Lithium has been investigated in detail as a possible acceptor impurity in zinc oxide for many years. Li interstitials are highly mobile in ZnO under Zn rich conditions and diffuse easily at temperatures as low as 300C. In contrast Li substituting for Zn sites is a known deep acceptor and is formed at much lower concentrations under O-rich conditions. . The high mobility of Li ions means that the formation of defect pairs between Li and various oppositely charged impurities should be easily formed. Defect pairs have been proposed as possible candidates for p-doping in ZnO as well as for quantum qubits, and therefore the subject is of general technological interest. In this work we study the low temperature photoluminescence (PL) of high quality ZnO single crystals grown by chemical vapor transport, and co-doped with Sn and Li. A well known donor bound exciton PL feature labeled I10 has recently been shown to include Sn substituting for Zn sites. In this study we show that an additional constituent, likely substitutional Li is also involved. Annealing Sn-doped samples under oxygen poor conditions results in a complete suppression of I10, while diffusion of Li in air at ~700C results in a clear enhancement of I10. Density functional theory within the GGA+U approximation has been used to investigate the formation energy and defect transition energies of SnZn-LiZn nearest neighbour complexes in ZnO. These calculations indicate that the Sn-Li complex is indeed a shallow donor as expected on simple charge considerations.
 K.E. Knutsen et al., J. Appl. Phys. 113, 023702 (2013)
 J. Cullen et al., Appl. Phys. Lett. 102, 192110 (2013)