Selectively grown GaN nanowalls and nanogrids for photocatalysis: growth and optical properties.

In this work, the selective area growth of GaN nanowalls and nanogrids on sapphire and GaN on sapphire by molecular beam epitaxy is investigated. We demonstrate the fabrication of homogeneous GaN nanowall arrays with different widths, distances and specific crystallographic side facets. Photoluminescence spectroscopy of as-grown GaN nanowalls reveals a high crystal quality and low defect density. Moreover, a distinct dependence of the nanowall width and the intensity of the donor-bound exciton emission on the crystal orientation of the sidewall facets is found and explained by different surface states for a-plane and m-plane GaN. The waveguide character of the GaN nanowalls, given by the large refractive index of GaN and the subwavelength size of the structures, is analysed by experimental transmission measurements and numerical simulations. Our results and the high epitaxial control achieved by selective area growth show the potential of tailor-made nanowall-based devices, e.g., in photocatalysis or nanofluidics.