Integration of Ferroelectric HfO2 onto a III-V Nanowire Platform

The discovery of ferroelectricity in CMOS-compatible oxides, such as doped hafnium oxide, has opened new possibilities for electronics by reviving the use of ferroelectric implementations on modern technology platforms. This thesis presents the ground-up integration of ferroelectric HfO2 on a thermally sensitive III-V nanowire platform leading to the successful implementation of ferroelectric transistors (FeFETs), tunnel junctions (FTJs), and varactors for mm-wave applications. As ferroelectric HfO2 on III-V semiconductors is a nascent technology, a special emphasis is put on the fundamental integration issues and the various engineering challenges facing the technology.The fabrication of metal-oxide-semiconductor (MOS) capacitors is treated as well as the measurement methods developed to investigate the interfacial quality to the narrow bandgap III-V materials using both electrical and operando synchrotron light source techniques. After optimizing both the films and the top electrode, the gate stack is integrated onto vertical InAs nanowires on Si in order to successfully implement FeFETs. Their performance and reliability can be explained from the deeper physical understanding obtained from the capacitor structures.By introducing an InAs/(In)GaAsSb/GaSb heterostructure in the nanowire, a ferroelectric tunnel field effect transistor (ferro-TFET) is fabricated. Based on the ultra-short effective channel created by the band-to-band tunneling process, the localized potential variations induced by single ultra-scaled ferroelectric domains and individual defects are sensed and investigated. By intentionally introducing a gate-source overlap in the ferro-TFET, a non-volatile reconfigurable single-transistor solution for modulating an input signal with diverse modes including signal transmission, phase shift, frequency doubling, and mixing is implemented.Finally, by fabricating scaled ferroelectric MOS capacitors in the front-end with a dedicated and adopted RF and mm-wave backend-of-line (BEOL) implementation, the ferroelectric behavior is captured at RF and mm-wave frequencies

Reconfigurable electronics based on metal-insulator transition:steep-slope switches and high frequency functions enabled by Vanadium Dioxide

The vast majority of disruptive innovations in science and technology has been originated from the discovery of a new material or the way its properties have been exploited to create novel devices and systems. New advanced nanomaterials will have a lasting impact over the next decades, providing breakthroughs in all scientific domains addressing the main challenges faced by the world today, including energy efficiency, sustainability, climate and health. The electronics industry relied over the last decades on the miniaturization process based on the scaling laws of complementary metal-oxide semiconductors (CMOS). As this process is approaching fundamental limitations, new materials or physical principles must be exploited to replace or supplement CMOS technology. The aim of the work in this thesis is to propose the abrupt metal-insulator transition in functional oxides as a physical phenomenon enabling new classes of Beyond CMOS devices. In order to provide an experimental validation of the proposed designs, vanadium dioxide (VO2) has been selected among functional oxides exhibiting a metal-insulator transition, due to the possibility to operate at room temperature and the high contrast between the electrical properties of its two structural phases. A CMOS-compatible sputtering process for uniform large scale deposition of stoichiometric polycrystalline VO2 has been optimized, enabling high yield and low variability for the devices presented in the rest of the thesis. The high quality of the film has been confirmed by several electrical and structural characterization techniques. The first class of devices based on the MIT in VO2 presented in this work is the steep-slope electronic switch. A quantitative study of the slope of the electrically induced MIT (E-MIT) in 2-terminal VO2 switches is reported, including its dependence on temperature. Moreover, the switches present excellent ON-state conduction independently of temperature, suggesting MIT VO2 switches as promising candidates for steep-slope, highly conductive, temperature stable electronic switches. A novel design for the shape of the electrodes used in VO2 switches has been proposed, targeting a reduction in the actuation voltage necessary to induce the E-MIT. The electrothermal simulations addressing this effect have been validated by measurements. The potential of the MIT in VO2 for reconfigurable electronics in the microwave frequency range has been expressed by the design, fabrication and characterization of low-loss, highly reliable, broadband VO2 radio-frequency (RF) switches, novel VO2 tunable capacitors and RF tunable filters. The newly proposed tunable capacitors overcome the frequency limitations of conventional VO2 RF switches, enabling filters working at a higher frequency range than the current state-of-the-art. An alternative actuation method for the tunable capacitors has been proposed by integrating microheaters for local heating of the VO2 region, and the design tradeoffs have been discussed by coupled electrothermal and electromagnetic simulations. The last device presented in this work operates in the terahertz (THz) range; the MIT in VO2 has been exploited to demonstrate for the first time the operation of a modulated scatterer (MST) working at THz frequencies. The proposed MST is the first THz device whose working principle is based on the actuation of a single VO2 junction, in contrast to commonly employed VO2 metasurfaces

Novel Computing Paradigms using Oscillators

This dissertation is concerned with new ways of using oscillators to perform computational tasks. Specifically, it introduces methods for building finite state machines (for general-purpose Boolean computation) as well as Ising machines (for solving combinatorial optimization problems) using coupled oscillator networks.But firstly, why oscillators? Why use them for computation?An important reason is simply that oscillators are fascinating. Coupled oscillator systems often display intriguing synchronization phenomena where spontaneous patterns arise. From the synchronous flashing of fireflies to Huygens' clocks ticking in unison, from the molecular mechanism of circadian rhythms to the phase patterns in oscillatory neural circuits, the observation and study of synchronization in coupled oscillators has a long and rich history. Engineers across many disciplines have also taken inspiration from these phenomena, e.g., to design high-performance radio frequency communication circuits and optical lasers. To be able to contribute to the study of coupled oscillators and leverage them in novel paradigms of computing is without question an interesting andfulfilling quest in and of itself.Moreover, as Moore's Law nears its limits, new computing paradigms that are different from mere conventional complementary metal–oxide–semiconductor (CMOS) scaling have become an important area of exploration. One broad direction aims to improve CMOS performance using device technology such as fin field-effect transistors (FinFET) and gate-all-around (GAA) FETs. Other new computing schemes are based on non-CMOS material and device technology, e.g., graphene, carbon nanotubes, memristive devices, optical devices, etc.. Another growing trend in both academia and industry is to build digital application-specific integrated circuits (ASIC) suitable for speeding up certain computational tasks, often leveraging the parallel nature of unconventional non-von Neumann architectures. These schemes seek to circumvent the limitations posed at the device level through innovations at the system/architecture level.Our work on oscillator-based computation represents a direction that is different from the above and features several points of novelty and attractiveness. Firstly, it makes meaningful use of nonlinear dynamical phenomena to tackle well-defined computational tasks that span analog and digital domains. It also differs from conventional computational systems at the fundamental logic encoding level, using timing/phase of oscillation as opposed to voltage levels to represent logic values. These differences bring about several advantages. The change of logic encoding scheme has several device- and system-level benefits related to noise immunity and interference resistance. The use of nonlinear oscillator dynamics allows our systems to address problems difficult for conventional digital computation. Furthermore, our schemes are amenable to realizations using almost all types of oscillators, allowing a wide variety of devices from multiple physical domains to serve as the substrate for computing. This ability to leverage emerging multiphysics devices need not put off the realization of our ideas far into the future. Instead, implementations using well-established circuit technology are already both practical and attractive.This work also differs from all past work on oscillator-based computing, which mostly focuses on specialized image preprocessing tasks, such as edge detection, image segmentation and pattern recognition. Perhaps its most unique feature is that our systems use transitions between analog and digital modes of operation --- unlike other existing schemes that simply couple oscillators and let their phases settle to a continuum of values, we use a special type of injection locking to make each oscillator settle to one of the several well-defined multistable phase-locked states, which we use to encode logic values for computation. Our schemes of oscillator-based Boolean and Ising computation are built upon this digitization of phase; they expand the scope of oscillator-based computing significantly.Our ideas are built on years of past research in the modelling, simulation and analysis of oscillators. While there is a considerable amount of literature (arguably since Christiaan Huygens wrote about his observation of synchronized pendulum clocks in the 17th century) analyzing the synchronization phenomenon from different perspectives at different levels, we have been able to further develop the theory of injection locking, connecting the dots to find a path of analysis that starts from the low-level differential equations of individual oscillators and arrives at phase-based models and energy landscapes of coupled oscillator systems. This theoretical scaffolding is able not only to explain the operation of oscillator-based systems, but also to serve as the basis for simulation and design tools. Building on this, we explore the practical design of our proposed systems, demonstrate working prototypes, as well as develop the techniques, tools and methodologies essential for the process

Silicon Integrated Arrays: From Microwave to IR

Integrated chips have enabled realization and mass production of complex systems in a small form factor. Through process miniaturization many novel applications in silicon photonics and electronic systems have been enabled. In this thesis I have provided several examples of innovations that are only enabled by integration. I have also demonstrated how electronics and photonics circuits can complement each other to achieve a system with superior performance.</p