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Three-state resistive switching in CoFe2O4/Pb(Zr0.52Ti0.48)O3/ZnO heterostructure

Ziwei Li, Mingxiu Zhou, Wangfeng Ding, Hang Zhou, Bo Chen et al.

Citation: Appl. Phys. Lett. 100, 262903 (2012); doi: 10.1063/1.4730965 View online: http://dx.doi.org/10.1063/1.4730965

View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v100/i26 Published by the American Institute of Physics.

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Three-state resistive switching in CoFe

₂

O

₄

/Pb(Zr

_0.52

Ti

_0.48

)O

₃

/ZnO heterostructure

Ziwei Li, Mingxiu Zhou, Wangfeng Ding, Hang Zhou, Bo Chen, Jian-Guo Wan,^a) Jun-Ming Liu, and Guanghou Wang

National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China

(Received 26 March 2012; accepted 9 June 2012; published online 25 June 2012)

The heterostructural film combining multiferroic CoFe₂O₄/Pb(Zr_0.52Ti_0.48)O₃ bilayer with semiconductor ZnO layer was prepared. Three-state resistive switching was demonstrated by time-dependent current measurements under different stimuli combination of voltage pulse and magnetic bias. The asymmetry diodelike current-voltage, capacitance-voltage, and polarization-voltage loops, which seriously depend on magnetic bias, were observed.

We revealed that three-state resistive switching was dominated by the changes in the charge carriers in the heterostructure, which were modulated by the magnetoelectric coupling between ferromagnetic and ferroelectric layers and interface polarization coupling between ferroelectric and semiconductor layers. This work provides promising candidates for developing advanced switchable devices with multifunctional memory.V^C 2012 American Institute of Physics.

[http://dx.doi.org/10.1063/1.4730965]

Recently, switchable resistive memory effect has attracted increasing attentions due to their great potential for applications in the next-generation data storage devices.¹ Several material systems have been demonstrated to show resistive switching, including organic compounds, transition- metal oxides, and heterostructures.^2–4 Among them, ferroelectric-semiconductor heterostructures (e.g., BaTiO3/ ZnO) are in particular receiving considerable interests due to their many attractive advantages such as high-density, low- power consumption and high-speed.^5–8Modulated by dipolar electric field, the charge carriers can be accumulated or depleted at the ferroelectric-semiconductor interface, which causes different conductivities. Reversible two-state resistive switching between high resistance state (HRS) and low resistance state (LRS) is thus achieved. A control mechanism of resistive switching, i.e., interface polarization coupling between semiconductor layer and ferroelectric layer, has been proposed.^5,7

Herein, we propose an available avenue to achieve three-state resistive switching by introducing ferromagnetic oxide layer into ferroelectric-ZnO heterostructure. ZnO is a n-type semiconducting oxide with wurtzite-structure and exhibits spontaneous polarization of 4.17.0 lC/cm² in addition to stress-induced piezoelectric effect.⁷ Neverthe- less, related to inherent crystal structure, the spontaneous polarization of ZnO is practically electrically irreversible.^7,9 In a common ferroelectric-ZnO heterostructure, the coupling between reversible polarization of ferroelectric layer and irreversible spontaneous polarization of ZnO layer dominates the switching of charge accumulation or depletion states. If a ferromagnetic layer is introduced, an additional ferromagnetic-ferroelectric heterostructure will form, which makes it possible for the system to generate

ferroelectric polarization under external magnetic field, i.e., magnetoelectric effect.^10,11So it is expected that the manipu- lation of multistate resistive switching in such ferromagnetic- ferroelectric-ZnO heterostructure may be achieved by the combined modulation of magnetoelectric effect and interface charge coupling effect upon external electric and magnetic field stimuli.

In this letter, we use a three-layered heterostructural film stacked sequentially by semiconductor ZnO, ferroelectric Pb(Zr0.52Ti0.48)O3 (PZT), and ferromagnetic CoFe2O4

(CFO) to demonstrate the three-state resistive switching, which are distinguished by various stimuli combination of electric voltage pulse and magnetic bias. The asymmetry diodelike current-voltage, capacitance-voltage, and ferroelectric polarization-voltage loops, which seriously depend on magnetic bias, are observed. Our results are interpreted in terms of the changes in the charge carrier states modulated by magnetoelectric coupling between CFO and PZT layers as well as interface polarization coupling between PZT and ZnO layers.

The CFO/PZT/ZnO heterostructural film was prepared by the typical sol-gel process and spin coating technique.

The 115 nm ZnO, 175 nm PZT, and 175 nm CFO in turn were grown on the Pt/Ti/SiO2/Si(100) wafer. The details of preparation can be found elsewhere.^12,13 For electric measurement, Au top electrodes with 0.2 mm in diameter were sputtered onto the surface of the film. Figure1(a)depicts the schematic drawing of the configuration of the heterostructure. Figure 1(b) presents the cross-sectional morphology image examined by scanning electron microscopy (SEM, LEO-1530VP). Clear interfaces between each layer are observed. The x-ray diffraction characterization confirmed that the film consisted of ZnO phase with highly preferred (002) orientation and polycrystalline PZT and CFO phases, as shown in Figure1(c). Highly preferred crystal orientation in ZnO layer indicates that it has a large spontaneous

a)Author to whom correspondence should be addressed. Electronic mail:

wanjg@nju.edu.cn.

0003-6951/2012/100(26)/262903/5/$30.00 100, 262903-1 VC2012 American Institute of Physics

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polarization close to the theoretical value.⁷ The magnetic hysteresis loops measured at room temperature by vibrating sample magnetometer showed that the heterostructure pos- sessed evident ferromagnetic characteristic with in-plane saturation magnetization of 180 emu/cm³and coercive field of460 Oe, respectively.

The time-dependent current behaviors of the heterostructure were measured using a Keithley 6517B ampere me- ter after it undergone various combined stimuli of electric voltage pulse and in-plane magnetic bias. Figure2(a) plots the stimuli sequence of electric voltage pulse and magnetic

bias. Theþ8V and8V write pulse were applied for the duration of 100 ms (the voltage is positive as the top Au electrode is grounded, and negative as the bottom Pt electrode is grounded), and the magnetic bias was set to H_bias¼1.0 kOe.

The voltage bias of þ1 V for the duration of 2000 ms was applied as read bias. Before external field stimuli, the current-voltage measurement gave a high resistance value of about 3.5 MX for the original film. After written by þ8 V pulse whatever the magnetic bias was applied or not, the film almost had no change in the current at read bias, as shown in Figure 2(b). Nevertheless, when the film was written by 8 V pulse at Hbias¼0, the current at read bias abruptly increased, indicating that the film was switched from original HRS to LRS. The resistance at LRS was approximately 6 times smaller than that at HRS. More surprisingly, when the film was written by 8 V pulse at Hbias¼1.0 kOe, the current further significantly increased, implying that the film turned to lower resistance state (LerRS). The resistance value at LerRS was approximately 20 times smaller than that at HRS. Accordingly, three-state resistive switching was achieved by different stimuli combination of electric voltage pulse and magnetic bias. Figure 2(c) further plots the enlarged curves of time-dependent current. It is seen that the three resistance states can retain at least for 10³ms just with little degradation. Different from the comment resistive switching materials only with two resistance states switched by electric field, our present heterostructure additionally permits the writing by combined stimuli of magnetic bias and electrical field and electrical non-destructive readout, which produces three resistance states and facilitates the design of switchable devices with multifunctional memory.

We then carried out the measurement of current (I)-voltage (V) behaviors at various magnetic bias to explore the influence of electric voltage and magnetic bias on the resistance state changes of the heterostructure, as shown in Figure 3(a). The magnetic bias was applied in the range of 0–5.8 kOe and the maximum sweeping voltage (V_max) was set to 10 V. The sweeping direction of the voltage was 0! þVmax

! Vmax !0. The film exhibits apparent asymmetric and diodelike I-V hysteresis behaviors at various magnetic bias, with HRS for positive voltage and LRS for negative voltage.

The resistance difference between positive and negative voltage increases with increasing the electric voltage. Moreover, the magnetic bias almost has no effect on the I-V behavior for positive voltage, while greatly influences it for negative voltage. Figure3(b)further plots the current as a function of magnetic bias measured at610 V. It is clear that atþ10 V the current is independent of magnetic bias, just fluctuating near 0.8lA, whereas at10 V the current sharply increases with increasing magnetic bias and reaches the saturation when Hbiasis beyond 2.0 kOe.

To study how the charge state in the heterostructure varies with magnetic bias, we subsequently performed the measurements of dielectric and ferroelectric polarization properties. Figure 4(a) presents the capacitance (C)–voltage (V) curves at various magnetic bias measured at constant frequency of 1.0 kHz using an impedance analyzer (HP4294).

The film exhibits significantly asymmetry C-V characteristic, with quite low capacitance for positive voltage and high capacitance for negative voltage. The capacitance peaks

FIG. 1. (a) A schematic drawing of the configuration, (b) cross-sectional SEM image, and (c) XRD characterization of the CFO/PZT/ZnO film.

FIG. 2. (a) Stimuli sequence of electric voltage pulse and magnetic bias for switching resistance state measurements. (b) Current as a function of time measured at fixed read voltage bias of 1 V. (c) Enlarged current-time curves extracted from (b). Eachþ8 V and8 V write pulse were applied for 100 ms. The magnetic bias was set at 1.0 kOe.

262903-2 Liet al. Appl. Phys. Lett.100, 262903 (2012)

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located at about2.8 V are observed, which should be asso- ciated with the ferroelectric polarization switching of the PZT layer.¹⁴When the magnetic bias is applied, the capacitance almost has no change for positive voltage, whereas gets large change for negative voltage. The capacitance value for negative voltage greatly decreases with increasing magnetic bias. Figure4(b)gives the ferroelectric polarization (P) vs electric voltage (V) hysteresis loops recorded at various magnetic bias using a Radiant Multiferroic Test System.

A standard bipolar triangular waveform with measuring frequency of 10 kHz was used so as to furthest reduce the influence of leakage current. The whole P-V loops resemble the rectifying behaviors, i.e., the polarization value is quite low and almost constant for positive voltage, whereas increases with decreasing the voltage for negative voltage. Note that the P-V loops at negative voltage region are still slightly dis- torted due to the influence of leakage current. The polarization value at10 V is about twice as large as that atþ10 V.

Similar to the C–V curves, the polarization value decreases with increasing magnetic bias for negative voltage, whereas has no change for positive voltage.

In previous investigations on the multiferroic CFO/PZT film, above significantly asymmetry C-V and P-V behaviors were not observed.¹²So we consider that the ZnO layer plays an important role on the asymmetry characteristics in the

present CFO/PZT/ZnO heterostructure. It is recognized that oxygen vacancies are closely related to the leakage current of PZT.¹⁵At room temperature the migration of oxygen vacancies hardly happens and electron holes arising from the incor- poration of ambient oxygen at oxygen vacancies during preparation actually act as carriers.¹⁶ The minority hole carriers in PZT layer deplete some electron carriers in ZnO layer at the PZT-ZnO interface so that a thin depletion layer forms.

When the heterostructure is exposed to positive voltage, electrons are withdrawn from the ZnO layer, causing the depletion layer width to become large. As a result, an additional small series capacitance is added to the overall heterostructure,¹⁷ which results in the deep drop of the capacitance of overall heterostructure. So most of potential drop is across the ZnO layer, whilst the potential drop across the PZT layer becomes so small. Our experimental results of low polarization and capacitance values at positive voltage, as seen in Figures4(a)and4(b), exactly reflects that no strong electric field is built up in the PZT layer. Furthermore, we performed the measurements of electric resistivity at the voltage of 4 V.

The resistivity of CFO was only710⁵Xcm, which was much lower than that of PZT (610⁹Xcm) and depleted ZnO (410¹⁰Xcm). Accordingly, the electrical conductivity of overall heterostructure is dominated by the depleted ZnO layer at positive voltage, and the heterostructure is thus switched to the HRS. In contrast, when the voltage becomes negative, the ZnO layer is switched to accumulation state and becomes conductive. The electric field drops practically fully across the PZT layer. Therefore, the ferroelectric polarization

FIG. 3. (a) I-V curves measured at various magnetic bias. (b) Current as a function of magnetic bias measured at610 V.

FIG. 4. (a) C-V curves measured at various magnetic bias and fixed frequency of 1.0 kHz. The arrows indicate the sweep direction. (b) P-V loops measured at various magnetic bias and fixed frequency of 10.0 kHz.

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of PZT layer develops into its typical hysteresis behavior. In this case, the electrical conductivity of the overall heterostructure is almost attributed to the PZT layer. Hence the heterostructure evolves to the LRS.

As reported in previous investigations, for the single PZT/ZnO junction, the charge depletion or accumulation states are generally dominated by the interface polarization coupling between ZnO and PZT, which is controlled only by external electric field.^6,7,17However, when the ferromagnetic CFO layer is introduced and an multiferroic CFO/PZT junction forms, the charge states will be also influenced by external magnetic field due to the magnetoelectric coupling between CFO and PZT, which changes both charge carrier distribution in the PZT layer and interface polarization coupling between PZT and ZnO. The further understanding of the effect of magnetoelectric coupling on the resistance state of the CFO/PZT/ZnO heterostructure is discussed as follows.

First, we analyse the case that the heterostructure is under negative voltage, where the depletion layer vanishes and space-charge accumulation region forms at the PZT/

ZnO interface. Upon negative voltage, the electrons in ZnO layer are transported into the PZT layer and subsequently combined with the holes. Meanwhile, the holes in CFO layer are injected into the PZT layer to maintain the hole concentration in PZT layer. A continuous electrical conduction thus forms in the whole heterostructure. Accordingly, the influence of magnetoelectric coupling on the electrical conductivity of the heterostructure can be understood in terms of the following two aspects. On one hand, for the ferroelectric PZT layer, its leakage current is dominated by the holes arising from oxygen vacancies, some of which are pinned in the ferroelectric domain walls due to the built-in potential caused by ferroelectric polarization.^15,16Upon external magnetic bias, the ferroelectric polarization of PZT is partially suppressed owing to the magnetoelectric coupling between CFO and PZT [as seen in Figure 4(b)].^19,20 This is a magneto-mechanical-electric coupling process. In detail, magnetic bias induces compressive mechanical stress in CFO layer due to negative magnetostrictive effect, which is subsequently transferred to the PZT layer by interface stress transfer. As a result, the PZT layer suffers an in-plane compressive stress, which causes the suppression of ferroelectric domain reversal accompanied by the decrease of ferroelectric polarization.¹⁸ Therefore, the built-in potential in PZT layer drops and parts of oxygen vacancies are released from the domain walls, consequently leading to the increase of the hole concentration in PZT layer. On the other hand, charge injection at the PZT/ZnO interface will occur at negative voltage, causing the formation of charge accumulation region. The interface polarization coupling between switchable ferroelectric polarization in PZT layer and nonswitch- able spontaneous polarization in ZnO layer causes a part of charges to be trapped in the interface region.¹⁷In addition, the work function difference between ZnO (AZnO¼4.5 eV) and PZT (APZT¼2.6–3.5 eV) causes an interface barrier, which further prevents the bound charge carriers from flow- ing across the interface under external electric field.^21,22This interface barrier will increase when the polarization of ferroelectric layer enhances.^15,23 When the heterostructure is exposed to external magnetic bias, however, the above

charge binding state at the interface can be effectively modulated. Due to the magnetoelectric coupling between CFO and PZT, the ferroelectric polarization in PZT layer is suppressed, leading to the weakening of the interface polarization coupling between PZT and ZnO. Thereby, the binding of space charge at the PZT/ZnO interface becomes weak.

Meanwhile, the suppression of ferroelectric polarization in PZT layer leads to the drop of built-in potential in PZT layer, indicating that the influence of ferroelectric polarization on the interface barrier becomes weak.²⁴These combined modulation of magnetoelectric effect permits more charge carries to flow across the PZT/ZnO interface, consequently resulting in lower resistance.

As for the depletion state (i.e., at positive voltage), the magnetoelectric coupling between CFO and PZT almost has no influence on the ferroelectric polarization under entire magnetic bias range, as shown in Figure 4(b). Both charge distribution in PZT layer and depletion layer width in ZnO layer are independence of magnetic bias, and the electrical conductive behavior of the whole heterostructure is only dominated by the depleted ZnO layer, so the heterostructure still remains the HRS. The resistance state change in the present heterostructure can also be further understood by energy band diagrams constructed by the Anderson model.^6,24

In summary, the CFO/PZT/ZnO film deposited on the Pt/Ti/SiO₂/Si(100) wafer was prepared and three resistance states switched by various stimuli combination of electric voltage pulse and magnetic bias were demonstrated. The heterostructure exhibited significantly asymmetry current- voltage, capacitance-voltage, and ferroelectric polarization- voltage loops, which seriously depended on magnetic bias.

The magnetoelectric coupling between CFO and PZT and interface polarization coupling between PZT and ZnO were responsible for the switching of the three resistance states.

The present work provides promising candidates to achieve multistate resistive switching controlled by multi-field stimuli, which is significant for developing high-performance devices with multifunctionality based on the resistive switching memory effect.

This work is supported by the National Key Projects for Basic Research of China (Grant Nos. 2010CB923401, 2009CB623303), the National Natural Science Foundation of China (Grant Nos. 50972055, 11134005), the PAPD pro- ject, and the Provincial Nature Science Foundation of Jiangsu in China (BK2008024).

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