Graphene-Boron Nitride-Graphene Cross-Point Memristors With Three Stable Resistive States

Memristor is a novel two-terminal electronic memory device with a simple metal-insulator-metal(MIM)sandwiched structure.Due to its excellent properties like small feature size,easy fabrication,three-dimensional stackability,low power consumption,high endurance,and long retention time,it has attracted worldwide interests.Thanks to the unique optical and electric property of two-dimensional(2D)materials,2D material-based memristors have shown several properties that are not shown by traditional ones,such as high transparency,robust mechanical strength and flexibility,superb chemical stability,enhanced thermal heat dissipation,ultralow power consumption,coexistence of bipolar and threshold resistive switching,and ultrastable relaxation when used as electronic synapse(among others).However,several electrical performances often required in memristive applications,such as the generation of multiple stable resistive states for high-density information storage,still have never been demonstrated.Here,we present the first 2D material-based memristors that exhibit three stable and well-distinguishable resistive states.By using a multilayer hexagonal boron nitride(h-BN)stack sandwiched by multilayer graphene(G)electrodes,we fabricate 5 μm×5μm cross-point Au/Ti/G/h-BN/G/Au memristors that can switch between each two or three resistive states,depending on the current limitation(CL)and reset voltage used.The use of graphene electrodes plus a small cross-point structure are key elements to observe the tristate operation,which has not been observed in larger(100 μm×100μm)devices with an identical Au/Ti/G/h-BN/G/Au structure nor in similar small(5 μm×5μm)devices without graphene interfacial layers(i.e.,Au/Ti/h-BN/Au).Basically,we generate an intermediate state between the high resistive state and the low resistive state(LRS),named soft-LRS(S-LRS),which may be related to the formation of a narrower conductive nanofilament across the h-BN because of the ability of graphene to limit metal penetration(at low CLs).Three resistive states have been fitted well with Ohm law,hybrid quantum point contact mode and Poole-Frenkel mode,respectively.In addition,combined with the cross-section transmission electron microscope(XTEM)and conductive atomic force microscope(C-AFM)characterization of h-BN,plus the data fitting of those three stable resistive states,we further confirmed the resistive switching mechanism is dominated by defective state and supplemented by metal penetration and boron vacancy generation.All the 2D materials have been fabricated using the scalable chemical vapor deposition(CVD)approach,which is an immediate advantage compared to other works using mechanical exfoliated 2D materials by enabling a large scale device fabrication.Benefit from this,we can further confirm the device reliability and low device-to-device variability.The successful implementation of this experiment makes the multi-level 2D material-based memristor possible.