  Using hybrid density functional theory combined with a semiempirical van der
Waals dispersion correction, we have investigated the structural and electronic
properties of vacancies and self-interstitials in defective few-layer
phosphorene. We find that both a vacancy and a self-interstitial defect are
more stable in the outer layer than in the inner layer. The formation energy
and transition energy of both a vacancy and a self-interstitial P defect
decrease with increasing film thickness, mainly due to the upward shift of the
host valence band maximum in reference to the vacuum level. Consequently, both
vacancies and self-interstitials could act as shallow acceptors, and this well
explains the experimentally observed p-type conductivity in few-layer
phosphorene. On the other hand, since these native point defects have moderate
formation energies and are stable in negatively charged states, they could also
serve as electron compensating centers in n-type few-layer phosphorene.
