Bradley Samuel R, Bersuker Gennadi, Shluger Alexander L
Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, UK.
J Phys Condens Matter. 2015 Oct 21;27(41):415401. doi: 10.1088/0953-8984/27/41/415401. Epub 2015 Sep 28.
Formation of metal rich conductive filaments and their rearrangements determine the switching characteristics in HfO2 based resistive random access memory (RRAM) devices. The initiation of a filament formation process may occur either via aggregation of pre-existing vacancies randomly distributed in the oxide or via generation of new oxygen vacancies close to the pre-existing ones. We evaluate the feasibility of vacancy aggregation processes by calculating the structures and binding energies of oxygen vacancy aggregates consisting of 2, 3 and 4 vacancies in bulk monoclinic (m)-HfO2 using density functional theory (DFT). We demonstrate that formation of neutral oxygen vacancy aggregates is accompanied by small energy gain, which depends on the size and shape of the aggregate. In the most strongly bound configurations, vacancies are unscreened by Hf cations and form voids within the crystal, with the larger aggregates having larger binding energy per vacancy (-0.11 to -0.18 eV). The negatively charged di-vacancy was found to have similar binding energies to the neutral one, while the positively charged di-vacancy was found to be unstable. Thus aggregation process of either neutral or negatively charged oxygen vacancies is energetically feasible.
富含金属的导电细丝的形成及其重排决定了基于HfO2的电阻式随机存取存储器（RRAM）器件的开关特性。细丝形成过程的起始可能通过氧化物中随机分布的预先存在的空位的聚集，或者通过在预先存在的空位附近产生新的氧空位而发生。我们使用密度泛函理论（DFT）计算体相单斜（m）-HfO2中由2、3和4个空位组成的氧空位聚集体的结构和结合能，以评估空位聚集过程的可行性。我们证明中性氧空位聚集体的形成伴随着小的能量增益，这取决于聚集体的大小和形状。在结合最紧密的构型中，空位未被Hf阳离子屏蔽，并在晶体内形成空隙，较大的聚集体每个空位具有更大的结合能（-0.11至-0.18 eV）。发现带负电的双空位具有与中性双空位相似的结合能，而带正电的双空位是不稳定的。因此，中性或带负电的氧空位的聚集过程在能量上是可行的。
