<p>In the last 30 years, the possibility of using polymeric thin film transistors (PTFTs) in flexible display, sensors, radio-frequency identification tag and the potential of using printing or low-cost reel-to-reel fabrication techniques has stimulated much research and technology development in these devices. However, the utilization of PTFTs needs better understanding of the organic semiconductor material’s properties and their physical and chemical mechanisms. In addition, the PTFTs show poor stability compared to the crystalline transistors. The PTFTs can have significant variations of threshold voltage, mobility, on/off ratio even when deposited using the same conditions on the same substrate. Therefore, better understanding of the PTFTs’ physical and chemical properties and the improvement of the characterization techniques are needed.</p> <p>The design and fabrication of the novel polymeric semiconductor, diketopyrrolopyrrole β-unsubstituted quaterthiophene (DKPP-βT) based bottom-gated top-contact PTFT and microfluidics PTFT are introduced in this thesis. The microfluidic PTFT consists of polydimethylsiloxane (PDMS) microchannel which guides liquids flowing over the top of the semiconductor channel.</p> <p>From consecutive electrical measurements, it was found that the threshold voltage (V_T) follows a logarithmic law function of the time. Illuminating the PTFTs results in shifts of the initial value of the threshold voltage linearly towards more positive value. The mobility is unaffected by time or by illumination. However, the off current increased proportionally with light. Also, the contact resistance extracted by the parameter compensated transmission line model (TLM) method is ohmic and gate bias independent for high gate biases.</p> <p>The novel microfluidic PTFT enables the study of the sensing property of the DKPP-βT PTFT of liquid analytes. The threshold voltage evolution in the deionized (DI) water measurements also follows logarithmic function of the time with a slightly steeper slope than in air. The mobility only slightly decreases initially on exposure to DI water. The off current in DI water measurements decrease compared with air measurements. In acid solution measurements, the threshold voltage remains stable and the mobility slightly increased, compared with measurements in water. Additionally, the subthreshold slope and off current in both acid solution and salt water measurements show similar results to the DI water measurements. While the base solution damages the device immediately. The stable performance of DKPP-βT PTFTs with DI water and low-concentration salt water in the microchannel makes it a promising biosensor.</p> / Master of Applied Science (MASc)

<p>GaAs/GaP heterostructure nanowires (NWs) were grown on GaAs(111)B and Si(111) substrates by gold (Au) assisted vapor-liquid-solid (VLS) growth in a molecular beam epitaxy (MBE) system. NW morphology and crystal structure were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Early results indicated substantial differences in the length and crystal structure of the GaAs/GaP heterostructures. Efforts to remove these inhomogeneities required an improved Au VLS seed deposition method as well as a better understanding of VLS growth across GaAs/GaP hetero-interfaces.</p> <p>Experiments with GaAs/GaP heterostructures yielded the observation of changes in crystal phase in GaP, including the first reported occurrence of the 4H polytype. These observations revealed the presence of transient growth behavior during the formation of the GaAs to GaP hetero-interface that was unique to the VLS technique. Further characterization required the need to move from VLS seeds formed by annealing thin Au films to Au particles formed precisely by electron beam lithography (EBL). NW growth using EBL patterned Au seeds was discovered to be inhibited by the formation of a thin silicon oxide layer, formed at low temperatures by Au-enhanced silicon oxidation. Elimination of this layer immediately prior to growth resulted in successful patterned VLS growth.</p> <p>A systematic study of the transient GaP growth behavior was then conducted using patterned arrays to grow GaAs/GaP heterostructure NWs with frequent, periodic oscillations in the group V composition. These oscillations were measured by high angle annular dark field (HAADF) to determine the instantaneous growth rate of many NWs. A phenomenological model was fit to the data and transient growth rate behavior following a GaAs to GaP hetero-interface was understood on the basis of transient droplet compositions, which arise due to the large difference in As or P alloy concentrations required to reach the critical supersaturation.</p> / Doctor of Philosophy (PhD)

<p>Epitaxy is a key technological process for the production of thin films and nanostructures for electronic and optoelectronic devices. The epitaxial process has been traditionally studied through the lens of lattice-matched and chemically similar material systems, specifically the III-V quaternary material systems. This work investigates the role energy and symmetry play at epitaxial interfaces for cases far different than those of typical epitaxy. In the realm of energy, the impact of chemically dissimilar epitaxial interfaces was investigated, specifically between semiconductors and oxides, noble metals and oxides, and polar-on-nonpolar epitaxy. For symmetry at epitaxial interfaces, the role of symmetry breaking, through surface reconstructions and asymmetric surfaces was investigated. Investigations into energy found two key insights: 1) epitaxy is possible between materials which one would expect to be very weakly interacting (gold on oxides) and, 2) epitaxial interfaces, while promoting single crystal growth, can be weakly bonded enough to allow controlled liftoff of single crystal epitaxial thin films. Investigations into symmetry at epitaxial interfaces found three key insights: 1) intentional symmetry breaking of the growth substrate through steps can suppress twinning of zincblende thin films, 2) asymmetric (211)-oriented substrates can accommodate strain of mismatched zincblende thin films, and 3) reconstructed oxide substrates can provide unique epitaxial templates for thin films which significantly differ from their bulk lattice. The results of this investigation provide a path towards the improvement of epitaxy through the manipulation of symmetry at epitaxial surfaces, and the production of free standing thin films through the epitaxial liftoff process.</p> / Doctor of Philosophy (PhD)

CuI is a p-type transparent conductive semiconductor with unique optoelectronic properties, including wide band gap (3.1 eV), high hole mobility (>40 cm2 V−1 s−1 in bulk), and large room-temperature exciton binding energy (62 meV). The difficulty in epitaxy of CuI is the main obstacle for its application in advanced solid-state electronic devices. Herein, room-temperature heteroepitaxial growth of CuI on various substrates with well-defined in-plane epitaxial relations is realized by reactive sputtering technique. In such heteroepitaxial growth the formation of rotation domains is observed and hereby systematically investigated in accordance with existing theoretical study of domain-epitaxy. The controllable epitaxy of CuI thin films allows for the combination of p-type CuI with suitable n-type semiconductors with the purpose to fabricate epitaxial thin film heterojunctions. Such heterostructures have superior properties to structures without or with weakly ordered in-plane orientation. The obtained epitaxial thin film heterojunction of p-CuI(111)/n-ZnO(00.1) exhibits a high rectification up to 2 × 109 (±2 V), a 100-fold improvement compared to diodes with disordered interfaces. Also a low saturation current density down to 5 × 10−9 Acm−2 is formed. These results prove the great potential of epitaxial CuI as a promising p-type optoelectronic material.

This work presents a new low-temperature fabrication process of metal oxide nanostructures that allows high-aspect ratio zinc oxide (ZnO) and titanium dioxide (TiO2) nanowires and nanotubes to be readily integrated with microelectronic devices for sensor applications. This process relies on a new method of forming a close-packed array of self-assembled high-aspect-ratio nanopores in an anodized aluminum oxide (AAO) template in a thin (2.5 µm) aluminum film deposited on a silicon and lithium niobate substrate (LiNbO3). This technique is in sharp contrast to traditional free-standing thick film methods and the use of an integrated thin aluminum film greatly enhances the utility of such methods. We have demonstrated the method by integrating ZnO nanowires, TiO2 nanowires, and multiwall TiO2 nanotubes onto the metal gate of a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor), and the delay line of a surface acoustic wave (SAW) device to form an integrated ChemFET (Chemical Field-Effect Transistor) and a orthogonal frequency coded (OFC) SAW gas sensor. The resulting metal oxide nanostructures of 1-1.7 µm in height and 40-100 nm in diameter offer an increase of up to 220X the surface area over a standard flat metal oxide film for sensing applications. The metal oxide nanostructures were characterized by SEM, EDX, TEM and Hall measurements to verify stoichiometry, crystal structure and electrical properties. Additionally, the electrical response of ChemFETs and OFC SAW gas sensors with ZnO nanowires, TiO2 nanowires, and multiwall TiO2 nanotubes were measured using 5-200 ppm ammonia as a target gas at room temperature (24ºC) showing high sensitivity and reproducible testing results.

InGaN heterostructures are at the core of blue light emitting diodes (LEDs) which are the basic building blocks for energy efficient and environment friendly modern white light generating sources. Through quantum confinement and electronic band structure tuning on the opposite end of the spectrum, Ge1−xSnx alloys have recently attracted significant interest due to its potential role as a silicon compatible infra-red (IR) optical material for photodetectors and LEDs owing to transition to direct bandgap with increasing Sn. This thesis is dedicated to establishing an understanding of the optical processes and carrier dynamics in InGaN heterostructures for achieving more efficient visible light emitters and terahertz generating nanocavities and in colloidal Ge1−xSnx quantum dots (QDs) for developing efficient silicon compatible optoelectronics. To alleviate the electron overflow, which through strong experimental evidence is revealed to be the dominating mechanism responsible for efficiency degradation at high injection in InGaN based blue LEDs, different strategies involving electron injectors and optimized active regions have been developed. Effectiveness of optimum electron injector (EI) layers in reducing electron overflow and increasing quantum efficiency of InGaN based LEDs was demonstrated by photoluminescence (PL) and electroluminescence spectroscopy along with numerical simulations. Increasing the two-layer EI thickness in double heterostructure LEDs substantially reduced the electron overflow and increased external quantum efficiency (EQE) by three fold. By incorporating δ p-doped InGaN barriers in multiple quantum well (MQW) LEDs, 20% enhancement in EQE was achieved due to improved hole injection without degrading the layer quality. Carrier diffusion length, an important physical parameter that directly affects the performance of optoelectronic devices, was measured in epitaxial GaN using PL spectroscopy. The obtained diffusion lengths at room temperature in p- and n-type GaN were 93±7 nm and 432±30 nm, respectively. Moreover, near field scanning optical microscopy was employed to investigate the spatial variations of extended defects and their effects on the optical quality of semipolar and InGaN heterostructures, which are promoted for higher efficiency light emitters owing to reduced internal polarization fields. The near-field PL from the c+ wings in heterostructures was found to be relatively strong and uniform across the sample but the emission from the c- wings was substantially weaker due to the presence of high density of threading dislocations and basal plane stacking faults. In case of heterostructures, striated regions had weaker PL intensities compared to other regions and the meeting fronts of different facets were characterized by higher Indium content due to the varying internal field. Apart from being the part and parcel of blue LEDs, InGaN heterostructures can be utilized in generation of coherent lattice vibrations at terahertz frequencies. In analogy to LASERs based on photon cavities where light intensity is amplified, acoustic nanocavity devices can be realized for sustaining terahertz phonon oscillations which could potentially be used in acoustic imaging at the nanoscale and ultrafast acousto-optic modulation. Using In0.03Ga0.97N/InxGa1-xN MQWs with varying x, coherent phonon oscillations at frequencies of 0.69-0.80 THz were generated, where changing the MQW period (11.5 nm -10 nm) provided frequency tuning. The magnitude of phonon oscillations was found to increase with indium content in quantum wells, as demonstrated by time resolved differential transmission spectroscopy. Design of an acoustic nanocavity structure was proposed based on the abovementioned experimental findings and also supported by full cavity simulations. Optical gap engineering and carrier dynamics in colloidal Ge1−xSnx QDs were investigated in order to explore their potential in optoelectronics. By changing the Sn content from 5% to 23% in 2 nm-QDs, band-gap tunability from 1.88 eV to 1.61 eV, respectively, was demonstrated at 15 K, consistent with theoretical calculations. At 15 K, time resolved PL spectroscopy revealed slow decay (3 − 27 μs) of luminescence, due to recombination of spin-forbidden dark excitons and effect of surface states. Increase in temperature to 295 K led to three orders of magnitude faster decay (9 − 28 ns) owing to the effects of thermal activation of bright excitons and carrier detrapping from surface states. These findings on the effect of Sn incorporation on optical properties and carrier relaxation and recombination processes are important for future design of efficient Ge1−xSnx QDs based optoelectronic devices. This thesis work represents a comprehensive optical study of InGaN heterostructures and colloidal Ge1−xSnx QDs which would pave the way for more efficient InGaN based LEDs, realization of terahertz generating nanocavities, and efficient Ge1−xSnx based silicon compatible optoelectronic devices.

The ability to control the bi-stable magnetization states of shape anisotropic single domain nanomagnets has enormous potential for spawning non-volatile and energy-efficient computing and signal processing systems. One of the most energy efficient switching methods is to adopt a system of a 2-phase multiferroic nanomagnet, where a voltage applied on the piezoelectric layer generates a strain in it and the strain is elastically transferred to the magnetostrictive nanomagnet which rotates the magnetization states of the nanomagnet at room temperature via the converse magnet-electric effect. Recently, it has been demonstrated that the magnetization of a Co nanomagnet can be switched between two stable orientations by this technique. The switching probability, however, is low due to the relatively small magnetostriction of Co. One possible way to improve the statistics is to use a better magnetostrictive material like Galfenol which has much higher magnetostriction and is therefore desirable, but it also presents unique material challenges owing to the existence of many phases. Nonetheless, there is a need to step beyond elemental ferromagnets and examine compound or alloyed ferromagnets with much higher magnetostriction to advance this field. There has not been much work in nanoscale FeGa magnets which are important for nanomagnetic logic and memory applications. Here, we have experimentally demonstrated switching of magnetization of Galfenol nanomagnets and proposed a core component of ultra-energy-efficient memory cell. We also demonstrated a bit writing scheme which completely reverses the magnetization with only strain, thus overcoming the fundamental obstacle of strain induced switching of magnetizations of nanomagnets.

A recent International Technology Roadmap for Semiconductors (ITRS) report (2.0, 2015 edition) has shown that Moore’s law is unlikely to hold beyond 2028. There is a need for alternate devices to replace CMOS based devices, if further miniaturization and high energy efficiency is desired. The goal of this dissertation is to experimentally demonstrate the feasibility of nanomagnetic memory and logic devices that can be clocked with acoustic waves in an extremely energy efficient manner. While clocking nanomagnetic logic by stressing the magnetostrictive layer of a multiferroic logic element with with an electric field applied across the piezoelectric layer is known to be an extremely energy-efficient clocking scheme, stressing every nanomagnet separately requires individual contacts to each one of them that would necessitate cumbersome lithography. On the other hand, if all nanomagnets are stressed simultaneously with a global voltage, it will eliminate the need for individual contacts, but such a global clock makes the architecture non-pipelined (the next input bit cannot be written till the previous bit has completely propagated through the chain) and therefore, unacceptably slow and error prone. Use of global acoustic wave, that has in-built granularity, would offer the best of both worlds. As the crest and the trough propagate in space with a velocity, nanomagnets that find themselves at a crest are stressed in tension while those in the trough are compressed. All other magnets are relaxed (no stress). Thus, all magnets are not stressed simultaneously but are clocked in a sequentially manner, even though the clocking agent is global. Finally, the acoustic wave energy is distributed over billions of nanomagnets it clocks, which results in an extremely small energy cost per bit per nanomagnet. In summary, acoustic clocking of nanomagnets can lead to extremely energy efficient nanomagnetic computing devices while also eliminating the need for complex lithography. The dissertation work focuses on the following two topics: Acoustic Waves, generated by IDTs fabricated on a piezoelectric lithium niobate substrate, can be utilized to manipulate the magnetization states in elliptical Co nanomagnets. The magnetization switches from its initial single-domain state to a vortex state after SAW stress cycles propagate through the nanomagnets. The vortex states are stable and the magnetization remains in this state until it is ‘reset’ by an external magnetic field. 2. Acoustic Waves can also be utilized to induce 1800 magnetization switching in dipole coupled elliptical Co nanomagnets. The magnetization switches from its initial single-domain ‘up’ state to a single-domain ‘down’ state after SAW tensile/compressive stress cycles propagate through the nanomagnets. The switched state is stable and non-volatile. These results show the effective implementation of a Boolean NOT gate. Ultimately, the advantage of this technology is that it could also perform higher order information processing (not discussed here) while consuming extremely low power. Finally, while we have demonstrated acoustically clocked nanomagnetic memory and logic schemes with Co nanomagnets, materials with higher magnetostriction (such as FeGa) may ultimately improve the switching reliability of such devices. With this in mind we prepared and studied FeGa films using a ferromagnetic resonance (FMR) technique to extract properties of importance to magnetization dynamics in such materials that could have higher magneto elastic coupling than either Co or Ni.

Dispositivos fotovoltaicos híbridos baseados em polímeros conjugados e semicondutores inorgânicos estão sendo utilizados nos últimos anos para a produção de células de energia solar com baixo custo. Para que haja uma alta eficiência é necessária dissociação eficiente de éxcitons, por isso é importante conhecer os níveis de energias dos componentes do dispositivo fotovoltaico. O presente estudos mostra que o sistema híbrido formado por nanofios cilíndricos preparados com heteroestrutura radial de camadas alternadas de GaAs/AlGaAs/GaAs recobertas com polímero conjugado poli-fenileno vinileno (PPV) forma uma opção alternativa para a fabricação de dispositivos fotovoltaicos. Os nanofios foram fabricados por Epitaxia por Feixe Molecular (MBE). Tanto potencial interno radial e modulação energética axial produzem a separação eficiente de elétrons e buracos fotoexcitados, que gera emissões de natureza e origem distintas e singulares nos nanofios: emissões envolvendo a impurezas aceitadoras no centro do núcleo de GaAs, bem como éxcitons indiretos presos a interface WZ e BZ e a interface da barreira estreita de AlGaAs na casca do nanofio. Medidas do decaimento temporal da emissão mostram uma forte dependência tempo de vida com o comprimento de onda, o que está associado com o afunilamento e distribuição energética destes estados emissivos. Medidas da emissão com a temperatura dão forte evidencia experimental de que a energia de ligação das impurezas tem uma forte dependência na direção radial. Este sistema híbrido funciona como coletor eficaz de luz tanto no visível quanto no infravermelho próximo. O trabalho demonstra também por espectroscopia resolvida no tempo que éxcitons são dissociados nas interfaces formadas por filmes ultrafinos de polímeros conjugados e nanofios e que esse material à base de arseneto de gálio (GaAs) atua como um forte receptor e separador de elétrons (alta afinidade eletrônica). / Hybrid photovoltaic devices based on conjugated polymers and inorganic semiconductors are being used in recent years to the production of solar cells at low cost. So there is a high efficiency is required efficient exciton dissociation, so it´s important to know the levels of energy of the components of the photovoltaic device. The present studies show that the hybrid system formed by cylindrical radial heterostructure nanowires prepared from alternating layers of GaAs / AlGaAs / GaAs covered with the conjugated polymer poly-phenylene vinylene (PPV) forms an alternative option for the manufacture of photovoltaic devices. Nanowires were manufactured by Molecular Beam Epitaxy (MBE). Both radial and axial inner potential energy modulation produce the efficient separation of electrons and photoexcited holes, which generates distinct and unique nature and source emissions in nanowires: emissions involving the acceptor impurities in the center core of GaAs and indirect excitons attached to the interface WZ and BZ and narrow barrier interface of AlGaAs on the shell of the nanowire. Measures the time decay of the issue show a strong dependence lifetime with the wavelength, which is associated with the bottleneck and energy distribution of emissive states. Emission measurements with temperature provide strong experimental evidence that the impurity binding energy has a strong dependence on the radial direction. This hybrid system works as an efficient collector of light both in the visible and near infrared. The work also shows for time resolved spectroscopy that excitons are dissociated at the interfaces formed by ultrathin conjugated polymers and films and nanowires that this material based on gallium arsenide (GaAs) acts as a strong receiver and electrons separator (high electron affinity ).

Este trabalho tem por objetivo investigar as propriedades ópticas de sistemas poliméricos luminescentes submetidos a diferentes doses de radiação gama. Foram preparadas soluções de poli[2-metóxi-5-(2\'etil-hexilóxi)-p-fenilenovinileno] - MEHPPV em clorofórmio e filmes finos depositados a partir de soluções deste polímero sobre substrato de vidro. As soluções e filmes finos tiveram suas propriedades investigadas por meio de medidas de absorção ultravioleta-visível (UV-VIS). Os filmes finos também foram analisados por medidas de espectroscopia de infravermelho (FTIR) além da cromatografia HPSEC. Os resultados experimentais foram analisados buscando elucidar tanto os efeitos da radiação nas propriedades ópticas dos polímeros, como também o estudo da viabilidade destes materiais como elementos ativos de dosímetros para baixas doses de radiação gama (< 1 kGy) especialmente para aplicações no tratamento de esterilização de bolsas de sangue (~ 25 Gy) e altas doses de radiação gama (até 25 kGy) para aplicações como esterilização de materiais médicos hospitalares. / This work aims the investigation of the optical properties of luminescent polymeric systems when submitted to different gamma radiation doses. Solutions of poly[2-methoxi-5-(2\'etil-hexiloxi)-p-phenilenevinilene] - MEH-PPV in chloroform were prepared. From those solutions, thin film layers of MEH-PPV were deposited on glass substrates. The polymer solutions and the thin-film samples of MEHPPV had their properties investigated by ultraviolet-visible (UV-Vis) absorption spectroscopy. The thin-film prepared samples were, additionally, investigated by InfraRed Fourier Transform (FTIR) spectroscopy and HPSEC chromatography. The experimental results were investigated to elucidate the radiation effects on the optical properties of these polymeric systems. From the experimental results, the feasibility of using this material as active element in gamma radiation dosimeters was investigated. The research was conducted aiming the low dose gamma radiation (< 1 kGy) specifically for application in the sterilization treatment of blood bags (~ 25 Gy) and high gamma radiation doses (up to 25 kGy) as used for the sterilization of medical and hospital supplies.