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 ${\mathrm{Zn}}_{\mathrm{O}},$ ${\mathrm{Zn}}_i,$ ${\mathrm{V}}_{\mathrm{O}},$ ${\mathrm{O}}_i,$ and $V_{\mathrm{Zn}}$ 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, ${\mathrm{Zn}}_i,$ 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 ${\mathrm{Zn}}_i$ (interstitial O, ${\mathrm{O}}_i,$ and Zn vacancy, $V_{\mathrm{Zn}}),$ 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 $({\mathrm{O}}_i,V_{\mathrm{Zn}})$ 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 ${(V}_{\mathrm{O}}{,\mathrm{}\mathrm{Zn}}_i,{\mathrm{Zn}}_{\mathrm{O}})$ have low formation enthalpies so these “hole killers” form readily. Furthermore, we identify electron-hole radiative recombination at the $V_{\mathrm{O}}$ 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.