Characterization of HfO 2 -based devices with indication of second order memristor effects

Resistive switching is investigated in TiN/Ti/HfO2(10 nm)/TiN devices in series with a NMOS transistor as selector in a 1T1R configuration. A complete electrical characterization of the devices is carried out using DC voltage loops, constant-voltage stressed and pulses with varying voltage amplitude and time width. Good control of the ON resistance is achieved by applying different transistor gate voltages and multiple OFF states are controllably reached by varying the maximum reset voltage. The on/off resistance ratio is successfully controlled by changing the pulse amplitude and time duration. However, significant memory effects are reported, showing that the switching of a device depends on its switching history. Exploring the on/off ratio for different set and reset voltage amplitudes in otherwise identical set/reset pulse experiments is shown to depend on the order of application of the different stress conditions so that the on/off resistance map in the VSET/VRESET space is not unique. We interpret these results as an evidence of second-order memristive effects.