The objective and motivation for this project is to design a low-power, low-noise oven-control circuit to optimize the stability of a MEMS oscillator. MEMS oscillators can be fabricated using conventional semiconductor manufacturing methods and can often be assembled in packages smaller than those of traditional crystal oscillators. However, one of their largest disadvantages currently is their high temperature coefficient of frequency (TCF), causing MEMS oscillators to be especially sensitive to temperature changes. Hence, this project focuses on designing a printed circuit board that will allow the user to manually tune a current passing through a resonator wire-bonded to the board to elevate the resonator temperature. This will ensure that the device's resonance frequency stays largely constant and that the oscillator provides a very stable signal.

Terahertz region in the electromagnetic spectrum is the region between Infrared and Microwave. As the Terahertz region has both wave and particle nature, it is difficult to make a room temperature, fast, and sensitive detector in this region. In this work, we fabricated a Palladium based IR detector and a CNT based THz bolometer. In Chapter 1, I give a brief introduction of the Terahertz region, the detectors already available in the market and different techniques I can use to test my detector. In Chapter 2, I explain about the Palladium IR bolometer, the fabrication technique I have used, and then we discuss the performance of the detector. In Chapter 3, I explained about the Roll to Roll based THz bolometer, its working and fabrication techniques, and at the end we discussed its performance.

<p>Metamorphic buffer (MB) layers were studied as a means to grow epilayers on top of a GaAs substrate which have different lattice constant than the GaAs. Growths were done by molecular beam epitaxy on a GaAs (001) substrate. The growths of step-graded InGaAs and InGaAsP MBs have been investigated using both linear and logarithmic grading profiles. The logarithmic grading profile shows slight improvement in the crystal quality over the linear grading profiles. This is an indication that instead of increasing the strain with the same grading rate, it may be helpful to have higher grading rate at the beginning and lower grading rate at the end of the buffer. InGaAsP graded buffers were grown where group III ratio was kept fixed. However due to the existence of phase separation and lower relaxation the quaternary growths exhibited no performance improvement as might have been expected from growths with only group V grading. Also, the effects of using an InGaP layer grown at low temperature before the MB were determined. Quantum wells (QW), which were grown on top of the MBs, were used to probe the optical emission properties. No significant difference was observed in photoluminescence between the samples with a low temperature layer and without a low temperature layer. Annealing enhanced the PL intensity but the crystal quality degraded due to the appearance of surface defects. Surface undulations, known as “cross-hatch” (CH), were observed in the top MB layers. Atomic force microscopy (AFM) was used to analyze the surface morphology and degree of polarization (DOP) measurement was used to analyze the strain features in the final MB layer. Similar patterns of both surface morphology and strain field indicate a correlation between these two. From analysis of the periodicity of strain field and the CH, evidence was found in the support of one of the existing models of CH evolution which implies that the CH appears before the formation of MDs and subsequently MDs form at some troughs in the undulation.</p> / Master of Applied Science (MASc)

<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)

A molecular dynamics coarse-grained block copolymer (BCP) model was developed and used to studied directed self-assembly (DSA), especially in regards to applications for semiconductor manufacturing. Most of the thesis is spent investigating the effect that guiding layer properties and block copolymer properties have on line roughness and defect density in a BCP-DSA process. These two effects are perhaps the most critical in making BCP-DSA a cost efficient industrial process. It is found that guiding patterns have little effect on line roughness and in fact that the BCP heals the majority of roughness in the underlying pattern. BCP properties have a larger effect on line roughness. Segregation strength (as measured by χN, where χ is the Flory- Huggins interaction parameter and N is the degree of polymerization) resulted in a larger than expected increase in line roughness when χN was low. Polydispersity resulted in a moderate increase in line roughness. In regards to equilibrium defect density, free energy calculations showed that χ was the primary determining factor, not χN as many expected. Equilibrium defect density was found to decrease exponentially with increasing χ. Defect density is also found to scale exponentially with polydispersity. Concerning defect heal rate, which can increase the real defect rate of a process if said rate is too low, it is found that increasing χN linearly increased the barrier to defect healing, which means that the defect heal rate decreases exponentially. However, for thin films this is only true for χN > ~ 50. Below χN ~ 50, the barrier is approximately constant. These results give excellent guidance to the type of materials and processes necessary to optimize a BCP-DSA process. A simulation technique designed to more efficiently sample over energy barriers called protracted noise dynamics for polymer systems was developed and studied. It was found that a decrease in simulation time of up to 4 orders of magnitude was achieved. The effect of box size on allowable pitches for a lamellar forming BCP was derived and demonstrated. It was found that more elongated boxes yielded more possible pitches and more accurate results. A short study on the effect of multiblock copolymers on the location of the order-disorder transition was also carried out and it was found that multiblock copolymers had small effect on the ODT. The distribution of chain conformations was also calculated.

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.