扫描电压幅值调控循环方向的NiO_x薄膜电阻开关特性

Reversed bipolar resistive switching behaviors modulated by scanning voltage amplitude in polycrystalline NiO_x films

本文利用射频磁控溅射方法在优化制备条件下沉积制备了沿[200]晶向择优取向的多晶Ni Ox薄膜.Ag/Ni Ox/Pt电容器结构的电流-电压曲线具有典型的双极性电阻开关特性,但循环方向与扫描电压幅值有关:当最大扫描电压小于2 V时,电流-电压曲线沿逆时针方向循环,高/低电阻值比大于50,可稳定维持超过50个循环周期.当最大扫描电压增大到2 V,电流-电压曲线变为沿顺时针方向循环,高/低电阻值比大于10,可稳定维持超过120个循环周期.电流-电压曲线循环方向随扫描电压幅值的改变揭示了其不同的电阻开关物理机制:低扫描电压下逆时针循环电阻开关特性主要归因于电场作用下氧离子定向漂移导致氧空位形成的导电细丝通道周期性地导通和截断;当最大扫描电压增加到2 V时,银离子扩散进入Ni Ox薄膜与定向漂移的氧离子发生氧化/还原反应合成/分解Ag Ox,使得薄膜导电性周期性地增大和减小,从而使得Ni Ox薄膜具有了顺时针循环电阻开关特性.

NiO-based metal-insulator-metal structures have been widely studied for resistive switching random access memory device applications since 1960s. Researchers prefer to improve their resistive switching parameters （ON/OFF ratio, stability, endurance and retention performances etc.） than discuss their switching direction changes. In this paper, we have studied the reverse of resistive switching directions in polycrystalline NiOx films modulated by scanning voltage amplitude. 60 nm-thick polycrystalline NiOx films with a [200] preferred orientation have been grown by radio frequency magnetron sputtering under optimized deposition conditions （deposited at 280℃ in 3 Pa Ar＋O2 mixed ambient with an oxygen partial pressure of 20%）. Ag/NiOx/Pt memory cells exhibit reversed bipolar resistive switching behaviors modulated by scanning voltage amplitude. When applying a scanning voltage lower than ＋2 V, they present anticlockwise current-voltage loops with good ON/OFF ratios （〉50）, but poor durability （-50 switching cycles）. Once the scanning voltage increases to ＋2 V, those memory cells obtain clockwise current-voltage loops with a lower ON/OFF ratio of 10, but better durability （~120 switching cycles）. The logarithmic plots and power-law fittings for current-voltage log curves demonstrate that ohmic conduction processes are dominated on both the low and high resistance state for the anticlockwise current-voltage loop. For the clockwise current-voltage loop, an ohmic conduction process is still dominated on the low resistance state, while a trap-controlled space-charge limited current conduction process is responsible for the current transport on the high resistance state when applying a high electric field. The formation and annihilation of filamentary conducting paths composed by oxygen vacancies are responsible for the anticlockwise resistive switching behaviors; while the synthesis and decomposition of AgOx due to the oxidation/reduction reactions between the diffusing silver ions and the drifting oxygen ions result in the clockwise resistive switching behaviors.