Tuning the valence and concentration of europium and luminescence centers in GaN through co-doping and defect association

Defect physics of europium (Eu) doped GaN is investigated using\nfirst-principles hybrid density-functional defect calculations. This includes\nthe interaction between the rare-earth dopant and native point defects (Ga and\nN vacancies) and other impurities (O, Si, C, H, and Mg) unintentionally present\nor intentionally incorporated into the host material. While the trivalent\nEu$^{3+}$ ion is often found to be predominant when Eu is incorporated at the\nGa site in wurtzite GaN, the divalent Eu$^{2+}$ is also stable and found to be\npredominant in a small range of Fermi-level values in the band-gap region. The\nEu$^{2+}$/Eu$^{3+}$ ratio can be tuned by tuning the position of Fermi level\nand through defect association. We find co-doping with oxygen can facilitate\nthe incorporation of Eu into the lattice. The unassociated Eu$_{\\rm Ga}$ is an\nelectrically and optically active defect center and its behavior is profoundly\nimpacted by local defect--defect interaction. Defect complexes such as Eu$_{\\rm\nGa}$-O$_{\\rm N}$, Eu$_{\\rm Ga}$-Si$_{\\rm Ga}$, Eu$_{\\rm Ga}$-H$_i$, Eu$_{\\rm\nGa}$-Mg$_{\\rm Ga}$, and Eu$_{\\rm Ga}$-O$_{\\rm N}$-Mg$_{\\rm Ga}$ can efficiently\nact as deep carrier traps and mediate energy transfer from the host into the\nEu$^{3+}$ $4f$-electron core which then leads to sharp red intra-$f$\nluminescence. Eu-related defects can also give rise to defect-to-band\nluminescence. The unassociated Eu$_{\\rm Ga}$, for example, is identified as a\npossible source of the broad blue emission observed in n-type,\nEu$^{2+}$-containing GaN. This work calls for a re-assessment of certain\nassumptions regarding specific defect configurations previously made for\nEu-doped GaN and further investigation into the origin of the photoluminescence\nhysteresis observed in (Eu,Mg)-doped samples.\n