Spin resonance investigations ofMn2+in wurtzite GaN and AlN films

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 ${}^{55}{\mathrm{Mn}}^{2+}$ centers with electronic spin $S=5/2\mathrm{}$ and nuclear spin $I=5/2.\mathrm{}$ Isotropic g factors $g=2.000\mathrm{}$ and hyperfine parameters $A=-69\mathrm{G}\times g{\mu }_B$ are observed both in GaN and AlN, while the fine-structure parameters vary from $D_{\mathrm{GaN}}=-218\mathrm{G}\times g{\mu }_B$ for strongly strained GaN films to $D_{\mathrm{GaN}}=-236\mathrm{G}\times g{\mu }_B$ for almost relaxed GaN films, and to $D_{\mathrm{AlN}}=-648\mathrm{G}\times g{\mu }_B$ 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 $|\Delta m_I|>0.\mathrm{}$ This is confirmed experimentally as well as by numerical simulations. It is concluded that ${\mathrm{Mn}}^{2+}$ impurities are present as isolated, paramagnetic centers at the investigated doping concentration of ${10}^{20}{\mathrm{cm}}^{-3},$ without any evidence for electrostatic or magnetic coupling to extended valence band states, which is a prerequisite of ferromagnetic exchange required for spintronic devices.