Strain Driven Growth of Zinc Oxide Nanowires on Sapphire: Controlling Horizontal vs. Standing Growth
B. Nikoobakht, S. Eustis
National Institute of Standards and Technology, US
Keywords: ZnO, Horizontal Nanowire, Cross-section, TEM, Strain
Abstract:Recently we showed large scale fabrication of field-effect transistors from horizontal ZnO nanowires (NWs) on a-plane sapphire. We are interested in understanding the factors that induce the unique growth direction of horizontal ZnO NWs and utilizing them for diversifying the horizontal growth to other semiconductors. Here, we present a high resolution transmission electron microscopy study on cross-sections of horizontal NWs on sapphire. Results show how horizontally grown ZnO NWs influence their underlying sapphire surface and how sapphire determines the growth directionality of the NWs. The growth direction of NWs on the sapphire crystal is the path which produces the lowest lattice mismatch. Analyses of the NW interface with sapphire show that single crystal nanowires grow epitaxially and semicoherently with much fewer misfit dislocations than theoretically expected. We attribute the formation of fewer dislocations at the interface to local relaxation of ZnO strain into sapphire surface. We also define a critical nanowire thickness beyond which the growth mode changes from horizontal to standing. Above the critical thickness, gold preferentially wets the ZnO nanocrystal and formation of misfit dislocations becomes energetically favorable. Combination of these two effects results in the change of the observed growth modes.