Contrasting analog and digital resistive switching memory characteristics in solution-processed copper(i) thiocyanate and its polymer electrolyte-based memristive devices

Abstract

Usually, resistive switching (RS) devices show digital RS memory (sharp SET and RESET process), which is most suitable for digital data storage applications. Some RS devices also manifest ideal memristive behavior or analog memory characteristics (gradual change in resistance states). The analog RS properties of memristive devices widen their application domain to a much broader field of neuromorphic computing. The tunability of memristive devices to digital or analog memory applications greatly depends upon the switching medium. In this work, we report a comparative study on RS properties of two kinds of memristive devices based upon copper(I) thiocyanate (CuSCN) and a solid polymer electrolyte (SPE) made up of CuSCN as ionic moieties in polyethylene oxide (PEO). The device (ITO/CuSCN/Cu), prepared by spin-coating a CuSCN layer between ITO and the copper electrode, shows simultaneous analog and digital RS characteristics. The RS property of the device is tunable by varying the thickness of the CuSCN layer. The current–voltage characteristics reveal that devices prepared at 3000 rpm (∼168 nm thick) during the spin-coating show only digital bipolar RS memory. In comparison, the devices deposited at 4000 rpm (∼120 nm thick) show both analog and digital RS memory. The conduction mechanism responsible for RS behavior in the CuSCN-based devices is Schottky emission-mediated charge trapping and de-trapping at the interfacial states. Contrastingly, when the same CuSCN is used as the electrolyte in SPE film, the device only shows bipolar digital non-volatile memory characteristics. The RS behavior is due to the electrochemical metallization (ECM) mechanism. The ON and OFF states are achieved by the formation and rupture of copper filaments due to the redox reactions at the interface.