Optical lineshapes of the C center in silicon from <i>ab initio</i> calculations: Interplay of localized modes and bulk phonons

In this work, we present a first-principles density functional theory (DFT) computational investigation of the luminescence and absorption lineshapes associated with the neutral carbon-oxygen interstitial pair (${\mathrm{C}}_{\mathit{i}}{\mathrm{O}}_{\mathit{i}}$) defect in silicon. We obtain the lineshapes of the defect in the dilute limit using a computational methodology that constructs dynamical matrices of supercells containing tens of thousands of atoms, utilizing systems directly accessible through DFT. Both perturbed bulk phonons and localized vibrations contribute to the phonon sideband. We achieve excellent agreement with experimental luminescence data. Our findings further reinforce the attribution of the well-known C-line in silicon to the neutral ${\mathrm{C}}_{\mathit{i}}{\mathrm{O}}_{\mathit{i}}$ complex.