Strain Driven Growth of Zinc Oxide Nanowires on Sapphire: Controlling Horizontal vs. Standing Growth

Recently we showed large scale fabrication of field-effect transistors from horizontal ZnO nanowires (NWs) on a-plane sapphire. In growth of horizontal ZnO NWs, the substrate experiences a compressive strain of approximate to 7% in its [0001](sap) direction (along the width of a NW) to minimize its lattice mismatch with the ZnO NW. Accordingly, ZnO expands along its width to improve its lattice match with the sapphire. Although this system is a highly mismatched one, our results show that horizontal ZnO nanowires grow semi coherently with much fewer misfit dislocations than theoretically expected. We attribute the formation of fewer dislocations to partial strain relaxation of zinc oxide NW to sapphire surface. A critical nanowire thickness is defined beyond which misfit energy largely relaxes to dislocations, changing the growth mode from horizontal to standing.