Geometrical selection of GaN nanowires grown by plas-ma-assisted MBE on polycrystalline ZrN layers

The results contained in the repository show that unidirectional supply of material fluxes in molecular beam epitaxy (MBE) together with the shadow effect favor growth of well-oriented NWs even despite random orientation of grains to which the NWs are epitaxially linked [1]. We propose a geometrical model describing an impact of MBE chamber geometry on the orientation of the NWs. Specifically, we calculated effective area of the nanowire’s top facet Seff that is in the line of sight of an effusion cell as a function of nanowire tilt relative to substrate normal (α) and substrate rotation angle (ω). The Seff value is equivalent to the number of atoms incorporated on the NW top facet, which controls axial growth rate of the NW if adatom surface diffusion from the NW sidewalls is negligible, as assumed in the model. For example, this is the case of metal-limited MBE growth of nitride nanowires. As expected, the maximum of Seff corresponds to the NW facing directly in the direction of the Ga effusion cell (α = 40º, ω = 0º). However, during rotation of the substrate the fastest growing nanowires are those perpendicular to the substrate. To verify the model we grew GaN NWs on ZrN polycrystalline buffers. Without substrate rotation the NWs grew uniformly arranged at α = 40º, ω = 0º, exactly as predicted by our model. However, standard growth with rotation of the substrate led to densely packed NWs with significantly limited tilt dispersion. We attribute this behavior to geometrical selection of NWs in densely packed NW array via the shadowing of strongly tilted NWs by preferentially grown vertical ones. A description of the results can be found in the file "readme.txt".