Device-circuit interactions and impact on TFT circuit-system design

This paper reviews the importance of device-circuit interactions (DCI) and its consideration when designing thin film transistor circuits and systems. We examine temperature- and process-induced variations and propose a way to evaluate the maximum achievable intrinsic performance of the TFT. This is aimed at determining when DCI becomes crucial for a specific application. Compensation methods are then reviewed to show examples of how DCI is considered in the design of AMOLED displays. Other designs such as analog front-end and image sensors are also discussed, where alternate circuits should be designed to overcome the limitations of the intrinsic device properties

Amorphous In-Ga-Zn-O Thin-Film Transistors for Next Generation Ultra-High Definition Active-Matrix Liquid Crystal Displays.

Next generation ultra-high definition (UHD) active-matrix flat-panel displays have resolutions of 3840x2160 (4K) or 7680x4320 (8K) pixels shown at 120 Hz. The UHD display is expected to bring about immersive viewing experiences and perceived realness. The amorphous In-Ga-Zn-O (a-IGZO) thin-film transistor (TFT) is a prime candidate to be the backplane technology for UHD active-matrix liquid crystal displays (AM-LCDs) because it simultaneously fulfills two critical requirements: (i) sufficiently high field-effect mobility and (ii) uniform deposition in the amorphous phase over a large area. We have developed a robust a-IGZO density of states (DOS) model based on a combination of experimental results and information available in the literature. The impact of oxygen partial pressure during a-IGZO deposition on TFT electrical properties/instability is studied. Photoluminescence (PL) spectra are measured for a IGZO thin films of different processing conditions to identify the most likely electron-hole recombination. For the first time, we report the PL spectra measured within the a IGZO TFT channel region, and differences before/after bias-temperature stress (BTS) are compared. To evaluate the reliability of a-IGZO TFTs for UHD AM-LCD backplane, we have studied its ac BTS instability using a comprehensive set of conditions including unipolar/bipolar pulses, frequency, duty cycle, and drain biases. The TFT dynamic response, including charging characteristics and feedthrough voltage, are studied within the context of 4K and 8K UHD AM-LCD and are compared with hydrogenated amorphous silicon technology. We show that the a-IGZO TFT is fully capable of supporting 8K UHD at 480 Hz. In addition, it is feasible to reduce a-IGZO TFT feedthrough voltage by controlling for non-abrupt TFT switch-off.PhDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111526/1/ekyu_1.pd

Amorphous Transition-Metal-Oxides for Transparent Flexible Displays: Device Fabrication and Characterization

This Ph.D. dissertation presents the development and demonstration of optically transparent back-channel etched flexible InGaZnO (IGZO) thin-film transistors (TFTs) using a conventional TFT process flow implemented at low-temperatures. The study includes the development of the transition metal oxide (TMO) channel layers by relating the materials properties, surface topography, and chemical composition of the channel layer to the process integration of IGZO TFTs. Investigation of the process parameters included process temperature and post processing thermal-anneal on the electronic properties of the semiconductor and the effect of the chemical composition of the gate-dielectric layer on the active channel layer of TMO TFTs. A bi-layer SiOx/SiNx gate dielectric was found to be an effective structure for high-performance IGZO-based TFTs compared to single dielectric layers. The SiOx capping layer within the dual dielectric structure was found to be an effective hydrogen (H) diffusion barrier, preventing H contamination into the overlying semiconducting IGZO layer during the IGZO deposition, minimizing the generation of H induce oxygen vacancy (Vo) formation in the active IGZO channel. A low-temperature, 150˚C plasma-enhanced chemical vapor deposition (PECVD) process was used to produce TFTs having field-effect mobility, μ, of 5.7 cm2/V.sec, sub-threshold swing, S.S., of 0.54 V/decade, and Ion/off > 106. The same dual-dielectric stack was also found to be an effective passivation layer for back-channel etched IGZO TFTs. A low-temperature approach employing a thin room-temperature-deposited e-beam SiOx barrier layer directly deposited onto the IGZO back-channel to prevent both the plasma damage and unintentional hydrogen (H) doping of the IGZO channel region. In order to complete the process integration for fully transparent flexible TFTs, the development of the dielectric layers of the TFT structure were augmented by an investigation of ohmic transparent contacts patterned using selective wet-chemical etching. A high-selectivity wet-etch patterning process was developed to take advantage of the etch-rate differences between polycrystalline Al-doped ZnO (AZO) and amorphous (IGZO) TMO thin-films. This patterning technique resulted in the fabrication of back-channel etched flexible transparent IGZO TFTs using a conventional TFT process flow implemented at low-temperatures. A selectivity of nearly 20 was found for dilute HCl solution in water for patterning AZO source/drain electrodes on IGZO channel layers. The resulting patterned electrodes had a low contact resistance of < 19 KΩ and high optical transparency of ~85%. The transparent back-channel etched flexible IGZO TFTs exhibited a μ of ~9.3 cm2/V.sec, VT of <5 V, and Ion/off ratio of ~107. Finally, the integration of the transparent semiconductor, dielectric, and conductive electrodes onto a flexible platform was demonstrated. Through a combination of the low-temperature processes developed in this work, the integration of transparent flexible TFTs onto polyethylene naphthalate (PEN) substrates was accomplished. The flexible IGZO TFTs had current-voltage (I-V) characteristics similar to their rigid counterparts. The fully encapsulated transparent devices had μ of ~6.7 cm2/V.sec, VT of ~1 V, and Ion/off ratio of >106. Electrical stability measurements of the flexible devices under tensile and compressive mechanical strain showed no appreciable change in the I-V characteristics during bending. The electrical characteristics under mechanical bending suggest that carrier transport is unaffected during mechanical strain due to the overlapping spherical s-orbitals in the IGZO conduction band. Testing under dc gate bias conditions, the electrical stability of the TFTs showed a positive VT shift of 3.8 V after 3600 s without any change in subthreshold-swing (S.S.). Pulsed-gate recovery measurements also showed rapid recovery of the drain current, both of which suggest that the dominant aging mechanisms is charge trapping in the back-channel etched transparent flexible IGZO TFTs

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

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

Backplane Circuit Design with Amorphous Silicon Thin-Film Transistors for Flexible Displays

In recent years, rapid advancement in LED fabrication has enabled the possibility of using GaN micro-LEDs to be the light media in a display panel. It has superior performance in many aspects when compared with OLED technology, such as high contrast, wide viewing angle, and low power consumption. These advantages have enabled a possibility of using micro-LED technology to realize flexible displays. Currently, OLEDs need high mobility low-temperature-poly-silicon (LTPS) TFTs to be the backplane driving circuit material because lower mobility TFTs are inadequate to drive OLEDs. However, LTPS TFTs have poor uniformity over a large area due to unpredictable grain sizes and require additional fabrication processes which prevent it from being integrated onto a large-area flexible platform. On the other hand, conventional amorphous silicon (a-Si:H) technology used on LCD panels have an edge in terms of uniformity over large-area and low-cost fabrication. Even though the field-effect mobility of a-Si:H TFTs is much less than LTPS technology, it is sufficient to power up micro-LEDs with decent pixel density, which is impossible with OLEDs. However, the nature of amorphous materials gives rise to electrical instability issues. The output current of a-Si:H TFTs gradually decreases over time under electrical stress, which results in dimmer micro-LEDs in pixels. Moreover, the lack of complementary p-type TFTs in a-Si:H limits the integration of driver and control circuits onto the flexible platform to realize a full "system-on-flex". To overcome such shortcomings of a-Si:H technologies, this thesis makes a contribution in providing a solution to compensate the output current degradation by a novel pixel circuit with simple control scheme, as well as bootstrapped logic circuits that can be used as row driver and control circuits on flexible substrates. The proposed compensation pixel and row driver circuits can be combined to facilitate the realization of a "system-on-flex" backplane for a display panel with a-Si:H and micro-LED technologies

The Characteristics and Reliability of In-Ga-Zn-O Thin-Film Transistors on Glass and Flexible Polyimide Substrate under Temperature and Illumination Stress

학위논문 (박사)-- 서울대학교 대학원 : 전기컴퓨터공학부, 2013. 2. 한민구.Recently, flexible displays have attracted considerable attention in the emerging electronic device market. Flexible plastic substrates have the advantages such as flexibility, ruggedness and light-weight and its low cost, compared to glass substrate. Indium-Gallium-Zinc-Oxide thin-film transistors (IGZO TFTs) are promising candidates for next generation display backplane due to high mobility, good uniformity, and low process temperature, which suitable for flexible display. In this thesis, the characteristics and reliability of flexible IGZO TFTs were presented and discussed. Firstly, the electrical characteristics and reliability of IGZO TFTs on glass substrate are discussed. The IGZO TFTs were fabricated on a glass substrate with an inverted staggered structure. The initial electrical characteristics and gate bias induced instability was investigated. And drain bias induced instability is investigated. Unique degradation phenomenon was observed under the high drain bias stress. After the high drain bias stress, the drain current, measured at the low drain bias, was significantly decreased. Based on the experimental results, I proposed a degradation model for the high drain bias induced degradation. And light-induced hysteresis of IGZO TFTs is investigated. Hysteresis was observed under the 450-nm illumination, and was increased with temperature. And hysteresis was increased with wavelength decrease. Light-induced hysteresis occurs due to increased sub-band gap states at the interface between the gate insulator layer and the active layer. Also, bias illumination stress induced instability is investigated. The transfer curve did not change after positive bias illumination stress. However, the transfer curve shifted to a negative direction after negative bias illumination stress. The transfer curve could be shifted to the negative direction after negative bias illumination stress due to the increase of VO2+ states. Secondly, the electrical characteristics and reliability of IGZO TFTs on flexible substrate are discussed. The IGZO TFTs were fabricated on a polyimide (PI) substrate with an inverted staggered structure. An inorganic buffer layer, composed of SiO2 and SiNx multi-layer, was employed, in order to prevent the environmental stress, such as water or oxygen molecules. The effects of PI and inorganic buffer layer on the characteristics and reliability of IGZO TFTs were investigated. And the effects of passivation layer on the electrical stability of IGZO TFTs with single passivation layer and double passivation layer fabricated on PI substrate were investigated. The positive bias stress and negative bias stress were applied to the IGZO TFTs at various temperatures from 20 oC to 80 oC. The threshold voltage shift of double passivation device was larger than that of single passivation device under NBTS. The threshold voltage shift of double passivation device was slightly less than that of single passivation device under PBTS. The threshold voltage shift of NBTS is considerably increased than that of PBTS at high temperature due to the difference between conduction band offset and valence band offset. Lastly, the effects of mechanical bending on the electrical stability of flexible IGZO TFTs were investigated.Abstract i Contents iv List of Tables vii List of Figures viii Chapter 1 Introduction 1 1.1 Evolution of display technology 2 1.2 Outline of this thesis 10 Chapter 2 Review of IGZO TFTs and flexible display technology 11 2.1 Recent issues of IGZO TFTs 12 2.1.1 Reliability under bias temperature stress 14 2.1.2 Reliability under negative bias illumination stress 18 2.1.3 Reliability under various environments 24 2.2 Various backplane materials for flexible display 30 Chapter 3 The electrical characteristics and reliability of IGZO TFTs on glass substrate 33 3.1 Overview 34 3.2 Fabrication process of IGZO TFTs on glass substrate 36 3.3 Electrical characteristics of IGZO TFTs 39 3.4 Gate bias induced instability without illumination 42 3.5 Drain bias induced instability without illumination 46 3.5.1 Introduction 46 3.5.2 Experimental methods 48 3.5.3 Experimental results and discussions 49 3.5.4 Conclusion 63 3.6 Light-Induced Hysteresis of IGZO TFTs with Various Wavelengths 64 3.6.1 Introduction 64 3.6.2 Experimental methods 65 3.6.3 Experimental results and discussions 66 3.6.4 Conclusion 75 3.7 Light-Induced Hysteresis of IGZO TFTs with Various Temperatures 76 3.7.1 Introduction 76 3.7.2 Experimental methods 78 3.7.3 Experimental results and discussions 79 3.7.4 Conclusion 89 3.8 Bias illumination stress induced instability 90 Chapter 4 The electrical characteristics and reliability of IGZO TFTs on flexible substrate 99 4.1 Overview 100 4.2 Fabrication process of IGZO TFTs on polyimide substrate 102 4.3 Comparison between IGZO TFTs on glass substrate and flexible substrate 105 4.4 Effects of the buffer layer on the electrical characteristics of flexible IGZO TFTs 109 4.5 Effects of passivation layer on the electrical stability of flexible IGZO TFTs 115 4.5.1 Introduction 115 4.5.2 Experimental methods 117 4.5.3 Experimental results and discussions 119 4.5.4 Conclusion 127 4.6 Effects of humidity on the electrical characteristics of IGZO TFTs 128 4.7 Effects of mechanical bending on the electrical stability of flexible IGZO TFTs 139 Chapter 5 Summary 154 Bibliography 156 초 록 175Docto

Interpretation and Physical Modeling of Electronic Transport and Defect States in IGZO Thin-Film Transistors

This work is a comprehensive study on the interpretation and modeling of electronic transport behavior and defect states in indium-gallium-zinc-oxide (IGZO) TFTs. Key studies have focused on advancing the state of IGZO TFTs by addressing several challenges in device stability, scaling, and device modeling. These studies have provided new insight on the associated mechanisms and have resulted in the realization of scaled thin-film transistors that exhibit excellent electrical performance and stability. This work has demonstrated the ability to scale the conventional inverted staggered IGZO TFT down to one micron channel length, with excellent on-state and off-state performance where the VT ≈1 V, µeff =12 cm2/Vs, Ileak ≤ 10-12 A/µm and SS ≈ 160 mV/dec. The working source/drain electrodes are direct metal contact regions to the IGZO, which requires several microns of gate overlap to provide ohmic behavior with minimal series resistance and ensure tolerance to overlay error. New results utilizing ion implantation for self-aligned source/drain regions present a path towards submicron channel length. This strategy offers a reduction in channel length as well as parasitic capacitance, which translates to improvement in RC delay and associated voltage losses due to charge-sharing. The realization of self-aligned TFTs using boron ion implantation for selective activation was introduced in a first-time report of boron-doped IGZO. Cryogenic measurements made on long-channel devices has revealed temperature-dependent behavior that is not explained by existing TCAD models employed for defect states and carrier mobility. A completely new device model using Silvaco Atlas has been established which properly accounts for the role of donor-like oxygen vacancy defects, acceptor-like band-tail states, acceptor-like interface traps, and a temperature-dependent intrinsic channel mobility. The developed model demonstrates a remarkable match to transfer characteristics measured at T = 150 K to room temperature. A power-law fit for the µch = f(T) relationship, which resembles 〖μ ~ T〗^((+3)⁄2) behavior consistent with ionized defect scattering. The mobility model is expressly independent of carrier concentration, without dependence on the applied gate bias. The device model is consistent with a compact model developed for circuit simulation (SPICE) that has been recently refined to include on-state and off-state operation. While IGZO is the only AOS technology mature enough for commercialization, the effective electron channel mobility µeff ~ 10 cm2/Vs presents a performance limitation. Other candidate AOS materials which have higher reported channel mobility values have also been investigated; specifically, indium-tungsten-oxide (IWO) and indium-gallium-tin-oxide (ITGO). These investigations serve as preliminary studies; device characteristics support the claims of high channel mobility; however the influence of defect states clearly indicates the need for further process development. The advancements realized in IGZO TFTs in this work will serve as a foundation for these alternative AOS materials

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

O2 플라즈마, 자외선 조사, Biased-H2O 어닐링을 통한 저온 용액공정을 이용한 산화물 박막트랜지스터의 특성 향상에 대한 연구

학위논문 (박사)-- 서울대학교 대학원 : 전기·컴퓨터공학부, 2013. 8. 한민구.Zinc tin oxide (ZTO)나 indium gallium zinc oxide (IGZO)를 이용한 용액공정형 산화물 박막 트랜지스터는 고이동도, 빛 투명성, 플렉서블 적합성, 물질의 광범위함, 분자 구성비에 의한 전기적 특성 조절 용이성 등의 장점 때문에 실리콘 기반의 박막 트랜지스터와 유기물 박막 트랜지스터를 대체하며 능동 매트릭스형 디스플레이의 구동 소자로서 상당한 주목을 받고 있다. 용액공정형 산화물 박막 트랜지스터는 능동 매트릭스형 액정표시장치와 능동 매트릭스형 유기발광다이오드 디스플레이 백플레인으로서 많은 문제점을 가지고 있는 실리콘 기반의 박막 트랜지스터와 유기물 박막 트랜지스터와 비교하여 우수한 특성을 보여주고 있다. 더욱이, 용액공정형 산화물 박막 트랜지스터는 우수한 균일성과 고처리량 덕분에 대면적 공정에 적합하다. ZnO 기반의 산화물 반도체 중에서, Sn 물질을 이용한 ZTO 박막 트랜지스터는 Sn이 널리 사용되고 있는 In 보다 상당히 저가의 물질이기 때문에 저가 공정을 확립하는데 유망한 소자이다. 추가적인 가격 절감과 플렉서블 디스플레이로의 응용성 확장을 위해서 용액공정형 ZTO 박막 트랜지스터는 저렴하고 플렉서블한 기판에 제작되어야 한다. 플렉서블한 기판은 고온에서 쉽게 손상되기 때문에 플렉서블한 기판에 용액공정형 ZTO 박막 트랜지스터가 제작되기 위해서는 저온 공정이 요구된다. 그러나, 저온에서 제작된 용액공정형 ZTO 박막 트랜지스터는 낮은 on-currnet, 높은 문턱 접압, 낮은 이동도 등의 열등한 특성을 가지므로, 우수한 특성의 용액공정형 ZTO 박막 트랜지스터를 제작하기 위해서는 500도 이상의 고온 공정이 필요하다. 저온에서 제작된 용액공정형 산화물 박막 트랜지스터의 소자 특성을 향상시키기 위해서는, 용액공정형 산화물 박막 트랜지스터에 대한 어닐링 온도의 영향성과 더불어 저온 공정에서 제작되더라도 우수한 소자 특성을 가지도록 하는 연구가 요구된다. 기존에 용액공정형 산화물 박막 트랜지스터에 대한 어닐링 온도의 영향상을 분석하려는 시도가 있었지만, 용액공정형 산화물 박막 트랜지스터에 대한 어닐링 온도 영향성의 전기적, 화학적 메커니즘은 거의 연구되지 않았다. 이 논문의 목적은 용액공정을 이용하여 다양한 어닐링 온도에서 산화물 박막 트랜지스터를 제작하여 문턱 전압, 포화 이동도, 신뢰성 등의 용액공정형 산화물 박막 트랜지스터의 전기적 특성에 대한 어닐링 온도의 영향성을 분석하고, 제안된 O2 플라즈마, 자외선 조사, Biased-H2O 어닐링 등의 방법을 통하여 능동 매트릭스형 디스플레이를 위한 저온 용액공정 산화물 박막 트랜지스터의 전기적 특성을 향상시키는 것이다.Solution-processed oxide thin film transistors (TFTs) with zinc-tin-oxide (ZTO) and indium-gallium-zinc-oxide (IGZO) have attracted considerable attention for the driving elements of active matrix display, instead of Si-based TFTs and organic TFTs, because of high mobility, visible light transparency, flexibility, wide range of materials, and controllability of electrical properties by atomic composition. Solution-processed oxide TFTs show superior performance for active matrix liquid crystal display (AMLCD) and active matrix organic light emitting diode (AMOLED) display backplanes, compared with solution-processed Si and organic TFTs which have a number of issues. Furthermore, solution-processed oxide TFTs are compatible with large area due to good uniformity and high throughput, so that could be a method for achieving low cost fabrication contrary to vacuum processes. Among various ZnO-based oxide semiconductors, ZTO TFTs employing tin (Sn) material maybe promising candidates for achieving low cost processes because Sn is a quite low cost material compared with widely used indium (In). Solution-processed ZTO TFTs need to be fabricated on inexpensive and flexible substrates such as glass and plastic for additional cost reduction and application extension to a flexible display. For solution-processed ZTO TFTs fabrication with these flexible substrates, low temperature processes are necessary because these substrates are easily damaged at high annealing temperatures. At low annealing temperature, however, solution-processed ZTO TFTs have poor performance such as low on-current, high threshold voltage and low mobility, so a rather high annealing temperature exceeding 500 °C is required in solution-processed ZTO TFTs. To improve the device characteristics of solution-processed oxide TFTs even at low annealing temperature on an active layer, a study of the effects of annealing temperature on the electrical characteristics of solution-processed oxide TFTs and the efforts to achieve high device characteristics of solution-processed oxide TFTs even at low annealing temperature on active layer are desired. There were some efforts to investigate the effects of annealing temperature on solution-processed oxide TFTs, but the electrical and chemical mechanisms of annealing temperature on solution-processed oxide TFTs have been scarcely studied. The purpose of this thesis is to fabricate oxide TFTs employing solution-process for an oxide semiconductor active layer with various annealing temperatures to investigate the effects of annealing temperature on the electrical characteristics of solution-processed oxide TFTs such as threshold voltage, saturation mobility, and reliability, and to improve the electrical characteristics of low temperature solution-processed oxide TFTs for low cost, stable, and flexible active matrix display backplane. The effects of annealing temperature on the bonding structure of ZTO active layer in solution-processed ZTO TFTs were investigated and the chemical formation equation of the ZTO active layer with regard to the annealing temperature was established. To improve the electrical characteristics of low temperature solution-processed oxide TFTs according to the investigation of effects of annealing temperature in regard of the chemical formation of ZTO active layer, O2 plasma treatment, UV radiation treatment, and the biased-H2O annealing were proposed to achieve high device characteristics of solution-processed oxide TFTs even at low annealing temperature. Moreover, the effects on electrical and chemical characteristics of solution-processed oxide TFTs with proposed methods were investigated in detail. These proposed methods to improve the electrical characteristics of low temperature solution-processed oxide TFTs would be suitable for the low cost, stable, and flexible active matrix display backplane.Abstract i Contents iv List of Tables vii List of Figures ix Chapter 1 Introduction 1 1.1 Recent flat panel display technology 2 1.2 Device parameter extraction 12 1.3 Dissertation organization 14 Chapter 2 Review of solution-processed oxide TFTs 16 2.1 Overview of oxide TFTs 17 2.2 Advantages of solution-process 25 2.3 Solution-processed oxide TFTs 30 Chapter 3 Optimization of the fabrication process of solution-processed oxide TFTs 36 3.1 Overview 37 3.2 Structure of solution-processed oxide TFTs 38 3.3 Stirring time on solution-processed oxide TFTs 47 3.4 Active layer thickness on solution-processed oxide TFTs 56 3.5 Effects of passivation on solution-processed oxide TFTs 60 3.6 Electrical characteristics of solution-processed oxide TFTs 63 3.6.1 Transfer characteristics 63 3.6.2 Reliability characteristics 68 Chapter 4 Effects of Annealing Temperature on Solution-processed oxide TFTs 75 4.1 Motivation 76 4.2 Fabrication of solution-processed ZTO TFTs with various annealing temperature 78 4.3 Electrical characteristics with the increase in annealing temperature 80 4.4 Dechlorination on threshold voltage with the increase in annealing temperature 83 4.5 Dechlorination and crystallization on saturation mobility with the increase in annealing temperature 89 4.6 Reliability characteristics with the increase in annealing temperature 93 4.7 Chemical formation equations with the increase in annealing temperature 95 4.8 Conclusion 99 Chapter 5 Improvement of low temperature solution-processed oxide TFTs 100 5.1 Improvement of low temperature solution-processed oxide TFTs employing O2 plasma treatment 101 5.1.1 Motivation 101 5.1.2 Fabrication of solution-processed ZTO TFTs employing O2 plasma treatment 104 5.1.3 Electrical characteristics with O2 plasma treatment 108 5.1.4 Preferential dissociation of Cl on threshold voltage by O2 plasma treatment 111 5.1.5 Increase of electron concentration on saturation mobility by O2 plasma treatment 116 5.1.6 Reliability characteristics with O2 plasma treatment 119 5.1.7 Conclusion 122 5.2 Improvement of low temperature solution-processed oxide TFTs employing Ultra-Violet radiation treatment 123 5.2.1 Motivation 123 5.2.2 Fabrication of solution-processed ZTO TFTs employing UV radiation treatment 126 5.2.3 Electrical characteristics with UV radiation treatment 130 5.2.4 Effects of UV radiation treatment on oxide active layer semiconductors 133 5.2.5 Generation of hydroxide(-OH) bonding by UV radiation treatment on oxide active layer semiconductors 137 5.2.6 Conclusion 141 5.3 Improvement of low temperature solution-processed oxide TFTs employing biaed-H2O annealing 142 5.3.1 Motivation 142 5.3.2 Effects of various annealing condition 145 5.3.3 Effects of H2O wet annealing according to the annealing temperature 148 5.3.4 Proposed biased-H2O annealing to improve low temperature solution-processed oxide TFTs 154 5.3.5 Conclusion 161 Chapter 6 Summary 162 Bibliography 171 초 록 191Docto