The Carbon State in Dilute Germanium Carbides

Conduction and valence band states for the highly mismatched alloy (HMA) Ge:C are projected onto Ge crystal states, Ge vacancy states, and Ge/C atomic orbitals, revealing that substitutional carbon not only creates a direct bandgap, but the new conduction band is optically active. Overlap integrals of the new Ge:C conduction band with bands of pure Ge shows the new band has almost no Ge band character. C sites structurally mimic uncharged vacancies in the Ge lattice, similar to Hjalmarson's model for other HMAs. C perturbs the entire Ge band structure even at the deepest crystal core energy levels. Projection onto atomic sites shows relatively weak localization compared with other HMAs, but does show a strong anisotropy in probability distribution. L-valley conduction band states in Ge are ruled out as major contributors to the carbon state in Ge:C, both by weak inner products between these states and by a negligible effect on optical transition strength when adding C.