Symmetric band structures and asymmetric ultrafast electron and hole relaxations in silicon and germanium quantum dots: time-domain ab initio simulation
Symmetric band structures and asymmetric ultrafast electron and hole relaxations in silicon and germanium quantum dots: time-domain ab initio simulation
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DOI：
10.1039/b909267f
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发表时间：
2009-01-01
影响因子：
4
通讯作者：
Prezhdo, Oleg V.
中科院分区：
文献类型：
--
作者：
Hyeon-Deuk, Kim;Madrid, Angeline B.;Prezhdo, Oleg V.
利用最新的时域密度泛函理论和非绝热（NA）分子动力学模拟研究了Ge和Si量子点（QDs）中光激发电子和空穴的声子诱导弛豫。弛豫与生产过程竞争，导致量子点太阳能电池的能量和电压损失。从头计算表明，量子约束使得Si和Ge量子点的电子和空穴密度比体量子点更对称。令人惊讶的是，尽管存在对称的DOS，电子和空穴的弛豫却是相当不对称的：电子比空穴衰减得快。这种不对称是由于NA在传导带（CB）中的耦合比在价带（VB）中的耦合更强。与空穴相比，电子更强的NA耦合是由高频Ge-H和Si-H表面钝化键相对于VB对CB的更大贡献来解释的。发现电子和空穴弛豫速率与CB和VB DOS之间的线性关系符合费米黄金法则。与Ge和Si量子点中的空穴相比，电子的弛豫速度更快是出乎意料的，这与大多数二元量子点（如CdSe）的相应动力学形成了对比。这表明，俄歇过程将能量从空穴转移到电子，而不是像CdSe那样以相反的方向转移，并且更大比例的光激发能量将转移到与电子耦合的声子而不是空穴。在锗量子点中，声子诱导的电子和空穴衰减速率的差异大于硅量子点，这表明在锗量子点中，俄歇过程应该是特别重要的。模拟结果直接证明了量子点表面的高频配体模式在半导体量子点的电子-声子弛豫动力学中起着关键作用。
State-of-the-art time domain density functional theory and non-adiabatic (NA) molecular dynamic simulations are used to study phonon-induced relaxation of photoexcited electrons and holes in Ge and Si quantum dots (QDs). The relaxation competes with productive processes and causes energy and voltage losses in QD solar cells. The ab initio calculations show that quantum confinement makes the electron and hole density of states (DOS) more symmetric in Si and Ge QDs compared to bulk. Surprisingly, in spite of the symmetric DOS, the electron and hole relaxations are quite asymmetric: the electrons decay faster than the holes. The asymmetry arises due to stronger NA coupling in the conduction band (CB) than in the valence band (VB). The stronger NA coupling of the electrons compared to the holes is rationalized by the larger contribution of the high-frequency Ge-H and Si-H surface passivating bonds to the CB relative to the VB. Linear relationships between the electron and hole relaxation rates and the CB and VB DOS are found in agreement with Fermi's golden rule. The faster relaxation of the electrons compared to the holes in the Ge and Si QDs is unexpected and is in contrast with the corresponding dynamics in the majority of binary QDs, such as CdSe. It suggests that Auger processes will transfer energy from holes to electrons rather than in the opposite direction as in CdSe, and that a larger fraction of the photoexcitation energy will be transferred to phonons coupled with electrons rather than holes. The difference in the phonon-induced electron and hole decay rates is larger in Ge than Si, indicating that the Auger processes should be particularly important in Ge QDs. The simulations provide direct evidence that the high-frequency ligand modes on the QD surface play a pivotal role in the electron-phonon relaxation dynamics of semiconductor QDs.