With the extensive integration of portable computers and smartphones with “Internet of Things” technology, further miniaturization, high reading/writing speed and big storage capacity are required for the new-generation non-volatile memory devices. Compared with traditional charge memory and magnetoresistive memory, resistive random access memory (RRAM) based on transition metal oxides is one of the promising candidates due to its low power consumption, small footprint, high stack ability, fast switching speed and multi-level storage capacity.

Inspired by the excellent resistive switching characteristics of NiO and HfO₂, NiO_x films are deposited by magnetron sputtering on the Pt\langle111\rangle layer and the polycrystalline HfO₂ film, respectively. Their microstructures, resistive switching characteristics and conductive mechanisms are studied. X-ray diffractometer data show the \langle111\rangle preferred orientation for the NiO_x film deposited on the Pt\langle111\rangle layer but the \langle100\rangle preferred one for the film deposited on the polycrystalline HfO₂ layer. X-ray photoelectron depth profile of Ni 2p core level reveals that the NiO_x film is the mixture of oxygen-deficient NiO and Ni₂O₃. NiO_x(111) films show bipolar resistive switching (RS) characteristics with a clockwise current-voltage (I-V) loop, but its ratio of the high resistance to the low resistance (R_H/R_L) is only ~10, and its endurance is also poor. The NiO_x(200)/HfO₂ stack exhibits bipolar RS characteristics with a counterclockwise I-V loop. The R_H/R_L is greater than 10⁴, the endurance is about 10⁴ cycles, and the retention time exceeds 10⁴ s. In the initial stage, the HfO₂/NiO_x(200)/HfO₂ stack shows similar bi-level RS characteristics to the NiO_x(200)/HfO₂ stack. However, in the middle and the last stages, its I-V curves gradually evolve into tri-level RS characteristics with a “two-step Setting process” in the positive voltage region, showing potential applications in multilevel nonvolatile memory devices and brain-like neural synapses. Its I-V curves in the high and the low resistance state follow the relationship of ohmic conduction ( I \propto V ), while the I-V curves in the intermediate resistance state are dominated by the space-charge-limited-current mechanism ( I \propto V^2 ). The tri-level RS phenomena are attributed to the coexistence of the oxygen-vacancy conductive filaments in the NiO_x(200) film and the space charge limited current in the upper HfO₂ film.