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 1B1 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 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 4A2 as the ground state and obtain the transition energy to 4T1 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. © 1995 The American Physical Society.