Local-field and excitonic effects in the calculated optical properties of semiconductors from first-principles

The recently developed GW approximation (GWA) based on the all-electron full-potential projector augmented wave method is used to study the local-field (LF) and electron-hole excitation effects in the optical properties of small-, medium-, and large-band-gap semiconductors: Si, InP, AlAs, GaAs, and diamond. It is found that while the use of the GWA energies instead of local-density approximation (LDA) eigenvalues has a tendency to align the calculated structures in the optical spectra with their experimental counterparts, the LF effects do not change these peak positions but systematically reduce the intensities of the so-called $E_1$ and $E_2$ structures in all the optical spectra. Taking into account the electron-hole interaction, shifts the theoretical oscillator strength towards lower photon energies and thereby improves considerably the comparison with experiment. It is also shown that the LDA static dielectric constant, a ground-state property, is considerably improved when the LF effects are included. On the other hand, as expected, the static dielectric function obtained using the GW quasiparticle energies, and including the LF effects, is underestimated for all the semiconductors. Including the excitonic effects in the theory is expected to correct this discrepancy with experiment.