Porosity Control for Plasma-Assisted Molecular Beam Epitaxy of GaN Nanowires

We report on the bottom-up fabrication, by plasma-assisted molecular beam epitaxy, of monocrystalline GaN solid, hollow, and c-shape nanowires deposited in a compact fashion. The shape exhibited by these nanostructures varies from solid to c-shape and hollow nanowires. They were epitaxially grown with their [0001] directions perpendicular with respect to different surfaces of Si substrates. Advanced studies of these GaN nanostructures were carried out by means of selected-area electron diffraction and scanning and high-resolution transmission electron microscopy evidencing their structure and epitaxial alignments with respect to the silicon. Through a comprehensive analysis of the growth conditions (substrate temperature and Ga and N* fluxes) we demonstrate that a local Ga-limited regime is the mechanism behind the particular shape of these nanostructures. Additionally, spectroscopic ellipsometry studies, applying a model based on Bruggeman effective medium approximations and taking into account several aspects related to the nature of these GaN nanostructures, were carried out to obtain valuable information about the evolution of the optical constants and the porosity along the layer. This work shows a way to control the porosity and shape of GaN nanowires by varying the growth conditions, which could open new horizons in the development of GaN nanostructures for future applications.