Polytype transition of N-face GaN:Mg from wurtzite to zinc-blende

We have investigated the polytype conversion of a GaN film from N-face wurtzite (2H-) to zinc-blende (3C-) structure due to Mg doping during growth by plasma-assisted molecular-beam epitaxy. Structural analysis by high-resolution transmission electron microscopy and high-resolution x-ray diffraction measurement revealed alignment of the cubic phase with the [111] axis perpendicular to the substrate surface. The optical characteristics of GaN:Mg layers are shown to be very sensitive to the presence of the cubic polytype. For low Mg doping, photoluminescence is dominated by a phonon-replicated donor-acceptor pair at ∼3.25 eV, related to the shallow Mg acceptor level, accompanied by a narrow excitonic emission. For high Mg doping, the photoluminescence spectra are also dominated by a line around 3.25 eV, but this emission displays the behavior of excitonic luminescence from cubic GaN. A cubic-related donor-acceptor transition at ∼3.16 eV is also observed, together with a broad blue band around 2.9 eV, previously reported in heavily Mg-doped 3C-GaN(001)