Tri-level resistive switching characteristics and conductive mechanism of HfO<sub>2</sub>/NiO<sub>x</sub>/HfO<sub>2</sub>

Abstract

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 of 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.<br>Inspired by the excellent resistive switching characteristics of NiO and HfO<sub>2</sub>, NiO<sub>x</sub> films were deposited by magnetron sputtering on the Pt<111> layer and the polycrystalline HfO<sub>2</sub> film, respectively. Their microstructures, resistive switching characteristics and conductive mechanisms were studied. X-ray diffractometer data shows the <111> preferred orientation for the NiO<sub>x</sub> film deposited on the Pt<111> layer but the <100> preferred one for the film deposited on the polycrystalline HfO<sub>2</sub> layer. X-ray photoelectron depth profile of Ni2p core level reveals that the NiO<sub>x</sub> film is the mixture of oxygen-deficient NiO and Ni<sub>2</sub>O<sub>3</sub>. NiO<sub>x</sub>(111) films show bipolar resistive switching(RS) characteristics with a clockwise current-voltage(<i>I-V</i>) loop, but its ratio of the high resistance to the low resistance(<i>R</i><sub>H</sub>/<i>R</i><sub>L</sub>) is only~10, and its endurance is also poor. The NiO<sub>x</sub>(200)/HfO<sub>2</sub> stack exhibits bipolar RS characteristics with a counterclockwise <i>I-V</i> loop. The <i>R</i><sub>H</sub>/<i>R</i><sub>L</sub> is greater than 10<sup>4</sup>, the endurance is about 10<sup>4</sup> cycles, and the retention time exceeds 10<sup>4</sup> s. In the initial stage, the HfO<sub>2</sub>/NiO<sub>x</sub>(200)/HfO<sub>2</sub> stack shows similar bi-level RS characteristics with that of the NiO<sub>x</sub>(200)/HfO<sub>2</sub> stack. However, in the middle and the last stages, it's <i>I-V</i> curves gradually evolve into tri-level RS characteristics with a "two-step Set process" in the positive voltage region, showing potential applications in multilevel nonvolatile memory devices and brain-like neural synapses. Its <i>I-V</i> curves in the high and the low resistance states follow the relationship of Ohmic conduction (<i>I</i>∝<i>V</i>), while the <i>I-V</i> curves in the intermediate resistance state are dominated by the space-charge-limited-current mechanism (<i>I</i>∝<i>V</i><sup>2</sup>). The tri-level RS phenomena are attributed to the coexistence of the oxygen-vacancy conductive filaments in the NiO<sub>x</sub>(200) film and the space charge limited current in the upper HfO<sub>2</sub> film.