Free carrier absorption in doped silicon from first-principles

Optoelectronic applications of silicon (Si) always involve doping to inject free carriers. It is thus important to understand the effects of free carrier absorption (FCA) on Si-based devices. In this work, we developed and applied first-principles computational approaches based on density functional and many-body perturbation theory to quantify the different free-carrier absorption mechanisms in Si, including direct, phonon-assisted, charged-impurity-assisted and conductivity-induced absorptions. We show that our calculated FCA coefficients agree with experimental measurements over a wide range of carrier densities and photon wavelengths for free electrons and free holes. We show that the different mechanisms of FCA dominate different photon wavelengths. Quantifying the effects of FCA on electron-hole induced current density, we show that FCA is an essential source of optical loss in Si especially for IR applications.