Impact of the cation composition on the electrical performance of solution-processed zinc tin oxide thin-film transistors

An artificially-intelligent cornea with tactile sensation enables sensory expansion and interaction

We demonstrate an artificially-intelligent cornea that can assume the functions of the native human cornea such as protection, tactile perception, and light refraction, and possesses sensory expansion and interactive functions. These functions are realized by an artificial corneal reflex arc that is constructed to implement mechanical and light information coding, information processing, and the regulation of transmitted light. Digitally-aligned, long and continuous zinc tin oxide (ZTO) semiconductor fabric patterns were fabricated as the active channels of the artificial synapse, which are non-toxic, heavy-metal-free, low-cost, and ensure superior comprehensive optical properties (transmittance >99.89%, haze <0.36%). Precisely-tuned crystal-phase structures of the ZTO fibers enabled reconfigurable synaptic plasticity, which is applicable to encrypted communication and associative learning. This work suggests new strategies for the tuning of synaptic plasticity and the design of visual neuroprosthetics, and has important implications for the development of neuromorphic electronics and for visual restoration.

Hydrogen-Doping-Enabled Boosting of the Carrier Mobility and Stability in Amorphous IGZTO Transistors

This study investigated the effect of hydrogen (H) on the performance of amorphous In-Ga-Zn-Sn oxide (a-In_0.29Ga_0.35Zn_0.11Sn_0.25O) thin-film transistors (TFTs). Ample H in plasma-enhanced atomic layer deposition (PEALD)-derived SiO₂ can diffuse into the underlying a-IGZTO film during the postdeposition annealing (PDA) process, which affects the electrical properties of the resulting TFTs due to its donor behavior in the a-IGZTO. The a-In_0.29Ga_0.35Zn_0.11Sn_0.25O TFTs at the PDA temperature of 400 °C exhibited a remarkably higher field-effect mobility (μ_FE) of 85.9 cm²/Vs, a subthreshold gate swing (SS) of 0.33 V/decade, a threshold voltage (V_TH) of -0.49 V, and an I_ON/OFF ratio of ∼10⁸; these values are superior compared to those of unpassivated a-In_0.29Ga_0.35Zn_0.11Sn_0.25O TFTs (μ_FE = 23.3 cm²/Vs, SS = 0.36 V/decade, and V_TH = -3.33 V). In addition, the passivated a-In_0.29Ga_0.35Zn_0.11Sn_0.25O TFTs had good stability against the external gate bias duration. This performance change can be attributed to the substitutional H doping into oxygen sites (H_O) leading to a boost in n_e and μ_FE. In contrast, the beneficial H_O effect was barely observed for amorphous indium gallium zinc oxide (a-IGZO) TFTs, suggesting that the hydrogen-doping-enabled boosting of a-IGZTO TFTs is strongly related to the existence of Sn cations. Electronic calculations of V_O and H_O using density functional theory (DFT) were performed to explain this disparity. The introduction of SnO₂ in a-IGZO is predicted to cause a conversion from shallow V_O to deep V_O due to the lower formation energy of deep V_O, which is effectively created around Sn cations. The formation of H_O by H doping in the IGZTO facilitates the efficient connection of atomic states forming the conduction band more smoothly. This reduces the effective mass and enhances the carrier mobility.

Low-Cost, High-Performance Spray Pyrolysis-Grown Amorphous Zinc Tin Oxide: The Challenge of a Complex Growth Process

Transparent conductive oxides (TCOs) are important materials for a wide range of optoelectronic devices. Amorphous zinc tin oxide (a-ZTO) is a TCO and one of the best nontoxic, low-cost replacements for more expensive amorphous indium-gallium-zinc oxide. Here, we employ spray pyrolysis (SP), an inexpensive and versatile chemical vapor deposition-based technique, to synthesize a-ZTO with an as-deposited conductivity of ≈300 S/cm-the highest value hitherto among the reported solution-processed films. Compositional analysis via X-ray photoelectron spectroscopy reveals a nonstoichiometric transfer of Zn and Sn from the dissolved precursors into the film, with the best electrical properties achieved at a film composition of x_film = 0.38 ± 0.04 ((ZnO)_x(SnO₂)_1-x (0 < x < 1)). The morphology of these films is compared to films synthesized by physical vapor deposition (PVD), and a strong correlation between morphology and electrical properties is revealed. The granular nature of the SP-grown films, which seems like a drawback at first glance, brings about the prospect of using a-ZTO in ink-jet-printed films from a nanoparticle suspension for the room-temperature deposition. Brief post-anneal cycles in N₂ gas improve the conductivity of the films by means of grain boundary (GB) passivation.

Importance of Local Bond Order to Conduction in Amorphous, Transparent, Conducting Oxides: The Case of Amorphous ZnSnOy

In this report, reactive and nonreactive sputtering of amorphous ZnSnO_y (a-ZnSnO_y) was investigated, and extensive composition maps have been measured by X-ray photoelectron spectroscopy. The comprehensive analysis of the ((ZnO)_x(SnO₂)_1-x) composition reveals that the best Zn/Sn ratio for high conductivity of the material can vary depending on the deposition technique utilized. Best conductivities of 225 S/cm were found to occur at x = 0.32 for reactive sputtering of a Sn target and x = 0.27 for nonreactive sputtering of a SnO₂ target. These values correspond to unstable polymorphs of a-ZnSnO_y, ZnSn₂O₅, and ZnSn₃O₇. Distinct local bonding arrangements have been confirmed by Raman spectroscopy.

Printed Electronics Based on Inorganic Semiconductors: From Processes and Materials to Devices

Following the ever-expanding technological demands, printed electronics has shown palpable potential to create new and commercially viable technologies that will benefit from its unique characteristics, such as, large-area and wide range of substrate compatibility, conformability and low-cost. Through the last few decades, printed/solution-processed field-effect transistors (FETs) and circuits have witnessed immense research efforts, technological growth and increased commercial interests. Although printing of functional inks comprising organic semiconductors has already been initiated in early 1990s, gradually the attention, at least partially, has been shifted to various forms of inorganic semiconductors, starting from metal chalcogenides, oxides, carbon nanotubes and very recently to graphene and other 2D semiconductors. In this review, the entire domain of printable inorganic semiconductors is considered. In fact, thanks to the continuous development of materials/functional inks and novel design/printing strategies, the inorganic printed semiconductor-based circuits today have reached an operation frequency up to several hundreds of kilohertz with only a few nanosecond time delays at the individual FET/inverter levels; in this regard, often circuits based on hybrid material systems have been found to be advantageous. At the end, a comparison of relative successes of various printable inorganic semiconductor materials, the remaining challenges and the available future opportunities are summarized.

Flexible Zinc-Tin Oxide Thin Film Transistors Operating at 1 kV for Integrated Switching of Dielectric Elastomer Actuators Arrays

Flexible high-voltage thin-film transistors (HVTFTs) operating at more than 1 kV are integrated with compliant dielectric elastomer actuators (DEA) to create a flexible array of 16 independent actuators. To allow for high-voltage operation, the HVTFT implements a zinc-tin oxide channel, a thick dielectric stack, and an offset gate. At a source-drain bias of 1 kV, the HVTFT has a 20 µA on-current at a gate voltage bias of 30 V. Their electrical characteristics enable the switching of DEAs which require drive voltages of over 1 kV, making control of an array simpler in comparison to the use of external high-voltage switching. These HVTFTs are integrated in a flexible haptic display consisting of a 4 × 4 matrix of DEAs and HVTFTs. Using a single 1.4 kV supply, each DEA is independently switched by its associated HVTFT, requiring only a 30 V gate voltage for full DEA deflection. The 4 × 4 display operates well even when bent to a 5 mm radius of curvature. By enabling DEA switching at low voltages, flexible metal-oxide HVTFTs enable complex flexible systems with dozens to hundreds of independent DEAs for applications in haptics, Braille displays, and soft robotics.

Low-Temperature Solution-Processed Zinc Tin Oxide Film as a Cathode Interlayer for Organic Solar Cells

In this study, Sn-doped ZnO (ZTO) is prepared by a sol-gel method and is employed as an electron transport material for organic solar cells (OSCs). After Sn modification, the fabricated ZTO films exhibited better charge transport properties and smoother surface morphology, especially for those processed at a low temperature of 120 °C. By incorporation of the high-temperature (200 °C) processed ZTO films, inverted OSCs showed the highest power conversion efficiency (PCE) of 9.32%, which is higher than those based on the same temperature processed ZnO films. For the devices based on the low-temperature processed ZTO, a high PCE over 9.0% with long-term stability was achieved, which is much better than those based on the same temperature processed ZnO (8.46% PCE). Here, the ZTO films can be fabricated without high-temperature annealing, demonstrating their great potential as electron transport layers for efficient flexible OSCs.

Influence of the Cation Ratio on Optical and Electrical Properties of Amorphous Zinc-Tin-Oxide Thin Films Grown by Pulsed Laser Deposition

Continuous composition spread (CCS) methods allow fast and economic exploration of composition dependent properties of multielement compounds. Here, a CCS method was applied for room temperature pulsed laser deposition (PLD) of amorphous zinc-tin-oxide to gain detailed insight into the influence of the zinc-to-tin cation ratio on optical and electrical properties of this ternary compound. Our CCS approach for a large-area offset PLD process utilizes a segmented target and thus makes target exchange or movable masks in the PLD chamber obsolete. Cation concentrations of 0.08-0.82 Zn/(Zn + Sn) were achieved across single 50 × 50 mm(2) glass substrates. The electrical conductivity increases for increasing tin content, and the absorption edge shifts to lower energies. The free carrier concentration can be tuned from 10(20) to 10(16) cm(-3) by variation of the cation ratio from 0.1 to 0.5 Zn/(Zn + Sn).