Abstract
Semiconductor alloys containing silicon and germanium are of growing importance for compact and highly efficient photonic devices due to their favorable properties for direct integration into silicon platforms and wide tunability of optical parameters. Here, we report the simultaneous direct and energy-resolved probing of ultrafast electron and hole dynamics in a silicon-germanium alloy with the stoichiometry Si0.25Ge0.75 by extreme ultraviolet transient absorption spectroscopy. Probing the photoinduced dynamics of charge carriers at the germanium M4,5-edge (∼30 eV) allows the germanium atoms to be used as reporter atoms for carrier dynamics in the alloy. The photoexcitation of electrons across the direct and indirect band gap into conduction band (CB) valleys and their subsequent hot carrier relaxation are observed and compared to pure germanium, where the Ge direct and Si0.25Ge0.75 indirect gaps () are comparable in energy. In the alloy, comparable carrier lifetimes are observed for the X, L, and Γ valleys in the conduction band. A midgap feature associated with electrons accumulating in trap states near the CB edge following intraband thermalization is observed in the Si0.25Ge0.75 alloy. The successful implementation of the reporter atom concept for capturing the dynamics of the electronic bands by site-specific probing in solids opens a route to study carrier dynamics in more complex materials with femtosecond and sub-femtosecond temporal resolution.
Highlights
Studying carrier dynamics in semiconductors has been an active research field for decades
In XUV transient absorption spectroscopy, a visible-to-near infrared (VIS-NIR) pump pulse that photoexcites carriers is followed after a time delay s by a broadband XUV probe pulse generated by high harmonic generation (HHG)14 [Fig. 1(b)]
XUV transient absorption measurements allow for probing of electron dynamics in a silicon-germanium alloy
Summary
Studying carrier dynamics in semiconductors has been an active research field for decades. Optical methods often do not separately resolve the spectral signatures of the electrons and holes directly and simultaneously, which renders capturing a full picture of the carrier dynamics difficult. Recent developments in transient absorption (TA) spectroscopy in the extreme ultraviolet (XUV) provide new capabilities for femtosecond to sub-femtosecond time resolution and for direct access to electronic structural features.. XUV ultrafast solid-state spectroscopy has opened up the possibility to study the dielectric response of insulators and electron dynamics in semiconductors.. 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 M4,5-edge of germanium.. The XUV pulse excites core-level electrons into the valence band (VB) and conduction band (CB), and the transient absorption of the XUV photons tracks the dynamics of excited carriers and possible band modifications in the material. The element specificity of core-level XUV absorption renders this technique advantageous for site-specific investigations in heteroatomic, ternary, and quaternary systems
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