My research regarded the fabrication of InSb and InAsSb large diameter nanowires for infrared applications. / InSb and InAsSb pillars, which are large diameter nanowires (NWs), were investigated as an alternative infrared (IR) detector technology to HgCdTe (MCT) for tunable multispectral IR detection with optical properties manipulated by pillar diameter and pitch. Undoped InSb and InAsSb thin films were grown on undoped Si (100) substrates by molecular beam epitaxy (MBE) with a thin AlSb buffer layer. A top-down etching method was used to fabricate pillars of diameters ranging from 300 nm to 1500 nm for InSb, and 1700 nm to 4000 nm for InAsSb. Pillar arrays were analyzed optically by Fourier transform IR spectroscopy (FTIR). The InSb and InAsSb pillars produced narrow absorption peaks with wavelength ranging from 1.61 μm to 6.86 μm for InSb and 8.1 μm to 16.2 μm for InAsSb. A 100 nm increase in pillar diameter corresponded to a 0.495 μm increase in peak absorption wavelength. InSb thin films were also grown on n-type (As doped, ≤ 0.005 Ω cm) Si (100) substrates to create a p-i-n junction, with an initial 2 μm thick undoped InSb region grown directly on the substrate, and a 0.5 μm thick p-type (Be doped, 2x1019 cm-3) InSb top layer. These films were used to create two devices; an interdigitated contact photoconductor with varying finger geometry, and a photovoltaic device with square top contacts of varying area. I-V characterization demonstrated trends in current with varying finger geometry. Photocurrent measurements were obtained for both the photoconductor and photovoltaic devices under IR and solar illumination. The photocurrent values were orders of magnitude higher for the photoconductive device compared to the photovoltaic device, indicative of potential photoconductive gain. Photocurrent generation in the InSb p-i-n structure introduces the possibility of diameter-dependent photocurrent generation in etched pillar devices. / Thesis / Master of Applied Science (MASc) / Infrared light (IR) falls in the wavelength range of 0.75 μm to 1000 μm, with IR based technology having numerous applications in society. With uses in the sciences, research, medicine, and general everyday technology, common IR ranges for material analysis range from 1.4 to 3 μm in the short-wavelength IR (SWIR), 3-5 μm in the mid-wavelength IR (MWIR), and 8-15 μm in the long-wavelength IR (LWIR). These ranges include IR absorption due to molecular vibrations, and includes wavelengths corresponding to the bandgaps of relevant semiconductor materials for IR detectors. To aid in light absorption in semiconductor materials, nanometer scale cylindrical structures called nanowires or pillars can be used on the detector surface, enhancing light absorption, and allowing for absorption wavelength manipulation by adjusting nanowire diameter. This work focuses on developing IR detectors with wavelength absorption in the 1-16 μm range, dependent on nanowire geometry.

Thermoelectric devices (TEDs) are useful in a variety of niche applications, but low efficiencies limit their broader application. Semiconductor nanowires (NWs) could be the key to efficient thermoelectrics, through the benefits of one-dimensional band structures and a greatly reduced thermal conductivity. This thesis explores the transport fundamentals, experimental characterization, and computational approaches relevant to prospective III-V NW TEDs. Predictive electronic transport models are outlined for NWs and bulk III-Vs. These models are used to determine the optimum carrier concentration for maximizing the thermoelectric figure of merit (𝑍𝑇) in the bulk and in NWs of arbitrary size. We demonstrate the physical mechanisms underlying electronic thermoelectric improvements in NWs and confirm the superior performance of InSb and InAs, among other III-Vs. Next, thermal conductivity reduction in structurally complex NWs is investigated as a means of improving 𝑍𝑇. We compare polytypic and twinning superlattice (TSL) GaAs NWs in measurements obtained by a novel application of the 3𝜔 method. We find thermal conductivities of 8.4 ± 1.6 W/m-K and 5.2 ± 1.0 W/m-K for the polytypic and TSL NWs, respectively, demonstrating a significant difference and an almost ten-fold reduction compared to 50 W/m-K of bulk GaAs. We employ molecular dynamics simulations and the atomistic Green’s function method to address phonon engineering in generalized GaAs NW structures. In comparing twinning NWs, we find that a TSL period of 50 Å minimizes the lattice thermal conductivity across all the diameters considered. Our results also illustrate the importance of NW surfaces versus the internal crystal structure. Phonon coherence lengths are obtained by analyzing thermal conductivity trends in periodic and aperiodic structures. Transmission spectra are calculated to reveal the phonon frequencies targeted by structural engineering in NWs. These findings explain the range of thermal conductivities obtained for GaAs NWs with various crystal phases. Finally, to inform future growths of TSL NWs, we study the influence of the substrate temperature and V/III flux ratio on TSL formation in Te-doped GaAs NWs. The crystal structure of several NWs is investigated using transmission electron microscopy, revealing a range of polytypic and TSL morphologies. We find that periodic TSLs form only at low V/III flux ratios of 0.5 and substrate temperatures of 492 to 537 °C. To explain these trends, we derive a phase diagram for TSL NWs based on a kinetic growth model. / Thesis / Doctor of Philosophy (PhD) / In a circuit of dissimilar conductors, temperature differences create voltage differences that can drive electrical currents. Similarly, electrical currents in such circuits inherently lead to heating and cooling. These phenomena are known as thermoelectric effects because they couple heat and charge transport (electricity) in a symmetric and reversible way. The goal of some thermoelectric devices (TEDs) is to exploit these effects to generate electrical power or to provide controlled cooling. However, greater conversion efficiencies are required to compete against other existing technologies. With the advent of nanofabrication, semiconductor nanowires (NWs) have emerged as an attractive material system for efficient TEDs. In this thesis, we explore their thermal and electronic properties. We demonstrate a novel way to measure the NW thermal conductivity and employ computational methods to examine heat transport in NWs with various crystal structures. Finally, we examine how synthesis conditions can determine the morphology of NWs.

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This thesis presents work to help enable the transition of sensitive nanoscale instruments from research laboratory demonstration to societal use. It focuses on nanomechanical resonators made by field-directed assembly, with contributions to understanding effects of materials, clamp geometries and laser measurement of motion, towards their use as commercial scientific instruments. Nanomechanical resonators in their simplest form are cantilevered or doubly- clamped nanowires or nanotubes made to vibrate near one of their resonant frequencies. Their small mass and high frequency enable extraordinary mass sensitivity, as shown in published laboratory-scale demonstrations of their use for detection of a few molecules of prostate cancer biomarker and of their response to mass equal to that of a single proton. However such sensitive devices have been prohibitively expensive for societal use, since the fabrication process cost scales with number of devices and the chip area covered, when they are made using standard electron beam lithography. Our laboratory has published new results for the method of field-directed assembly, in which the nanofabrication process cost is independent of the number of devices. While drastically lowering the cost, this method also broadens the range of device materials and properties that can be used in instrument applications for sensitive mass and force detection. Unanswered questions affecting the performance of devices made by this method are studied in this thesis. Clamping variability can cause uncertainties in the device resonant frequency (effective stiffness), raising manufacturing metrology costs to track reduced homogeneity in performance. Using a numerical model, we quantify how compliant clamp material and insufficient clamp depth reduce the effective stiffness and resonance frequency. Obliquely clamped nanowires and defects at the clamp-nanowire interface break the symmetry and split the resonance frequency into fast and slow modes. The difference of resonance frequency between the fast and slow modes corresponds to the degree of asymmetry and must be controlled in fabrication to keep device error bounded. Optical transduction has been used for measuring the nanoresonator frequency spectrum; however, the influence of the laser in the measurement process is only recently receiving attention and is not well understood. We found that the measured spectrum is significantly influenced by laser-nanowire interaction. Variation of input laser power could result in resonance peak shifts in the kHz range for a resonance frequency in the MHz range, which could reduce device mass resolution by a factor of 100 or greater. As the laser power is increased, the resonance frequency decreases. The heating effect of the laser on temperature-dependent Young’s modulus could explain this phenomenon. To our surprise, we also found that the amplitude and frequency of the resonance peak signal vary significantly with the angle made by the plane of laser polarization with the nanowire axis. Our measurements established that the maximum signal amplitude is seen when the plane of the linearly polarized laser is parallel to SiNW or perpendicular to RhNW. Maximum resonance frequency was found when laser is polarized perpendicular to SiNW or parallel to RhNW. / Graduate / 0537 / 0548 / 0752

No description available.

The conventional floating gate NVSM will suffer some limitations for continued scaling of the device structure. Therefore, the silicon-oxide-nitride-oxide-silicon (SONOS) and the nanocrystal nonvolatile memory devices, have been investigated to overcome the limit of the conventional floating gate NVSM. For driving device application, we have used multilayer ONO gate dielectrics to make change the effective dielectric constant. The proposed TFT with ONO gate dielectrics have better gate control ability. On the other hand, nanowire has larger electric-field in the corner region at the same voltage. The SONOS-TFT with multiple nanowire channels have superior electrical characteristic, such as lower threshold voltage, higher On/Off ratio, steeper subthreshold slope, and superior driving ability. The memory characteristic of standard SONOS-TFT, channel width of the device is 1£gm, was compared with the nanowires SONOS-TFT, each channel width of the device is 65nm. The SONOS-TFT with multiple nanowires structure (NW SONOS-TFT) has good program/erase efficiency, retention, transfer characteristics and can suppress gate injection effectively. These characteristics are due to the larger electric field at the corner region and more number of corners. The NW SONOS-TFTs can be treated as high performance devices and also as high program/erase efficiency nonvolatile memory under adequate voltage range operation. In this thesis, the P/E characteristics at different temperatures will also be measured and discussed. The fabrication of SONOS-TFTs with nano-wire channels is quite easy and involves no additional processes. Such a SONOS-TFT is there by highly promising for application in the future system-on-panel display applications. The SONOS-TFTs combined the TFT and memory properties at the same time. Furthermore, the process flow is compatible with conventional poly-Si TFTs fabrication without additional process steps. Hence, the application of SONOS TFTs structure can reach the goal of system on panel (SOP) in the future.

Thermal transport in nanowires and nanotubes has attached much attention due to their use in various functional devices and their use as a model system for low dimensional transport phenomena. The precise control of the crystal structure, defects, characteristic size, and electronic properties of nanowires has allowed for fundamental studies of phonon and electron transport in a variety of nanoscale systems. The thermal conductivity in nanostructured materials can vary greatly compared to bulk values owing to classical and quantum size effects. In this work, two model systems for investigating fundamental phonon transport were investigated for potential use in thermoelectric and thermal management applications. The thermoelectric properties of twin defect indium arsenide nanowires and the thermal conductivity of polythiophene nanofibers with improved polymer chain crystallinity were measured with a microfabricated measurement device. The effects of twin planes on reducing the mean free path of phonons in indium arsenide and the effects of improved chain alignment in increasing the thermal conductivity in polymer fibers is discussed. / text

For many decades silicon based CMOS technology has made continual increase in drive current to achieve higher speed and lower power by scaling the gate length and the gate insulator thickness. The scaling becomes increasingly challenging because the devices are approaching physical quantum limits. Three-dimensional electronic devices, such as double gate, tri-gate and nanowire field-effect-transistors (FETs) provide an alternative solution because the ultra-thin fin or nanowire provides better electrostatic control of the device channel. Also high-[kappa] oxides lower the gate leakage current significantly, due to larger thickness for the same equivalent oxide thickness (EOT) compared with SiO₂ beyond the 22 nm node. Moreover, metal gate that avoids the poly-depletion effect in poly-Si gate has become mainstream semiconductor technology. The enabler technologies for high-[kappa] / metal gate 3D transistors include fabrication of high quality, vertical nanowire arrays, conformal metal and dielectric deposition and vertical patterning. One of the main focuses of this dissertation is developing a fabrication process flow to realize high performance MOSFETs with high-[kappa] oxide and metal gate on vertical silicon nanowire arrays. A variety of approaches to fabricating highly ordered silicon nanowire arrays have been achieved. Deep silicon etching process was developed and optimized for nanowire FETs. Process integration and patterning mythologies for high-[kappa] / metal gate were investigated and accomplished. 3-D electronic devices including nanowire capacitors, nanowire FETs and double gate MOSFETs for power applications were fabricated and characterized. The second part of this dissertation is about flexible electronics. Mechanically flexible integrated circuits (ICs) have gained increasing attention in recent years with emerging markets in portable electronics. Although a number of thin-film-transistor (TFT) IC solutions have been reported, challenges still remain for fabrication of inexpensive, high performance flexible devices. We report a simple and straightforward solution: mechanically exfoliating a thin Si film containing ICs. Transistors and circuits can be pre-fabricated on bulk silicon wafer with conventional CMOS process flow without additional temperature or process limitations. The short channel MOSFETs exhibit similar electrical performance before and after exfoliation. This exfoliation process also provides a fast and economical approach to produce thinned silicon wafers, which is a key enabler for three-dimensional (3D) silicon integration based on Through Silicon Vias (TSVs). / text

This study is dedicated to the implementing of Electron-Beam-Induced Current (EBIC) microscopy to study the behavior of metal oxide semiconducting (MOS) nanowire (NW) gas sensor in situ under exposure to different environment. First, we reported the development of a single nanowire gas sensor compatible with an environmental cell. The major component of the device we use in this study is a single SnO2 nanowire attached to an electron transparent SiN membrane (50-100 nm thick), which was used for mounting nanowire working electrodes and surface imaging of NW. First the NW's conductivity is investigated in different temperatures. Higher temperature is proved to cause higher conductivity of NW. We also found that often the Schottky barrier is formed at the nanowire's contacts with Au and Au/Cr electrodes. Then NW's responses to gas and electron beam (from SEM) are analyzed quantitatively by current measurement. Electron-Beam-Induced Current technique was introduced for the first time to characterize the conductivity behavior of the nanowire during the gas sensing process. Resistive contrast was observed in the EBIC image.

abstract: Zinc oxide nanowires ( NWs) have broad applications in various fields such as nanoelectronics, optoelectronics, piezoelectric nanogenerators, chemical/biological sensors, and heterogeneous catalysis. To meet the requirements for broader applications, the growth of high-quality ZnO NWs and functionalization of ZnO NWs are critical. In this work, specific types of functionalized ZnO NWs have been synthesized and correlations between specific structures and properties have been investigated. Deposition of δ-Bi2O3 (narrow band gap) epilayers onto ZnO (wide band gap) NWs improves the absorption efficiency of the visible light spectrum by 70%. Furthermore, the deposited δ-Bi2O3 grows selectively and epitaxially on the {11-20} but not on the {10-10} facets of the ZnO NWs. The selective epitaxial deposition and the interfacial structure were thoroughly investigated. The morphology and structure of the Bi2O3/ZnO nanocomposites can be tuned by controlling the deposition conditions. Various deposition methods, both physical and chemical, were used to functionalize the ZnO NWs with metal or alloy nanoparticles (NPs) for catalytic transformations of important molecules which are relevant to energy and environment. Cu and PdZn NPs were epitaxially grown on ZnO NWs to make them resistant to sintering at elevated temperatures and thus improved the stability of such catalytic systems for methanol steam reforming (MSR) to produce hydrogen. A series of Pd/ZnO catalysts with different Pd loadings were synthesized and tested for MSR reaction. The CO selectivity was found to be strongly dependent on the size of the Pd: Both PdZn alloy and single Pd atoms yield low CO selectivity while Pd clusters give the highest CO selectivity. By dispersing single Pd atoms onto ZnO NWs, Pd1/ZnO single-atom catalysts (SACs) was synthesized and their catalytic performance was evaluated for selected catalytic reactions. The experimental results show that the Pd1/ZnO SAC is active for CO oxidation and MSR but is not desirable other reactions. We further synthesized ZnO NWs supported noble metal (M1/ZnO; M=Rh, Pd, Pt, Ir) SACs and studied their catalytic performances for CO oxidation. The catalytic test data shows that all the fabricated noble metal SACs are active for CO oxidation but their activity are significantly different. Structure-performance relationships were investigated. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2017