Ultrafast electron intervalley scattering and hole dynamics in amorphous and crystalline Ge

Understanding the role of holes in energy transport mechanisms and determining electron intervalley scattering times is crucial for optimising Ge-based optoelectronic devices such as transistors, photodetectors, power devices, and light emitters. By employing our recently developed, ultrafast 20-fs resolution ellipsometer, we demonstrate that the absorption ( ɛ 2 ) in various types of Ge continues to decrease in the first picosecond after optical excitation, proving that the revival of the absorption is limited by the intrabandlike relaxation of holes (near the Γ point), instead of the intervalley scattering of the electrons. The observed decay times depend on the probing photon energies attributed to the different lifetimes of the holes in the light, split-off and heavy-hole bands. With gradually decreasing degree of crystallinity of our single crystalline, semicrystalline and amorphous Ge samples, the recovery times also decrease, which is related to the appearance and intensification of real-space scattering events over reciprocal space scattering. There is excellent correspondence for electron relaxation times for different samples with our two-temperature model, which is thoroughly analyzed via band-structure calculations. Our results contribute to the understanding of the excitation and decay processes in not only crystalline semiconductors, but also semicrystalline and amorphous semiconductors, resolving corresponding debates.