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 first-principles hybrid density-functional defect calculations. This includes the interaction between rare-earth dopant and native point defects (Ga N vacancies) other impurities (O, Si, C, H, Mg) unintentionally present or intentionally incorporated into host material. While trivalent Eu$^{3+}$ ion often found to be predominant when Eu at Ga site in wurtzite GaN, divalent Eu$^{2+}$ also stable a small range Fermi-level values band-gap region. The Eu$^{2+}$/Eu$^{3+}$ ratio can tuned by tuning position Fermi level through association. We find co-doping with oxygen facilitate incorporation lattice. unassociated Eu$_{\rm Ga}$ an electrically optically active center its behavior profoundly impacted local defect--defect interaction. complexes such as Ga}$-O$_{\rm N}$, Ga}$-Si$_{\rm Ga}$, Ga}$-H$_i$, Ga}$-Mg$_{\rm N}$-Mg$_{\rm efficiently act deep carrier traps mediate energy transfer from $4f$-electron core which then leads sharp red intra-$f$ luminescence. Eu-related give rise defect-to-band for example, identified possible source broad blue emission observed n-type, Eu$^{2+}$-containing GaN. work calls re-assessment certain assumptions regarding specific configurations previously made Eu-doped further investigation origin photoluminescence hysteresis (Eu,Mg)-doped samples.