201 (2007) 9275–9278www.elsevier.com/locate/surfcoat

Surface & Coatings Technology

Metalorganic chemical vapor deposition of metal oxide films exhibitingelectric-pulse-induced resistance switching

Toshihiro Nakamura ⁎, Kohei Homma, Takashi Yakushiji,Ryusuke Tai, Akira Nishio, Kunihide Tachibana

Department of Electronic Science and Engineering, Kyoto University, Kyotodaigaku-Katsura, Nishikyo-ku, Kyoto 615-8510, Japan

Available onl

ine 3 May 2007

Abstract

Pr1−xCaxMnO3 (PCMO) films with the desired atomic composition were deposited on Pt/SiO2/Si substrates by metalorganic chemical vapordeposition (MOCVD) using in situ infrared spectroscopic monitoring. The I–V characteristics exhibited nonlinear, asymmetric, and hystereticbehavior in PCMO-based devices with top electrode of Al or Ti. The electric-pulse-induced resistance switching was observed in the PCMO-baseddevices. The resistance change was dependent on the Pr/Ca composition ratio of the PCMO films and the kind of the top electrodes.© 2007 Elsevier B.V. All rights reserved.

Keywords: MOCVD; Resistance switching; Resistance random access memory; Manganite

1. Introduction

Magnetoresistive manganites have been attracting consider-able interest for their unique magnetic and electric propertiessuch as half-metallicity and colossal magnetoresistance (CMR).Recently, a large resistance change by the application of anelectric pulse was observed at room temperature in metal oxidessuch as Pr1−xCaxMnO3 (PCMO) [1–19]. This effect provides apossibility of a next-generation nonvolatile memory, calledresistance random access memory (ReRAM). ReRAM is highlyexpected due to its low power consumption, small bit cell size,and fast switching speed. Thin films of perovskite manganitesincluding PCMO have been prepared by various depositionmethods. From the viewpoint of practical use in deviceprocesses, metalorganic chemical vapor deposition (MOCVD)is regarded as one of the most promising techniques for thedeposition of magnetoresistive manganite films because of itsexcellent step coverage, its applicability to the large areadeposition, and the ease in changing the atomic composition.

In this work, PCMO filmswith the desired atomic compositionwere deposited by liquid-source MOCVD. The atomic compo-

⁎ Corresponding author. Tel./fax: +81 75 383 3079.E-mail address: tosihiro@kuee.kyoto-u.ac.jp (T. Nakamura).

0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2007.04.090

sition of the deposited film, such as the Ca/(Pr + Ca) ratio, wascontrolled using the precursor densities obtained by in situinfrared spectroscopic measurements [20–22]. We preparedlayered structures composed of PCMO sandwiched between Ptbottom electrode and top electrodes of Al or Ti. The electrical-pulse-induced resistance switching behavior was investigated inthe PCMO-based devices with different atomic composition ratioof Ca/(Pr + Ca).

2. Experimentals

Fig. 1 shows a schematic diagram of the liquid-sourceMOCVD apparatus. The experimental setup for the liquid-source CVD was the same as that described previously [19–22].We used tris(dipivaloylmethanato)praseodymium [Pr(DPM)3],bis(dipivaloylmethanato)calcium [Ca(DPM)2], and tris(dipiva-loylmethanato)manganese [Mn(DPM)3] as the source materials.These source materials were dissolved in tetrahydrofuran (THF,C4H8O) at a concentration of 0.1 mol/l. After each dissolvedsource was introduced into a vaporizer by N2 carrier gas at200 sccm, the vaporized source was transported into theMOCVD reactor and subsequently mixed with O2 oxidant gas.The flow rate of Ca(DPM)2/THF solution was changed from 0.1to 0.7 sccm, while the flow rates of the Pr(DPM)3/THF and Mn(DPM)3/THF solutions were fixed at constant values of 0.1 and

Fig. 1. Schematic diagram of the liquid-source CVD apparatus.

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0.2 sccm, respectively. The optimal proportion of the flow ratesof the liquid sources was determined using in situ infraredspectroscopic monitoring [20–22]. The pressure in the reactorwas maintained at 5 Torr. The films were deposited at 480 °C onPt/SiO2/Si substrates. The deposited films were then annealed inflowing oxygen at 600 °C for 5 h. All films were about 300 nmthick. Metallic electrodes of Al or Ti were deposited on top ofgrown films by thermal evaporation. In addition, electricalpulses were applied to the sample through the electrodes and theresistance was measured after each pulse. All the electricmeasurements have been done at room temperature.

Fig. 2. Atomic composition in the 600 °C annealed films of PCMO as a functionof the flow rate of Ca(DPM)2/THF solution.

3. Results and discussion

The atomic composition of the PCMO films was evaluatedby X-ray photoelectron spectroscopy (XPS) after etching of thefilm surface. Fig. 2 shows the atomic composition of the PCMOfilms annealed at 600 °C. No incorporation of carbon wasdetected by XPS measurements. The Pr1−xCaxMnO3 films withthe desired composition between x = 0.05 and 0.64 can beobtained by changing the flow rate of Ca(DPM)2/THF solution.

Fig. 3. I–V curves of the Al/PCMO/Pt device. The PCMO film was deposited atthe Ca(DPM)2/THF flow rate of 0.3 sccm and then annealed at 600 °C.

Fig. 4. Resistance switching behavior of the Al/PCMO/Pt device. The PCMOfilm was deposited at the Ca(DPM)2/THF flow rate of 0.3 sccm and thenannealed at 600 °C.

Fig. 6. Resistance switching behavior of the Ti/PCMO/Pt device. The PCMOfilm was deposited at the Ca(DPM)2/THF flow rate of 0.5 sccm and thenannealed at 600 °C.

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Fig. 3 shows typical I–V characteristic in the Al/PCMO/Ptdevice. The I–V curves exhibited two stable resistance states.The resistance change of the PCMO films was measured byapplying electric pulses. The five pulses with the same polaritywere applied before the polarity was changed. The amplitude ofthe applied pulses was 1.5 V. Positive or negative pulsesreversibly switched the resistance of the PCMO films betweenthe high resistance state and the low resistance state. Fig. 4shows the resistance switching in the Al/PCMO/Pt device. Thedependence of the resistance switching in the Al/PCMO/Ptdevices on the Ca/(Pr + Ca) ratio of the film was investigated bychanging the flow rate of Ca(DPM)2/THF solution from 0.1 to0.7 sccm. The largest resistance change in the Al/PCMO/Ptdevices was obtained at the Ca(DPM)2/THF flow rate of0.3 sccm.

Electrical voltage was applied to the sample through Ti topelectrode. Fig. 5 shows typical I–V characteristics in the Al/PCMO/Pt device. The I–V characteristics exhibited nonlinear,asymmetric, and hysteretic behavior. The resistance switchingin the Ti/PCMO/Pt device is shown in Fig. 6. The applied pulseamplitude was 1.2 V. The resistance change in the Ti/PCMO/Ptdevices reached its maximum value at the Ca(DPM)2/THF flow

Fig. 5. I–V curves of the Ti/PCMO/Pt device. The PCMO film was deposited atthe Ca(DPM)2/THF flow rate of 0.5 sccm and then annealed at 600 °C.

rate of 0.5 sccm. The Ti/PCMO/Pt devices show largerresistance change than the Al/PCMO/Pt devices.

4. Conclusions

Praseodymium manganite films with the appropriate amountof the doped calcium were deposited by liquid-source MOCVD.PCMO filmswith the desired atomic composition can be obtainedwithout any incorporation of carbon. We prepared layeredstructures composed of PCMO sandwiched between Pt bottomelectrode and top electrodes of Al or Ti. The I–V characteristicsfor the PCMO-based devices exhibited nonlinear, asymmetric,and hysteretic behavior. The application of the electric pulsesreversibly switched the resistance of the PCMO films between thehigh resistance state and the low resistance state. The pulseamplitude required for the resistance switching was lower than1.5 V. MOCVD is expected as a promising deposition techniqueof metal oxide films for the ReRAM devices.

Acknowledgements

We would like to thank the Kyoto University VentureBusiness Laboratory (KU-VBL) for the support. This work waspartly supported by Kyoto Nanotechnology Cluster, Nanotech-nology Support Project in Kyoto University, and Grant-in-Aidfor Young Scientists (no. 16760617) from the Ministry ofEducation, Culture, Sports, Science and Technology of Japan.

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