Low-temperature amorphous oxide semiconductors for thin-film transistors and memristors: physical insights and applications

While amorphous oxides semiconductors (AOS), namely InGaZnO (IGZO), have found market application in the display industry, their disruptive properties permit to envisage for more advanced concepts such as System-on-Panel (SoP) in which AOS devices could be used for addressing (and readout) of sensors and displays, for communication, and even for memory as oxide memristors are candidates for the next-generation memories. This work concerns the application of AOS for these applications considering the low thermal budgets (< 180 °C) required for flexible, low cost and alternative substrates. For maintaining low driving voltages, a sputtered multicomponent/multi-layered high-κ dielectric (Ta2O5+SiO2) was developed for low temperature IGZO TFTs which permitted high performance without sacrificing reliability and stability. Devices’ performance under temperature was investigated and the bias and temperature dependent mobility was modelled and included in TCAD simulation. Even for IGZO compositions yielding very high thermal activation, circuit topologies for counteracting both this and the bias stress effect were suggested. Channel length scaling of the devices was investigated, showing that operation for radio frequency identification (RFID) can be achieved without significant performance deterioration from short channel effects, which are attenuated by the high-κ dielectric, as is shown in TCAD simulation. The applicability of these devices in SoP is then exemplified by suggesting a large area flexible radiation sensing system with on-chip clock-generation, sensor matrix addressing and signal read-out, performed by the IGZO TFTs. Application for paper electronics was also shown, in which TCAD simulation was used to investigate on the unconventional floating gate structure. AOS memristors are also presented, with two distinct operation modes that could be envisaged for data storage or for synaptic applications. Employing typical TFT methodologies and materials, these are ease to integrate in oxide SoP architectures

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

Yttrium and Scandium in Solution-processed Oxide Electronic Materials.

Large area electronics are critical for many novel applications such as smart windows, wearable electronics and Internet of Things. Among candidate materials, metal oxides have relatively good performance and stability and can be deposited by low-cost solution processes. This thesis explores the roles of rare-earth elements yttrium and scandium in solution-processed metal oxide thin films including semiconducting scandium- or yttrium-doped ZTO, conducting scandium- or yttrium-doped zinc oxide, and insulating yttrium-scandium oxide. Yttrium and scandium can act as oxygen getters and stabilizers, and the use of higher-order alloys can improve film thermal stability and electrical performance. First, thin film transistors (TFTs) are used to characterize undoped ZTO films as a baseline. The device performance of solution-processed ZTO TFTs depends on ink Zn to Sn ratio and annealing temperature, optimized to be 7:3 and 480⁰C, respectively. The optimized ZTO has a shallow donor energy level of 7meV and a steep exponential subgap band tail with a percolation energy of 3meV. Sputtered Mo forms an excellent ohmic contact to solution-processed ZTO with a width-normalized contact resistance of 8.7Ω•cm and a transfer length of 0.34μm, making the technology suitable for future sub-micron channel length devices. Yttrium enhances performance of ZTO TFTs at low concentrations (3at%). High yttrium concentrations slightly improve TFT negative bias illumination stress stability by reducing oxygen vacancy-related defects. Second, the introduction of scandium or yttrium in solution-processed ZnO decreases the conductivity by three orders of magnitude, which is ascribed to formation of insulating structures along grain boundaries. Scandium or yttrium also make the resistivity of ZnO more thickness-dependent than undoped ZnO after forming gas anneal, by causing surface depletion and grain disruptions in the film. Third, solution-processed (YxSc1-x)2O3 insulating alloys have comparable dielectric performance to vacuum deposited (YxSc1-x)2O3, with high breakdown field > 4MV/cm, low leakage current and low dielectric frequency dispersion. Even after 900°C anneals induce crystallization, the alloys maintain a high breakdown field. The yttrium- and scandium- doped solution-processed oxides developed here form a complete suite of electronic materials suitable for fabrication of future large-area electronic devices.PhDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/133324/1/wbhu_1.pd

On improvements in metal oxide based flexible transistors through systematic evaluation of material properties

Thin-film metal oxide (MOx) semiconductors have opened the way to a new generation of electronics based on their unique properties. With mobilities, mu, of up to 80 cm2V-1s-1, metal oxides do not rival crystalline silicon (mu~1000 cm2V-1s-1) for complex applications. But such oxides do have three unique characteristics driving great interest: their mobilities persist in the amorphous form, contrary to the thousandfold drop seen in silicon; they are transparent; and they can be processed at, or near, room temperature. Most work on MOx semiconductors, in particular indium gallium zinc oxide (IGZO), has focused on display applications, where MOx thin-film transistors (TFTs) are used to drive individual pixels, reducing power consumption by blocking less light than alternatives, and allowing smaller pixels due to reduced TFT sizes. Such work has seen great advances in IGZO, but has generally not considered the thermal budget during production. By utilising the low temperature processing possible with MOx, a new world of applications becomes possible: flexible electronics. This work aims to improve the characteristics of TFTs based on amorphous IGZO (a-IGZO) through detailed study of the thin-film structure in relation to functional performance, looking at the material structure of three critical layers in an a-IGZO TFT. A study of optimisation of a dielectric layer of Al2O3, deposited by atomic layer deposition (ALD), is presented. This dielectric, between the a-IGZO and the gate electrode, shows a three-layer substructure in what has previously been regarded as a single homogeneous layer. A study of the insulating Al2O3 buffer layer below the a-IGZO compared the properties of Al2O3 deposited by ALD and sputtering. Sputtered material has a more complex structure than ALD, consisting of multiple sublayers that correlate with the sputtering process. The structure of the two materials is discussed, and the impact on device performance considered. A detailed systematic study of the effects of annealing of a-IGZO shows a strong dependence of the density on both time and temperature. A two mechanism model is proposed which consists of structural relaxation of the amorphous material followed by absorption of oxygen from the environment. Finally, investigation of the influence of the buffer material on the a-IGZO, and the structure of this interface showed little difference in the growth of the a-IGZO, but did reveal some changes in the interface, while a systematic study of annealing effects on the a-IGZO-dielectric interface showed some interesting changes in this structure, both of which are likely to significantly impact the operational characteristics of TFT devices

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

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

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

Gated Hall and Field-Effect Transport Characterization of E-mode ZnO TFTs

Amorphous and nano-crystalline metal oxide semiconductors are an important class of materials under continuing investigation for emerging technologies. Accurate measurements of electron mobility in these materials is critical for furthering overall device development. This is complicated due to the fact that device measurements such as current response, transistor input and output characteristics, as well as mobility are affected by transport-limiting factors, such as charge trapping effects at the dielectric / active layer interface, and restriction of electronic transport across grain boundaries. In this work, we focus on the binary metal oxide thin film transistor (ZnO TFT), a normally-off (e-mode) transistor with a positive threshold voltage (Vth) and a large ( \u3e 10 MΩ)) sheet resistance at gate voltage below threshold (VG \u3c Vth), Field-effect and Hall (extrinsic and intrinsic mobilities) were measured on the same device at the same time (concurrent mobility measurements of our gated Hall system) at device relevant dimensions (25 nm Al2O3, gate dielectric, 50 nm ZnO active layer), at typical transistor gate and drain bias device operating conditions (VG \u3c 10 V and VD biased in the linear region), on the same device (100 × 100 µm Van der Pauw). The large sheet resistance (RS) of the material requires electrostatic doping (by gate bias) in order to modulate resistance and increase Hall test current. However, as VG interacts with VD and VS, resulting vertical electric fields EGS and EGD must remain below dielectric breakdown (EBD). The result of meeting these test requirements led to a fully automated gated Hall test system capable of making measurements and comparisons of mobility across the allowable test bias spectrum (VG and VD). A design of experiment in which test wafers were compared between in situ deposition and exposure to clean room ambient air between the dielectric and active layer depositions (by atomic layer deposition) was used to examine interface effects. Post temperature oven annealing was used to compare differences in grain boundary effects by increasing grain size. A simple model of two transport regimes was developed (localized and non-localized transport) to fit several contradictory trends observed in the measured data sets

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