Interpretation and Regulation of Electronic Defects in IGZO TFTs Through Materials & Processes

The recent rise in the market for consumer electronics has fueled extensive research in the field of display. Thin-Film Transistors (TFTs) are used as active matrix switching devices for flat panel displays such as LCD and OLED. The following investigation involves an amorphous metal-oxide semiconductor that has the potential for improved performance over current technology, while maintaining high manufacturability. Indium-Gallium-Zinc-Oxide (IGZO) is a semiconductor material which is at the onset of commercialization. The low-temperature large-area deposition compatibility of IGZO makes it an attractive technology from a manufacturing standpoint, with an electron mobility that is 10 times higher than current amorphous silicon technology. The stability of IGZO TFTs continues to be a challenge due to the presence of defect states and problems associated with interface passivation. The goal of this dissertation is to further the understanding of the role of defect states in IGZO, and investigate materials and processes needed to regulate defects to the level at which the associated influence on device operation is controlled. The relationships between processes associated with IGZO TFT operation including IGZO sputter deposition, annealing conditions and back-channel passivation are established through process experimentation, materials analysis, electrical characterization, and modeling of electronic properties and transistor behavior. Each of these components has been essential in formulating and testing several hypotheses on the mechanisms involved, and directing efforts towards achieving the goal. Key accomplishments and quantified results are summarized as follows: • XPS analysis identified differences in oxygen vacancies in samples before and after oxidizing ambient annealing at 400 °C, showing a drop in relative integrated area of the O 1s peak from 32% to 19%, which experimentally translates to over a thousand fold decrease in the channel free electron concentration. • Transport behavior at cryogenic temperatures identified variable range hopping as the electron transport mechanism at temperature below 130 K, whereas at temperature greater than 130 K, the current vs temperature response followed an Arrhenius relationship consistent with extended state transport. • Refinement of an IGZO material model for TCAD simulation, which consists of oxygen vacancy donors providing an integrated space charge concentration NVO = +5e15 cm-3, and acceptor-like band-tail states with a total integrated ionized concentration of NTA = -2e18 cm-3. An intrinsic electron mobility was established to be Un = 12.7 cm2/V∙s. • A SPICE-compatible 2D on-state operation model for IGZO TFTs has been developed which includes the integration of drain-impressed deionization of band-tail states and results in a 2D modification of free channel charge. The model provides an exceptional match to measured data and TCAD simulation, with model parameters for channel mobility (Uch = 12 cm2/V∙s) and threshold voltage (VT = 0.14 V) having a close match to TCAD analogs. • TCAD material and device models for bottom-gate and double-gate TFT configurations have been developed which depict the role of defect states on device operation, as well as provide insight and support of a presented hypothesis on DIBL like device behavior associated with back-channel interface trap inhomogeneity. This phenomenon has been named Trap Associated Barrier Lowering (TABL). • A process integration scheme has been developed that includes IGZO back-channel passivation with PECVD SiO2, furnace annealing in O2 at 400 °C, and a thin capping layer of alumina deposited via atomic layer deposition. This process supports device stability when subjected to negative and positive bias stress conditions, and thermal stability up to 140 °C. It also enables TFT operation at short channel lengths (Leff ~ 3 µm) with steep subthreshold characteristics (SS ~ 120 mV/dec). The details of these contributions in the interpretation and regulation of electronic defect states in IGZO TFTs is presented, along with the support of device characteristics that are among the best reported in the literature. Additional material on a complementary technology which utilizes flash-lamp annealing of amorphous silicon will also be described. Flash-Lamp Annealed Polycrystalline Silicon (FLAPS) has realized n-channel and p-channel TFTs with promising results, and may provide an option for future applications with the highest performance demands. IGZO is rapidly emerging as the candidate to replace a-Si:H and address the performance needs of display products produced by large panel manufacturing

Indium-Gallium-Zinc Oxide Thin-Film Transistors for Active-Matrix Flat-Panel Displays

Amorphous oxide semiconductors (AOSs) including amorphous InGaZnO (a-IGZO) areexpected to be used as the thin-film semiconducting materials for TFTs in the next-generation ultra-high definition (UHD) active-matrix flat-panel displays (AM-FPDs). a-IGZO TFTs satisfy almost all the requirements for organic light-emitting-diode displays (OLEDs), large and fast liquid crystal displays (LCDs) as well as three-dimensional (3D) displays, which cannot be satisfied using conventional amorphous silicon (a-Si) or polysilicon (poly-Si) TFTs. In particular, a-IGZO TFTs satisfy two significant requirements of the backplane technology: high field-effect mobility and large-area uniformity.In this work, a robust process for fabrication of bottom-gate and top-gate a-IGZO TFTs is presented. An analytical drain current model for a-IGZO TFTs is proposed and its validation is demonstrated through experimental results. The instability mechanisms in a-IGZO TFTs under high current stress is investigated through low-frequency noise measurements. For the first time, the effect of engineered glass surface on the performance and reliability of bottom-gate a-IGZO TFTs is reported. The effect of source and drain metal contacts on electrical properties of a-IGZO TFTs including their effective channel lengths is studied. In particular, a-IGZO TFTs with Molybdenum versus Titanium source and drain electrodes are investigated. Finally, the potential of aluminum substrates for use in flexible display applications is demonstrated by fabrication of high performance a-IGZO TFTs on aluminum substrates and investigation of their stability under high current electrical stress as well as tensile and compressive strain

High-performance Zinc Oxide Thin-Film Transistors For Large Area Electronics

The increasing demand for high performance electronics that can be fabricated onto large area substrates employing low manufacturing cost techniques in recent years has fuelled the development of novel semiconductor materials such as organics and metal oxides, with tailored physical characteristics that are absent in their traditional inorganic counterparts such as silicon. Metal oxide semiconductors, in particular, are highly attractive for implementation into thin-film transistors because of their high charge carrier mobility, optical transparency, excellent chemical stability, mechanical stress tolerance and processing versatility. This thesis focuses on the development of high performance transistors based on zinc oxide (ZnO) semiconducting films grown by spray pyrolysis (SP), a low cost and highly scalable method that has never been used before for the manufacturing of oxide-based thin-film transistors. The physical properties of as-grown ZnO films have been studied using a range of techniques. Despite the simplicity of SP, as-fabricated transistors exhibit electrical characteristics comparable to those obtained from ZnO devices produced using highly sophisticated deposition processes. In particular, electron mobility up to 25 cm2/Vs has been achieved in transistors based on pristine ZnO films grown at 400 °C onto Si/SiO2 substrates utilising aluminium source-drain (S-D) electrodes. A strong dependence of the saturation mobility on the work function of S-D electrodes and the transistor channel length (L) has been established. Short channel transistors are found to exhibit improved performance as compared to long channel ones. This was attributed to grain boundary effects that tend to dominate charge transport in devices with L < 40 μm. High mobility, low operating voltage (<1.5 V) ZnO transistors have also been developed and characterised. This was achieved through the combination of SP, for the deposition of ZnO, and thermally stable solution-processed self-assembling monolayer gate dielectrics. Detailed study of the temperature dependence of the operating characteristics of ZnO transistors revealed a thermally activated electron transport process that was described by invoking the multiple trapping and release model. Importantly, ZnO transistors fabricated by SP are found to exhibit highly stable operating characteristics with a shelf lifetime of several months. The simple SPbased fabrication paradigm demonstrated in this thesis expands the possibilities for the development of advanced simple as well as multi-component oxide semiconductors far beyond those accessible by traditional deposition methods such as sputtering. Furthermore, it offers unprecedented processing scalability hence making it attractive for the manufacturing of future ubiquitous oxide electronics

Recent Advances in Thin Film Electronic Devices

This reprint is a collection of the papers from the Special Issue “Recent Advances in Thin Film Electronic Devices” in Micromachines. In this reprrint, 1 editorial and 11 original papers about recent advances in the research and development of thin film electronic devices are included. Specifically, three research fields are covered: device fundamentals (5 papers), fabrication processes (5 papers), and testing methods (1 paper). The experimental data, simulation results, and theoretical analysis presented in this reprint should benefit those researchers in flat panel displays, flat panel sensors, energy devices, memories, and so on

Development of oxide semiconductors : materials, devices, and integration

The aim of this dissertation is to develop oxide semiconductors by radio-frequency sputtering for thin-film transistor (TFT) applications. A variety of oxide semiconductors are used as the TFT channel layer, including indium gallium oxide (IGO), zinc tin oxide (ZTO), and indium gallium zinc oxide (IGZO). The variety of materials used underscores the abundance of materials options available within this nascent technology, with each material exhibiting unique chemical, mechanical, and electrical properties. The influence of several deposition parameters is explored; oxygen partial pressure of the deposition ambient is found to have a profound effect on the electrical characteristics of each material. With optimized deposition conditions, TFTs based on these materials exhibit excellent electrical properties, even when annealed at low-temperature (175 °C). Specifically, ZTO-based TFTs which are subjected to a 175 °C post-deposition anneal exhibit a channel mobility near 9 cm²V⁻¹s⁻¹. However, advancement of this technology also requires research in integration-related issues. Therefore, the effect of channel layer passivation and of TFT stability is evaluated. Passivation of the oxide semiconductor surface is required for circuits which employ multiple levels of interconnect and for mechanical/chemical protection of devices. Here, successful passivation of IGO, ZTO, and IGZO-based TFTs is demonstrated using SU-8, a negative tone epoxy-based photoresist. To appraise TFT stability, a constant voltage bias stress test of 1000 minutes is utilized, where the drain current, ID, is monitored throughout the duration of testing and the turn-on voltage, Von, is evaluated before and after stressing. TFT stability is found to be correlated to the turn-on voltage of a device and to the thickness of the semiconductor layer. IGZO-based TFTs with excellent stability are demonstrated, exhibiting almost no decrease in ID or any shift in Von throughout the duration of bias stress testing

Investigation of Hysteresis, Off-Current, and Instability in In-Ga-Zn Oxide Thin Film Transistors Under UV Light Irradiation

학위논문 (박사)-- 서울대학교 대학원 : 전기·컴퓨터공학부, 2013. 8. 한민구.Amorphous oxide-based thin film transistors (TFTs), for instance, amorphous indium gallium zinc oxide (IGZO) TFTs, are expected to meet emerging technological demands where conventional silicon-based TFTs confront with the limitation of the electrical performance such as field-effect mobility, uniformity, and process temperature. However, the variation of characteristics and the stability in IGZO TFTs under light illumination still needs to be verified for further application. In this thesis, the characteristics and reliability of IGZO TFTs under light illumination were investigated. Furthermore, the effect of mechanical bending on flexible IGZO TFTs was analyzed for flexible displays. First, the effects of light on initial characteristics of IGZO TFTs were studied. Under illuminated condition, significant hysteresis and off-current (Ioff) were observed due to the creation of donor-like interface states near conduction band energy level arising from ionized oxygen vacancy (Vo2+). From hysteresis, the response time (~10^0 s) of Vo2+ at the interface was obtained, which is important parameter for analyzing hysteresis. On the contrary to conventional mechanism of photo-current, the change in Ioff increased with increasing light intensity. The increase of Ioff occurs because Vo2+ at the interface prevents carrier depletion with Fermi-level pinning. Second, the reliability of IGZO TFTs under the conditions combined with negative gate bias stress and light illumination were investigated. Under illumination, negative shift of threshold voltage (Vth) is accelerated by the photo-induced holes and Vo2+. In TFTs featuring passivation layer, a long characteristic time (~10^2 s) for Vo2+ generation in IGZO bulk was extracted. It was also found that the charge trapping probability of single carrier did not change. Finally, the reliability of flexible IGZO TFTs was analyzed when the bending radius was 10 mm, 4 mm, and 2 mm. The device characteristics were hardly changed under mechanical strain unless the gate bias stress was applied. However, Vth shift was increased by mechanical strain under the gate bias stress due to valence band energy level shift.Abstract i Contents iv List of Tables vii List of Figures viii Chapter 1 Introduction 1 1.1 Recent flat panel display 1 1.2 Dissertation Organization 8 Chapter 2 Review of IGZO TFTs 9 2.1 Oxide semiconductor for TFT application 10 2.2 Reliability of IGZO TFTs 17 2.3 Passivation layer in IGZO TFTs 24 Chapter 3 Effect of light on initial characteristics of IGZO TFTs 27 3.1 Experiment 29 3.2 Electrical Characteristics of IGZO TFT under light illumination 33 3.3 Conclusion 58 Chapter 4 Effect of UV light on reliability of IGZO TFTs 61 4.1 Reliability of IGZO TFTs depending on gate insulator layer 63 4.2 IGZO TFT with SiO2 gate insulator layer 67 4.3 IGZO TFT with SiNx gate insulator layer 81 4.4 Conclusion 96 Chapter 5 Characteristics of IGZO TFT on Flexible Substrate 99 5.1 Overview of flexible TFT 100 5.2 Fabrication and Experiment of Flexible IGZO TFT 107 5.3 The effect of mechanical bending on electrical characteristics of Flexible IGZO TFT 112 5.4 The effect of mechanical bending on stability of Flexible IGZO TFT 119 5.5 The effect of light on flexible IGZO TFTs 131 5.6 Conclusion 136 Chapter 6 Summary 139 Appendix A Design and Fabrication of Simultaneous Emission AMOLED Pixel Circuit 143 Bibliography 165 초 록 177Docto

Miniaturized Transistors

What is the future of CMOS? Sustaining increased transistor densities along the path of Moore's Law has become increasingly challenging with limited power budgets, interconnect bandwidths, and fabrication capabilities. In the last decade alone, transistors have undergone significant design makeovers; from planar transistors of ten years ago, technological advancements have accelerated to today's FinFETs, which hardly resemble their bulky ancestors. FinFETs could potentially take us to the 5-nm node, but what comes after it? From gate-all-around devices to single electron transistors and two-dimensional semiconductors, a torrent of research is being carried out in order to design the next transistor generation, engineer the optimal materials, improve the fabrication technology, and properly model future devices. We invite insight from investigators and scientists in the field to showcase their work in this Special Issue with research papers, short communications, and review articles that focus on trends in micro- and nanotechnology from fundamental research to applications

Solid State Circuits Technologies

The evolution of solid-state circuit technology has a long history within a relatively short period of time. This technology has lead to the modern information society that connects us and tools, a large market, and many types of products and applications. The solid-state circuit technology continuously evolves via breakthroughs and improvements every year. This book is devoted to review and present novel approaches for some of the main issues involved in this exciting and vigorous technology. The book is composed of 22 chapters, written by authors coming from 30 different institutions located in 12 different countries throughout the Americas, Asia and Europe. Thus, reflecting the wide international contribution to the book. The broad range of subjects presented in the book offers a general overview of the main issues in modern solid-state circuit technology. Furthermore, the book offers an in depth analysis on specific subjects for specialists. We believe the book is of great scientific and educational value for many readers. I am profoundly indebted to the support provided by all of those involved in the work. First and foremost I would like to acknowledge and thank the authors who worked hard and generously agreed to share their results and knowledge. Second I would like to express my gratitude to the Intech team that invited me to edit the book and give me their full support and a fruitful experience while working together to combine this book