Recent Advances in Thin Film Electronic Devices

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

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

Solution Processed Electrolyte-Gated Thin Film Transistors and their Sensing applications

The thin film transistor (TFT) is one of the most important fundamental building blocks in modern electronic devices. Examples of TFT applications are in integrated circuits (ICs), amplifiers, addressing of flat panel displays, and also as chemical sensors, e.g. as ion- selective field effect transistors (ISFETs). Although the most common semiconductor materials used in thin film transistors (TFTs) is silicon (Si), the versatility of TFTs allows other semiconductors to be used instead of Si-based materials. Recent research effort has been directed towards alternative TFT semiconductors, for example solution-processable semiconductors that enable new manufacturing options. Since the 1990s, soluble semiconducting polymers have been intensely researched for TFT applications. The possibility to engineer their transport (HOMO/LUMO) levels via chemical synthesis, their low temperature processing from solution, and their mechanical flexibility offers the potential for economical device production, as well as large-area and flexible applications. In the last view years, the interest as promising materials for the next generation of TFTs has shifted towards oxide based semiconductors such as ZnO due to their high performance and relative stability to ambient conditions. The potential of ZnO was boosted by the discovery that it can derived by pyrolysis of an organic precursor that is soluble in polar organic solvents (e.g. alcohols, ketones) thus enabling solution processing of ZnO TFTs and other devices. In addition, it has been demonstrated that both organic thin films and ZnO films can be gated by a field effect with very low threshold using deionized (DI) water as an electrolytic gate medium, leading to the ‘water- gated thin film transistors’ (WGTFTs). This discovery marks the beginning of a new method for the sensing of waterborne analytes, which differs from the classic ISFET in one significant point: Here, the aqueous sample under test is an active portion of the transducer. A number of works have since been undertaken using this discovery as a novel sensor concept for detecting waterborne analytes. Such sensors rely in the integration of analyte- specific sensitisers into the TFT architecture. For the selective sensing of ions, one of the most prominent families of water- insoluble ion sensitisers (‘ionophores’) are the calixarenes, a family of organic macrocycles. Calixarenes can be designed to selectively to complex specific cations, anions and neutral molecules

Novel Semiconducting Materials and Thin Film Technologies for High Energy Radiation Detection

Nowadays the development of real-time ionizing radiation detection system operating over large areas is crucial. Increasing quest for flexible, portable, low cost and low power consumption sensors pushed the scientific community to look for alternative materials and technologies able to fulfill these new requirements. In this thesis the potentiality of organic semiconductors and metal oxides as material platforms for novel ionizing radiation sensors is demonstrated. In particular, organic semiconductors are human tissue-equivalent and this represents a unique and desirable property for the development of dosimeters to be employed in the medical field. The ionizing radiation sensors described in this thesis have been designed, fabricated and characterized during my PhD research and are realized onto polymeric foils leading to flexible devices operating at low voltages, in ambient condition and able to directly detect X-rays, gamma-rays and protons. Following the study of the properties and of the mechanisms of interaction between the radiation and the active layers of the sensors, several strategies have been adopted to enhance the efficiency of these detectors. X-rays dosimeters based on organic semiconductors have been realized presenting record sensitivity values compared with the state of the art for large area radiation detection. The unprecedentedly reported performance led to the possibility to testing these devices in actual medical environments. Moreover, the proof-of-principle demonstration of a dosimetric detection of proton beams by organic-based sensors is reported. Finally, a new sensing platform based on metal oxides is introduced. Combining the advantages of amorphous high mobility oxide semiconductors with a multilayer dielectric, novel devices have been designed, capable of providing a sensitivity one order of magnitude higher than the one shown by the standard RADFETs. Thanks to their unique properties, these sensors have been integrated with a wireless readout system based on a commercial RFID tag and its assessment is presented

On the minimization of contact resistance in organic thin-film transistors

Organic semiconductors have been implemented in a variety of electronic devices ranging from organic light-emitting diodes, organic photovoltaic devices, and organic transistors. In all of these devices, the efficient injection and/or extraction of charges across interfaces with conducting contacts is an essential requirement for device performance. The high contact resistance in organic transistors, which limits their usefulness in high-frequency electronics applications, has been a particularly challenging problem to solve. The contact resistance can depend strongly on various parameters, including the transistor architecture and the mismatch between the contact work function and the transport levels of the organic semiconductor. In this work, it is shown that using a thin gate-dielectric layer (around 5 nm) in a thin-film transistor (TFT) in combination with contacts modified using a chemisorbed monolayer to tune the contact-semiconductor interface properties yields record-low contact resistance in organic transistors (as small as 10 Ωcm). This approach was then extended to small-scale TFTs and circuits leading to additional record results in the dynamic performance, including voltage-normalized transit frequency of 7 MHz/V. Finally, strong evidence is shown that Fermi-level pinning limits the effectiveness of tuning the contact work function with chemisorbed monolayers to improve the contact resistance further

Solution Processed Electrolyte-Gated Thin Film Transistors and their Sensing applications

The thin film transistor (TFT) is one of the most important fundamental building blocks in modern electronic devices. Examples of TFT applications are in integrated circuits (ICs), amplifiers, addressing of flat panel displays, and also as chemical sensors, e.g. as ion- selective field effect transistors (ISFETs). Although the most common semiconductor materials used in thin film transistors (TFTs) is silicon (Si), the versatility of TFTs allows other semiconductors to be used instead of Si-based materials. Recent research effort has been directed towards alternative TFT semiconductors, for example solution-processable semiconductors that enable new manufacturing options. Since the 1990s, soluble semiconducting polymers have been intensely researched for TFT applications. The possibility to engineer their transport (HOMO/LUMO) levels via chemical synthesis, their low temperature processing from solution, and their mechanical flexibility offers the potential for economical device production, as well as large-area and flexible applications. In the last view years, the interest as promising materials for the next generation of TFTs has shifted towards oxide based semiconductors such as ZnO due to their high performance and relative stability to ambient conditions. The potential of ZnO was boosted by the discovery that it can derived by pyrolysis of an organic precursor that is soluble in polar organic solvents (e.g. alcohols, ketones) thus enabling solution processing of ZnO TFTs and other devices. In addition, it has been demonstrated that both organic thin films and ZnO films can be gated by a field effect with very low threshold using deionized (DI) water as an electrolytic gate medium, leading to the ‘water- gated thin film transistors’ (WGTFTs). This discovery marks the beginning of a new method for the sensing of waterborne analytes, which differs from the classic ISFET in one significant point: Here, the aqueous sample under test is an active portion of the transducer. A number of works have since been undertaken using this discovery as a novel sensor concept for detecting waterborne analytes. Such sensors rely in the integration of analyte- specific sensitisers into the TFT architecture. For the selective sensing of ions, one of the most prominent families of water- insoluble ion sensitisers (‘ionophores’) are the calixarenes, a family of organic macrocycles. Calixarenes can be designed to selectively to complex specific cations, anions and neutral molecules

High Resolution Active Pixel Sensor X-Ray Detectors for Digital Breast Tomosynthesis

Current large area x-ray detectors for digital breast tomosynthesis (DBT) are based on the amorphous silicon (a-Si:H) passive pixel sensor (PPS) technology. However, PPS detectors suffer from a limited resolution and high electronic noise. In this dissertation, we propose high resolution large area active pixel sensor (APS) x-ray detectors based on the complementary metal-oxide-semiconductor (CMOS) and amorphous In-Sn-Zn-O (a-ITZO) thin-film transistor (TFT) technologies to improve the imager resolution and noise properties. We evaluated the two-dimensional (2D) x-ray imaging performance as measured by the modulation transfer function (MTF), noise power spectrum (NPS) and detective quantum efficiency (DQE) for both 75 µm (Dexela 2923 MAM) and 50 µm pixel pitch (DynAMITe) CMOS APS x-ray detectors. Excellent imaging performance (DQE in the range of 0.7 – 0.3) has been achieved over the entire spatial frequency range (0 – 6.7 mm-1) at low air kerma below 10 µGy using the 75 µm pixel pitch Dexela 2923 MAM detector. The 50 μm pixel pitch DyAMITe detector has further extended the spatial resolution of the detector to 10 mm-1 with a low electronic noise of 150 e-. Also, a 2D cascaded system analysis model has been developed to describe the signal and noise transfer for the CMOS APS x-ray imaging systems. We also implemented three-dimensional (3D) cascaded system analysis to simulated the 3D MTF, NPS and DQE characteristics using DBT radiation conditions and acquisition geometries. The 3D cascaded system analysis for the DynAMITe detector was integrated with an object task function, a medical imaging display model, and the human eye contrast sensitivity function to calculate the detectability index and area under the ROC curve (AUC). It has been demonstrated that the display pixel pitch and zoom factor should be optimized to improve the AUC for detecting high contrast objects such as microcalcifications. Also, detector electronic noise of smaller than 300 e- and a high display maximum luminance (>1000 cd/cm2) are desirable to distinguish microcalcifications of 150 µm or smaller in size. For low contrast object detection, a medical imaging display with a minimum of 12 bits gray levels is needed to realize accurate luminance levels. A wide projection angle range (≥ ±30°) combined with the image gray level magnification could improve the detectability for low contrast objects especially when the anatomical background noise is high. CMOS APS x-ray detectors demonstrate both a high pixel resolution and low electronic noise, but are challenging to be fabricated in a large detector size greater than the wafer scale. Alternatively, current-mode APS (C-APS) based on a-ITZO TFTs was proposed for DBT due to the high gain, low noise, and capability to realize a large detector area. Specifically, we fabricated a-ITZO TFTs and achieved a high field-effect mobility of >30 cm2/Vs. We have also evaluated the electrical performance of a 50 µm pixel pitch a-ITZO TFT C-APS combined with an a-Si:H p+-i-n+ photodiode using SPICE simulation. The proposed C-APS circuit demonstrates a high charge gain of 885 with data line loadings considered. A pixel circuit layout and fabrication process have also been suggested. Finally, noise analysis has been applied to the a-ITZO TFT C-APS. A low electronic noise of around 239 e- has been established. The research presented in this thesis indicates that APS x-ray detectors based on both CMOS and a-ITZO TFT technologies are promising for next generation DBT systems.PHDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/136983/1/zhaocm_1.pd

Development and design of polymer circuits based on polymer thin-film transistors

Before 1870, Britain had been able to rely on naval and financial strength to protect her empire where necessary until such time as the expected separation of the settlement colonies into independent polities occurred. However, a series of changes in geopolitics, and in naval technology had undermined this security and had resulted in an increased awareness in military, naval, colonial and imperial circles - among those characterised as "Jingoes" - of the dangers to which the empire was exposed. Spurred on by the Balkan crisis of 1876-78, a far more active approach was taken to war planning, which resulted in a more integrated approach to imperial defence under Lord Carnarvon. It is also posited that Gladstone's Bulgarian agitation and the struggle between Disraeli, Derby and Salisbury over the correct response to the Balkan crisis destroyed the confidence of many colonial, imperial, naval and military figures in elected politicians and thus encouraged the disobedience of Lytton and Frere. Disasters at Isandlwana and Kabul however discredited the defence establishment and allowed a programme of anti-imperialist reaction to emerge, under the leadership of Gladstone. The subsequent working out of this policy caused such disquiet that Gladstone's cabinet was to break up over imperial defence issues and resulted in the commitment of Britain to a more positive espousal of imperial virtue and a determination to see the empire as an asset and defend it accordingly. Thus, within the period 1870- 1885, Britain underwent a revolution in Imperial policy, from a rejection of empire, to a virtuous acceptance of it