[论文解读] First Principles Study of Intrinsic and Extrinsic Point Defects in Monolayer WSe2

Yujie Zheng, Su Ying Quek|arXiv (Cornell University)|Jan 16, 2019

2D Materials and Applications参考文献 3被引用 5

一句话总结

本项基于第一性原理的密度泛函理论研究探讨了单层WSe2中的本征与外在点缺陷，揭示了硒空位（Sevac）为最稳定的本征缺陷，其具有非自旋极化的带隙态。氧在Sevac位点的吸附可完全消除这些带隙态，显著提升缺陷稳定性，表明氧钝化的WSe2是光电器件与自旋电子学应用的有前途候选材料。

ABSTRACT

We present a detailed first principles density functional theory study of intrinsic and extrinsic point defects in monolayer (ML) WSe2. Among the intrinsic point defects, Se vacancies (Sevac) have the lowest formation energy (disregarding Se adatoms that can be removed with annealing). The defects with the next smallest formation energies (at least 1 eV larger) are SeW (Se substituting W atoms in an antisite defect), Wvac (W vacancies) and 2Sevac (Se divacancies). All these intrinsic defects have gap states that are not spin-polarized. The presence of a graphite substrate does not change the formation energies of these defects significantly. For the extrinsic point defects, we focus on O, O2, H, H2 and C interacting with perfect WSe2 and its intrinsic point defects. The preferred binding site in perfect WSe2 is the interstitial site for atomic O, H and C. These interstitial defects have no gap states. The gap states of the intrinsic defects are modified by interaction with O, O2, H, H2 and C. In particular, the gap states of Sevac and 2Sevac are completely removed by interaction with O and O2. This is consistent with the significantly larger stability of O-related defects compared to H- and C-related defects. The preferred binding site for O is Sevac, while that for H is SeW. H bonded to SeW results in spin-polarized gap states, which may be useful in defect engineering for spintronics applications. The charge transition levels and ionization energies of these defects are also computed. H in the interstitial site is an effective donor, while all the other defects are deep donors or acceptors in isolated WSe2 ML.

研究动机与目标

• 确定单层WSe2中热力学最稳定的本征与外在点缺陷。
• 理解外在掺杂原子（O、H、C）如何改变本征缺陷的电子结构。
• 评估基底（如石墨）对缺陷形成能的影响。
• 评估缺陷工程在自旋电子学与光电子学应用中的潜力。
• 计算关键缺陷的电荷转变能级与电离能。

提出的方法

• 采用广义梯度近似（GGA-PBE）泛函结合DFT+U校正处理过渡金属d电子。
• 在不同化学势条件下计算本征缺陷（Sevac、Wvac、SeW、2Sevac）的形成能。
• 研究O、H、C、O2与H2在完整与缺陷WSe2中的间隙与取代位点的结合行为。
• 通过投影态密度（PDOS）与带隙态分析研究电子结构变化。
• 利用电荷自旋转变能级法计算电荷转变能级与电离能。
• 通过采用超胞模型模拟WSe2在石墨层上的结构，评估基底效应。

实验结果

研究问题

• RQ1 在单层WSe2中，哪些本征点缺陷具有最低的形成能？
• RQ2 外在掺杂原子（O、H、C）如何与本征缺陷相互作用并改变其电子性质？
• RQ3 氧在钝化硒空位相关带隙态中起什么作用？
• RQ4 氢与SeW缺陷结合是否能诱导自旋极化的带隙态，以用于自旋电子学应用？
• RQ5 缺陷的电荷转变能级与电离能如何影响其在电子器件中的行为？

主要发现

• 硒空位（Sevac）在所有本征缺陷中具有最低的形成能，且石墨基底对其稳定性无显著稳定作用。
• 其次最稳定的本征缺陷——SeW、Wvac与2Sevac——其形成能至少比Sevac高1 eV。
• 氧在Sevac位点的吸附可完全消除带隙态，使体系热力学稳定性显著优于H或C钝化的缺陷。
• 氢与SeW缺陷结合可诱导自旋极化的带隙态，表明其在缺陷工程自旋电子器件中具有潜力。
• 间隙氢表现为有效施主，而其他缺陷（包括O与C钝化缺陷）则为深能级施主或受主。
• 氧优先结合于Sevac位点，而氢则优先结合于SeW反位缺陷，O2的钝化效率与原子氧相当。