Small non-uniform basal crystal fields in HVPE free-standing GaN:Mg as evidenced by angular dependent and frequency-dependent EPR.

We studied thin-film and free-standing Mg-doped GaN using multi-frequency electron paramagnetic resonance (EPR) at 3-3.5 K and 9.4-130 GHz. Free-standing samples exhibit a highly anisotropic intensity, varying by a factor of 20 from 0<sup>o</sup> to 60<sup>o</sup>. In contrast, the intensity of the thin-film samples is significantly more isotropic, varying by no more than 10% over the same range of angles. The angular dependent intensity can be modeled in both free-standing and thin-film samples similarly to the g-factor anisotropy reported for thin films, supporting the theoretical predictions that the hole is on a basal site around the Mg acceptor. In addition, frequency-dependent transmission EPR measurements reveal a distribution of g<sub>⊥</sub> in free-standing samples, indicating that the local basal crystal field is non-uniform.