| The influence of AlN nucleation layer (NL) growth temperature and time on the structural and optical properties of AlN layer deposited on the (0001) sapphire by metal organic chemical vapor deposition (MOCVD) has been examined by X-ray diffraction (XRD), atomic force microscopy (AFM), transmission electron microscopy (TEM) and photoluminescence (PL). The crystal quality and polarity of AlN layers are strongly dependent on the surface morphology and thickness of the NL, which in turn depend on the deposition time and temperature. The narrowest X-ray rocking curve and flat surface are obtained when AlN NLs are grown at 510°C for 30 seconds. Under these conditions, We are able to obtain the lowest (0002) FWHM of 251 arc seconds, which is less than the 330 arc seconds reported previously for best AlN films. AFM shows that the surface morphology of overlayer improves as the roughness of the NL is decreased. AlN layer grown on the optimal NL has very low density of inversion domain (ID)s, whereas other films have high density of IDs. Cross-sectional TEM images reveal that AlN deposited on the smooth NL surface has a smooth over-layer surface. Conversely, AlN deposited on the rough NL surface has a rough over-layer surface and high ID density. Rough NLs have higher density of islands. Islands have (0001) flat top surfaces and pyramidal side facets which are composed of {lcub}1102{rcub} planes. Since wurtzite structure is non-centro symmetric, each plane is Al or N-terminated. ID boundaries originate from the coalescence of growths from Al and N-polar side facets. The origins of threading dislocation (TD)s in AlN layers have also been investigated. Results show that the majority of TDs do not form during the coalescence of islands. Instead, the growth faults in NL are identified a possible source. AlN layers with high IDs show better optical property. PL results show that the exciton peak is stronger in the case of high ID density AlN films. The inversion domains in the mixed polar films may confine carriers and thus enhance their recombination, resulting in increased luminescence. |
