This PDF file contains the front matter associated with SPIE Proceedings Volume 7768, including the Title Page, Copyright information, Table of Contents, Introduction, and the Conference Committee listing.

We discuss the present state-of-the-art concerning the growth mechanism, optical luminescence and electrical properties for GaN nanowires grown with catalyst-free molecular beam epitaxy. These nanowires are essentially defect-free and display long photoluminescence lifetimes and carrier mobilities relative to epitaxially grown GaN films. The exclusion of crystalline defects comes from the ease with which strain-relieving dislocations can reach the sidewalls and terminate. The growth mechanism is based on variations in Ga sticking coefficients and surface energies of the sidewall planes and end facet planes. With control of the nucleation process through selective epitaxy on patterned substrates, a high degree of diameter, length and position control can be achieved. Common difficulties with interpretation of optical and electrical data with regard to internal quantum efficiency and mobility are also addressed.

The vast majority of research on epitaxial quantum dots use compressive strain as the driving force for self-assembly on the (001) surface, with InAs/GaAs(001) and Ge/Si(001) being the best-known examples. In this talk, I will discuss our work on determining the feasibility of growing coherent, tensile-strained III-V nanostructures on a (110) surface. GaP on GaAs(110) was chosen as an initial test system. It is hoped that our efforts on self-assembled, tensile-strained dots on a (110) surface will lead the way to new devices exploiting the fundamental differences between the (110) and (001) surfaces. Furthermore it is anticipated that this work will form the first step towards a more general description of self-assembled nanostructure growth under tensile strain.

Semiconductor nanomembranes, single-crystal sheets as thin as ten nanometers, offer many opportunities for novel devices and new science. The most interesting involve epitaxy to introduce strain at both local and global levels. Coming into play are membrane thinness, access to both sides of a sheet, transferability, and enhanced compliancy. Advances in Group IV optoelectronics, thermoelectrics, and photonics may be achievable by combining epitaxy with Si and Ge nanomembranes. Nanoepitaxy allows formation of new strained materials, periodic strain lattices, and mix and match membranes with hybrid orientations or compositions.

High performance HgCdTe IR detector fabrication on silicon substrates first requires low defect density CdTe buffer layers to be grown on silicon. The objective of this paper is to demonstrate dislocation reduction in CdTe epitaxial layers grown on silicon substrate by using intermediate nanocrystalline CdTe buffer layers. Colloidal synthesis of high quality CdTe nanocrystals was accomplished and spin coating of these CdTe nanocrystals as buffer layers on silicon substrates was carried out. CdTe layers were grown on these buffered substrates by metalorganic chemical vapor deposition (MOCVD). However, the incomplete removal of SiO₂ on silicon substrate (by chemical treatment) prevented the exact orientation of the nanocrystals with the silicon substrate and over layer growth of continuous single crystal CdTe epitaxial film. Two new approaches were further investigated: (i) First a thin film of Ge was grown on Si, followed by the deposition of thin SiO₂ followed by nanopatterning using block co-polymer (BCP) lithography. Transmission electron microscopy (TEM) showed defect reduction in the CdTe layers grown on these substrates, but the x-ray rocking curves over a larger area gave wider full width half maximum values compared to that of layers grown on blanket surfaces. This was attributed to non uniform nanopatterning in these initial studies; (ii) SiO₂ coated silicon substrates were nanopatterned using interference lithography with a honeycomb array of holes. These substrates will be used for the selective growth of germanium and CdTe by MOCVD.

Detailed electron transport analysis is performed for an ensemble of conical indium phosphide nanowires bridging two hydrogenated n+-silicon electrodes. The current-voltage (Id-Vd) characteristics exhibit a Coulomb staircase in dark with a period of ~ 1 V at room temperature. The staircase is found to disappear under light illumination. This observation can be explained by assuming the presence of a tiny Coulomb island, and its existence is possible due to the large surface depletion region created within contributing nanowires. Electrons tunnel in and out of the Coulomb island, resulting in the Coulomb staircase Id-Vd. Applying light illumination raises the electron quasi-Fermi level and the tunneling barriers are buried, causing the Coulomb staircase to disappear.

For high speed and performance field effect transistor with high carrier mobility, vertically aligned Si <110> nanowires is demonstrated by chemical vapor deposition via a vapor-liquid-solid growth mechanism. We found that the orientation of NWs was changed from <111> direction to <110> direction on a Si (110) substrate with increasing the growth temperature above ~ 610°C by changing Au-Si eutectic phase. These vertically aligned <110> oriented SiNWs with significantly high carrier mobility opens up new opportunities for high speed and performance future electronic device applications.

Radial p-n silicon nanowire (SiNW) solar cells are of interest as a potential pathway to increase the efficiency of crystalline silicon photovoltaics by reducing the junction length and surface reflectivity. Our studies have focused on the use of vapor-liquid-solid (VLS) growth in combination with chemical vapor deposition (CVD) processing for the fabrication of radial p-n junction SiNW array solar cells. High aspect ratio p-type SiNW arrays were initially grown on gold-coated (111) Si substrates by CVD using SiCl₄ as the source gas and B2H6 as the p-type dopant source. The epitaxial re-growth of n-type Si shell layers on the Si nanowires was then investigated using SiH4 as the source gas and PH3 as the dopant. Highly conformal coatings were achieved on nanowires up to 25 μm in length. The microstructure of the Si shell layer changed from polycrystalline to single crystal as the deposition temperature was raised from 650oC to 950oC. Electrical test structures were fabricated by aligning released SiNWs onto pre-patterned substrates via fieldassisted assembly followed by selective removal of the n-type shell layer and contact deposition. Current-voltage measurements of the radial p-n SiNWs diodes fabricated with re-grown Si shell layers at 950°C demonstrate rectifying behavior with an ideality factor of 1.93. Under illumination from an AM1.5g spectrum and efficiency for this single SiNW radial p-n junction was determined to be 1.8%, total wire diameter was 985 nm.

We present the tuning of electrical characteristics of ZnO nanowire field effect transistors (FETs) by controlling surface morphology and size of nanowires and by introducing proton-irradiation-assisted manipulation and further demonstrate their logic inverter circuit. The FETs made from surface-architecture-controlled ZnO nanowires exhibit two different types of operation modes, which are distinguished as depletion and enhancement modes in terms of the polarity of the threshold voltage. We also explain that the electrical transport behaviors are associated with the influence of surface states. In addition, we demonstrate the proton irradiation effects on the electrical characteristics of two different types of FET device structures in which the ZnO nanowires are placed on the substrate or suspended above the substrate. The photoluminescence studies of the ZnO nanowires provide substantial evidence that the observed threshold voltage shift in nanowire transistors can be explained by a surface-band-bending through the gate electric field modulation, resulting from the irradiation-induced charges. Finally, as a practical approach, we demonstrate the logic inverter circuits made from the operation mode-controlled ZnO nanowire FETs.

Nanowires based on the III nitride materials system have attracted attention as potential nanoscale building blocks in optoelectronics, sensing, and electronics. However, before such applications can be realized, several challenges exist in the areas of controlled and ordered nanowire synthesis, fabrication of advanced nanowire heterostructures, and understanding and controlling the nanowire electrical and optical properties. Here, recent work is presented involving the aligned growth of GaN and III-nitride core-shell nanowires, along with extensive results providing insights into the nanowire properties obtained using advanced electrical, optical and structural characterization techniques.

Surface enhanced Raman scattering (SERS) is a powerful technique for the detection of submonolayer coverage of gold or silver surfaces. The magnitude of the effect and the spectral wavelength of the peak depend on the metal nanoparticles used and its geometry. In this paper we show that the use of chemicals that bind to gold or silver can lead to the clustering of nanoparticles. We used well defined Au nanoparticles in our experiments and add cysteamine to solutions containing the nanoparticles. The plasmonic response of the nanoparticles is measured by transmission Surface Plasmon Resonance (SPR) spectroscopy. We observed significant changes to the SPR spectra that are characteristics of close coupled nanoparticles. The time evolution of these changes indicates the formation of gold nanoparticles clusters. The SERS response of these clustered nanoparticles is observed to red shift from the designed peak wavelength in the green to the red. In addition, the placement of these clusters on dielectric surfaces shifts the SPR even more into the red. The experimental results are supported by calculations of the electromagnetic fields using finite difference methods.

We have investigated the formation of silicon nanowire arrays by the use of a simple chemical etching approach. The etching characteristics of silicon nanowire arrays using wafers with diverse doping levels and several orientations have been examined. Furthermore, the etching solution, etching time and temperature were also considered in order to optimize the etching conditions to produce thinner and more orderly silicon nanowire arrays in registry with the substrate. Since this process takes advantage of a silver catalyst, we have also investigated various ways of forming the initial silver catalyst on the silicon surfaces, and we show that electroless Ag deposition, as well as e-beam thin film deposition of Ag, results in successful, highly aligned and ordered Si nanowire arrays after the etching step. In addition, we have also performed Surfaced Enhanced Raman Scattering (SERS) measurements on the nanowire arrays and on nanowires removed from the substrate.

We demonstrate a post-growth in-situ chlorine passivation for suppressing surface-dominant transport in Si nanowires (SiNWs). The leakage current of bridged SiNWs suppressed more than five orders of magnitude as a result of chlorine passivation while the shape and structural properties of the bridging NWs remain unaffected by the post-growth in-situ HCl passivation. The chlorine passivated SiNW surfaces were found to be beneficial to enhance the high immunity to environmental degradation.

We present a novel nanostructure, metallic nanocomb structures, electro-chemically self assembled on nanoporous alumina templates. Gold and silver nanowires between 5-15 nm have been produced through the ordered nano-scale pores of alumina templates prepared by anodization of aluminum foils. These ultrathin nanowires prefer growing in the inner surfaces of the hexagonal pores in alumina, resulting in the nanocomb structure with remarkable long-range hexagonal close packed order, similar to those found in the nanoporous template. We report here the typical processing conditions and microstructure of this novel material as observed with SEM and EDX. We also provide preliminary field emission data, and indicate possible applications for which they can make remarkable impact.