Mobility and current boosting of In-Ga-Zn-O thin-film transistors with metal capping layer oxidation.

This study investigates the effect of an oxidized Ta capping layer on the boosting of field-effect mobility (µFE) of amorphous In-Ga-Zn-O (a-IGZO) Thin-film transistors (TFTs). The oxidation of Ta creates additional oxygen vacancies on the a-IGZO channel surface, leading to increased carrier density. We investigate the effect of increasing Ta coverage on threshold voltage (Vth), on-state current,µFEand gate bias stress stability of a-IGZO TFTs. A significant increase inµFEof over 8 fold, from 16 cm2Vs-1to 140 cm2Vs-1, was demonstrated with the Ta capping layer covering 90% of the channel surface. By partial leaving the a-IGZO uncovered at the contact region, a potential barrier region was created, maintaining the low off-state current and keeping the threshold voltage near 0 V, while the capped region operated as a carrier-boosted region, enhancing channel conduction. The results reported in this study present a novel methodology for realizing high-performance oxide semiconductor devices. The demonstrated approach holds promise for a wide range of next-generation device applications, offering new avenues for advancement in metal oxide semiconductor TFTs.

Improved high voltage operation of amorphous In-Ga-Zn-O thin film transistor by carrier density enhancement.

We report on improved high voltage operation of amorphous-In-Ga-Zn-O (a-IGZO) thin film transistors (TFTs) by increasing carrier density and distributing the high bias field over the length of the device which utilizes an off-set drain structure. By decreasing the O2partial pressure during sputter deposition of IGZO, the channel carrier density of the high voltage a-IGZO TFT (HiVIT) was increased to ∼1018cm-3. Which reduced channel resistance and therefore the voltage drop in the ungated offset region during the on-state. To further decrease the electric field in the offset region, we applied Ta capping and subsequent oxidation to locally increase the oxygen vacancy levels in the offset region thereby increasing local carrier density. The reduction of the drain field in the offset region from 1.90 Vµm-1to 1.46 Vµm-1at 200 V drain voltage, significantly improved the operational stability of the device by reducing high field degradation. At an extreme drain voltage of 500 V, the device showed an off-state current of ∼10-11A and on-state current of ∼1.59 mA demonstrating that with further enhancements the HiVIT may be applicable to thin-film form, low leakage, high voltage control applications.

Microelectronic current-sourcing device based on band-to-band tunneling current.

A new stable current-sourcing transistor is developed using the band-to-band tunneling phenomenon. A heterojunction between thin film WS2and heavily hole-doped bulk silicon converts a section of the WS2contacting the silicon into a hole-doped WS2inside the WS2channel, and band-to-band tunneling occurs between the electron-doped and hole-doped WS2. The output current is regulated by the tunneling barrier thickness. The thickness depends on the gate bias for device switching, but is less sensitive to the source bias, enabling stable output currents. The minimum line sensitivity is 2.6%, and the temperature coefficient is 1.4 × 103ppm°C-1. The device can be operated as a current sourcing device with an ultralow output current and power consumption.

An Ultralow Power Mixed Dimensional Heterojunction Transistor Based on the Charge Plasma pn Junction.

Development of a reliable doping method for 2D materials is a key issue to adopt the materials in the future microelectronic circuits and to replace the silicon, keeping the Moore's law toward the sub-10 nm channel length. Especially hole doping is highly required, because most of the transition metal dichalcogenides (TMDC) among the 2D materials are electron-doped by sulfur vacancies in their atomic structures. Here, hole doping of a TMDC, tungsten disulfide (WS2 ) using the silicon substrate as the dopant medium is demonstrated. An ultralow-power current sourcing transistor or a gated WS2 pn diode is fabricated based on a charge plasma pn heterojunction formed between the WS2 thin-film and heavily doped bulk silicon. An ultralow switchable output current down to 0.01 nA µm-1 , an off-state current of ≈1 × 10-14 A µm-1 , a static power consumption range of 1 fW µm-1 -1 pW µm-1 , and an output current ratio of 103 at 0.1 V supply voltage are achieved. The charge plasma heterojunction allows a stable (less than 3% variation) output current regardless of the gate voltage once it is turned on.

Low-temperature n-type doping of insulating ultrathin amorphous indium oxide using H plasma-assisted annealing.

Low-temperature process compatibility is a key factor in successfully constructing additional functional circuits on top of pre-existing circuitry without corrupting characteristics thereof, a technique that typically requires die-to-die (wafer-to-wafer) stacking and interconnecting. And against thermal annealing, which is mandatory and is possible only globally for activating amorphous oxide semiconductors, the selective control of electrical characteristics of the oxide thin-films for integrated circuit applications is challenging. Here, a low-temperature process that enables n-type doping of the designed region of insulating In2O3thin-film is demonstrated. A short hydrogen plasma treatment followed by low-temperature annealing is used to increase interstitial and substitutional hydrogen associated bond states creating shallow donor levels in the insulating In2O3surface to transform the thin-film into an n-type semiconductor. As a result, an In2O3thin-film transistor with a high on/off current ratio (>108), a field-effect mobility of 3.8 cm2V-1s-1, and a threshold voltage of ∼3.0 V has been developed. Compared to performing just thermal annealing, the H-plasma assisted annealing process resulted in an n-type In2O3thin-film transistor showing similar characteristics, while the processing time was reduced by ∼1/3 and the plasma-untreated area still remained insulating. With further development, the hydrogen plasma doping process may make possible a monolithic planar process technology for amorphous oxide semiconductors.

Tactile Interaction Sensor with Millimeter Sensing Acuity.

In this article we report on a 3 × 3 mm tactile interaction sensor that is able to simultaneously detect pressure level, pressure distribution, and shear force direction. The sensor consists of multiple mechanical switches under a conducting diaphragm. An external stimulus is measured by the deflection of the diaphragm and the arrangement of mechanical switches, resulting in low noise, high reliability, and high uniformity. Our sensor is able to detect tactile forces as small as ~50 mgf along with the direction of the shear force. It also distinguishes whether there is a normal pressure during slip motion. We also succeed in detecting the contact shape and the contact motion, demonstrating potential applications in robotics and remote input interfaces. Since our sensor has a simple structure and its function depends only on sensor dimensions, not on an active sensing material, in comparison with previous tactile sensors, our sensor shows high uniformity and reliability for an array-type integration.

Enrichment of Circulating Tumor Cells from Whole Blood Using a Microfluidic Device for Sequential Physical and Magnetophoretic Separations.

Based on their high clinical potential, the isolation and enrichment of rare circulating tumor cells (CTCs) from peripheral blood cells has been widely investigated. There have been technical challenges with CTC separation methods using solely cancer-specific surface molecules or just using physical properties of CTCs, as they may suffer from heterogeneity or lack of specificity from overlapping physical characteristics with leukocytes. Here, we integrated an immunomagnetic-based negative enrichment method that utilizes magnetic beads attached to leukocyte-specific surface antigens, with a physical separation method that utilizes the distinct size and deformability of CTCs. By manipulating the pressure distribution throughout the device and balancing the drag and magnetic forces acting on the magnetically labeled white blood cells (WBCs), the sequential physical and magnetophoretic separations were optimized to isolate intact cancer cells, regardless of heterogeneity from whole blood. Using a breast cancer cell line in whole blood, we achieved 100% separation efficiency for cancer cells and an average of 97.2% for WBCs, which resulted in a 93.3% average separation purity. The experimental results demonstrated that our microfluidic device can be a promising candidate for liquid biopsy and can be a vital tool for aiding future cancer research.

Biomimetic Tactile Sensors with Bilayer Fingerprint Ridges Demonstrating Texture Recognition.

In this article, we report on a biomimetic tactile sensor that has a surface kinetic interface (SKIN) that imitates human epidermal fingerprint ridges and the epidermis. The SKIN is composed of a bilayer polymer structure with different elastic moduli. We improved the tactile sensitivity of the SKIN by using a hard epidermal fingerprint ridge and a soft epidermal board. We also evaluated the effectiveness of the SKIN layer in shear transfer characteristics while varying the elasticity and geometrical factors of the epidermal fingerprint ridges and the epidermal board. The biomimetic tactile sensor with the SKIN layer showed a detection capability for surface structures under 100 µm with only 20-µm height differences. Our sensor could distinguish various textures that can be easily accessed in everyday life, demonstrating that the sensor may be used for texture recognition in future artificial and robotic fingers.

Graphene surface contacts of tin disulfide transistors for switching performance improvement and contact resistance reduction.

We investigated the performance improvement of tin disulfide channel transistors by graphene contact configurations. From its two-dimensional nature, graphene can make electric contacts only at the outermost layers of the channel. For intralayer current flow, two graphene flakes are contacted at the channel's top or bottom layer. For interlayer current flow, one flake is contacted at the top and bottom of each layer. We compared the transistor performance in terms of current magnitude, mobility, and subthreshold swing between the configurations. From such observations, we deduced that device characteristics depend on resistivity or doping level of individual graphene flakes. We also found that interlayer flow excels in the on-current magnitude and the mobility, and that top-contact configuration excels in the subthreshold swing.

Layered deposition of SnS2 grown by atomic layer deposition and its transport properties.

In this work, we report on the layered deposition of few-layer tin disulfide (SnS2) using atomic layer deposition (ALD). By varying the ALD cycles it was possible to deposit poly-crystalline SnS2 with small variation in layer numbers. Based on the ALD technique, we developed the process technology growing few-layer crystalline SnS2 film (3-6 layers) and we investigated their electrical properties by fabricating bottom-gated thin film transistors using the ALD SnS2 as the transport channel. SnS2 devices showed typical n-type characteristic with on/off current ratio of ∼8.32 × 106, threshold voltage of ∼2 V, and a subthreshold swing value of 830 mV decade-1 for the 6 layers SnS2. The developed SnS2 ALD technique may aid the realization of two-dimensional SnS2 based flexible and wearable devices.

Highly Sensitive Tactile Shear Sensor Using Spatially Digitized Contact Electrodes.

In this article, we report on a highly sensitive tactile shear sensor that was able to detect minute levels of shear and surface slip. The sensor consists of a suspended elastomer diaphragm with a top ridge structure, a graphene layer underneath, and a bottom substrate with multiple spatially digitized contact electrodes. When shear is applied to the top ridge structure, it creates torque and deflects the elastomer downwards. Then, the graphene electrode makes contact with the bottom spatially digitized electrodes completing a circuit producing output currents depending on the number of electrodes making contact. The tactile shear sensor was able to detect shear forces as small as 6 µN, detect shear direction, and also distinguish surface friction and roughness differences of shearing objects. We also succeeded in detecting the contact slip motion of a single thread demonstrating possible applications in future robotic fingers and remote surgical tools.

Buckled carbon nanotube network thin-film fabricated using chemically swelled elastomer substrates.

We report on the fabrication of buckled carbon nanotube thin-film networks (CNTN) that increases in conductivity with applied tactile pressure. When tactile pressure was applied, the buckled nanotubes collapsed and increased in interconnected density and as a result increased the thin-film conductivity. Unlike conventional methods using mechanically expanded elastomers, we utilize chemically swollen elastomers as the expanded substrate to transfer the CNTN. As the chemical evaporates, it compresses the CNTN causing the thin-film to buckle. The CNTN compression can be controlled by using organic solvents with differing elastomer absorption rates. Our method requires no mechanical instruments and shows in-plane multi-axial uniform strain for the entire substrate surface. Since the buckling was controlled chemically, the buckled CNTN can be produced reliably, furthering the possibility of its application as the active sensing material for highly sensitive tactile pressure sensors.

Continuous Separation of Circulating Tumor Cells from Whole Blood Using a Slanted Weir Microfluidic Device.

The separation of circulating tumor cells (CTCs) from the peripheral blood is an important issue that has been highlighted because of their high clinical potential. However, techniques that depend solely on tumor-specific surface molecules or just the larger size of CTCs are limited by tumor heterogeneity. Here, we present a slanted weir microfluidic device that utilizes the size and deformability of CTCs to separate them from the unprocessed whole blood. By testing its ability using a highly invasive breast cancer cell line, our device achieved a 97% separation efficiency, while showing an 8-log depletion of erythrocytes and 5.6-log depletion of leukocytes. We also developed an image analysis tool that was able to characterize the various morphologies and differing deformability of the separating cells. From the results, we believe our system possesses a high potential for liquid biopsy, aiding future cancer research.

Deterministic Capture of Individual Circulating Tumor Cells Using a Flow-Restricted Microfluidic Trap Array.

Circulating tumor cells (CTCs) are regarded as a strong biomarker which includes clinically valuable information. However, CTCs are very rare and require precise separation and detection for effective clinical applications. Furthermore, downstream analysis has become necessary to identify the distinct sub-population of CTCs that causes metastasis. Here, we report a flow-restricted microfluidic trap array capable of deterministic single-cell capture of CTCs. The extent of flow restriction, correlating with the device geometry, was then optimized using a highly invasive breast cancer cell line (LM2 MDA-MB-231) to achieve 97% capture efficiency with a single-cell capture rate of 99%. Single-cell capture of CTCs from mice with full-blown metastasis was also demonstrated. The single-CTC capturing ability of the flow-restricted trap array not only showed cell enumerating ability but also high prospects for application in future automated downstream analysis.

Fabrication of high crystalline SnS and SnS2 thin films, and their switching device characteristics.

Representative tin sulfide compounds, tin monosulfide (SnS) and tin disulfide (SnS2) are strong candidates for future nanoelectronic devices, based on non-toxicity, low cost, unique structures and optoelectronic properties. However, it is insufficient for synthesizing of tin sulfide thin films using vapor phase deposition method which is capable of fabricating reproducible device and securing high quality films, and their device characteristics. In this study, we obtained highly crystalline SnS thin films by atomic layer deposition and obtained highly crystalline SnS2 thin films by phase transition of the SnS thin films. The SnS thin film was transformed into SnS2 thin film by annealing at 450 °C for 1 h in H2S atmosphere. This phase transition was confirmed by x-ray diffractometer and x-ray photoelectron spectroscopy, and we studied the cause of the phase transition. We then compared the film characteristics of these two tin sulfide thin films and their switching device characteristics. SnS and SnS2 thin films had optical bandgaps of 1.35 and 2.70 eV, and absorption coefficients of about 105 and 104 cm-1 in the visible region, respectively. In addition, SnS and SnS2 thin films exhibited p-type and n-type semiconductor characteristics. In the images of high resolution-transmission electron microscopy, SnS and SnS2 directly showed a highly crystalline orthorhombic and hexagonal layered structure. The field effect transistors of SnS and SnS2 thin films exhibited on-off drain current ratios of 8.8 and 2.1 × 103 and mobilities of 0.21 and 0.014 cm2 V-1 s-1, respectively. This difference in switching device characteristics mainly depends on the carrier concentration because it contributes to off-state conductance and mobility. The major carrier concentrations of the SnS and SnS2 thin films were 6.0 × 1016 and 8.7 × 1013 cm-3, respectively, in this experiment.

A Portable Stiffness Measurement System.

A new stiffness measurement method is proposed that utilizes the lateral deformation profile of an object under indentation. The system consists of a force measurement module between a pair of equidistant touch sensing modules. Unique feature of the method is that by adjusting the touch module separation, indenter protrusion, and spring constant of the force sensing module, one can choose a desired sensing range for the force module. This feature helps to enhance the stiffness differentiation between objects of similar hardness and avoids measurement saturation. We devised a portable measurement system based on the method, and tested its performance with several materials including polymer foams and human skin.

Simultaneous Detection of Displacement, Rotation Angle, and Contact Pressure Using Sandpaper Molded Elastomer Based Triple Electrode Sensor.

In this article, we report on a flexible sensor based on a sandpaper molded elastomer that simultaneously detects planar displacement, rotation angle, and vertical contact pressure. When displacement, rotation, and contact pressure are applied, the contact area between the translating top elastomer electrode and the stationary three bottom electrodes change characteristically depending on the movement, making it possible to distinguish between them. The sandpaper molded undulating surface of the elastomer reduces friction at the contact allowing the sensor not to affect the movement during measurement. The sensor showed a 0.25 mm −1 displacement sensitivity with a ±33 μm accuracy, a 0.027 degree −1 of rotation sensitivity with ~0.95 degree accuracy, and a 4.96 kP −1 of pressure sensitivity. For possible application to joint movement detection, we demonstrated that our sensor effectively detected the up-and-down motion of a human forefinger and the bending and straightening motion of a human arm.

Selective Dirac voltage engineering of individual graphene field-effect transistors for digital inverter and frequency multiplier integrations.

The ambipolar band structure of graphene presents unique opportunities for novel electronic device applications. A cycle of gate voltage sweep in a conventional graphene transistor produces a frequency-doubled output current. To increase the frequency further, we used various graphene doping control techniques to produce Dirac voltage engineered graphene channels. The various surface treatments and substrate conditions produced differently doped graphene channels that were integrated on a single substrate and multiple Dirac voltages were observed by applying a single gate voltage sweep. We applied the Dirac voltage engineering techniques to graphene field-effect transistors on a single chip for the fabrication of a frequency multiplier and a logic inverter demonstrating analog and digital circuit application possibilities.

Reduction of hole doping of chemical vapor deposition grown graphene by photoresist selection and thermal treatment.

The doping effect on graphene by photoresists were studied in this article. Polymethyl methacrylate (PMMA) is the usual choice for graphene transfer, but it is known to leave a significant amount of residue. PMMA results in strong hole doping and reduction of mobility of the graphene devices. Not only PMMA, but photoresists also leave residues during the lithographic steps and dope the graphene in strong hole-doping states along with water and oxygen molecules. In this article, we tested three types of photoresists for their effects on graphene's electrical properties. It was found that a specific photoresist can significantly reduce the amount of hole-doping of the graphene transistor more than other photoresists. The use of hydrophobic substrates and additional thermal treatment can help reducing the hole-doping further.

Clogging-free microfluidics for continuous size-based separation of microparticles.

In microfluidic filtration systems, one of the leading obstacles to efficient, continuous operation is clogging of the filters. Here, we introduce a lateral flow microfluidic sieving (µ-sieving) technique to overcome clogging and to allow continuous operation of filter based microfluidic separation. A low frequency mechanical oscillation was added to the fluid flow, which made possible the release of aggregated unwanted polystyrene (PS) particles trapped between the larger target PS particles in the filter demonstrating continuous µ-sieving operation. We achieved collection of the target PS particles with 100% separation efficiency. Also, on average, more than 98% of the filtered target particles were retrieved after the filtration showing high retrieval rates. Since the oscillation was applied to the fluid but not to the microfluidic filter system, mechanical stresses to the system was minimized and no additional fabrication procedures were necessary. We also applied the µ-sieving technique to the separation of cancer cells (MDA-MB-231) from whole blood and showed that the fluidic oscillations prevented the filters from being blocked by the filtered cancer cells allowing continuous microfluidic separation with high efficiency.