Helium Ion Beam Irradiation of MoS2: Fabrication of 2D Memristors

Abstract

A memristor is an electronic device which exhibits a non-volatile and reversible resistance change in response to an applied electric field. These properties make the memristor a potential prime component in the field of non-volatile resistive memory (RRAM) as well as in bioinspired neuromorphic computing systems. Two-dimensional (2D) layered materials such as molybdenum disulfide (MoS2) offer some distinct advantages over traditional metal oxide-based RRAM components such as enhanced electrostatic control of the 2D channel, mechanical flexibility and optical transparency. However, challenges remain in the controllable sub-micron fabrication of memristive circuit components from these novel materials. This work demonstrates a novel method of fabricating on-chip lateral memristors from single crystal, monolayer MoS2 by irradiation with a finely focused helium ion probe (< 3 nm). Site-specific irradiation in the helium ion microscope (HIM) creates a population of sulfur vacancies in the MoS2 lattice. These act as mobile dopants which reversibly migrate under an applied electric field, modulating the resistance of the semiconducting channel, enabling versatile memristive functionality on the nanoscale. Electrical characterisation of the devices show stable, repeatable bipolar switching between high and low resistance states for hundreds of cycles and with retention time of > 12 h. Furthermore, owing to the 2D nature of these devices, the current levels, set voltage and on/off ratio can be further tuned by the application of a back-gate which is not possible for higher dimensional memristive systems. Neuromorphic functionalities of the devices such as pulsed potentiation and depression and heterosynaptic plasticity are also demonstrated. Finally, the evolution of conductivity of MoS2 under accumulating localised helium ion irradiation was investigated. Electrical characterisation of MoS2 devices was performed in-situ in the HIM chamber. The devices exhibit the emergence of dose-dependent hysteresis in the current-voltage (I-V) curves. The ion irradiation introduces defects to the material which act as charge traps. The density of traps increases approximately linearly with delivered dose before saturating above 2.5 pC &#956;m^(-1). This work enables future down-scaling of 2D memristive devices and furthers our understanding of the modification of 2D materials with the helium ion beam.