Prospects for carrier-mediated ferromagnetism in GaN

Theoretical predictions of room-temperature ferromagnetism in Mn-doped GaN and other wide band gap semiconductors suggest that these materials might be useful for spintronic applications. In this short review, we summarize recent observations on the gap states of GaN: Mn, which make it impossible that the two main prerequisites of these predictions can be fulfilled at the same time, which are (1) a large concentration of localized Mn2+ spins coexisting with (2) a high density of free holes in the valence band. Such conditions have been observed in only a few materials like e.g. GaAs:Mn. More typically, transition-metal impurities act as traps for free carriers, thus pinning the Fermi level in the semiconductor band gap far from the valence or conduction band. Alternatively to ferromagnetism mediated by free carriers, the interactions between bound magnetic polarons and the double-exchange mechanism have been suggested to possibly lead to ferromagnetism in GaN: Mn. Because of the energy position and the character of its gap states, Mn seems to be rather unsuitable for these two mechanisms. Better candidates would be GaN: Fe: Mg for a system of magnetic polarons, and GaN: Cr for a double-exchange ferromagnet. Because of its short-range nature, the double-exchange interaction requires rather concentrated alloys and does not offer significant advantages of GaN: Mn over the established ferromagnetic materials. However, microscopic ferromagnetic inclusions observed in many SQUID measurements of GaN: Mn could possibly help to achieve spin injection in nitride semiconductors. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim