X. Wu, R. Ge, M. Kim, D. Akinwande, J.C. Lee
The University of Texas at Austin,
Keywords: non-volatile memory, two-dimensional materials, flexible electronics, rf switch
Summary:In the recent years, tremendous efforts have been made to develop the next-generation electron devices, optoelectronics, flexible electronics and bioelectronics using two-dimensional (2D) materials. As one of the focused areas, non-volatile memory has drawn much attention in both academia and industry to meet the continuously increasing demand of data storage. Researchers have reported that various 2D materials, including graphene oxide, solution-processed transition metal dichalcogenides (TMDs), degraded black phosphorus, and multilayer hexagonal boron nitride (h-BN) can exhibit non-volatile resistance switching (NVRS) phenomenon, in which the resistance can be reversibly switched between a high-resistance state (HRS) and a low resistance state (LRS) via an external electrical bias and maintained without power supply. Monolayer 2D materials were not believed to have NVRS behavior due to the excessive leakage current. The monolayer MoS2, a representative TMD material, was reported to exhibit the NVRS effect in lateral device structure with the assistance of grain boundaries in the film. However, for practical integration and 3D stacking capability, the vertical metal-insulator-metal (MIM) devices are preferred. In our work, we first discovered the NVRS phenomenon in a collection of TMDs (MoS2, MoSe2, WS2 and WSe2) and h-BN atomic sheets based on the desired vertical device configuration. These devices can be collectively labelled as “atomristor”, which means the memristor effect in atomically thin nanomaterials. Crossbar devices and litho- and transfer-free devices are fabricated, showing forming-free characteristics and the coexistence of both bipolar and unipolar switching. The atomristors have the advantages of low switching voltages (down to < 1V) and fast switching speed via pulse operation (< 15ns), making it a promising low-power switch. In addition, a large on/off current ratio (up to 6 orders of magnitude) can be achieved in these devices, which enables potential applications in multi-bit storage and neuromorphic computing. Ab-initio simulation was conducted to reveal favorable substitution of metal ions into localized vacancies of the 2D layers, which corresponds to the switching from HRS to LRS, indicating a conductive-bridge-like switching mechanism. Possible application of atomristors in flexible non-volatile memory devices was demonstrated on polyimide substrate, revealing that the resistance states can afford mechanical bending cycles with reproducible switching properties. Benefit from the atomically thin active layer of atomristors, a novel application, radio frequency (RF) switch, was realized using MoS2 and h-BN. The atomristor-based RF switches showed great performance with low insertion loss (less than 0.2dB) and high isolation (more than 15dB) up to 110 GHz. Compared with other emerging switch technology based on MEMS, memristor and phase-change memory, the monolayer MoS2 switch achieved the best cutoff frequency of 70 THz. Considering the wide range of operating frequency over RF, 5G and mm-wave bands, the switch using monolayer 2D materials can be a good candidate for diverse communication and connectivity systems. Our work on 2D NVRS behavior indicates a potential universal resistance switching effect in 2D non-conductive monolayers, illustrates the fruitful interactions between defects and metal ions at the interfaces, and advances emerging applications on ultrathin flexible memory, zero-power RF switch and neuromorphic computing system.