Strain-Driven Growth of Zinc Oxide Nanowires on Sapphire: Transition from Horizontal to Standing Growth

Abstract

Recently, we showed large-scale fabrication of field-effect transistors from horizontal ZnO nanowires (NWs) on a-plane sapphire. Here, in examining the cross sections of such nanodevices, we use high-resolution transmission electron microscopy (HRTEM) and large-angle, convergent-beam electron diffraction (LACBED). We show how horizontally grown ZnO NWs influence their underlying sapphire surface and how substrate influences the growth directionality of the NWs. As a NW grows on sapphire, the substrate experiences a compressive strain of ≈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. The growth direction of (11̅00) is suggested to be the direction that produces a lower lattice strain between ZnO and sapphire. Analyses of NW/sapphire interfaces show that single-crystal NWs grow epitaxially and semicoherently with many fewer misfit dislocations than theoretically expected. We attribute the formation of fewer dislocations at the interface to local relaxation of zinc oxide strain into the sapphire surface. This relaxation is in agreement with the observed deformation of the sapphire underneath the NWs. We also define a critical NW thickness beyond which the growth mode changes from horizontal to standing. Results indicate that below this thickness, gold nanodroplets partially wet both sapphire and ZnO crystals. Above the critical thickness, gold preferentially wets the ZnO nanocrystal, and formation of misfit dislocations at the interface becomes energetically favorable. Combination of these two effects is used to explain the observed change in the growth modes of the NWs.

Scalable synthesis and device integration of self-registered one-dimensional zinc oxide nanostructures and related materials

“…5a shows the deformation of the interface due to this strain. 98 This observation opposes the existence of low strain paths along the growth directions according to the model above. 98 In the better matched case of ZnO on GaN, this stress is significantly reduced when viewed from the crosssection (Fig.…”

“…98 This observation opposes the existence of low strain paths along the growth directions according to the model above. 98 In the better matched case of ZnO on GaN, this stress is significantly reduced when viewed from the crosssection (Fig. 5b); however, from top view, when their c-planes are overlapped, no lattice match in any crystallographic direction is observed.…”

Zn2SnO4–SnO2 heterojunction nanocomposites for dye-sensitized solar cells

“…Dislocations were observed at the ZTO-SnO 2 interface to relax the misfit stress of two phases and reduce the interfacial elastic energy. The dislocation formation was widely reported for crystal growth in lattice-mismatched system [23] and alloy evolution under thermal exposure [24].…”

Inclined and ordered ZnO nanowire arrays developed on non-polar ZnO seed-layer films

“…23 For ZnO, the surface free energies are 0.91, 1.64, 1.58 and 1.74 J m 22 for {101 ¯0}, {112 ¯0}, {101 ¯1} and {0001} facets, respectively. 31 Thus, the relationship of growth velocities is V(0001) . V(112 ¯0) .…”