Coexistence of Nonvolatile Unipolar Memory and Volatile Threshold Resistance Switching In LaMnO 3 Thin Films

Coexistence of nonvolatile unipolar memory and volatile threshold resistive switching was observed in Pt/LaMnO 3 /Pt system. Specifically, nonvolatile unipolar memory was achieved by applying a negative bias, while volatile threshold resistive switching was obtained at a positive bias. The formation/rupture of conducting filaments and insulator-metal transition are supposed to induce nonvolatile unipolar memory and volatile threshold resistive switching, respectively. The convenient transition between nonvolatile and volatile switching by polarity is very useful for applications in in-memory computing technology.


Introduction
The resistance switching (RS) can be divided into non-volatile memory resistance switching and volatile threshold resistance switching [1,2]. In nonvolatile memory RS, both high and low resistance state can be retained under zero bias, which is useful in non-volatile memory devices. In contrast, in volatile threshold RS, only high resistance state (HRS) is stable under zero bias, which hinders its application in nonvolatile memory [2], but its frontier application is in neuromorphic devices [3,4], novel true random number generators [5] or voltage selectors in resistive random access memories [6,7]. In perspective of in-memory computing, the cross-point array memories are preferred for the size reduction of bit units, leading to a number of unwanted sneak-path currents [8,9]. This sneak current can be avoided by using a device with the coexistence of unipolar memory and threshold resistance switching.
LaMnO3 (LMO) compound as a Mott insulator and an A-type antiferromagnetic insulating material (with ferromagnetic spin correlations on a (001) crystal plane and antiferromagnetism between these crystal planes) [10] has been widely studied due to resistive switching characteristics by controlling oxygen content or electronic reconstruction [11,12]. The mutually transition between the nonvolatile unipolar memory switching and volatile threshold switching have been achieved by measurement temperature [13], oxygen content [1], compliance current [14], and electrical pulse polarity [15]. In our present work, we report on the transition between nonvolatile unipolar memory and volatile threshold RS in a simple LaMnO3 thin film at room temperature by dc voltage polarity. The nonvolatile unipolar memory 3 switching and the volatile threshold switching may originate from the opposite migration direction of oxygen vacancies under different voltage polarity. For many actual applications, endurance is a key parameter. TaOx [16] and VO2 [17] have endurance of over 10 10 cycles as nonvolatile and volatile memory, respectively, but only over 100 cycles when nonvolatile memory and volatile threshold switching coexist [18,19]. In the present work, LaMnO3 exhibits a stable threshold switching over 1000 cycles and unipolar memory switching over 500 cycles on the same device, and the convenient voltage polarity control mode are very useful for applications in in-memory computing technology.

Methods
Polycrystalline LaMnO3 films were deposited on Pt/Ti/TiO2/SiO2/Si substrate by pulsed laser deposition (PLD) using a KrF excimer laser (248 nm, 25 ns pulse duration, COMPexPro201, Coherent) at an energy of 300 mJ and frequency of 5 Hz, with the base vacuum of 2×10 −4 Pa. During deposition, the substrate temperature was kept at 750 ℃, and the target-substrate distance was 6.5 cm. The oxygen partial pressure was 5×10 −4 Pa, and the growth time was 20 min. After growth, the sample was annealed in situ for 20 min, and then, the temperature was reduced to room temperature at 10 ℃/min within a vacuum environment. In order to measure electrical properties, top electrode Pt (0.04 mm 2 ) were sputtered on LaMnO3 thin films through a shadow mask by DC sputtering at room temperature. Then, the sample was again annealed in PLD chamber, at 500 ℃ under 5×10 −4 Pa oxygen pressure for 30 min. The x-ray diffraction (XRD, DX-2700) was used to measure the structure of LMO film. 4 The atomic force microscope (AFM) and scanning electron microscope (SEM) were employed to characterize the surface morphology and cross-section image of film by Burke Multimode 8 and JSM-7001F instrument, respectively. Electrical properties were measured using Keithley 2400 source meter, the corresponding structure of our device is shown in the inset of Fig. 1a.

Results and Discussion
From XRD patterns shown in Fig. 1a the corresponding electric field is (2.43±0.29)×10 5 V/cm, which is comparable with previous reports [5,6]. Figure 2c shows the cumulative distributions of Vth and Vhold, the on-switching voltages (Vth) vary within the 0.50 V range, but the self-off-switching voltages (Vhold) are almost stable at 2.35 V, the variations of the Vth and Vhold is similar to that of VO2 [6]. The Vth and Vhold change with the threshold switching cycles in Fig. 2d, the threshold voltages decrease as the number of cycles increases, however, there is no such change in the self-off-switching voltages. That is, the Vhold is more stable than Vth in the threshold switching, which has also been observed in other Mott two-terminal devices, such as VO2 [6].
It is worth noting that the volatile threshold switching under a forward voltage is converted into nonvolatile unipolar memory switching by applying a reverse voltage. Figure 3a shows 500 unipolar switching cycles obtained on the same device, by sweeping the voltage from zero to a negative value, a low current is achieved, the current increases abruptly and it turns back to a low resistance (on state) at a certain 6 voltage (set voltage Vset), during this time, a compliance current was used to avoid the dielectric breakdown of the LaMnO3 film layer. While sweeping the negative voltage again, originally, a high current is measured, but the current suddenly drops to the initial value (off state) at a certain voltage (reset voltage Vreset), the reset voltage is much lower than set voltage. The statistical distribution and corresponding Lorentz fitting results of the Vset and Vreset are shown in the Fig. 3b, the set and reset voltages are -7.46 ± 3.26 V and -2.65 ± 1.85 V, respectively, the corresponding electric field (10 5 V/cm) is lower than that of memory resistance switching Ni/NiO core-shell nanowires (10 6 V/cm), which is helpful for reducing the energy consumption in memory devices [2]. Figure 3c shows the cumulative distributions of the Vset and Vreset, a slightly wider distribution of the Vset and Vreset than that in memory resistance switching Ni/NiO core-shell nanowires, which might be caused by more measurement cycles of Pt/LaMnO3/Pt device, while there is only about 25 cycles of Ni/NiO core-shell nanowires [2]. The resistance values of the HRS and LRS were on the order of 10 4 Ω and about 60 Ω, respectively, as shown in Fig. 3d, and the ROFF/RON ratio is close to 10 3 . By applying the forward voltage again, the LaMnO3 film recovers to reproducible threshold switching behaviors after setting unipolar memory switching to the HRS.
As stated above, the LaMnO3 film was fabricated and annealed under a low oxygen pressure (5×10 −4 Pa), so there are a large amount of oxygen vacancies, which has been confirmed by synchrotron-based X-ray diffraction (SXRD) and aberration-corrected scanning transmission electron microscopy (STEM) [11]. 7 Meanwhile, the rectification characteristics in the lower right inset in Fig. 1d suggests that the top Pt/LaMnO3 interface is responsible for the resistance switching. Due to the presence of oxygen vacancies, a pristine low resistance state has been obtained in our device. As it is known that the oxygen-deficient LMO is generally considered as an n-type semiconductor, because the oxygen vacancies act as donors in oxides and become positively charged after releasing electrons to the conduction band [11,20].
Furthermore, based on the rectification direction, as shown in the lower right inset of  Figure 4a shows the schematic band structure of the HRS of Pt/LMO/Pt after applying a forward bias. Under a positive bias, the oxygen vacancies drift away from the top electrode interface, which results in a wider and higher barrier at the top Pt/LMO interface, leading to a LRS to HRS transition [11]. Meanwhile, and the concentration of oxygen vacancies in bulk LaMnO3 films enhances accompanied with the oxygen vacancies drifting into the bulk. It is well known that in the stoichiometric LaMnO3, the oxidation state of Mn is +3 [21,22]. When there are a lot of oxygen vacancies in LaMnO3 film, charge disproportionation effect of the Mn 3+ →Mn 2+ possibly happens, and the concentration enhancement of oxygen vacancies will lead to an increase of Mn 2+ concentration [21]. Then the double exchange (DE) effect occurs in Mn 2+ -O-Mn 3+ in our oxygen-deficient LaMnO3 electron-doped films, just as in the case in Mn 3+ -O-Mn 4+ in the hole-doped manganites [21,23]. Thus the ferromagnetic 8 metal phase increases due to the enhancement of double exchange effect between the Mn 2+ and Mn 3+ , which is unevenly distributed in the charge ordered antiferromagnetic insulator phase of LaMnO3 film [23]. Furthermore, the resistivity of the ferromagnetic state is much larger than that of ordinary metals, which may be caused by electrons localization from magnetic disorder [24]. Accordingly, an intermediate resistance state appears during the initial positive reset transition in our Pt/LaMnO3/Pt device, as shown in Fig. 1d. In the present oxygen-deficient bulk LMO films, more ferromagnetic metal clusters phase coexist with antiferromagnetic insulating phase.
The intermediate resistance state is higher than the initial low resistance state, which is called bad metallic state [25]. When applying a positive bias, a percolation type transition between the insulating phase and bad metallic phase plays a major role, as shown in the schematic diagram of Fig. 4b, c, so the insulator-metal transition occurs, and the device shows the characteristics of threshold switching with a comparable threshold electric field of VO2 [23]. The schematic band structure of Pt/LMO/Pt at LRS after a negative bias is shown in Fig. 4d. Under a negative bias, the oxygen vacancies drift toward the top electrode interface, leading to a decrease in the height and width of the Schottky barrier and the device is set to the LRS of the unipolar memory switching. Simultaneously, the concentration of oxygen vacancies in bulk LaMnO3 films reduces, and the ratio of Mn 2+ /Mn 3+ decreases in the bulk LaMnO3 film, the concentration of ferromagnetic metal phase reduces due to the reduction of the double exchange effect between the Mn 2+ and Mn 3+ , so the formation and fracture of oxygen vacancies conducting filaments should be responsible for the unipolar 9 switching. The oxygen vacancies conducting channel between the top and bottom electrodes will be formed under negative bias, as can be seen from the Fig. 4e, f, when the negative bias is applied again, the oxygen vacancies conductive filaments is disconnected with the help of Joule heating, and the device switches from the low resistance state to the high resistance state.

Conclusions
In summary, coexistence of the nonvolatile unipolar memory and volatile threshold resistance switching was observed in LaMnO3 film, which is dependent on the bias polarity. The voltage polarity affects the migration direction/distribution of oxygen vacancies and the width/height of the top interface barrier, resulting in a transition between nonvolatile unipolar switching and volatile threshold switching.
Because of its excellent endurance, our device is expected to be used in in-memory computing technology and accelerate the development of post-Moore industry.

Availability of Data and Materials
All the data and materials are available by contacting the corresponding author.