This research project is focused on a new strategy for the creation of nanowire based semiconductor devices. The main goal is to understand and optimize the electrical and optical properties of two types of nanoscale devices; in first type lithographically patterned nanowire electrodeposition (LPNE) method has been utilized to fabricate nanowire field effect transistors (NWFET) and second type involved the development of light emitting semiconductor nanowire arrays (NWLED). Field effect transistors (NWFETs) have been prepared from arrays of polycrystalline cadmium selenide (pc-CdSe) nanowires using a back gate configuration. pc-CdSe nanowires were fabricated using the lithographically patterned nanowire electrode- position (LPNE) process on SiO2 /Si substrates. After electrodeposition, pc-CdSe nanowires were thermally annealed at 300 °C x 4 h either with or without exposure to CdCl2 in methanol- a grain growth promoter. The influence of CdCl 2 treatment was to increase the mean grain diameter as determined by X-ray diffraction pattern and to convert the crystal structure from cubic to wurtzite. Transfer characteristics showed an increase of the field effect mobility ([mu] [subcript] eff) by an order of magnitude and increase of the Ion/Ioff ratio by a factor of 3-4. Light emitting devices (NW-LED) based on lithographically patterned pc-CdSe nanowire arrays have been investigated. Electroluminescence (EL) spectra of CdSe nanowires under various biases exhibited broad emission spectra centered at 750 nm close to the band gap of CdSe (1.7eV). To enhance the intensity of the emitted light and the external quantum efficiency (EQE), the distance between the contacts were reduced from 5 [mu]m to less than 1 [mu]m which increased the efficiency by an order of magnitude. Also, increasing the annealing temperature of nanowires from 300 °C x 4 h to 450 °C x1h enhanced grain growth confirmed by structural characterization including X-ray diffraction (XRD), Scanning electron microscopy (SEM) and Raman Spectroscopy. Correspondingly the light emission intensity and EQE improved due to this grain growth. Kelvin probe force microscopy (KPFM) was utilized to understand mechanism of light emission in CdSe nanowires. Arrays of CdTe nanowires were electrodeposited using LPNE process where the elec- trodeposition of pc-CdTe was carried out at two temperatures: 20 °C (cold) and 55 °C (hot). Transmission electron microscopy (TEM) and X-ray diffraction (XRD) re- sults revealed higher crystallinity, larger grain size and presence of Te for nanowires prepared at 55 °C compared to nanowires deposited at 20 °C. Nanowires prepared at 55 °C showed higher electrical conductivity and enhanced electroluminescence proper- ties, including higher light emission intensity and improved External Quantum Efficiency (EQE). Electrical conduction mechanism also investigated for CdTe nanowires. Thermionic emission over schottky barrier height was identified as the dominant charge transport mechanism in pc-CdTe nanowires.

Abstract: Over the past 15 years, nanowires (NWs) and nanotubes have drawn great attention since the application of VLS growth mechanism into the synthesis of one dimensional structures. Semiconductor nanowires exhibit novel electrical and optical properties. With a broad selection of composition and band structures, these one-dimensional semiconductor nanostructures are considered to be the critical components in a wide range of potential nanoscale device applications. To fully exploit these one-dimensional nanostructures, current research has focused on synthetic control of one-dimensional nanoscale building blocks, characterization of their novel properties, device fabrication based on nanowire building blocks, and integration of nanowire elements into complex functional architectures. Progress has been made in past two decades. However, there are still challenges in NWs growth controls, such as size, shape, position, stoichiometry and defects. Due to the dimensionality and possible quantum confinement effects of nanowires, there are also challenges in characterization and device fabrication. A systematic study of controlled growth of nanowires has been conducted in this dissertation. The first part of this dissertation presents various synthesis techniques of semiconductor nanowires via metal catalyzed vapor-liquid-solid (VLS) growth mechanism. Pulse laser deposition (PLD) with arsenic over pressure method has been successfully utilized for GaAs nanowires. Challenges such as uniformity issue commonly seen in MOCVD and MBE systems, morphology and stoichiometry issues commonly seen in conventional PLD systems have been overcome. Si nanowires fabrication via ultrahigh vacuum magnetron sputtering has reported for the first time, which also provides an alternate route for Si nanowires synthesis. The second part of this dissertation discusses optical properties of ensemble direct band gap nanowires. Photoluminescence spectra have been measured on an ensemble of random orientated InP nanowires. Polarization anisotropy has been explored on ensemble nanowires and oxide-coated nanowires. Our calculation for randomly oriented nanowires agrees well with experimental results. The control of polarization anisotropy of nanowires is realized by coating nanowires with an oxide layer composed of matching dielectric constant media. This opens a path to optical spin injection and detection on direct band gap nanowires.

Semiconductor nanowires promise to provide the building blocks for a new generation of nanoscale electronic and optoelectronic devices. Semiconductor Nanowires: Materials, Synthesis, Characterization and Applications covers advanced materials for nanowires, the growth and synthesis of semiconductor nanowires—including methods such as solution growth, MOVPE, MBE, and self-organization. Characterizing the properties of semiconductor nanowires is covered in chapters describing studies using TEM, SPM, and Raman scattering. Applications of semiconductor nanowires are discussed in chapters focusing on solar cells, battery electrodes, sensors, optoelectronics and biology. - Explores a selection of advanced materials for semiconductor nanowires - Outlines key techniques for the property assessment and characterization of semiconductor nanowires - Covers a broad range of applications across a number of fields

Volume 1, Metal and Semiconductor Nanowires covers a wide range of materials systems, from noble metals (such as Au, Ag, Cu), single element semiconductors (such as Si and Ge), compound semiconductors (such as InP, CdS and GaAs as well as heterostructures), nitrides (such as GaN and Si3N4) to carbides (such as SiC). The objective of this volume is to cover the synthesis, properties and device applications of nanowires based on metal and semiconductor materials. The volume starts with a review on novel electronic and optical nanodevices, nanosensors and logic circuits that have been built using individual nanowires as building blocks. Then, the theoretical background for electrical properties and mechanical properties of nanowires is given. The molecular nanowires, their quantized conductance, and metallic nanowires synthesized by chemical technique will be introduced next. Finally, the volume covers the synthesis and properties of semiconductor and nitrides nanowires.

With the rapid evolution of semiconductor technologies, the size of the fundamental device components is already approaching nanometer scale. In order to fabricate even smaller and faster yet more power efficient devices, new materials or designs are required. As one of the best candidate for future electronic and photonic applications, semiconductor nanowires have created substantial interest in the last decade. Variety of researches has been conducted to understand its growth and fundamental properties. Among the nanowires with different materials and designs, hetero-structure nanowires are especially attractive due to their capability of realizing band gap engineering without forming interface defects. In Chapter 2, we use a combination of optical, electronic and electron-beam measurements as well as theoretical simulation to obtain a clear picture of a GaP/GaAs core/shell nanowire hetero-interface strain distribution and relaxation. Micro-Raman spectroscopy is primarily used to map the high resolution strain distribution. A compressive strain is observed on GaAs, while a tensile strain is observed on GaP. The tension on GaP becomes smaller as core/shell size ratio grows. Selected-area electron diffraction (SAED) is also performed to study the strain, which is consistent with Raman. Due to the strain and stress, the band structure of either GaP or GaAs is modified. A band structure calculation along the core/shell nanowire is performed based on strain measured by Raman, which is consistent with photo-current measurement. Finally, comparing the experimental strain and the finite-element method simulation strain, a relaxation of the strain is observed and it is correlated to the hetero-interface dislocation densities observed by TEM measurements. When designing new electronic or photonic devices based on nanowires, the understandings of carrier dynamics are critical in optimizing their performance. In Chapter 3, transient Rayleigh scattering (TRS) experiment is performed to study the carrier dynamics of complex band structure InP nanowires. Different band structures of zinc blende and wurtzite InP nanowires are clearly observable. More interestingly, a fitting model based on band to band transition theory is developed to extract the carrier densities and temperatures as a function of time after initial excitation. Based on the carrier density or temperature relaxation, electron/hole recombination or thermalization process could be analyzed respectively. Comparing the carrier thermalization behavior of InP nanowires to other materials, like GaAs nanowires, a unique hot phonon effect is observed due to InP's special phonon band structures (huge band gap between optical and acoustic branches). In addition to the visible to near-IR wavelength range we have been studying for long time, near~mid IR wavelength materials nanowires become interesting recently due to their potential opto-electronic applications. In Chapter 4, an infrared modified TRS system is developed and optimized to obtain high quality ultra-fast TRS data across wavelength range 500~2500nm with a simple diode (InGaAs or InSb). The electronic band structures and carrier relaxation dynamics are obtained for a variety of nanowires (i.e. Zn3As2, GaAs1-xSbx, GaSb). For bare Zn3As2 nanowire data, a substantially long carrier relaxation process is observed, which indicates low Zn3As2 surface recombination velocity. For GaAs11-x/subSbx samples, the nanowire obtains 2-order of magnitude longer carrier lifetime after InP surface passivation. All of these measurements provide informative feedback to the growth and design of near~mid IR nanowires for future applications.

One dimensional electronic materials are expected to be key components owing to their potential applications in nanoscale electronics, optics, energy storage, and biology. Besides, compound semiconductors have been greatly developed as epitaxial growth crystal materials. Molecular beam and metalorganic vapor phase epitaxy approaches are representative techniques achieving 0D–2D quantum well, wire, and dot semiconductor III-V heterostructures with precise structural accuracy with atomic resolution. Based on the background of those epitaxial techniques, high-quality, single-crystalline III-V heterostructures have been achieved. III-V Nanowires have been proposed for the next generation of nanoscale optical and electrical devices such as nanowire light emitting diodes, lasers, photovoltaics, and transistors. Key issues for the realization of those devices involve the superior mobility and optical properties of III-V materials (i.e., nitride-, phosphide-, and arsenide-related heterostructure systems). Further, the developed epitaxial growth technique enables electronic carrier control through the formation of quantum structures and precise doping, which can be introduced into the nanowire system. The growth can extend the functions of the material systems through the introduction of elements with large miscibility gap, or, alternatively, by the formation of hybrid heterostructures between semiconductors and another material systems. This book reviews recent progresses of such novel III-V semiconductor nanowires, covering a wide range of aspects from the epitaxial growth to the device applications. Prospects of such advanced 1D structures for nanoscience and nanotechnology are also discussed.