Unusual charge states and lattice sites of Fe in Alx Ga1-x N:Mn

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