Epitaxial La 0.7 Sr 0.3 Mn O 3 thin films with two in-plane orientations on siliconsubstrates with yttria-stabilized zirconia and Y Ba 2 Cu 3 O 7 − δ as buffer layersWei Chuan Goh, Kui Yao, and C. K. Ong Citation: Journal of Applied Physics 97, 073905 (2005); doi: 10.1063/1.1876577 View online: http://dx.doi.org/10.1063/1.1876577 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/97/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Enhanced magnetoelectric effect in La0.67Sr0.33MnO3/PbZr0.52Ti0.48O3 multiferroic nanocomposite films witha SrRuO3 buffer layer J. Appl. Phys. 113, 164106 (2013); 10.1063/1.4803057 Magnetotransport properties of c -axis oriented La 0.7 Sr 0.3 MnO 3 thin films on MgO-buffered SiO 2 / Sisubstrates J. Appl. Phys. 105, 07D711 (2009); 10.1063/1.3065974 Structural, magnetic, and electric properties of La 0.7 Sr 0.3 MnO 3 / PbZr x Ti 1 − x O 3 heterostructures J. Appl. Phys. 104, 063908 (2008); 10.1063/1.2980322 Microcrack-free epitaxy of thick YBa 2 Cu 3 O 7 − δ films on vicinal r-cut sapphire buffered with CeO 2 Appl. Phys. Lett. 86, 192507 (2005); 10.1063/1.1926415 Large low-field magnetoresistance observed in twinned La 2 ∕ 3 Ca 1 ∕ 3 MnO 3 thin films epitaxially grown onyttria-stabilized zirconia-buffered silicon on insulator substrates Appl. Phys. Lett. 86, 112514 (2005); 10.1063/1.1875766

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Epitaxial La 0.7Sr0.3MnO3 thin films with two in-plane orientations on siliconsubstrates with yttria-stabilized zirconia and YBa 2Cu3O7−das buffer layers

Wei Chuan GohDepartment of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542

Kui Yaoa!

Institute of Materials Research and Engineering (IMRE), 3 Research Link, Singapore 117602

C. K. OngDepartment of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542

sReceived 19 July 2004; accepted 28 January 2005; published online 18 March 2005d

Epitaxial La0.7Sr0.3MnO3 sLSMOd thin films were fabricated on silicon substrates by pulsed laserdeposition utilizing yttria-stabilized zirconiasYSZd and YBa2Cu3O7−d sYBCOd films as bufferlayers. Structural characterization showed that the epitaxial LSMO films weres001d 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 ands001d-oriented grains, without thecolumnar growth structure. The role of the YBCO layer for achieving thec-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. fDOI: 10.1063/1.1876577g

INTRODUCTION

The perovskite La0.7Sr0.3MnO3 sLSMOd has potentialuse in various magnetic applications, such as magnetic-fieldsensors,1 hard disk read heads,2 infrared devices,3 and micro-wave active components, to name a few, due to its largecolossal magnetoresistancesCMRd. In addition, the highelectrical conductivity of LSMO and its lattice matchingwith other perovskite ferroelectric oxides, such as lead zir-conate titanatesPZTd and barium strontium titanatesBSTd,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 param-eters, including SrTiO3 and LaAlO3,

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 LSMO

film 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 zirconiasYSZd 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 YBa2Cu3O7−d sYBCOd on the top of the YSZ layer, wecan promote the epitaxial growth ofs001d-oriented LSMOthin films. The epitaxial growth mechanism, crystal struc-tures, surface morphology, resistivity, magnetoresistance,and the various interrelationships between these characteris-tics are discussed.

EXPERIMENTAL PROCEDURE

s100d-oriented silicon substrates were cleaned using ni-tric acid 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 depositionsPLDdmethod, in which a KrF excimer laser was usedspulse dura-tion 30 ns, wavelength 248 nm, Lambda Physik Lextra 200d.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/

adAuthor to whom correspondence should be addressed; electronic mail:k-yao@imre.a-star.edu.sg

JOURNAL OF APPLIED PHYSICS97, 073905s2005d

0021-8979/2005/97~7!/073905/5/$22.50 © 2005 American Institute of Physics97, 073905-1

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YSZ/Si. In addition, a LSMO film grown on a lattice-matching single-crystal LaAlO3 sLAOd substrate was fabri-cated for comparison purposes.

Both u-2u and f-scan x-ray diffractionsXRD, Bruker,D8-ADVANCEd studies were conducted to determine thecrystalline phase and the crystal orientation of the films.Scanning electron microscopysSEM, JEOL 6700Fd was car-ried out to investigate the morphology of the films. A four-probe resistance measurement systemsKeithley 220 currentsource, Keithley 182 voltmeter, and Lakeshore 330 tempera-ture controllerd was used to analyze the electrical propertiesof the samples from room temperature to 77 K. Magnetore-sistancesMRd experiments with magnetic field up to 3000 Gshomemade electromagnetd were also carried out at 77 K toassess the magnetic properties of the LSMO films.

RESULTS

A. Crystalline structure

XRD u-2u scans for the LSMO/YSZ/Si and LSMO/YBCO/YSZ/Si samples are shown in Fig. 1. An incompletes001d-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 completes001d orientation and no unidentified phase is present. Wedid not observe strong XRD peaks of the YBCO layers be-cause the power of the x ray was lows20 kV,5 mAd.

XRD f scans for the LSMO/YBCO/YSZ/Si and LSMO/YSZ/Si samples are presented in Figs. 2sad and 2sbd, respec-tively. The YSZ layers exhibit a cube-on-cube epitaxial rela-tionship to thes100d-oriented single-crystal silicon substrates

for both samples, as revealed by the matched fourfold sym-metry for both YSZ and silicon in Figs. 2sad and 2sbd. TheLSMO film on the YSZ layer, which shows the incompletes001d orientation in theu-2u scan in Fig. 1, exhibits a weakfour-fold in-plane symmetry with broad diffraction peaks ofplaneh113j, as shown in Fig. 2sbd. 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, thef scan ofthe LSMO h103j plane exhibits strong eight-fold diffractionpeaks, as shown in Fig. 2sad. This confirms that there are two

TABLE I. Summary of PLD processing parameters for the YSZ, YBCO, and LSMO thin films.

Layer

Depositiontemperature

s°Cd

Oxygenambience

smbard

LaserfluencesJ/cm2d

RepetitionratesHzd

Depositiontimesmind

Thicknesss310−9 md

YSZ 670 5310−4 5 5 13 10YBCO 650 5310−1 3.5 3 8 200LSMO 670 2310−1 5 5 45 1000

FIG. 1. X-ray diffraction patternssu-2ud for the LSMO thin films depositedon YBCO/YSZ/Si and YSZ/Si substrates.

FIG. 2. sad X-ray diffraction patternssf scand for sId Si h113j, sII d YSZh113j, andsIII d LSMO h113j peaks for the LSMO/YBCO/YSZ/Si multilayer.The inset figure shows the relativef angle relationship between YBCOh103j and LSMO h103j. sbd X-ray diffraction resultssf scand for sId Sih113j, sII d YSZ h113j, and sIII d LSMO h113j peaks for the LSMO/YSZ/Simultilayer.

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sets of in-plane orientations in the LSMO layer with an in-plane shift 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 thef scan results ofthe h103j planes for both LSMO and YBCO layers in theinset figure in Fig. 2sad. As Fig. 1 shows a completes001dorientation, 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. Morphology

SEM images of cross sections through the LSMO/YBCO/YSZ/Si and LSMO/YSZ/Si multilayer structures arepresented in Figs. 3sad and 3sbd, respectively. The LSMOfilm on the YBCO layer exhibits a columnar growth struc-ture, as shown in Fig. 3sad, while the LSMO film grown onthe YSZ/Si substrate does not show a columnar structure, asapparent from Fig. 3sbd. 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 surfacessnot shownd. 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 microscopysAFMd. Our LSMO films werealways rather granular due to the nature of substantial struc-tural mismatch and imperfect epitaxial growth by the pulsedlaser deposition.

C. Resistivity and magnetoresistance

Figure 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 MRs,16%d than the LSMO film

on the YBCO/YSZ/Si substrates,2%d. The results are con-sistent with the report by Guet al.8 who found that LSMOfilms with a higher density of grain boundaries showed alarger MR.

DISCUSSION

The growth of YSZ ons100d silicon is based on af100g i f100g 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 strongc-axis orientation along the thickness di-rection and two in-plane orientations with a 45° shift fromeach other, i.e., YBCOf100gs001d iYSZf100gs001d andYBCOf110gs001d iYSZf100gs001d, as indicated in Fig. 2sad.A similar dual-in-plane orientation for YBCO thin films wasalso reported in the literature.10,11 We also observed that theorientation of LSMOf100gs001d is more preferred when theLSMO film is grown on the YBCO layer with the dual-in-plane orientation, i.e., YBCOf100gs001d iYSZf100gs001dand YBCOf110gs001d iYSZf100gs001d, as its XRD intensityis much stronger than the orientation of LSMOf110gs100d inFig. 2sad. Without introducing the YBCO film, the LSMO

FIG. 3. SEM images of cross sections throughsad LSMO/YBCO/YSZ/Siand sbd LSMO/YSZ/Si.

FIG. 4. Temperature dependence of LSMO thin films deposited onsad LAO,sbd YBCO/YSZ/Si, andscd YSZ/Si substrates.

FIG. 5. MagnetoresistancesMRd vs magnetic field for LSMO thin filmsdeposited onsad YBCO/YSZ/Si andsbd YSZ/Si substrates.

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film on the YSZ did not exhibit the same in-plane orientationrelationship with the underlying YSZ layer, as shown in Fig.2sbd. The epitaxial growth of YBCOf110gs001d iYSZf100g3s001d is obviously due to the small misfit of 6.24% for thes110d lattice of YBCO ands100d lattice of YSZ. However, itis interesting to note that YBCOf100gs001d iYSZf100gs001depitaxial growth occurred while LSMOf100g3s001d iYSZf100gs001d did not. Schematic diagrams to il-lustrate both 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. 6sad, the Cu–O plane of the YBCO is grown onto theO–O plane of the YSZsat 3/4 of an YSZ unit celld for theYBCOf100g iYSZf100g orientation. In Fig. 6sbd, the Cu–Oplane of the YBCO is turned 45° in plane on the Zr–Zr planeof YSZ to realize the YBCOf110g iYSZf100g epitaxialgrowth.

The structures of YBCO and LSMO need to be analyzedto understand the reason why YBCO exhibitsYBCOf100g iYSZf100g-type in-plane orientation whileLSMO, which apparently has similar lattice parameters, doesnot grow with the same in-plane orientation. The misfits forYBCOf100g iYSZf100g and LSMOf100g iYSZf100ggrowths are 24.90% and 24.71%, respectively. In the ortho-rhombic YBCO crystal structure, as shown in Fig. 7sad, 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. 7sbd. The bending of the Cu–O planein YBCO is better suited for the epitaxial growth, as depictedin Fig. 6sad, because it better tolerates the lattice misfit andprovides a more angular flexibility for forming chemicalbonds between copper and oxygen at the interface. In addi-tion, the electron configuration 3d104s1 of copper could alsoform Cu+ that electrically neutralizes oxygen vacancies at theinterface of the YSZ and YBCO, and provides more struc-tural flexibility than LSMO. It is therefore essential to havethe additional YBCO template layer in order to grow com-

pletely c-axis-oriented LSMO films through the dual-in-plane orientation 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 struc-ture of 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 con-sideration of electron polarization and scattering mecha-nisms. Inside the magnetic domain, the conduction electronsare spin polarized, i.e., having the same magnetic dipole di-rection, and electrons are easily transferred between pairs ofMn3+ and Mn4+ ions. However, when these electrons travelacross the grain boundaries, they experience strong spin-dependent scattering.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 completec-axis orientation with thecolumnar grain structure. The smaller, randomly orientedgrains in the LSMO/YSZ/Si form many more grain bound-aries. 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 de-crease in 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.

CONCLUSION

s001d-oriented epitaxial LSMO thin films with two in-plane orientations were fabricated on silicon substrates by

FIG. 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: sad YBCO f100gs001d iYSZf100gs001d, in which theCu–O plane of the YBCO is grown on the O–O plane of the YSZs3/4height of the unit cell of YSZd; sbd YBCO f110gs001d iYSZf100gs001d, theCu–O plane of the YBCO is turned 45° in plane on the Zr–Zr plane of YSZ.

FIG. 7. Structure ofsad orthorhombic YBCO crystal andsbd perovskiteLSMO crystal.

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PLD with YSZ and YBCO films as buffer layers. The epi-taxial LSMO 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 orien-tation relationship among the LSMO, YBCO, and YSZ filmscan be expressed as, LSMO/YBCOf110gs001d iYSZf100g3s001d and LSMO/YBCOf100gs001d iYSZf100gs001d. Incontrast, when LSMO films were deposited directly onYSZ/Si without using the YBCO template layer, it containeda mixture of randomly oriented polycrystalline grains ands001d-oriented grains. Therefore, introduction of the YBCOlayer was essential to obtainc-axis-oriented epitaxial LSMOfilms through the dual-in-plane orientation growth mecha-nism of YBCO on the YSZ/Si substrate. The difference be-tween the 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 LSMO

film on the YSZ/Si substrate, which had more randomly ori-ented grains 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 substan-tially higher magnetoresistance.

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