Figure 1.

The most stable and representative doping structures: (a) ${\mathrm{Cu}}_1$, (b) ${\mathrm{Cu}}_2$, (c) ${\mathrm{Cu}}_2{+\mathrm{Cu}}_2-\mathrm{N}$, (d) ${\mathrm{Cu}}_{\mathrm{C}}$, (e) ${\mathrm{Cu}}_{\mathrm{B}}{+\mathrm{Cu}}_1-\mathrm{N}$ and (f) ${\mathrm{Cu}}_{\mathrm{C}}{+\mathrm{Cu}}_{\mathrm{C}}-\mathrm{N}$. The red, green, and blue balls represent O, Ga, and Cu atoms, respectively.