Antimony thin films demonstrate programmable optical non-linearity

The use of metals of nanometer dimensions to enhance and manipulate light-matter interactions for a range of emerging plasmonics-enabled nanophotonic and optoelectronic applications is an interesting, yet not highly explored area of research outside of plasmonics1,2. Even more importantly, the concept of an active metal, i.e. a metal that can undergo an optical non-volatile transition has not been explored. Nanostructure-based applications would have unprecedented impact on both the existing and future of optics with the development of active and nonlinear optical tunabilities in single elemental metals3-5. Compared to alloys, pure metals have the material simplicity and uniformity; however single elemental metals have not been viewed as tunable optical materials, although they have been explored as viable electrically switchable materials. In this paper we demonstrate for the first time that antimony (Sb), a pure metal, is optically distinguishable between two programmable states as nanoscale thin films. We then show that these states are stable at room temperature, and the states correspond to the crystalline and amorphous phases of the metal. Crucially from an application standpoint, we demonstrate both its optoelectronic modulation capabilities as well as speed of switching using single sub-picosecond (ps) pulses. The simplicity of depositing a single metal portends its potential for use in applications ranging from high speed active metamaterials to photonic neuromorphic computing, and opens up the possibility for its use in any optoelectronic application where metallic conductors with an actively tunable state is important

A Study of the Scaling and Advanced Functionality Potential of Phase Change Memory Devices

As traditional volatile and non-volatile data storage and memory technologies such as SRAM, DRAM, Flash and HDD face fundamental scaling challenges, scientists and engineers are forced to search for and develop alternative technologies for future electronic and computing systems that are relatively free from scaling issues, have lower power consumptions, higher storage densities, faster speeds, and can be easily integrated on-chip with microprocessor cores. This thesis focuses on the scaling and advanced functionality potential of one such memory technology i.e. Phase Change Memory (PCM), which is a leading contender to complement or even replace the above mentioned traditional technologies. In the first part of the thesis, a physically-realistic Multiphysics Cellular Automata PCM device modelling approach was used to study the scaling potential of conventional and commercially-viable PCM devices. It was demonstrated that mushroom-type and patterned probe PCM devices can indeed be scaled down to ultrasmall (single-nanometer) dimensions, and in doing so, ultralow programming currents (sub-20 μA) and ultrahigh storage densities (~10 Tb/in2) can be achieved via such a scaling process. Our sophisticated modelling approach also provided a detailed insight into some key PCM device characteristics, such as amorphization (Reset) and crystallization (Set) kinetics, thermal confinement, and the important resistance window i.e. difference in resistances between the Reset and Set states. In the second part of the thesis, the aforementioned modelling approach was used to assess the feasibility of some advanced functionalities of PCM devices, such as neuromorphic computing and phase change metadevices. It was demonstrated that by utilizing the accumulation mode of operation inherent to phase change materials, we can combine a physical PCM device with an external comparator-type circuit to deliver a ‘self-resetting spiking phase change neuron’, which when combined with phase change synapses can potentially open a new route for the realization of all-phase change neuromorphic computers. It was further shown that it is indeed feasible to design and ‘electrically’ switch practicable phase change metadevices (for absorber and modulator applications, and suited to operation in the technologically important near-infrared range of the spectrum). Finally, it was demonstrated that the Gillespie Cellular Automata (GCA) phase change model is capable of exhibiting ‘non-Arrhenius kinetics of crystallization’, which were found to be in good agreement with reported experimental studies

Growth and Oxidation of Graphene and Two-Dimensional Materials for Flexible Electronic Applications

The non-volatile storage of information is becoming increasingly important in our data-driven society. Limitations in conventional devices are driving the research and development of incorporating new materials into conventional device architectures to improve performance, as well as developing an array of emerging memory technologies based on entirely new physical processes. The discovery of graphene allowed for developing new approaches to these problems, both itself and as part of the larger, and ever-expanding family of 2D materials. In this thesis the growth and oxidation of these materials is investigated for implementing into such devices, exploiting some of the unique properties of 2D materials including atomic thinness, mechanical flexibility and tune-ability through chemical modification - to meet some challenges facing the community. This begins with the growth of graphene by chemical vapour deposition for a high quality flexible electrode material, followed by oxidation of graphene for use in resistive memory devices. The theme of oxidation is then extended to another 2D material, HfS2, which is selectively oxidised for use as high-k dielectric in Van der Waals heterostructures for FETs and resistive memory devices. Lastly, a technique for fabrication of graphene-based devices directly on the copper growth substrate is demonstrated for use in flexible devices for sensing touch and humidity

RESISTIVE SWITCHING CHARACTERISTICS OF NANOSTRUCTURED AND SOLUTION-PROCESSED COMPLEX OXIDE ASSEMBLIES

Miniaturization of conventional nonvolatile (NVM) memory devices is rapidly approaching the physical limitations of the constituent materials. An emerging random access memory (RAM), nanoscale resistive RAM (RRAM), has the potential to replace conventional nonvolatile memory and could foster novel type of computing due to its fast switching speed, high scalability, and low power consumption. RRAM, or memristors, represent a class of two terminal devices comprising an insulating layer, such as a metal oxide, sandwiched between two terminal electrodes that exhibits two or more distinct resistance states that depend on the history of the applied bias. While the sudden resistance reduction into a conductive state in metal oxide insulators has been known for almost 50 years, the fundamental resistive switching mechanism is a complex phenomenon that is still long-debated, complex process. Further improvements to existing memristor performance require a complete understanding of memristive properties under various operation conditions. Additional technical issues also remain, such as the development of facile, low-cost fabrication methods as an alternative to expensive, ultra-high vacuum (UHV) deposition methods. This collection of work explores resistive switching within metal oxide-based memristive material assemblies by analyzing the fundamental physical insulating material properties. Chapter 3 aims to translate the utility and simplicity of the highly ordered anodic aluminum oxide (AAO) template structure to complex, yet more functional (memristive) materials. Functional oxides possessing ordered, scalable nanoporous arrays and nanocapacitor arrays over a large area is of interest to both the fields of next-generation electronics and energy storing/harvesting devices. Here their switching performance will be evaluated using conductive atomic force microscopy (C-AFM). Chapter 4 demonstrates a convective self-assembly fabrication method that effectively enables the synthesis of a low-cost solution processed memristor comprising binary oxide and perovskite ABO3 nanocrystals of varying diameter. Chapter 5 systematically compares the influence of inter-nanoparticle distance on the threshold switching SET voltage of hafnium oxide (HfO2) memristors. Utilizing shorter phosphonic acid ligands with higher binding affinity on the nanocrystal surface enabled a record-low SET voltage to be achieved. Chapter 6 extends the scope to the fine tuning of solution processed memristors with two types of perovskites nanocrystals. The primary advantage of nanocrystal memristors is the ability to draw from additional degrees of freedom by tuning the constituent nanocrystal material properties. Recent advancement of solution phase techniques enables a high degree of controllability over the nanocrystal size and structure. Thus, this work found in this dissertation aims to understand and decouple the effects of the geometric size and substitutional nanocrystal parameters on resistive switching

Toward a new generation of photonic devices based on the integration of metal oxides in silicon technology

[ES] La búsqueda de nuevas soluciones e ideas innovadoras en el campo de la fotónica de silicio mediante la integración de nuevos materiales con prestaciones únicas es un tema de alta actualidad entre la comunidad científica en fotónica y con un impacto potencial muy alto. Dentro de esta temática, esta tesis pretende contribuir hacia una nueva generación de dispositivos fotónicos basados en la integración de óxidos metálicos en tecnología de silicio. Los óxidos metálicos elegidos pertenecen a la familia de óxidos conductores transparentes (TCO), concretamente el óxido de indio y estaño (ITO) y el óxido de cadmio (CdO), y materiales de cambio de fase (PCM) como el dióxido de vanadio (VO2). Dichos materiales se caracterizan especialmente por una variación drástica de sus propiedades optoelectrónicas, tales como la resistividad o el índice de refracción, frente a un estímulo externo ya sea en forma de temperatura, aplicación de un campo eléctrico o excitación óptica. De esta forma, nuestro objetivo es diseñar, fabricar y demostrar experimentalmente nuevas soluciones y dispositivos clave tales como dispositivos no volátiles, desfasadores y dispositivos con no linealidad óptica. Tales dispositivos podrían encontrar potencial utilidad en diversas aplicaciones que comprenden las comunicaciones ópticas, redes neuronales, LiDAR, computación, cuántica, entre otros. Las prestaciones clave en las que se pretende dar un salto disruptivo son el tamaño y capacidad para una alta densidad de integración, el consumo de potencia, y el ancho de banda.[CA] La recerca de noves solucions i idees innovadores al camp de la fotònica de silici mitjançant la integració de nous materials amb prestacions úniques és un tema d'alta actualitat entre la comunitat científica en fotònica i amb un impacte potencial molt alt. D'aquesta temàtica, aquesta tesi pretén contribuir cap a una nova generació de dispositius fotònics basats en la integració d'òxids metàl·lics en tecnologia de silici. Els òxids metàl·lics elegits pertanyen a la família d'òxids conductors transparents (TCO), concretament l'òxid d'indi i estany (ITO) i l'òxid de cadmi (CdO), i materials de canvi de fase (PCM) com el diòxid de vanadi (VO2). Aquests materials es caracteritzen especialment per una variació dràstica de les propietats optoelectròniques, com ara la resistivitat o l'índex de refracció, davant d'un estímul extern ja siga en forma de temperatura, aplicació d'un camp elèctric o excitació òptica. D'aquesta manera, el nostre objectiu és dissenyar, fabricar i demostrar experimentalment noves solucions i dispositius clau com ara dispositius no volàtils, desfasadors i dispositius amb no-linealitat òptica. Aquests dispositius podrien trobar potencial utilitat en diverses aplicacions que comprenen les comunicacions òptiques, xarxes neuronals, LiDAR, computació, quàntica, entre d'altres. Les prestacions clau en què es pretén fer un salt disruptiu són la grandària i la capacitat per a una alta densitat d'integració, el consum de potència i l'amplada de banda.[EN] The search for new solutions and innovative ideas in the field of silicon photonics through the integration of new materials featuring unique optoelectronic properties is a hot topic among the photonics scientific community with a very high potential impact. Within this topic, this thesis aims to contribute to a new generation of photonic devices based on the integration of metal oxides in silicon technology. The chosen metal oxides belong to the family of transparent conducting oxides (TCOs), namely indium tin oxide (ITO) and cadmium oxide (CdO), and phase change materials (PCMs) such as vanadium dioxide (VO2). These materials are characterized by a drastic variation of their optoelectronic properties, such as resistivity or refractive index, in response to an external stimulus either in the form of temperature, application of an electric field, or optical excitation. Therefore, our objective is to design, fabricate and experimentally demonstrate new solutions and key devices such as non-volatile devices, phase shifters, and devices with optical nonlinearity. Such devices could find potential utility in several applications, including optical communications, neural networks, LiDAR, computing, and quantum. The key features in which we aim to take a leapfrog are footprint and capacity for high integration density, power consumption, and bandwidth.This work is supported in part by grants ACIF/2018/172 funded by Generaliltat Valenciana, and FPU17/04224 funded by MCIN/AEI/10.13039/501100011033 and by “ESF Investing in your future”.Parra Gómez, J. (2022). Toward a new generation of photonic devices based on the integration of metal oxides in silicon technology [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/19088

Structural and electrical characterization of phase‐change memory line cells

De afgelopen tien jaar is Flash geheugen veel toegepast in o.a. USB sticks en later ook in smartphones en tablets. Een geheugentype waarvan verwacht werd dat ze Flash zou kunnen vervangen is Phase-change RAM of PRAM. NXP, dat toendertijd nog bekend stond als Philips Semiconductors, introduceerde het lijncelgeheugen dat bedoeld was voor embedded toepassingen, bijvoorbeeld voor chips in paspoorten of chipkaarten. Veel verschillende aspecten van PCMs en PRAM lijncellen geproduceerd door NXP zijn onderzocht, wat tot nieuwe en verassende resultaten heeft. De PRAM geheugencellen van NXP konden met elektrische pulsen van honderdmiljoen keer worden geschakeld tussen de SET en RESET toestand. Zeker in vergelijking met Flash geheugen, dat maar tienduizend keer worden geschakeld wat het gebruikt in solid state schijven erg gecompliceerd maakt, heeft PRAM op dit vlak zeer een streepje voor. Daarnaast werden verschillende celeigenschappen gemeten tussen het schakelen door zodat de celeigenschappen gerelateerd werden aan het aantal keer dat de cel was geschakeld. Deze resultaten zijn van belang voor het beter kunnen voorspellen van hoe een PRAM geheugen zich gedraagt bij daadwerkelijk gebruik. Ook werden elektrische resultaten gecorreleerd met elektronenmiscroscoopbeelden van de structuur van dezelfde PRAM geheugencel. In het bijzonder werd ontdekt dat wanneer de PRAM cel werd geprogrammeerd met een te hoge spanning dat het geschakelde (amorfe) gebied buiten het actieve gedeelte van de cel schuift. Daarnaast limiteren vernauwingen binnen het actieve gedeelte van de PRAM lijncel die kunnen ontstaan bij cel productie de levensduur doordat door onder hoge elektrisch velden het actieve material uiteenvalt. Deze resultaten zijn van belang voor de ontwikkeling en het op de markt brengen van PRAM geheugen