We demonstrate spontaneous growth of a high density of self-organized, oriented, and epitaxial GaN low-dimensional nanostructures on Al 2 O 3 (0001) by sheer kinetic control without involving lithography, catalysts, buffer layers, or any surface pretreatment and consequently reducing process steps. GaN thin films grown by plasma assisted-molecular beam epitaxy (PA-MBE) form wurtzite GaN as a nanowall hexagonal network of flat 2-D films. In a narrow parametric window, 1-D nanocolumns of high density (1 Â 10 8 cm À2 ) with excellent structural and optical properties are observed. The reduced adatom diffusion in the high nitrogen rich conditions is proposed to cause supersaturation and nucleation at edge and screw dislocations, forming nanowalls and nanocolumns.

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We elucidate the atomistic level details of the formation of 1-D GaN nanostructures on Si(001) by molecular beam epitaxy. In a multitechnique study, we understand the epitaxy and role of the unintentional interfacial SiN x layer in determining the shape, structure, and organization of the nanofeatures. The 1-D GaN features are seen to be mfaceted and grow along the Si(111) planes and thus are tilted with a 4-fold symmetry on the cubic Si(100) surface. The interfacial mismatch induced dislocations are shown to provide the nucleation centers for the spiral 1-D growth, while their local density determines their evolution into solid rods, tubes, or c-tubes. The unintentional interfacial nitridation of the substrate forms discontinuous amorphous mounds that provide the epitaxial contact to the substrate and also laterally isolate the rods/tubes, enabling them to grow laterally in their equilibrium hexagonal wurtzite structures. We consolidate the results into a schematic model to unveil the underlying mechanism and demonstrate the subtle relationship between the kinetics of growth and the interfacial properties.

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