Picosecond relaxation of X-ray excited GaAs

In this paper we present the current status of our theoretical studies on ultrafast relaxation of X-ray/XUV excited gallium arsenide. First, we discuss our previous approach, the unified model based on rate equations, two-temperature model and the extended Drude approach. By fitting the model to the available experimental data, we obtained realistic estimates on transient electronic temperature and electron-lattice thermalization timescale. Next, we make a step towards a rigorous description of the relaxation process with our hybrid code, XTANT. We extend the XTANT to include the band-specific effect of the suppression of collisional processes in GaAs, and perform dedicated simulations. We find that the extended model correctly describes the predicted transient non-isothermality of conduction and valence bands, however, currently, it cannot reproduce the experimentally observed reflectivity overshooting at 5-10 ps. The reason for this discrepancy is that the electron-phonon coupling rate implemented in XTANT, although successfully applied for diamond and silicon, clearly underestimates the strength of this coupling in GaAs. The outline for a respective model improvement is discussed. (C) 2017 Elsevier B.V. All rights reserved.