In the context of spintronic applications we studied Mn, Fe and Gd doped GaN and ZnO. Samples with different transition metal concentrations and n or p co-doping were investigated by means of optical and magnetic experiments. The results allow us to elucidate the following issues that are of crucial significance on the way to eventually realize a ferromagnetic coupling at room temperature: (1) Which charge states the transition metals are found in, (2) whether and where they from levels within the band gap, (3) the formation of bound states consisting of a hole localized at a transition metal ion in the charge state 2+. We also assess the influence of the transition metal concentration on the structural, electronic and optical properties of GaN and ZnO. The origin of room temperature ferromagnetism of Gd doped GaN is still an unsolved puzzle. In this contribution we present a comprehensive array of experimental results that help to shed light on this issue. While the incorporation of Gd barely affects the structural and electrical properties of GaN, it leads to a magnetization of 20 emu/cm3 as determined by vibrating sample magnetometry.

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1. Cathodoluminescence centres in ZnO