Unipolar resistive switching behaviors of Ru∕HfOx∕TiN devices with Ru as anode were investigated. Wide dispersion of switching operation parameters was observed. The conduction mechanisms in low and high resistance states of the devices were characterized to be Ohmic-like and tunneling, respectively. The band offset of the Ru∕HfOx interface was extracted from the measured tunneling current versus voltage characteristics. Instability of the band offset at the anodic interface was observed and may be responsible for the wide fluctuation of the operation voltage in the Ru∕HfOx∕TiN device at a high resistance state. The possible mechanism for these unstable characteristics of band offset at the Ru∕HfOx interface is also discussed.

The degradation behavior of the Ti/HfO x bipolar resistive random access memory (RRAM) during endurance cycles, and the operational parameters, which induce the endurance failure, are studied through the two proposed stressing methods. The over-RESET energy is considered to be the key electrical parameter to induce endurance failure in the memory device. When the device suffers the over-RESET energy, a gradually reduced memory window is observed associated with endurance cycles, and the overall degradation will include two stages. The first stage can be explained by the worn filament model and is mainly due to imbalance energy between SET and RESET process. The occurrence of unusual resistance-voltage (R-V) patterns at positive and negative voltage seep in the memory device under the second stage degradation demonstrates the existence of complementary resistive switching (CRS) in the single Ti/HfO x bipolar RRAM. After analyzing the operation conditions to activate the self-CRS in memory device with one transistor-one resistor (1T-1R) configuration, the mechanism about the second stage degradation in the RRAM originated from over-RESET energy is also discussed. A mechanism based on the worn filament model and the induction of CRS is proposed to explain the endurance failure induced by over-RESET in the Ti/HfO x RRAM with 1T-1R configuration. With an appropriate RESET energy, a robust reliability for endurance cycles is expected.

The advantage of a nonlinear bidirectional selector is its high access ratio with multiple transport mechanisms in a wide bias range. The multilayer structure of TiN/Al 2 O 3 /TiO 2 /Al 2 O 3 /TiN with a nonlinear varistor characteristic is proposed. The features of the structure include its low-temperature process (>250 °C) for stackable applications, non-requirement of rare materials such as Pt, and its high access ratio (>3200). The varistor characteristic of the nonlinear selector is discussed and attributed to carrier tunneling and thermionic emission for small biases (OFF state) and large biases (ON state), respectively. The selector shows scaling feasibility owing to its property of inertness to SiO 2 encapsulation and is compatible with the current IC and memory industry.