Solid State Communications 143 (2007) 382–385www.elsevier.com/locate/ssc

Study on the resistive switching properties of epitaxialLa0.67Sr0.33MnO3 films

Lina Huanga, Bingjun Qua,∗, Litian Liua, Liuwan Zhangb

a Institute of Microelectronics, Tsinghua University, Beijing 100084, People’s Republic of Chinab Physics Department, Laboratory of Advanced Materials, Tsinghua University, Beijing 100084, People’s Republic of China

Received 5 March 2007; accepted 17 June 2007 by D.D. SarmaAvailable online 21 June 2007

Abstract

The hysteretic and reversible polarity-dependent resistive switching effect has been studied in epitaxial La0.67Sr0.33MnO3 (LSMO) films underDC bias stress and voltage pulses. A distinct current–voltage characteristic of the Ag/LSMO system with pronounced nonlinearity, asymmetryand hysteresis was observed, which is considered to be a precursor sign of the resistance switching. The pulsed voltage amplitude and durationdependence of the nonvolatile resistive switch were provided. Reproducible switching properties, involving non-symmetrical R–V hysteresisloop, active pulse width window and stepwise multilevel switchable capability, demonstrate well controllability with respect to future nonvolatilememory applications.c© 2007 Elsevier Ltd. All rights reserved.

PACS: 73.50.Fq; 73.40.-c; 81.15.-z

Keywords: A. La0.67Sr0.33MnO3; A. Thin film; D. Resistive switching; E. Electric-pulse-induced

1. Introduction

The perovskite R1−xAxMnO3-based (R and A being trivalentrare-earth and divalent alkaline-earth ions, respectively)manganites, one family of the large group of correlated electronmaterials, have been attracting substantial interest due to theirunique room temperature electric-pulse-induced resistance(EPIR) change effect discovered recently [1–8], in which theresistance of the compound can reversibly switch betweentwo stable resistive states with applied short electric pulsesof different polarity, and the modified resistance retains evenafter removing the pulse source. Well developed resistiveswitching phenomena have been further reported in other oxidematerials, such as titanates [9,10], zirconates [11–13] andniobic oxides [14,15], exhibiting possible universality [16]. Thefascinating features of the EPIR effect manifest a great potentialfor digital device applications, e.g. nonvolatile resistance

∗ Corresponding author. Tel.: +86 010 62789151 319; fax: +86 01062771130.

E-mail address: qubj@mail.tsinghua.edu.cn (B.J. Qu).

0038-1098/$ - see front matter c© 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.ssc.2007.06.006

random access memories with high density, fast switchingspeed and low power consumption.

Up to now, various researches have been undertaken inorder to elucidate the mechanism underlying the EPIR effect,involving field-driven lattice distortions [1], Schottky barrierswith interfacial states [17], electrochemical migration at themetal/oxide interface [2,3,18], and phase separation [19], etc.Although partial experimental results can be qualitativelyexplained based on the above models, the exact nature of theEPIR effect still remains vague. It is hence of vital importanceto systematically study the diverse behaviours of the resistiveswitch, which will provide another clue to understanding theeffect. In this work, we investigated the polarity-dependentresistive switch properties of Ag/La0.67Sr0.33MnO3/SrTiO3system, and found that the nonlinear, asymmetrical DCcurrent–voltage characteristic with an obvious hysteresis maybe a precursor sign of the resistive switching. The switchingproperties, which share a key feature of nonsymmetry, arestrongly affected by different selections of pulse parameters,thus showing flexible controllability and multilevel storagecapability as for nonvolatile memory applications.

L. Huang et al. / Solid State Communications 143 (2007) 382–385 383

Fig. 1. The X-ray diffraction θ–2θ pattern of in-situ grown LSMO filmdeposited on STO (001) substrate. The inset shows the rocking curve of theLSMO (002) reflection.

2. Experimental details

200 nm thick La0.67Sr0.33MnO3 (LSMO) films employinga stoichiometric ceramic LSMO target were grown on (001)SrTiO3 (STO) single crystal substrates by a pulsed laser depo-sition technique. The substrate temperature was held at 770 ◦Cunder an oxygen pressure of 48 Pa during ablation. The focusedlaser fluence at the surface of the rotating target was approxi-mately 1.8 J/cm2 with a repetition rate of 3 Hz. After depo-sition, the film was furnace cooled to the ambient temperaturewith a cooling rate of 7 ◦C/min in one atmosphere of oxygen.Silver contact pads of radius r ≈ 100 µm with 250 µm spacedapart were attached on the top of the LSMO layer using silverpaint.

A schematic diagram of the device structure and themeasurement circuit is shown in the inset of Fig. 2. Voltagepulses were produced by a Tektronix TM 5003 pulse generator,with the positive (+) pulse direction defined as from pad1 to pad 2. The two-terminal resistance was checked aftereach pulse by feeding a 5 µA direct current and readingthe corresponding voltage using Keithley 2400 source meterunit. Current–voltage characteristics were also examined bythe Keithley measurement unit. Film crystallinity of the as-deposited films was characterized by a four-circle X-raydiffractometer (D/MAX-RB) with Cu Kα radiation.

3. Results and discussions

Epitaxial growth of LSMO films have been confirmed in theanalysis of X-ray diffraction θ–2θ scan, as shown in Fig. 1. TheLSMO film is of single phase and highly c-axis orientated, witha strong peak identified as LSMO (002) besides the STO (002)diffraction peak. The full width at half maximum (FWHM) ofthe LSMO (002) peak is as narrow as 0.28◦ in the rocking curve,as given in the inset of Fig. 1, indicating an especially highdegree of in-plane atomic ordering of the film.

The I –V characteristics obtained at room temperatureare plotted in Fig. 2, with two consecutive measurementsperformed to verify the reproducibility. The voltage bias was

Fig. 2. I –V characteristic of Ag/LSMO/STO structure. The upper inset showsresistive switching effect of the sample with applied single pulse of ±15 Vamplitude and 150 ns duration. The lower one shows a schematic configurationof the device. Arrows in the figure denote the voltage scan direction.

Fig. 3. Hysteretic resistance change as a function of pulsed voltage. Arrowsindicate the clockwise measurement direction.

swept as 0V → +2V → 0V → −2V → 0V . It can be seenthat the curve exhibits nonlinear and asymmetrical behavioursalong with a clear hysteresis both at positive and negativebias, reasonably consistent with the previous reports [17,20,21].The nonlinear and asymmetrical responses suggest possibleSchottky barriers at the Ag/LSMO interface although it wouldplay a minor role in the resistive switching. The hysteresiscaptured here is due to a relaxation of trapped carriers nearthe interface and may be a precursor sign of the resistancetransition. It should be pointed out that the virginal resistanceof the sample is on the order of k� in magnitude, as illustratedin Fig. 2. Consecutive I –V sweep (>20 cycles) can graduallyreduce the resistance, and stabilize it at ∼330 � finally. Sucha bias stress process to get the stable resistance state withhysteretic I –V characteristics is suggested to be related to onekind of accumulation/alignment process of interfacial carriers.

A typical resistive switching result driven by electric pulsesis displayed in the upper inset of Fig. 2. Single pulse of ±15 Vfor 150 ns duration switches the Ag/LSMO sample alternativelybetween high (∼330 �) and low (∼210 �) resistance state, andboth of the states are nonvolatile at room temperature. Fig. 3

384 L. Huang et al. / Solid State Communications 143 (2007) 382–385

Fig. 4. Active pulse width window of the Ag/LSMO/STO sample. The EPIRratio is defined as ∆R/RMIN = (RMAX − RMIN)/RMIN, where RMAX andRMIN are the maximum and minimum resistance induced by electric pulses,respectively.

shows the resistance dependence on the pulse amplitude. Ahysteretic resistance change loop with applied single pulse of1 µs duration is obtained. The high resistance state of about340 � is stable until the pulsed voltage reaches the positivethreshold voltage (Vth+) of +5 V. Then the resistance dropsslowly with increasing pulsed voltage and a low resistancestate (∼200 �) is attained at +15 V. In the voltage decreaseprocess, the resistance shows hysteretic behaviour and remainsunchanged until another threshold voltage (Vth−) of −7 V isachieved. Accordingly, the resistance has a noticeable increaseand returns to the original high value.

The rapid transition between high and low resistance stateis clearly denoted by the almost rectangular hysteresis loop.However, it is important to note here that the hystereticresistance change loop has a key feature of “non-symmetry”,that is, the threshold voltages and the switching rates fortransitioning the resistive states up and down are both different.One possible reason is that the Ag/LSMO switching systemhas some intrinsic anisotropy, e.g. point-to-point variation of

the metal/oxide contact [2], thus resulting in different switchingbehaviours with regard to the opposite pulse polarity.

The sample can only be switched within a certain range ofpulse width, as illustrated in Fig. 4. The active pulse widthwindow is typically from 25 ns to 4 ms and wider pulse canhardly switch the resistance efficiently, which is in contrast withthe previous results [22,23]. It is proved that applying shorterelectric pulse can restore the resistive switching. As is shownin Fig. 4, a large EPIR ratio can be addressed within a widerange of 500 ns–5 µs duration, indicating ideal programmingefficiency as for nonvolatile memory applications.

The effect of unidirectional pulses on the resistive switchingis further investigated. It is confirmed from Fig. 5 that agiven resistive state increases (or decreases) cumulatively withapplication of multiple negative (or positive) voltage pulsesof 100 ns duration, exhibiting a stair step-up or step-downlike resistance transition. Upon reversing the pulse direction,a complete return to the opposite resistive state takes place.As is seen from Fig. 5, the sample can be set to differentdiscrete intermediate resistance levels and each of the resistancelevels has a specific state of memory. As a result, multilevelswitchable states can be extracted from the Ag/LSMO/STOmemory unit. It is expected that any target resistance state canbe addressed at will by application of appropriate pulse numberand pulse amplitude.

Compared to the positive-pulse-driven step-down resistancetransition (Fig. 5(b)), the step-up transition (Fig. 5(a)) starts ata larger threshold voltage and is sharper indicated by distinctdiscontinuity points at large pulsed voltage, which is in goodagreement with the nonsymmetrical result shown in Fig. 3.It is therefore believed that the non-symmetrical switchingcharacteristic is likely intrinsic to the Ag/LSMO test sample.

In the case of Ag/LSMO/STO system, there exist di-verse resistance switching behaviours modulated flexibly bypulse number, pulsed voltage amplitude and duration, thusdemonstrating a considerable potential for controlling this phe-nomenon for device applications. Reproducible nonsymmet-rical switching properties are probably due to the built-inanisotropy arising from film inhomogeneity [24] and different

Fig. 5. The step-up (a) and step-down (b) resistance changes versus pulse number and pulse amplitude by application of unidirectional single pulse of 100 ns.

L. Huang et al. / Solid State Communications 143 (2007) 382–385 385

contact near two metal/oxide interfaces. This phenomenon isexpected to be another clue to elucidating the resistive switch-ing mechanism.

4. Conclusions

The reversible polarity-dependent resistive switching prop-erties of epitaxial La0.67Sr0.33MnO3 thin films have been stud-ied under DC bias stress and voltage pulses. A reproduciblenonlinear and asymmetrical current–voltage characteristic witha clear hysteresis was observed. The hysteresis is likely ascribedto a relaxation of interfacial trapped carriers. By different se-lections of electric pulses, diverse behaviours of the resistiveswitching were also obtained, which show a great potential forthe well control of the EPIR effect in response to practical de-vice applications. The physical details and the optimization ofthe resistive switch are now under further investigation.

Acknowledgments

The authors are pleased to acknowledge C. Yang andG.L. Xie for giving support to measurement. This work issupported by National Natural Science Foundation (NNSF)Grant No. 90407013, and in part by the Ministry of Scienceand Technology of the People’s Republic of China (863) GrantNo. 2006AA03Z317.

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