Unusual charge states and lattice sites of Fe in Al_xGa_1-x N:Mn

Charge states and lattice sites of Fe ions in virgin and Mn-doped AlxGa1−xN samples were investigated using 57Fe emission Mössbauer spectroscopy following radioactive 57Mn+ ion implantation at ISOLDE, CERN. In the undoped AlxGa1−xN, Fe2+ on Al/Ga sites associated with nitrogen vacancies and Fe3+ on substitutional Al/Ga sites are identified. With Mn doping, the contribution of Fe3+ is considerably reduced and replaced instead by a corresponding emergence of a single-line-like component consistent with Fe4+ on Al/Ga sites. Density functional theory calculations confirm the Fe4+ charge state as stabilised by the presence of substitutional Mn2+ in its vicinity. The completely filled spin up orbitals in Mn2+ (3d5) are expected to enhance magnetic exchange interactions. The population of the Fe4+ state is less pronounced at high Al concentration in AlxGa1−xN: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 Fe4+ and Mn2+, 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