Excited-State Properties of Thin Silicon Nanowires

Calculating many-electron interactions is crucial in determining excited-state properties of low-dimensional structures and is often beyond the capability of density functional theory, which is a ground-state theory. First-principles manybody perturbation theory can better describe many-electron effects and has been successfully employed to study excited-state properties of a wide range of solids. Such properties include quasiparticle energies, excitonic effects, and optical absorption spectra. In one-dimensional ultrathin silicon nanowires, which have been regarded as building blocks of nanoscale devices, many-electron interactions are substantially enhanced because of reduced dimensionality and weak electronic screening. If we employ many-body perturbation theory based on GW calculations for a silicon nanowire with a diameter of 1.2 nm, the quasiparticle band gap is 3.2 eV. This value is twice of that calculated from density functional theory. Excitonic effects and optical absorption spectra, which cannot be obtained solely from GW calculations, are obtained by solving the Bethe-Salpeter equation. The calculated electron-hole binding energy is 1.2 eV for the same sized silicon nanowire. This binding energy is about two orders of magnitude larger than those found in bulk semiconductors. Our results agree well with recent measurements.