Deposition-Parameter-Determined Resistive Switching Characteristics in TiO x / Pb ( Zr 0 . 52 Ti 0 . 48 ) O 3 Bilayers

Pb(Zr 0.52 Ti 0.48 )O 3 (PZT) ferroelectric thin films were deposited epitaxially, by pulsed laser deposition, on (001) SrTiO 3 substrates bufferedwith La 0.7 Sr 0.3 MnO 3 (LSMO) electrodes. AmorphousTiO x thin filmswere deposited on top of PZT at various temperatures and oxygen chamber pressures. Bipolar resistive switching characteristics of Pt/TiO x /PZT/LSMOheterostructures are found to vary with TiO x deposition parameters, from an interface controlled ferroelectric diode behavior to a bulk-controlled conductive filament behavior. The observations are discussed in terms of the concentration and migration of oxygen vacancies in the TiO x layer.


Introduction
Although memories based on FLASH techniques are ubiquitous nowadays, novel nonvolatile memories, such as phase-change memories, resistive switching memories, and magnetic and ferroelectric tunnel-junction-memories, have attracted much attention recently to achieve lower power consumption and higher data storage density.Among these emerging memories proposed, resistive switching memories have shown advantages such as scalability [1,2], high speed [3], long data retention, high-endurance [4], and low energy consumption [5].
Resistive switching behaviors have been reported in a number of materials due to various mechanisms.In binary transition metal oxides, resistive switching has been extensively studied and modulations on filamentary conduction have been widely accepted.Formation and rupture of conductive TaO 1− filaments, responsible for resistive switching in Pt/SiO 2 /a-Ta 2 O 5− /a-TaO 2− /Pt heterostructures, have been directly observed by transmission electron microscopy [6].Salaoru et al. and Bousoulas et al. proposed that concentration and distribution of oxygen vacancies have a great impact on the diameter of the conducting filaments and hence the operation of TiO 2− -based memory devices [7,8].However, other resistive switching mechanisms have also been proposed.For example, resistive switching in ZrO 2 has been ascribed to space charge limited currents modulated by Zr + traps [9].Electrically induced modulations on oxide/electrode interface barriers may also result in resistive switching [10,11].Although such modulations can be achieved through electric-field-driven migrations of charged defects, ferroelectric switching provides a unique modulation mechanism in this respect, the switchable ferroelectric diode behavior, as first observed in BiFeO 3 -based devices [12,13].In these devices, Schottky barriers are formed at both interfaces and the barrier heights can be switched to a high or a low level as a response to the switched bound charges at the interface due to ferroelectric switching.
In this report, we show that resistive switching in Pt/ Pb(Zr 0.52 Ti 0.48 )O 3 (PZT)/Pt devices can be tuned from a ferroelectric diode behavior to an oxygen vacancy controlled behavior by inserting a TiO  interfacial layer between PZT and Pt top electrodes.Deposition parameters of the TiO  interfacial layer, such as substrate temperature and oxygen chamber pressure, are demonstrated significant to the observed resistive switching behavior.

Experimental Section
The heterostructures studied were deposited by pulsed laser deposition (Adnano Corp. Crystal structures of the multilayers were characterized by X-ray diffraction (XRD) using a Rigaku ULTIMA-III diffractometer with a Cu K radiation.Surface topography was studied using an Asylum Research Cypher atomic force microscope (AFM) in contact mode.Electrical measurements were conducted using a Keithley 2400 source-measure unit.Voltage was applied to the Pt top electrodes and the bottom LSMO electrodes were always grounded.Chemical analysis was carried out ex situ by X-ray photoelectron spectroscopy (XPS) using a Thermo Fisher K-Alpha spectrometer equipped with an Al K (1486.7 eV) X-ray source.accompanied with peaks from the substrate.This indicates that PZT and LSMO are grown coherently to the STO substrates and the TiO  films are amorphous.The -axis lattice parameter of PZT, obtained from the XRD patterns, is 4.090 Å, greater than its bulk value 4.082 Å.This can be ascribed to the in-plane epitaxial strain exerted by the LSMO layer underneath.Amorphous TiO  is a result of low deposition temperature [14].Surface morphology of the samples was checked by AFM.All the samples show a smooth surface, with a root-mean-square roughness less than 3.0 Å.A representative example is shown in Figure 1(b), where the top TiO  layer was deposited at 80 ∘ C in 2.0 Pa oxygen.Figure 1(c) shows the ferroelectric hysteresis loop of a 50 nm PZT film.The coercive voltage is about 3.0 V and the remnant polarization is about 40 C/cm 2 , in agreement with those reported in the literature [15,16].Current-voltage (-) characteristics of Pt/PZT/LSMO heterostructures are shown in Figure 2(a).The voltage was set to sweep from 0 V to 10.0 V, then from 10.0 V to −10.0 V, and finally from −10.0 V back to 0 V. Pronounced bipolar resistive switching can be observed.The device is initially in the high resistance (low current) state.It is switched to a low resistance (high current) state when the voltage exceeds about 3.2 V.When negative voltages are applied, the device behaves in a similar way, switching from the high resistance state to the low resistance state at about −3.4 V.Note that the resistance state switches at the coercive voltage of the PZT film.Figures 2(b)-2(e) show - curves of Pt/TiO  /PZT/LSMO heterostructures, in which the TiO  layer was deposited at various temperatures in 2.0 Pa oxygen.In Figure 2(b), when the TiO  layer is deposited at 30 ∘ C, the resistance switching behavior of Pt/TiO  /PZT/LSMO is similar to that of Pt/PZT/LSMO.The inset of Figure 2(b) shows the - curves of the same device, measured after poling the PZT layer by a 10.0 or a −10.0 V pulse.Rectifying characteristics, switched by the poling voltage, are observed.This is a typical ferroelectric diode behavior, in which the resistance, read by a certain positive or negative voltage, is controlled by the polarization direction in the ferroelectric layer [17,18].However, the resistance switching behavior varies with the increase of TiO  deposition temperature.When the TiO  deposition temperature is above 150 ∘ C, the device exhibits an initial low resistance state, which can be switched into a high resistance state when the voltage exceeds 6.8 V.This high resistance state can be maintained until a negative voltage stronger than −5.5 V is applied, at which the device is reset to the low resistance state.Similar resistance switching sequence is observed in Pt/TiO  /LSMO heterostructures, as shown in Figure 2(f).This is typical for bipolar resistance switching observed in NiO, Ta 2 O 5 and TiO 2 [19][20][21], which is ascribed to the formation and rupture of conductive filaments [6].

Results and Discussion
These observations indicate an evolution of the resistive switching behavior from an interface controlled to a bulkcontrolled mechanism, by increasing the deposition temperature of TiO  layers.It has been reported, in TiO  films, that oxygen deficiency might be enhanced at elevated deposition temperatures [22].Therefore, the conductive filaments in TiO  /PZT heterostructures may probably be formed by oxygen vacancies.In heterostructures where the TiO  layers are deposited at 30 and 80 ∘ C, the initial high resistance state may be a result of the highly resistive PZT layer.With increasing TiO  deposition temperature, the concentration of oxygen vacancies increases.These charged defects may diffuse into PZT during deposition and form the conductive filaments.Therefore, the initial state becomes a low resistance state.When a positive voltage is applied, more oxygen vacancies are driven into PZT and the diameter of the filaments may grow.However, Joule heating at positive high voltages may fuse the conductive filaments and set the device into a high resistance state.When excessive oxygen vacancies are driven back into TiO  by a negative voltage, the conductive path in PZT may be formed again.This resets the device into the low resistance state.To check the assumption that the observed resistance switching behavior is associated with oxygen vacancies in TiO  , TiO  layers were deposited on top of PZT in background vacuum to increase the oxygen vacancy concentration deliberately.As shown in Figure 3(a), the resistance switching behavior follows the conductive filament picture with an initial low resistance state, when the TiO  layer is deposited at 30 ∘ C.This is in contrast to the ferroelectric diode behavior observed in the device with the TiO  layer deposited in flowing oxygen at 30 ∘ C (Figure 2(b)).When the deposition temperature is increased to 150 ∘ C, no resistance switching behavior is observed due to the low resistance of the overall heterostructure, as shown in Figure 3(b).These indicate that low chamber oxygen pressure and high TiO  deposition temperature indeed introduce more oxygen vacancies into the heterostructure.
XPS analysis was performed on various TiO  /PZT/LSMO heterostructures to study the valance states of Ti ions in TiO  .Photoelectron spectra emitted from Ti 2p orbital are shown in Figure 4. Major features are two peaks at 458.5 and 464.1 eV, which can be assigned to the spin-orbital split of Ti 4+ 2p orbital.The Ti 2p1/2 and Ti 2p3/2 split is 5.6 eV and the intensity ratio is about 2 : 1.These are in agreement with previous reports on TiO 2 [23,24].However, photoelectron emissions from Ti 3+ ions appear at binding energies 1.4 eV lower than their Ti 4+ counterparts.The appearance of reduced Ti ions implies the existence of oxygen vacancies.It is also observable that the intensity of Ti 3+ components increases with increasing deposition temperature and decreasing oxygen pressure during TiO  deposition.Therefore, the oxygen vacancy concentration increases accordingly.As summarized in Figure 5, the Ti 3+ components increase from 6.5% to 8.8% in TiO  deposited in 2.0 Pa oxygen when deposition temperature increases from 30 to 150 ∘ C. For TiO  deposited at 80 ∘ C, the Ti 3+ percentage is 1.8 times greater in samples deposited in background vacuum than those deposited in 2.0 Pa oxygen.

Conclusions
In summary, resistive switching characteristics of Pt/TiO  / PZT/LSMO heterostructures have been studied.An evolution from an interface controlled ferroelectric diode behavior to a bulk-controlled conductive filament behavior has been observed by modulating the oxygen vacancy concentration in the TiO  layer, which increases with increasing deposition temperature and decreasing oxygen pressure.In heterostructures with less oxygen vacancies, the PZT layers are highly resistive and the electrical transport is limited by electron injection from the interface Schottky barrier, which can be controlled by polarization switching in PZT.However, in heterostructures with more oxygen vacancies, the diffusion of oxygen vacancies into PZT results in a resistive switching behavior controlled by the formation and rupture of conductive filaments composed of conglomerated oxygen vacancies.These observations suggest that the resistive switching characteristics of bilayer heterostructures could be tuned by deposition parameters.

Figure 1 :
Figure 1: (a) XRD patterns of TiO  /PZT/LSMO/STO heterostructures, in which the TiO  layers were deposited in 2.0 Pa oxygen at various temperatures from 30 to 300 ∘ C; (b) typical surface morphology of TiO  /PZT/LSMO/STO heterostructures; (c) hysteresis loop of a 50 nm thick PZT film.