Resistive switching properties of Ce and Mn co-doped BiFeO3 thin films for nonvolatile memory application

The Ce and Mn co-doped BiFeO3 (BCFMO) thin films were synthesized on Pt/Ti/SiO2/Si substrates using a sol-gel method. The unipolar resistive switching (URS) and bipolar resistive switching (BRS) behaviors were observed in the Pt/BCFMO/Pt device structure, which was attributed to the formation/rupture of metal filaments. The fabricated device exhibits a large ROFF/RON ratio (>80), long retention time (>105 s) and low programming voltages (<1.5 V). Analysis of linear fitting current-voltage curves suggests that the space charge limited leakage current (SCLC) and Schottky emission were observed as the conduction mechanisms of the devices.


I. INTRODUCTION
Resistive random access memory (RRAM), which is based on resistive switching induced by external electrical stimulations, has attracted great attention due to its potential application in next generation nonvolatile memory (NVM) devices.The RRAM devices possess a series of advantages such as nonvolatility, fast response speed, low power consumption, long retention time, low cost, high endurance and simple device structure etc. [1][2][3][4] And furthermore, varieties of materials presenting resistive switching (RS) characteristics have been found, including transition metal oxide materials which are compatible with present semiconductor manufacturing processes, such as TiO 2 , ZrO 2 , Cu x O, ZnO, HfO 2 , perovskites such as SrTiO 3 , Pr 0.7 Ca 0.3 MnO 3 , and organic material as well as silicon oxide etc. [5][6][7][8][9][10][11][12][13] To understand RS mechanism, a lot of models have been presented and studied intensively.Usually, the RS mechanisms are mainly divided into two categories, including bulk effect and interface effect using defects or trapped carriers, oxygen vacancies migrating. 14,15 cently, the considerable and stable resistive switching characteristics have been observed with certain ferroelectric systems, which have attracted increasing attention as candidates for the resistive information storage. 16BiFeO 3 thin film possesses a giant remanent polarization, a room-temperature magnetization, and a high Curie temperature (T C ∼ 1103 K), promising as a candidate material for several technologically demanding applications. 17,18 urthermore, the switchable ferroelectric diode-like and resistive hysteresis behavior have been reported for the BiFeO 3 thin films and the resistive switching in the metal-ferroelectrics-metal structure is very important for real technical applications. 19Therefore, it is of considerable interests to investigate the feasibility of realizing the resistive behavior for the BiFeO 3 thin films.1][22] The RS effects have been found in Cedoped BiFeO 3 films, which were suggested to result from the formation/rupture of metal filaments due to the diffusion of the top electrodes under a bias voltage. 23Luo et al reported the nonpolar resistive switching in Mn-doped BiFeO 3 thin films by chemical solution deposition. 24However, until now there were few systematic studies on the resistive switching behaviour of metal/Ce and Mn codoped BiFeO 3 /metal sandwiched structure.Among all dopants, Ce should be a promising one since the ionic radii of Ce 3+ (1.18 Å) and Ce 4+ (1.02 Å) are closer to that of Bi 3+ (1.20 Å).Appropriate Ce cations might be used to substitute for A site Bi 3+ in the BiFeO 3 matrix and Ce cations could stabilize oxygen octahedron that helps decrease Bi volatilization and improve the crystallization. 25,26 oreover, doping at the Fe site with aliovalent atoms, such as Mn, is found to be advantageous in reducing leakage current and improving both electrical and magnetic properties of BiFeO 3 films. 27,28 he substitution of Mn ions is expected to affect the number of oxygen vacancies as well as the chemical states of Fe ions.All the above arguments motivate the authors to investigate the movement and interaction of the Ce and Mn codoped BiFeO 3 thin films.In this study, we report the resistive switching behavior of BCFMO thin films prepared by using the sol-gel method, which has the advantages that include low cost, easy stoichiometric control and high uniformity.And, the BCFMO thin films were found to have the reversible switching behavior and the memory effect.

II. EXPERIMENTAL DETAILS
The pure BiFeO 3 (BFO), Ce doped Bi 1−x Ce x FeO 3 (BCFO), Ce and Mn co-doped BiFeO 3 (Bi 0.85 Ce 0.15 Fe 0.95 Mn 0.05 O 3 ) films were fabricated on commercially available Pt/Ti/SiO 2 /Si substrates by using the sol-gel process.Compared with Si substrates, the Pt/Ti/SiO 2 /Si substrates have some advantages such as decreasing stress and improving the crystallization with a buffer of Ti/SiO 2 .Bismuth nitrate [Bi(NO 3 ) 3 O] were used as the starting materials to prepare the BFO, BCFO and BCFMO precursor solution.Excess 2 mol% Bi(NO 3 ) 3 • 5H 2 O was used to attempt to compensate for the expected loss of volatile Bi during the following heat treatment.The solution was spin-coated at a rate of 4000 rpm for 30 s, followed by a drying process at 220 • C for 3 min and a pyrolysis process at 400 • C for 3 min.The process was repeated several times until the film thickness reached the desired thickness (250 nm).Finally, thin films were annealed at 550 • C for 10 min in air ambient to enhance the crystalline quality.
The crystalline orientation, microstructure of the film samples were characterized using X-ray diffraction (XRD, D/Max 2550, Japan), Raman spectra (RM1000, Renishaw Co.), scanning electron microscopy (SEM, S4800, Japan), energy dispersive spectroscopy (EDS) attached to the SEM and atomic force microscopy (AFM, SPI4000 PROBE STATM, Japan).Thermogravimetric curve of BCFMO powder was determined by TGA (DSC/TG, Q50, USA).For electrical measurement, the top Pt electrodes were deposited by rf magnetron sputtering with a metal shadow mask to form the metal-insulator-metal structure (MIM) structure device cells.The electrical measurements were performed by semiconductor parameter analyzer (Keithley 4200-SCS, and Agilent B1500A, USA).

III. RESULTS AND DISCUSSION
Various concentrations of Ce (x = 0, 0.05, 0.1, 0.15 and 0.2) doped Bi 1−x Ce x FeO 3 and the Ce and Mn co-doped BiFeO 3 (Bi 0.85 Ce 0.15 Fe 0.95 Mn 0.05 O 3 ) thin films were investigated to analyze the memory performance for future NVM application.The performance of resistive switching and leakage current among all concentrations of Ce in BiFeO 3 , also the Ce and Mn co-doped BiFeO 3 are listed in Table I.After comparison, we found that the BCFMO film displays the better resistive switching behavior by taking electric properties into account comprehensively.Therefore, in this work, we report the reversible switching behavior and the memory effect of Bi 0.85 Ce 0.15 Fe 0.95 Mn 0.05 O 3 thin films in detail.Fig. 1(a) shows the typical XRD patterns of the BFO and BCFMO films.All the peaks are indexed according to the standard powder diffraction date of BiFeO 3 complied in the JCPDS card which consisted of hexagonal structure (JCPDS no.20-0169).As Ce and Mn elements doped to BFO thin films, the diffraction peaks of (111), ( 200) and (002), (431) appear for CeO 2 and Mn-(BiFe)O 3-x phase, respectively.These results indicate that the Ce and Mn elements begin to crystallize in BCFMO thin film and demonstrate that the structure of BFO is changed.In addition, as shown in the inset, a peak around 2θ = 46 • in the pattern of the BCFMO films shift and was split into three peaks, indexed by tetragonal symmetry, which results clearly indicated that a phase transition from the distorted hexagonal structure for the pure BFO film to tetragonal structure for the BCFMO films was taken place. 29As indicated in Fig. 1(b), the thermal decomposition process and the DSC curve where phonon mode assignments are given in square brackets. 30,31 ompared to the pure BFO counterparts, Raman peaks of the BCFMO film located at 79 cm  32,33 And, new bands which appear at 272 cm −1 , 282 cm −1 and 425 cm −1 assign to the interior phonon modes of CeO 2 and Mn-BiFeO 3−x , which show that the bending vibration of Bi-O and Fe-O were largely affected by the substitution of Ce and Mn. 34The EDS spectra acquired from BFO and BCFMO thin films on Pt/Ti/SiO 2 /Si substrates were examined in Figs.2(b) and 2(c), respectively.It can be seen clearly that the main element contents changes among Bi, Fe, O, Pt and Si.Moreover, the strong peaks of Ce and Mn elements in EDS spectra were observed with Ce and Mn codoping into BiFeO 3 thin film shown in Fig. 2(c).The SEM images of BFO and BCFMO thin films in the insets of Figs.2(b) and 2(c) showed that the smoothness and surface quality improved with Ce and Mn codoping into BiFeO 3 thin film.
As depicted in Fig. 3 the thickness is about 210 nm for the BCFMO film on Pt/Ti/SiO 2 /Si substrate, which was measured by SEM through scanning cross section.The AFM image of BCFMO/Pt/Ti/SiO 2 /Si structure is shown in the inset, which could demonstrate the BCFMO thin film have a relative smooth surface and a small crystalline size.As shown in Fig. 4(a), the BRS behavior is observed in the Pt/BCFMO/Pt structure device.The current values were measured while the voltage is applied from −5 V to +5 V and back to −5 V under the sweeping sequence of 1→2→3→4.The current compliance is fixed at 0.001 A to prevent permanent dielectric breakdown.The current hysteresis indicates that the device has bistable resistance states: switching from HRS to LRS (set process) and the reverse switching (reset process), a set process occurs at about −1.5 V (V set ). Afterward, by sweeping the voltage to a positive value, the device current is abruptly decreased at about 1.4 V (V reset ), thus, the device switches from the low resistance state (LRS) to the HRS (reset process) again.Subsequently, the Pt/BCFMO/Pt structure device exhibits reproducibly BRS behavior.The HRS and LRS are still stable after 60 testing cycles at room temperature, shown in Fig. 4(b).Furthermore, the electroforming process was not needed in the initial state in the Pt/BCFMO/Pt structure device.This property is beneficial to realize high-density RRAM device applications. 2As shown in Fig. 4(c), when applying a positive sweep voltage, the set process occurs at about 4.9 V, and the reset process takes place at about 1.3 V with the same polarity bias, which are lower than the present reports. 22,24 oreover, the reset current is larger than that of the BRS.After several set-reset processes of URS, the resistance evolution of HRS and LRS is also demonstrated in 60 switching cycles with the same reading voltage of 1V (Fig. 4(d)).In order to clarify leakage current mechanism of RS behaviors, the relation of current versus sweeping voltage was presented in a logarithmic plot, and the linear fitting results were shown in Fig. 5.The I-V characteristics of LRS in both URS and BRS exhibit a linearly Ohmic behavior with a slope of 1.This suggests that our devices belong to conductive filaments formation/rupture model.On the other hand, the fitting results of HRS suggest that the current density shows nonlinear dependence on electric field.Generally, the traditional nonlinear leakage mechanisms are classified into two categories: bulk-limited conduction and interface-limited conduction.The bulk-limited mechanisms include the Ohmic conduction, Poole-Frenkel emission, and the SCLC (space chargelimited current).For the SCLC mechanism, the current density arises due to the excess space charges which are injected into the conduction band. 35The current density is governed by the Child's law: 19,36 Where ε 0 is the permittivity of free space, ε r is the dielectric constant of the insulator, μ is the mobility of the charge carriers, V is the applied voltage, d is the film thickness.The interface-limited mechanism includes Schottky emission which arises from a difference in Fermi levels between a metal and an insulator or a semiconductor.The Schottky emission can be expressed as: Where A is the Richardson constant, is the height of the Schottky barrier, ε 0 is the permittivity of free space, K is the dielectric constant of the film, K B is the Boltzmann constant, V is the applied voltage, and d is the sample thickness.The BRS behaviors of Pt/BCFMO/Pt device are changed by varying the voltage polarity (positive and negative), the analysis of I-V curve fitting should be carried out under different bias voltage (positive and negative voltage), as shown in Figs.5(a) and 5(b).The fitting lines of negative voltage exhibit three regions with slope values at different magnitudes of applied field.At the lower voltage region, I-V curve of the first region exhibits linear behavior with a slope (∼1) and the leakage current behavior is governed by Ohmic law.At the second region, a narrow trap free region with a slope (∼2) is observed.As the high applied voltage increase, the leakage is observed to be dominated by SCLC with a slope (>2) in the third region.In the annealing progress, it is inevitable that some defects such as metallic defects, oxygen vacancies exist in the polycrystalline thin films, which can induce the valence change from Fe 2+ ions to Fe 3+ ions, leading to the disruption of filaments. 2,36,37 Wen a negative voltage applied to the top electrode, the oxygen vacancies migrate to the top electrode and pile up on the top electrode, and enough oxygen vacancies accumulated will make the Schottky barrier at the top interface eliminated; the top interface between the top electrode and film became an Ohmic contact after the action of the negative voltage, and the oxygen vacancies filaments grow toward bottom electrode, at last the device was switched from HRS to LRS.A positive voltage sweep applied to the top electrode reject positive charged vacancies in the films to move from the top electrode toward the bottom electrode; high-density oxygen vacancies migrate away the top electrode under positive voltage and make the Schottky barrier wider, the device was reversed back to HRS.Since the localized recovery of filament can be determined by the migration of oxygen vacancies, which is dependent on the distribution of electric field, oxygen vacancies concentration and temperature, the created conducting filament would be different in each RS cycle. 24According to Eq. ( 1) and ( 2), the results show that the conduction mechanisms are dominated by the space charge limited leakage current and the Schottky emission.For URS with the Pt top electrode, conductive filaments formation/rupture controlled by thermal effects mechanism and oxygen vacancies concentration are proposed to explain the URS behavior, which was reported in our previous work. 13,24,38,39 Threfore, for either URS or BRS behavior in the BCFMO thin films, there consists of three portions: the Ohmic region, the Child's law region, and the steep current increase region.These results can be well explained by space charge limited conduction (SCLC) mechanism.
To further study the conduction mechanism, the temperature and cell area dependences of both LRS and HRS are shown in Fig. 6, the on-state's current increased with the rise in temperature, indicating that that the filamentary path is not a metallic filament.A weak dependence of resistance in LRS on the geometrical size of the cell, as shown in Fig. 6(b), shows that the current is distributed although the device as locally confined filament paths rather than as a homogeneously distributed.To understand the intrinsic mechanism and the relationship between ferroelectric properties and RS behaviors of multiferroic BFO-based films, the ferroelectric performance of BiFeO 3 , Bi 0.95 Ce 0.05 FeO 3 , Bi 0.85 Ce 0.15 FeO 3 and Bi 0.85 Ce 0.15 Fe 0.95 Mn 0.05 O 3 thin films were investigated as shown in Fig. 7.At an applied voltage of 12 V, P-E hysteresis loops with remanent polarization (P r ) 24.98 μC/cm 2 and saturated polarization (P s ) 31.96 μC/cm 2 for pure BiFeO 3 thin film, P r ∼ 28.9 μC/cm 2 and P s ∼ 36.1 μC/cm 2 for Bi 0.95 Ce 0.05 FeO 3 thin film, P r ∼ 28.9 μC/cm 2  Bi 0.85 Ce 0.15 FeO 3 thin film, P r ∼ 12.6 μC/cm 2 and P s ∼ 15.0 μC/cm 2 for Bi 0.85 Ce 0.15 Fe 0.95 Mn 0.05 O 3 thin film were observed, respectively.From Fig. 7, it can be conjectured that Ce doping in small quantity could be effective to improve ferroelectric properties of BFO-based films, which is consistent with that reported by Liu et al. 29 However, the ferroelectric performance of the vast Ce doped or Ce and Mn co-doped BFO thin films became bad judged from E c and P r .The measured results could be understood as follows: as Ce doping in small quantity in BFO-based films, some oxygen vacancies are suppressed and thereby a reduced leakage current and the ferroelectric properties are primarily caused. 40However, when a vast doped Ce or co-doped Ce and Mn were observed in BFO films, the local and migratory oxygen vacancies and defects were enhanced and thereby their pileup near metal electrodes were demonstrated, which could lead to the increscent leakage current and the improved RS behaviors. 22,41 lthough the mechanisms of resistive switching in BFO-based thin films are interesting and still controversial, more and more investigations suggested that the filamentary conduction mechanism could explain the resistive switching behavior associated with the presence of oxygen vacancies.Moreover, the resistive switching behaviors and ferroelectric properties might be transformed in the doping BFO-based thin films due to the local change in oxygen vacancy concentration and defects. 22,42,43 Iorder to further analyze the memory performance, the retention performances of the memory cells of BFO and BCFMO thin films at 85 • C are displayed in Fig. 8, one can see that the retention time of 10 5 s has been demonstrated for the HRS and LRS without obvious degradation, while the information storages in this device cells are likely to persist for an even longer time judging from the present trend of data.According to the stable RS performance, low operation voltage and the improved R OFF /R ON ratio by comparison, the BCFMO thin films are appropriate as the switching layer for RRAM devices.

IV. CONCLUSION
In summary, the BCFMO thin films are grown on Pt/Ti/SiO 2 /Si substrate.With Ce and Mn codoping into BiFeO 3 thin films, the Pt/BCFMO/Pt device exhibits better resistive switching behaviors in terms of the improved R OFF /R ON ratio of more than 80, good retention characteristics (>10 5 s) and low operation voltage (<1.5 V) owing to the mobility of local oxygen vacancies.The dominant conduction mechanisms of LRS and HRS currents are Ohmic, SCLC and Schottky emission behavior, respectively.Due to the electrical characteristics of RRAM device based on BCFMO thin films, we suggest that this device could be a promising candidate for the next generation nonvolatile memory applications.

FIG. 3 .
FIG. 3. The typical cross section of BCFMO thin film on Pt/Ti/SiO 2 /Si substrate, and the AFM image of BCFMO thin film in the inset.

FIG. 6 .
FIG. 6.The temperature (a) and cell area (b) dependences of both LRS and HRS.

TABLE I .
The performance of resistive switching and leakage current of BFO-based thin films.