Controlling the Growth Direction of ZnO Nanowires on c-Plane Sapphire (original) (raw)
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Epitaxial growth of ZnO nanowires on a- and c-plane sapphire
Journal of Crystal Growth, 2005
Epitaxial ZnO nanowires were grown on a-and c-plane sapphire substrates by metalorganic chemical vapor deposition without metal catalysts or templates. Nanowires with monodisperse diameters grow in dense arrays perpendicular to a-plane sapphire and with in-plane rotational alignment due to [0 0 0 1] ZnO J[1 12 0] sapphire , ½1 12 0 ZnO ½0 0 0 1 sapphire epitaxy. On c-plane sapphire, multiple possible epitaxial relations give a mixture of nanowire orientations. The majority of the nanowires grow in one of the three directions all at an angle of 51.81 off the substrate plane with [0 0 0 1] ZnO J[1 01 4] sapphire , ½1 01 0 ZnO J[12 1 0] sapphire epitaxy. A small fraction of the nanowires grow perpendicular to the substrate with [0 0 0 1] ZnO J[0 0 0 1] sapphire .
Journal of Physical Chemistry C, 2009
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 (11 j 00) 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.
Growth of Vertically Aligned ZnO Nanowire Arrays Using Bilayered Metal Catalysts
Journal of Nanomaterials, 2012
Vertically aligned, high-density ZnO nanowires (NWs) were grown for the first time on c-plane sapphire using binary alloys of Ni/Au or Cu/Au as the catalyst. The growth was performed under argon gas flow and involved the vapor-liquid-solid (VLS) growth process. We have investigated various ratios of catalyst components for the NWs growth and results indicate that very thin adhesion layers of Ni or Cu deposited prior to the Au layer are not deleterious to the ZnO NW array growth. Significant improvement of the Au adhesion on the substrate was noted, opening the potential for direct catalyst patterning of Au and subsequent NW array growth. Additionally, we found that an increase of in thickness of the Cu adhesion layer results in the simultaneous growth of NWs and nanoplates (NPs), indicating that in this case the growth involves both the VLS and vapor-solid (VS) growth mechanisms. Energy dispersive X-ray spectroscopy (EDX) and surface-enhanced Raman scattering (SERS) studies were als...
Vertically Well-Aligned ZnO Nanowires on c-Al2O3 and GaN Substrates by Au Catalyst
ETRI Journal, 2006
Hyun-Kyu Park et al. 787 ABSTRACT⎯In this letter, we report that vertically wellaligned ZnO nanowires were grown on GaN epilayers and cplane sapphire via a vapor-liquid-solid process by introducing a 3 nm Au thin film as a catalyst. In our experiments, epitaxially grown ZnO nanowires on Au-coated GaN were vertically wellaligned, while nanowires normally tilted from the surface when grown on Au-coated c-Al 2 O 3 substrates. However, pre-growth annealing of the Au thin layer on c-Al 2 O 3 resulted in the growth of well-aligned nanowires in a normal surface direction. Highresolution transmission electron microscopy measurements showed that the grown nanowires have a hexagonal c-axis orientation with a single-crystalline structure.
Vertical ZnO nanowire growth on metal substrates
Nanotechnology, 2012
Vertical growth of ZnO nanowires is usually achieved on lattice-matched substrates such as ZnO or sapphire using various vapor transport techniques. Accomplishing this on silicon substrates requires thick ZnO buffer layers. Here we demonstrate growth of vertical ZnO nanowires on FeCrAl substrates. The pre-annealing prior to growth appears to preferentially segregate Al and O to the surface, thus leading to a self-forming, thin pseudo-buffer layer, which then results in vertical nanowire growth as on sapphire substrates. Metal substrates are more suitable and cheaper than others for applications in piezoelectric devices, and thin self-forming layers can also reduce interfacial resistance to electrical and thermal conduction.
Patterned growth of aligned ZnO nanowire arrays on sapphire and GaN layers
Superlattices and Microstructures, 2004
Patterned growth of vertically aligned ZnO nanowire arrays on the micrometer and nanometer scale on sapphire and GaN epilayers is reported. In order to control the position and distribution density of the ZnO nanowires, Au seeding nanodots are defined, as regular arrays, with the assistance of deposition shadow masks. Electron micrographs reveal that the wires are single crystals having wire axes along the hexagonal c-axes. The epitaxial growth of ZnO nanowires on sapphire and GaN films on Si substrates was further verified by cross sectional electron microscopy investigations. Compared to the sapphire case, the perfect epitaxial growth on a GaN film on a Si substrate is believed to be more suitable for potential electronic device applications of ZnO nanowire arrays.
Growth habits and defects in ZnO nanowires grown on GaN/sapphire substrates
Applied Physics Letters, 2005
Growth habits and defects in epitaxial ZnO nanowires grown from Au catalyst on ͑00.1͒ GaN/ sapphire substrate using the vapor-liquid-solid ͑VLS͒ technique were studied using electron microscopy and x-ray diffraction. The results revealed presence of both horizontal ͑crawling-like͒ and vertical nanowires having similar orientation relationship to the substrate ͑00.1͒ ZnO ʈ ͑00.1͒ GaN , ͓11.0͔ ZnO ʈ ͓11.0͔ GaN . The crawling-like growth precedes the vertical growth, and the coalescence and overgrowth of the crawling nanowires produce a highly defective layer which separates the substrate and vertical nanorods. Transmission electron microscopy revealed a high density of planar defects in this interfacial layer. A significant density of stacking faults residing on the ͑0001͒ planes was also observed in the shorter vertical nanorods. The crawling nanowires are under residual compressive strain, whereas the vertical nanorods grow strain-free.
Journal of Physics and Chemistry of Solids, 2008
Vertically well-aligned ZnO nanowires were synthesized on c -Al 2 O 3 substrates at 950 °C by the vapor-phase epitaxy (VPE) method. An Au thin film with a thickness of 3 nm was used as a catalyst. The growth direction and length of the ZnO nanowires were successfully controlled by adjusting the ramping rate. Tilted nanowires with a shorter length were grown by increasing the ramping rate, while vertically well-aligned nanowires with a longer length were uniformly formed by decreasing the ramping rate. The X-ray diffraction (XRD) and high-resolution transmission electron microscope measurements showed that the vertically well-aligned ZnO nanowires on the c -Al 2 O 3 substrate have a single-crystalline hexagonal structure and preferred c -axis growth orientation.
Journal of Crystal Growth, 2012
We report on the epitaxial growth of ZnO nanosheets and nanowires on a-and c-plane sapphire substrates by Au-assisted pulsed laser deposition. The epitaxial relationship of the nanostructures was determined by x-ray diffraction (XRD) pole figure measurements. On c-plane sapphire, the ZnO nanowires grew along the ZnO c-axis and were inclined to the substrate surface normal with an angle of about 371. The ZnO(0001) plane of the wires aligned with Al 2 O 3 ð1014Þ of the sapphire substrate via two degenerate in-plane configurations, promoted by low lattice mismatch (0.05%). ZnO nanosheets grown on c-plane sapphire exhibited no preferential orientation on the substrate and no epitaxial relationship could be unambiguously identified. On a-plane sapphire, ZnO nanowires grew vertically along the ZnO c-axis with a single epitaxial configuration, whereas ZnO nanosheets seemed to grow along ZnO½1010 in two preferred in-plane orientations, 721-741 apart. These configurations could be explained by two distinct alignments of the ZnOð1011Þ plane on the a-plane sapphire substrate surface, promoted by low lattice mismatches.
Selective growth of vertical ZnO nanowires on ZnO:Ga/Si[sub 3]N[sub 4]/SiO[sub 2]/Si templates
Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 2005
High density vertical single crystal ZnO nanowires were selectively grown on ZnO : Ga/ Si 3 N 4 / SiO 2 / Si templates at various temperatures by a two-step oxygen injection process of self-catalyzed vapor-liquid-solid ͑VLS͒ technology. It was found that tips of the ZnO nanowires are hexagonal. It was also found that average length of the ZnO nanowires increased while the average tip diameter of the ZnO nanowires decreased as the growth temperature increased. Furthermore, it was found that the ZnO nanowires grown at 500°C were "tube-shaped" while the ZnO nanowires grown at 700°C were "cone-shaped." Photoluminescence ͑PL͒, x-ray diffraction ͑XRD͒, and energy depersive x-ray ͑EDX͒ results all indicate that the quality of our ZnO nanowires is good.