Engineering a Subnanometer Interface Tailoring Layer for Precise Hydrogen Incorporation and Defect Passivation for High-End Oxide Thin-Film Transistors.

Top-gate self-aligned structured oxide thin-film transistors (TFTs) are suitable for the backplanes of high-end displays because of their low parasitic capacitances. The gate insulator (GI) deposition process should be carefully designed to manufacture a highly stable, high-mobility oxide TFT, particularly for a top-gate structure. In this study, a nanometer-thick Al2O3 layer via plasma-enhanced atomic layer deposition (PE-ALD) is deposited on the top-gate bottom-contact structured oxide TFT as the interface tailoring layer, which can also act as the hydrogen barrier to modulate carrier generation from hydrogen incorporation into the active layer of the TFT during the following process such as postannealing. Al-doped InSnZnO (Al/ITZO) with an Al/In/Sn/Zn atomic ratio composition of 1.7:24.3:40:34 was used for high mobility oxide semiconductors, and an Al2O3/Si3N4 bilayer was used for the GI. The degradation issue due to the excellent barrier characteristics of Al2O3 and Si3N4 can be minimized. An oxide TFT fabricated without the interface tailoring layer exhibits conductor-like characteristics owing to the excessive carrier generation by hydrogen incorporation. However, TFTs with additional interface layers exhibit reasonable characteristics and distinct trends in electrical characteristics depending on the thicknesses of the interface layers. The optimized devices exhibit an average turn-on voltage (Von) of -0.31 V with 33.63 cm2/(V s) of high mobility and 0.09 V/dec of subthreshold swing value. The interfaces between the active layer and hydrogen barriers were investigated using a high-resolution transmission electron microscope, contact angle measurement, and secondary ion mass spectroscopy to reveal the origin of the trends in properties between the devices. The top-gate device with a hydrogen barrier using the four-cycle deposition exhibits optimum electrical characteristics of both high mobility and good stability with only a 0.04 V shift of Von under positive-bias temperature stress (PBTS). We realize a high-end, self-aligned TFT with high mobility [34.7 cm2/(V s)] and negligible Von shift of -0.06 V under PBTS by applying a subnanometer hydrogen barrier.

Atomically Flat, 2D Edge-Directed Self-Assembly of Block Copolymers.

Nanoscale shape engineering is an essential requirement for the practical use of 2D materials, aiming at precisely customizing optimal structures and properties. In this work, sub-10-nm-scale block copolymer (BCP) self-assembled nanopatterns finely aligned along the atomic edge of 2D flakes, including graphene, MoS2 , and h-BN, are exploited for reliable nanopatterning of 2D materials. The underlying mechanism for the alignment of the self-assembled nanodomains is elucidated based on the wetting layer alternation of the BCP film in the presence of intermediate 2D flakes. The resultant highly aligned nanocylinder templates with remarkably low levels of line edge roughness (LER) and line-width roughness (LWR) yield a sub-10-nm-wide graphene nanoribbon (GNR) array with noticeable switching characteristics (on-to-off ratio up to ≈6 × 104 ).

An immunosensor based on a high performance dual-gate oxide semiconductor thin-film transistor for rapid detection of SARS-CoV-2.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the agent of an infectious disease that has led the WHO to declare its highest level (6) pandemic. The coronavirus disease 2019 (COVID-19) has spread rapidly around the world, and the number of confirmed cases has passed 246 million as of November 2021. Therefore, precise and fast virus detection protocols need to be developed to cope with the rapid spread of the virus. Here, we present a high performance dual-gate oxide semiconductor thin-film transistor (TFT)-based immunosensor for detecting SARS-CoV-2. The immunosensor has an indium tin oxide sensing membrane to which the antibody against the SARS-CoV-2 spike S1 protein can be immobilized through functionalization. The dual-gate TFT was stable under ambient conditions with near-zero hysteresis; capacitive coupling yields a 10.14 ± 0.14-fold amplification of the surface charge potential on the sensing membrane and improves the pH sensitivity to 770.1 ± 37.74 mV pH-1 above the Nernst limit. The immunosensor could rapidly detect the SARS-CoV-2 spike S1 protein and cultured SARS-CoV-2 in 0.01× PBS with high antigen selectivity and sensitivity. Our immunosensor can accurately measure the electrical changes originated from SARS-CoV-2, without the need for polymerase chain reaction tests or labeling.

Effects of Al Precursors on the Characteristics of Indium-Aluminum Oxide Semiconductor Grown by Plasma-Enhanced Atomic Layer Deposition.

Atomic layer deposition (ALD) has attracted much attention, particularly for applications in nanoelectronics because of its atomic-level controllability and high-quality products. In this study, we developed a plasma-enhanced atomic layer deposition (PEALD) process to fabricate a homogeneous indium aluminum oxide (IAO) semiconductor film. Trimethylaluminum (TMA) and dimethylaluminum isopropoxide (DMAI) were used as Al precursors, which yielded different compositions. Density functional theory (DFT) calculations on the surface reactions between indium and aluminum precursors showed that while highly reactive TMA would etch In, DMAI with lower reactivity would allow indium to persist in the films, resulting in a more controlled doping of Al. The In/Al composition ratio could be further precisely controlled by adjusting the indium precursor dose time to sub-saturation. IAO based on DMAI was applied to fabricate thin-film transistors (TFTs), showing that Al can be a carrier suppressor of indium oxide. TFTs with PEALD IAO containing 3.8 atomic % Al showed a turn-on voltage of -0.4 ± 0.3 V, a subthreshold slope of 0.09 V/decade, and a field effect mobility of 18.9 cm2/(V s).

Hetero-Dimensional 2D Ti3C2Tx MXene and 1D Graphene Nanoribbon Hybrids for Machine Learning-Assisted Pressure Sensors.

Hybridization of low-dimensional components with diverse geometrical dimensions should offer an opportunity for the discovery of synergistic nanocomposite structures. In this regard, how to establish a reliable interfacial interaction is the key requirement for the successful integration of geometrically different components. Here, we present 1D/2D heterodimensional hybrids via dopant induced hybridization of 2D Ti3C2Tx MXene with 1D nitrogen-doped graphene nanoribbon. Edge abundant nanoribbon structures allow a high level nitrogen doping (∼6.8 at%), desirable for the strong coordination interaction with Ti3C2Tx MXene surface. For piezoresistive pressure sensor application, strong adhesion between the conductive layers and at the conductive layer/elastomer interface significantly diminishes the sensing hysteresis down to 1.33% and enhances the sensing stability up to 10 000 cycles at high pressure (100 kPa). Moreover, large-area pressure sensor array reveals a high potential for smart seat cushion-based posture monitoring application with high accuracy (>95%) by exploiting machine learning algorithm.

Oxygen vacancy control as a strategy to achieve highly reliable hafnia ferroelectrics using oxide electrode.

Recently, hafnia ferroelectrics with two spontaneous polarization states have attracted marked attention for non-volatile, super-steep switching devices, and neuromorphic application due to their fast switching, scalability, and CMOS compatibility. However, field cycling-induced instabilities are a serious obstacle in the practical application of various low-power electronic devices that require a settled characteristic of polarization hysteresis. In this work, a large reduction in the field cycling-induced instabilities and significantly improved ferroelectric properties were observed in a Hf0.5Zr0.5O2 (HZO) thin film with a RuO2 oxide electrode. The oxide electrode can supply additional oxygen to the HZO film, consequently minimizing the oxygen vacancies at the interface which is the origin of low reliability. From the material and electrical analysis results, we verified that HZO with the RuO2 electrode has less non-ferroelectric dead layers and fewer oxygen vacancies at the interface, resulting in excellent switching properties and improved reliability. This result suggests a beneficial method to produce high-quality hafnia thin films free from interfacial defects and with stable field cycling electrical properties for actual applications.

Ultrathin Nanofibrous Membranes Containing Insulating Microbeads for Highly Sensitive Flexible Pressure Sensors.

Highly sensitive and flexible pressure sensors were developed based on dielectric membranes composed of insulating microbeads contained within polyvinylidene fluoride (PVDF) nanofibers. The membrane is fabricated using a simple electrospinning process. The presence of the microbeads enhances porosity, which in turn enhances the sensitivity (1.12 kPa-1 for the range of 0-1 kPa) of the membrane when used as a pressure sensor. The microbeads are fixed in position and uniformly distributed throughout the nanofibers, resulting in a wide dynamic range (up to 40 kPa) without any sensitivity loss. The fluffy and nonsticky PVDF nanofiber features low hysteresis and ultrafast response times (∼10 ms). The sensor has also demonstrated reliable pressure detection over 10 000 loading cycles and 250 bending cycles at a 13 mm bending radius. These pressure sensors were successfully applied to detect heart rate and respiratory signals, and an array of sensors was fabricated and used to recognize spatial pressure distribution. The sensors described herein are ultrathin and ultralight, with a total thickness of less than 100 µm, including the electrodes. All of the materials comprising the sensors are flexible, making them suitable for on-body applications such as tactile sensors, electronic skins, and wearable healthcare devices.

Highly Sensitive Active-Matrix Driven Self-Capacitive Fingerprint Sensor based on Oxide Thin Film Transistor.

The fingerprint recognition has been widely used for biometrics in mobile devices. Existing fingerprint sensors have already been commercialized in the field of mobile devices using primarily Si-based technologies. Recently, mutual-capacitive fingerprint sensors have been developed to lower production costs and expand the range of application using thin-film technologies. However, since the mutual-capacitive method detects the change of mutual capacitance, it has high ratio of parasitic capacitance to ridge-to-valley capacitance, resulting in low sensitivity, compared to the self-capacitive method. In order to demonstrate the self-capacitive fingerprint sensor, a switching device such as a transistor should be integrated in each pixel, which reduces a complexity of electrode configuration and sensing circuits. The oxide thin-film transistor (TFT) can be a good candidate as a switching device for the self-capacitive fingerprint sensor. In this work, we report a systematic approach for self-capacitive fingerprint sensor integrating Al-InSnZnO TFTs with field-effect mobility higher than 30 cm2/Vs, which enable isolation between pixels, by employing industry-friendly process methods. The fingerprint sensors are designed to reduce parasitic resistance and capacitance in terms of the entire system. The excellent uniformity and low leakage current (<10-12) of the oxide TFTs allow successful capture of a fingerprint image.

Interface tailoring through the supply of optimized oxygen and hydrogen to semiconductors for highly stable top-gate-structured high-mobility oxide thin-film transistors.

Self-aligned structured oxide thin-film transistors (TFTs) are appropriate candidates for use in the backplanes of high-end displays. Although SiN x is an appropriate candidate for use in the gate insulators (GIs) of high-performance driving TFTs, direct deposition of SiN x on top of high-mobility oxide semiconductors is impossible due to significant hydrogen (H) incorporation. In this study, we used AlO x deposited by thermal atomic layer deposition (T-ALD) as the first GI, as it has good H barrier characteristics. During the T-ALD, however, a small amount of H from H2O can also be incorporated into the adjacent active layer. In here, we performed O2 or N2O plasma treatment just prior to the T-ALD process to control the carrier density, and utilized H to passivate the defects rather than generate free carriers. While the TFT fabricated without plasma treatment exhibited conductive characteristics, both O2 and N2O plasma-treated TFTs exhibited good transfer characteristics, with a V th of 2 V and high mobility (∼30 cm2 V-1 s-1). Although the TFT with a plasma-enhanced atomic layer deposited (PE-ALD) GI exhibited reasonable on/off characteristics, even without any plasma treatment, it exhibited poor stability. In contrast, the O2 plasma-treated TFT with T-ALD GI exhibited outstanding stability, i.e., a V th shift of 0.23 V under positive-bias temperature stress for 10 ks and a current decay of 1.2% under current stress for 3 ks. Therefore, the T-ALD process for GI deposition can be adopted to yield high-mobility, high-stability top-gate-structured oxide TFTs under O2 or N2O plasma treatment.

Low-Temperature Growth of Indium Oxide Thin Film by Plasma-Enhanced Atomic Layer Deposition Using Liquid Dimethyl(N-ethoxy-2,2-dimethylpropanamido)indium for High-Mobility Thin Film Transistor Application.

Low-temperature growth of In2O3 films was demonstrated at 70-250 °C by plasma-enhanced atomic layer deposition (PEALD) using a newly synthesized liquid indium precursor, dimethyl(N-ethoxy-2,2-dimethylcarboxylicpropanamide)indium (Me2In(EDPA)), and O2 plasma for application to high-mobility thin film transistors. Self-limiting In2O3 PEALD growth was observed with a saturated growth rate of approximately 0.053 nm/cycle in an ALD temperature window of 90-180 °C. As-deposited In2O3 films showed negligible residual impurity, film densities as high as 6.64-7.16 g/cm3, smooth surface morphology with a root-mean-square (RMS) roughness of approximately 0.2 nm, and semiconducting level carrier concentrations of 1017-1018 cm-3. Ultrathin In2O3 channel-based thin film transistors (TFTs) were fabricated in a coplanar bottom gate structure, and their electrical performances were evaluated. Because of the excellent quality of In2O3 films, superior electronic switching performances were achieved with high field effect mobilities of 28-30 and 16-19 cm2/V·s in the linear and saturation regimes, respectively. Furthermore, the fabricated TFTs showed excellent gate control characteristics in terms of subthreshold swing, hysteresis, and on/off current ratio. The low-temperature PEALD process for high-quality In2O3 films using the developed novel In precursor can be widely used in a variety of applications such as microelectronics, displays, energy devices, and sensors, especially at temperatures compatible with organic substrates.

Effect of hydrogen on dynamic charge transport in amorphous oxide thin film transistors.

Hydrogen in zinc oxide based semiconductors functions as a donor or a defect de-activator depending on its concentration, greatly affecting the device characteristics of oxide thin-film transistors (TFTs). Thus, controlling the hydrogen concentration in oxide semiconductors is very important for achieving high mobility and minimizing device instability. In this study, we investigated the charge transport dynamics of the amorphous semiconductor InGaZnO at various hydrogen concentrations as a function of the deposition temperature of the gate insulator. To examine the nature of dynamic charge trapping, we employed short-pulse current-voltage and transient current-time measurements. Among various examined oxide devices, that with a high hydrogen concentration exhibits the best performance characteristics, such as high saturation mobility (10.9 cm(2) v(-1) s(-1)), low subthreshold slope (0.12 V/dec), and negligible hysteresis, which stem from low defect densities and negligible transient charge trapping. Our finding indicates that hydrogen atoms effectively passivate the defects in subgap states of the bulk semiconductor, minimizing the mobility degradation and threshold voltage instability. This study indicates that hydrogen plays a useful role in TFTs by improving the device performance and stability.

High-performance hybrid complementary logic inverter through monolithic integration of a MEMS switch and an oxide TFT.

A hybrid complementary logic inverter consisting of a microelectromechanical system switch as a promising alternative for the p-type oxide thin film transistor (TFT) and an n-type oxide TFT is presented for ultralow power integrated circuits. These heterogeneous microdevices are monolithically integrated. The resulting logic device shows a distinctive voltage transfer characteristic curve, very low static leakage, zero-short circuit current, and exceedingly high voltage gain.

Negative gate bias and light illumination-induced hump in amorphous InGaZnO thin film transistor.

While observing the transfer characteristics of a-IGZO TFTs, it was noticed that a hump occurred in the subthreshold regime after light and bias stress. This study analyzes the mechanism of the hump occurrence. It was determined that hump characteristics were related with parasitic TFTs which formed at the peripheral edges parallel with the channel direction. It seems that the negative shift of the transfer characteristics of parasitic TFTs was larger than that of the main TFT under light and bias stress. Therefore, the difference in the negative shift between the main TFT and the parasitic TFT was the origin of the hump occurrence. We investigated the instability of a-IGZO TFTs under negative gate bias with light illumination for various channel structures in order to verify the above mechanism.

Solution-processed flexible fluorine-doped indium zinc oxide thin-film transistors fabricated on plastic film at low temperature.

Transparent flexible fluorine-doped indium zinc oxide (IZO:F) thin-film transistors (TFTs) were demonstrated using the spin-coating method of the metal fluoride precursor aqueous solution with annealing at 200°C for 2 hrs on polyethylene naphthalate films. The proposed thermal evolution mechanism of metal fluoride aqueous precursor solution examined by thermogravimetric analysis and Raman spectroscopy can easily explain oxide formation. The chemical composition analysed by XPS confirms that the fluorine was doped in the thin films annealed below 250°C. In the IZO:F thin films, a doped fluorine atom substitutes for an oxygen atom generating a free electron or occupies an oxygen vacancy site eliminating an electron trap site. These dual roles of the doped fluorine can enhance the mobility and improve the gate bias stability of the TFTs. Therefore, the transparent flexible IZO:F TFT shows a high mobility of up to 4.1 cm(2)/V·s and stable characteristics under the various gate bias and temperature stresses.

Structural and optical properties of Cu doped ZnO thin films by co-sputtering.

This paper reports on the structural and optical properties of ZnCuO thin films that were prepared by co-sputtering for the application of p-type-channel transparent thin-film transistors (TFTs). Pure ceramic ZnO and metal Cu targets were prepared for the co-sputtering of the ZnCuO thin films. The effects of the Cu concentration on the structural, optical, and electrical properties of the ZnCuO films were investigated after their heat treatment. It was observed from the XRD measurements that the ZnCuO films with a Cu concentration of 7% had ZnO(002), Cu2O(111), and Cu2O(200) planes. The 7% Cu-doped ZnO films also showed a band-gap energy of approximately 2.05 eV, an average transmittance of approximately 62%, and a p-type carrier density of approximately 1.33 x 10(19) cm-3 at room temperature. The bottom-gated TFTs that were fabricated with the ZnCuO thin film as a p-type channel exhibited an on-off ratio of approximately 6. These results indicate the possibility of applying ZnCuO thin films with variable band-gap energies to ZnO-based optoelectronic devices.