Spin resonance investigations of Mn2+ in wurtzite GaN and AlN films -: art. no. 165215

High quality Mn-doped GaN and AlN films grown by molecular beam epitaxy have been investigated with X-band electron spin resonance (ESR). The observed resonance patterns are well described by the spin Hamiltonian for isolated Mn-55(2+) centers with electronic spin S = 5/2 and nuclear spin I = 5/2. Isotropic g factors g = 2.000 and hyperfine parameters A = -69 Gxgmu(B) are observed both in GaN and AlN, while the fine-structure parameters vary from D-GaN = -218 Gxgmu(B) for strongly strained GaN films to D-GaN = -236 Gxgmu(B) for almost relaxed GaN films, and to D-AlN = -648 Gxgmu(B) for relaxed AlN films. At intermediate orientations of the crystalline c axis with respect to the magnetic field, intermixing occurs between the nuclear spin eigenstates due to off-diagonal elements in the spin Hamiltonian, which strongly enhances the transition probabilities of usually forbidden ESR transitions with \Deltam(I)\>0. This is confirmed experimentally as well as by numerical simulations. It is concluded that Mn2+ impurities are present as isolated, paramagnetic centers at the investigated doping concentration of 10(20) cm(-3), without any evidence for electrostatic or magnetic coupling to extended valence band states, which is a prerequisite of ferromagnetic exchange required for spintronic devices.