Horizontal Nanowires Research Articles

Kelvin probe force microscopy for characterizing doped semiconductors for future sensor applications in nano- and biotechnology

Kelvin probe force microscopy (KPFM) is one of the most promising non-contact electrical nanometrology techniques to characterize doped semiconductors. By applying a recently introduced explanation of measured KPFM signals, we show the applicability of KPFM to determine and control surface-near electrostatic forces in planar doped silicon and in doped silicon nanostructures. Surface-near electrostatic forces may be used for the immobilization of nano- and biomaterials in future sensor applications in nano- and biotechnology. Additionally, the influence of the electrostatic potential distribution in doped semiconductor nanostructures, e.g. in horizontal Si nanowires, and its influence on the surface-near electrostatic forces are discussed. It is explained how drift and diffusion of injected electrons and holes in intrinsic electric fields influence the detected KPFM signal. For example KPFM is successfully employed to locate p+p and n+p junctions along B-doped and As-doped p-Si nanowires, respectively. As an outlook the physical immobilization and the transport of biomaterials above arrays of separately addressable doped semiconductor cells will be discussed.

Three-dimensionally kinked high-conducting CoGe nanowire growth induced by rotational twinning

We have synthesized single-crystalline horizontal and free-standing monoclinic CoGe nanowire (NW) arrays on high-k dielectric Y-stabilized ZrO2 (110) substrates via a chemical vapor transport process without using any catalysts. Horizontal NWs are grown epitaxially on the substrate. Three-dimensionally (3D)-kinked NWs are grown from the tip of the horizontal NWs homoepitaxially initiated by rotational twinning. Electrical measurements show that both horizontal and 3D-kinked CoGe NWs have low resistivity. The 3D-kinked NWs as well as free-standing metallic CoGe NWs integrated on Y-stabilized ZrO2 substrates could find applications as effective on-chip interconnects and nanoelectrodes for highly integrated nanoelectronic devices and as platforms for fuel cells and as efficient catalysts.

Step-edge induced ordered growth: targeting to assemble super long horizontal nanowire alignment in large-scale

Nowadays, the development of nano-synthesis has turned to controllable design for specific demands in micro-nano device application, to be integrated into functional units more conveniently with low-cost and efficiency principles. In this case, an appropriate approach for directly obtaining horizontally aligned nanowires in a large scale would be of great significance in future micro-nano device integration. Here, on the HOPG surface, we managed to achieve this. The approach is versatile to various kinds of materials. Horizontally aligned nanowires of Al-C based materials, such as Al4C3 and Al4O4C, were achieved. All of the nanowires exhibit a high degree ordered alignments and possess super aspect ratios with uniform widths of about 100 nm and lengths on the millimeter level. We believe the assembly mechanism lies in a step-edge induced ordered growth process, through which quaternary Al-Si-O-C nanoball alignment could also be obtained. It is expected that this method could be beneficial to adjust many useful materials for micro device integration in the future.

Near-ultraviolet zinc oxide nanowire sensor using low temperature hydrothermal growth

Two near-ultraviolet (UV) sensors based on solution-grown zinc oxide (ZnO) nanowires (NWs) which are only sensitive to photo-excitation at or below 400 nm wavelength have been fabricated and characterized. Both devices keep all processing steps, including nanowire growth, under 100 °C for compatibility with a wide variety of substrates. The first device type uses a single optical lithography step process to allow simultaneous in situ horizontal NW growth from solution and creation of symmetric ohmic contacts to the nanowires. The second device type uses a two-mask optical lithography process to create asymmetric ohmic and Schottky contacts. For the symmetric ohmic contacts, at a voltage bias of 1 V across the device, we observed a 29-fold increase in current in comparison to dark current when the NWs were photo-excited by a 400 nm light-emitting diode (LED) at 0.15 mW cm−2 with a relaxation time constant (τ) ranging from 50 to 555 s. For the asymmetric ohmic and Schottky contacts under 400 nm excitation, τ is measured between 0.5 and 1.4 s over varying time internals, which is ∼2 orders of magnitude faster than the devices using symmetric ohmic contacts.

Guided Growth of Horizontal ZnO Nanowires with Controlled Orientations on Flat and Faceted Sapphire Surfaces

The large-scale integration of nanowires into practical devices is hindered by the limited ability to controllably assemble these nanoscale objects on surfaces. Following our first report on the guided growth of millimeter-long horizontal nanowires with controlled orientations, here we demonstrate the generality of the guided growth approach by extending it from GaN nanowires to ZnO nanowires. We describe the guided growth of horizontally aligned ZnO nanowires with controlled crystallographic orientations on eight different planes of sapphire, including both flat and faceted surfaces. The growth directions, crystallographic orientation, and faceting of the nanowires are constant for each surface plane and are determined by their epitaxial relation with the substrate, as well as by a graphoepitaxial effect that guides their growth along surface steps and grooves. These horizontal ZnO nanowires exhibit optical and electronic properties comparable to those of vertically grown nanowires, indicating a low concentration of defects. While the guided growth of ZnO nanowires described here resembles the guided growth of GaN nanowires in its general aspects, it also displays notable differences and qualitatively new phenomena, such as the controlled growth of nanowires with vicinal orientations, longitudinal grain boundaries, and thickness-dependent orientations. This article proves the generality of the guided growth phenomenon, which enables us to create highly controlled nanowire structures and arrays with potential applications not available by other means.

A Monte Carlo simulation study on growth mechanism of horizontal nanowires on crystal surface

With the kinetic Monte Carlo simulation of smooth (001) surface of symmetry-broken simple cubic crystal in fluid with low supersatuaration rate, the mechanism of nanowire growth based on crystal nuclei on the surface is discovered and the morphology of nanowire is obtained. The dependences of nanowire morphology on thermal roughness in the longitudinal and latitudinal direction and growth time on the anisotropic surface of the crystal are further discussed. The relations of nanowire growth rate with thermal roughness, supersaturation rate, surface size and diffusion rate on the surface are then systematically studied.

Fabrication of horizontal silicon nanowire arrays on insulator by ion irradiation

We report a simple and potentially mass productive technique to fabricate horizontal single crystalline Si nanowire arrays on insulating substrate based on a self-organized pattern formation mechanism during Xe+ ion beam irradiation of Si-on-insulator material. A periodic ripple surface pattern is created by ion irradiation at 67o incidence angle to the surface normal. The transfer of this pattern to the oxide interface results in an array of electrically disconnected parallel ordered Si nanowires on the insulating oxide. Doping of the nanowires was demonstrated by boron ion implantation and annealing. The morphology and resistivity of the narrow nanowires with large aspect ratio were analysed by cross sectional transmission electron microscopy and scanning spreading resistance microscopy, respectively. Physical reasons of the observed low carrier activation are discussed.

High responsivity ZnO nanowires based UV detector fabricated by the dielectrophoresis method

ZnO nanowires UV photodetectors with different interdigital electrode distances were fabricated by using a dielectrophoresis method in this work. The multiple horizontal nanowires array integrated that photodetectors were composed by lots of paralleled ZnO nanowires. Experimental results showed the responsivity of the detector with the electrode distance of 6.5μm could reach 40A/W at 10V bias. It was also observed that the rising and decaying stages of the time-resolve photocurrent were both two processes, which was possibly attributed to the relaxation processes of the surface states and the deep level traps.

Guided Growth of Millimeter-Long Horizontal Nanowires with Controlled Orientations

The large-scale assembly of nanowires with controlled orientation on surfaces remains one challenge preventing their integration into practical devices. We report the vapor-liquid-solid growth of aligned, millimeter-long, horizontal GaN nanowires with controlled crystallographic orientations on different planes of sapphire. The growth directions, crystallographic orientation, and faceting of the nanowires vary with each surface orientation, as determined by their epitaxial relationship with the substrate, as well as by a graphoepitaxial effect that guides their growth along surface steps and grooves. Despite their interaction with the surface, these horizontally grown nanowires display few structural defects, exhibiting optical and electronic properties comparable to those of vertically grown nanowires. This paves the way to highly controlled nanowire structures with potential applications not available by other means.

TEM Investigation of Horizontal-Grown Silicon Nanowires

Horizontal silicon nanowires were grown using thermal annealing of thin layers of aluminum on silicon substrates. The template to grow these wires was the thin aluminum layer. As the samples reached the eutectic temperature during annealing, silicon dissolved rapidly in the aluminum layer and deposited at the grain boundary in a form of silicon nanowires. Cross sectional TEM analysis indicated the crystalline structure of these wires.

Stereoaligned Epitaxial Growth of Single-Crystalline Platinum Nanowires by Chemical Vapor Transport

Epitaxial Pt nanowire (NW) arrays are synthesized for the first time by a chemical vapor transport method by using a metal halide as a precursor. Here we report that the epitaxial growth direction of NWs can be steered by seed crystal morphology. Octahedral seeds grow into inclined NWs possessing six growth directions, whereas half-octahedral seeds grow into vertical and horizontal NWs. Interfacial energies between the seed material and the substrate are critical in determining the morphology of seed crystals. We also demonstrate that non-SERS-active Pt NWs can show strong surface-enhanced Raman scattering (SERS) spectra by placing them on Ag films. The active SERS observation would help to elucidate platinum-catalyzed chemical reactions.

Fabrication of horizontally grown silicon nanowires using a thin aluminum film as a catalyst

We present a new method for the fabrication of horizontal silicon nanowires for application in nanoelectronic devices. A web of horizontally connected silicon nanowires is grown on a silicon substrate using a thin aluminum film as a catalyst. A thin layer of oxide is thermally grown on a silicon substrate. The oxide layer is then selectively etched using photolithography. A thin layer of aluminum is thermally evaporated on the substrate with the patterned oxide layer. When the sample is annealed above the eutectic temperature, we show that the silicon gets deposited along the grain boundaries of aluminum in the form of thin nanowires. We show that this phenomenon is due to the high solubility of silicon in aluminum at high temperatures. The surface morphology was analyzed using Scanning Electron Microscopy (SEM). The compositional analysis was done using Energy Dispersive X-ray spectroscopy (EDX).

Self-catalytic growth of horizontal and straight Si nanowires on Si substrates using a sputter deposition technique

We report on the growth of horizontal and straight Si nanowires (NWs) on Si substrate using sputter deposition of the Si layer followed by thermal annealing at 1000 °C and above. The growth of horizontal NWs was achieved without the use of any metal catalyst. Uniform cylindrical shaped Si NWs with a diameter in the range of 50–60 nm and a length of up to 8 μm were synthesized. The as-synthesized Si NWs have a Si core covered with a thin amorphous native oxide layer, as revealed by high resolution transmission electron microscopy. The aspect ratio of these Si NWs is in the range of 100–160. Micro-Raman studies on the NWs reveal a tensile strain on the Si NW core due to presence of a thin oxide layer. From the Raman shift, we calculate a strain of 1.0% for the catalyst free Si NW. FTIR analysis indicates the presence of interstitial oxygen atoms in the Si NWs, as expected from oxidation of Si NWs. For comparison, metal catalyst (Au) assisted Si NWs have also been grown on Si(100) substrate by a similar process. These NWs have a similar diameter and a marginally higher aspect ratio. A model for the growth mechanism of horizontal NWs is presented. This represents one of the first examples of direct horizontal growth of straight Si NWs on commonly used Si substrates suitable for nanoelectronic device fabrication.

Pattern-Selective Epitaxial Growth of Twin-Free Pd Nanowires from Supported Nanocrystal Seeds

We report that twin-free single-crystalline Pd nanowire (NW) arrays grow epitaxially in a selected pattern on a substrate. Parallel aligned Pd NWs are synthesized on a SrTiO(3) (110) substrate in a very high density. On a SrTiO(3) (001) substrate, Pd NWs grow horizontally in two perpendicular directions. Vertical Pd NWs are synthesized instead of horizontal NWs when a c-cut sapphire substrate is employed. We reveal that the atomic structure of the substrate surface determines the geometry and orientation of seeds, which in turn direct the growth patterns of the NWs. The interface energy between the NW material and the substrate is also critical in determining the NW growth pattern. Polarization-dependent localized surface plasmon resonance of as-synthesized epitaxial Pd NW arrays is investigated for application as a plasmonic platform.

Steering Epitaxial Alignment of Au, Pd, and AuPd Nanowire Arrays by Atom Flux Change

We have synthesized epitaxial Au, Pd, and AuPd nanowire arrays in vertical or horizontal alignment on a c-cut sapphire substrate. We show that the vertical and horizontal nanowire arrays grow from half-octahedral seeds by the correlations of the geometry and orientation of seed crystals with those of as-grown nanowires. The alignment of nanowires can be steered by changing the atom flux. At low atom deposition flux vertical nanowires grow, while at high atom flux horizontal nanowires grow. Similar vertical/horizontal epitaxial growth is also demonstrated on SrTiO(3) substrates. This orientation-steering mechanism is visualized by molecular dynamics simulations.

Patterned Growth of Horizontal ZnO Nanowire Arrays

We report an approach to fabricating patterned horizontal ZnO nanowire arrays with a high degree of control over their dimensionality, orientation, and uniformity. Our method combines electron beam lithography and a low temperature hydrothermal decomposition. This approach opens up possibilities to fabricate ZnO NW array based strain and force sensors, two-dimensional photonic crystals, integrated circuit interconnects, and alternative current nanogenerators.

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

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.

Self-connected horizontal silicon nanowire field effect transistor

We propose a technique to fabricate self-connected horizontal Si nanowire (NW) field effect transistors (FETs) by a self-assembly mechanism. We show direct growth of Si NWs between two predefined metallic electrodes along the SiO 2 gate oxide using the vapour–liquid–solid (VLS) growth mode. In our approach, the gold catalyst layer is covered by the contact metal, giving rise to selective and localized catalytic activity and growth of NWs from the gold edges. The diameter of the NWs can be adjusted by the thickness of the catalyst layer. Using such a process, we demonstrate field effect operation on the conductivity of a non-intentionally doped 20 nm diameter Si NW. This technique can be implemented in three dimensions, paving the way to three-dimensionalD integration using vertical stacks of self-connected FETs.

A Scalable Platform for Integrating Horizontal Nanochannels with Known Registries to Microchannels

Controlling the location and physical orientation of nanobuilding blocks in assemblies of nanodevices become challenging tasks when it comes to the use of “bottom-up” chemical approaches for nanodevice fabrication. Here, a strategy is reported for in situ fabrication of horizontal nanochannels, which allows control over their orientation and average pore size to an unprecedented range of ∼5−20 nm. Using this approach, we selectively fabricated nanochannels with controlled average pore sizes at known positions on a sapphire surface. Equally important, nanochannels, either single or group, are addressable photolithographically and integrable to microchannels without the use of any high resolution lithography. This technique uses horizontal ZnO nanowires (NWs) as a sacrificial template and could provide the capability of fabrication of complex micro- and nanofluidic platforms. Cross-sections of nanowires and nanochannels are prepared via a focused-ion beam and examined by electron microscopy and energy-dispe...

Selective Growth of Vertical and Horizontal ZnO Nanowires for Device Applications

Selective Growth of Vertical and Horizontal ZnO Nanowires for Device Applications