Excitonic transitions in MBE grown h-GaN with cubic inclusions

Undoped and magnesium doped MBE grown GaN epilayers on sapphire substrates show a particular variety of near-bandgap luminescent transitions. Despite the large lattice mismatch to the substrate, pronounced free- and bound-exciton transitions allow for an estimation of the excitonic binding energies. For the given thickness range (about 1 μm), we find an almost strain-relaxed situation with the main exciton transition energies well corresponding to the bulk values. On their low-energy side, we identify lines having been tentatively assigned to stacking fault excitons, and interface-related exciton transitions correlated to extended defects and/or dislocations in this spatial region. Evidence of cubic inclusions of a size up to 500 nm is doubtless given by observing sharp C-GaN related donor-bound exciton emission and respective structures in transmission electron microscope investigations.