of the YSZ substrate, whereas the CZO film onLaAlO3 was almost relaxed, because its in-plane latticeparameter was close to the reported value for the bulk, asindicated by the red dashed-dotted line in Figure S7. Thistrend can be explained by considering the critical thickness(Figure S4). The critical thicknesses of the CZO films onYAlO3, YSZ, and LaAlO3 were estimated to be 15, 3, and 2 nm,respectively. Consequently, fluorite-structured CZO bufferlayers with different lattice spacings between YSZ and bulkCZO were successfully fabricated. Based on the obtainedlattice spacings, short-axis-oriented 7YHO films can be grownand their lattice strain can be controlled.Film Growth on the Buffer Layer. Figure 6 shows theout-of-plane XRD 2θ−ω scans near the {200} and {300} peaksfor the films on CZO-buffered (100)pcLaAlO3, (100)pcYAlO3,and (100)YSZ. The XRD profiles were fitted by using multipleGaussian functions. The peak positions of the CZO bufferlayers were assumed based on the results shown in Figure S5.A 7YHO 200 peak was observed for the film on the CZO-buffered LaAlO3, whereas the 020/002 and 030 peaks were notclearly observed. The existence of the long-axis orientation(Orientations I and/or II) for the films on CZO-bufferedLaAlO3 is suggestive considering the 7YHO {200} peakpositions, but the existence of the short-axis orientation shouldbe carefully considered with other XRD scans. However, the020/002 and 030 peaks were both observed, indicating that thefilms on CZO-buffered YAlO3 and CZO-buffered YSZ mayadopt a short-axis orientation (Orientations III−IV and/or V−VI).Figure 7 shows the composite XRD 2θ−ψ scans for the7YHO films, analogous to that in Figure 4. The presence of theorthorhombic phase in all of the films was confirmed by theobservation of 110 spots. The 7YHO 110 spots of the films onCZO-buffered YAlO3 and CZO-buffered YSZ were mostintense with ψ < 45°, also indicating the short-axis orientation(Orientations III−IV and/or V−VI). Remarkably, the ψ angleinformation corroborates the absence of the long-axisorientation of 7YHO, which is hidden by the diffractionpeaks of the CZO buffer layer in Figure 6.Figure 8 shows three types of in-plane XRD 2θχ−φ scans forthe 7YHO film: (1) near {200}, (2) {300} along the directionof <100>pc YHO, and (3) near 110 along the direction of<110>pc YHO. The XRD profiles were fitted to multipleGaussian functions, and the peak positions of the buffer layerswere assumed based on the results in Figure S6. The peaks ofthe substrates were calculated from the following latticeconstants: a = 0.536 nm for LaAlO3, a = 0.522 nm for YAlO3,and a = 0.514 nm for YSZ.The measurements for the film on CZO-buffered(100)pcLaAlO3 showed the presence of 7YHO 020/002 and7YHO 030 peaks but no 7YHO 110 peak. These resultscorrespond to Orientations I−II, the coexistence of which wasconfirmed by the observation of the 030 peak in the in-planeXRD 2θχ−φ scans along the direction <010>pc of YHO.For the films on CZO-buffered (100)pcYAlO3 and CZO-buffered (100)YSZ, the observation of both the 7YHO 200and 7YHO 020/002 peaks indicates that the films have short-axis and long-axis orientation in the in-plane direction (Figures6 and 8). YHO 030 and 7YHO 110 peaks were also observed.The data indicate Orientation V−VI. These results are notinconsistent with the deduction of Orientation III−IV based onthe out-of-plane XRD 2θ−ω scans in Figure 6. In summary,XRD analysis of the films on CZO-buffered (100)pcYAlO3 andCZO-buffered (100)YSZ confirmed the existence of Orienta-tions III, IV, V, and VI, suggesting the existence of a 90°polydomain structure (see also Figure 1). The data differ fromthe results for the film deposited directly on YSZ, where onlyOrientations III and IV were present in the in-plane 90°domain.■ DISCUSSIONFigure 9 shows the out-of-plane and in-plane lattice parametersof 7YHO, calculated from the XRD 2θ−ω and 2θχ−φ scans,respectively, as a function of the lattice spacing of theunderlying layer. Table 1 shows the calculated latticeparameters and orientation of the 7YHO films on eachsubstrate. Short-axis-oriented 7YHO films were obtained whenthe in-plane lattice parameter was in the range of 0.518−0.523nm, where the lattice mismatch with the long axis ranged from−1.8% to −0.2%, as shown in Figure 9. Selective growth of thelong-axis-oriented 7YHO films was achieved by selecting anunderlying layer with a lattice spacing >0.525 nm or latticemismatch with the long-axis > 0.8%. Lattice strain relaxationwas observed in these long-axis-oriented films, which resultedin almost relaxed short-axis and long-axis lattice parameters of0.506 and 0.523 nm, respectively. It was reported that epitaxialunstrained (111)-oriented films of ferroelectric (Hf,Zr)O2 onstrain-controlled (La,Sr)MnO3 electrodes grow epitaxiallythrough an unconventional mechanism of epitaxy. Meanwhile,the observation of unclamped lattice parameters for ortho-rhombic HfO2-based materials was experimentally achieved indifferent (100) orientation films in this work.Figure 6. XRD 2θ−ω patterns near (1) {200} and (2) {300} peaksfor 7YHO films grown on (a) CZO-buffered (100)pcLaAlO3, (b)CZO-buffered (100)pcYAlO3, and (c) CZO-buffered (100) YSZsubstrates. Blue dashed lines: based on the CZO {200} peak shown inFigure S5, green lines: 7YHO 020/002 peak, red lines: 7YHO 200peak.ACS Applied Electronic Materials pubs.acs.org/acsaelm Articlehttps://doi.org/10.1021/acsaelm.4c00368ACS Appl. Electron. Mater. 2024, 6, 5525−55355530https://pubs.acs.org/doi/suppl/10.1021/acsaelm.4c00368/suppl_file/el4c00368_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsaelm.4c00368/suppl_file/el4c00368_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsaelm.4c00368/suppl_file/el4c00368_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsaelm.4c00368/suppl_file/el4c00368_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsaelm.4c00368/suppl_file/el4c00368_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsaelm.4c00368/suppl_file/el4c00368_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsaelm.4c00368/suppl_file/el4c00368_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsaelm.4c00368/suppl_file/el4c00368_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig6&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig6&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig6&ref=pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsaelm.4c00368/suppl_file/el4c00368_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig6&ref=pdfpubs.acs.org/acsaelm?ref=pdfhttps://doi.org/10.1021/acsaelm.4c00368?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asThe growth range of short-axis-oriented films is limited. Thegrowth of long-axis-oriented films with a large lattice mismatchcan be explained by the anisotropy of the imposed strain:isotropic biaxial strain is imposed on the long-axis-orientedfilms and anisotropic strain is imposed on the short-axis-oriented films. As shown in Figure 5(d), YSZ has a latticeparameter of 0.518 nm, which lies between the long axis cT andthe short axes aT. Considering that the in-plane latticeFigure 7. XRD 2θ−ψ patterns of 7YHO films and the CZO buffer layer deposited on (a) (100)pcLaAlO3, (b) (100)pcYAlO3, and (c) (100) YSZsubstrates. The dotted line shows a tilt angle of 45°. The area framed by the dashed line is the area where the 110 spots of the perovskite structuresubstrates were observed by rotating the film 45° in the φ direction.Figure 8. XRD 2θχ−φ patterns near (1) {200} and (2) {300} peaks along the direction of <100>pc YHO and (3) near 110 peaks along thedirection of <110>pc YHO for the 7YHO films grown on (a) (100)pcLaAlO3, (b) (100)pcYAlO3, and (c) (100) YSZ substrates. Blue fitting lines:(Ce0.35Zr0.65)O2 peak {200}, green lines: 7YHO 020/002 peak, 7YHO 030 peak, and red lines: 7YHO 200 peak.ACS Applied Electronic Materials pubs.acs.org/acsaelm Articlehttps://doi.org/10.1021/acsaelm.4c00368ACS Appl. Electron. Mater. 2024, 6, 5525−55355531https://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig7&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig7&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig7&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig7&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig8&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig8&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig8&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig8&ref=pdfpubs.acs.org/acsaelm?ref=pdfhttps://doi.org/10.1021/acsaelm.4c00368?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asparameter of the films on YSZ was almost the same as that ofYSZ, anisotropic strain (i.e., tensile strain) was imposed on theshort axis co and compressive strain was imposed on the longaxis ao. Anisotropic strain with tensile strain on the short axis coand long axis ao is not energetically favorable, as experimentallyshown by the growth of the long-axis under an isotropic tensilestrain. One can easily imagine that the strain relaxation due todomain formation can possibly occur to decrease the elasticenergy penalty of lattice mismatching and to lower thethickness where strain relaxation takes place, as described asdomain III/IV, IV/V, V/VI, and III/VI in Figure 1. For thisclarification, further studies with high-resolution transmissionelectron microscopy are needed. Nevertheless, the trend ofgrowth orientation can be understood in terms of latticematching.The growth orientation of films may be affected by otherfactors. As Speck mentioned, the depolarization field cancontribute lattice strain relaxation or ferroelectric domainformation, and the extent of the depolarization fields dependson the insulating substrate or conductive substrate.39−41Therefore, it is probable that the resultant remnant strain onconductive buffer layers differs from this study. Although thefilm adopted short-axis bo orientation (Orientations III−IV) inthis study, both short-axis bo and co orientations were coexistentin the films (Orientations III−IV and/or V−VI) in a previousreport.22 This may be caused by the difference in thedeposition conditions, film thickness, or conductivity ofunderlying films; thus, these influences need to be furtherinvestigated in the future.In addition to demonstrating orientation-selective growth, itis worth mentioning that the lattice strain for the short-axis-and long-axis-oriented 7YHO films is governed by the latticespacing of the underlying layers. Manipulation of the latticespacing using CZO buffer layers provides a window for filmgrowth with a short-axis orientation and control of the strainstate. Orientation control on electrodes is very important tounderstand the more detailed information. Based on thepresent result, further investigation of the growth on variousconductive materials with suitable lattice matching is necessary.In addition, the electrical properties of the strained 7YHOfilms will be investigated as the next step.Figure 9. Lattice parameter of 7YHO films as a function of substrate lattice spacing along [100] at the deposition temperature of 700 °C. Closedsymbols: films grown directly on substrates; open symbols: (Ce0.35Zr0.65)O2-buffered substrates; square symbols: out-of-plane lattice parameters oflong-axis-oriented films; diamond symbols: out-of-plane lattice parameters of short-axis-oriented films; inverse triangle symbols: in-plane latticeparameters of long-axis-oriented films; triangle symbols: in-plane lattice parameters of short-axis-oriented films. The dashed dotted lines representthe out-of-plane lattice parameter (red line) and the in-plane lattice parameter (blue line).Table 1. Calculated Lattice Parameters and Orientation of7YHO Films on Each SubstrateSubstrateOut-of-plane latticeparameter of 7YHO film[nm]In-plane latticeparameter of7YHO film [nm]7YHO filmorientationITO//YSZ 0.520 0.506 (100)YSZ 0.507 0.518 (010)YAlO3 0.522 0.505 (100)LaAlO3 0.523 0.504 (100)SrRuO3//SrTiO30.521 0.505 (100)CZO//YSZ 0.508 0.521 (010)/(001)CZO//YAlO30.511 0.523 (010)/(001)CZO//LaAlO30.517 0.513 (100)ACS Applied Electronic Materials pubs.acs.org/acsaelm Articlehttps://doi.org/10.1021/acsaelm.4c00368ACS Appl. Electron. Mater. 2024, 6, 5525−55355532https://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig9&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig9&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig9&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368?fig=fig9&ref=pdfpubs.acs.org/acsaelm?ref=pdfhttps://doi.org/10.1021/acsaelm.4c00368?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-as■ CONCLUSIONS{100}-Oriented, epitaxial, orthorhombic (Hf0.93Y0.07)O2(7YHO) films were grown at 700 °C on various substrates,with and without (Ce0.35Zr0.65)O2 (CZO) buffer layers, usingpulsed laser deposition. XRD measurements revealed that out-of-plane short-axis-oriented films were obtained on (100)YSZ,CZO-buffered (100)pcYAlO3, and CZO-buffered (100)YSZsubstrates, whereas out-of-plane long-axis-oriented films wereobtained on (100)pcSrRuO3//SrTiO3, (100)pcLaAlO3, and(100)pcYAlO3 substrates. The unclamped lattice constants ofthe short and long axes (0.506 and 0.523 nm, respectively) ofthe orthorhombic 7YHO films were experimentally observedfor the first time on (100)pcSrRuO3//SrTiO3, (100)pcLaAlO3,and (100)pcYAlO3. Short-axis-oriented 7YHO films wereobtained when the in-plane lattice parameter was within therange of 0.518−0.523 nm. The lattice parameters werecontrolled by inserting CZO buffer layers. Consequently,{100} orientation control and manipulation of the lattice strainwere achieved by modulating the in-plane lattice spacing of theunderlying layers. These findings provide a basis for under-standing the fundamental ferroelectric properties of HfO2-based materials.■ EXPERIMENTAL METHODSEpitaxial (Hf0.93Y0.07)O2 (7YHO) films with thickness ranging from10 to 20 nm were grown at 700 °C under an oxygen pressure of 10mTorr by pulsed laser deposition using a KrF excimer laser (λ = 248nm). The power and repetition of KrF excimer laser were 35 mJ and 3Hz, respectively, and the laser was focused on the target with anoptical lens. The target-to-substrate distance of 55 mm was used.Single crystals of (100)YSZ, (100)pcYAlO3, (100)pcLaAlO3, and(100)SrTiO3 covered by (100)pcSrRuO3 were used as substrates. Theepitaxial SrRuO3 film was deposited by radio frequency magnetronsputtering.Approximately 15 nm thick epitaxial (Ce0.35Zr0.65)O2 buffer layerswere grown before the deposition of 7YHO films. The deposition of(Ce0.35Zr0.65)O2 layers was implemented by using pulsed laserdeposition. The laser condition, deposition temperature, oxygenpressure, and the target-to-substrate distance were same as for thedeposition of 7YHO.The crystal phase and the orientation were determined based onfour types of XRD measurements: (1) 2θ−ω scans along the z-direction (PANalytical X’Pert MRD system), (2) 2θ−ψ scans alongthe y-z-direction (Bruker, D8-discover), (3) 2θχ−φ scans along the y-direction (Rigaku Smartlab), and (4) 2θχ−φ scans along the x-ydirection (Rigaku Smartlab). An X-ray diffractometer (Bruker, D8-discover) equipped with a large-area 2D detector (Vantec-500) withCuKα radiation was also used to evaluate the in-plane and out-of-plane crystal structure. In the in-plane XRD measurements, theparallel Soller slit with 0.5°, which is relatively wider than typicallyused, was employed to achieve observation of faint signals ofdiffraction peaks from the orthorhombic 7YHO. The orientationdetermination based on these XRD measurements was illustrated inFigure S1.■ ASSOCIATED CONTENT*sı Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acsaelm.4c00368.Additional experiments, schematics of the crystalorientations and domains, unit cells, lattice parametersof (AxZr1−x)O2 (fluorite structure) as a function of x, thecalculated critical thickness of (Ce0.35Zr0.65)O2 films,out-of-plane and in-plane XRD patterns, and in-planelattice spacing of the buffer layer as a function of thelattice spacing of the substrate (PDF)■ AUTHOR INFORMATIONCorresponding AuthorsHiroshi Funakubo − Department of Materials Science andEngineering, Tokyo Institute of Technology, Yokohama,Kanagawa 226-8502, Japan; orcid.org/0000-0002-1106-200X; Email: funakubo.h.aa@m.titech.ac.jpKazuki Okamoto − Department of Materials Science andEngineering, Tokyo Institute of Technology, Yokohama,Kanagawa 226-8502, Japan; Email: okamoto.k.ar@m.titech.ac.jpAuthorsYoshiki Maekawa − Department of Materials Science andEngineering, Tokyo Institute of Technology, Yokohama,Kanagawa 226-8502, JapanKoji Hirai − Department of Materials Science and Engineering,Tokyo Institute of Technology, Yokohama, Kanagawa 226-8502, JapanTakao Shimizu − National Institute for Materials Science,Tsukuba, Ibaraki 987-6543, Japan; orcid.org/0000-0001-9508-7601Complete contact information is available at:https://pubs.acs.org/10.1021/acsaelm.4c00368Author ContributionsThe manuscript was written through contributions of allauthors. All authors have given approval to the final version ofthe manuscript.NotesThe authors declare no competing financial interest.■ ACKNOWLEDGMENTSThis work was supported by the MEXT Initiative to EstablishNext-Generation Novel Integrated Circuits Centers (X-NICS)(JPJ011438), the MEXT Program: Data Creation andUtilization Type Material Research and Development(JPMXP1122683430), and by the Japan Society for thePromotion of Science (JSPS) KAKENHI Grant Nos.21H01617, 22K18307, 23K13364, and 24H00375.■ REFERENCES(1) Müller, J.; Böscke, T. S.; Bräuhaus, D.; Schröder, U.; Böttger, U.;Sundqvist, J.; Kücher, P.; Mikolajick, T.; Frey, L. FerroelectricZr0.5Hf0.5O2 Thin Films for Nonvolatile Memory Applications. Appl.Phys. Lett. 2011, 99 (11), 112901.(2) Müller, J.; Böscke, T. S.; Müller, S.; Yurchuk, E.; Polakowski, P.;Paul, J.; Martin, D.; Schenk, T.; Khullar, K.; Kersch, A.; Weinreich,W.; Riedel, S.; Seidel, K.; Kumar, A.; Arruda, T. M.; Kalinin, S. 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