Time-resolved observation of band gap shrinking and electron-lattice thermalization in x-ray excited GaAs

It is a long-standing puzzle in material sciences how fast excited hot electrons exchange energy with the atomic lattice. Differing results are obtained with various theoretical approaches. Their experimental verification is a challenge; so far experiments have been limited mostly to an excitation regime near room temperature. Our method enables an access to the unprecedented regime of electron temperatures of a few eVs. It is based on the use of the unique properties of X-ray free-electron laser, e. g., FLASH at DESY Hamburg, to excite electrons within a semiconducting material to high energies on a hundred femtosecond timescale (10^-13 s), and to follow their relaxation on a few picosecond timescale (10^-12 s). During this relaxation the solid can be heated up to a high temperature. The responsible mechanism is the electron-lattice energy exchange (electron-phonon coupling). In order to measure how fast this energy exchange progresses, we propose to use its specific signature which is the shrinking of the so called 'band-gap' in a heated semiconductor. The band-gap is the quantum-mechanically forbidden energy region separating valence electrons from the conduction band electrons. The characteristic band-gap shrinking at high temperatures can be used to accurately determine the rate of electron-lattice energy exchange. Our experimental scheme uses the X-ray free-electron laser pump - optical laser probe scheme to measure transient optical properties of a semiconductor which are very sensitive to the actual band-gap width. The proposed method is of general applicability. We expect it to inspire dedicated quantitative studies of electron-lattice coupling in X-ray excited GaAs and other narrow band-gap semiconductors, with a high impact on this research field.

B.Ziaja., N. Medvedev, V. Tkachenko, T. Maltezopoulos, W. Wurth,
Sci. Rep. 5, 18068 (2015)