The intrinsic large electronegativity of $\text{O}2p$ character of the valence-band maximum (VBM) of ZnO renders it extremely difficult to be doped $p$ type. We show from density functional calculation that such VBM characteristic can be altered by compensated donor-acceptor pairs, thus improve the $p$-type dopability. By incorporating $\left(\text{Ti}+\text{C}\right)$ or $\left(\text{Zr}+\text{C}\right)$ into ZnO simultaneously, a fully occupied impurity band that has the $\text{C}2p$ character is created above the VBM of host ZnO. Subsequent doping by N in $\text{ZnO}:\left(\text{Ti}+\text{C}\right)$ and $\text{ZnO}:\left(\text{Zr}+\text{C}\right)$ lead to the acceptor ionization energies of 0.18 and 0.13 eV, respectively, which is about 200 meV lower than it is in pure ZnO.

• Received 28 July 2009

DOI:https://doi.org/10.1103/PhysRevB.80.153201