基于HfO_2的阻变存储器中Ag导电细丝方向和浓度的第一性原理研究

Orientation and concentration of Ag conductive filament in HfO_2-based resistive random access memory:first-principles study

采用基于密度泛函理论的第一性原理方法,研究了基于HfO_2的阻变存储器中Ag导电细丝浓度以及方向性.通过计算Ag杂质5种方向模型的分波电荷态密度等势面图、形成能、迁移势垒和分波电荷态密度最高等势面值,发现[-111]方向最有利于Ag导电细丝的形成,这对器件的开启电压、形成电压和开关比有很大影响.本文基于最佳的[-111]导电细丝方向,设计了4种Ag浓度结构.计算4种Ag浓度结构的分波电荷态密度等势面图,得出Ag浓度低于4.00 at.%时晶胞结构中无导电细丝形成且无阻变现象.当Ag浓度从4.00 at.%增加到4.95 at.%时,晶胞结构中发现有导电细丝形成,表明Ag浓度高于4.00 at.%时,晶胞中可以发生阻变现象.然而,通过进一步对比计算这两种晶胞结构中Ag的形成能、分波电荷态密度最高等势面值、总态密度与Ag的投影态密度发现,Ag浓度越大,导电细丝却不稳定,并且不利于提高阻变存储器的开关比.本文的研究结果可为改善基于HfO_2的阻变存储器的性能提供一定理论指导.

HfO_2-based resistive random access memory takes advantage of metal dopants defects in its principle of operation.Then, it is significantly important to study the performance of metal dopants in the formation of conductive filament.Except for the effects of the applied voltage, the orientation and concentration mechanism of the Ag dopants are investigated based on the first principle. First, five possible models of Ag in HfO_2 are established in [001], [010], [100],[-111] and [110] directions, in each of which adequate and equal dopants of Ag are ensured. The isosurface plots of partial charge density, formation energy, highest isosurface value and migration barrier of Ag dopants are calculated and compared to investigate the promising formation direction of Ag in the five established orientation systems. The formations of conductive filament are observed in [100], [010], [001] and [-111] directions in the unit cell structure from the isosurface plots of partial charge density. But no filament is formed in [110] direction. And the highest isosurface value of Ag dopant is largest in [-111] direction. This indicates that the most favorable conductive filament formation takes place in this direction. The formation energy of Ag in the different direction is different, and the values in [-111]and [100] direction are minimum and close to each other, which shows that it is easy to form conductive filaments in these two directions. In addition, the smallest migration barrier of Ag in [-111] direction reveals that the [-111] orientation is the optimal conductive path of Ag in HfO_2, which will effectively influence the SET voltage, formation voltage and the ON/OFF ratio of the device. Next, based on the results of orientation dependence, four different concentration models（HfAg_xO_2, x = 2, 3, 4, 5） are established along the [-111] crystal orientation. The isosurface plots of partial charge density about those concentration models are compared, showing that the resistive switching phenomenon cannot be observed for the samples deposited in a mixture with less than 4.00 at.% of Ag content（Hf Ag_4O_2）. The RS behavior is improved with Ag content increasing from 4.00 at. % to 4.95 at.%. However, the formation energy and highest isosurface value are calculated and it is found that the conductive filaments cannot be switched into a stable state when Ag content becomes greater than 4.00 at.%. Then, the total electron density of states and the projected electron density of states are also calculated for the two models. It indirectly shows that the conductive filament is mainly comprised of Ag atoms,rather than Hf atoms or oxygen vacancy. Also, it is not helpful to improve the ON/OFF ratio of the device when the Ag dopant concentration is higher than 4.00 at.%. Therefore, the best doping concentration of Ag is 4.00 at.% and it is more advantageous to change the resistance memory storage features. This work may provide a theoretical guidance for improving the performances of HfO_2-based resistive random access memory.