Direct and Simultaneous Observation of Ultrafast Electron and Hole Dynamics in Germanium

TL;DR

This study employs attosecond transient absorption spectroscopy to directly and simultaneously observe ultrafast electron and hole dynamics in germanium nanocrystals, revealing detailed relaxation processes crucial for semiconductor device development.

Contribution

It introduces a method to directly and simultaneously measure electron and hole dynamics in germanium using ATAS, overcoming spectral overlap issues in traditional spectroscopy.

Findings

Electron and hole relaxation times are distinguished as a function of energy.

A ~1 ps decay indicates Shockley-Read-Hall recombination.

Simultaneous observation enables detailed carrier dynamics analysis.

Abstract

Understanding excited carrier dynamics in semiconductors is crucial for the development of photovoltaics and efficient photonic devices. However, overlapping spectral features in optical/NIR 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 attosecond transient absorption spectroscopy (ATAS) in the extreme ultraviolet at the germanium M_{4,5}-edge (~30 eV). We decompose the ATAS spectra into contributions of electronic state blocking and photo-induced band shifts at a carrier density of 8*10^{20}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 ATAS paves the way for investigating few to sub-femtosecond dynamics of both holes and electrons in complex semiconductor materials and across junctions.