Ultrafast scattering processes of hot electrons in InSb studied by time- and angle-resolved photoemission spectroscopy

Ultrafast scattering processes of hot electrons photoinjected into the conduction band of InSb have been studied using time- and angle-resolved photoemission spectroscopy. The nascent distributions of hot-electron packets are captured directly in energy and momentum spaces, and their ultrafast scattering processes are traced at femtosecond temporal resolution on a state-resolved basis. Hot electrons injected in the Γ valley with excess energies above the minimum of the L valley show ultrafast intervalley scattering, with transition times of the order of 40 fs. The relaxation processes in the L valley are resolved in energy and momentum spaces, including their backscattering into the Γ valley during relaxation. In contrast, relaxation of hot electrons with excess energy below the minimum of the L valley is governed by the direct impact ionization (IMP). We reveal state-selective features of the IMP process, and we have determined the direct IMP rate to be 7×1012s-1 for hot electrons with excess energy in the range of 0.35 to 0.6 eV. The direct IMP process results in a rapid increase, within 300 fs after excitation, of the electron density at the conduction band minimum (CBM), and phonon-assisted IMP by hot electrons scattered in the L valley and those backscattered into the Γ valley persistently enhances the electron density up to 8 ps after excitation. By analyzing correlations between the IMP rates of hot electrons and the electron densities near the CBM, an important role of a transient Auger recombination is proposed to quantify the yield of low-energy electrons generated in the IMP process.