Unusual charge states and lattice sites of Fe in Al$_{x}$Ga$_{1−x}$N:Mn

Charge states and lattice sites of Fe ions in virgin and Mn-doped Al$_{x}$Ga$_{1−x}$N samples were investigated using $^{57}$Fe emission Mössbauer spectroscopy following radioactive $^{57}$Mn$^{+}$ ion implantation at ISOLDE, CERN. In the undoped Al$_{x}$Ga$_{1−x}$N, Fe$^{2+}$ on Al/Ga sites associated with nitrogen vacancies and Fe$^{3+}$ on substitutional Al/Ga sites are identified. With Mn doping, the contribution of Fe$^{3+}$ is considerably reduced and replaced instead by a corresponding emergence of a single-line-like component consistent with Fe$^{4+}$ on Al/Ga sites. Density functional theory calculations confirm the Fe$^{4+}$ charge state as stabilised by the presence of substitutional Mn$^{2+}$ in its vicinity. The completely filled spin up orbitals in Mn$^{2+}$ (3d$^{5}$) are expected to enhance magnetic exchange interactions. The population of the Fe$^{4+}$ state is less pronounced at high Al concentration in Al$_{x}$Ga$_{1−x}$N:Mn, a behaviour attributable to hybridisation effects of 3d states to the semiconductor bands which weakens with increasing (decreasing) Al (Ga) content. Our results demonstrate that co-doping promotes the co-existence of unusual charge states of Fe$^{4+}$ and Mn$^{2+}$, whereas their trivalent charge states prevail with either transition metal incorporated independently in III-nitrides. Co-doping thus opens up a new avenue for tailoring novel magnetic properties in doped semiconductors.