Abstract
Understanding excited carrier dynamics in semiconductors is crucial for the development of photovoltaics and efficient photonic devices. However, overlapping spectral features in optical pump-probe spectroscopy often render assignments of separate electron and hole carrier dynamics ambiguous. Here, ultrafast electron and hole dynamics in germanium nanocrystalline thin films are directly and simultaneously observed by ultrafast transient absorption spectroscopy in the extreme ultraviolet at the germanium M4,5 edge. We decompose the spectra into contributions of electronic state blocking and photo-induced band shifts at a carrier density of 8 × 1020 cm−3. Separate electron and hole relaxation times are observed as a function of hot carrier energies. A first-order electron and hole decay of ∼1 ps suggests a Shockley–Read–Hall recombination mechanism. The simultaneous observation of electrons and holes with extreme ultraviolet transient absorption spectroscopy paves the way for investigating few- to sub-femtosecond dynamics of both holes and electrons in complex semiconductor materials and across junctions.
Highlights
Understanding excited carrier dynamics in semiconductors is crucial for the development of photovoltaics and efficient photonic devices
In ultrafast XUV transient absorption spectroscopy, a visible-to-near infrared (VIS-NIR) pump pulse excites carriers first, and after a given time delay t, a broadband pulse in the XUV consisting of an isolated attosecond pulse or a short attosecond pulse train generated by high harmonic generation[12] excites core-level electrons into the valence and conduction bands (CBs)
We show how the carrier dynamics can be extracted by separating spinorbit components in the XUV
Summary
Understanding excited carrier dynamics in semiconductors is crucial for the development of photovoltaics and efficient photonic devices. Ultrafast transient absorption spectroscopy in the extreme ultraviolet (XUV) is an important technique for studying electron dynamics at the sub-femtosecond and few-femtosecond timescale[6,7] This technique has been successfully applied to investigate the dielectric response of insulators[8,9] and carrier dynamics in semiconductors[10,11] down to sub-femtosecond timescales. In ultrafast XUV transient absorption spectroscopy, a visible-to-near infrared (VIS-NIR) pump pulse excites carriers first, and after a given time delay t, a broadband pulse in the XUV consisting of an isolated attosecond pulse or a short attosecond pulse train generated by high harmonic generation[12] excites core-level electrons into the valence and conduction bands (CBs).
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