Ab initio investigation of the native defects in diamond and self-diffusion

Local-density-functional pseudopotential theory is used to investigate native defects in diamond, their structure, electronic, vibrational, and diffusive properties. We find the only truly stable structure for an interstitial atom to be the 〈100〉 split interstitial defect. This conclusion holds in the neutral, -1, +1, and +2 charge states. We analyze the multiplet structure of this defect, finding ${}_{}{}^1{}_{}{}^{}$${\mathit{B}}_1$ to be the lowest in energy. However, a Jahn-Teller distortion is also possible in all but the +2 charge state, giving considerable reductions in energy (0.6 eV for the neutral case). The tetrahedral, hexagonal, bond-centered, and 〈110〉 split interstitial structures are shown to be unstable. An upper bound for the energy barrier to the motion of the 〈100〉 split interstitial is found to be 1.7 eV. This is lower than that of vacancy diffusion although movement of interstitial atoms is usually ignored in considering self-diffusion. For the vacancy in diamond, we find that the surrounding four atoms relax outwards by 0.2 Å in both the neutral and negative charge states. The neutral vacancy undergoes a Jahn-Teller distortion with an energy gain of 0.36 eV. This effect is known to be dynamic at room temperature. For the negative vacancy we obtain ${}_{}{}^4{}_{}{}^{}$${\mathit{A}}_2$ as the ground state and obtain the transition energy to ${}_{}{}^4{}_{}{}^{}$${\mathit{T}}_1$ to be 3.3 eV, in good agreement with the observed ND1 band at 3.149 eV. Energies of other multiplets are estimated. We find the migration energies of the neutral and negative vacancies to be 2.8 and 3.4 eV, respectively. Finally, we calculate the formation energy for a vacancy-〈100〉 split interstitial pair to be 20 eV.