Morphological and Microstructural Evolution and Related Impurity Incorporation in Non-Polar a-Plane GaN Grown on r-Sapphire Substrates

Effects of the growth temperature on morphological and microstructural evolution of a-plane GaN films grown on r-plane sapphires by metal organic chemical vapor deposition are investigated by atomic force microscopy and secondary ion mass spectroscopy （SIMS）. Surface morphology, structural quality and related impurity incorpora- tion are very sensitive to the growth temperature. A significant difference of yellow luminescence is observed and attributed to the incorporation of carbon into GaN films, which is confirmed by SIMS analysis. Our results show that the sample with triangular-pit morphology has sample with pentagon-like pit morphology, which is significantly higher concentrations of oxygen than the other induced by the existence of an N-face in triangular pits.

Surface Carbonization of GaN and the Related Structure Evolution during the Annealing Process

To explain the stabilization mechanism of the carbon-ion-implanted GaN under the diamond growth environment,the luminescence characteristics and structure evolution correlative with sites＇ carbon atoms located for highfluence carbon-ion-implanted Ga N are discussed. GaN is implanted with carbon ion using fluence of 2×10^（17） cm^（-2） and energy of 45 keV. Then the implanted samples are annealed at 800℃ for 20 min and 1 h under the N_2 atmosphere. The luminescence characteristics of carbon-ion-implanted GaN are evaluated by photoluminescence spectrum at wavelength 325 nm. The lattice damage of Ga N is characterized by Raman spectrum and the corresponding vacancy-defect evolution before and after annealing is measured by slow positron annihilation. The results show that most of the carbon atoms will be located at the interstitial sites after carbon ion implantation due to the weak mobility. As the implanted samples are annealed, strong yellow luminescence is emitted and the vacancies for Ga（V_（Ga）） are reduced resulting from the migration of interstitial carbon（C_i） and formation of complexes（CGaand/or C_（Ga）-C_i） between them. As the annealing time is prolonged, the carbon ions accommodated by the vacancies are saturated, vacancy clusters with carbon atoms appear and the concentration of C_（Ga） diminishes, which will have an adverse effect on the diamond film nucleation and growth.

EVOLUTIONAL FEATURES OF INTERANNUAL LOW-FREQUENCY OSCILLATIONS AND THEIR RELATION TO THE OCCURRENCE OF EL NINO

A simplified data set with 8°×8° grid system in a region of 32°S--32°N from 1951 to 1979 for the elements of sea surface temperature (SST), zonal wind at sea level (U), sea level pressure (SLP) and total cloud amount (CA) is made from the COADS. The oscillation components with periods of 2 years (QBO), 3.5 years (SO) and 5.5 years (FYO) in interannual low-frequency oscillations have been studied by using the methods of extended EOF (EEOF) and lag correlation analysis with the oscillational components of SST in the equator of eastern Pacific as the reference element. In our paper, the relationship between oscilla- tion components and occurrence of El Nino is also investigated.