Intrinsic n

ZnO typifies a class of materials that can be doped via native defects in only one way: either n type or p type. We explain this asymmetry in ZnO via a study of its intrinsic defect physics, including ZnO, Zni, VO, Oi, and VZn and n-type impurity dopants, Al and F. We find that ZnO is n type at Zn-rich conditions. This is because (i) the Zn interstitial, Zni, is a shallow donor, supplying electrons; (ii) its formation enthalpy is low for both Zn-rich and O-rich conditions, so this defect is abundant; and (iii) the native defects that could compensate the n-type doping effect of Zni (interstitial O, Oi, and Zn vacancy, VZn), have high formation enthalpies for Zn-rich conditions, so these ''electron killers'' are not abundant. We find that ZnO cannot be doped p type via native defects (Oi,VZn) despite the fact that they are shallow (i.e., supplying holes at room temperature). This is because at both Zn-rich and O-rich conditions, the defects that could compensate p-type doping (VO,Zni,ZnO) have low formation enthalpies so these ''hole killers'' form readily. Furthermore, we identify electron-hole radiative recombination at the VO center as the source of the green luminescence. In contrast, a large structural relaxation of the same center upon double hole capture leads to slow electron-hole recombination (either radiative or nonradiative) responsible for the slow decay of photoconductivity