Epitaxial La0.7Sr0.3MnO3 thin films with two in-plane orientations ...

JOURNAL OF APPLIED PHYSICS 97, 073905 2005Epitaxial La 0.7 Sr 0.3 MnO 3 thin films with two in-plane orientations on siliconsubstrates with yttria-stabilized zirconia and YBa 2 Cu 3 O 7−as buffer layersWei Chuan GohDepartment of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542Kui Yao aInstitute of Materials Research and Engineering (IMRE), 3 Research Link, Singapore 117602C. K. OngDepartment of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542Received 19 July 2004; accepted 28 January 2005; published online 18 March 2005Epitaxial La 0.7 Sr 0.3 MnO 3 LSMO thin films were fabricated on silicon substrates by pulsed laserdeposition utilizing yttria-stabilized zirconia YSZ and YBa 2 Cu 3 O 7− YBCO films as bufferlayers. Structural characterization showed that the epitaxial LSMO films were 001 oriented withtwo in-plane orientations and exhibited a columnar growth structure. In contrast, when LSMO wasdeposited directly on the YSZ/Si substrate without the YBCO template layer, it was characterized asa mixture of randomly oriented polycrystalline grains and 001-oriented grains, without thecolumnar growth structure. The role of the YBCO layer for achieving the c-axis-oriented epitaxialLSMO film by introducing the dual-in-plane orientation mechanism on the YSZ/Si substrate hasbeen analyzed. In addition, it was found that the LSMO film with the dual-in-plane orientations onthe YBCO/YSZ/Si substrate exhibited a much lower resistivity compared to the LSMO film directlydeposited on the YSZ/Si substrate. This effect is attributed to the existence of the randomly orientedgrains in the latter, which resulted in more significant electron scattering at the grain boundaries. Thehigher density of grain boundaries in the LSMO film on YSZ/Si also led to a substantially highermagnetoresistance. © 2005 American Institute of Physics. DOI: 10.1063/1.1876577INTRODUCTIONThe perovskite La 0.7 Sr 0.3 MnO 3 LSMO has potentialuse in various magnetic applications, such as magnetic-fieldsensors, 1 hard disk read heads, 2 infrared devices, 3 and microwaveactive components, to name a few, due to its largecolossal magnetoresistance CMR. In addition, the highelectrical conductivity of LSMO and its lattice matchingwith other perovskite ferroelectric oxides, such as lead zirconatetitanate PZT and barium strontium titanate BST,make LSMO an attractive candidate as the bottom electrodematerial for ferroelectric perovskite oxide thin films. Assingle-crystal materials often exhibit superior electrical andmagnetic properties, epitaxial LSMO thin films are desirablefor both electrical and magnetic applications.Although epitaxial LSMO thin films can be grown onsingle-crystal oxide substrates with lattice-matching parameters,including SrTiO 3 and LaAlO 3 , 4 reports of LSMO thinfilms grown on silicon substrates are very limited in theliterature. 5–7 There is no direct crystal lattice match betweensilicon and LSMO, and chemical reactions between LSMOand silicon could also occur to form interfacial phases. Thus,appropriate buffer layers need to be introduced in order toachieve high-quality epitaxial LSMO film growth. It is alsoof interest to investigate the epitaxial mechanism of LSMOa Author to whom correspondence should be addressed; electronic mail:k-yao@imre.a-star.edu.sgfilm growth on buffer-layered silicon substrates to clarifyhow the structure develops and how the buffer layers affectthe film properties.In this paper, we describe the fabrication of LSMO filmson an yttria-stabilized zirconia YSZ buffer layer on silicon.The YSZ layer has good lattice matching with the LSMOand prevents the formation of an interfacial layer betweenLSMO and silicon. By introducing an additional buffer layerof YBa 2 Cu 3 O 7− YBCO on the top of the YSZ layer, wecan promote the epitaxial growth of 001-oriented LSMOthin films. The epitaxial growth mechanism, crystal structures,surface morphology, resistivity, magnetoresistance,and the various interrelationships between these characteristicsare discussed.EXPERIMENTAL PROCEDURE100-oriented silicon substrates were cleaned using nitricacid in an ultrasonic cleaner for 5 min to remove anynatural oxide layer or oxide contaminant on the surface. Thesubstrates were subsequently cleaned with de-ionized water,acetone, and ethanol. All the YSZ, YBCO, and LSMO thinfilms were prepared by the pulsed laser deposition PLDmethod, in which a KrF excimer laser was used pulse duration30 ns, wavelength 248 nm, Lambda Physik Lextra 200.The detailed processing parameters for all the three films aresummarized in Table I. Two kinds of multilayer sampleswere prepared, namely, LSMO/YSZ/Si and LSMO/YBCO/0021-8979/2005/977/073905/5/$22.5097, 073905-1© 2005 American Institute of PhysicsDownloaded 04 Apr 2005 to 137.132.123.76. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp

073905-2 Goh, Yao, and Ong J. Appl. Phys. 97, 073905 2005TABLE I. Summary of PLD processing parameters for the YSZ, YBCO, and LSMO thin films.LayerDepositiontemperature°COxygenambiencembarLaserfluenceJ/cm 2 RepetitionrateHzDepositiontimeminThickness10 −9 mYSZ 670 510 −4 5 5 13 10YBCO 650 510 −1 3.5 3 8 200LSMO 670 210 −1 5 5 45 1000YSZ/Si. In addition, a LSMO film grown on a latticematchingsingle-crystal LaAlO 3 LAO substrate was fabricatedfor comparison purposes.Both -2 and -scan x-ray diffraction XRD, Bruker,D8-ADVANCE studies were conducted to determine thecrystalline phase and the crystal orientation of the films.Scanning electron microscopy SEM, JEOL 6700F was carriedout to investigate the morphology of the films. A fourproberesistance measurement system Keithley 220 currentsource, Keithley 182 voltmeter, and Lakeshore 330 temperaturecontroller was used to analyze the electrical propertiesof the samples from room temperature to 77 K. MagnetoresistanceMR experiments with magnetic field up to 3000 Ghomemade electromagnet were also carried out at 77 K toassess the magnetic properties of the LSMO films.for both samples, as revealed by the matched fourfold symmetryfor both YSZ and silicon in Figs. 2a and 2b. TheLSMO film on the YSZ layer, which shows the incomplete001 orientation in the -2 scan in Fig. 1, exhibits a weakfour-fold in-plane symmetry with broad diffraction peaks ofplane 113, as shown in Fig. 2b. From these results we canconclude that the LSMO film on the YSZ consists mostly ofrandomly oriented grains with a small number of epitaxialgrains. For the LSMO/YBCO/YSZ/Si sample, the scan ofthe LSMO 103 plane exhibits strong eight-fold diffractionpeaks, as shown in Fig. 2a. This confirms that there are twoRESULTSA. Crystalline structureXRD -2 scans for the LSMO/YSZ/Si and LSMO/YBCO/YSZ/Si samples are shown in Fig. 1. An incomplete001-orientation preference and some unidentified phasesare observed in the LSMO film grown on YSZ/Si, whereasthe LSMO film grown on YBCO/YSZ/Si exhibits a complete001 orientation and no unidentified phase is present. Wedid not observe strong XRD peaks of the YBCO layers becausethe power of the x ray was low 20 kV,5 mA.XRD scans for the LSMO/YBCO/YSZ/Si and LSMO/YSZ/Si samples are presented in Figs. 2a and 2b, respectively.The YSZ layers exhibit a cube-on-cube epitaxial relationshipto the 100-oriented single-crystal silicon substratesFIG. 1. X-ray diffraction patterns -2 for the LSMO thin films depositedon YBCO/YSZ/Si and YSZ/Si substrates.FIG. 2. a X-ray diffraction patterns scan for I Si 113, II YSZ113, and III LSMO 113 peaks for the LSMO/YBCO/YSZ/Si multilayer.The inset figure shows the relative angle relationship between YBCO103 and LSMO 103. b X-ray diffraction results scan for I Si113, II YSZ 113, and III LSMO 113 peaks for the LSMO/YSZ/Simultilayer.Downloaded 04 Apr 2005 to 137.132.123.76. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp

073905-3 Goh, Yao, and Ong J. Appl. Phys. 97, 073905 2005FIG. 3. SEM images of cross sections through a LSMO/YBCO/YSZ/Siand b LSMO/YSZ/Si.sets of in-plane orientations in the LSMO layer with an inplaneshift of 45° from each other. The two sets of in-planeorientation in the LSMO layer are found to result from thecorresponding two sets of the in-plane orientation in theYBCO template layer, as indicated by the scan results ofthe 103 planes for both LSMO and YBCO layers in theinset figure in Fig. 2a. As Fig. 1 shows a complete 001orientation, the LSMO film on the YBCO/YSZ/Si substrateis an epitaxial film but has two sets of in-plane orientation,with a shift of 45° from each other.B. MorphologySEM images of cross sections through the LSMO/YBCO/YSZ/Si and LSMO/YSZ/Si multilayer structures arepresented in Figs. 3a and 3b, respectively. The LSMOfilm on the YBCO layer exhibits a columnar growth structure,as shown in Fig. 3a, while the LSMO film grown onthe YSZ/Si substrate does not show a columnar structure, asapparent from Fig. 3b. The columnar grains in the LSMOfilm on the YBCO/YSZ/Si substrate are around 100–175 nmin width across the film. The LSMO film on the YSZ/Sisubstrate has more grain boundaries than the LSMO film onthe YBCO/YSZ/Si substrate, which is also apparent fromSEM images of the film surfaces not shown. The SEMimages indicate better epitaxial quality for the LSMO film onthe YBCO/YSZ/Si substrate than that on the YSZ/Si, whichis consistent with the XRD results. Both of the LSMO filmsshow a root-mean-square surface roughness of around 10 nmby atomic force microscopy AFM. Our LSMO films werealways rather granular due to the nature of substantial structuralmismatch and imperfect epitaxial growth by the pulsedlaser deposition.C. Resistivity and magnetoresistanceFigure 4 shows the temperature dependence of theLSMO film resistivity deposited on LAO, YBCO/YSZ/Si,and YSZ/Si substrates. The resistivity of the LSMO film onthe YBCO/YSZ/Si substrate is much lower than that of theLSMO film on the YSZ/Si, which has randomly orientedgrains, but is still higher than the resistivity of the epitaxialLSMO film grown on the lattice-matched substrate LAO.The MR of the LSMO films deposited on YBCO/YSZ/Siand YSZ/Si substrates is given in Fig. 5. A magnetic field ofup to 3000 G was applied perpendicular to the surface of thesamples at 77 K. In Fig. 5, the LSMO film on the YSZ/Sisubstrate has much larger MR 16% than the LSMO filmFIG. 4. Temperature dependence of LSMO thin films deposited on a LAO,b YBCO/YSZ/Si, and c YSZ/Si substrates.on the YBCO/YSZ/Si substrate 2%. The results are consistentwith the report by Gu et al. 8 who found that LSMOfilms with a higher density of grain boundaries showed alarger MR.DISCUSSIONThe growth of YSZ on 100 silicon is based on a100100 cube-on-cube mechanism, which is the same asreported previously by our group. 9 When the YBCO film wassubsequently deposited onto the YSZ film, the YBCO filmexhibited a strong c-axis orientation along the thickness directionand two in-plane orientations with a 45° shift fromeach other, i.e., YBCO100001YSZ100001 andYBCO110001YSZ100001, as indicated in Fig. 2a.A similar dual-in-plane orientation for YBCO thin films wasalso reported in the literature. 10,11 We also observed that theorientation of LSMO100001 is more preferred when theLSMO film is grown on the YBCO layer with the dual-inplaneorientation, i.e., YBCO100001YSZ100001and YBCO110001YSZ100001, as its XRD intensityis much stronger than the orientation of LSMO110100 inFig. 2a. Without introducing the YBCO film, the LSMOFIG. 5. Magnetoresistance MR vs magnetic field for LSMO thin filmsdeposited on a YBCO/YSZ/Si and b YSZ/Si substrates.Downloaded 04 Apr 2005 to 137.132.123.76. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp

073905-4 Goh, Yao, and Ong J. Appl. Phys. 97, 073905 2005FIG. 6. A schematic diagram of the proposed epitaxial growth mechanismsof the YBCO thin film for the two observed in-plane orientations on theYSZ buffer layer: a YBCO 100001YSZ100001, in which theCu–O plane of the YBCO is grown on the O–O plane of the YSZ 3/4height of the unit cell of YSZ; b YBCO 110001YSZ100001, theCu–O plane of the YBCO is turned 45° in plane on the Zr–Zr plane of YSZ.FIG. 7. Structure of a orthorhombic YBCO crystal and b perovskiteLSMO crystal.film on the YSZ did not exhibit the same in-plane orientationrelationship with the underlying YSZ layer, as shown in Fig.2b. The epitaxial growth of YBCO110001YSZ100001 is obviously due to the small misfit of 6.24% for the110 lattice of YBCO and 100 lattice of YSZ. However, itis interesting to note that YBCO100001YSZ100001epitaxial growth occurred while LSMO100001YSZ100001 did not. Schematic diagrams to illustrateboth of the in-plane orientations of YBCO on YSZare shown in Fig. 6. The Cu–O plane layers are depicted tobe in contact with YSZ as a proposed growth mechanism. InFig. 6a, the Cu–O plane of the YBCO is grown onto theO–O plane of the YSZ at 3/4 of an YSZ unit cell for theYBCO100YSZ100 orientation. In Fig. 6b, the Cu–Oplane of the YBCO is turned 45° in plane on the Zr–Zr planeof YSZ to realize the YBCO110YSZ100 epitaxialgrowth.The structures of YBCO and LSMO need to be analyzedto understand the reason why YBCO exhibitsYBCO100YSZ100-type in-plane orientation whileLSMO, which apparently has similar lattice parameters, doesnot grow with the same in-plane orientation. The misfits forYBCO100YSZ100 and LSMO100YSZ100growths are 24.90% and 24.71%, respectively. In the orthorhombicYBCO crystal structure, as shown in Fig. 7a, theCu–O bond at the second and third planes is slightly bent,whereas the Mn–O plane in the perovskite LSMO structureis flat, as shown in Fig. 7b. The bending of the Cu–O planein YBCO is better suited for the epitaxial growth, as depictedin Fig. 6a, because it better tolerates the lattice misfit andprovides a more angular flexibility for forming chemicalbonds between copper and oxygen at the interface. In addition,the electron configuration 3d 10 4s 1 of copper could alsoform Cu + that electrically neutralizes oxygen vacancies at theinterface of the YSZ and YBCO, and provides more structuralflexibility than LSMO. It is therefore essential to havethe additional YBCO template layer in order to grow completelyc-axis-oriented LSMO films through the dual-inplaneorientation mechanisms. With that being said, we dounderstand that there could be other oxide candidates to beused as a buffer layer instead of YBCO. However, our resultsshow that the relatively complicated and more tolerant structureof YBCO may promote the epitaxial growth betweentwo materials with substantial difference in structure, such asLSMO and YSZ described in this paper.The difference in the resistivities of the LSMO/YBCO/YSZ/Si and LSMO/YSZ/Si films can be understood by considerationof electron polarization and scattering mechanisms.Inside the magnetic domain, the conduction electronsare spin polarized, i.e., having the same magnetic dipole direction,and electrons are easily transferred between pairs ofMn 3+ and Mn 4+ ions. However, when these electrons travelacross the grain boundaries, they experience strong spindependentscattering. 11 A large density of grain boundarieswould lead to an increase in scattering and hence result in anincrease of the resistivity. The lower resistivity of the LSMOfilm grown on YBCO/YSZ/Si as compared to the LSMO filmon YSZ/Si is due to its complete c-axis orientation with thecolumnar grain structure. The smaller, randomly orientedgrains in the LSMO/YSZ/Si form many more grain boundaries.It is therefore understandable that the LSMO filmsgrown on YBCO/YSZ/Si show a larger resistivity whencompared to the epitaxial LSMO film on LAO substrate,which has only one in-plane orientation.With the application of a magnetic field, the randomlyoriented magnetic domains in the LSMO film on the YSZbuffer layer are reoriented, which leads to a significant decreasein the resistivity. Therefore, a larger MR was observedin the LSMO film deposited on the YSZ/Si substrate thanthat on the YBCO/YSZ/Si substrate, which has no substantialamount of randomly oriented grains.CONCLUSION001-oriented epitaxial LSMO thin films with two inplaneorientations were fabricated on silicon substrates byDownloaded 04 Apr 2005 to 137.132.123.76. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp

073905-5 Goh, Yao, and Ong J. Appl. Phys. 97, 073905 2005PLD with YSZ and YBCO films as buffer layers. The epitaxialLSMO films on the YBCO layers had a cross-sectionalcolumnar growth structure. The dual-in-plane orientations ofthe LSMO film were formed by epitaxial growth from dualorientations in the YBCO template layer. The crystal orientationrelationship among the LSMO, YBCO, and YSZ filmscan be expressed as, LSMO/YBCO110001YSZ100001 and LSMO/YBCO100001YSZ100001. Incontrast, when LSMO films were deposited directly onYSZ/Si without using the YBCO template layer, it containeda mixture of randomly oriented polycrystalline grains and001-oriented grains. Therefore, introduction of the YBCOlayer was essential to obtain c-axis-oriented epitaxial LSMOfilms through the dual-in-plane orientation growth mechanismof YBCO on the YSZ/Si substrate. The difference betweenthe in-plane orientations of the LSMO and YBCOfilms on the YSZ layer is mainly attributed to the disparity intheir crystallographical structure. The LSMO film with thedual-in-plane orientations on the YBCO/YSZ/Si substrateexhibited a much lower resistivity as compared to the LSMOfilm on the YSZ/Si substrate, which had more randomly orientedgrains and hence more electron scattering at the grainboundaries. The greater density of grain boundaries in theLSMO film on the YSZ/Si substrate also led to a substantiallyhigher magnetoresistance.1 S. Jin, M. McCormack, T. H. Tiefel, and R. Ramesh, J. Appl. Phys. 76,6929 1994.2 K. Derbyshire and E. Korczynski, Solid State Technol. 38, 571995.3 A. Lisauskas, S. I. Khartsev, and A. Grishin, Appl. Phys. Lett. 77, 7562000.4 J. Li, C. K. Ong, Q. Zhan, and D. X. Li, J. Phys.: Condens. Matter 14,6341 2002.5 Z. Tranjanovic et al., Appl. Phys. Lett. 69, 1005 1996.6 J. Y. Gu et al., Appl. Phys. Lett. 70, 17631997.7 J.-H. Kim, S. I. Khartsev, and A. M. Grishin, Appl. Phys. Lett. 82, 42952003.8 J. Y. Gu et al., Appl. Phys. Lett. 72, 1113 1998.9 S. J. Wang, S. Y. Xu, L. P. You, S. L. Lim, and C. K. Ong, Supercond. Sci.Technol. 13, 362 2000.10 S. M. Garrison, N. Newman, B. F. Cole, K. Char, and R. W. Barton, Appl.Phys. Lett. 58, 2168 1991.11 A. Gupta et al., Phys. Rev. B 54, 015629 1996.Downloaded 04 Apr 2005 to 137.132.123.76. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp