Spin resonance investigations of Mn²⁺ in wurtzite GaN and AIN films

High quality Mn-doped GaN and A1N 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 ⁵⁵Mn²⁺ centers with electronic spin S = 5/2 and nuclear spin I = 5/2. Isotropic g factors g = 2.000 and hyperfine parameters A = -69 G×gμ_B are observed both in GaN and A1N, while the fine-structure parameters vary from D_GaN = -218 G×gμ_B for strongly strained GaN films to D_GAN = -236 G ×gμB for almost relaxed GaN films, and to D_AIN = 648 G×gμ_B for relaxed AIN 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 |Δm₁|>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 10²⁰ 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.