Impedance Spectroscopy of Hafnium Oxide: Memristive and Memcapacitive Switching With Annealing

In this study, we report binary resistive and capacitive switching in hafnium oxide (HfOX) memristors. Given that annealed devices are widely recognized in the literature for exhibiting improved switching properties, we compared the behavior of as-deposited and annealed samples. Both devices exhibit pinched hysteresis with bipolar switching; however, annealed devices show significantly enhanced ON/OFF ratios for both resistive (1:100) and capacitive (1:3) states. The enhanced performance is attributed to post-deposition annealing at 400 °C, which induces a monoclinic phase and hence results in a film that exhibits a mixed amorphous and crystalline phase. To investigate the dynamic, frequency-dependent characteristics of these devices, we performed impedance spectroscopy (IS) analysis to get a Bode and Nyquist plots. The Bode plot indicates that these devices function as tuneable low-pass filters, with as-deposited devices providing a bandwidth of 0.82 MHz and annealed devices exhibiting a significantly expanded bandwidth of 9.43 MHz. The Nyquist plots reveal the presence of a constant phase element (CPE) in both devices, with a stronger presence in annealed devices. The modeled parameters align well with experimental data within acceptable error margins. In addition, LTspice simulations verify the model and the presence of nonideal parameters. Our comprehensive study underscores the potential of HfOX memristors as reliable and tuneable memcapacitors, exhibiting improved performance for annealed devices.