Design of shallow acceptors in ZnO through early transition metals codoped with N acceptors

We perform first-principles density-functional theory calculations to study the atomic and electronic properties of early transition metals (Zr, Ti, Y, and Sc) codoped with N in wurtzite ZnO. By incorporating early transition metals Ti, Zr, Y, and Sc with N into ZnO simultaneously, we find that forming complexes (Zr-2N), (Ti-2N), (Y-N), and (Sc-N) induces fully occupied impurity bands with the N 2p character above the valence band maximum of host ZnO. With further doping of N in ZnO, the systems (Zr-2N):N, (Ti-2N):N, (Y-N):N, or (Sc-N):N have acceptor ionization energies lower than that of the isolated N acceptor in ZnO. Under different growth conditions (i.e., using an N₂O or NO source for the nitrogen atoms), we calculate the formation energies of the defect complexes and compare the dopability of the selected codoped systems. Our results show that the valence band maximum characteristic of ZnO can be altered by compensated donor-acceptor pairs, thus improving the p-type dopability.