Abstract
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 3d5/2 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.
Highlights
The optically excited carrier relaxation and structural dynamics of semiconductors govern their optoelectronic properties and functionality in emerging device applications.[1]
The differential absorption spectrum at a delay time of 400 fs provides a direct observation of photoexcited holes in the valence band (VB), which are identified by short-lived Te 3d5/2 ! VB absorption transitions appearing $1 eV below the 3d5/2 ! conduction band (CB) absorption edge
The transient spectrum is characterized by a negative DOD signal at the 3d5/2 ! CB energies due to excited electrons in the CB, bandgap renormalization, and lattice-heating effects
Summary
The optically excited carrier relaxation and structural dynamics of semiconductors govern their optoelectronic properties and functionality in emerging device applications.[1] Femtosecond timeresolved x-ray spectroscopy and scattering are powerful techniques to track the electronic and structural dynamics of such materials in real time. Scitation.org/journal/sdy ultraviolet (XUV) sources using x-ray free electron lasers (XFELs) and high-harmonic generation (HHG), new opportunities are being explored for investigating charge-carrier dynamics in the condensed phase with carrier-, element-, and oxidation-state specificity.[2,3,4,5] The ability to probe the valence electronic structure via localized core levels using x-ray/XUV absorption spectroscopy (XAS) has been exploited to capture electron and hole carrier relaxation dynamics separately in bulk semiconductors[6,7,8,9,10,11,12] and to measure layer-specific dynamics within multi-component heterojunctions.[13] These early successes have been led primarily by applications of table-top HHG sources for XUV transient absorption and reflection spectroscopy.[3]
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