  P mono-doped and (P, N) co-doped ZnO are investigated by the first-principles
calculations. It is found that substitutive P defect forms a deep acceptor
level at O site (PO) and it behaves as a donor at Zn site (PZn), while
interstitial P (Pi) is amphoteric. Under equilibrium conditions, these defects
contribute little to the p-type conductivity of ZnO samples since the formation
energy of PZn is much lower than that of Pi or PO when EF is below mid-gap (a
prerequisite p-type condition). Zinc vacancies (VZn) and PZn-2VZn complex are
demonstrated to be shallow acceptors with ionization energies around 100 meV,
but they are easily compensated by PZn defect. Fortunately, PZn-4NO complexes
may have lower formation energy than that of PZn under Zn rich condition by
proper choices of P and N sources. In addition, the neutral PZn-3NO passive
defects may form an impurity band right above the valence band maximum of ZnO
as in earlier reported (Ga,N) or (Zr,N) doped ZnO. This significantly reduces
the acceptor level of PZn-4NO complexes, and helps improving the p-type
conductivity in ZnO.
