Electronic band structure and optical absorption of CdSe doped with interstitial oxygen molecules

We have investigated the electronic structure and optical properties of CdSe doped with molecular oxygen by using first-principle local density-functional theory (DFT) based on a hybrid functional. We find that interstitial oxygen molecule introduces two kinds of states in CdSe: filled states which are degenerate and strongly hybridized with the top of the valence bands and one empty state lying in the middle of the gap. The origin of these states can be traced to the 1πg molecular orbitals of O2. We compared the optical absorption spectra of CdSe with and without molecular oxygen. States introduced by oxygen molecules lower the absorption onset of CdSe to ∼1.0eV while hybridization causes redistribution of the oscillator strengths of CdSe transitions from the ultraviolet to the infrared. Our results demonstrate that doping small molecules into semiconductors can alter the electronic and optical properties of the host in ways different from atomic dopants.