Intrinsicn-type versusp-type doping asymmetry and the defect physics of ZnO

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 Zn₎, Zn₈, V₎, O₈, and Vₙ₍ 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, Zn₈, 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 Zn₈ (interstitial O, O₈, and Zn vacancy, Vₙ₍), 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 (O₈, Vₙ₍) 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 (V₎, Zn₈, Zn₎) have low formation enthalpies so these ``hole killers'' form readily. Furthermore, we identify electron-hole radiative recombination at the V₎ 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.