Department of Materials Science and Engineering and Inter-University Semiconductor Research Center, Seoul National University, Seoul 151-744, Republic of Korea.
Nanoscale. 2017 May 11;9(18):6010-6019. doi: 10.1039/c7nr01243h.
The endurance of switching cycles, which is a critical measure of device reliability, in an ultra-thin (1.5 nm) TaO and HfO resistive random access (ReRAM) memory cell with a 28 nm lateral dimension was studied using current-voltage (I-V) sweep and closed-loop pulse switching (CLPS) tests. The two devices showed the typical oxygen-deficient conducting-filament (CF)-mediated bipolar resistance switching behaviour, which was induced by the asymmetric electrode configuration: Ta as the oxygen vacancy (V) source/reservoir and TiN as the inert electrode. In these device geometries, the CF is supposed to initiate at the oxide/TiN interface and to grow towards the Ta electrode during the switch-on process, while the switch-off process was induced by the contraction of the CF from the Ta/oxide interface. Both devices, however, showed inversion (anomalous SET; switching from the off- to on-state) behaviour in the RESET (switching from the on- to off-state) process, which can be explained by the authors' previous model of the hourglass-shaped CF. In this model, once the CF is ruptured, the RESET polarity bias makes the lower portion of the CF regrow to slightly reconnect such a CF through the accelerated migration of V from the upper-portion CF to the lower-portion CF, which induces switching performance degradation. In the I-V sweeps, the on- and off-states of the devices showed an overall conductance difference approximately corresponding to the integer multiple values of quantum point contact (G), but there were arbitrary 0.25 and 0.125G differences in the conductance values of the on-state for the TaO and HfO devices, respectively. This suggests that these are the minimal units of conductance variation even for a given CF with a standard G. Although the precise reason for the emergence of such an abnormal conductance unit is not yet understood, its implication for the reliability is critical. Reliable resistive switching was achieved only for the cases where the minimum point conductance was retained even in the off-state; in the other cases, over-SET and over-RESET were induced, which eventually degraded the device reliability. The detailed quantitative analysis of the device failure revealed that the increasing concentration of V within the non-CF region in the cell decreased the resistance values of that region, which eventually resulted in the over-SET and over-RESET behaviours during the CLPS tests.
采用电流-电压（I-V）扫描和闭环脉冲切换（CLPS）测试研究了具有 28nm 横向尺寸的超薄（1.5nm）TaO 和 HfO 电阻式随机存取（ReRAM）存储器单元中切换循环的耐久性，这是设备可靠性的关键衡量标准。这两个器件表现出典型的缺氧导电线（CF）介导的双极性电阻开关行为，这是由不对称的电极结构引起的：Ta 作为氧空位（V）源/储库，TiN 作为惰性电极。在这些器件结构中，CF 应该在氧化层/TiN 界面处开始，并在开关导通过程中向 Ta 电极生长，而开关关断过程则是由 CF 从 Ta/氧化层界面收缩引起的。然而，这两个器件在重置（从导通状态切换到截止状态）过程中都表现出反转（异常 SET；从截止状态切换到导通状态）行为，这可以用作者之前的沙漏形 CF 模型来解释。在该模型中，一旦 CF 破裂，重置偏压使 CF 的下部重新生长，通过 V 从上部 CF 加速迁移到下部 CF，略微重新连接这样的 CF，从而导致开关性能下降。在 I-V 扫描中，器件的导通和截止状态显示出总体电导差，大致对应于量子点接触（G）的整数倍，但 TaO 和 HfO 器件的导通状态电导值分别有任意的 0.25 和 0.125G 差异。这表明，即使对于具有标准 G 的给定 CF，这也是电导变化的最小单元。尽管出现这种异常电导单元的确切原因尚不清楚，但它对可靠性的影响至关重要。只有在保留最小导通点电导的情况下，才能实现可靠的电阻开关；在其他情况下，会导致过 SET 和过重置，最终降低器件可靠性。对器件失效的详细定量分析表明，单元中非 CF 区域内 V 浓度的增加降低了该区域的电阻值，最终导致 CLPS 测试中的过 SET 和过重置行为。
