n-type dopants and conduction-band electrons in diamond: Cluster molecular-orbital theory

A molecular-orbital study has been made of substitutional (s) atomic Li, N, and P and interstitial (i) Li in diamond through the use of cluster models of the diamond lattice. The small binding energy calculated for P(s) indicates extremely small solubility for P in the lattice. Calculated electronic structures, including lattice relaxations, show Li(s) is an acceptor and P(s) is a donor that should fully ionize in bulk diamond. Li(i) is either a shallow donor or fully ionized. N(s) is a deep donor and its thermal excitation energy is significantly lower than the optical energy because of stabilization from relaxation around N⁺. In clusters of the size studied, with up to 98 carbon atoms, conduction-band electrons become trapped in donor C-C bond stretch regions, occupying a C-C σ* orbital. Electron transport in such clusters would be activated by polaron hopping, with local lattice distortions around the localized electron orbital. In the bulk limit this donor level will be stablized slightly but the bottom of the conduction band lowers to the point that free-electron transport should take over.