Tunable resistive switching behaviour in ferroelectric–ZnO bilayer films

Journal of Physics D: Applied Physics

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Tunable resistive switching behaviour in ferroelectric–ZnO bilayer films

To cite this article: Ming-Xiu Zhou et al 2013 J. Phys. D: Appl. Phys. 46 165304

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J. Phys. D: Appl. Phys.46(2013) 165304 (8pp) doi:10.1088/0022-3727/46/16/165304

Tunable resistive switching behaviour in ferroelectric–ZnO bilayer films

Ming-Xiu Zhou, Zi-Wei Li, Bo Chen, Jian-Guo Wan and Jun-Ming Liu

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

E-mail:wanjg@nju.edu.cn

Received 22 November 2012, in final form 18 February 2013 Published 3 April 2013

Online atstacks.iop.org/JPhysD/46/165304 Abstract

Pb(Zr_0.52Ti_0.48)O₃/ZnO bilayer films with various ZnO-layer thicknesses were prepared by a sol–gel process, and their phase structures, electric conduction and polarization behaviour were measured. The results showed that the preferential crystal orientation of the ZnO layer changed with a change in its thickness. The strong dependence of both asymmetric current–voltage and polarization–voltage characteristics on the ZnO-layer thickness was observed. The resistance ratio of the high-resistance state (HRS) to the low-resistance state (LRS) increased with increasing ZnO-layer thickness, and a high rectification ratio was obtained in the bilayer film with an optimized ZnO-layer thickness. The combined effects of interface polarization coupling and energy band structure on the resistive switching behaviour of the bilayer films were revealed, and the electric conduction mechanisms of the bilayer films at both HRS and LRS were analysed in detail. This work presents an effective method to modulate the resistive switching behaviour of ferroelectric–ZnO heterostructures, which is significant in designing high-performance ferroelectric–semiconductor heterostructures for actual applications.

(Some figures may appear in colour only in the online journal)

1. Introduction

In the past few decades, ferroelectric materials have drawn continuous attention due to their switchable ferroelectric polarization properties. They have been widely used in many fields such as sensors, actuators, memories, etc [1,2].

Recently, to further widen their applications, some kinds of heterostructures composed of ferroelectric oxides and other functional materials have been developed. Among them, metal–ferroelectric–semiconductor (MFS) heterostructures have received considerable attention due to their unique bistable and switchable resistive behaviour, which exhibits potential applications in resistance-based random access memories (RRAMs) and ferroelectric field-effect transistors [3,4]. In an MFS heterostructure, charge carriers are accumulated or depleted at the ferroelectric–semiconductor interface by dipolar electric field stimuli, which causes different conductivities. So reversible resistive switching between a high-resistance state (HRS) and a low-resistance state (LRS) is achieved [5,6].

In previous investigations of MFS heterostructures, ZnO was usually used as the semiconductor layer. ZnO is an n-type

direct gap semiconductor with a wurtzite crystal structure.

It has a spontaneous polarization of−4.1 to−7.0µC cm⁻² in addition to a stress-induced piezoelectric effect [7,8].

Nevertheless, related to the inherent crystal structure, the spontaneous polarization of ZnO is in practice electrically irreversible [8]. In ZnO-based MFS heterostructures (e.g.

Pt/BaTiO₃/ZnO), the coupling between reversible polarization of the ferroelectric layer and irreversible spontaneous polarization of the ZnO layer can lead to diode-like current–

voltage behaviour by interface polarization coupling [5,6]. So far several MFS heterostructures composed of ferroelectric oxides and semiconductor ZnO have been developed, all of which have been demonstrated to possess unique diode- like resistive switching behaviour [5,6,9]. The formation of the depletion layer occurs at the ZnO–ferroelectric interface, which is crucial for the formation of different resistive states.

To modulate the resistive switching behaviour, a conventional method is to change the depletion layer width, which can be achieved by changing the layer stacking sequences or a variety of possible arrangements of the ferroelectric and ZnO layers [6]. Another method is to modulate the interface polarization coupling between the ferroelectric and

J. Phys. D: Appl. Phys.46(2013) 165304 M-X Zhouet al

semiconductor layers, which can be tuned by changing the amount of polarization value of the ferroelectric layer [10]. Various ferroelectric oxides with different ferroelectric properties, such as BaTiO3, Pb(Zr0.52Ti0.48)O3 (PZT) and BiFeO3, have been used in MFS structures [5,6,9]. On the other hand, the electric conduction process and its physical origin for the observed resistive hysteresis are important in designing ferroelectric–semiconductor heterostructures for actual applications. Nevertheless, the electric conduction mechanisms at both HRS and LRS for ZnO-based MFS heterostructures are still unclear.

In this work, we report on the modulation of resistive switching behaviour of ZnO-based MFS heterostructural films by tuning the ZnO-layer thickness. We choose PZT as the ferroelectric layer, which has excellent ferroelectric behaviour, and prepare a series of PZT/ZnO heterostructural films with various ZnO-layer thicknesses. We demonstrate that the variation of ZnO-layer thickness brings about a change in the preferential crystal orientation in the ZnO layer, resulting in a change in the amount of spontaneous polarization of the ZnO layer. The strong dependence of both current–

voltage and polarization–voltage characteristics on the ZnO- layer thickness is observed, and the energy band structures are constructed for the heterostructures with various ZnO-layer thicknesses. Based on the combined effects of energy band structure and interface polarization coupling, the modulation mechanism of resistive switching for the heterostructures with various ZnO-layer thicknesses is analysed. Furthermore, the electric conduction mechanisms of the heterostructures at both HRS and LRS are revealed. This work not only presents an effective method to modulate the resistive switching behaviour of ferroelectric–ZnO heterostructures, but also gives a deep insight into the behaviour of resistive switching and electric conduction. The results are significant in designing high- performance MFS heterostructures for actual applications.

2. Experimental

The PZT/ZnO bilayer films were deposited onto the Pt/Ti/SiO2/Si(1 0 0) wafers by a sol–gel process and the spin coating technique. First, a series of ZnO gel films with different number of gel layers (2, 3, 4, 5 and 6 layers) were spin-coated onto Pt/Ti/SiO₂/Si(1 0 0) wafers using a ZnO sol solution, respectively, and then annealed at 650^◦C to form ZnO films by a rapid annealing process under oxygen atmosphere.

Subsequently, four-layer PZT gels were spin-coated onto these ZnO films and annealed by the same process. Details of the preparation procedure can be found elsewhere [11,12]. Cross- sectional morphology examination on a scanning electron microscope (SEM, LEO-1530VP) showed that the PZT-layer thickness for all the bilayer films was 175 nm, and the ZnO- layer thickness was 35 nm (Z2P), 65 nm (Z3P), 95 nm (Z4P), 125 nm (Z5P) and 155 nm (Z6P), respectively. The phase structure characterizations were carried out by x-ray diffraction (XRD) on a D/MAX-RA diffractometer using Cu Kαradiation.

For electric measurements, 100 nm Au top electrodes with a diameter of 0.2 mm were sputtered onto the surface of the films through a shadow mask. The current–voltage (I–V)

Figure 1.(a) XRD spectra of the PZT/ZnO bilayer film with various ZnO-layer thicknesses (tZnO). (b) Diffraction intensity ratio (γ) of the (0 0 2) peak to the (1 0 1) peak as a function oftZnO.

characteristics were measured using a Keithley 6517B ampere meter (the voltage is positive as the top Au electrode is grounded, and negative as the bottom Pt electrode is grounded).

The maximum sweeping voltage (Vmax) was set to 5.0 V.

The sweeping direction of the voltage was 0 → +Vmax →

−V_max → 0. Ferroelectric polarization measurements were carried out by a Radiant multiferroic test system.

3. Results and discussion

3.1. Phase structures of the PZT/ZnO bilayer films

Figure1(a) shows the XRD patterns of the PZT/ZnO bilayer films with various ZnO-layer thicknesses. All peaks can be identified. Two evident sets of well-defined peaks are observed and there are no additional or intermediate phase peaks apart from ZnO and PZT. The PZT layers exhibit polycrystalline structures with random orientations for all the samples, while the crystal orientation of the ZnO layer varies with its thickness.

With increasing ZnO-layer thickness, the diffraction intensity 2

Figure 2.(a) Log(I) versus voltage (V) for the PZT/ZnO bilayer films with various ZnO-layer thicknesses at room temperature.

(b) Rectification ratio as a function of ZnO-layer thickness for the PZT/ZnO bilayer films.

along the (0 0 2) plane increases, meanwhile the intensity along the (1 0 1) plane decreases. Figure1(b) plots the diffraction intensity ratio (γ) of the (0 0 2) peak to the (1 0 1) peak in the ZnO layer as a function of ZnO-layer thickness (t_ZnO). It is seen that theγ value increases with increasingtZnO and gradually reaches the saturation point ast_ZnO>125 nm. The maximum γvalue appears in the Z5P film, indicating the highly preferred crystal orientation along the (0 0 2) plane in the ZnO layer. In addition, from figure1(a) we observe that the peak intensity of PZT (1 1 1) varies with the change in ZnO-layer thickness.

This indicates that the ZnO-layer thickness also influences the crystalline structures of the PZT layer to a certain degree.

3.2. Current–voltage characteristics of the PZT/ZnO bilayer films

Figure 2(a) presents log(I) versus voltage (V) for the PZT/ZnO bilayer films with various ZnO-layer thicknesses measured at room temperature. The films exhibit different I–V characteristics depending on the ZnO-layer thickness.

When the ZnO layer is very thin (Z2P), the I–V curve is nearly symmetrical, similar to that of pure PZT films.

However, with increasing ZnO-layer thickness, the I–V curve gradually becomes asymmetric and exhibits apparent diode-like rectification behaviour, i.e. the current is evidently suppressed (the film is in the HRS) at a positive voltage, while it remains at a high value (the film is in the LRS) at a negative voltage. The current at a positive voltage is much lower than that at a negative voltage, and their difference becomes more obvious with increasing ZnO-layer thickness. Herein, we define the rectification ratio ε =

| −I_max|/| +I_max|, i.e. the current ratio at the positive and negative voltages with the same value, to characterize such a difference. Figure2(b) plots the rectification ratio as a function of ZnO-layer thickness measured atV = ±5.0 V. It is seen that theεvalue increases with increasing ZnO-layer thickness, and nearly reaches saturation when the ZnO-layer thickness is beyond∼125 nm, at which the maximum rectification ratio is as high asε∼1750.

Previous investigations have demonstrated that the forma- tion of HRS and LRS in a ferroelectric–ZnO heterostructure is generally dominated by the interface polarization coupling between switchable ferroelectric polarization in the ferroelec- tric layer and nonswitchable spontaneous polarization in the ZnO layer [5,6,10,13]. The electrical conductivity of the het- erostructure changes if the charge carriers in the heterostructure are accumulated or depleted at the ferroelectric–ZnO interface.

In the present PZT/ZnO films, since ZnO is an n-type semi- conductor and PZT is a p-type material, where electron holes are the majority carriers and dominate the electrical conduc- tion of PZT [14], the PZT/ZnO film is actually similar to a p–n junction. The spontaneous polarization of the ZnO layer leads to the formation of a thin depleted ZnO layer at the interface even at zero electric voltage [5]. When the bilayer film is ex- posed to a positive voltage, electrons are withdrawn from the ZnO layer, causing the depletion layer width to become large.

Accordingly, the electrical conductivity of the bilayer film is dominated by the depleted ZnO layer. As the depleted ZnO layer has larger resistivity compared with the PZT layer [10], the film is switched to the HRS. In contrast, when the applied voltage is negative, the ZnO layer is switched to the accumula- tion state and becomes conductive. In this case, the electrical conductivity of the overall heterostructure is almost attributed to the PZT layer. Hence, the film evolves to the LRS.

3.3. Energy band structures of the PZT/ZnO bilayer films To further understand the ZnO-layer thickness dependence of I–V behaviour, we analysed the energy band structures of the PZT/ZnO films. The band gap energies (E_g) of both ZnO and PZT layers are crucial for the construction of the energy band structure of the PZT/ZnO films. In general, theE_g value of oxides is related to the crystal structures, component, preparing process and so on [15]. So it is necessary to experimentally measure theE_gvalue for both PZT and ZnO films. We first prepared a pure 175 nm PZT film on the quartz substrate by a sol–gel process, and its optical transmittance spectra are shown in figure3(a). It is seen that the transmittance for PZT

J. Phys. D: Appl. Phys.46(2013) 165304 M-X Zhouet al

Figure 3.(a) Optical transmittance of the PZT film deposited on the quartz substrate. (b) (αhν)²versushνcurves for the PZT film.

gradually decreases on decreasing the operation wavelength.

The transmittance is given by the expressionT =exp(−αd), which can be converted into the absorption coefficient α =

−lnT /d, wheredis the film thickness. Thus, theE_gvalue can be derived assuming a direct transition between valence and conduction bands. For a direct gap material, the absorption coefficient can be expressed as [16]

(αhν)² =C(hν−E_g) (1) wherehis the Planck constant,hνis the incident photon energy andC is a constant. Figure 3(b) plots the(αhν)² versushν curve for the PZT film. A linear region can be fitted, which gives the evidence of the assumption that there is a direct transition between valence and conduction bands of PZT as a direct gap material [17]. An extrapolation of the linear region to(αhν)²=0 givesE_g∼3.68 eV for the PZT film.

Similarly, five pure ZnO films with various thicknesses of 35, 65, 95, 125 and 155 nm were prepared on the quartz substrates by the sol–gel process, and their E_g values were obtained by the measurements of optical transmittance spectra, as listed in table1. It is clear that theE_gvalue of the ZnO film decreases with increasing thickness, and almost reaches the

Table 1.The measured band gap energy (Eg) values for the ZnO films with various thicknesses (tZnO).

tZnO(nm) 35 65 95 125 155 Eg(eV) 3.33 3.30 3.26 3.22 3.23

Figure 4.Schematic energy band diagram for the PZT/ZnO bilayer film.

saturation point ast_ZnO>125 nm. ThisE_gdependence on the ZnO-layer thickness should be correlated with the change in the substrate-induced stress in the ZnO layer [18,19].

To construct the band diagram of the PZT/ZnO films, the electron affinities of ZnO and PZT (i.e.χ_PZT = 3.5 eV and χ_ZnO =4.35 eV) [9], and the work function of ZnO (ϕ_M,ZnO= 4.45 eV) [20] were also used. We suppose that the PZT Fermi level is in the middle of the energy band gap [21], accordingly, the work function (ϕM,PZT) of the PZT film is 5.34 eV. Figure4 shows the band diagram of the PZT/ZnO film. Ideally, because of the difference in work functions between PZT and ZnO, a built-in potentialV_D equal to 0.89 eV is produced, which is obtained byV_D =ϕ_M,PZT−ϕ_M,ZnO. Electrons and holes are accumulated at the PZT–ZnO interface due to the existence of conduction and valence band offsets. To calculate the barrier heights for electrons and holes in such a p–n junction, the band offsets have to be taken into account. The band offset of the conduction band isE_c = 0.85 eV according toE_c=χ_ZnO−χ_PZT. The band offset of the valence band is denoted byE_v=χ_ZnO−χ_PZT+Eg,ZnO−E_g,PZT, whose value varies with the ZnO-layer thickness. VariousE_vvalues of 0.5 eV for Z₂P, 0.47 eV for Z₃P, 0.43 eV for Z₄P, 0.39 eV for Z5P and 0.40 eV for Z6P are thus obtained. Accordingly, the barrier height for electrons (Ec+V_D) is 1.74 eV, while the barrier height for holes (Ev+V_D) decreases with increasing ZnO-layer thickness.

As the barrier height for electron is the same for all the PZT/ZnO bilayer films, the I–V behaviour is actually dominated by the barrier height for holes. Typically, if the barrier height for holes is 0.2 eV smaller than that for electrons, the hole current will be approximately a factor of 10⁴ larger than the electron current [21]. When the bilayer film is in the LRS (at a negative voltage), the electrical conductivity of the overall film is almost caused by the hole current of the p- type PZT layer, and the current increases with decreasing hole barrier. The thicker the ZnO layer, the smaller is the barrier 4

Figure 5.(a) Polarization versus electric voltage hysteresis loops of the PZT/ZnO bilayer films with various ZnO-layer thicknesses. (b), (c) Polarization dependence of the bilayer film on ZnO-layer thickness measured at±5.0 V.

height for holes. Therefore, diode-likeI–V behaviour of the bilayer film becomes more remarkable when the ZnO-layer thickness is larger. It is notable that, although the work function between the top (Au) and bottom (Pt) electrodes cannot be completely ruled out, their influence on the asymmetricI–V behaviour is small compared with the influence of the ZnO- layer thickness [22]. So we do not take into account the effect of electrode layers on the band structure of the heterostructures in this work.

3.4. Polarization behaviour of the PZT/ZnO bilayer films We first explore the influence of ZnO-layer thickness on the polarization behaviour of the PZT/ZnO bilayer films.

Figure5(a) presents the polarization (P) versus voltage (V) hysteresis loops of the bilayer films with various ZnO-layer thicknesses. Asymmetric P–V loops are observed, and the polarization values at the positive voltage are much smaller than those at the negative voltage. The asymmetry of P– V loops becomes more obvious with increasing ZnO-layer thickness. For the Z5P and Z6P films, the polarization values in the whole positive region are even close to zero, while theirP–V loops remain having the regular ferroelectric

shape at the negative voltage. Figures 5(b) and (c) further plot the polarization dependence on the ZnO-layer thickness (measured at ±5.0 V). At +5.0 V voltage, the polarization value quickly decreases with increasing ZnO-layer thickness, e.g. from 18.6µC cm⁻² (Z2P) to 1.4µC cm⁻² (Z5P), and almost reaches saturation as the ZnO-layer thickness is beyond 125 nm (Z5P). At−5.0 V voltage, however, the polarization value increases with increasing ZnO-layer thickness, e.g.

from −18.8µC cm⁻² (Z2P) to −28.8µC cm⁻² (Z5P), and also reaches saturation as the ZnO-layer thickness is beyond 125 nm.

The above results indicate that the ZnO-layer thickness seriously influences the polarization behaviour of the PZT/ZnO film, which can be understood by the interface polarization coupling between PZT and ZnO layers [10,13]. Since the PZT layers for all the films have similar polycrystalline structures with the same thickness, their ferroelectric polarization characteristics are almost the same. Nevertheless, the spontaneous polarization value of the ZnO layer varies with its thickness. It is well known that ZnO crystallizes into the wurtzite structure together with the ionic nature of the Zn–

O bond, producing a net dipole moment along thec-axis of the unit cell. These dipole moments cause equal and opposite bound polarization charges on the Zn-polar and O-polar faces, that is, the spontaneous polarization of ZnO is irreversible and closely related to thec-axis orientated crystal structure [23].

According to our XRD data shown in figure1(a), as the ZnO- layer thickness increases, its preferential orientation along the (0 0 2) plane (orc-axis ) becomes strong, indicating the enhancement of spontaneous polarization in the ZnO layer.

Such a spontaneous polarization dependence on the ZnO- layer thickness significantly influences the polarization of the PZT/ZnO bilayer film. We first take into account the case that the film is under a positive voltage, and understand it in terms of the following two points. (i) At positive voltage, the spontaneous polarization orientation of the ZnO layer is opposite to the ferroelectric polarization orientation of the PZT layer, so the superposition of both polarizations results in a decrease in the polarization of the bilayer film. The stronger the spontaneous polarization of the ZnO layer, the smaller is the polarization of the bilayer film. (ii) The positive voltage causes the depleted ZnO layer to broaden due to the withdrawing of electrons from the ZnO layer. This depletion layer has a very small capacitance, which is in series in the overall film. So most of the potential drop is across the depletion layer, whilst the potential drop across the PZT layer becomes so small, consequently leading to the polarization drop of the overall film. If the spontaneous polarization of the ZnO layer is enhanced, the depletion layer becomes wider and thus the potential drop across the depletion layer further increases. As a result, the polarization of the overall film becomes smaller. However, when the applied voltage is negative, the depletion layer vanishes and the ZnO layer becomes conductive. The potential drop is practically fully across the PZT layer, so the bilayer still exhibits a typical ferroelectric hysteresis loop with large polarization. Moreover, at negative voltage, the spontaneous polarization orientation of the ZnO layer is the same as the ferroelectric polarization

J. Phys. D: Appl. Phys.46(2013) 165304 M-X Zhouet al

orientation of the PZT layer, indicating that the enhanced spontaneous polarization of the ZnO layer could further result in the polarization enhancement of the bilayer film. It is worth noting that, at the negative voltage, since the ZnO layer is switched to the accumulation state, the charges accumulated at the ZnO–PZT interface may influence the shape of theP–V loop of the PZT layer to a certain degree. As a result, theP– V loops of the bilayer films measured at the negative voltage become somewhat irregular, as shown in figure5(a).

3.5. Electric conduction mechanisms of the PZT/ZnO bilayer films

According to the above results and discussion, we suggest that the asymmetric diode-like current–voltage behaviour of the PZT/ZnO bilayer film, i.e. HRS at a positive voltage and LRS at a negative voltage, can be modulated by the combined effects of interface polarization coupling and energy band structure. On the one hand, the interface polarization coupling in the bilayer film is closely associated with its polarization behaviour, which actually dominates the resistive behaviour in the HRS. At a positive voltage, the increase in the ZnO-layer thickness causes an enhanced spontaneous polarization of the ZnO layer, leading to the broadening of the depletion layer. And so the current decreases, accompanied by the resistance rise of the whole heterostructure. On the other hand, the resistive behaviour of the bilayer film in the LRS is determined by the barrier height for holes. Under the applied negative voltage, the increase in the ZnO-layer thickness produces a smaller barrier height for holes, so the current flowing at the interface increases and the resistance of the bilayer film drops. In conclusion, the asymmetric diode-likeI–V behaviour of the bilayer film can be well tuned by the combined effects of interface polarization coupling and energy band structure, which cause an enlarged rectification ratio as the ZnO-layer thickness increases. In this work, the maximum spontaneous polarization of the ZnO layer appears in the Z₅P film (as shown in figure1(b)), in which the ZnO layer has the highest preferential orientation along the c-axis. So the film has the strongest interface polarization coupling, causing the smallest current at the HRS. Meanwhile, the maximum current appears at the LRS as its barrier height for holes is the lowest. As a result, the maximum rectification ratio is obtained in this film.

Furthermore, we explored the electric conduction process of the PZT/ZnO bilayer films for deeply understanding their asymmetric I–V characteristics as well as their ZnO-layer thickness dependence at the HRS and LRS. Figures6 and7 present the log(I) versus log(V) curves for the PZT/ZnO bilayer films with various ZnO-layer thicknesses at the HRS and LRS, respectively. We made a linear fitting for all curves in figure6 and found that all films had the same conduction mechanisms at the HRS. In detail, in the low electric field (0–2 V), the current linearly increases with an exponential of β ∼1, so the ohmic conduction is dominant in the low electric field. On further increasing the electric field (>2 V), theβ value becomes∼2, which is in good agreement with the space- charge-limited current (SCLC) behaviour [24,25]. When the films are at the LRS, however, the conduction mechanism

Figure 6.(a)–(e) Log(I) versus Log(V) curves for the PZT/ZnO bilayer films with various ZnO-layer thicknesses measured at the HRS. Linear fittings are conducted to extract the slopes to identify whether the ohmic or SCLC behaviour occurs.

becomes different, as shown in figure 7. When the ZnO layer is very thin (e.g. the Z₂P film), the conduction is in agreement with the ohmic behaviour (β = 0.8) in the low electric field and the SCLC process (β = 1.92) in the high electric field, respectively. However, for the Z₃P, Z₄P, Z₅P and Z₆P films which have thicker ZnO layers, although the ohmic conduction is still dominant in the low electric field, the conduction deviates significantly from the SCLC behaviour in the high electric field since theβ values are all much higher thanβ = 2. Accordingly, we suggest that another different conduction mechanism should be responsible for suchI–V behaviour in the high electric field. In a relatively high electric field, injection of charge carriers into the ferroelectric layer from the electrode layer may occur by tunnelling through an interfacial energy barrier, which is called Fowler–Nordheim 6

Figure 7.(a)–(e) Log(I) versus Log(V) curves for the PZT/ZnO bilayer films with various ZnO-layer thicknesses measured at the LRS, where voltage is taken as an absolute value. Linear fittings are conducted to extract the slopes to identify whether the ohmic or SCLC behaviour occurs.

(FN) tunnelling and can be described as [26]

J_FN=CE² exp



−D²

ϕ³_i E



 (2)

whereCandDare constants,Eis the applied electric field and ϕ_iis the potential barrier height. Figure8plots the log(J /E²) versus 1/Ecurves. The data fittings according to equation (2) were conducted to identify whether FN tunnelling process exists. It is seen that the log(J /E²) versus 1/Eplots exhibit a good linear relationship for all the films in the high electric field (>2,V), indicating that the FN tunnelling dominates the current–voltage behaviour in the high electric field at the LRS.

Figure 8.(a)–(d) Log(J /E²) versus (1/E) curves for the PZT/ZnO bilayer films with various ZnO-layer thicknesses measured at the LRS. Linear fittings are conducted to identify whether the FN tunnelling dominates the current–voltage behaviour in high electric fields.

Such an FN tunnelling process is actually associated with the hole carriers flowing across the PZT–ZnO interface, which becomes easier if the ZnO layer is thicker since the barrier height for holes becomes smaller. This further confirms that, for the ferroelectric–ZnO heterostructure with a thicker ZnO layer, its resistive behaviour at the LRS is dominated by the PZT–ZnO interface, which is associated with the energy band structure of the heterostructure. Finally, it is worth mentioning that, according to our calculations, the other two conventional electric conduction processes in the high electric field, i.e.

the bulk-limited Poole–Frenkel (PF) emission caused by the trap effect in the ferroelectric layer and the interface-limited Schottky emission controlled by the interface between the ferroelectric layer and the metal layer, could not occur in the present PZT/ZnO bilayer films.

4. Conclusions

In summary, PZT/ZnO bilayer films with various ZnO- layer thicknesses are prepared on Pt/Ti/SiO2/Si(1 0 0) wafers.

Asymmetric diode-like current–voltage characteristics are observed in all the films, which strongly depend on the ZnO-layer thickness. The rectification ratio of the bilayer film increases with increasing ZnO-layer thickness, which is attributed to the enhanced interface polarization coupling and energy band structure differences caused by the ZnO- layer thickness. The electric conduction mechanism in low electric fields is ohmic conduction for all the films; however, in high electric fields, space-charge-limited current process is dominant at the high-resistance state, while the Fowler–

Nordheim tunnelling process is responsible for the electric

J. Phys. D: Appl. Phys.46(2013) 165304 M-X Zhouet al

conduction process at the low-resistance state when the ZnO layer is thicker. This work presents an effective way to modulate the resistive switching behaviour of ferroelectric–

ZnO heterostructures, which is significant in designing high- performance ferroelectric–semiconductor heterostructures for actual applications such as resistance-based random access memories and ferroelectric field-effect transistors.

Acknowledgments

This work was supported by the National Key Projects for Basic Research of China (Grant Nos 2010CB923401 and 2009CB623303), the National Natural Science Foundation of China (Grant Nos 50972055 and 11134005) and the PAPD project of Jiangsu in China.

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