This PDF file contains the front matter associated with SPIE Proceedings Volume 8820, including the Title Page, Copyright Information, Table of Contents, and the Conference Committee listing.

Nanowire bridges have been almost dormant in a nanostructured device community due to the challenges in reproducible growth and device fabrication. In this work, we present simple methods for creating silicon nanobridge arrays with repeatability, and demonstrate integration of gate-all-around field-effect-transistors in the arrays. P-type silicon nanowires air-bridges were synthesized using gold nanoparticles via the VLS technique on the array of predefined silicon electrode-pairs, and then surrounding gates were formed on the suspended air-bridge nanowires. The nanowire air-bridge field-effect-transistors with the surrounding gate exhibited p-type accumulation-mode characteristics with a subthreshold swing of 187 mV/dec and an on/off current ratio of 1.6×10⁶. Despite the surrounding gate that helps gate biases govern the channel, off current substantially increased as drain bias increases. This ambipolar current-voltage property was attributable to gate-induced-drain-leakage at the overlap of gate and drain electrodes and trap-assisted tunneling at the nanowire and electrode connection.

As electronic devices shrink to the one-dimensional limit, unusual device physics can result, even at room temperature. Nanoscale conductors like single-walled carbon nanotubes (SWNTs) are particularly useful tools for experimentally investigating these effects. Our characterization of point defects in SWNTs has focused on these electronic consequences. A single scattering site in an otherwise quasi-ballistic SWNT introduces resistance, transconductance, and chemical sensitivity, and here we investigate these contributions using a combination of transport and scanning probe techniques. The transport measurements determine the two-terminal contributions over a wide range of bias, temperature, and environmental conditions, while the scanning probe work provides complementary confirmation that the effects originate at a particular site along the conduction path in a SWNT. Together, the combination proves that single point defects behave like scattering barriers having Poole-Frenkel transport characteristics. The Poole-Frenkel barriers have heights of 10 – 30 meV and gate-dependent widths that grow as large as 1 μm due to the uniquely poor screening in one dimension. Poole-Frenkel characteristics suggest that the barriers contain at least one localized electronic state, and that this state primarily contributes to conduction under high bias or high temperature conditions. Because these localized states vary from one device to another, we hypothesize that each might be unique to a particular defect’s chemical type.

A concept for a nanowire-based photovoltaic (PV) device is presented along with the requirements for achieving high photoconversion efficiency including nanowire morphology, crystalline structure, nanowire dimensions (diameter, period (spacing) and length), avoidance of misfit dislocations, low resistance contacts, controlled doping for p-n junctions, surface passivation, and current-matching. The state of the nanowire PV device field is presented.

The epitaxial growth of <110> silicon nanowires on (110) Si substrates by the vapor-liquid-solid growth process was investigated using SiCl₄ as the source gas. A high percentage of <110> nanowires was obtained at high temperatures and reduced SiCl₄ partial pressures. Transmission electron microscopy characterization of the <110> Si nanowires revealed symmetric V-shaped {111} facets at the tip and large {111} facets on the sidewalls of the nanowires. The symmetric {111} tip faceting was explained as arising from low catalyst supersaturation during growth which is expected to occur given the near-equilibrium nature of the SiCl₄ process. The predominance of {111} facets obtained under these conditions promotes the growth of <110> SiNWs.

Semiconductor-oxide nanostructure devices can be a very intriguing material platform if optoelectronic properties of the original semiconductor nanostructures can be tuned by explicitly controlling properties of the oxide coating. This paper describes our finding that optical properties of semiconductor nanowires can be tuned by depositing a thin layer of metal oxide. In this experiment, indium phosphide nanowires were grown by metal organic chemical vapor deposition on silicon substrates with gold catalyst. The nanowires formed three-dimensional nanowire networks from which collective optical properties were obtained. The nanowire network was coated with an aluminum oxide thin film deposited by plasma-enhanced atomic layer deposition. We studied the dependence of the peak wavelength of photoluminescence spectra on the thickness of the oxide coatings. We observed continuous blue shift in photoluminescence spectra when the thickness of the oxide coating was increased. The observed blue shift is attributed to the Burstein-Moss effect due to increased carrier concentration in the nanowire cores caused by repulsion from an intrinsic negative fixed charge from the oxide surface. Samples were further characterized by scanning electron microscopy, transmission electron microscopy, and selective area diffractometry in an attempt to explain the physical mechanisms for the blue shift.

SiC nanowires (NWs) are attractive building blocks for the next generation electronic devices since silicon carbide is a wide bandgap semiconductor with high electrical breakdown strength, radiation resistance, mechanical strength, thermal conductivity, chemical stability and biocompatibility. Epitaxial growth using metal-catalyst-based vapor-liquid-solid mechanism was employed for SiC NW growth in this work. 4H-SiC substrates having different crystallographic orientations were used in order to control NW alignment and polytype. A new technique based on vapor-phase delivery of the metal catalyst was developed to facilitate control of the NW density. Both 4H and 3C polytypes with a strong stacking disorder were obtained. The 4H and 3C NWs had different orientations with respect to the substrate. 4H NWs grew perpendicular to the c-plane of the substrate. The stacking faults (SFs) in these nanowires were perpendicular to the [0001] nanowire axes. All 3C NWs grew at 20° with respect to the substrate c-plane, and their projections on the c-plane corresponded to one of the six equivalent ⟨101-0⟩ crystallographic directions. All six orientations were obtained simultaneously when growing NWs on the (0001) substrate surface, while only one or two NW orientations were observed when growing NWs on any particular crystallographic plane parallel to the c-axis of the substrate. Growth on {101-0} surfaces resulted in only one NW orientation, thereby producing well-aligned NW arrays. Preliminary measurements of the NW electrical conductivity are reported utilizing two-terminal device geometry.

The ability to make a good electrical/thermal contact to a large area filled with semiconductor nanowires has been a major engineering challenge in developing this type of thermoelectric devices. A practical fabrication process of a top electrical/thermal contact onto a network of randomly oriented intersecting semiconductor nanowires was designed by implementing a sequence of two separated metal organic chemical vapor deposition processes for indium phosphide. In the first step, a nanowire network was grown on a substrate with indium phosphide nanowires grown axially. Subsequently, growth temperature and pressure were altered to change the axial growth to lateral growth that promoted the formation of indium phosphide extending over multiple nanowires. Possible growth mechanisms during the lateral growth and structural properties of the laterally grown segment will be discussed.

In this paper, we review our recent development and validation of the ultrasensitive electronic biomolecular assays enabled by our novel amplifying nanowire field-effect transistor (nwFET) biosensors. Our semiconductor nwFET biosensor platform technology performs extreme proximity signal amplification in the electrical domain that requires neither labeling nor enzymes nor optics. We have designed and fabricated the biomolecular assay prototypes and developed the corresponding analytical procedures. We have also confirmed their analytical performance in quantitating key protein biomarker in human serum, demonstrating an ultralow limit of detection and concurrently high output current level for the first time.

We present a novel approach for the fabrication of lithium-ion microbattery electrodes which deliver high energy and high power density. The key enabling technology is the use of self-assembled Tobacco mosaic virus (TMV) nanoforests as a template for active battery materials. The self-assembling TMV is a genetically modified biological nanorod with increased metal binding properties for enhanced manufacturability. High energy density is achieved due to the active surface area increase within a given footprint by combining TMV with three-dimensional (3D) microfabricated structures. The TMV nanostructure enables high power density through larger electrode/electrolyte contact area and faster charge transport. The electrodes consist of an array of electroplated gold micropillars. The pillars are coated with the self-assembled nanoscale TMV template and subsequently metalized in-place. Active battery material (V₂O₅) is conformally deposited using atomic layer deposition (ALD) on the hierarchical micro/nano network. Electrochemical testing of these electrodes indicates a 3-5 fold increase in energy density, compared to the TMV-templated electrodes without micropillars, without increasing footprint area or reducing rate performance. Further increase in energy density can be achieved by increasing surface area of 3D microelements as demonstrated by fabrication and electrochemical testing of the electrodes with hollow gold micropillars. Scaling up energy density by increasing active material thickness beyond 100 nm revealed some loss in surface area which highlighted the importance of nanoscale engineering for achieving maximum energy and power density in energy storage systems.

The authors report on the differences in ferromagnetic MnAs nanocluster formation on GaAs, GaAs/AlGaAs, GaAs/GaAsP, and InAs nanowire templates by combing selective-area metal-organic vapor phase epitaxy of semiconducting nanowires and endotaxial nanoclustering of MnAs. To characterize the dependences of MnAs nanocluster formation on semiconducting materials of the nanowire templates, GaAs, GaAs/AlGaAs core-shell, and GaAs/GaAsP core-shell nanowires have been grown at 750 °C, whereas InAs nanowires have been grown at 580 °C. MnAs nanoclusters are commonly and most frequently formed at six ridges between two {0-11} crystal facets on hexagonal prisms of III-V semiconducting nanowires. That is presumably because many atomic steps exist between the crystal facets. Here, MnAs nanoclusters are grown “into” the nanowires, as a result of the phenomenon of “endotaxy”. Manganese atoms on the nanowires surface form chemical bonds mainly with arsenic atoms of the nanowires, because only manganese organometallic source and hydrogen are supplied, i.e. no supply of arsenic hydride source during the endotaxy of MnAs. In the case of GaAs/GaAsP core-shell and InAs nanowires, however, MnAs nanoclusters are formed on the top {111}B surfaces of the nanowires, as well as at six ridges of the hexagonal prisms. The results obtained in the current work possibly show that the endotaxy of MnAs depends on the thermal stability of the nanowires and/or the strength of atomic bonds in the host materials of nanowires.

An urgent demand remains in astronomy for high-reflectivity silver mirrors that can withstand years of exposure in observatory environments. The University of California Observatories Astronomical Coatings Lab has undertaken development of protected silver coatings suitable for telescope mirrors that maintain high reflectivity at wavelengths from 340 nm through the mid-infrared spectrum. We present initial results of an investigation into whether plasma-enhanced atomic layer deposition (PEALD) can produce superior protective layers of transparent dielectrics. Several novel coating recipes have been developed with ion-assisted electron beam deposition (IAEBD) of materials including yttrium fluoride, and oxides of yttrium, hafnium, and titanium. Samples of these mirror coatings were covered with conformal layers of aluminum oxide (AlOx) deposited by PEALD using trimethylaluminum as a metal precursor and oxygen as an oxidant gas activated by remote plasma. Samples of coating recipes with and without PEALD oxide undergo aggressive environmental testing, including high temperature/high humidity (HTHH), in which samples were exposed to an environment of 80% humidity at 80°C for ten days in a simple test set-up. HTHH testing show visible results suggesting that the PEALD oxide offers enhanced robust protection against chemical corrosion and moisture from an accelerated aging environment. Mirror samples are further characterized by reflectivity/absorption and atomic force microscopy before and after deposition of oxide coatings. AlOx is suitable for many applications and has been the initial material choice for this study, although we also tried TiOx and HfOx. Further experimentation based on these initial results is on-going.

Here we describe the development of optical coatings for silicon-based detectors for astronomy, planetary and terrestrial applications. We have used atomic layer deposition (ALD) to develop broadband (i.e. 320-1000 nm) antireflection (AR) coatings on silicon substrates with the ultimate goal of incorporating these AR coatings with existing detector technologies. Materials characterization was used to study film and interface quality of these coatings. We are able to achieve precision growth of single and multilayer films to significantly reduce reflection losses for this region of spectrum and provide tailored, repeatable performance targeted for specific applications.

Indium phosphide (InP) nanowire networks coated with gold were characterized by Raman spectroscopy. First, InP nanowire networks were grown via metal organic chemical vapor deposition (MOCVD) on silicon substrates with gold catalyst. Subsequently, gold was deposited by thermal evaporation on the grown InP nanowire networks. Different nominal thicknesses of gold were deposited, and then the goal coated InP nanowire networks were annealed in vacuum. Raman spectroscopy was used to study the dependence of InP phonon modes on the thickness of the gold coating. The study shows the gold coating decreases the longitudinal optical phonon mode signal of InP as the thickness increases.


Publisher’s Note: This paper, originally published on 19 September 2013, was replaced with a corrected/revised version on 18 October 2013. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance.

The growth of silicon core-shell nanowires with a crystalline-core and a polycrystalline-shell on copper substrates pretreated with carbon via Plasma Enhanced Chemical Vapor Deposition (PECVD) was demonstrated. The nanowire diameters range from 120 to 250nm with 10-20nm crystalline cores. The overall large diameter enables easier methods of forming an electrical/thermal contact while the small core maintains the benefits of nanowires. By altering the copper surface with carbon, highly dense silicon nanowire networks can be directly grown on copper substrates, which could allow for efficient and economical incorporation of silicon nanowires into such applications as thermoelectric devices.