Impacts of anisotropic lattice relaxation on crystal mosaicity and luminescence spectra of m-plane Al_xGa_1-xN films grown on m-plane freestanding GaN substrates by NH₃ source molecular beam epitaxy

In-plane anisotropic lattice relaxation was correlated with the crystal mosaicity and luminescence spectra for m -plane Al_xGa_1-xN films grown on a freestanding GaN substrate by NH₃-source molecular beam epitaxy. The homoepitaxial GaN film exhibited A - and B -excitonic emissions at 8 K, which obeyed the polarization selection rules. For Al _xGa_1-xN overlayers, the m -plane tilt mosaic along c -axis was the same as the substrate as far as coherent growth was maintained (x≤0.25). However, it became more severe than along the a -axis for lattice-relaxed films (x≥0.52). The results are explained in terms of anisotropic lattice and thermal mismatches between the film and the substrate. Nonetheless, all the Al_xGa_1-xN films exhibited a near-band-edge emission peak and considerably weak deep emission at room temperature.