Direct determination of impact-ionization rates near threshold in semiconductors using soft-x-ray photoemission

The line shape of the Al 2p core-level photoemission peak in ${\mathrm{Al}}_{0.9}$ ${\mathrm{Ga}}_{0.1}$As is found to be strongly dependent on the electron kinetic energy. Using Monte Carlo transport simulations it is shown that the observed strong, asymmetric line broadening is caused by electron-phonon scattering. In agreement with the experimental observation, the simulations predict that this phonon-induced line broadening is rapidly suppressed as the core line is shifted through the impact-ionization threshold. Thus, it becomes possible to infer from the core-level line shape the energy-dependent impact-ionization rates near the impact-ionization threshold.