Variable range hopping transport in ferromagnetic GaGdN epitaxial layers

Electrical-transport properties of ferromagnetic GaGdN layers grown by molecular-beam epitaxy on highly resistive 6H-SiC(0001) substrates have been investigated. It is found that doping with low concentrations of Gd increases the resistivity by several orders of magnitude as compared to unintentionally doped GaN. In the measurable temperature range between 5 and 120 K two different temperature dependences of the resistivity (T^-1/2 and T^-1/4) are observed, both of which are characteristic of variable-range-hopping conductivity in an impurity band of localized states. The experimental evidence, that room-temperature ferromagnetism and impurity-band hopping transport are coexistent, is discussed in connection with recent models proposing defect-induced magnetism in wide-band-gap III-Nitrides.