This PDF file contains the front matter associated with SPIE Proceedings Volume 7406, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

Novel ternary nanostructures (ZnO-Ga₂O₃ nanobrushes, SnO₂-Ga₂O₃ heterostructures and Sn-doped Ga2O3 nanowires) are excellent materials for gas sensing applications due to their large surface areas and structural defects. Also, these nanostructures consist of different materials with different degrees of crystallinity and defect densities thus broadening their gas sensing capabilities. Gas sensing devices, developed in our laboratory based on room temperature capacitance measurements, were first fabricated by standard photolithography and lift-off techniques to pattern platinum (Pt) pads and interdigitated fingers acting as conducting paths. The nanostructures, which were characterized by electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and photoluminescence (PL), were then incorporated by the catalyst-assisted growth directly onto the devices. The most efficient devices were those with high yield of nanostructures and with low-resistivity of the Pt pads. To achieve that, different catalysts (nickel, Ni; copper, Cu, and gold, Au) were used for different nanostructures. For example, the best catalyst for the device fabrication of Sn-doped Ga₂O₃ nanowires was Ni whereas for nanostructures with high Sn content Cu was the best catalyst. Challenges and successes of device fabrication for capacitance-based gas sensing devices are discussed in this work together with some sensing results for such analytes as acetone, acetic acid, isopropanol, dichoropentane, nitrotolouene and nitromethane.

This paper describes a simple and yet rapid flame synthesis method to produce one dimensional metal oxide nanostructures by directly oxidizing metals in the post-flame region of a flat flame. α-Fe₂O₃ nanoneedles grow in the post-flame region by a solid diffusion mechanism, are highly crystalline, and are aligned perpendicularly to the substrate, with a large surface coverage density. The growth rate of the nanostructures is almost two orders of magnitude larger than those demonstrated previously in furnaces or on hotplates. The rapid growth rate is attributed to the large initial heating rate of the metal substrate in the flame, which generates thin and porous oxide layers that greatly enhance the diffusion of the deficient species to the nanostructure growth site.

The growth of crystalline 1D nanowires of semiconductors on non-epitaxial surfaces holds the promise to overcome many of the current challenges of heteroepitaxial material synthesis and device fabrication for a wide range of electronic and photonic applications. Nano-heteroepitaxial bridging of CVD grown nanowires potentially enables a low cost and mass-manufacturable approach to nanowire based device fabrication. Here we report the synthesis and bridging of lateral silicon nanowires between a pair of vertical non-single crystal surfaces and application of this technique in the design and fabrication of waveguide-integrated photodetectors. The device consists of a number of 1D nanowires laterally grown across gaps etched into rib optical waveguides with an amorphous silicon oxynitride core and silicon oxide claddings. A pair of phosphorous-doped polysilicon electrodes was deposited on the walls of the waveguide gap for electrical interfacing of the nanowires to collect the photocurrent under optical excitation. Characterization results demonstrated good waveguide characteristics, high electrical isolation between the electrodes, low leakage current and distinct photoresponse from the bridged nanowires. This implementation of silicon nanowires on polysilicon combines the characteristics of crystalline 1D nanowires with the flexible fabrication processes on non-single-crystal silicon platforms facilitating advances in silicon photonics and beyond.

The novel properties of semiconductor nanowires, along with their potential for device applications in areas including nanoscale lasers and thermoelectrics, have led to a resurgence of interest in their growth and characterization over the past decade. However, the further development and optimization of nanowire-based devices will depend critically on an understanding of carrier relaxation in these nanostructures. For example, the operation of GaN-based photonic devices is often influenced by the presence of a large defect state concentration. Ultrafast optical spectroscopy can address this problem by measuring carrier transfer into and out of these states, which will be important in optimizing device performance. In this work, we use ultrafast wavelength-tunable optical spectroscopy to temporally resolve carrier dynamics in semiconductor nanowires. Wavelength-tunable optical pump-probe measurements enable us to independently measure electron and hole dynamics in Ge nanowires, revealing that the lifetime of both electrons and holes decreases with decreasing nanowire diameter. Similar measurements on CdSe nanostructures reveal that the surface-to-volume ratio strongly influences carrier relaxation. Finally, ultrafast optical experiments on GaN nanowires probe carrier dynamics in the defect states that influence device operation. These experiments provide fundamental insight into carrier relaxation in these nanosystems and reveal information critical to optimizing their performance for applications.

Material incompatibilities among dissimilar group III-V compound semiconductors (III-V CSs) often place limits on combining epitaxial thin films, however low-dimensional epitaxial structures (e.g., quantum dots and nanowires) demonstrate coherent growth even on incompatible surfaces. First, InAs QDs grown by molecular beam epitaxy on GaAs are described. Two-dimensional to three-dimensional morphological transition, lateral size evolution and vertical alignment of InAs QDs in a single and multiple stacks will be illustrated. Second, InP nanowires grown on non-single crystalline surfaces by metal organic chemical vapor deposition are described with the view toward applications where III-V CSs are functionally integrated onto various material platforms.

Nanostructures such as carbon nanotubes, nanowires and graphene nanoribbons are being intensively explored for future nanoelectronic and nanophotonic applications. In order for these nanosystems to progress from the research laboratory to technology, it is critical to precisely understand and control charge injection at the contacts and subsequent charge transport. In this paper, we discuss recent experimental and theoretical results on electrical contacts to Ge nanowires and electronic transport in GaN and InAs nanowires. It is shown that both the properties of the nanocontacts and the charge transport differ significantly from those of bulk systems.

Epitaxy can be used to direct nanowire chemical vapor deposition and to influence the crystallographic orientation of nanowires during their nucleation and growth via the vapor-liquid-solid mechanism. Under some circumstances, the influence of epitaxy competes with capillary effects and the influence of nanoparticle catalyst coarsening and surface impurities on nanowire orientation selection. We have investigated rapid thermal chemical vapor deposition of epitaxial Ge nanowires and have used it to separately study nanowire nucleation and growth. This has given important insights into deep-subeutectic Ge nanowire growth using Au catalyst particles. These Ge nanowires have also been studied as the cores in epitaxial Ge core/Si shell nanowires. We have studied the conditions under which strain driven surface roughening and dislocations formation occurs in these coaxial nanowire heterostructures. Recent results indicate that suppression of Si shell surface roughening can lead to fully strained, coherent core/shell nanowires.

In addition to future applications in electronics, optoelectronics, and biophotonics, synthesis of nanostructures such as nanowires, nanorods, and quantum dots offer insights regarding the governing principle of crystal growth that can be applied to a wide range of mesoscopic phenomena. The basis for understanding the morphology of GaN nanosystems during epitaxy is the (kinetic) Wulff theorem which incorporates the concept of energy minimization into a set of geometrical rules depicting shape evolution. An appreciation of the Wulff plot for GaN, a three-dimensional diagram (v-plot) where the radial distance is proportional to the growth velocity along that direction, not only assists the interpretation but also facilitates a detailed control of nanoepitaxial processes. To map out the kinetic Wulff diagram, we carried out selective-area growth (SAG) of GaN on polar, nonpolar, and semipolar surfaces under a wide range of conditions (temperature, pressure, and V/III ratio). Salient features on the kinetic Wulff plot include cusps, saddle points, and apexes, which all have implications in shaping the nano-objects. Examples will be given to illustrate the utility of Wulff plots in explaining the topography of nanorods and quantum dots and in aiding a rational design of GaN nonpolar and semipolar growth for solid state lighting applications.

We report the growth and characterization of Ge-Si_xGe_1-x core-shell nanowires. Using a combination of vapor-liquid-solid nanowire growth and ultra-high-vacuum chemical vapor deposition conformal growth, we demonstrate the realization of epitaxial Ge-Si_xGe_1-x core-shell nanowire heterostructures with tunable shell content. We investigate the intrinsic electronic properties of Ge-Si_xGe_1-x core-shell nanowires using back-gate dependent two- and four-terminal resistance measurements, and demonstrate high performance Ge-Si_xGe_1-x core-shell nanowire field-effect transistors with highly doped source and drain.

We present here a novel design to form an artificial quantum dot with electrical confinement and apply it to a Quantum Cascade Laser structure to realize a Quantum Dot Cascade Laser. A two-dimensional finite element method has been used to numerically simulate the novel design of electrical formation of an artificial quantum dot. The size of the quantum dot is electrically tunable and can be applied to quantum cascade laser structure to reduce the non-radiative LO-phonon relaxation. Numerical modeling with cylindrical symmetry is custom developed using Comsol multiphysics to evaluate the electrical performance of the device and optimize it by varying design parameters, namely, the doping density of different layers and thickness of the cladding and active regions. The typical s-, p-, d- and f- wave functions have been calculated. Numerical simulations show that the energy level separation could be as large as 50 meV by electrical confinement. We also demonstrate the road map for the fabrication of such a device using a maskless super lens photolithography technique. We have achieved a uniform array of nano-contacts of size ~ 200nm, required for the device, using photolithographic technique with a UV source of λ ~ 400nm. The entire processing involves 7 photolithographic steps. This new device - "Quantum dot cascade laser", promises low threshold current density and high wall-plug efficiency.

Nanofibers made from para-hexaphenylene (p6P) molecules hold unique optoelectronic properties, which make them interesting candidates as elements in electronic and optoelectronic devices. Typically these nanofibers are grown on specific single-crystalline substrates, on which long, mutually parallel nanofibers are formed. However, the lack of ability to further process these substrates restrains their use in devices. In this work, a novel method for in-situ growth of p6P nanofibers on nano- and micro-structured gold surfaces is presented. The substrates are prepared by conventional microfabrication techniques such as lithography, etching and metal deposition, which increase their potential as device platforms. The results presented here demonstrate, that both the growth direction and the nanofiber length can be controlled by placement of nano- and micro-structured lines on the substrate. It is shown that the preferred growth direction of the nanofibers is perpendicular to these structures whereas their length scales are limited by the size and placement of the structures. This work therefore demonstrates a new technique, which can be useful within future organic nanofiber based applications.

There has been significant interest in a variety of nanowire (NW) systems for various sensing applications. We had developed highly sensitive dielectric core/metal sheath nanowires composites which serve as surface-enhanced Raman scattering (SERS) substrates. Previously, our composites were fabricated using e-beam deposition, which has the problem of incomplete coverage. Here we report an electroless (EL) plating approach to cover the NWs with a silver sheath, producing the core/metal NW structures for the SERS measurements. In comparison with the common silver deposition via e-beam evaporation, electroless coating can result in the full metal coverage on NWs. Therefore, this approach provides a way to fully cover nanostructures with Ag, including NWs arrays, regardless of the orientations and shapes of the nanostructures. SERS measurements on EL produced Ag/NWs composites show stronger signals than those produced by e-beam evaporation. Electric field calculations suggest that the strong SERS signal is due to plasmonic coupling of neighboring closely spaced islands.