The migration of interstitial H in diamond and its pairing with substitutional B and N: Molecular orbital theory

We present results of atom superposition and electron delocalization molecular orbital (ASED-MO) calculations of interactions of interstitial H with substitutional B and N in diamond. Nearest-neighbor and next-nearest-neighbor C atoms were relaxed in geometry depending on the cluster size, XC₃₄H₃₆ or XC₇₀H₆₀, respectively, where X = B or N and the H atoms saturate the surface dangling radical orbitals of the models. A small Jahn-Teller distortion occurs for interstitial B, a shallow acceptor which, in the B₋ state, sits in a tetrahedral lattice site. For interstitial N distortions are large, with a long C-N distance which stabilizes a ŝ^∗ orbital that would otherwise be in the conduction band. This orbital has one electron in it and has its greatest amplitude on C; the bonding counterpart has its greatest amplitude on N and is similar to the N lone-pair orbital in amines. The calculations indicate that N is a deep donor and N⁺ relaxes to the tetrahedral lattice site. Interstitial H is a mid-band-gap donor and is possibly also an acceptor with a high 1.9 eV calculated activation energy barrier to migration. Interstitial H⁺ is expected to be very mobile, with a migration barrier of 0.1 eV. H⁻ is predicted to be relatively immobile with an activation barrier for migration of 2.5 eV. The mobility of bond-inserted H around B in BH pairs should be high, with a calculated activation energy of 0.13 eV, but for N the comparable process has an activation energy of 2.50 eV. In NH pairs the interstitial H has formed a bond with the radical orbital on the C, so donation would be from the lone-pair orbital on N, which lies deep in the band gap; hence, the donor property is passivated.