Semiconductor quantum dots are promising candidates as bright, indistinguishable, single-photon sources---making them desirable for applications in quantum computing and quantum cryptography protocols. By embedding the quantum dots in III-V nanowires, the collection efficiency from the quantum dot is greatly increased. Our goal is to develop a platform that allows for the stable and efficient generation of single-photons on chip. This on-chip design offers an enhanced degree of stability and miniaturization, important in many applications involving the processing of quantum information. In this thesis, we demonstrate the efficient coupling of quantum light generated in a III-V photonic nanowire to a silicon-based photonic integrated circuit. We use high quality SiN waveguide devices fabricated by a foundry (LIGENTEC) to minimize coupling and propagation losses through the waveguide. A hybrid integration of these single-photon sources with a photonic integrated circuit is developed by employing a "pick & place" method which uses a nanomanipulator in a scanning electron microscope setup. By tailoring the nanowire geometry, we are able to maximize the efficient coupling between the optical mode of the photonic nanowire and an accompanying SiN waveguide through evanescent coupling. To determine the effectiveness of our integration method, we compare our hybrid devices with free-standing nanowires on their growth substrate. For each set, we measured the optical properties (brightness, spectral purity, lifetime, and single-photon purity) and efficiencies of the devices. We have shown that using tapered nanowires with embedded quantum dots coupled to on-chip photonic structures is a viable route for the fabrication of stable, high-efficiency, single-photon sources. Although the measured collection efficiencies from device to device were substantially different 9.6%~93%, we have found that the optical properties of the hybrid devices were hardly impacted from the transfer process. In fact, from the same nanowire that achieved 93% coupling efficiency, we were able to measure a single photon purity of 97%. By comparing the amount of emitted light collected from both ends of the nanowire (taper and base), we confirmed that the coupling efficiency of the devices have a strong dependence on the geometry of the nanowire as collection from the taper yielded count rates at least 10x greater than from the base. From our promising results, we can envision integrating the nanowire devices with different types of photonic structures such as ring resonators.

No description available.

No description available.

In order to completely assess the potential of semiconductor nanowires for multifunctional applications such as flexible electronics, nanoelectromechanical systems (NEMS), and composites, a full characterization of their properties must be obtained. While many of their physical properties have been well studied, explorations of mechanical, electromechanical, and optical properties of semiconductor nanowires remain relatively sparse in the literature. Two major hurdles to the elucidation of these properties are: (1) the development of experimental techniques which are capable of mechanical and electromechanical measurements coupled with detailed structural analysis, and (2) the synthesis of high quality nanowires with the high yields necessary to produce the quantities needed for composite fabrication. These issues are addressed in this dissertation by utilizing the supercritical fluid-liquid-solid (SFLS) synthesis method to produce germanium (Ge) nanowire specimens for mechanical and electromechanical measurements coupled with high-resolution transmission electron microscopy (HRTEM). In addition, excellent dispersibility and large quantities allow for optical measurements of dispersions and composites. Ge cantilever nanoelectromechanical resonators were fabricated and induced into resonance. From the frequency response, the Young's modulus of the nanowires was determined to be insensitive to diameter and on par with the literature values for bulk Ge. The mechanical quality factors of the resonators were found to decrease with decreasing diameter. The data indicate that energy dissipation from the oscillating cantilevers occurs predominantly via surface losses. The mechanical strengths of individual Ge nanowires were measured by in situ nanomanipulation in a scanning electron microscope (SEM). The nanowires were found to tolerate diameter-dependent flexural strains more than two orders of magnitude higher than bulk Ge. Corresponding bending strengths were in agreement with the ideal strength of a perfect Ge crystal, indicative of a reduced presence of extended defects. The nanowires also exhibited plastic deformation at room temperature, becoming amorphous at the point of maximum strain. The optical absorbance spectra of Ge nanowires were measured and found to exhibit spectra markedly different from bulk Ge. Simulations using a discrete dipole approximation (DDA) model suggest that the difference in light absorption results from light trapping within the nanowires. / text

For the last decade, semiconducting nanowires synthesized by bottom-up methods have opened up new opportunities, stimulated innovative scientific research, and led to applications in materials science, electronics, optics, and biology at the nanoscale. Notably, nanowire building blocks with precise control of size, structure, morphology, and even composition in one, two, and three dimensions can successfully demonstrate high-performance electrical characteristics of field-effect transistors (FETs) and highly sensitive, selective, label-free, real-time biosensors in the fields of nanoelectronics and nano-biosensing, respectively. This thesis has focused on the design, synthesis, assembly, fabrication and electrical characterization of nanowire heterostructures for a proof-of-concept nanoprocessor and morphology-modulated kinked nanowire molecular nanosensor. / Physics

No description available.

No description available.

III–V nitride quantum-confined structures embedded in nanowires (NWs), also known as quantum-disks-in-nanowires (Qdisks-in-NWs), have recently emerged as a new class of nanoscale materials exhibiting outstanding properties for optoelectronic devices and systems. It is promising for circumventing the technology limitation of existing planar epitaxy devices, which are bounded by the lattice-, crystal-structure-, and thermal- matching conditions. This work presents significant advances in the growth of good quality GaN, InGaN and AlGaN Qdisks-in-NWs based on careful optimization of the growth parameters, coupled with a meticulous layer structure and active region design. The NWs were grown, catalyst-free, using plasma assisted molecular beam epitaxy (PAMBE) on silicon (Si) substrates. A 2-step growth scheme was developed to achieve high areal density, dislocation free and vertically aligned NWs on Ti/Si substrates. Numerical modeling of the NWs structures, using the nextnano3 software, showed reduced polarization fields, and, in the presence of Qdisks, exhibited improved quantum-confinement; thus contributing to high carrier radiative-recombination rates. As a result, based on the growth and device structure optimization, the technologically challenging orange and yellow NWs light emitting devices (LEDs) targeting the ‘green-yellow’ gap were demonstrated on scalable, foundry compatible, and low-cost Ti coated Si substrates. The NWs work was also extended to LEDs emitting in the ultraviolet (UV) range with niche applications in environmental cleaning, UV-curing, medicine, and lighting. In this work, we used a Ti (100 nm) interlayer and Qdisks to achieve good quality AlGaN based UV-A (320 - 400 nm) device. To address the issue of UV-absorbing polymer, used in the planarization process, we developed a pendeo-epitaxy technique, for achieving an ultra-thin coalescence of the top p-GaN contact layer, for a self-planarized Qdisks-in-NWs UV-B (280 – 320 nm) LED grown on silicon. This process constitutes a significant advancement in simplifying the UV-B and UV-C fabrication process favoring light extraction. Addressing the issue of poor white light quality in the conventional blue laser diode (LD) and YAG:Ce3+ technology, a number of applications related investigations was conducted. Notably, the orange and yellow emitting InGaN/GaN Qdisks-in-NWs LEDs were implemented as an “active phosphor” to achieve intensity- and bandwidth-tunability for high color-quality solid-state lighting.

Thesis advisor: Michael Graf / Bismuth is an interesting element to study because the low effective mass of its charge carriers makes the material sensitive to quantum confinement effects. When bismuth is reduced to the nanoscale two interesting phenomena may occur: it may transition from a semimetal to a semiconductor, or charge carriers in special surface states may begin to dominate the behavior of the material. Arrays of bismuth nanowires of various diameters were studied to investigate these possibilities. The magnetoresistance of the arrays was measured and the period of Shubnikov-de Haas oscillations suggested an increase in the effective mass and density of the material’s charge carriers for small nanowire diameters. These increases suggested that electrons were present in surface states and strongly influenced the material’s behavior when its dimensions were sufficiently reduced. The magnetization of the nanowire arrays was also measured and the lack of de Haas-van Alphen oscillations for certain diameter nanowires suggested that electrons were not present in surface states and that instead the material was transitioning from a semimetal to a semiconductor. Heat capacity measurements were planned to reconcile the two experiments. My detailed calculations demonstrated that heat capacity measurements were feasible to determine the presence, or absence, of surface charge carriers. Because the electronic contribution to the material’s heat capacity is small a calorimeter platform was constructed with ultra-low heat capacity components. / Thesis (BS) — Boston College, 2009. / Submitted to: Boston College. College of Arts and Sciences. / Discipline: College Honors Program. / Discipline: Physics.

No description available.