Chuangui Wu

Key concepts behind forming-free resistive switching incorporated with rectifying transport properties

This work reports the effect of Ti diffusion on the bipolar resistive switching in Au/BiFeO3/Pt/Ti capacitor-like structures. Polycrystalline BiFeO3 thin films are deposited by pulsed laser deposition at different temperatures on Pt/Ti/SiO2/Si substrates. From the energy filtered transmission electron microscopy and Rutherford backscattering spectrometry it is observed that Ti diffusion occurs if the deposition temperature is above 600°C. The current-voltage (I–V) curves indicate that resistive switching can only be achieved in Au/BiFeO3/Pt/Ti capacitor-like structures where this Ti diffusion occurs. The effect of Ti diffusion is confirmed by the BiFeO3 thin films deposited on Pt/sapphire and Pt/Ti/sapphire substrates. The resistive switching needs no electroforming process, and is incorporated with rectifying properties which is potentially useful to suppress the sneak current in a crossbar architecture. Those specific features open a promising alternative concept for nonvolatile memory devices as well as for other memristive devices like synapses in neuromorphic circuits.

Control of Rectifying and Resistive Switching Behavior in BiFeO3 Thin Films

BiFeO3 thin films have been grown on Pt/Ti/SiO2/Si substrates with pulsed laser deposition using Au as the top electrode. The resistive switching property of the Au/BiFeO3/Pt stack has been significantly improved by carefully tuning the oxygen pressure during the growth, and a large switching ratio of ~4500 has been achieved. The deposition pressure modifies the concentration of oxygen vacancies and the rectifying behavior of the Au/BiFeO3 junction, and consequently influences the resistive switching behavior of the whole stack. The switching takes place homogeneously over the entire electrode, and shows a long-term retention.

Nonvolatile Multilevel Resistive Switching in

Low-energy Ar+ ion irradiation has been applied to an Au/BiFeO3/Pt capacitor structure before deposition of the Au top electrode. The irradiated thin film exhibits multilevel resistive switching (RS) without detrimental resistance degradation, which makes the intermediate resistance states more distinguishable, as compared with the nonirradiated thin film. The stabilization of resistance states after irradiation is discussed based on the analysis of the conduction mechanism during the RS, which was investigated by means of temperature-dependent current-voltage measurement from room temperature to 423 K.

\hbox{Ar}^{+}

BiFeO3 thin films have been deposited on Pt/sapphire and Pt/Ti/SiO2/Si substrates with pulsed laser deposition using the same growth conditions. Au was sputtered as the top electrode. The microscopic structure of the thin film varies by changing the underlying substrate. Thin films on Pt/sapphire are not resistively switchable due to the formation of Schottky contacts at both the top and the bottom interfaces. However, thin films on Pt/Ti/SiO2/Si exhibit an obvious resistive switching behavior under forward bias. The conduction mechanisms in BiFeO3 thin films on Pt/sapphire and Pt/Ti/SiO2/Si substrates are discussed to understand the different resistive switching behaviors.

Irradiated

In this letter, we report the resistive switching properties of ion-exfoliated LiNbO3 thin films. After annealing in Ar or in vacuum, electro-forming has been observed on the thin films, and the oxygen gas bubbles can be eliminated by tuning the annealing conditions in order to prevent the destruction of top electrodes. The thin films show rectifying filamentary resistive switching after forming, which is interpreted by a simplified model that the local filament does not penetrate throughout the LiNbO3 thin film, resulting in asymmetric contact barriers at the two interfaces. The well controlled electro-forming step and the highly reproducible switching properties are attributed to the more homogeneous distribution of defects in single crystalline materials and the specific geometry of filament.

\hbox{BiFeO}_{3}

Dilute ferromagnetic semiconductors such as III-Mn-V have lately attracted amount of attention, due to the considerable potential for spintronic device [1]. From the application point of view, the current-induced magnetization reversal originating from a spin transfer torque (STT) raises a special need for preparing DMS materials with a large perpendicular magnetic anisotropy (PMA) [2, 3]. The PMA in GaMnAs has been realized by two approaches: The mostly used one is to tune the strain from compressive to tensile state in GaMnAs either by applying InGaAs as buffer [2, 3]. Different from GaMnAs, homoepitaxial InMnAs on InAs is subject to tensile strain, therefore, should exhibit PMA. Moreover, ferromagnetic InMnAs films in principle provide a more suitable test-bed for STT phenomena in magnetic semiconductors due to its larger spin-orbit coupling [4]. We present the preparation of homoepitaxial InMnAs on InAs by ion implantation and pulsed laser melting. We show that a Curie temperature of 77 K (higher than reported up to now) is achieved in In1-xMnxAs with PMA at a Mn concentration of only x=0.08. The magnetic anisotropy in InMnAs is confirmed by magnetization and x-ray magnetic circular dichroism (XMCD) measurements. The pronounced PMA in this system is attributed to the tensile strain arising from the Mn ions incorporation.