First-principles approach to closing the 10-100 eV gap for charge-carrier thermalization in semiconductors

Since the 1960s and the first observations of radiation-induced disruption of electronic devices in space, the study of the effects of ionizing radiation on electronics has grown into an extensive field of its own. The present paper is concerned with studying accurately the energy-loss processes that control the thermalization of hot carriers (electrons, holes, and/or electron-hole pairs) that are generated by high-energy radiation in wurtzite GaN, using an ab initio approach. Current physical models of the nuclear/particle physics community cover thermalization in the high-energy range (kinetic energies exceeding -100 eV), and the electronic-device community has studied extensively carrier transport in the low-energy range (below -10 eV). However, the processes that control the energy losses and thermalization of electrons and holes in the intermediate energy range of about 10-100 eV (which we define as the "10-100 eV gap") are poorly known. The aim of this research is to close this gap. To this end, we utilize density functional theory (DFT) to obtain the band structure and dielectric function of GaN for energies up to about 100 eV. We also calculate charge-carrier scattering rates for the major charge-carrier interactions (phonon scattering, impact ionization, and plasmon emission), using the DFT results and first-order perturbation theory (Fermi's golden rule/first Born approximation). With this information, we study the thermalization of electrons starting at 100 eV using the Monte Carlo method to solve the semiclassical Boltzmann transport equation. Full thermalization of electrons and holes is complete within -1 and 0.5 ps, respectively. Hot electrons dissipate about 90% of their initial kinetic energy to the electron-hole gas (90 eV) during the first -0.1 fs, due to rapid plasmon emission and impact ionization at high energies. The remaining energy is lost more slowly as phonon emission dominates at lower energies (below -10 eV). During the thermalization, hot electrons generate pairs with an average energy of -8.9 eV/pair (11-12 pairs per hot electron). Additionally, during the thermalization, the maximum electron displacement from its original position is found to be on the order of 100 nm.