In this paper, the resistive switching characteristics in a Cu/HfO(2):Cu/Pt sandwiched structure is investigated for multilevel non-volatile memory applications. The device shows excellent resistive switching performance, including good endurance, long retention time, fast operation speed and a large storage window (R(OFF)/R(ON)>10(7)). Based on the temperature-dependent test results, the formation of Cu conducting filaments is believed to be the reason for the resistance switching from the OFF state to the ON state. By integrating the resistive switching mechanism study and the device fabrication, different resistance values are achieved using different compliance currents in the program process. These resistance values can be easily distinguished in a large temperature range, and can be maintained over 10 years by extrapolating retention data at room temperature. The integrated experiment and mechanism studies set up the foundation for the development of high-performance multilevel RRAM.

A high-κ based charge trap flash (CTF) memory structure using bandgap engineered trapping layer HfO2/Al2O3/HfO2 (HAH) has been demonstrated for multilevel cell applications. Compared to a single HfO2 trapping layer, a CTF memory device based on the HAH trapping layer exhibits a larger memory window of 9.2 V, faster program/erase speed, and significantly improved data retention. Enhancements of memory performance and reliability are attributed to the modulation of charge distribution by bandgap engineering in trapping layer. The findings provide a guide for future design of CTF.

We report the direct electrical measurement of multiple resistance steps in the ZrO2-based solid electrolyte nonvolatile memory device using the refined dc I-V method with a very small voltage increasing rate. The results demonstrate that multiple conductive filaments are formed successively between the bottom and top metal electrodes through the insulating layer while increasing the bias voltage, which are consistent with the electrical field simulation results based on the solid electrolyte theory. The inverse relationship between resistance steps and the filament formation sequence are obtained, which helps understand the switching mechanism of the multiple conductive filaments.