Impact of the Stacking Order of HfOx and AlOx Dielectric Films on RRAM Switching Mechanisms to Behave Digital Resistive Switching and Synaptic Characteristics

Resistive random access memory (RRAM) devices with analog resistive switching are expected to be beneficial for neuromorphic applications, and consecutive voltage sweeps or pulses can be applied to change the device conductance and behave synaptic characteristics. In this paper, RRAM devices with a reverse stacking order of 6-nm-thick HfO
_x
and 2-nm-thick AlO
_x
dielectric films were fabricated. The device with TiN/Ti/AlO
_x
/HfO
_x
/TiN stacked layers exhibited digital resistive switching, while the other device with TiN/Ti/HfO
_x
/AlO
_x
/TiN stacked layers could demonstrate synaptic characteristics that were analog set and reset processes under consecutive positive and negative voltage sweeps or a train of potentiation and depression pulses. Moreover, this device could also implement synaptic learning rules, spike-timing-dependent plasticity (STDP). Varying temperature measurements and linear fittings of the measured data were conducted to analyze current conduction mechanisms. As a result, the variation of resistive switching behavior between these two devices is attributed to the varying effectiveness of the oxygen scavenging ability of the Ti layer when put into contact with either AlO
_x
or HfO
_x
. Moreover, AlO
_x
functioned as a diffusion limiting layer (DLL) in the device with TiN/Ti/HfO
_x
/AlO
_x
/TiN stacked layers, and gradual modulation of the production and annihilation of oxygen vacancies is the cause of synaptic characteristics.