Passivation of Amorphous Indium-Gallium-Zinc Oxide (IGZO) Thin-Film Transistors

Thin-film transistors (TFTs) with channel materials made out of hydrogenated amorphous silicon (a-Si:H) and polycrystalline silicon (poly-Si) have been extensively investigated. Amorphous silicon continues to dominate the large-format display technology; however newer technologies demand a higher performance TFT which a-Si:H cannot deliver due to its low electron mobility, µn ~ 1 cm2/V*s. Metal-oxide materials such as Indium-Gallium-Zinc Oxide (IGZO) have demonstrated semiconductor properties, and are candidates to replace a Si:H for TFT backplane technologies. This work involves the fabrication and characterization of TFTs utilizing a-IGZO deposited by RF sputtering. An overview of the process details and results from recently fabricated IGZO TFTs following designed experiments are presented, followed by analysis of electrical results. The investigated process variables were the thickness of the IGZO channel material, passivation layer material, and annealing conditions. The use of electron-beam deposited Aluminum oxide (alumina or Al2O3) as back-channel passivation material resulted in improved device stability; however ID VG transfer characteristics revealed the influence of back-channel interface traps. Results indicate that an interaction effect between the annealing condition (time/temperature) and the IGZO thickness on the electrical behavior of alumina-passivated devices may be significant. A device model implementing fixed charge and donor-like interface traps that are consistent with oxygen vacancies (OV) resulted in a reasonable match to measured characteristics. Modified annealing conditions have resulted in a reduction of back-channel interface traps, with levels comparable to devices fabricated without the addition of passivation material

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

An amorphous oxide semiconductor thin-film transistor route to oxide electronics

Amorphous oxide semiconductor (AOS) thin-film transistors (TFTs) invented only one decade ago are now being commercialized for active-matrix liquid crystal display (AMLCD) backplane applications. They also appear to be well positioned for other flat-panel display applications such as active-matrix organic light-emitting diode (AMOLED) applications, electrophoretic displays, and transparent displays. The objectives of this contribution are to overview AOS materials design; assess indium gallium zinc oxide (IGZO) TFTs for AMLCD and AMOLED applications; identify several technical topics meriting future scrutiny before they can be confidently relied upon as providing a solid scientific foundation for underpinning AOS TFT technology; and briefly speculate on the future of AOS TFTs for display and non-display applications

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

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

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

Zinc tin oxide thin-film transistor circuits

The primary objective of this thesis is to develop a process for fabricating integrated circuits based on thin-film transistors (TFTs) using zinc tin oxide (ZTO) as the channel layer. ZTO, in contrast to indium- or gallium-based amorphous oxide semiconductors (AOS), is perceived to be a more commercially viable AOS choice due to its low cost and ability to be deposited via DC reactive sputtering. In the absence of an acceptable ZTO wet etch process, a plasma-etching process using Ar/CH₄ is developed for both 1:1 and 2:1 ZTO compositions. An Ar/CH₄ plasma etch process is also designed for indium gallium oxide (IGO), indium gallium zinc oxide (IGZO), and indium tin oxide (ITO). Ar/CH₄ dry etches have excellent selectivity with respect to SiO₂, providing a route for obtaining patterned ZTO channels. A critical asset of ZTO process integration involves removing polymer deposits after ZTO etching without active layer damage. A ZTO process is developed for the fabrication of integrated circuits which use ZTO channel enhancement-mode TFTs. Such ZTO TFTs exhibit incremental and average mobilities of 23 and 18 cm²V⁻¹s⁻¹, respectively, turn-on voltages approximately 0 to 1.5 V and subthreshold swings below 0.5 V/dec when annealed in air at 400 °C for 1 hour. Several types of ZTO TFT circuits are realized for the first time. Despite large parasitic capacitances due to large gate-source and gate-drain overlaps, AC/DC rectifiers are fabricated and found to operate in the MHz range. Thus, they are usable for RFID and other equivalent-speed applications. Finally, a ZTO process for simultaneously fabricating both enhancement-mode and depletion-mode TFTs on a single substrate using a single target and anneal step is developed. This dual-channel process is used to build a high-gain two-transistor enhancement/depletion inverter. At a rail voltage of 10 V, this inverter has a gain of 10.6 V/V, the highest yet reported for an AOS-based inverter. This E/D inverter is an important new functional block which will enable the realization of more complex digital logic circuits

Study on the Zn-Sn-O field effect transistors for the application to transparent and flexible display devices

학위논문 (박사)-- 서울대학교 대학원 : 공과대학 재료공학부, 2018. 2. 김형준.In recent years, the display devices focused on user experience such as curved, flexible and transparent features. Since conventional silicon-based thin film transistors (TFTs) have a limit in flexibility and transparency, it is necessary to develop new materials with these properties. Therefore, amorphous oxide semiconductors (AOSs) such as InGaZnOx (IGZO) and ZnSnOx (ZTO) have attracted attention as a new active-layer materials of the TFTs in display devices due to the need for channel materials that have flexibility and productivity. ZTO is especially taking the spotlight as it has none of expensive rare-earth elements as well as high mobility and superior productivity. ZTO has superior electrical properties, but there are still problems to be solved to apply them to the flexible devices. To fabricate a flexible device, it is necessary to introduce polymer substrates with thermal resistance, such as polyethylene naphthalate (PEN) or poly arylate (PAR), which resist heats up to 200 °C. Whereas conventional process for fabrication of ZTO TFTs need a thermal treatment over 350 °C, which is excessively high than the heat resistance temperature of the polymer substrate. Therefore, it is essential to develop a process which can achieve the transistor properties of oxide semiconductor even at low temperature under the limit of polymer substrate. Also, there is a lack of understanding of the phenomenon that occurs when a flexible display device using the ZTO exposed to external stress. Generally, the external stresses applied to the flexible display devices are the illumination from the back light unit and the mechanical stress. In this dissertation, the studies on the ZTO TFTs for the application to transparent and flexible devices were conducted. First, the experiments carried out to lower the maximum process temperature in order to make the ZTO devices applicable to the polymer substrates which have process temperature limit of 200 °C or less. Conventional ZTO should be annealed at 350 °C or higher after the deposition using an RF sputtering system. This annealing process makes atomic bonds tight and enhances to react the oxygen vacancies with oxygen. Instead of post annealing process, the inter-bonding among atoms and relief of oxygen vacancy occurred through heating the substrate and adding oxygen in the process chamber. As a result, it was possible to fabricate devices that show excellent electrical characteristics with field effect electron mobilities of 5.8 – 27.1 cm2 V–1 s–1, Ion/Ioff of 108, and subthreshold swing of 0.15~1.7 V dec-1, which are similar to those of the devices fabricated by the conventional process, even though lowering the process temperature by more than 150 °C. As a result of measuring the thin film density by X-ray reflectometry, the films deposited at room temperature had the density of 5.5 g cm-3, 5.7 g cm-3 and 5.9 g cm-3 after deposition, post-heat treatment at 200 °C and 350 °C, respectively. The more dense films were formed as the annealing temperature increased. In the case of the in-situ annealing deposition, the density of 5.9 and 6.0 g cm-3 was observed at the 150 ° and 200 ° processes, respectively. It was confirmed that a dense thin film can be formed even at a low temperature through the in-situ annealing deposition. In addition, the surface roughness using an atomic force microscope was also improved with in-situ annealing. The ZTO thin film deposited at room-temperature after post-annealing at 150 °C had a root mean square roughness of 1.81nm, whereas the thin films after in-situ annealing had the roughness of 0.33 ~ 0.43nm, which are improved more than four times better than that of the conventional post annealing method. Transmission electron microscopy analysis showed the effects of in-situ annealing process indisputably. In the samples deposited at room temperature, a complete amorphous phase was observed even with the post deposition annealing at 350 °C. These results inform that the ZTO clusters deposited on the heated substrate have a sufficient kinetic energy due to high temperature and be movable filling the physical vacancies and forming the crystalline phase which stabilize the electrical properties of ZTO TFTs . Therefore, it has been confirmed that the density increase and the improvement of the roughness occurs together. These results show that the ZTO clusters deposited on the heated substrate have sufficient kinetic energy due to thermal energy and move to fill the physical pores, and thus the density increases and the roughness decreases. The ultimate goal of lowering the process temperature was to fabricate the ZTO TFT devices on polymer substrate which have transparency and flexibility. ZTO-TFTs possessing transparency and flexibility were successfully fabricated on the 125 μm thick PEN substrates. They showed electrical properties with a mobility of 9.7 cm2 V–1 s–1, Ion/Ioff of 108, and subthreshold swing of 0.28 V dec-1. The devices on the polymer substrates performed good enough to be applied to commercial electronic devices. Secondly, the effects of external stress on the properties of flexible ZTO TFTs were studied. As mentioned above, ZTO TFTs are widely used in display devices based on the advantages such as transparency and flexibility. When applied to a flexible display device, the ZTO TFTs are exposed to the illumination stress from the back light unit and the mechanical stress caused by the bending. Therefore, it is very important to analyze the reliability of the devices under such stress conditions. The device was fixed in convex and concave jigs with a radius of curvature of 20 mm, and performed reliability analysis by irradiating the light of 450 and 500 nm wavelength under the condition of mechanical stress. As a result, when light was illuminated on the convex jig, the threshold voltage of the TFT moved more toward lower voltage with the tensile stress. Tensile stress tended to deteriorate the reliability of the ZTO TFT devices. On the contrary, when the compressive stress was applied to the devices by fixing on the concave jig, the threshold voltage shift was lowered in the stress state. To analyze this phenomenon, the way for applying different stress states to the equally deposited ZTO thin films on a transparent PEN substrate was studied, and a method of independently differentiating stress without changing the chemical composition was suggested. The optical band gap measurement using UV-vis spectrometer and elemental analysis using ultra-violet photoelectron spectrometer were performed to analyze the valence band maximum (VBM) and work function. As a result, when the compressive stress was applied up to 0.6 %, the bandgap and VBM increased. It is found that the energy differences between conduction band minimum and fermi level (EF) decrease with increasing compression. And the reliability of the devices have correlation with the applied mechanical stress which means that it can be possible to enhance the photo-bias stability by adopting appropriate mechanical stress.Chapter 1. Introduction 1 1.1 Overview 1 Chapter 2. Literature Review 9 2.1 Oxide semiconductor thin-film transistors 9 2.1.1 Device structure 9 2.1.2 Operation of thin-film transistors 12 2.1.3 Electrical properties of thin film transistors 14 2.2 Electrical instability of the oxide TFTs 18 2.2.1 Overview 18 2.2.2 Charge trapping model 19 2.2.3 Carrier generation models 20 Chapter 3. Experiments and Analyses 32 3.1 Sputter deposition of oxide films 32 3.2 Deposition of ZTO and ITO films 32 3.3 Thin film transistor fabrication 36 3.4 Analysis Methods 39 Chapter 4. Low-temperature fabrication of amorphous zinc-tin-oxide thin film transistors with in-situ annealing process 42 4.1 Introduction 42 4.2 Experimental 46 4.3 Results & Discussion 47 4.3.1 Effects of post annealing temperature in traditional thermal annealing process 47 4.3.2 Effects of in-situ annealing in ZTO-TFTs 51 4.3.3 Fabrication of transparent flexible TFTs on PEN substrates . 66 4.4 Conclusions 70 Chapter 5. Reliability analysis of Zinc-tin-oxide thin film transistor under mechanical stress and negative biased illuminated stress condition 71 5.1 Introduction 71 5.2 Experiment 74 5.2.1 Flexible ZTO TFTs 74 5.2.2 Negative biased illumination stress (NBIS) conditions for reliability test of ZTO TFTs 74 5.2.3 Analyzing the band structure of ZTO layer and the effects of the mechanical stress 77 5.3 Results & Discussion 79 5.3.1 Effects of mechanical stress in electrical properties of ZTO-TFTs 79 5.3.2 Effects of mechanical stress on the ZTO layer 81 5.4. Conclusions 89 Chapter 6. Conclusions 90 References 93 Curriculum Vitae 100 List of publication 103 Abstract (in Korean) 106Docto

Flexible oxide thin film transistors: fabrication and photoresponse

Gli ossidi amorfi semiconduttori (AOS) sono nuovi candidati per l’elettronica flessibile e su grandi aree: grazie ai loro legami prevalentemente ionici hanno una mobilità relativamente alta (µ > 10cm^2/Vs) anche nella fase amorfa. Transistor a film sottile (TFT) basati sugli AOS saranno quindi più performanti di tecnologie a base di a-Si e più economici di quelle a base di silicio policristallino. Essendo amorfi, possono essere depositati a basse temperature e su substrati polimerici, caratteristica chiave per l’elettronica flessibile e su grandi aree. Per questa tesi, diversi TFT sono stati fabbricati e caratterizzati nei laboratori del CENIMAT all’Università Nova di Lisbona sotto la supervisione del Prof. P. Barquinha. Questi dispositivi sono composti di contatti in molibdeno, un canale semiconduttivo di ossido di zinco, gallio e indio (IGZO) e un dielettrico composto da 7 strati alternati di SiO2 e SiO2+Ta2O5. Tutti i dispositivi sono stati depositati mediante sputtering su sostrati flessibili (fogli di PEN). Le misure tensione-corrente mostrano che i dispositivi mantengono alte mobilità (decine di 10cm^2/Vs) anche quando fabbricati a temperature inferiori a 200°C. Si è analizzato il funzionamento dei dispositivi come fototransistor rilevando la risposta alla luce ultravioletta e in particolare la loro responsività e spostamento della tensione di soglia in funzione della lunghezza d’onda incidente. Questi risultati consentono di formulare ipotesi sul comportamento dei dispositivi alla scala microscopica. In particolare, indicano che i) la mobilità del canale non è influenzata dall’illuminazione, ii) sia l'IGZO sia il Ta2O5 contribuiscono al processo di fotoconduttività e iii) il processo di fotogenerazione non è adiabatico. La tesi contiene inoltre una descrizione del processo di ricombinazione e presenta un’applicazione pratica di tali dispositivi in un circuito per RFID. Infine, esplora la possibilità di migliorarne la flessibilità e le prestazioni