The foundation of vacuum nanoelectronics was laid as early as in 1961 when Kenneth Shoulders proposed the development of vertical field-emission micro-triodes. After years of conspicuous stagnancy in the field much interest has reemerged for the vacuum nanoelectronics in recent years. Electron field emission under high electric field from conventional and exotic nanoemitters, which have now been made possible with the use of modern day technology, has been the driving force behind this renewal of interest in vacuum nanoelectronics. In the research reported in this thesis self-assembled silicon nanostructures were studied as a potential source of field emission for vacuum nanoelectronic device applications. Whiskerlike protruding silicon nanostructures were grown on untreated n- and p-type silicon surfaces using electron-beam annealing under high vacuum. The electrical transport characteristics of the silicon nanostructures were investigated using conductive atomic force microscopy (C-AFM). Higher electrical conductivities for the nanostructured surface compared to that for the surrounding planar silicon substrate region were observed. Non-ideal diode behaviour with high ideality factors were reported for the individual nanostructure-AFM tip Schottky nanocontacts. This demonstration, indicative of the presence of a significant field emission component in the analysed current transport phenomena was also detailed. Field emission from these nanostructures was demonstrated qualitatively in a lift-mode interleave C-AFM study. A technique to fabricate integrated field emission diodes using silicon nanostructures in a CMOS process technology was developed. The process incorporated the nanostructure growth phase at the closing steps in the process flow. Turn-on voltages as low as ~ 0.6 V were reported for these devices, which make them good candidates for incorporation into standard CMOS circuit applications. Reproducible I V characteristics exhibited by these fabricated devices were further studied and field emission parameters were extracted. A new consistent and reliable method to extract field emission parameters such as effective barrier height, field conversion factor, and total emitting area at the onset of the field emission regime was developed and is reported herein. The developed parameter extraction method used a unified electron emission approach in the transition region of the device operation. The existence of an electron-supply limited current saturation region at very high electric field was also confirmed. Both the C-AFM and the device characterization studies were modelled and simulated using the finite element method in COMSOL Multiphysics. The experimental results – the field developed at various operating environments – are explained in relation to these finite element analyses. Field enhancements at the atomically sharp nanostructure apexes as suggested in the experimental studies were confirmed. The nanostructure tip radius effect and sensitivity to small nanostructure height variation were investigated and mathematical relations for the nanostructure regime of our interest were established. A technique to optimize the cathode-opening area was also demonstrated. Suggestions related to further research on field emission from silicon nanostructures, optimization of the field emission device fabrication process, and fabrication of field emission triodes are elaborated in the final chapter of this thesis. The experimental, modelling, and simulation works of this thesis indicate that silicon field emission devices could be integrated into the existing CMOS process technology. This integration would offer goods from both the worlds of vacuum and solid-sate nanoelectronics – fast ballistic electron transport, temperature insensitivity, radiation hardness, high packing density, mature technological backing, and economies of scale among other features.

Aggressive scaling required to augment device performance has caused conventional electrode materials to approach their physical scaling limits. Alternative metal gate/high dielectric constant (MG/High-[kappa]) stacks have been implemented successfully in commercial devices and hold promise for further scaling based performance advances. Existing MG/High-[kappa] technology does not achieve a single metal n-MOS to p-MOS effective work function (EWF) tuning range suitable for bulk silicon (Si) device applications. Dual metal gates (DMGs) utilizing a separate metal for n-MOS and p-MOS electrodes increases the cost and complexity of fabrication. The research presented herein introduces a method by which the cost and complexity of MG/High-[kappa] device fabrication may be reduced. Innovative fin field effect transistors (FinFETs) incorporating 3 dimensional ultra thin body silicon on oxide (3-D UTB-SOI) technology display superior electrical characteristics compared to bulk Si devices at the nanometer (nm) dimension and require only a +/-200meV n-MOS to p-MOS EWF tuning range around the Si mid-gap. Single metals capable of achieving this +/-200meV EWF tuning range have been evaluated herein and the tuning mechanisms investigated and engineered to develop a single MG/High-[kappa] FinFET the fabrication complexity of which is reduced by 40%. More specifically, the research shows that the metal thickness of titanium nitride/hafnium silicon oxide (TiN/HfSiOx) gate stack may be engineered to achieve an n-MOS (thinner TiN) to p-MOS (thicker TiN) appropriate FinFET EWF tuning range. FinFETs may be fabricated by depositing a single p-MOS appropriate TiN thickness which may be selectively etched back to achieve thinner, n-MOS appropriate films. Similar electrical behavior is exhibited by etched back and as deposited TiN electrode FinFETs. The single metal etch back fabrication method removes many of the additional steps required for DMG fabrication and preserves the integrity of the MG/High-[kappa] interface between n-MOS and p-MOS devices. These advantages result in reduced fabrication complexity and improved reliability and reproducibility. / text

No description available.

Developing effective methods of generating thin metal oxide films are important for sensing and separations applications. An obstacle to device fabrication is controlling the size and spatial orientation of domain level pores while retaining the ability to generate arbitrary device level patterns. Well-ordered hexagonally packed cylindrical pores were created by taking advantage of block copolymer self-assembly followed by selective condensation of silica precursors using supercritical carbon dioxide as the solvent. It was possible to control the pore size by choosing PEO-PPO-PEO (Pluronic® series) triblock copolymers of differing molecular weights. These processes were then incorporated with conventional lithographic techniques to generate patterns on the device scale. The first route involves replacement of the organic acid catalyst with a photoacid generator that restricts acid formation by masking pre-determined regions then exposing to UV light. The second route is similar except that addition of a cross-linking agent limits acid diffusion while reversing the tone of the final pattern. The third route avoids acid diffusion altogether and generates the pattern through reactive ion etching through a sacrificial photoresist. A completely different fourth route was taken and nanoimprint lithography was used to generate sub-micron patterns with alternate block copolymers. The feasibility of the preliminary devices generated in this thesis has been examined through particle diffusion experiments. Samples were soaked in a fluorescent dye then exposed to multiple sizes of gold nanoparticles. Fluorescence quenching was then monitored to determine pore accessibility.

Al–O monolayers embedded in ultrathin SiO₂ were shown previously to contain Al-induced acceptor states, which capture electrons from adjacent silicon wafers and generate a negative fixed charge that enables efficient Si-surface passivation. Here, we show that this surface passivation is just in part attributed to field-effect passivation, since the electrically active interface trap density Dit itself at the Si/SiO₂ interface is reduced by the presence of the acceptor states. For sufficiently thin tunnel-SiO₂ films between the Si-surface and the Al–O monolayers, Dit is reduced by more than one order of magnitude. This is attributed to an interface defect deactivation mechanism that involves the discharge of the singly-occupied dangling bonds (Pb0 defects) into the acceptor states, so that Shockley-Read-Hall-recombination is drastically reduced. We demonstrate that the combined electronic and field-effect passivation allows for minority carrier lifetimes in excess of 1 ms on n-type Si and that additional H₂-passivation is not able to improve that lifetime significantly.

<p>Developing a hybrid platform that combines nanophotonic circuits and atomic physic may provide new chip-scale devices for quantum application or versatile tools for exploring photon-mediated long-range quantum systems. However, this challenging project demands the excellent integration of cold atom trapping and manipulation technology with cutting-edge nanophotonics circuit design and fabrication. In this thesis project, we aim to develop a novel suspended membrane platform that serves as a quantum interface between laser-cooled, trapped atoms in an ultrahigh vacuum and the photons guided in the nanophotonic circuits based on high-quality silicon nitride microring resonators fabricated on a transparent membrane substrate. </p> <p><br></p> <p>The proposed platform meets the stringent performance requirements imposed by nanofabrication and optical physics in an ultra-high vacuum. These include a high yield rate for mm-scale suspended dielectric photonic devices, minimization of the surface roughness to achieve ultrahigh-optical quality, complete control of optical loss/in-coupling rate to achieve critical photon coupling to a microring resonator, and high-efficiency waveguide optical input/output coupler in an ultrahigh vacuum environment. This platform is compatible with laser-cooled and trapped cold atoms. The experimental demonstration of trapping and imaging single atoms on a photonic resonator circuit using optical tweezers has been demonstrated. Our circuit design can potentially reach a record-high cooperativity parameter C$>$500 for single atom-photon coupling, which is of high importance in realizing a coherent quantum nonlinear optical platform and holds great promise as an on-chip atom-cavity QED platform.</p>

<div>Silicon based semiconductor technology is facing more and more challenges to continue the Moore's law due to its fundamental scaling limitations. To continue the pace of progress of device performance for both logic and memory devices, researchers are exploring new low-dimensional materials, e.g. nanowire, nanotube, graphene and hexagonal boron nitride. Transition metal dichalcogenides (TMDs) are attracted considerable attention due their atomically thin nature and proper bandgap at the initial study. Recently, more and more interesting properties are found in these materials, which will bring out more potential usefulness for electronic applications. Competing with the silicon device performance is not the only goal in the potential path finding of beyond silicon. Low-dimensional materials may have other outstanding performances as an alternative materials in many application realms. </div><div><br></div><div>This thesis explores the potential of TMD based devices in memory and logic applications. For the memory application, TMD based vertical devices are fully studied. Two-terminal vertical transition metal dichalcogenide (TMD) based memory selectors were firstly built and characterized, exhibiting better overall performance compared with some traditional selectors. Polymorphism is one of unique properties in TMD materials. 2D phase engineering in TMDs attracted great attention. While electric switching between semiconductor phase to metallic phase is the most desirable. In this thesis, electric field induced structural transition in MoTe₂ and Mo_1-xW_xTe₂ is firstly presented. Reproducible bipolar resistive random access (RRAM) behavior is observed in MoTe₂ and Mo_1-xW_xTe₂ based vertical devices. Direct confirmation of a phase transition from a 2H semiconductor to a distorted 2H_d metallic phase was obtained after applying an electric field. Set voltage is changed with flake thickness, and switching speed is less than 5 ns. Different from conventional RRAM devices based on ionic migration, the MoTe₂-based RRAMs offer intrinsically better reliability and control. In comparison to phase change memory (PCM)-based devices that operate based on a change between an amorphous and a crystalline structure, our MoTe₂-based RRAM devices allow faster switching due to a transition between two crystalline states. Moreover, utilization of atomically thin 2D materials allows for aggressive scaling and high-performance flexible electronics applications. Both of the studies shine lights on the new application in the memory field with two-dimensional materials.<br></div><div><br></div><div>For the logic application, the ultra thin body nature of TMDs allows for more aggressive scaling compared with bulk material - silicon. Two aspects of scaling properties in TMD based devices are discussed, channel length scaling and channel width scaling. A tunability of short channel effects in MoS₂ field effect transistor (FET) is reported. The electrical performance of MoS₂ flakes is governed by an unexpected dependence on the effective body thickness of the device which in turn depends on the amount of intercalated water molecules that exist in the layered structure. In particular, we observe that the doping stage of a MoS₂ FET strongly depends on the environment (air/vacuum). For the channel width scaling, the impact of edge states in three types of TMDs, metallic T_d-phase WTe₂ as well as semiconducting 2H-phase MoTe₂ and MoS₂ were explored, by patterning thin flakes into ribbons with varying channel widths. No obvious charge depletion at the edges is observed for any of these three materials, which is different from what has been observed in graphene nanoribbon devices. </div>

In the recent years area organic photovoltaics is generating a lot of interests because whole process of synthesis and fabrication is less energy intensive process as well as it is cost effective compared to conventional inorganic Si based photovoltaic technology. This work mainly deals with the fabrication and optimization of device fabrication conditions for organic photovoltaic materials. In first part of the work, the solar cell fabrication conditions were optimized for the commonly used system P3HT – PCBM. The fabricated device was optimized for the solvents used for the active material, concentration of the active material solution, donor- acceptor ratio of the active material, annealing conditions of the active layer and the metal evaporation conditions for the cathode. All the optimization procedures were carried out in controlled atmosphere to minimize the environmental effect inference during fabrication of the solar cell devices. All the characterization was carried out at ambient conditions. The efficiency of the solar cell was improved from 0.009% to 6.2%. the environmental stability of the fabricated devices were carried out after encapsulating it with epoxy based resin in both ambient conditions as well as extreme conditions like 85% RH at 25°C inside the humidity chamber. It was observed that both the data matches well with each other indicating proper encapsulation required to safe guard the device for the better performance over the period of time. Second part of this work mainly deals with understanding the structure property relationship of thiophene based donor- acceptor- donor molecule 2,5-dithienyl-3,4-(1,8-naphthylene) cyclopentadienone (DTCPA), which is highly crystalline, low band gap organic molecule which absorbs over entire visible region of the solar spectra. DTCPA crystals of various morphologies were prepared by various recrystallization routes. It was observed that macro scale morphology of these crystals differs from each other. Also depending on the method of recrystallization sizes of the crystals also varies. All the recrystallized DTCPA shows strong orientation toward (001) direction. However, it was observed that lattice parameters of these crystals slightly differ from each other owing to the recrystallization methodology. These variations in crystal parameters are more than 0.02 which is significant. It was also observed that the crystallite sizes depend on the recrystallization routes. Slow evaporation of concentrated solution (SEC) grown crystals has the larger crystallite size of 170nm. It was observed that absorption range of these crystals slightly differ from each other owing to the change in the crystallite sizes and crystal parameters. Third part of this work deals with the fabrication and optimization of thermal evaporation process of DTCPA for photovoltaic applications. DTCPA is stable at higher temperatures as well as has sharp melting point which make it ideal candidate for thermal evaporation. In this work films of DTCPA were fabricated for various evaporation rates by thermal evaporation technique. Chemical integrity of the molecules upon evaporation is found to be intact as observed from FTIR spectroscopy. XRD shows that at lower (25 W/m2) as well as higher (40 W/m2) films are oriented to (001), (400) as well as (311) directions, at 30 W/m2 and 35 W/m2 there is a strong orientation towards (311) and (001) directions respectively. Photo luminescence studies indicate that there is strong 410 nm emission for films deposited at the power of 25 W/m2 and 40 W/m2. Microscopic studies confirm that morphology is dependent on the deposition rates as it changes with the change in deposition rate. This in turn reflects in the device characteristics of these films. It was observed that films deposited at high deposition rates show better device characteristics with high VOC and current density values. All these device fabrication and characterizations were carried out in ambient conditions. Fourth part of this work deals with P3HT - DTCPA composites which exhibit wide range of light absorption. It was observed that DTCPA act as nucleating centers for the P3HT molecules and increases crystallinity in the composite. Furthermore, DTCPA helps in exciton separation because of donor and acceptor moieties present in the molecule. It also helps in charge transportation because of its crystalline nature and further it induces molecular ordering in the P3HT matrix. The band diagram of P3HT- DTCPA suggests that the band edges of both materials are ideal for charge separation. In addition, crystalline nature of the DTCPA molecule helps in effective charge transportation. J-V characteristics shows that there is large built in potential in the devices from these blends leading to large Voc. Composites with lower DTCPA loadings show higher efficiency than with higher loadings. These devices were prepared in ambient conditions and needs to be optimized for obtaining better device properties. In the fifth part of the work two types of system were studied to understand the band edge matching on the photovoltaic properties, carbazole based copolymers and DTCPA based copolymers. In the case of carbazole based copolymers it was observed that by copolymerizing carbazole with thiophene based derivatives lowers the band gap and modifies the HOMO and LUMO levels for better suit for the photovoltaic device fabrication. It was observed that that is two orders of improvements in the efficiency by co polymerizing carbazole with benzothiodizole as improves the JSC and VOC. Also the copolymerization of carbazole with both benzothiodiazole and bithiophene results in better light harvesting as the optical band gap was lowered. In the case of DTCPA copolymers with DTBT and DHTBT as both are random copolymers the solubility was low as well as their HOMO band edge was mismatched with the PEDOT: PSS which is a hole transport layer. However, the alternate polymerization of DTCPA with DTBT improved the band edge matching and also the solubility. As a result there was tenfold improvement in the charge collection and hence the efficiency was improved from 0.02% to 2.4%. Many of the conducting polymers have good material property but poor filmability. In the sixth part of this work deals with fabrication of device quality films by alternate deposition technique like pulsed laser deposition. Two types of system were studied in this work (i) polypyrrole- MWCNT nanocomposites and (ii) Poly DTCPA polymer. In both the cases it was observed that chemical integrity of the polymer retained during ablation. PolyDTCPA films were fabricated by pulsed laser deposition by both IR (Nd-YAG) and UV (KrF) laser source. Morphological studies indicate that IR laser ablated films were particulate in nature whereas UV laser ablated films were grown as continuous layers as polyDTCPA absorbs better in UV region. As a result the IV characteristics indicate that IR laser ablated films are resistive in nature and UV laser ablated films are good rectifiers indicating the suitability of the process for fabrication of device quality films.

In this thesis, the strength of the LBIC system is enhanced in different aspects that includes its feasibility as a non-destructive characterization tool, the signal analysis and development of analytical solution to have better understanding on the defects and inhomogeneities in the quantum dot based hetero-structures for device applications, finally understanding its limits due to the size of the laser beam and interpretation of artefacts in the signal appearance due to the presence of co-devices. Chapter#1 provides the introduction and literature survey of the LBIC system. It covers the importance and area of application of the LBIC. Chapter#2 various tools and instrumentation are discussed briefly for the systems that are developed in the lab as well as standard tools utilised for the material characterization. A LBIC instrumentation a novel colloidal quantum dots (CQD) thin film deposition system is discussed. In the last part along with the standard characterization systems a software tool (semiconductor device simulator) is discussed, which is used to visualize and understand the LBIC profile that is obtained experimentally. Chapter#3 provides the information of colloidal synthesis of PbS and HgxCd1-xTe quantum dots. Device fabrication process is explained step by step for the following devices. p-n junction silicon diodes, PbS-CQD/Si hetero-structures, ITO/PbS-CQD/Al crossbar structures and HgCdTe-CQD/Si hetero-structures. Chapter#4 deals with the major constraints imposed on the LBIC due to the need of Ohmic contacts. To overcome this major limitation, in this work, the origin of the signal is studied with the remote contact geometry for silicon p-n junction devices. It was observed that the signals can be collected with the capacitively coupled remote contacts, where LBIC was ultimately demonstrated as contactless measurement tool without any compromise on the measurements and thus obtained physical parameters. The effect of finite laser beam size is also described, which was found to have effect on the actual dimensions measured with the LBIC images. LBIC utility is further enhanced with the Si/CQD based hetero-structure devices, which are the potential candidates in the evolving device technology to be utilized in various modular systems such as PDs and LED applications. Chapter#5 discusses the origin and possible mechanisms for lateral photo-voltage which is closely monitored in the PbS-CQD/Si hetero-junction device systems. Interestingly, it is observed that there are two different line profiles for n and p type Si substrates. Different mechanisms that give rise to this kind of profiles were found to be distinct and are related to the band alignment of the CQD/Si hetero-structure. It lead to the revelation of an interesting phenomenon and believed to be universally observed irrespective of the materials involved in the formation of hetero-junction. Simulations and experimental results are quite consistent and in agreement with each other, which confirm the underlying physical mechanism that connects the LBIC anomalies with the band alignment. Chapter#6 deals with the spatial variations in the transverse photocurrent in the PbS-CQD film which is studied as a function of applied bias. Analytical equation is setup for the photocurrent in the CQD film under applied bias with the help of available transport mechanism and equations from the literature. The spatial non-uniformity that exists in the photocurrent proved to be the result of spatial inhomoginities in the physical parameters. By correlating the spatial data to the analytical equation, it is shown that the inhomoginities can be predicted. This approach is important for the devices, where monolithic detectors are fabricated by depositing CQD film on Read-Out-Integrated-Circuit (ROIC), where the manifestation of non-uniformity can be understood and probably fixed. Chapter#7 HgCdTe CQD based devices are studied for the purpose of photo-detector applications in MWIR (3  5 μm) region. HgxCd1-xTe Colloidal quantum dots are technologically important due to their wide absorption range that covers different regions of the atmospheric window. HgxCd1-xTe are successfully synthesised, which covers the absorption edge up to ~6.25 m in the IR region. Absorption and photo-response studies are carried out on HgxCd1-xTe/Si hetero-junctions under incident IR radiation. It is observed that the band gap of the quantum dots can be tuned easily by controlling the growth time as a parameter, thus moulded HgxCd1-xTe CQD/Si hetero-structures were found to have good photo-response. Chapter#8 the summary and the future direction and scope of the work is discussed. This includes the interesting observations during this thesis work which are not reported here in details.

<div>Power semiconductor devices play an important role in many areas, including household</div><div>appliances, electric vehicles, high speed trains, electric power stations, and renewable energy</div><div>conversion. In the modern era, silicon based devices have dominated the semiconductor</div><div>market, including power electronics, because of their low cost and high performance. The</div><div>applications of devices rated 600 V - 6.5 kV are still dominated by silicon devices, but they</div><div>are nearly reaching fundamental material limits. New wide band gap materials such as silicon</div><div>carbide (SiC) offer significant performance improvements due to superior material properties</div><div>for such applications in and beyond this voltage range. 4H-SiC is a strong candidate</div><div>among other wide band gap materials because of its high critical electric field, high thermal</div><div>conductivity, compatibility with silicon processing techniques, and the availability of high</div><div>quality conductive substrates.</div><div>Vertical DMOSFETs and insulated gate bipolar transistors (IGBT) are key devices for</div><div>high voltage applications. High blocking voltages require thick drift regions with very light</div><div>doping, leading to specific on-resistance (R_ON,SP ) that increases with the square of blocking</div><div>voltage (V_BR). In theory, superjunction drift regions could provide a solution because of a</div><div>linear dependence of R_ON,SP on V_BR when charge balance between the pillars is achieved</div><div>through extremely tight process control. In this thesis, we have concluded that superjunction</div><div>devices inevitably have at least some level of charge imbalance which leads to a quadratic</div><div>relationship between V_BR and R_ON,SP . We then proposed an optimization methodology to</div><div>achieve improved performance in the presence of this inevitable imbalance.</div><div>On the other hand, an IGBT combines the benefits of a conductivity modulated drift</div><div>region for significantly reduced specific on-resistance with the voltage controlled input of a</div><div>MOSFET. Silicon carbide n-channel IGBTs would have lower conduction losses than equivalent</div><div>DMOSFETs beyond 6.5 kV, but traditionally have not been feasible below 15 kV. This</div><div>is due to the fact that the n+ substrate must be removed to access the p+ collector of the</div><div>IGBT, and devices below 15 kV have drift layers too thin to be mechanically self-supporting.</div><div>In this thesis, we have demonstrated the world’s first functional 10 kV class n-IGBT with</div><div>a waffle substrate through simulation, process development, fabrication and characterization.</div><div><div>The waffle substrate would provide the required mechanical support for this class of devices.</div><div>The fabricated IGBT has exhibited a differential R_ON,SP of 160 mohm</div><div>.cm², less than half of</div><div>what would be expected without conductivity modulation. An extensive fabrication process</div><div>development for integrating a waffle substrate into an active IGBT structure is described</div><div>in this thesis. This process enables an entirely new class of moderate voltage SiC IGBTs,</div><div>opening up new applications for SiC power devices.</div></div>