PhD, University of Cincinnati, 2016, Arts and Sciences: Physics

In this dissertation, the dynamics of excitons in hybrid metal/organic/nanowire structures possessing nanometer thick deposited molecular and metal films on top of InP and GaAs nanowire (NW) surfaces were investigated. Optical characterizations were carried out as a function of the semiconductor NW material, design, NW size and the type and thickness of the organic material and metal used. Hybrid organic and plasmonic semiconductor nanowire heterostructures were fabricated using organic molecular beam deposition technique. I investigated the photon emission of excitons in ~150 nm diameter polytype wurtzite/zincblende InP NWs and the influence of a few ten nanometer thick organic and metal films on the emission using intensity- and temperature-dependent time-integrated and time resolved (TR) photoluminescence (PL). The plasmonic NWs were coated with an Aluminum quinoline (Alq3) interlayer and magnesium–silver (Mg0.9:Ag0.1) top layer. In addition, the nonlinear optical technique of heterodyne four-wave mixing was used (in collaboration with Prof. Wolfgang Langbein, University of Cardiff) to study incoherent and coherent carrier relaxation processes on bare nanowires on a 100 femtosecond time-scale. Alq3 covered NWs reveal a stronger emission and a longer decay time of exciton transitions indicating surface state passivation at the Alq3/NW interface. Alq3/Mg:Ag NWs reveal a strong quenching of the exciton emission which is predominantly attributed to Forster energy-transfer from excitons to plasmon oscillations in the metal cluster film. Changing the Mg:Ag to gold and the organic Alq3 spacer layer to PTCDA leads to a similar behavior, but the PL quenching is strongly increased. The observed behavior is attributed to a more continuous gold deposition leading to an increased Forster energy transfer and to a metal induced band-bending. I also investigated ensembles of bare and gold/Alq3 coated GaAs-AlGaAs-GaAs core shell NWs (open full item for complete abstract)

Committee: Hans Peter Wagner Ph.D. (Committee Chair); Philip Argyres Ph.D. (Committee Member); Carlos Bolech Ph.D. (Committee Member); Howard Everett Jackson Ph.D. (Committee Member) Subjects: Physics

PhD, University of Cincinnati, 2012, Arts and Sciences: Physics

This work uses a broad range of optical spectroscopies and electron microscopy to characterize the structure and electronic states of nanowires. We place an emphasis on understanding how to alter the electronic properties using strain and quantum confinement. We seek to develop a comprehensive understanding of NW properties through comparisons with model predictions. In addition, we adapt optical techniques traditionally used with larger structures to obtain a sub-micron measurement of nanowire diffusion and mobility. First, we extend our optical techniques by spatially resolving the diffusion of excitons along the long axis of a nanowire using a solid immersion lens (SIL). By sampling the time decays as a function of distance along the nanowire, we can measure the diffusion of excitons directly. The extracted diffusion constants for defect free single crystal GaAs measured between 45-100 cm^2/s with resultant mobilities of 52,000-116,000 cm^2/eV s. In contrast, a mixed phase InP nanowire shows a much shorter spatial diffusion limited by defect states with measured diffusion constants of 22 cm^2/s and mobilities of 29,000 cm^2/eV s. Turning our focus novel NW morphologies in Chapter 3-5, we first study the strain effects from a series of a lattice mismatched (3.6%) GaAs/GaP core shell NWs. Strain on a semiconductor creates deformations in the lattice of the material which in turn effect the electronic states and possibly the material quality. We compare our PL energies with theoretical predictions and find that our measurements are lower than predicted. We next exploit correlations between PL emission and TO2 phonon emission to predict the hydrostatic and sheer strains in cases when the light hole emission is not visible and/or TO1 phonon cannot be resolved. In chapter 4, we investigate the material quality issues with these strained nanowires and find that the presence of dislocations results in non-radiative recombination centers which causes the electron-hole (open full item for complete abstract)

Committee: Leigh Smith PhD (Committee Chair); Howard Everett Jackson PhD (Committee Member); Kay Kinoshita PhD (Committee Member); Michael Ma PhD (Committee Member); Jan Yarrison-Rice PhD (Committee Member) Subjects: Nanotechnology

Master of Science, Miami University, 2013, Physics

Gold nanoparticles and semiconductor nanowires are widely studied for novel optical and electrical properties, and for possible application in nano-chemical sensors and high efficiency photovoltaic arrays. Herein, we pursue a finite difference time domain analysis of several varieties of gold nanoparticles and semiconductor nanostructures. We present the optical absorption of single particles which compares well to the optical extinction of particle colloids and photocurrent measurements of single nanostructures. We also present local electric fields generated by excited metallic and semiconductor particles.

Committee: Jan Yarrison-Rice (Advisor); James Clemens (Committee Member); Herbert Jaeger (Committee Member) Subjects: Nanoscience; Physics

MS, University of Cincinnati, 2009, Engineering : Electrical Engineering

As device dimensions continue to shrink into the nanometer length regime, conventional complementary metal-oxide semiconductor (CMOS) technology will approach its fundamental physical limits. Further miniaturization based on conventional scaling appears neither technically nor economically feasible. New strategies, including the use of novel materials and one-dimensional device concepts, innovative device architectures, and smart integration schemes need to be explored. They are crucial to extending current capabilities and maintaining momentum beyond the end of the technology roadmap. Semiconducting nanowires are an attractive and viable option for channel structures. By virtue of their potential one-dimensionality, such nanoscale structures introduce quantum confinement effects, thus enabling new functionalities and device concepts. In this thesis we study performance limits of Indium Arsenide nanowire Field Effect Transistors (InAs NWFETs) in a Gate All Around (GAA) structure and examine its upper limits of performance. InAs in particular is an attractive candidate for NW-based electronic devices because of its very high electron mobility at room temperature of 30,000 cm2/Vs in comparison to silicon's mobility of 1480 cm2/Vs. The device simulations were carried out using MultiGate Nanowire (Nanowire MG) simulator made available at NanoHUB (www.nanohub.org) by Network for Computational Nanotechnology (NCN). The InAs NWFET was simulated for variations in channel diameter, channel length, oxide thickness and the corresponding Id – Vg characteristics were analyzed. Short Channel Effects (SCEs) namely Drain Induced Barrier Lowering (DIBL) and threshold voltage roll off were studied. Sub-threshold slope and ON/OFF current variations were analyzed for variations in device dimensions. Finally the device characteristics of Silicon Nanowire Field Effect Transistors (Si NWFETs) were simulated for the same variations in channel diameter, channel length and oxide thickness (open full item for complete abstract)

Committee: Kenneth Roenker PhD (Committee Chair); Marc Cahay PhD (Committee Member); Punit Boolchand PhD (Committee Member) Subjects: Electrical Engineering

Doctor of Philosophy, The Ohio State University, 2021, Physics

Quantum information science is rapidly advancing and it is likely that society will experience a major shift in the way we handle information in the coming years. As critical advancements such as quantum computation and quantum cryptography progress, the full manifestation of a second quantum revolution is on the horizon. Photons will be an integral part of quantum information science, especially for communication purposes, so we will need to develop excellent single-photon detector technologies. In this thesis, I describe a promising single-photon detector candidate, superconducting nanowire single-photon detectors, and provide a new theoretical basis for understanding these detectors. Specifically, I explore the relationship between various detector parameters and the readout signal and show that its rising edge can be described using a characteristic time that is proportional to the square root of the length of the detector and is inversely proportional to the square root of the absorbed photon number n and bias current. Therefore, this work provides a theoretical basis for photon-number resolution in a conventional single-pixel superconducting nanowire single-photon detector. Based on these predictions, I describe a demonstration of photon-number resolution in superconducting nanowire single-photon detectors that correctly identifies more than 99.7% of n=1 events and more than 98% of n>1 events when distinguishing between n=1 versus n>1, as well as explore the effects of other parameters via experiment and by drawing from the literature. While seeking to understand the quality of the demonstrated photon-number resolution, I find that there is likely some spatial variation in the intrinsic detector parameters. I predict that this may result in some below critical temperature resistance in these detectors and verify this experimentally. The resistance ranges from around 50 Ohms at 0.9 K to multiple kOhms closer to the critical temperature. This is (open full item for complete abstract)

Committee: Daniel Gauthier (Advisor); Gregory Lafyatis (Committee Member); Ezekiel Johnston-Halperin (Committee Member); Richard Furnstahl (Committee Member) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 2019, Materials Science and Engineering

Self-assembled nanowires are attractive because of their innate ability to effectively strain relax without the creation of extended defects. This allows for interesting heteroepitaxial growths and extreme heterostructures. III-Nitride nanowires are of particular interest because of the wide range of direct bandgaps available in the material system, spanning form the infrared to the deep ultraviolet finding uses in sensors, photovoltaics, lasers and LEDs. The work presented here will be focused on nanowire LEDs with emission in the ultraviolet grown by molecular beam epitaxy. The first part of this work will discuss the possible inhomogeneities present in self-assembled nanowires and how these manifest themselves in ensemble devices. The effect of nonuniformities (specifically shorts) on the current spreading in devices where many individual diodes are wired in parallel is then addressed, and the use of a short-term-overload bias is shown as a way to reduce the presence of nonuniformities, increasing the efficiency of ensemble devices. Next, alternative substrates are investigated, with the growth of high-quality GaN nanowires being demonstrated on polycrystalline foils, the fabrication of the first UV LED grown directly on metal foil follows. The final portion of this work begins by addressing the grain-dependent uniformity issues present with growth on bulk polycrystalline foils through the use of thin nanocrystalline metal films and amorphous metals. Finally, a different nanowire LED structure is discussed in which the upper portion of the nanowires is coalesced to form a “thin-film” transparent conductive layer, enabling the substitution of the traditional fully conformal thin metal top contact with only a current spreading grid.

Committee: Myers Roberto (Advisor); Siddharth Rajan (Committee Member); Tyler Grassman (Committee Member) Subjects: Electrical Engineering; Materials Science

PhD, University of Cincinnati, 2018, Arts and Sciences: Physics

Semiconductor nanowires and nanorods are ideal building blocks for the synergetic combination of semiconductor properties with the plasmonic behavior of metals. Excitons and electron-hole pairs are the most prominent optical excitations and controlling their emission is an important step towards new plasmonic devices including subwavelength nanowire waveguides, metamaterials and plasmonic lasers. In the first part of my thesis, I studied the modification of the excitonic emission of gold coated GaN nanorods using an organic interlayer. The carrier dynamics in GaN nanorods was investigated with time-integrated and time-resolved photoluminescence (PL) and compared with multilevel model calculations providing the essential time constants of the dynamics and their weight. Based on these findings the PL spectra and time-resolved emission traces from aluminum quinoline (Alq3) and gold/Alq3 coated GaN nanorods were analyzed and interpreted. The physical processes which are responsible for the modified carrier dynamics in plasmonic nanorods were identified with the developed model. Particularly, it was found that metal induced band-bending leads to an enhanced ionization of impurity bound excitons which caused a significant quenching of the excitonic emission. Forster energy transfer from the semiconductor excitons to metal plasmons additionally contributes to the faster lifetime of the excitonic emission. Both effects decrease with increasing Alq3 interlayer thickness. In the second part of my thesis, I investigated the exciton dynamics of both bare and gold coated ZnO tetrapods comprising a 3 nm Al2O3 top coating as a function of temperature and laser excitation intensities. An increase of the excitation intensity for bare and gold coated tetrapods increased the lattice temperature leading to a longer exciton lifetime due to the thermal ionization of donor bound excitons. Like in the gold coated GaN nanorods, metal induced band bending as well as Forster energy transf (open full item for complete abstract)

Committee: Hans Peter Wagner Ph.D. (Committee Chair); Carlos Bolech Ph.D. (Committee Member); Howard Everett Jackson Ph.D. (Committee Member); Alexandre Sousa Ph.D. (Committee Member) Subjects: Optics

Master of Science, The Ohio State University, 2016, Electrical and Computer Engineering

In this MS thesis study, we investigate the low frequency noise characteristics of ZnO nanowire field effect transistors (FETs). Three low frequency noise models including Hooge's model, Carrier Number Fluctuation (CNF) model, and CNF correlated with mobility fluctuation model are applied to the nanowire FETs case to study the mechanism of the low frequency noise. The 1/f noise power spectrum density is observed in both room temperature and low temperature down to 10K.

Committee: Wu Lu (Advisor); Siddharth Rajan (Committee Member) Subjects: Electrical Engineering

PhD, University of Cincinnati, 2016, Arts and Sciences: Physics

We are facing variability problems for modern semiconductor transistors due to the fact that the performances of nominally identical devices in the scale of 10~100 nm could be dramatically different attributed to the small manufacturing variations. Different doping strategies give statistical variations in the number of dopant atom density ND in the channel. The material size gives variations in wire diameter dW. And the immediate environment of the material leads to an additional level of variability. E.g. vacuum-semiconductor interface causes variations in surface state density Ds, metal-semiconductor interface causes variations in Schottky barrier and dielectric semiconductor interface induces dielectric confinement at small scales. To approach these variability problems, I choose Si-doped GaAs nanowires as an example. I investigate transport in Si-doped GaAs nanowire (NW) samples contacted by lithographically patterned Gold-Titanium films as function of temperature T. I find a drastically different temperature dependence between the wire resistance RW, which is relatively weak, and the zero bias resistance RC, which is strong. I show that the data are consistent with a model based on a sharp donor energy level slightly above the bottom of the semiconductor conduction band and develop a simple method for using transport measurements for estimates of the doping density after nanowire growth. I discuss the predictions of effective free carrier density neff as function of the surface state density Ds and wire size dW. I also describe a correction to the widely used model of Schottky contacts that improves thermodynamic consistency of the Schottky tunnel barrier profile and show that the original theory may underestimate the barrier conductance under certain conditions. I also provide analytical calculations for shallow silicon dopant energy in GaAs crystals, and find the presence of dielectrics (dielectric screening) and free carriers (Coulomb screening) caus (open full item for complete abstract)

Committee: Andrei Kogan Ph.D. (Committee Chair); Carlos Bolech Ph.D. (Committee Member); Howard Everett Jackson Ph.D. (Committee Member); Alexandre Sousa Ph.D. (Committee Member) Subjects: Condensation

PhD, University of Cincinnati, 2016, Arts and Sciences: Physics

We have used electrical transport measurements, photocurrent spectroscopy (PC), photoluminescence (PL) and photoluminescence excitation spectroscopies (PLE) to investigate electronic band structure and quantum confined states in single (MOCVD) grown semiconductor nanowires. The nanowires used in this study include Zn2As3, GaAs, GaAsSb, GaAs/AlGaAs core-shell and GaAs/AlGaAs core-multishell (Quantum well tube (QWT)) nanowires. Single nanowire devices were fabricated using photolithography to deposit metal contacts on either end of the nanowire to allow optoelectronic measurements. Electrical transport and photocurrent spectroscopy were used to characterize a novel II-V semiconductor nanowire device, Zn3As2, at room and low (10 K) temperature. Employing metal-semiconductor-metal modeling and self-consistent fitting, we have extracted relevant intrinsic semiconductor parameters from room temperature current-voltage characteristics. The extracted acceptor doping densities for nanowire and nanoplatelate devices are of 1.67×1018cm-3 and 7.41×1018cm-3 respectively. These values agree well with doping densities determined using transient Rayleigh scattering. The photocurrent spectra measurements reported here allow an estimation of the band gap of this material to be 1.13 eV which is in the near-infrared at 10 K. The electronic band structure of single GaAs nanowires was also characterized using photocurrent spectroscopy at room and low (10 K) temperature. In bare 100 nm diameter GaAs nanowire devices we observed the non-linear dark current arising from back-to-back Schottky behavior. We observed saturation of photocurrent as we increased the bias for fixed photon energy above the band gap at room temperature. We attributed this saturation to the diffusion length of minority carriers resulting in the complete extraction of photogenerated carriers at high biases. Photocurrent spectra at 10 K exhibit a peak near the band edge of GaAs ~1.5eV, in both bare GaAs core and GaAs/ (open full item for complete abstract)

Committee: Leigh Smith Ph.D. (Committee Chair); Philip Argyres Ph.D. (Committee Member); Howard Everett Jackson Ph.D. (Committee Member); Frank Pinski Ph.D. (Committee Member) Subjects: Physics

PhD, University of Cincinnati, 2016, Arts and Sciences: Physics

Strain distribution in the core and the shell of a semiconductor nanowire (NW) and its effect on band structures including carrier recombination dynamics of individual Wurtzite (WZ) In1-xGaxAs/InP and Zincblende (ZB) GaAs1-xSbx/InP strained core-shell NWs are investigated using room temperature Raman scattering and transient Rayleigh scattering (TRS) optical spectroscopy techniques. In addition, the electrical transport properties of individual ZB InP NWs are explored using gate-dependent current-voltage (I-V) measurements. Micro-Raman scattering from individual In1-xGaxAs NWs show InAs like TO and GaAs like TO modes with frequencies which are consistent with the 35% Ga concentration determined from the growth parameters. Calculations showed that the In0.65Ga0.35As core is under compressive strain of 0.26% while the InP shell is in tensile strain of 0.42% in In0.65Ga0.35As/InP NWs. TRS measurements of single NWs show clear evidence for a strong band resonance in the WZ In0.65Ga0.35As NW at 0.819 eV which is estimated to be a ~186 meV blue-shift with respect to bulk ZB In0.65Ga0.35As. Furthermore, both Raman scattering and TRS measurements are on excellent agreement with the band gap shift of In0.65Ga0.35As/InP core-shell NWs with respect to the core only NW by 46~48 meV which experimentally confirmed the InP shell induced compression of the core. The time decays of the resonance are observed to be long (~125 ps) for core-shell NWs while it is short (~31 ps) for core only NWs consistent with a larger nonradiative recombination rate. Optical phonon modes of GaAs1-xSbx are observed to be red-shifted with increasing Antimony fraction in GaAs1-xSbx NWs which can be expected in an alloy with increasing concentration of a heavier atom in the lattice. Using TRS measurements, the GaAs0.71Sb0.29 band gap for the core-shell NW is observed to be reduced by 0.04 eV with respect to the core only NW because of the tensile strain in the core. Raman experiments show a blue-shi (open full item for complete abstract)

Committee: Leigh Smith Ph.D. (Committee Chair); Carlos Bolech Ph.D. (Committee Member); Howard Everett Jackson Ph.D. (Committee Member); L.C.R. Wijewardhana Ph.D. (Committee Member) Subjects: Materials Science

MS, University of Cincinnati, 2016, Arts and Sciences: Physics

As lower dimensionality becomes more accessible for semiconductor devices, a thorough understanding of size effects is crucial. In this thesis, we review the literature to examine these effects, particularly surface state depletion, dielectric mismatch and quantum confinement, in a wide range of conditions including doping, surrounding environment, external field, and temperature in order to illustrate the general influence on properties such as activation energy, mobility, electron concentration, and resistivity. Methods of determining the sizes at which they become apparent are given. Ultimately, we remark that size effects are as important to degenerate-nondegenerate and metal-nonmetal transitions as doping and temperature, and describe how electrical transport properties of low dimensional semiconductors can be tuned by adjusting size, applying surface treatments, and altering the surrounding environment.

Committee: Andrei Kogan Ph.D. (Committee Chair); F Paul Esposito Ph.D. (Committee Member); Leigh Smith Ph.D. (Committee Member) Subjects: Physics

PhD, University of Cincinnati, 2016, Arts and Sciences: Physics

In this dissertation, a wide variety of optical spectroscopies including photoluminescence (PL), photoluminescence excitation (PLE), photoluminescence imaging, time-resolved photoluminescence (TRPL), as well as photocurrent (PC) spectroscopy have been used to explore the optical and electronic structures of Zn-doped GaAs twinning superlattices (TSL) and GaAs/AlGaAs quantum well tube (QWT) nanowires. Low Temperature PL spectra of a single Zn-doped GaAs TSL nanowire reveal a broad band low energy emission centerd at 1.47 eV in addition to the free exciton emission of zincblende GaAs. We do not observe any changes in the relative ratio of these emissions as a function of excitation power, suggesting that the low energy emission is not related to defects or carbon impurities, this is further confirmed by PC measurement. A simple theoretical model with holes confined in the wurtzite segment is developed in order to understand the origin of the emissions. In GaAs/AlGaAs QWT nanowires, low temperature PL spectra of 8 and 4 nm QWTs show that the exciton confinement energy have been shifted up by 57 meV and 185 meV with respect to the GaAs core. A first excited state transition has been observed in PLE spectra of only the 8 nm QWT. A simple theoretical modeling using a cylindrical quantum well shows the calculations are in good agreement with both direct and indirect transitions observed in PL and PLE measurements. A more detailed model including the hexagonal symmetric facets shows that the electron and holes ground states are strongly localized to the corners, while the excited states are confined to the long facets of the QWT. In order to gain more information about quantum confined energy in these radial heterostructures, we then studied a series of QWTs with thicknesses ranging from 1.5 to 8 nm. High resolution spatially-resolved PL images reveal several ultranarrow emission lines (localized states) distributed at different spatial positions along the long axis of (open full item for complete abstract)

Committee: Leigh Smith Ph.D. (Committee Chair); Howard Everett Jackson Ph.D. (Committee Member); Randy Johnson Ph.D. (Committee Member); Nayana Shah Ph.D. (Committee Member) Subjects: Condensation

PhD, University of Cincinnati, 2016, Arts and Sciences: Physics

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 Chapter3, transient Rayleigh scattering (TRS) experiment i (open full item for complete abstract)

Committee: Leigh Smith Ph.D. (Committee Chair); Carlos Bolech Ph.D. (Committee Member); Howard Everett Jackson Ph.D. (Committee Member); Alexandre Sousa Ph.D. (Committee Member) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 2015, Electrical and Computer Engineering

The history of the semiconductor industry is a story of Moore's Law. However, the end of Moore's Law has been predicted for the near future as the transistor's overly-scaled gate length eventually loses control of current flow in the channel. Gate-all-around transistors with one-dimensional nanowires (NWs) as the device channel surrounded by a gate to control the flow of current are considered as one of potential candidates for next generation electronics. In addition, their unique properties also make NW an ideal candidate for resonant tunneling devices (RTDs) with extremely high switching speed (in terahertz range) for future high frequency and THz communications. Before this becomes a reality, however, unless some fundamental issues of semiconductor NWs are clarified, it is hard to realize breakthroughs on device performance. This Ph.D research aims to address some of these issues. The research highlights and key innovations are summarized as below: 1. Metallic contacts: We show that surface states in ZnO NWs can affect the properties of metal contacts to NWs, causing Fermi level pinning with a fixed Schottky barrier height and high contact resistance. A circuit model is developed to characterize metal contacts for NW FETs. The results show that surface states in ZnO NWs is oxygen vacancy related. A general model is also developed to study interfacial traps between NWs and the gate dielectrics applied. 2. Electron transport: Our results show that the electron transport of ZnO NW FETs is governed by the space charge limited model at temperatures below a trap temperature. Above the trap temperature, the electron transport is thermionic emission dominated. A method is developed for NW trap density extraction through the Arrhenius plot. Based on the space charge limited model, we show that the conventional field effect mobility extraction method over estimates the mobility in NW FETs. An accurate method that takes into account the surface trap related scattering (open full item for complete abstract)

Committee: Wu Lu (Advisor); Siddharth Rajan (Committee Member); Marvin White (Committee Member); Haijun Su (Committee Member) Subjects: Electrical Engineering

Master of Science (M.S.), University of Dayton, 2015, Electro-Optics

Although silicon photodetectors are widely used in the manufacture of consumer cameras and light sensors, their fabrication requires a large number of process steps, equipment and resources. In order to study novel device concepts, such as the inclusion of silicon nanowires, quantum-confinement, nanostructured moth-eye structures or on-chip optical filtering, we need control over critical fabrication steps, which is not possible if we rely only on commercially produced devices. In this work, we have designed, fabricated and characterized silicon photodiodes starting from bare silicon wafers to completely packaged chips. We considered two major configurations – front-side illuminated detectors on standard SSP silicon wafers, and back-side illuminated detectors with ultrathin DSP silicon wafers. Ion implantation process was used for creating the p-n junctions, but we also acquired a diffusion furnace and developed our own process for thermal diffusion from a solid source. We also fabricated silicon nanowires on the front side of the diodes using a gold metal-assisted chemical etching (MACE) process to examine their effects on the optical and electrical performances of the devices. The fabricated devices were tested on a probe station, and then they were packaged, wire-bonded and tested for optical responsivities and quantum efficiencies.

Committee: Andrew Sarangan (Committee Chair); Imad Agha (Committee Member); Joseph Haus (Committee Member) Subjects: Electrical Engineering; Engineering; Optics; Physics

Doctor of Philosophy, Case Western Reserve University, 2014, EMC - Mechanical Engineering

Energy harvesting using nanoscale devices is gaining increased attention because of the interest in scavenging power from the environment and redirecting it locally to power sensors or other devices. This Ph.D. dissertation focuses on nanostructured devices based on InAs nanowires (NWs), carbon nanotubes (CNTs) and graphene. The nanostructures were fabricated and incorporated into flow channels to investigate (i): the phenomenon of flow-induced voltage generation and its mechanism, (ii): the potential for nanosensor system applications and energy harvesting. In particular, the effects of fluid flow on electrical current/potential changes on nanostructured devices with/out source-drain voltages driving the device were investigated, and the voltage generations of NW, CNT and graphene-based energy harvesters were assessed. Indium Arsenide (InAs) nanowire (NW) and graphene field effect transistors (FETs) were incorporated into a microfluidic channel to detect the flow rate change as well as to harvest fluid flow energy for electric power generation. Discrete changes in the electric current through InAs NW FETs and graphene FETs were observed upon flow rate changes at steps of 1 ml/hr (equivalent to ~3 mm/s change in average linear velocity). The current also showed a sign change upon reversing flow direction. By comparing the response of device with and without a driving voltage between source-drain electrodes, it was concluded that the dominant contribution in the response was the streaming potential tuned conductance of NW/graphene. In the absence of a source-drain voltage, it was further demonstrated that ionic transport caused by the flow enabled generation of a ~mV electrical potential (or ~nA electrical current) inside the InAs NW per ml/hr increase in flow rate. This is most likely due to a charge dragging effect. Oriented multi-walled carbon nanotube (MWCNT) arrays mounted on a silicon (Si) substrate were subjected to a stagnation-type flow confi (open full item for complete abstract)

Committee: Iwan Alexander (Advisor); Xuan Gao (Committee Member); Alexis Abramson (Committee Member); Ozan Akkus (Committee Member) Subjects: Mechanical Engineering

Doctor of Philosophy, University of Akron, 2013, Polymer Engineering

Photodetectors are light responsive devices that convert optical signals into electric signals. Photodetectors have wide applications in image sensing, environmental monitoring, day- and night-surveillance, chemical and biological detection, industrial process control, communication, planetary probing and so on. Currently, photodetectors based on GaN, ZnO, Si, InGaAs and bulk PbS cover different sub-bands from UV to infrared region. These photodetectors are expensive and some of them require to be operated at low temperature, which certainly limits their applications. Polymer photodetectors made with conjugated polymers possess the unique features, including room-temperature operation, high sensitivity, low working voltage, low cost, thin profile, large area and flexibility. Ultrasensitive polymer photodetectors with high response speed and spectral response ranging from UV to near infrared have been demonstrated. However, new device structure, high responsivity and stable polymer photodetectors needs to be developed. In my dissertation, we reported various methods to enhance the performance of polymer photodetectors. By solvent annealing and post-production thermal annealing, we were able to demonstrate that polymer photodetectors possess comparable responsivity to inorganic counterparts. We have, for the first time, developed the inverted device structure for polymer photodetectors. By utilizing inorganic nanowires and quantum dots as either cathode or anode buffer layer, we were able to demonstrate robust polymer photodetectors. We also investigate the device performance versus energy offset between the workfunction of anode electrode and the valance band of conjugated polymers, band offset at the heterojunction and purity of conjugated polymers.

Committee: Xiong Gong Dr. (Advisor); Alamgir Karim Dr. (Committee Chair); Stephen Cheng Dr. (Committee Member); Matthew Becker Dr. (Committee Member); Yi Pang Dr. (Committee Member) Subjects: Electrical Engineering; Materials Science; Polymers

Doctor of Philosophy, Case Western Reserve University, 2014, Physics

In this dissertation, we present band structure studies of three types of materials, which are wurtzite GaAs, atomically thin MoS2, and highly correlated rare-earth nitrides, by using the quasiparticle self-consistent GW (QSGW) method. First, we report results for wurtzite GaAs, which is found to be stable and coexists with the zinc-blende crystal structure (more stable in bulk) in nanowires. We provide detailed band structure parameters, such as effective mass parameters, band gap, crystal field splitting, spin-orbit splitting, etc., of wurtzite GaAs. This information on the bulk band structure is needed for the study of the nanowire specific electronic states, which can be obtained within the envelope function or effective mass type theories. The strain effects on the band structure parameters are also studied, because a nanowire could be under strain due to surface tension and tension caused by the matching of the lattice constant between wurtzite and zinc-blende sections in the wire. The band structure parameters of more well known zinc-blende GaAs was also calculated in order to test the validity of our QSGW calculations. We also present the band structure of 4H GaAs as a guideline of how the band parameters might change when there is mixing between hexagonal structure (wurtzite) and cubic structure (zinc blende) in the same nanowire. Second, we present the results of bulk and atomically thin MoS2. Our QSGW results confirm the transition of the band gap nature from indirect gap to direct gap when the form of MoS2 changes from bulk to monolayer. However, the QSGW significantly overestimates the direct gaps at the K point of monolayer and bilayer MoS2 due to the very strong excitonic effect in this two dimensional material, which is not taken into account in the QSGW method. Therefore, we also estimated the exitonic effect by using the Mott-Wannier effective mass theory, and obtained a large ground state exciton binding energy for both monolayer and bilayer. Our (open full item for complete abstract)

Committee: Walter Lambrecht (Advisor); Philip Taylor (Committee Member); Jie Shan (Committee Member); Clemens Burda (Committee Member) Subjects: Physics

PhD, University of Cincinnati, 0, Arts and Sciences: Physics

Photocurrent from photoexcitation of charged carriers in CdS nanosheets (NSs) and GaP/GaAs heterostructured nanowires (NWs) has been studied. Devices were fabricated based on single CdS NS and GaP/GaAs NW and their optoelectronic properties fully characterized using various light sources and at different wavelengths. The CdS NSs are grown using a gold catalyst-assisted vapor phase transport growth method at 800°C for 20min. Metal-semiconductor-metal nanodevices are made with both Schottky and Ohmic contacts using photolithography followed by Ti/Al (20nm/200nm) metal evaporation and lift-off. Ohmic contacts are formed by Ar+ bombardment before the metal deposition to create donor sulfur vacancies which increases the electron concentration. Scanning photocurrent microscopy (SPCM) is used to obtain spatial imaging of the photocurrent. An Ar+ laser with emission at 488nm was used for above band-gap excitation, whereas a Ti:S mode-locked laser with emission ranging 700nm to 1000nm is used for sub band-gap excitation. Spatial imaging of the photocurrent shows that the photosensitive regions are localized at the reverse biased contact for Schottky type contacts and uniformly distributed throughout the nanosheet for Ohmic contacts. A polarization analysis shows that the photocurrent is maximized for laser excitation polarized perpendicular to the c-axis of the NS. Photocurrent spectra excited above the CdS band-gap using a filtered monochromatic white light source at low temperatures reveal optical transitions between the A, B, C valence bands and the conduction band of the CdS NS at energies of 2.552eV, 2.569eV and 2.635eV respectively, which are in agreement with the accepted values for these bands. At room temperature these resonance peaks shift by ~0.053eV towards lower energies because of temperature dependence of the band-gap. The photocurrent increases linearly with power for above gap excitation. Photocurrents excited by a below-gap laser pulse increase nonli (open full item for complete abstract)

Committee: Leigh Smith Ph.D. (Committee Chair); Howard Everett Jackson Ph.D. (Committee Member); Kay Kinoshita Ph.D. (Committee Member); Frank Pinski Ph.D. (Committee Member); Jan Yarrison-Rice Ph.D. (Committee Member) Subjects: Physics