Femtosecond carrier dynamics in layered 2H-MoTe₂ semiconductor crystals have been investigated using soft x-ray transient absorption spectroscopy at the x-ray free-electron laser (XFEL) of the Pohang Accelerator Laboratory. Following above-bandgap optical excitation of 2H-MoTe₂, the photoexcited hole distribution is directly probed via short-lived transitions from the Te 3d_5/2 core level (M₅-edge, 572–577 eV) to transiently unoccupied states in the valence band. The optically excited electrons are separately probed via the reduced absorption probability at the Te M₅-edge involving partially occupied states of the conduction band. A 400 ± 110 fs delay is observed between this transient electron signal near the conduction band minimum compared to higher-lying states within the conduction band, which we assign to hot electron relaxation. Additionally, the transient absorption signals below and above the Te M₅ edge, assigned to photoexcited holes and electrons, respectively, are observed to decay concomitantly on a 1–2 ps timescale, which is interpreted as electron–hole recombination. The present work provides a benchmark for applications of XFELs for soft x-ray absorption studies of carrier-specific dynamics in semiconductors, and future opportunities enabled by this method are discussed.

中文翻译：

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飞秒软X射线吸收光谱法（使用X射线自由电子激光）观察到的2H-MoTe2中特定于载流子的动力学

在浦项加速器实验室的X射线自由电子激光器（XFEL）上使用软X射线瞬态吸收光谱法研究了层状2H-MoTe ₂半导体晶体中的飞秒载流子动力学。在2H-MoTe ₂的带隙以上光学激发之后，通过从Te 3 d _5/2核心能级（M ₅边缘，572-577 eV）到瞬态未占据状态的短暂跃迁直接探测了光激发空穴的分布。价带。通过在Te M ₅处降低的吸收概率分别探测光激发电子-边缘涉及导带的部分占据状态。与导带内较高的状态相比，在导带最小值附近的该瞬态电子信号之间观察到400±110 fs的延迟，我们将其分配给热电子弛豫。此外，观察到分别在Te M ₅边缘以下和之上的瞬态吸收信号分别分配给光激发空穴和电子，它们在1-2 ps的时间尺度上伴随衰减，这被解释为电子-空穴复合。本工作为XFEL在半导体中特定于载流子动力学的软X射线吸收研究中的应用提供了基准，并讨论了该方法带来的未来机会。