We have investigated the electrical transport properties of 8 nm thick
La0.67Ca0.33MnO3 films, sputter-deposited on SrTiO3 (STO), and etched into 5
micrometer-wide bridges by Ar-ion etching. We find that even slight overetching
of the film leads to conductance of the STO substrate, and asymmetric and
non-linear current-voltage (I-V) characteristics. However, a brief oxygen
plasma etch allows full recovery of the insulating character of the substrate.
The I-V characteristics of the bridges are then fully linear over a large range
of current densities. We find colossal magnetoresistance properties typical for
strained LCMO on STO but no signature of non-linear effects (so-called
electroresistance) connected to electronic inhomogeneites. In the metallic
state below 150 K, the highest current densities lead to heating effects and
non-linear I-V characteristics.Comment: 3 pages, 5 figure

Aarts, J.

Beekman, C.

Hesselberth, M.

Komissarov, I.

English

arXiv

Kommissarov, I.

Hesselberth, M.B.S.

Leiden University Scholary Publications

arXiv:0705.1962v1  [cond-mat.mtrl-sci]  14 May 2007Transport properties of microstructured ultrathin films of La0.67Ca0.33MnO3 on SrTiO3C. Beekman, I. Komissarov, M. Hesselberth and J. AartsKamerlingh Onnes Laboratory, Leiden University,P.O. box 9504, 2300RA Leiden, The Netherlands(Dated: February 24, 2013)We have investigated the electrical transport properties of 8 nm thick La0.67Ca0.33MnO3 films,sputter-deposited on SrTiO3 (STO), and etched into 5 µm-wide bridges by Ar-ion etching. We findthat even slight overetching of the film leads to conductance of the STO substrate, and asymmetricand non-linear current-voltage (I-V) characteristics. However, a brief oxygen plasma etch allows fullrecovery of the insulating character of the substrate. The I-V characteristics of the bridges are thenfully linear over a large range of current densities. We find colossal magnetoresistance propertiestypical for strained LCMO on STO but no signature of non-linear effects (so-called electroresistance)connected to electronic inhomogeneites. In the metallic state below 150 K, the highest currentdensities lead to heating effects and non-linear I-V characteristics.PACS numbers: 75.47.Lx,73.50.-h,71.30.+hDoped manganese oxides such as La1−xCaxMnO3are of interest since, in a certain range of doping,a combined insulator-to-metal and paramagnetic-to-ferromagnetic transition can take place. One conse-quence is the well-known Colossal Magnetoresistance ef-fect, but another is the fundamentally interesting phe-nomenon of phase separation. The susceptibility of thephase transition to chemical and crystallographic dis-order (doping disorder, oxygen non-stoichiometry, de-fects from strain relaxation, twinning, grain boundaries)can lead to an inhomogeneous state in which the in-sulating and metallic phases coexist on a variety oflength scales. In such systems, the percolative natureof the conductance may lead to strongly non-linear be-havior and a large sensitivity to electric fields, whichcan be useful for a variety of applications. Lately,therefore, there has been renewed focus on conduc-tance issues, leading to various different observations.Non-linearities, presented as a strongly decreasing resis-tance as function of increasing current density, were re-portedly measured on microbridges made from films ofLa0.7Ca0.3MnO3 and La0.85Ba0.15MnO3 grown on STO[1]. Similar observations were reported on samples madewith La0.7Ca0.3MnO3 [2]. In both cases it was suggestedthat these so-called electroresistance (ER) effects are dueto phase separation. In other experiments, microbridgeswere subjected to high currents (”current processing”),and non-linear as well as asymmetric current-voltage (I-V) characteristics were subsequently found in the two-point resistance [3, 4]. This was tentatively ascribed tothe formation of junction-like structures in the films, andtherefore intrinsic, although modification of the interfacebetween the metal electrodes and the oxide film by thecurrent was not fully ruled out. The interface is a knowncomplication in 2-point geometries; non-linear and asym-metric I-V characteristics were demonstrated in rectify-ing Ti/Pr0.7Ca0.3MnO3 contacts [5], in a p-n heterostruc-ture involving La0.7Ca0.3MnO3 and Nb-doped SrTiO3(STO) [6], and in Ag-La0.7Ca0.3MnO3 heterostructures[7]. However, 4-point measurements on La0.8Ca0.2MnO3microbridges also showed current-induced ER [8, 9], andit was concluded that high currents can change the bal-ance in the coexistence of the different phases. All of theabove microbridges are still relatively large, with typi-cal film thicknesses of 100 nm and bridge widths around50 µm. Phase separation phenomena may be found downto very small length scales, in particular when strain andstrain relaxation also play a role [10, 11]. The questionthen arises whether similar ER effects can be seen insmaller bridges and thinner films. Here we note thatspecial care has to be taken in the structuring. The com-monly used Ar+-etching technique easily damages theSTO substrate, which results in a conducting surfacelayer after etching [12]. Current leakage through thislayer interferes with the transport measurements and in-trinsic current effects will be obscured. This problemcan be overcome by a brief oxygen plasma etch, as willbe shown below.Epitaxial films of La0.67Ca0.33MnO3 with a typicalthickness of 8 nm were grown on (001)STO substratesby DC sputtering in an oxygen pressure of 300 Pa, ata growth temperature of 840◦C. The substrate surfacewas treated to have single termination of TiO2, and hada misorientation of 1◦ towards [010] in order to improvethe smoothness of the film. A resist mask was patternedby e-beam lithography to yield a structure for 4-pointmeasurements, with a bridge width of 5 µm, a distancebetween the voltage contacts of typically 16 µm and theorientation of the bridge perpendicular with respect tothe step edges of the substrate. The film was etchedwith Ar-ions (beam current: 10 mA, beam voltage: 350V) during 40 s; with a calibrated etch rate 0.31 nm/s,this means overetching of 14 s, in order to be certainto remove all of the film. Temperature and magneticfield regulation were done in a Physical Property Mea-surement System (PPMS: Quantum Design) but externalcurrent sources and nanovoltmeters were used to performmost of the transport measurements.I-V characteristics were measured on the microbridgesin the temperature range of 10 K - 300 K using currents2-0.6 -0.3 0.0 0.3 0.6 0.9-2-10120 50 100 150 200 250 3000481216-0.075 0.000 0.075-101V (volts)I (mA)T = 140 K (a) (b) T (K) R (M)FIG. 1: (a) I-V curve at T = 140 K of an LCMO bridge on STOafter 14 sec overetching; insert : behavior near zero current); (b)Resistance measured as averaged voltage (see text) versus T of thesame sample.up to 0.6 mA, which corresponds to a current densityJ = 1.5×1010 A/m2. A typical one, taken at 140 K,is shown in Fig.1a. It shows nonlinearity and a largeasymmetry between opposite current directions, as wellas hysteresis at high J . Similar curves could be observedat all temperatures. Still, by simply averaging voltagesat small positive and negative currents (0.1 µA; this wasperformed with the PPMS electronics) the ’resistance’ R(Fig.1b) shows a sharp phase transition at 150 K, as ex-pected for films under tensile strain [13]. Apparently, themeasurements at least partly probe the bridge structure,but since this is a 4-point measurement, the asymmetryindicates that not all current is flowing between the volt-age contacts. It is known that Ar etching of STO causesthe formation of a conducting surface layer by the re-moval of oxygen [12]. To conclusively show this in oursystem, we etched a w = 1.5 mm wide STO substrate for30 sec. After etching, 4 Au/MoGe contacts [14] wheresputtered on top (ℓ = 2.6 mm between the voltage con-tacts). The I-V curves (Fig.2) are symmetric down to 150K, although below this temperature they become slightlynonlinear. Fig.2 also shows the T-dependent sheet resis-tance R = Rw/ℓ obtained at low bias. The dependenceis metallic, R is very close to the number reported inref. [12], but also important to note is that the absolutevalue of R is in the range of (only) 1 kΩ - 10 kΩ, whichmakes it substantially smaller than the expected peakvalue for R in our microbridge, or even for R in signifi-cantly wider and thicker bridges.In order to restore the insulating properties of the sub-strate, post-annealing in an oxygen environment wouldprobably be possible, but this is often unwanted as a pro-cess step; in our case, it might lead to strain relaxationand additional defects in the film. Instead, after the Ar-etch we subjected structures with the resist still in placeto an O2 plasma for typically 1 - 2 min. and found thatthe substrate had become insulating again while resistwas still present. Generally, the procedure appears towork for small amount of etching of the STO. Substrateswhich were Ar-etched for more than 1 min. did not showrecovery anymore, even after 4 min. of plasma treatment-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6-4-20240 50 100 150 200 250 3000510152025V (volts)I (mA)(b)T = 140 K(a) R (k/)T (K)Ibias = 20 AFIG. 2: (a) I-V curve of 30 sec Ar-etched STO (1.5 x 2.6 mm2)at T = 140 K; (b) R-T plot of the same sample obtained from theI-V curves at 20 µA.(by which time the resist layer had been removed). Wesurmise that oxygen loss can be recovered by the plasma,but that more structural damage to the STO (amorphi-sation) renders this impossible.In Fig. 3 we show an I-V curve and the R(T) plot fora microbridge which was overetched by 4 sec and thenplasma-treated. The result clearly shows that when theSTO substrate is restored to its insulating state, the mi-crostructured LCMO thin film has linear and symmetricI-V characteristics in four point measurements. No elec-troresistance is observed in our LCMO bridges. Note that-15 -10 -5 0 5 10 15-4-3-2-1012340 50 100 150 200 250 3000.00.10.20.30.40.50.6V (volts)I ( A)T = 140 K(b)(a) T (K)R (M )  H = 0 T  H = 5 TFIG. 3: (a) I-V curve at T = 140 K of an LCMO bridge on STOafter 4 sec overetching and O2-plasma treatment. The thin linesindicate the voltage limit for the measurement; (b) R-T plot atmagnetic field B = 0 T and 5 T of the same sample.the current densities we used, between 2.5x107 A/m2 and1.5x1010 A/m2, lie within the range for J where large re-sistance variations were reported in refs. [1, 2]. In thosecases no details are given about I-V characteristics or theeffects of microstructuring, but the samples are differentfrom ours, since they are typically 100 nm thick. A possi-ble explanation for the quite strong discrepancies is thatour films are very homogeneous even on submicrometerscales, in particular since strain relaxation has not yetset in. It seems probable that the grain structure andthe disorder in the films determine possible ER effects toa large degree, as was surmised in ref [2].To observe possible effects of the contacts we also mea-sured the treated sample in a two-probe configuration,with current injected through the voltage pads. The re-sults are shown in Fig. 4 where we compare the I-V curves3-200 -100 0 100 200-0.3-0.2-0.10.00.1-200 -100 0 100 200-6-303-200 0 20001200-200 0 2000600002-pt. measurementV (volts)I ( A)4-pt. measurementI ( A)V (volts)(a) (b) dV/dI dV/dIFIG. 4: I-V curves of an LCMO bridge on STO at T = 10 K (a)in 4-point configuration; (b) in 2-point configuration. Insets showderivatives dV /dI.-100 -50 0 50 100-2-10120.0 0.5 1.0 1.5 2.00102030I (mA)V (volts)2-pt. measurementI (mA)T (K)(a) (b)FIG. 5: a)I-V curve of an LCMO bridge on STO at T = 50 Kmeasured in a 2-point configuration. b) Calculation of the expectedJoule heating (see text) represented as temperature increase ∆Tversus applied current I.for two and four point measurements taken at 10 K. The4-point measurement shows a linear and symmetric IVcurve as expected. The 2-point resistance is significantlylarger (around a factor of 5 after correction for the extralead resistance in the contact pads). This can be at-tributed to a large contact resistance. Another feature isthe nonlinearity of the I-V curve, with the correspondingpeak in the derivative dV/dI (see inset) clearly visibleand most probably caused by the presence of a barrier atthe contact-film interface.In the 2-point measurements in the metallic state wealso found nonlinear effects in the I-V characteristicswhich we ascribe to Joule heating. A measurement at50 K is shown in Fig.5a where the voltage-driven systemsuddenly switches to a lower current (higher resistance).It is not straightforward to make an estimate of the ef-fect. The measured resistance is dominated by the con-tacts, but the area of the contacts is much larger thanthe bridge so that it is not a priori clear in which part ofthe structure the heating occurs. If we still assume it isthe bridge, we can estimate the temperature increase ∆Tfrom a current I using the following equation taken from[15], ∆T (T, I) = (2I2ρ(T + ∆T ))/(Sκsub(T + ∆T ), withρ(T) the specific resistance of the bridge at temperatureT , S its cross-section, and κ the thermal conductance ofthe substrate. Taking ρ(T) in the metallic state indepen-dent of temperature and use the value ρ = 260 µΩcmfound in the 4-point measurement, then with κ = 16WK−1m−1 [16], we find ∆(T) ≈ 30 K at 2 mA (Fig.5b).The model is quite crude, but the result at least indicatesthat heating effects cannot be negelected in our bridges.The mechanism leading to the switching behavior is notyet understood, however, and needs further investigation.In summary we conclude that the observed peculiari-ties in the I-V characteristics of our films are caused bythe Ar-etching and are not an intrinsic feature. We canrestore the insulating STO surface layer by an O2 plasmatreatment and then find no electroresistance effects in ourthin and homogeneous films. We further demonstratedthat contact resistance and Joule heating can introducenon-linear effects which are not intrinsic to the materialunder study.This research is in part supported by NanoNed, anationale nanotechnology program coordinated by theDutch Ministry of Economic Affairs, and in part bythe ’Stichting voor Fundamenteel Onderzoek der Ma-terie (FOM)’, which is financially supported by the ’Ned-erlandse Organisatie voor Wetenschappelijk Onderzoek(NWO)’[1] J. Gao, S. Q. Shen, T. K. Li and J. R. Sun, Appl. Phys.Lett. 82, 4732 (2003).[2] Y.G. Zhao, Y.H. Wang, G.M Zhang, B. Zhang, X.P.Zhang, C.X. Yang, P.L. Lang and M.H. Zhu, Appl. Phys.Lett. 86, 122502 (2005).[3] J.R. Sun, G.J. Liu, S.Y. Zhang, H.W. Zhang, X.F. Hanand B.G. Shen, Appl. Phys. Lett. 86, 242507 (2005).[4] Y. W. Xie, J. R. Sun, D. J. Wang, S. Liang, W. M. Lüand B. G. Shen, Appl. Phys. Lett. 89, 172507 (2006).[5] A. Sawa, T. Fujii, M. Kawasaki and Y. Tokura, Appl.Phys. Lett. 85, 4073 (2004).[6] X. P. Zhang, B. T. Xie, Y. S. Xiao, B. Yang, P. L. Langand Y. G. Zhao, Appl. Phys. Lett. 87 072506 (2005).[7] D. S. Shang, L. D. Shen, Q. Wang, W. Q. Zhang, Z. H.Wu and X. M. Li, Appl. Phys. Lett. 89 172102 (2006).[8] F. X. Hu and J. Gao, Phys. Rev. B 69, 212413 (2004).[9] F. X. Hu and J. Gao, Appl. Phys. Lett. 87, 152504(2005).[10] M. Fäth, S. Freisem, A.A. Menovsky, Y. Tomioka, J.Aarts, and J.A. Mydosh, Science 285, 1540 (1999).[11] A. Biswas, M. Rajeswari, R. C. Srivastava, Y. H. Li, T.Venkatesan, R. L. Greeneand A. J. Millis, Phys. Rev. B61 9665 (2000).[12] D. Kan, T. Terashima, R. Kanda, A. Masuno, K. Tanaka,S. Chu, H. Kan, A. Ishizumi, Y. Kanemitsu, Y. Shi-makawa and M. Takano, Nature Materials 4, 816 (2005).[13] J. Aarts, S. Freisem, R. Hendrikx and H. W. Zandbergen,Appl. Phys. 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Transport properties of microstructured ultrathin films of La0.67Ca0.33MnO3 on SrTiO3

C. Beekman

I. Komissarov

M. Hesselberth

J. Aarts

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Transport properties of microstructured ultrathin films of
  La0.67Ca0.33MnO3 on SrTiO3

Transport properties of microstructured ultrathin films of La0.67Ca0.33MnO3 on SrTiO3

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