Fully "Erase-free" Multi-Bit Operation in HfO2-Based Resistive Switching Device.

Fully "Erase-free" multi-bit operation was demonstrated in a W/HfO2/TiN-stacked resistive switching device. The term Erase-free means that a digital state in a multi-bit operation can be achieved without initializing the device resistance state when the device moves to another digital state. Because initializing the resistance state of a resistive switching device causes high energy consumption, omitting this sequence can achieve energy efficient multi-bit operation during rewriting of the resistance state of the device. Experimentally, an operational energy savings of up to 75% was confirmed. For stable and reliable Erase-free operation, several prerequisites are required, such as gradual resistance change with electric pulse stimuli during both writing and erasing, predictable operational voltages for certain resistance states, and high reliability of resistive switching. These prerequisites could be achieved by adopting a W top electrode in a W/HfO2/TiN-stacked resistive switching device. These results can pave the way to future nonvolatile memory applications.