Long-lasting deformation potential effect in Ge induced by UV photoexcitation

Abstract

Understanding ultrafast response of a semiconductor is necessary for next-generation optoelectronic device applications. Here, we investigate the ultrafast response of an archetypal semiconductor Ge [111] crystal upon photoexcitation at two pump wavelengths, 800 nm (fundamental) and 400 nm (second harmonic, UV radiation), using time-resolved x-ray diffraction (TXRD). The simulated TXRD profiles using a proposed four-layer model and Takagi–Taupin equations reveal that the strain propagation is primarily due to electron diffusion. Intriguingly, the initial (<100 ps) fast decrease in the induced strain for second harmonic excitation suggests that the strain originates from the deformation potential (DP) effect. The higher photon energy (second harmonic pump) excites the electrons to higher conduction band valleys, resulting in DP-induced strain compared to the fundamental pump, which only generates thermo-elastic strain. The DP strain, estimated from the DP coefficient (simulated via density functional theory-based electronic structure simulations) and electron density (simulated from experimental results), shows an excellent agreement with the observed strain. The strain after ∼100 ps delay for second harmonic excitation and strain from fundamental excitation are attributed to the thermo-elastic effect as confirmed by the density-dependent two-temperature model. Our study reveals the long-lasting DP effect in Ge [111], which provides an opportunity to avoid heating in optoelectronic devices due to the thermo-elastic effect at the initial time scale.