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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使用 X 射线自由电子激光通过飞秒软 X 射线吸收光谱观察 2H-MoTe2 中的载流子特定动力学


浦项加速器实验室的 X 射线自由电子激光器 (XFEL) 使用软 X 射线瞬态吸收光谱研究了层状 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）到瞬时未占据状态的短暂跃迁，直接探测光激发空穴分布。价带。通过Te M ₅边缘处涉及导带部分占据状态的降低吸收概率来单独探测光激发电子。与导带内较高的状态相比，在导带最小值附近的瞬态电子信号之间观察到 400 ± 110 fs 的延迟，我们将其归因于热电子弛豫。此外，观察到 Te M ₅边缘下方和上方的瞬态吸收信号分别分配给光激发空穴和电子，在 1-2 ps 时间尺度上同时衰减，这被解释为电子空穴复合。目前的工作为 XFEL 在半导体载流子特定动力学的软 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。