Thin film transistors (TFT's) on flexible large-area substrates enable large-scale deployment... more Thin film transistors (TFT's) on flexible large-area substrates enable large-scale deployment of form-fitting embedded and tactile sensors. However, the combination of insulating substrates (e.g., glass, plastics), long metal traces for distributed sensors and circuits over large areas, plasma processing and packaging/assembly for hybrid (CMOS-TFT) systems makes anomalous breakdown in TFT gate dielectrics a prominent limiter of yield in complex systems. In this work, we use layout modifications, shielding layers, and temporary “shorting bars” to enable high-yield processing and assembly of distributed sensor-acquisition circuits, in which 161 ZnO TFT's are used per sensor (Fig. 1) to implement compressed sensing (i.e., matrix projection). Although this is a large number of TFT's, compressed sensing greatly enhances critical system metrics, e.g., reduces the number of acquisition cycles and physical interfaces to a readout CMOS IC, as demonstrated in a tactile force-sensing system [1].
2017 75th Annual Device Research Conference (DRC), 2017
Besides traditional RFID [1], an emerging application of large-area electronics is hybrid systems... more Besides traditional RFID [1], an emerging application of large-area electronics is hybrid systems [2], where we need to transfer power and signals from one flexible sheet to another. This is driven by manufacturing considerations: sheets providing different functionality, via different materials and devices, can simply be laminated together. Hard-wired metallurgical bonds between flexible sheets are expected to be problematic, especially for reliability, motivating the use of non-contact inductively-coupled interfaces between adjacent sheets [2-3]. For efficiency of such interfaces, high-frequency thin-film diodes (HF-TFDs) are required, since inductive losses are lower at high frequencies [3].
Damage detection, localization and prognosis are fundamentally important for a comprehensive stru... more Damage detection, localization and prognosis are fundamentally important for a comprehensive structural health monitoring (SHM) process. Damage is often associated with an anomalous strain changes or jumps. These changes are best captured if sensors are in direct contact with the damage. However, one-dimensional direct sensing is often expensive to cover the whole structure. Two-dimensional sensors, on the other hand, show promise to allow direct sensing on a large scale. Researchers at Princeton University have developed a flexible thin-film resistive strain sensor based on the principles on large area electronics (LAE). These two-dimensional sheets are suitable for covering large areas ( 10m2) and, hence, could be used for direct damage detection. In this work, the authors explore the suitability of such sheets for real-life applications. Results of laboratory experiments conducted using an aluminum cantilever beam for evaluation of performance in ideal conditions are presented. T...
Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems 2019, 2019
Damage characterization often requires direct sensing due to the localization of the anomalous be... more Damage characterization often requires direct sensing due to the localization of the anomalous behavior near the cracks. Direct sensing, however, is expensive because of the need to deploy a dense array of individual sensors. Sensing sheets based on Large Area Electronics (LAE) and Integrated Circuits (ICs) are a novel solution to this problem. Such sensing sheets could span several square meters, with a dense array of strain sensors embedded on a polyimide substrate along with the relevant electronics allowing for direct sensing while keeping the costs low. Current studies on LAE based sensing sheets are limited to laboratory experiments. This paper explores the question of suitability of the sensing sheets as a viable option for real-life SHM based on LAE and ICs. Results of laboratory experiments on an aluminum beam are provided to demonstrate the performance of sensing sheets in ideal conditions. Then, the sensing sheets are employed on a pedestrian bridge already equipped with fiber-optic sensors. The strain measurements from the sensing sheets and the fiber-optic sensors are compared and sources of differences are discussed.
IABSE Congress, New York, New York 2019: The Evolving Metropolis, 2019
Damage detection in structures is an important part of structural health monitoring (SHM). Two ap... more Damage detection in structures is an important part of structural health monitoring (SHM). Two approaches for detecting damages are indirect and direct sensing. Indirect sensing uses sparse array of sensors and complex algorithms to determine the extent and localization of damage. Crack initiation can best be captured with direct sensing as it provides resolved information about the anomalous behavior near cracks. Direct sensing, however, is expensive because of the need to install a large array of densely packed sensors. A novel solution developed recently is the use of two-dimensional sensing sheets designed to cover large areas of structures. Such sheets are based on large area electronics (LAE) with flexible thin film resistive strain sensors embedded in polyimide substrate along with the relevant electronics. This paper explores the use of sensing sheets for damage detection using static and dynamic measurement. Laboratory testing on aluminum beam is used to demonstrate the per...
The tremendous value artificial intelligence (AI) is showing across a broad range of applications... more The tremendous value artificial intelligence (AI) is showing across a broad range of applications is driving it from cyber-systems to systems pervading every aspect of our lives. But real-world data challenges the efficiency and robustness with which AI systems of today can perform, due to the highly dynamic and noisy scenarios they face. While algorithmic solutions are required, this paper also explores technological solutions based on largescale sensing. Specifically, Large-Area Electronics (LAE) is a technology that can make large-scale, form-fitting sensors possible for broad deployment in our lives. System-design principles, architectural approaches, supporting circuits, and underlying technological concerns surrounding LAE and its use in emerging systems for intelligent sensing are explored.
Damage significantly influences response of a strain sensor only if it occurs in the proximity of... more Damage significantly influences response of a strain sensor only if it occurs in the proximity of the sensor. Thus, two-dimensional (2D) sensing sheets covering large areas offer reliable early-stage damage detection for structural health monitoring (SHM) applications. This paper presents a scalable sensing sheet design consisting of a dense array of thin-film resistive strain sensors. The sensing sheet is fabricated using flexible printed circuit board (Flex-PCB) manufacturing process which enables low-cost and high-volume sensors that can cover large areas. The lab tests on an aluminum beam showed the sheet has a gauge factor of 2.1 and has a low drift of 1.5 μ ϵ / d a y . The field test on a pedestrian bridge showed the sheet is sensitive enough to track strain induced by the bridge’s temperature variations. The strain measured by the sheet had a root-mean-square (RMS) error of 7 μ ϵ r m s compared to a reference strain on the surface, extrapolated from fiber-optic sensors embedd...
IEEE Transactions on Biomedical Circuits and Systems, 2019
Tactile sensing requires form-fitting and dense sensor arrays over large-areas. Hybrid systems, c... more Tactile sensing requires form-fitting and dense sensor arrays over large-areas. Hybrid systems, combining Large-Area Electronics (LAE) and silicon-CMOS ICs to respectively provide diverse sensing and high-performance computation/control, enable a platform for such sensing. A key challenge is that hybrid systems require a large number of interfaces between the LAE and CMOS domains, particularly as the number of sensors scales. This paper presents an architecture that exploits the attribute of signal sparsity, commonly exhibited in large-scale tactile-sensing applications, to reduce the interfacing complexity to a level set by the sparsity rather than the number of sensors. This enhances scalability compared to sequential-scanning and active-matrix approaches. The architecture implements compressed sensing via thin-film-transistor (TFT) switches, and is demonstrated in a force-sensing system with 20 force sensors, a TFT die (with 161 ZnO TFTs) per sensor, and a custom CMOS IC for system readout and control. Acquisition error of 0.7 kΩ RMS is achieved over a 100kΩ-20kΩ sensing range, at energy and rate of 2.46 µJ/frame and 31 fps.
The advent of flexible substrates with thin film transistors (TFTs) over large areas (meters) mak... more The advent of flexible substrates with thin film transistors (TFTs) over large areas (meters) makes large-area electronics (LAE) an attractive platform for integrating very large numbers of sensors onto surfaces over large areas. While TFT's may directly interface to sensors and may be used for sensor addressing, to realistically communicate with the outside world, IC's will probably be bonded onto the "sensor sheets" to create a "hybrid" LAE/IC system. This paper examines novel architectures to minimize the number of physical interfaces to the IC, beyond the typical TFT-based active-matrix approach. Approaches demonstrated include (i) high-frequency TFTbased analog oscillators, and (ii) implementing elements of machine learning into TFT circuitry, so a higher-level information is sent to the IC's, thus requiring fewer physical connections.
Sensing sheets based on Large Area Electronics (LAE) and Integrated Circuits (ICs) are novel sens... more Sensing sheets based on Large Area Electronics (LAE) and Integrated Circuits (ICs) are novel sensors designed to enable reliable early-stage detection of local unusual structural behaviors. Such a device consists of a dense array of strain sensors, patterned onto a flexible polyimide substrate along with associated electronics. Previous tests performed on steel specimens equipped with sensing sheet prototypes and subjected to fatigue cracking pointed to a potential issue: individual sensors that were on or near a crack would immediately be damaged by the crack, thereby rendering them useless in assessing the size of the crack opening or to monitor future crack growth. In these tests, a stiff adhesive was used to bond the sensing sheet prototype to the steel specimen. Such an adhesive provided excellent strain transfer, but it also caused premature failure of individual sensors within the sheet. Therefore, the aim of this paper is to identify an alternative adhesive that survives min...
2016 Tenth IEEE/ACM International Symposium on Networks-on-Chip (NOCS), 2016
Hybrid systems combine Large-Area Electronics (LAE) with high-performance technologies (e.g., sil... more Hybrid systems combine Large-Area Electronics (LAE) with high-performance technologies (e.g., silicon CMOS) [1]. With architectural concepts for hybrid systems broadening to match the range of emerging applications, this paper examines modular approaches for multi-sheet, multi-technology integration. It identifies the interfaces required as a critical backbone. For interfaces associated with various system functionalities (sensing, processing, powering), specific approaches are surveyed and analyzed, taking from insights derived from several previous experimental demonstrations of complete hybrid systems.
IEEE Transactions on Device and Materials Reliability, 2016
Part I of this paper introduced a two-stage model for reliability characterization and lifetime p... more Part I of this paper introduced a two-stage model for reliability characterization and lifetime prediction of amorphoussilicon thin-film transistors (a-Si TFTs) under low gate-field stress that includes both charge trapping in the silicon nitride (SiN x) gate dielectric and defect generation in the a-Si channel. In part II, the model is used to experimentally reduce the drain current instability under room temperature operation of a-Si TFTs under a prolonged gate bias of 5 V. Deposition conditions for the SiN x gate insulator and the a-Si channel layer were varied, and TFTs were fabricated with all reactive-ion-etch steps, or with all wet-etch steps. The stability of the a-Si channel also depends on the deposition conditions for the underlying SiN x gate insulator, and TFTs made with wet etching are more stable than TFTs made with reactive ion etching. Combining the various improvements raised the extrapolated 50% lifetime of the drain current of backchannel-passivated dry-etched TFTs under continuous operation in saturation at 20 • C with V GS = 5 V from 3 × 10 4 s (9.2 h) to 4 × 10 7 s (1.4 years). We also extend the model, so that parameters from the degradation at one gate voltage can be used to estimate the degradation at other low gate voltages.
We report on the development of UV range photodetector based on molybdenum disulfide nanocrystals... more We report on the development of UV range photodetector based on molybdenum disulfide nanocrystals (MoS 2-NCs). The inorganic MoS 2-NCs are produced by pulsed laser ablation technique in deionized water and the colloidal MoS 2-NCs are characterized by transmission electron microscopy, Raman spectroscopy, X-ray diffraction and UV/VIS absorption measurements. The photoresponse studies indicate that the fabricated MoS 2-NCs photodetector (MoS 2-NCs PD) operates well within 300-400 nm UV range, with diminishing response at visible wavelengths, due to the MoS 2-NCs absorption characteristics. The structural and the optical properties of laser generated MoS 2-NCs suggest promising applications in the field of photonics and optoelectronics.
We present ZnO-channel thin-film transistors with actively tunable photocurrent in the visible sp... more We present ZnO-channel thin-film transistors with actively tunable photocurrent in the visible spectrum, although ZnO band edge is in the ultraviolet. ZnO channel is deposited by atomic layer deposition technique at a low temperature (80 C), which is known to introduce deep level traps within the forbidden band of ZnO. The gate bias dynamically modifies the occupancy probability of these trap states by controlling the depletion region in the ZnO channel. Unoccupied trap states enable the absorption of the photons with lower energies than the bandgap of ZnO. Photoresponse to visible light is controlled by the applied voltage bias at the gate terminal.
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2015
ZnO thin film transistors (TFTs) are fabricated on Si substrates using atomic layer deposition te... more ZnO thin film transistors (TFTs) are fabricated on Si substrates using atomic layer deposition technique. The growth temperature of ZnO channel layers are selected as 80, 100, 120, 130, and 250 °C. Material characteristics of ZnO films are examined using x-ray photoelectron spectroscopy and x-ray diffraction methods. Stoichiometry analyses showed that the amount of both oxygen vacancies and interstitial zinc decrease with decreasing growth temperature. Electrical characteristics improve with decreasing growth temperature. Best results are obtained with ZnO channels deposited at 80 °C; Ion/Ioff ratio is extracted as 7.8 × 109 and subthreshold slope is extracted as 0.116 V/dec. Flexible ZnO TFT devices are also fabricated using films grown at 80 °C. ID–VGS characterization results showed that devices fabricated on different substrates (Si and polyethylene terephthalate) show similar electrical characteristics. Sub-bandgap photo sensing properties of ZnO based TFTs are investigated; it...
Thin film transistors (TFT's) on flexible large-area substrates enable large-scale deployment... more Thin film transistors (TFT's) on flexible large-area substrates enable large-scale deployment of form-fitting embedded and tactile sensors. However, the combination of insulating substrates (e.g., glass, plastics), long metal traces for distributed sensors and circuits over large areas, plasma processing and packaging/assembly for hybrid (CMOS-TFT) systems makes anomalous breakdown in TFT gate dielectrics a prominent limiter of yield in complex systems. In this work, we use layout modifications, shielding layers, and temporary “shorting bars” to enable high-yield processing and assembly of distributed sensor-acquisition circuits, in which 161 ZnO TFT's are used per sensor (Fig. 1) to implement compressed sensing (i.e., matrix projection). Although this is a large number of TFT's, compressed sensing greatly enhances critical system metrics, e.g., reduces the number of acquisition cycles and physical interfaces to a readout CMOS IC, as demonstrated in a tactile force-sensing system [1].
2017 75th Annual Device Research Conference (DRC), 2017
Besides traditional RFID [1], an emerging application of large-area electronics is hybrid systems... more Besides traditional RFID [1], an emerging application of large-area electronics is hybrid systems [2], where we need to transfer power and signals from one flexible sheet to another. This is driven by manufacturing considerations: sheets providing different functionality, via different materials and devices, can simply be laminated together. Hard-wired metallurgical bonds between flexible sheets are expected to be problematic, especially for reliability, motivating the use of non-contact inductively-coupled interfaces between adjacent sheets [2-3]. For efficiency of such interfaces, high-frequency thin-film diodes (HF-TFDs) are required, since inductive losses are lower at high frequencies [3].
Damage detection, localization and prognosis are fundamentally important for a comprehensive stru... more Damage detection, localization and prognosis are fundamentally important for a comprehensive structural health monitoring (SHM) process. Damage is often associated with an anomalous strain changes or jumps. These changes are best captured if sensors are in direct contact with the damage. However, one-dimensional direct sensing is often expensive to cover the whole structure. Two-dimensional sensors, on the other hand, show promise to allow direct sensing on a large scale. Researchers at Princeton University have developed a flexible thin-film resistive strain sensor based on the principles on large area electronics (LAE). These two-dimensional sheets are suitable for covering large areas ( 10m2) and, hence, could be used for direct damage detection. In this work, the authors explore the suitability of such sheets for real-life applications. Results of laboratory experiments conducted using an aluminum cantilever beam for evaluation of performance in ideal conditions are presented. T...
Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems 2019, 2019
Damage characterization often requires direct sensing due to the localization of the anomalous be... more Damage characterization often requires direct sensing due to the localization of the anomalous behavior near the cracks. Direct sensing, however, is expensive because of the need to deploy a dense array of individual sensors. Sensing sheets based on Large Area Electronics (LAE) and Integrated Circuits (ICs) are a novel solution to this problem. Such sensing sheets could span several square meters, with a dense array of strain sensors embedded on a polyimide substrate along with the relevant electronics allowing for direct sensing while keeping the costs low. Current studies on LAE based sensing sheets are limited to laboratory experiments. This paper explores the question of suitability of the sensing sheets as a viable option for real-life SHM based on LAE and ICs. Results of laboratory experiments on an aluminum beam are provided to demonstrate the performance of sensing sheets in ideal conditions. Then, the sensing sheets are employed on a pedestrian bridge already equipped with fiber-optic sensors. The strain measurements from the sensing sheets and the fiber-optic sensors are compared and sources of differences are discussed.
IABSE Congress, New York, New York 2019: The Evolving Metropolis, 2019
Damage detection in structures is an important part of structural health monitoring (SHM). Two ap... more Damage detection in structures is an important part of structural health monitoring (SHM). Two approaches for detecting damages are indirect and direct sensing. Indirect sensing uses sparse array of sensors and complex algorithms to determine the extent and localization of damage. Crack initiation can best be captured with direct sensing as it provides resolved information about the anomalous behavior near cracks. Direct sensing, however, is expensive because of the need to install a large array of densely packed sensors. A novel solution developed recently is the use of two-dimensional sensing sheets designed to cover large areas of structures. Such sheets are based on large area electronics (LAE) with flexible thin film resistive strain sensors embedded in polyimide substrate along with the relevant electronics. This paper explores the use of sensing sheets for damage detection using static and dynamic measurement. Laboratory testing on aluminum beam is used to demonstrate the per...
The tremendous value artificial intelligence (AI) is showing across a broad range of applications... more The tremendous value artificial intelligence (AI) is showing across a broad range of applications is driving it from cyber-systems to systems pervading every aspect of our lives. But real-world data challenges the efficiency and robustness with which AI systems of today can perform, due to the highly dynamic and noisy scenarios they face. While algorithmic solutions are required, this paper also explores technological solutions based on largescale sensing. Specifically, Large-Area Electronics (LAE) is a technology that can make large-scale, form-fitting sensors possible for broad deployment in our lives. System-design principles, architectural approaches, supporting circuits, and underlying technological concerns surrounding LAE and its use in emerging systems for intelligent sensing are explored.
Damage significantly influences response of a strain sensor only if it occurs in the proximity of... more Damage significantly influences response of a strain sensor only if it occurs in the proximity of the sensor. Thus, two-dimensional (2D) sensing sheets covering large areas offer reliable early-stage damage detection for structural health monitoring (SHM) applications. This paper presents a scalable sensing sheet design consisting of a dense array of thin-film resistive strain sensors. The sensing sheet is fabricated using flexible printed circuit board (Flex-PCB) manufacturing process which enables low-cost and high-volume sensors that can cover large areas. The lab tests on an aluminum beam showed the sheet has a gauge factor of 2.1 and has a low drift of 1.5 μ ϵ / d a y . The field test on a pedestrian bridge showed the sheet is sensitive enough to track strain induced by the bridge’s temperature variations. The strain measured by the sheet had a root-mean-square (RMS) error of 7 μ ϵ r m s compared to a reference strain on the surface, extrapolated from fiber-optic sensors embedd...
IEEE Transactions on Biomedical Circuits and Systems, 2019
Tactile sensing requires form-fitting and dense sensor arrays over large-areas. Hybrid systems, c... more Tactile sensing requires form-fitting and dense sensor arrays over large-areas. Hybrid systems, combining Large-Area Electronics (LAE) and silicon-CMOS ICs to respectively provide diverse sensing and high-performance computation/control, enable a platform for such sensing. A key challenge is that hybrid systems require a large number of interfaces between the LAE and CMOS domains, particularly as the number of sensors scales. This paper presents an architecture that exploits the attribute of signal sparsity, commonly exhibited in large-scale tactile-sensing applications, to reduce the interfacing complexity to a level set by the sparsity rather than the number of sensors. This enhances scalability compared to sequential-scanning and active-matrix approaches. The architecture implements compressed sensing via thin-film-transistor (TFT) switches, and is demonstrated in a force-sensing system with 20 force sensors, a TFT die (with 161 ZnO TFTs) per sensor, and a custom CMOS IC for system readout and control. Acquisition error of 0.7 kΩ RMS is achieved over a 100kΩ-20kΩ sensing range, at energy and rate of 2.46 µJ/frame and 31 fps.
The advent of flexible substrates with thin film transistors (TFTs) over large areas (meters) mak... more The advent of flexible substrates with thin film transistors (TFTs) over large areas (meters) makes large-area electronics (LAE) an attractive platform for integrating very large numbers of sensors onto surfaces over large areas. While TFT's may directly interface to sensors and may be used for sensor addressing, to realistically communicate with the outside world, IC's will probably be bonded onto the "sensor sheets" to create a "hybrid" LAE/IC system. This paper examines novel architectures to minimize the number of physical interfaces to the IC, beyond the typical TFT-based active-matrix approach. Approaches demonstrated include (i) high-frequency TFTbased analog oscillators, and (ii) implementing elements of machine learning into TFT circuitry, so a higher-level information is sent to the IC's, thus requiring fewer physical connections.
Sensing sheets based on Large Area Electronics (LAE) and Integrated Circuits (ICs) are novel sens... more Sensing sheets based on Large Area Electronics (LAE) and Integrated Circuits (ICs) are novel sensors designed to enable reliable early-stage detection of local unusual structural behaviors. Such a device consists of a dense array of strain sensors, patterned onto a flexible polyimide substrate along with associated electronics. Previous tests performed on steel specimens equipped with sensing sheet prototypes and subjected to fatigue cracking pointed to a potential issue: individual sensors that were on or near a crack would immediately be damaged by the crack, thereby rendering them useless in assessing the size of the crack opening or to monitor future crack growth. In these tests, a stiff adhesive was used to bond the sensing sheet prototype to the steel specimen. Such an adhesive provided excellent strain transfer, but it also caused premature failure of individual sensors within the sheet. Therefore, the aim of this paper is to identify an alternative adhesive that survives min...
2016 Tenth IEEE/ACM International Symposium on Networks-on-Chip (NOCS), 2016
Hybrid systems combine Large-Area Electronics (LAE) with high-performance technologies (e.g., sil... more Hybrid systems combine Large-Area Electronics (LAE) with high-performance technologies (e.g., silicon CMOS) [1]. With architectural concepts for hybrid systems broadening to match the range of emerging applications, this paper examines modular approaches for multi-sheet, multi-technology integration. It identifies the interfaces required as a critical backbone. For interfaces associated with various system functionalities (sensing, processing, powering), specific approaches are surveyed and analyzed, taking from insights derived from several previous experimental demonstrations of complete hybrid systems.
IEEE Transactions on Device and Materials Reliability, 2016
Part I of this paper introduced a two-stage model for reliability characterization and lifetime p... more Part I of this paper introduced a two-stage model for reliability characterization and lifetime prediction of amorphoussilicon thin-film transistors (a-Si TFTs) under low gate-field stress that includes both charge trapping in the silicon nitride (SiN x) gate dielectric and defect generation in the a-Si channel. In part II, the model is used to experimentally reduce the drain current instability under room temperature operation of a-Si TFTs under a prolonged gate bias of 5 V. Deposition conditions for the SiN x gate insulator and the a-Si channel layer were varied, and TFTs were fabricated with all reactive-ion-etch steps, or with all wet-etch steps. The stability of the a-Si channel also depends on the deposition conditions for the underlying SiN x gate insulator, and TFTs made with wet etching are more stable than TFTs made with reactive ion etching. Combining the various improvements raised the extrapolated 50% lifetime of the drain current of backchannel-passivated dry-etched TFTs under continuous operation in saturation at 20 • C with V GS = 5 V from 3 × 10 4 s (9.2 h) to 4 × 10 7 s (1.4 years). We also extend the model, so that parameters from the degradation at one gate voltage can be used to estimate the degradation at other low gate voltages.
We report on the development of UV range photodetector based on molybdenum disulfide nanocrystals... more We report on the development of UV range photodetector based on molybdenum disulfide nanocrystals (MoS 2-NCs). The inorganic MoS 2-NCs are produced by pulsed laser ablation technique in deionized water and the colloidal MoS 2-NCs are characterized by transmission electron microscopy, Raman spectroscopy, X-ray diffraction and UV/VIS absorption measurements. The photoresponse studies indicate that the fabricated MoS 2-NCs photodetector (MoS 2-NCs PD) operates well within 300-400 nm UV range, with diminishing response at visible wavelengths, due to the MoS 2-NCs absorption characteristics. The structural and the optical properties of laser generated MoS 2-NCs suggest promising applications in the field of photonics and optoelectronics.
We present ZnO-channel thin-film transistors with actively tunable photocurrent in the visible sp... more We present ZnO-channel thin-film transistors with actively tunable photocurrent in the visible spectrum, although ZnO band edge is in the ultraviolet. ZnO channel is deposited by atomic layer deposition technique at a low temperature (80 C), which is known to introduce deep level traps within the forbidden band of ZnO. The gate bias dynamically modifies the occupancy probability of these trap states by controlling the depletion region in the ZnO channel. Unoccupied trap states enable the absorption of the photons with lower energies than the bandgap of ZnO. Photoresponse to visible light is controlled by the applied voltage bias at the gate terminal.
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2015
ZnO thin film transistors (TFTs) are fabricated on Si substrates using atomic layer deposition te... more ZnO thin film transistors (TFTs) are fabricated on Si substrates using atomic layer deposition technique. The growth temperature of ZnO channel layers are selected as 80, 100, 120, 130, and 250 °C. Material characteristics of ZnO films are examined using x-ray photoelectron spectroscopy and x-ray diffraction methods. Stoichiometry analyses showed that the amount of both oxygen vacancies and interstitial zinc decrease with decreasing growth temperature. Electrical characteristics improve with decreasing growth temperature. Best results are obtained with ZnO channels deposited at 80 °C; Ion/Ioff ratio is extracted as 7.8 × 109 and subthreshold slope is extracted as 0.116 V/dec. Flexible ZnO TFT devices are also fabricated using films grown at 80 °C. ID–VGS characterization results showed that devices fabricated on different substrates (Si and polyethylene terephthalate) show similar electrical characteristics. Sub-bandgap photo sensing properties of ZnO based TFTs are investigated; it...
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Papers by Levent E Aygun