低维半导体中电子、声子及等离激元散射对热载流子寿命的影响

Photoelectric conversation efficiency (PCE) plays a key role in semiconductors. The photo-excited hot carriers could relax fast to the band edges through coupling with lattice vibrations and other interactions, which losing the energy as heat. Slowing down the relaxation of hot carriers is a important way to enhance the PCE. For low-dimensional semiconductors, the spacing of energy levels and phonon dispersion are different from the bulk counterparts due to the quantum confinement. Meanwhile, the hot carriers suffer from unique scattering owing to the relative weak screening. In this project, we aim to investigate the lifetime of the hot carriers, from atomic scale, resulting from the interaction of electron, phonon and plasmon via many body perturbation theory in low-dimensional semiconductors. Additionally, we will discuss the relaxation process and its dependence on the temperature through manipulating the electronic structures and phonon vibration modes, designing the hetero-junction and so on. Our findings will shed new light on extracting the extra energy of the hot carriers in low-dimensional materials.

半导体中，光电转换效率是衡量其光电性能的重要指标。受光激发形成的热载流子会通过与晶格振动耦合和其它相互作用快速的回到能带边缘，从而释放出热量。而减缓这一过程，是提高光电转换效率的重要途径。在低维半导体中，量子限制效应导致的能级间距和声子色散关系与块体材料大不相同。同时，较弱的屏蔽效应也使得热载流子受到的散射作用也独具特点。本项目中，我们利用多体微扰理论从原子尺度研究低维半导体中电子、声子及等离激元相互作用对热载流子寿命的影响，并在此基础上通过调控电子结构、声子振动方式及设计异质结结构来调节热载流子的弛豫过程，并给出其对温度的依赖关系，从理论上对低维半导体中热载流子能量的提取进行预测与设计。

热载流子的寿命对光电转换现象极其重要。本项目中，我们从多体微扰理论出发，研究了过渡金属二硫化物及其异质结、新型碳材料和单元素或其化合物二维半导体中热载流子的散射性质。研究结果发现在非对称的Janus MoSSe中，内在的电偶极矩导致声子对载流子的散射作用增强，对应的带边热载流子的寿命减小，平均自由程小于MoS2；二维范德华异质结g-C3N4/MoS2中热空穴的弛豫过程涉及到了相同层和不同层之间的转移，导致其衰减速度大于热电子；两类新型碳材料中，由于声子对空穴的散射强烈，导致热空穴平均自由程大于对应的热电子平均自由程；在AsSb二维材料中，横向光学声子对载流子的散射贡献很大，因此是影响其迁移率的主要因素。本项目的结论对于这些材料在光电子转换和热载流子收集方面的应用给予了有力的理论支撑。