Large-area flexible electronics based on low-temperature solution-processed oxide semiconductors

Due to their high charge carrier mobility, optical transparency and mechanical flexibility, thin-film transistors (TFTs) based on metal oxide semiconductors represent an emerging technology that offers the potential to revolutionise the next-generations of large-area electronics. This thesis focuses on the development of high-performance TFTs based on low-temperature, solution-processed metal oxide semiconductors that are compatible with inexpensive flexible plastic substrates. The first part of the dissertation describes an ultraviolet light assisted processing method suitable for room-temperature activation of ZnO nanoparticles and their application as semiconducting channels in TFTs. The impact of the semiconductor/dielectric interface on electrical performance is studied using different device configurations and dielectric surface-passivation methods. Furthermore, a nanocomposite concept is proposed in order to assist electron transport between different crystalline domains. Using this approach, TFTs with electron mobilities of ~3 cm2/Vs are demonstrated. The second part of this work explores a carbon-free, aqueous-based Zn-ammine complex route for the synthesis of polycrystalline ZnO thin-films at low temperature and their subsequent use in TFTs. Concurrently, the development of a complementary high-κ oxide dielectric system enables the demonstration of high-performance ZnO TFTs with electron mobilities >10 cm2/Vs and operation voltage down to ~1.2 V. This low-temperature aqueous chemistry is further explored using a facile n-type doping approach. Enhancement in electrical performance is attributed to the different crystallographic properties of the Al-doped ZnO layers. The final part of the thesis introduces a novel TFT concept that exploits the enhanced electron transport properties of low-dimensional polycrystalline quasi-superlattices (QSLs), consisting of sequentially spin-cast layers of In2O3, Ga2O3 and ZnO deposited at temperatures 40 cm2/Vs - an order of magnitude higher than devices based on single binary oxide layers. Based on temperature dependent electron transport and capacitance-voltage measurements, it is reasoned that the enhanced electrical performance arises from the presence of quasi two-dimensional electron gas-like systems formed at the carefully engineered oxide heterointerfaces buried within the QSLs.Open Acces

Materials, Device, and System Integration of Amorphous Oxide Semiconductor TFTs

Amorphous oxide semiconductor (AOS) thin-film transistors (TFTs) have great potential for use in the next-generation of electronics. AOS TFTs can be used to fabricate circuits and sensors on various substrates, due to unique properties, including high mobility, excellent uniformity, and it requiring a low-temperature process. Currently, indium gallium zinc oxide (IGZO) is the predominant AOS used in the display industry as a TFT semiconductor. Although the IGZO technology is very mature, the development of AOS continues. Additional AOSs are being investigated to reduce cost and improve stability. Considering availability and the potential of materials, indium silicon oxide (ISO) was selected for this project. ISO uses silicon to suppress the instability originating from the oxygen vacancy. The silicon-oxygen bond has a higher dissociation energy, which improves retention of oxygen atoms in the film, and thus, increases the transistor’s stability. This detailed study follows a bottom-up approach. It starts with the fabrication and characterisation of materials. Basic material properties of the ISO film are discussed within, including amorphicity, bandgap, stoichiometry, and Hall-effect parameters. Based on the characterisation results, different deposition recipes for the TFT were developed and tested. The interface quality and etching selectivity were investigated. Uniformity and stability data were extracted from a TFT array using the developed photo-lithography process, which was used to verify and quantitate the capability of the process in system integration and circuit design. A Monte-Carlo simulation environment was established based on the extracted data. The two urgent challenges in all-TFT analogue circuit design, the lack of proper active load and the large parasitic capacitance, were investigated. In-depth analysis on these two issues and applicable solutions were presented. Investigation on system integration of TFT circuits and sensors were conducted, since the device demonstrated the required performance and uniformity. An all-TFT differential-input amplifier was designed and verified, as the first mixed signal all-TFT circuit.China Scholarship Council Cambridge Trus

Miniaturized Transistors, Volume II

In this book, we aim to address the ever-advancing progress in microelectronic device scaling. Complementary Metal-Oxide-Semiconductor (CMOS) devices continue to endure miniaturization, irrespective of the seeming physical limitations, helped by advancing fabrication techniques. We observe that miniaturization does not always refer to the latest technology node for digital transistors. Rather, by applying novel materials and device geometries, a significant reduction in the size of microelectronic devices for a broad set of applications can be achieved. The achievements made in the scaling of devices for applications beyond digital logic (e.g., high power, optoelectronics, and sensors) are taking the forefront in microelectronic miniaturization. Furthermore, all these achievements are assisted by improvements in the simulation and modeling of the involved materials and device structures. In particular, process and device technology computer-aided design (TCAD) has become indispensable in the design cycle of novel devices and technologies. It is our sincere hope that the results provided in this Special Issue prove useful to scientists and engineers who find themselves at the forefront of this rapidly evolving and broadening field. Now, more than ever, it is essential to look for solutions to find the next disrupting technologies which will allow for transistor miniaturization well beyond silicon’s physical limits and the current state-of-the-art. This requires a broad attack, including studies of novel and innovative designs as well as emerging materials which are becoming more application-specific than ever before

Roadmap on quantum nanotechnologies

Quantum phenomena are typically observable at length and time scales smaller than those of our everyday experience, often involving individual particles or excitations. The past few decades have seen a revolution in the ability to structure matter at the nanoscale, and experiments at the single particle level have become commonplace. This has opened wide new avenues for exploring and harnessing quantum mechanical effects in condensed matter. These quantum phenomena, in turn, have the potential to revolutionize the way we communicate, compute and probe the nanoscale world. Here, we review developments in key areas of quantum research in light of the nanotechnologies that enable them, with a view to what the future holds. Materials and devices with nanoscale features are used for quantum metrology and sensing, as building blocks for quantum computing, and as sources and detectors for quantum communication. They enable explorations of quantum behaviour and unconventional states in nano- and opto-mechanical systems, low-dimensional systems, molecular devices, nano-plasmonics, quantum electrodynamics, scanning tunnelling microscopy, and more. This rapidly expanding intersection of nanotechnology and quantum science/technology is mutually beneficial to both fields, laying claim to some of the most exciting scientific leaps of the last decade, with more on the horizon

Electron transport in molecular nanostructures

The past decades have seen huge efforts in trying to integrate carbon nanomaterials into electronic devices. Although microelectronic processors running on carbon nanotubes have demonstrated excellent performance and low power consumption, there are still limitations inherent to these materials, which restrict their applicability and integrability into devices. The lack of a bandgap in graphene, the mixture of metallic and semiconducting behaviours in carbon nanotubes, and tendency of nanotube to aggregate, are some examples of why it is necessary to devise alternative carbon nanostructures. In this thesis, I investigate the electronic properties of nanodevices where molecularly defined graphene nanoribbons have been integrated as functional elements. These systems offer several advantages: they are intrinsically semiconducting, with atomically precise topology that guarantees, in principle, low scattering and ultraclean behaviour, as well as access to topological states. On the other hand, their integration into devices is just in its infancy. After detailing the background of these systems, and of the most advanced devices that can be created out of them, I will describe the instrumentation and device architectures used to measure the electronic-transport properties. I then demonstrate a molecular quantum singleelectron transistor that can work up to room temperature, and detail the transport regimes that appear in the different temperature regions. These findings are not only the first observation of quantum operation at room temperature in a molecular structure, but also clarify the physics of how such devices work. I then address the second issue that has plagued devices based on carbon nanotubes: the presence of aggregates that can compromise, by intermolecular forces, device performance. I demonstrate how chemical functionalisation of the edges by the appropriate side groups can overcome these issues, allowing us to produce some the cleanest electronic nanodevices and features of single-electron transport, ever obtained. The clarity of the transport features is comparable of what has been seen for carbon nanotubes after more than 10 years of efforts, and allow us to address fundamental interactions that can regulate the transport, such as the electron–vibration coupling. I observe very strong vibronic coupling, with clear features of a Franck–Condon blockade. Eventually I examine how to control the chemical potential of molecular nanoribbons, and dope them at will. This is first addressed by investigating dopant groups attached to the ribbon edges. The energy shift induced in the nanoribbons by the dopant is, however, weak and impractical at the device level. I then present measurements in the first field-effect transistor of a unique class of materials, where dopants and metal atoms are inserted directly into the honeycomb lattice, and I propose possible improvements. These results offer a first step towards the realisation of practical and extremely clean electronic devices based on molecular nanoribbons. They thus open the path to the use of electronics for the investigation of spin states, topological effects, and the physics of quantum systems at high temperatures. They offer a picture of the transport mechanisms at play, and, from the practical level, offer the first recipes to overcome longstanding issues that have hindered the development of carbon-based electronic devices, and may thus be instrumental in the development of the first nanoribbon-based microprocessors

Strain-Engineered MOSFETs

This book brings together new developments in the area of strain-engineered MOSFETs using high-mibility substrates such as SIGe, strained-Si, germanium-on-insulator and III-V semiconductors into a single text which will cover the materials aspects, principles, and design of advanced devices, their fabrication and applications. The book presents a full TCAD methodology for strain-engineering in Si CMOS technology involving data flow from process simulation to systematic process variability simulation and generation of SPICE process compact models for manufacturing for yield optimization

GeSn semiconductor for micro-nanoelectronic applications

Within the last few years the steady electronic evolution lead the semiconductor world to study innovative device architectures and new materials able to replace Si platforms. In this scenario Ge1-xSnx alloy attracts the interest of the scientific community due to its ability to tune the material bandgap as a function of Sn content and its extreme compatibility with Si processing. Although the enhanced optical properties of Ge1-xSnx are evident, the augmented electrical properties such as the higher electron and holes mobility are also beneficial for metal oxide semiconductor. Therefore the alloy is expected to be a potential solution to integrate both electrical and optical devices. On one hand, several theoretical and experimental works depict the Ge1-xSnx alloy as a novel and fascinating solution to replace Si; on the other hand the material novelty forces us to enhance the knowledge of its fundamental physical and chemical properties, re-adapting the processing steps necessary to develop electronic and optical devices. In this dissertation a comprehensive study on Ge1-xSnx has been undertaken and discussed analysing a wide range of topics. The first chapter provides a detailed theoretical study on the electronic properties of the GeSn performed using first principle methods; subsequently the data obtained have been inserted into a TCAD software in order to create and calibrate a library used to simulate electrical devices. It is important to note, that at the beginning of this PhD GeSn was not an available material in the Synopsys device software, and thus it had to be defined from scratch As a next point, since the ever decreasing device size push toward the definition of Ohmic contacts, different stanogermanide films have been thoroughly analysed using various metals (Ni, Pt and Ti) annealed with two distinct methodologies (Rapid Thermal Annealing and Laser Thermal Annealing). Subsequently, considering the material limitation such as the limited thermal budget and the Sn segregation, an exhaustive study on the material doping has been firstly discussed theoretically and after experimentally characterized using both classical ion implantation and layer deposition techniques. The different building blocks of Field Effect Transistors have been investigated and tuned individually with the aim to develop FET devices with bottom up approach. Then, Field Effect Transistor devices using GeSn NWs grown by a VLS methodology with Sn composition ranging from (0.03-0.09 at.%) have been developed and extensively characterized with the state of the art present in literature. Finally the analysis of highly selective etch recipes lead to the development of sub-nm device configuration such as Gate-All-Around (GAA) structure obtained using classical top down lithography approach. The innovative structure was electrically characterized highlighting the possibility to obtain decananometer device architecture with this innovative alloy. Lastly thesis summary and final outlooks were reported with the aim to outline the thesis contribution and the future material investigations

Development of high-performance, cost-effective quantum dot lasers for data-centre and Si photonics applications

Photonic technologies have been considered new methods to achieve high bandwidth data communication and transmission. Si-photonics was proposed to address the discrepancy between bulky photonic devices and advanced electronics and create high-density integrated photonics. One of the challenges is integrating all the components necessary for full-functionality photonic integrated circuits (PIC). Great efforts have been devoted to overcoming the inherent limitations of Group-IV materials to provide sufficient gain, efficient modulation and sensitive detections. Making Si the host material for efficient light emission poses the most stringent requirements and is the primary missing component in the Si-photonics platform. Incorporating III-V materials with the Si photonics platform and quantum dot (QD) structure is a promising solution to the problem of a fully-integrated and high-functioning PIC. High-performance QD lasers on III-V substrate or epitaxially on silicon have been developed in the last few decades with low threshold current density, low-temperature sensitivity, great reliability and large injection efficiency. Moreover, from the dynamic aspect, the intrinsic frequency of direct modulated laser and noise intensity is important for its applications in a data centre. QD is considered an alternative to quantum wells (QWs); however, the demonstrated QD laser has not fulfilled initial expectations, mainly due to its high gain compression and low differential gain. Another feature that needs to be noticed is feedback sensitivity, as the properties of semiconductor lasers are greatly degraded by reflection from external reflectors, such as the fibre connects and facets of integrated devices. QD devices are predicted to have stronger feedback resistance due to their large damping and small linewidth enhancement factor (LEF). These properties have attracted much research, and high-performance QD devices have been developed. In this thesis, we comprehensively investigated QD laser performance and applied our QD laser in the optical module instead of the commercial QW distributed feedback (DFB) laser. The background of Si photonics, the development of QD devices, and the fundamentals of QD lasers are presented in Chapter 1. The basic static and dynamic performances are demonstrated in Chapters 2 and 3. The GaAs-based QD laser provides a low threshold, high-temperature stability, and low noise operation with a limited small signal bandwidth. Chapter 4 provides a comprehensive study of the feedback resistance of the QD laser. The onset of coherence collapse is determined as -14 dB, verified by the static optical and electrical spectra and small signal response. Based on previous measurements, the QD laser is proven to be a high-performance, low-cost candidate for the Si-photonics module. In Chapter 5, the QD laser is used in practical applications, including a large signal transmission system with and without feedback and a commercial optical module. Although the intrinsic bandwidth of the QD laser is limited to around 5GHz due to the large damping and unoptimised capacitance, 30 Gbps data transmission has been demonstrated by a directly modulated QD laser. Large, high-speed signal modulation is achieved due to its high gain compression factor. Regarding the laser with intentional feedback, there is little degradation in the eye diagram under the whole feedback level up to -8dB. We also replaced the commercial QW DFB laser in 100G data-centre reach (DR)-1 optical module with our QD Fabry Perot (FP) laser without an isolator which gives a clear eye diagram under 53 Gbps 4-level pulse amplitude modulation (PAM4) with an extinction ratio (ER) of 4.7 dB. In conclusion, this thesis verifies the feasibility of adopting the QD laser as a light source for the Si-photonics module. The QD laser is selected over other lasers because of its low threshold, high-temperature stability and maximum operating temperature, and strong tolerance to unintentional feedback. This is the first project to measure critical feedback levels with different characteristics and to theoretically analyse the inconsistent value. More importantly, this thesis’ most original contribution is investigating the commercial applications of QD lasers in a Si-photonics module in an isolator-free state. In summary, the QD laser has been proven to be a feasible solution for the next-generation optical system