epitaxial ca2ruo4+δ thin films grown on (001) laalo3 by pulsed laser deposition

007) 3946–3951www.elsevier.com/locate/tsf

Thin Solid Films 515 (2

Epitaxial Ca2RuO4+δ thin films grown on (001) LaAlO3

by pulsed laser deposition

Y. Xin a,⁎, X. Wang b,1, Z.X. Zhou a,2, J.P. Zheng b

a National High Magnetic Field Laboratory, Tallahassee, FL 32310, USAb Department of Electrical and Computer Engineering, FAMU-FSU College of Engineering, FL 32310, USA

Received 12 May 2006; received in revised form 14 August 2006; accepted 20 September 2006Available online 25 October 2006

Abstract

Ca2RuO4+δ single crystal has shown intriguing physical properties that are the focus of many recent studies. In this work, epitaxial Ca2RuO4+δ

thin films have been grown by pulsed laser deposition technique. The crystallinity and microstructures of the films were studied by both X-raydiffraction and transmission electron microscopy. Under carefully controlled conditions, Ca2RuO4+δ grows epitaxially on (001) LaAlO3 substratewith a in-plane relationship of ⟨110⟩film$⟨100⟩sub. The lattice parameters of films are consistent with a strained long phase. The growth conditionsthat influence the film quality, such as substrate temperature, target composition, oxygen pressure and flow rate, have been systematically studied.The thin film's magnetic and electrical transport properties differ drastically from the bulk counterpart. In contrary to the bulk, our films showoverall metallic behavior with no magnetic transition in temperature range of 4–300 K. Aweak metal to non-metal transition at about 200 K wasobserved, which could be correlated to the imperfect microstructures of the thin films.© 2006 Elsevier B.V. All rights reserved.

Keywords: Pulsed laser deposition; Calcium ruthenate

1. Introduction

Transition metal oxides as one class of electroceramicmaterials have shown a rich spectrum of physical properties. Inparticular, perovskite-like transition metal oxides have attractedwide attention for their novel and complex physical propertiesand potential device applications. They display a broad range oftechnologically important phenomena, including ferromagne-tism, ferroelectricity, and superconductivity. One fascinatingtype is the Ruddlesdon-Popper series with a general chemicalformula An+1MnO3n+1, with M as transition metals such as Ti,Mn and Ru, and A the alkaline elements, n an integer. Amongthem, the 4d-electron ruthenates have begun to arouse much

⁎ Corresponding author. Tel.: +1 850 644 1529; fax: +1 850 644 0867.E-mail address: xin@magnet.fsu.edu (Y. Xin).

1 Present address: Microsoft Corporation, Redmond, WA 98052, USA.2 Present address: Materials Science and Technology Division, Oak Ridge

National Laboratory, USA.

0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2006.09.028

interest due to a wide variety of intriguing phenomena showinga strong interplay of structural, magnetic and electronic degreesof freedom. These layered ruthenates have a formula ofAn+1RunO3n+1 where A is CaxSr1−x, n=1,2, 3 …, ∞, and x=0to 1.0. They are highly correlated electron systems, and exhibitan astonishing variety of dimensionality-dependent physicalproperties including metal-to-insulator transitions and antifer-romagnetism (Ca2RuO4) [1,2], ferromagnetism (SrRuO3, andSr4Ru3O10) [3,4], superconductivity (Sr2RuO4) [5], metamag-netism and magnetoresistivity (Ca3Ru2O7) [6]. A characteristicfeature of these layered materials is that their physical propertiescan be tuned or manipulated by chemical doping, strainintroduction, and external fields. Due to the underlyingstructural and chemical similarities of these materials, it ispossible to take advantage of such diverse behavior in epitaxialheterostructures.

For the layered ruthenate series, thin film growth of SrRuO3,CaRuO3, and Sr2RuO4 has been achieved [7,8]. In this paper,we report the thin film growth of Ca2RuO4 which is structurallyanalogous to the unusual superconductor Sr2RuO4, but has very

Fig. 1. X-ray diffraction of a 20 nm thick Ca2RuO4+δ thin film grown at 890 °C,with O2 pressure of 2.67 Pa and O2 flow rate of 7 sccm: (a) θ−2θ scan. Thesubstrate peaks are marked S. (b) Pole figure taken at 2θ=70.87°. Peaks labeledF are from the film.

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different properties. Bulk Ca2RuO4+δ is a Mott-insulator, whichundergoes a first order metal to insulator transition betweenabove room temperature and 105 K depending on the oxygencontent, and is antiferromagnetic with Néel temperature ofabout 110 K [9]. Its properties are very sensitive to chemicaldoping [10–12] and hydrostatic pressure [13]. For example,trivalent doping of La3+, isovalent doping of Sr2+ or certainhydrostatic pressure can reduce it from an insulating state to ametallic state, and induce weak ferromagnetism.

Here we present our study of the epitaxial thin film ofCa2RuO4+δ grown on (001) LaAlO3 substrates by pulsed laserdeposition (PLD). Thin film quality was optimized by varyinggrowth parameters including substrate temperature, targetcomposition, oxygen pressure. It was found that the epitaxialgrowth of Ca2RuO4+δ requires higher growth temperature andmore stringent growth conditions than the n=∞ series (such asCaRuO3). The films were characterized by X-ray diffractometryand transmission electron microscopy (TEM). Physical proper-ties were measured and compared with their bulk counterpart.

2. Experimental details

All the films were grown by PLD using a KrF Excimer laser(Lambda Physik COMPex 102) with a wavelength of 248 nmand a fluence of ∼3 J/cm2. The system uses on-axis geometryand the target to substrate distance is ∼5 cm. The substrate wasdegreased in a series of ultrasonic baths of toluene, acetone, andmethanol for 10 min each, then thoroughly rinsed withdeionized water. It was attached to the ceramic heater bymixed platinum and silver paste during the growth. The basepressure in the growth chamber was 1.3×10−3 Pa. O2 wasintroduced into the chamber after the substrate reached thegrowth temperature. The O2 flow rate and chamber pressurewere controlled by adjusting both O2 gas inlet valve and thepumping gate valve.

The PLD targets were fabricated by solid state reaction usingthe same conditions as for making polycrystalline bulk samples[2]. The mixed powder was first pressed into a pellet at apressure of 155 Pa, and subsequently calcined in air at 900 °Cfor 8 h. After regrinding, the powder was pressed into a 2.5 cmdiameter disk at about 300 MPa. It was then sintered at 1370 °Cfor 24 h in a flow of a gas mixture comprising 1% O2 and 99%Ar. Targets were sanded between growths and pre-ablatedimmediately prior to film deposition. The target was rotatedduring the deposition process. Most of our films were ∼20 nmthick and grown at a rate of 0.2 nm/s.

The growth temperature was measured by a thermocoupleadhered to the heater surface close to the substrate by the mixedplatinum and silver paste. Various growth conditions includinglaser energy density, laser repetition rate, growth temperature,chamber oxygen pressure, O2 flow rate, and cooling conditions(under vacuum or oxygen pressure, and cooling rate) werevaried.

A Philips X'Pert PW3040 MRD X-ray diffractometer withCu Kα radiation was used to investigate the crystal structureand quality of thin films. θ–2θ scan and ω scan were primarilyused as the characterization technique to compare the quality of

different films. The full width at half maximum (FWHM) of theω scan of 002 peak was used as indicators of the film quality. AJeol-2011 transmission electron microscope, operated at 200 kVwith a point resolution of 0.23 nm, was used to study themicrostructure of the films and the film/substrate interface. Bothcross-sectional view and plan view TEM samples were preparedat room temperature by mechanical thinning and ion milling.High resolution TEM image simulation was carried out usingjems software [14]. Superconducting Quantum InterferenceDevice (SQUID) was used to measure the magnetic property.The electrical transport properties were measured between 4 and300 K in different magnetic fields using the standard four probemethod.

3. Results and discussion

3.1. Structural and microstructual characterization

Fig. 1(a) shows an X-ray diffraction (XRD) θ–2θ patternfrom a typical film. The peaks from the film are indexed and allsubstrate peaks are labeled “S”. From this data, we conclude

Fig. 2. Cross-sectional TEM images. (a) HRTEM image of the film down[110]film$[100]sub. The arrows indicating the film/substrate interface. (b)Selected area diffraction pattern of the film down [110] zone axis. (c) FFTfiltered HRTEM image of the region inside the film. (d) FFT filtered HRTEMimage of the film/interface region. The rectangles represent Ca2RuO4+δ unit cell,and the squares represent LaAlO3 unit cell.

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that the film's c-plane is parallel to the substrate surface, withc=12.20(9) Å. The FWHM of the 002 rocking curve (notshown here) was 0.33° and 0.5° along two substrate ⟨100⟩directions respectively. Fig. 1(b) shows a pole figure taken at2θ=70.87°. The four spots located at Ψ=30° are 208 from thefilm, and four peaks at Ψ=45° are 202 from the substrate. Thein-plane 45° rotation between substrate diffractions and the filmones indicates that the film ⟨110⟩ is 45° rotated with respect to⟨110⟩ of substrate. From XRD, it is evident that the film issingle crystal, and epitaxially grown on the substrate with⟨110⟩film$⟨100⟩sub. The in-plane lattice parameters a and bwere measured using 2D mapping of reciprocal space. Withinthe uncertainty of the measurement, a=b=5.352(8) Å [15].

It is known that bulk Ca2RuO4+δ has two distinct phases, i.e.stoichiometric short phase (δ=0.0) (a=5.4097 Å, b=5.4924 Å,c=11.9613 Å) and excess oxygen long phase (δN0.0)(a=5.3292 Å, b=5.3194 Å, c=12.3719 Å) [9]. Their crystalstructures are related, with the RuO2 octahedra having differentdegrees of flattening, tilting and rotation, which in turndetermine their somewhat different physical properties. Thelattice parameters of our thin film is close to the long phase, buthas a slightly shorter c and larger a and b than the bulk values.Since the grown film has unit cell volume (349.39 Å3) veryclose to that of the bulk long phase (350.72 Å3) andsignificantly smaller than that of the short phase (355.40 Å3),it suggests that the thin film is strained non-stoichiometric longphase with excessive oxygen. Due to the lattice mismatchbetween the film and the substrate (−0.7%), the film has slighttensile strain in-plane, and compressive strain out-of-plane.

The 002 XRD peak intensity ratio can be used as an indicatorof crystalline ordering quality. The theoretically calculated 002and 006 peak intensity ratio of the long phase Ca2RuO4+δ is1.54, while the I002/I006 ratio from our experimental data is lessthan 1.0. This discrepancy may be speculated to arise from theruthenium deficiency and some degree of cation mixing. Thecomposition analysis from the energy dispersive spectroscopyin TEM shows Ru deficient in some area of the films with Ca/Ru ratio about 2.1:0.8. Calculated I002/I006 ratio drops whenassuming Ru deficiency, or Ca, Ru mixing, or both. Due to theionic size difference between Ca2+ and Ru4+, it is more likelyRu deficiency plays a bigger role in the disorder.

Fig. 2(a) shows a cross-sectional high resolution TEM(HRTEM) image of a sample along [110]film and [100]sub zoneaxis. The stacking of the atomic layers in the film is well orderedwith few planar defects or intergrowths. The film/substrateinterface, indicated by the two arrows, is abrupt and atomicallyflat. Fig. 2(b) is a selected area diffraction pattern from the filmdown [110] zone axis. The pattern is indexed and consistentwith the simulation of the long phase. Fig. 2(c) is a fast Fouriertransform (FFT) filtered HRTEM image of the film, which hasatomic layers with bright dots and layers with weaker dots.Fig. 3 is HRTEM image simulations of a long phase Ca2RuO4+δ

along [110] using the Bloch-wave method (jems). It consists ofa panel of 49 simulations with different defocus values (δf) andspecimen thicknesses (d). Comparing with the experimentalimage, the best parameters that produce similar contrast ared=5–7.5 nm, δf=90–105 nm. By superimposing the unit cell

onto the simulation image (not shown here), it is evident that thelayers with bright dots are RuO2, and the layers with weakerdots are CaO layers. In Fig. 2(d), the unit cell of Ca2RuO4+δ isindicated by a long rectangular box, and the substrate unit cellby the square box. By extending both the unit cells to theinterface, it is determined that the first layer of the film is RuO2,and the growth proceeds with a subsequent layers of CaO–CaO–RuO2, etc.

3.2. Film quality dependence on deposition parameters

In order to investigate the effect of the growth parameters onthe quality of the films, the growth conditions includingsubstrate temperature, target composition, O2 pressure and O2

flow rate and cooling procedure were systematically studied.Both the XRD 002 peak rocking curve FWHM and thenormalized 002 peak intensities are used as indicators for thefilm quality (only FWHM data are presented below).

3.2.1. Target composition and growth temperature dependenceEven though PLD process is known for its ability to produce

films whose compositions are very close to those of the targets,special attention must be paid to the target composition for somematerial systems that have volatile elements especially whengrown at high temperatures. For instance, PLD growth of

Fig. 4. XRD 002 peak FWHM vs. growth temperature with different Ru targetcomposition. Other parameters were kept the same: O2 pressure=2.67 Pa, O2

flow rate=7 sccm, laser repetition=10 Hz, laser fluence ∼3 J/cm2.

Fig. 3. HRTEM simulations of the long phase Ca2RuO4+δ along [110]. d issample thickness, δf is the defocus. Other parameters Cs=1 mm, Cc=1.4 mm,and energy spread 1.4 eV were used in the simulation.

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SrBi2Ta2O9 requires Bi rich target to achieve stoichiometricfilms [16]. Similarly, the volatility of ruthenium oxidesconstitutes a potential problem during both target fabricationand PLD processes. Therefore, the effect of target compositionwas studied using CaCO3/RuO2 mole ratios of 2:1.04, 2:1.2,and 2:1.35.

Fig. 4 summarized the film quality represented by theFWHM of the 002 rocking curve for different target as afunction of growth temperature between 870 and 960 °C.Ca2RuO4+δ cannot be stabilized below 870 °C or above 950 °C,as there is no X-ray diffraction peaks observed from the films.At temperatures below 870 °C, CaRuO3 inclusions formed. The20% Ru rich target Ca2Ru1.2O4 produces the best quality filmwith the narrowest 002 FWHM in the widest temperaturewindow. Ru richer target (35%) moves the growth temperaturewindow towards higher temperatures, as the higher tempera-tures assist the evaporation of the excessive Ru.

3.2.2. O2 ambient pressure and O2 flow dependenceO2 chamber pressure and O2 flow rate are often important

growth parameters for oxide films. We studied this effect usingO2 pressure 0 to 5.3 Pa. The films grown in vacuum did notshow X-ray peaks even after annealing at high temperature inair. Low O2 pressure (b1.3 Pa) resulted in CaO inclusions in thefilm as revealed by TEM. The film quality increases with O2

pressure and saturates after 2.67 Pa. But the growth ratedecreases with increasing O2 ambient pressure. To avoid thedetrimental effect of high O2 pressure on the PLD vacuumsystem, we used 2.67 Pa for most of our film growths.

The O2 flow during growth showed a similar effect. For thisexperiment, we used 0, 7, and 30 sccm flow rate while keepingthe O2 pressure at 2.67 Pa. The film grown at 0 sccm flow ishighly disordered indicated by absence of XRD peaks. Both 7and 30 sccm O2 flow rates result in good quality films. Thedependence of film quality on O2 pressure and flow indicatesthat the sufficient presence O2 in the ambient during growth iscrucial. The oxygen in the target alone is not sufficient forgrowing quality films.

Films for further physical property studies were grown usingfollowing optimized growth parameters, growth temperaturesbetween 880 and 910 °C, target of 20% ruthenium rich, an O2

pressure of 2.67 Pa, a flow rate of 7 sccm, a laser repetition rateof 10 Hz, energy of 200 mJ/pulse, fluence of ∼3 J/cm2, andfree-fall cooling under vacuum.

3.3. Magnetic and electrical transport properties

The samples for the physical property studies are ∼20 nmthick grown under the optimized growth conditions. Forcomparison, a La doped sample was prepared using aLa0.1Ca1.9Ru1.2O4 target under the same growth conditions.

The magnetization versus temperature was measured bySQUID from 4 to 300 K with applied magnetic field up to 1 T.The bulk Ca2RuO4 magnetization data [1] were used to estimatethe expected magnetic signal and ensure that signal to noiseratio would be above SQUID's detection limit. In contrast to themagnetic property of its bulk counterpart which showsantiferromagnetic transition at 110 K [1], no magnetic phasetransition was observed in our film.

Fig. 5(a) is the in-plane resistivity (ρ) measured as a functionof temperature (T) for samples with and without La doping. Forthe undoped sample, the ρ vs. T curve can be divided into threeregions. Above 200 K, ρ increases very slowly with temperaturesuggesting, in part, the electron scattering due to phonons.Between 20 and 200 K, ρ decreases with increasingtemperature. However, unlike the bulk crystal whose resistivitychanges by a few orders of magnitude within the sametemperature range [1,2], the resistivity of the film isconsiderably smaller and its change with T is less than oneorder of magnitude. This behavior seems to imply a thermalactivation process. However, ρ vs. 1/T in this temperature rangecannot be fitted by a single exponential function. Attempts to fita straight line to lnρ vs. Tα in this temperature region, where αis −1/2 or −1/4, failed. If we fit the lnρ vs. 1/T data withmultiple exponential functions, the activation energy rangesfrom 1 to 10 meV. This energy scale is considerably smallerthan that in the bulk crystal. Similarly behavior has beenobserved in BaRuO3 thin films, where Ba and Ru mixing wasspeculated to be the cause [17]. We speculate that the lowactivation energy is linked to the band broadening due to thedisorder in our films. Below 20 K, the ρ is almost independent

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of temperature as better shown in the inset of Fig. 5(a) where lnρis plotted against 1/T. Some films even show decreasingresistivity with decreasing temperature in this temperatureregion [15].

Doping of trivalent La causes the band filling of the system[10,11]. In bulk samples, LaN0.15 doping is needed for it tobecome metallic. However, the La=0.10 doped film is alreadymetallic as shown in Fig. 5(a). The difference is consistent withthe film's smaller activation energy.

To better understand its transport properties, we measured ρvs. T in magnetic fields. The results are shown in Fig. 5(b).Above 100 K, magnetic field up to 13 T has no significant effecton the resistivity temperature dependence. Below 20 K, themagnetoresistance increase dramatically as shown in the inset ofFig. 5(b) where the magnetoresistance is plotted vs. T in a 13 Tfield. This coincides with the plateau in ρ vs. T below 20 K. Thetransport and magnetotransport behavior in the low temperatureregion are yet to be understood.

Furthermore, Hall measurement was carried out by sweepingthe magnetic field from 0 to 9 T. The Hall coefficient RH wasobtained by linear fitting of Hall voltage vs. field data. Fig. 6(a)shows the RH temperature dependence of the undoped film. Themajority carriers are holes at all temperatures judging by thepositive RH. This is consistent with the previous work onslightly Sr2+ doped bulk material [18], indicating that extra

Fig. 6. (a) Hall coefficient measured as a function of T for the undoped film;inset is the 1/RH vs. T; Positive RH indicates hole conduction. (b) Hallcoefficient vs. T for the La=0.1 doped film; RH becomes negative below 40 K.

Fig. 5. (a) The in-plane electrical resistivity as a function of temperature for 4 Kto 300 K of the undoped and La=0.1 doped films; Inset: A plot of lnρ v. 1/T,showing the resistivity plateau at low temperature below 20 K. (b) The electricalresistivity under magnetic field of 0, 4 and 13 T. The inset is themagnetoresistance vs. T in 13T magnetic field.

oxygen and ruthenium deficiency provided the hole carriers inthis system. Assuming a single carrier system, we plotted thecarrier density ρ=1/RH vs. T in the inset. The fact that the holedensity decreases with decreasing temperature can explain the ρvs. T behavior between 20 and 200 K. Below 20 K when the ρvs. T plateau occurred, however, no anomaly in hole densitywas observed.

For the La=0.1 doped sample, the Hall coefficient [Fig. 6(b)]RH decreases with decreasing temperature from positive tonegative crossing 0 at 40 K. Similar behavior was also observedin Sr2RuO4 bulk materials where a multiband two-carrier modelwas used to explain the change of sign in the RH at a certaintemperature [19].

The most significant difference in transport behaviorbetween the bulk sample and the thin film is the lack of sharpmetal to insulator transition in the thin films. As demonstratedabove, this is related to lower activation energy. Several factorsmay contribute to it. Mott–Hubbard model is useful inunderstanding this strongly correlated electron system. Theratio of electron–electron repulsion energy U to the bandwidthW determines the metal to insulator transition behavior of thesystem. It is known that relatively small Ca2+ ion leads to thedistortion of the crystal structure that induces rotation and tiltingof the RuO2 octahedra, which narrows the bandwidth. As aresult, U/W increases and the system becomes insulating incontrast to Sr2RuO4 at low temperatures. We speculate that W

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increases for the thin film due to the strain and the disorder, andtherefore a smaller U/W makes the material more metallic.Previous theoretical electronic structure calculations and someexperimental evidence [13,20] show that the flattening of theRuO2 octahedra favors the insulating ground state. The a–bplane tensile strain in our films may assist the elongation of theRuO2 octahedra, or result in less tilting/rotation of RuO2

octahedra and lead to the metallic state. In addition, theinevitable disorder, such as structural defects and impuritiesmight result in energy levels inside the band gap. All thesefactors might play parts towards lowering the activation energyand making the films more metallic.

4. Conclusions

In conclusion, Ca2RuO4+δ thin films were epitaxially grownon (001) LaAlO3 substrate by PLD. The film/substrateorientation relationship is determined to be [110]film$[100]suband [11̄0]film$[010]sub (in-plane), [001]film$[001]sub (out-of-plane) by both TEM and XRD. The film lattice constants weremeasured as a=b=5.352(8) Å, and c=12.20(9) Å. The highestquality film has a FWHM of 0.3° in XRD (002) rocking curve.Growth conditions such as the growth temperature, targetcomposition, O2 pressure and O2 flow were optimized. The bestfilm were grown at 880 to 910 °C, 2.67 Pa O2 pressure, 7 sccmO2 flow rate, and free-fall cooling in vacuum after growth. Thetarget with 20% excessive Ru (CaO/RuO2=2:1.2) yields bestresults. The magnetic and electrical transport property of thefilm differs drastically from its bulk counterpart. While nomagnetic transition was observed, the film is metallic, with onlyweak metal to non-metal transition occurring at about 200 K.Strain, disorder of the thin films is believed to be the cause forthis behavior.

Acknowledgements

The authors would like to acknowledge Florida StateUniversity Cornerstone PEG Grant No. 5024699 and NationalHigh Magnetic Field Laboratory under NSF DMR-0084173,

and NSF grant No. DMR-9625692. We also gratefully thankDmitry Shulyatev for his help with the preparation of the target,and Jun Lu and Luis Balicas from NHMFL for helpfuldiscussions.

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