Papers by Author: Min Su Yi

Abstract: The crystallization of α-Fe2O3/α-Al2O3(0001) thin films has been studied using real-time synchrotron x-ray scattering and atomic force microscope. In the sputter-grown amorphous films of various thicknesses at room temperature, we find the coexistence of α-Fe2O3 and Fe3O4 interfacial crystallites (~50-Å-thick), well aligned to the α-Al2O3[0001] direction. The amorphous precursor is crystallized to the epitaxial α-Fe2O3 grains in three steps with annealing temperature; i) the growth of the well aligned α-Fe2O3 interfacial crystallites to approximately 200-Å-thick, together with the transformation of the Fe3O4 crystallites to the α-Fe2O3 crystallites (< 400°C), ii) the growth of the less aligned α-Fe2O3 grains on top of the well aligned grains (> 400°C), and iii) the nucleation of the different less aligned α-Fe2O3 grains directly on the α-Al2O3 substrate (> 600°C). The surface evolution of the amorphous precursor films after annealing is consistent with the microstructure evolution during the crystallization.

Abstract: ZnO nanowalls and nanocolumns were synthesized on Si3N4 (50 nm)/Si (001) substrates at low growth temperature (350 and 400 oC) by metalorganic chemical vapor deposition (MOCVD) with no metal catalysts. ZnO nanowalls with extremely small wall thicknesses below 10 nm and nanocolumns with diameters over 100 nm were formed on the Si3N4/Si substrates relying on MOCVD-growth temperature. It was found that ZnO nanowalls have a strong c-axis preferred orientation with a hexagonal structure, while ZnO nanocolumns have a weak c-axis preferred orientation with broken stacking orders in synchrotron x-ray scattering experiments. In addition, strong free-exciton emission from the ZnO nanowalls was clearly observed in photoluminescence measurements. On the other hand, we could not observe any emission bands from the ZnO nanocolumn samples.

Abstract: The initial GaN growth mode on stepped sapphires by plasma enhanced metal organic molecular beam epitaxy (PEMOMBE) has been analyzed using in-situ, real time synchrotron x-ray diffraction and x-ray absorption. The sapphire substrate annealed at high temperature had flat terraces and regular atomic steps. The crystal quality and the vicinal angle of sapphire substrate had an effect on the width of terraces and the step arrangement. The initial growth mode of the GaN film on the regular atomic step (AS) surface was the layer-by-layer mode and changed to the 3D growth mode within 2 bilayer thickness. In the meanwhile, the growth mode of the GaN film grown on the sapphire with random roughness (RR) surface made the flat surface in the early stage and changed the 3D growth mode. As increasing the film thickness, the nucleation layer grows strain-free hexagonal GaN on stepped sapphires