Direct and simultaneous observation of ultrafast electron and hole dynamics in germanium

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.

Ultrafast carrier thermalization and trapping in silicon-germanium alloy probed by extreme ultraviolet transient absorption spectroscopy

“…An upper bound for the instrumental response function was determined to be ∼6 fs by transient absorption measurements in argon. An in-depth description of the apparatus and more technical details are reported elsewhere 13 . A detailed characterization of the optical setup can be found in supplementary material, Sec.…”

“…Furthermore, since the germanium M 4,5 -edge has contributions by two 3 d core-level states that exhibit a spin-orbit splitting (${\Delta E}_{\text{so}}=0.58\hspace{0.17em}\text{eV}$, Ref. 39) comparable to the band gap in the materials considered here (${\Delta E}_{\text{gap},\text{Ge},\text{direct}}=0.8\hspace{0.17em}\text{eV}$, ${\Delta E}_{\text{gap},{\text{Si}}_{0.25}{\text{Ge}}_{0.75}\text{indirect}}=0.95\hspace{0.17em}\text{eV}$), it is necessary to separate these contributions, which is done by a Fourier method 13 . In the following, we restrict ourselves to describing the energy and time-dependent state blocking of the contribution due to the 3 d 5/2 core-level ${\Delta A}_{\text{SB},{3d}_{5/2}}\left(E,\tau \right)$, obtaining a direct representation of the dynamics of electrons and holes versus energy and time delay.…”

“…XUV ultrafast solid-state spectroscopy has opened up the possibility to study the dielectric response of insulators 6,7 and electron dynamics in semiconductors 8–12 . Recently, it was demonstrated that this technique can be employed for tracking electrons and holes as well as the energy shift of bands simultaneously at the M 4,5 -edge of germanium 13 . In XUV transient absorption spectroscopy, a visible-to-near infrared (VIS-NIR) pump pulse that photoexcites carriers is followed after a time delay τ by a broadband XUV probe pulse generated by high harmonic generation (HHG) 14 [Fig.…”

Efficient table-top dual-wavelength beamline for ultrafast transient absorption spectroscopy in the soft X-ray region

“…Therefore, a trade-off comprising a continuous spectrum extending above the carbon K-edge with high flux and few-femtosecond temporal resolution would be desirable. Moreover, transient absorption in the XUV typically employs a fraction of energy of the fewcycle visible-near IR pulses used for HHG as a pump via strong-field 4,6 or single-photon excitation 5,[8][9][10] . Due to the smaller photon energy in the SWIR, using a similar approach for SXR transient absorption considerably reduces the pumping excitation capabilities.…”

Attosecond x-ray transient absorption in condensed-matter: a core-state-resolved Bloch model

“…Attosecond transient absorption (ATA) is a ultrafast technique resulting from such technology. ATA has been successfully applied to studies of electron dynamics in material systems, allowing to understand petahertz conductivity changes in dielectrics [1], tunneling excitation induced by intense laser pulses [2], sub-femtosecond dynamics of both holes and electrons in complex semiconductor materials [3], among other interesting ultrafast studies [4][5][6][7][8].…”