In this dissertation, GaN growth on porous templates by metalorganic chemical vapor deposition (MOCVD) was studied. The motivation of this research is pursuing an effective reduction of defects in GaN by its submicron-scale and nano-scale epitaxial lateral overgrowth (ELO) on these porous templates, which included porous TiN/GaN (P-TiN), imprint lithography patterned Ti/GaN (IL-Ti), carbon-face nano-porous SiC (C-PSC), and silicon-face nano-porous SiC (Si-PSC). The porous TiN/GaN was formed in situ in MOCVD reactor by annealing a Ti-covered GaN seed layer. This simplicity makes the GaN ELO on the P-TiN more cost-efficient than the conventional ELO which requires ex situ photolithography and/or etching. Both the GaN nano-ELO and the GaN micron-ELO could be realized on P-TiN by controlling the GaN nucleation scheme. The reduction efficacy of edge threading dislocation (TD) was ~15 times. The optical characterization indicated that the non-radiative point-defects in GaN grown were reduced significantly on the P-TiN.The imprint lithography patterned Ti/GaN had uniformly distributed submicron Ti pads on GaN seed layer. These Ti pads acted as GaN ELO masks. The TD reduction efficacy of the IL-Ti was only ~2 due to the low coverage of Ti (~25%) on the GaN seed layer and the low pressure (30 Torr) employed during GaN ELO. Even with a small reduction of TDs, the point-defects in GaN were effectively lowered by the IL-Ti. Hydrogen polishing, sacrificial oxidation, and chemical mechanical polishing were employed to remove surface damage on the PSC substrates. Nitrogen-polarity GaN grown on the C-PSC was highly dislocated because the rough surface of C-PSC induced strong misorientation between GaN nucleation islands. The efficacy of Si-PSC on defect reduction primarily depended on the GaN nucleation schemes. A high density of GaN nano-nucleation-islands was required to realize the GaN nano-ELO extensively. With such a nucleation scheme, the GaN grown on Si-PSC had a ~20 times reduction on the density of the mixed and screw TDs compared with control sample. This growth method is promising for effective defect reduction within a small GaN thickness. Reducing the GaN nucleation density further lowered the TD density but also diminished the efficacy of Si-PSC. These results were explained by a growth model based on the mosaic structure of GaN.

In this study, template-based methods are used for the fabrication of various nanostructures such as nandots, nanorods, nanowires, nanotubes, and core-shell structures. Porous alumina membranes were employed as templates and metal nanostructures were synthesized in the templates by electrodeposition. By using lithography techniques, controlled patterned nanostructures were also fabricated on alumina templates. The magnetic properties of the various metal nanostructures were investigated. The pore size, interpore distance, and pore geometry highly affect magnetic properties of nanostructures grown in the templates. Hexagonally ordered porous alumina templates can be fabricated by two-step anodization. The pore diameters and interpore distances were readily controlled by appropriately changing anodization conditions and pore widening time. Alumina templates with various pore geometries were also successfully synthesized by changing applied voltage, increasing and decreasing, during a third anodization step. To understand magnetic properties of nanostructures with different aspect rations in the form of nanodots, nanorods, or nanowires, Fe nanostructures were fabricated in the templates by controlling of electrodeposition times. The coercivity of nanostructures increased with increasing aspect ratio. The anisotropy of the arrays was governed by the shape anisotropy of the magnetic objects with different aspect ratios. nanowires in mild-hard alumina and conventional alumina templates showed distinct differences in the squareness of hysteresis loops and coercivity both as a function of pore structure and magnetic component. Iron oxide nanotubes with a unique inner-surface were also fabricated by an electrodeposition method. β-FeOOH nanotubes were grown in alumina templates and transformed into hematite and magnetite structures during various heating processes. Hematite nanotubes are composed of small nanoparticles less than 20 nm diameters and the hysteresis loops and FC-ZFC curves show superparamagnetic properties without the Morin transition. In the case of magnetite nanotubes, which consist of slightly larger nanoparticles, hysteresis loops show ferromagnetism with weak coercivity at room temperature while FC-ZFC curves exhibit the Verwey transition at 125 K. For the patterning of nanowires, lithography techniques including nanosphere lithography and e-beam lithography were used. Nanosphere lithography used self-assembled PS spheres as a mask creates holes between spheres and the size of the holes is determined by the size and geometry of ordered PS spheres on the templates. This method can grow patterned nanowires arrays and also produce unique cup-shaped nanostructures with sizes ranging from micrometer down to several nanometers. E-beam lithography was also combined with template-based electrodeposition. Of these two lithographic methods, this one is the most powerful in the fabrication of patterned nanostructures with high aspect ratios. Various features and the sizes of patterned structures can be readily controlled. By the directing the pore diameters and interpore distances of the alumina template, the size and number of patterned nanowires are also adjustable.