Improved surface morphology in GaN homoepitaxy by NH3-source molecular-beam epitaxy

GaN homoepitaxial layers of improved surface morphology were obtained by a NH3-source molecular-beam epitaxy method supplying a proper reactive NH3-to-Ga flux ratio (V/III ratio) on the growing surface, combined with a procedure to prevent the surface roughening of the GaN/(0001) Al2O3 epitaxial templates, which were prepared by metalorganic vapor-phase epitaxy. In situ monitoring on the heated templates revealed their thermal decomposition above 700 °C in ultrahigh vacuum, which gave rise to surface unevenness of both initially atomically flat templates and the homoepitaxial layers. The surface flatness was maintained by depositing the “flux-modulated” GaN prior to the high-temperature growth, where Ga flux was supplied intermittently during heating the template under continuous NH3 flow. The increase in V/III ratio greatly improved the surface flatness. As a result of the epilayer/template interface control and growth optimization, the epilayers exhibited higher mobility, smaller full width at half-maximum value (34 meV) of excitonic photoluminescence (PL) peak, and longer PL lifetime of 180 ps at 293 K.