Effects of electrode materials and bias polarities on breakdown behaviors of oxide dielectrics and their mechanisms

Abstract

<sec>The memristors and the energy storage capacitors have the same sandwich structure, but the operating voltages required by the two devices are significantly different. Therefore, in the same device, it is necessary to study the influencing factors of operating voltage and adjust the operating voltage of the devices to realize the applications of the device in diverse fields. The polycrystalline ZrO<sub>2</sub> and amorphous TaO<sub><i>x</i></sub> thin films are deposited on ITO conductive glass and Pt/Si substrates by reactive magnetron sputtering technology. Au, Ag and Al metal materials are selected as the top electrodes to construct a variety of metal/insulator/metal sandwich capacitors. The breakdown strengths of these devices under different bias polarities are studied. </sec><sec>The results demonstrate that the breakdown strength is slightly larger for the ZrO<sub>2</sub> based capacitor with ITO as the bottom electrode than for the Pt electrode device under negative bias. The breakdown electric field of the device with Ag as the top electrode shows obvious dependence on bias polarity, no matter whether the bottom electrode is ITO or Pt. The breakdown strength is reduced by more than an order of magnitude under a positive bias (2.13 MV/cm) compared with under a negative bias (0.17 MV/cm) of Ag/ZrO<sub>2</sub>/ITO device. The breakdown strength of the Al/TaO<sub><i>x</i></sub>/Pt device is enhanced under the forward bias (3.6 MV/cm), contrary to the Ag electrode device, which is nearly twice higher than the breakdown electric field under the negative bias (1.81 MV/cm). The different breakdown behaviors of the above devices can be explained by the migration and rearrangement of oxygen between the oxide electrode and the dielectric interface layer; the dissolution, migration and reduction of the electrochemically active metal electrode; and the redox reaction between the chemically active metal electrode and the oxide dielectric interface. </sec><sec>The ZrO<sub>2</sub> based capacitor with ITO electrode undergoes a redox reaction of Sn<sup>4+</sup> in the ITO under negative bias, forming an insulating layer at the interface between the dielectric layer and the ITO electrode, which contributes a larger breakdown electric field. In addition, the electrochemical metallization process happens to the Ag electrode device under positive bias, and the breakdown electric field is smaller than negative bias due to the large diffusion coefficient of Ag ions in the film, while breakdown is dominated by the defect characteristics of the dielectric film under negative bias. The Al/TaO<sub><i>x</i></sub>/Pt devices can form AlO<sub><i>x</i></sub> oxide layer under positive bias, spontaneously, which can inhibit the leakage current, and also act as a series resistance to disperse part of the voltage and enhance the breakdown voltage of the device. The experimental results have guided significance in designing and operating the devices with different operating voltage requirements, such as memristors and dielectric energy storage capacitors.</sec>