Time-Dependent Impedance Switching in HfO2 Memcapacitors

International audienceResistive Random-Access Memories (ReRAMs, or memristors), are attracting industry attention as a new technology to replace Flash nonvolatile memories. ReRAMs make use of resistance switching in metal-insulator-metal (MIM) structures which are subjected to voltage biasing. Up to now, attention was focused on dc resistance transition. Less attention has been paid to the impedance variation, i.e. to the influence of voltage biasing on ac conductance and capacitance in ReRAM devices.In a recent work on HfO2 ReRAMs [1], we demonstrated that the capacitance, and more generally the imaginary part of RRAM devices, can also be switched by external bias. This paves the way for the realization of capacitive switching memories, or memcapacitors. Besides digital memory applications, memcapacitors could also be used in a broad range of electronic applications, such as reconfigurable RF circuits. From the fundamental point of view, impedance studies can provide information about conduction mechanisms and related switching phenomena.This work aims to better understand the mechanisms of ac conductance and capacitance variation in ReRAM devices. Studied devices are HfO2 MIM structures elaborated by atomic layer deposition. Devices are subjected to a constant dc voltage bias, and the impedance (real and imaginary parts) is recorded as a function of time. This allows us to follow kinetics of impedance switching, i.e. of both the ac conductance and the capacitance transition. Measurements are carried out at different frequencies and at different temperatures. The frequency and temperature dependence of the impedance in the high and low resistive states is discussed along ac conduction and polarization models implying intrinsic and bias-induced defects. Study of the capacitance switching time, as a function of bias, brings further information about the generation rate of defects which control impedance transition