Nanogap Device: Fabrication and Applications

A nanogap device as a platform for nanoscale electronic devices is presented. Integrated nanostructures on the platform have been used to functionalize the nanogap for biosensor and molecular electronics. Nanogap devices have great potential as a tool for investigating physical phenomena at the nanoscale in nanotechnology. In this dissertation, a laterally self-aligned nanogap device is presented and its feasibility is demonstrated with a nano ZnO dot light emitting diode (LED) and the growth of a metallic sharp tip forming a subnanometer gap suitable for single molecule attachment. For realizing a nanoscale device, a resolution of patterning is critical, and many studies have been performed to overcome this limitation. The creation of a sub nanoscale device is still a challenge. To surmount the challenge, novel processes including double layer etch mask and crystallographic axis alignment have been developed. The processes provide an effective way for making a suspended nanogap device consisting of two self-aligned sharp tips with conventional lithography and 3-D micromachining using anisotropic wet chemical Si etching. As conventional lithography is employed, the nanogap device is fabricated in a wafer scale and the processes assure the productivity and the repeatability. The anisotropic Si etching determines a final size of the nanogap, which is independent of the critical dimension of the lithography used. A nanoscale light emitting device is investigated. A nano ZnO dot is directly integrated on a silicon nanogap device by Zn thermal oxidation followed by Ni and Zn blanket evaporation instead of complex and time consuming processes for integrating nanostructure. The electrical properties of the fabricated LED device are analyzed for its current-voltage characteristic and metal-semiconductor-metal model. Furthermore, the electroluminescence spectrum of the emitted light is measured with a monochromator implemented with a CCD camera to understand the optical properties. The atomically sharp metallic tips are grown by metal ion migration induced by high electric field across a nanogap. To investigate the growth mechanism, in-situ TEM is conducted and the growing is monitored. The grown dendrite nanostructures show less than 1nm curvature of radius. These nanostructures may be compatible for studying the electrical properties of single molecule

Functional 2D nanoparticle/polymer array : interfacial assembly, transfer, characterization, and coupling to photonic crystal cavities

We developed a universal, facile and robust method to prepare free-standing, ordered and patternable nanoparticle/polymer monolayer arrays by evaporation-induced self-assembly at a fluid interface. The ultra-thin monolayer nanoparticle/polymer arrays are sufficiently robust that they can be transferred to arbitrary substrates, even with complex topographies. More importantly, the Poly (methyl methacrylate) (PMMA) in the system serves as a photoresist enabling two modes of electron beam (e-beam) nanoparticle patterning. These ultra-thin films of monolayer nanoparticle arrays are of fundamental interest as 2D artificial solids for electronic, magnetic and optical properties and are also of technological interest for a diverse range of applications in micro- and macro-scale devices including photovoltaics, sensors, catalysis, and magnetic storage. By co-assembly with block co-polymers, the nanoparticles were selectively positioned in one specific phase, representing a high throughput route for creating nanoparticle patterns. The self-assembly process was investigated by combined in-situ grazing incidence small angle x-ray scattering (GISAXS) and numerical simulation. By e-beam irradiation of free-standing 2D NP/polymer arrays, anisotropic nanowire arrays have been fabricated. Additionally, preliminary investigation on assembly of binary nanoparticle arrays has also been introduced, serving as promising future directions of interfacial assembly. viii Controlling the rate of spontaneous emission and thus promoting the photon generation efficiency is a key step toward fabrication of Quantum dot based single-photon sources, and harnessing of light energy from emitters with a broad emitting spectrum. Coupling of photo emitters to photonic cavities without perturbing the optical performance of cavities remains as a challenge in study of Purcell effect based on quantum electrodynamics. Taking advantage of interfacial assembly and transfer, we have achieved controlled deposition of quantum dots into high Q photonic microcavities and studied the modification of their optical properties. Anomalous enhanced spontaneous emission and Fabry-Perot resonance have been observed

Gate-Controlled Quantum Dots in Two-Dimensional Tungsten Diselenide and One-Dimensional Tellurium Nanowires

This work focuses on the investigation of gate-defined quantum dots in two-dimensional transition metal dichalcogenide tungsten diselenide (WSe2) as a means to unravel mesoscopic physical phenomena such as valley-contrasting physics in WSe2 flakes and its potential application as qubit, as well as realizing gate-controlled quantum dots based on elementaltellurium nanostructures which may unlock the topological nature of the host material carriers such as Weyl states in tellurium nanowires.The fabrication and characterization of gate-defined hole quantum dots in monolayer and bilayer WSe2 are reported. The gate electrodes in the device design are located above and below the WSe2 nanoflakes to accumulate a hole gas. For some devices we additionally used gates to deplete the gas to define the dot. Temperature dependence of Coulomb-blockade peak height complies with single-level transport and the small size of the dot leads to observation of excited states in the Coulomb diamond measurements. Further, magnetic field dependence of the excited states in the bilayer devices provides a lower bound for g factors. For the chiral crystals of elemental Te, the intriguing property of combining Weyl physics with a small semiconducting bandgap enables the creation of gate-tunable devices to probe and utilize the topological properties of Te. The formation of gate-defined quantum dots in Te would allow Coulomb blockade spectroscopy to provide information about the strength of exchange interaction, spin-orbit coupling, and g-factors associated with discrete quantum states in Te nanostructures. Using low-pressure physical vapor deposition, Te nanowires are grown that permits local control of carrier density using electrostatic gates. While atomically flat hexagonal boron nitride (hBN) gate dielectrics haves been widely used for high quality layered material devices, the relatively weak adhesion to Te nanowires makes hBN-insulated Te device assembly challenging. Therefore, the configuration of the device underwent a few iterations. The gate electrodes design and insulating strategy compare different methods involving more traditional dielectrics, as well as a hybrid approach that uses a global Si backgate and hBN-insulated local top gates for these Weyl semiconductor devices. Early measurements of Te devices demonstrate density control in these devices. Future work must be aimed at quantum transport measurements in Te dots

Gate control, g-factors and spin orbit energy of p-type GaSb nanowire quantum dot devices

Proposals for quantum information applications are frequently based on the coherent manipulation of spins confined to quantum dots. For these applications, p-type III-V material systems promise a reduction of the hyperfine interaction while maintaining large $g$-factors and strong spin-orbit interaction. In this work, we study bottom-gated device architectures to realize single and serial multi-quantum dot systems in Schottky contacted p-type GaSb nanowires. We find that the effect of potentials applied to gate electrodes on the nanowire is highly localized to the immediate vicinity of the gate electrode only, which prevents the formation of double quantum dots with commonly used device architectures. We further study the transport properties of a single quantum dot induced by bottom-gating, find large gate-voltage dependent variations of the $g^*$-factors up to $8.1\pm 0.2$ as well as spin-orbit energies between $110$-$230\,\mu$eV.Comment: 7 pages, 4 figure

Methodologies for Achieving 1D ZnO Nanostructures Potential for Solar Cells

One-dimensional (1D) nanostructures are generally used to describe large aspect ratio rods, wires, belts, and tubes. The 1D ZnO nanostructures have become the focus of research owing to its unique physical and technological significance in fabricating nanoscale devices. When the radial dimension of the 1D ZnO nanostructures decreases to some lengths (e.g., the light wavelength, the mean of the free path of the phonon, Bohr radius, etc.), the effect of the quantum mechanics is definitely crucial. With the large surface-to-volume ratio and the confinement of two dimensions, 1D ZnO nanostructures possess the captivating electronic, magnetic, and optical properties. Furthermore, 1D ZnO nanostructure’s large aspect ratio, an ideal candidate for the energy transport material, can conduct the quantum particles (photons, phonons, electrons) to improve the relevant technique applications. To date, many methods have been developed to synthesize 1D ZnO nanostructures. Therefore, methodologies for achieving 1D ZnO nanostructures are expressed, and the relevant potential application for solar cells are also present to highlight the attractive property of 1D ZnO nanostructures

Development of Zinc Oxide Nanowires and Quantum Dot Incorporation for Photovoltaic Applications

Heterojunctions of metal oxide semiconductors with quantum dots (QD) have been deployed in a number of advanced electronic devices. Improvement in the devices’ performance requires in-depth studies on charge carrier transfer dynamics. In this work, charge carrier dynamics, at the interface on zinc oxide nanowires (ZnO NW) with cadmium selenide QDs, were investigated. ZnO NWs were synthesized and characterized through the chemical vapor deposition (CVD) and hydrothermal methods. Both methods yielded highly crystalline ZnO structures. The hydrothermally grown NWs were doped with aluminum (Al) and the spectroscopy analyses showed that Al was successfully incorporated into the ZnO crystalline structure. Colloidal cadmium selenide/zinc sulfide (CdSe/ZnS) core/shell QDs were incorporated into synthesized ZnO NW arrays. The interaction and wettability of two different QD ligands (Octadecylamine and oleic acid) on the self-assembly of QDs in the NW spacing were investigated using electron microscopy. Afterwards, the charge carrier transfer dynamics at the heterojunction of NW/QD were studied employing time resolved photoluminescence spectroscopy (TRPL). A hypothesis on charge transfer kinetics, based on the experimental measurements, was provided. It was realized that photocharging of QDs is the main reason for substantial PL quench, when holes are not effectively removed from the photoexcited QDs by a hole-transporting medium. Furthermore, the TRPL measurements showed that the hole transfer rate by a polysulfide electrolyte is slower than that of an electron; one main reason in impeding the device performance in quantum dot-sensitized solar cells (QDSSC). The NW/QD heterojunction was deployed in the structure of a QDSSC. The current-voltage behavior of the cells under various conditions was characterized in both dark and light conditions. The underlying problems hindering the device performance were identified by these characterizations. Heterojunction of ZnO NWs with a GaN thin film was also deployed in the structure of an LED. The NWs were grown on GaN film using the hydrothermal method. The fabricated device exhibited light emission under both forward and reverse bias injection currents. The electroluminescence and PL characterizations revealed that the light emission from the fabricated device depends on the point defects and interface states of the two semiconductors

Epitaxial growth of iii-nitride nanostructures and their optoelectronic applications

Light-emitting diodes (LEDs) using III-nitride nanowire heterostructures have been intensively studied as promising candidates for future phosphor-free solid-state lighting and full-color displays. Compared to conventional GaN-based planar LEDs, III-nitride nanowire LEDs exhibit numerous advantages including greatly reduced dislocation densities, polarization fields, and quantum-confined Stark effect due to the effective lateral stress relaxation, promising high efficiency full-color LEDs. Beside these advantages, however, several factors have been identified as the limiting factors for further enhancing the nanowire LED quantum efficiency and light output power. Some of the most probable causes have been identified as due to the lack of carrier confinement in the active region, non-uniform carrier distribution, and electron overflow. Moreover, the presence of large surface states and defects contribute significantly to the carrier loss in nanowire LEDs. In this dissertation, a unique core-shell nanowire heterostructure is reported, that could overcome some of the aforementioned-problems of nanowire LEDs. The device performance of such core-shell nanowire LEDs is significantly enhanced by employing several effective approaches. For instance, electron overflow and surface states/defects issues can be significantly improved by the usage of electron blocking layer and by passivating the nanowire surface with either dielectric material / large bandgap energy semiconductors, respectively. Such core-shell nanowire structures exhibit significantly increased carrier lifetime and massively enhanced photoluminescence intensity compared to conventional InGaN/GaN nanowire LEDs. Furthermore, AlGaN based ultraviolet LEDs are studied and demonstrated in this dissertation. The simulation studies using Finite-Difference Time-Domain method (FDTD) substantiate the design modifications such as flip-chip nanowire LED introduced in this work. High performance nanowire LEDs on metal substrates (copper) were fabricated via substrate-transfer process. These LEDs display higher output power in comparison to typical nanowire LEDs grown on Si substrates. By engineering the device active region, high brightness phosphor-free LEDs on Cu with highly stable white light emission and high color rendering index of \u3e 95 are realized. High performance nickel?zinc oxide (Ni-ZnO) and zinc oxide-graphene (ZnO-G) particles have been fabricated through a modified polyol route at 250?C. Such materials exhibit great potential for dye-sensitized solar cell (DSSC) applications on account of the enhanced short-circuit current density values and improved efficiency that stems from the enhanced absorption and large surface area of the composite. The enhanced absorption of Ni-ZnO composites can be explained by the reduction in grain boundaries of the composite structure as well as to scattering at the grain boundaries. The impregnation of graphene into ZnO structures results in a significant increase in photocurrent consequently due to graphene\u27s unique attributes including high surface area and ultra-high electron mobility. Future research directions will involve the development of such wide-bandgap devices such as solar cells, full color LEDs, phosphor free white-LEDs, UV LEDs and laser diodes for several applications including general lighting, wearable flexible electronics, water purification, as well as high speed LEDs for visible light communications

Strain Engineering of InGaN/GaN Nanopillars for Optoelectronic Applications.

The InGaN/GaN material system is critical for optoelectronic applications because it has direct band gap and large oscillator strength. The bandgap can be tuned by the alloy composition, and the emission wavelength covers the entire visible spectrum and extends into ultraviolet and near infrared regions. Due to the large lattice mismatch between InGaN and GaN, a large built-in strain exists in the InGaN quantum wells and induces a piezoelectric field across the wells. The piezoelectric field leads to the quantum-confined Stark effect which red-shifts the emission wavelength and degrades the recombination efficiency. It is known that nanostructures have large surface-to-volume ratio and can help relax strain via free surfaces. In this work, we present top-down InGaN/GaN nanostructures to manipulate the strain and serve as a building block to engineer the strain effect for novel optoelectronic functionalities. First, we demonstrate the emission colors from top-down nanopillars can be tuned from blue to red by changing the nanopillar diameter. The wavelength shift is well-described by an analytical model. We also demonstrate electrical nanoLED devices based on the nanopillars. It provides a simple solution to monolithic integration of multiple color pixels on a single chip. Second, we discuss the benefits of strain engineering for quantum light source applications. We focus on the intrinsic control of photon polarization states via asymmetric strain. Experimental data is provided to show that pre-defined polarization states of single photons with high degree of linear polarization can be achieved by engineering quantum dot geometry and strain. It suggests the potential of top-down InGaN quantum dots for quantum information applications. Finally, the non-ideal factors in our top-down quantum dots, including random alloy fluctuation and well-width fluctuation, are discussed. These effects modify the potential landscape and impose a fundamental limit to the quantum dot inhomogeneity, especially for ternary alloys. A methodology to model random alloy distribution and random well-width fluctuation is developed. The modeling results suggest that the strain-relaxation-induced potential is the dominant effect of lateral confinement even with the presence of random indium fluctuation and well-width fluctuation. The results are also compared to experimental data and show very good agreement.PhDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/120736/1/chteng_1.pd