Giuseppe Cantarella

Metal oxide semiconductor thin-film transistors for flexible electronics

The field of flexible electronics has rapidly expanded over the last decades, pioneering novel applications, such as wearable and textile integrated devices, seamless and embedded patch-like systems, soft electronic skins, as well as imperceptible and transient implants. The possibility to revolutionize our daily life with such disruptive appliances has fueled the quest for electronic devices which yield good electrical and mechanical performance and are at the same time light-weight, transparent, conformable, stretchable, and even biodegradable. Flexible metal oxide semiconductor thin-film transistors (TFTs) can fulfill all these requirements and are therefore considered the most promising technology for tomorrows electronics. This review reflects the establishment of flexible metal oxide semiconductor TFTs, from the development of single devices, large-area circuits, up to entirely integrated systems. First, an introduction on metal oxide semiconductor TFTs is given, where the history of the field is revisited, the TFT configurations and operating principles are presented, and the main issues and technological challenges faced in the area are analyzed. Then, the recent advances achieved for flexible n-type metal oxide semiconductor TFTs manufactured by physical vapor deposition methods and solution-processing techniques are summarized. In particular, the ability of flexible metal oxide semiconductor TFTs to combine low temperature fabrication, high carrier mobility, large frequency operation, extreme mechanical bendability, together with transparency, conformability, stretchability, and water dissolubility is shown. Afterward, a detailed analysis of the most promising metal oxide semiconducting materials developed to realize the state-of-the-art flexible p-type TFTs is given. Next, the recent progresses obtained for flexible metal oxide semiconductor-based electronic circuits, realized with both unipolar and complementary technology, are reported. In particular, the realization of large-area digital circuitry like flexible near field communication tags and analog integrated circuits such as bendable operational amplifiers is presented. The last topic of this review is devoted for emerging flexible electronic systems, from foldable displays, power transmission elements to integrated systems for large-area sensing and data storage and transmission. Finally, the conclusions are drawn and an outlook over the field with a prediction for the future is provided.

Low-temperature spray-deposited indium oxide for flexible thin-film transistors and integrated circuits

Indium oxide (In2O3) films were deposited by ultrasonic spray pyrolysis in ambient air and incorporated into bottom-gate coplanar and staggered thin-film transistors. As-fabricated devices exhibited electron-transporting characteristics with mobility values of 1 cm2V−1s−1 and 16 cm2V−1s−1 for coplanar and staggered architectures, respectively. Integration of In2O3 transistors enabled realization of unipolar inverters with high gain (5.3 V/V) and low-voltage operation. The low temperature deposition (≤250 °C) of In2O3 also allowed transistor fabrication on free-standing 50 μm-thick polyimide foils. The resulting flexible In2O3 transistors exhibit good characteristics and remain fully functional even when bent to tensile radii of 4 mm.

Flexible In–Ga–Zn–O Thin-Film Transistors on Elastomeric Substrate Bent to 2.3% Strain

In this letter, a photolithographic fabrication process is used to manufacture indium-gallium-zinc-oxide thin-film transistors (TFTs) with mobilities > 10 cm2/Vs directly on a 80 μm thick polydimethylsiloxane (PDMS) substrate. Once the fabrication is completed, the PDMS is detached from a silicon wafer used as carrier substrate. Due to the thermal mismatch between silicon and PDMS, the release results in a reduction of the PDMS area by 7.2%, which leads to the formation of out-of-plane wrinkles on the TFT surface. The reflattening of the wrinkles under tensile strain enables device functionality, while the TFTs are bent up to 2.3% strain. Mechanical stability of the TFTs with our wrinkled approach is shown by electrically characterizing them at bending radii down to 6 mm.

Solution-processed p-type copper(I) thiocyanate (CuSCN) for low-voltage flexible thin-film transistors and integrated inverter circuits

We report on low operating voltage thin-film transistors (TFTs) and integrated inverters based on copper(I) thiocyanate (CuSCN) layers processed from solution at low temperature on freestanding plastic foils. As-fabricated coplanar bottom-gate and staggered top-gate TFTs exhibit hole-transporting characteristics with average mobility values of 0.0016 cm2 V1 s 1 and 0.013 cm2 V1 s 1 , respectively, current on/off ratio in the range 102 –104 , and maximum operating voltages between 3.5 and 10 V, depending on the gate dielectric employed. The promising TFT characteristics enable fabrication of unipolar NOT gates on flexible free-standing plastic substrates with voltage gain of 3.4 at voltages as low as 3.5 V. Importantly, discrete CuSCN transistors and integrated logic inverters remain fully functional even when mechanically bent to a tensile radius of 4 mm, demonstrating the potential of the technology for flexible electronics.

Metal‐Halide Perovskites for Gate Dielectrics in Field‐Effect Transistors and Photodetectors Enabled by PMMA Lift‐Off Process

Metal-halide perovskites have emerged as promising materials for optoelectronics applications, such as photovoltaics, light-emitting diodes, and photodetectors due to their excellent photoconversion efficiencies. However, their instability in aqueous solutions and most organic solvents has complicated their micropatterning procedures, which are needed for dense device integration, for example, in displays or cameras. In this work, a lift-off process based on poly(methyl methacrylate) and deep ultraviolet lithography on flexible plastic foils is presented. This technique comprises simultaneous patterning of the metal-halide perovskite with a top electrode, which results in microscale vertical device architectures with high spatial resolution and alignment properties. Hence, thin-film transistors (TFTs) with methyl-ammonium lead iodide (MAPbI3 ) gate dielectrics are demonstrated for the first time. The giant dielectric constant of MAPbI3 (>1000) leads to excellent low-voltage TFT switching capabilities with subthreshold swings ≈80 mV decade-1 over ≈5 orders of drain current magnitude. Furthermore, vertically stacked low-power Au-MAPbI3 -Au photodetectors with close-to-ideal linear response (R2 = 0.9997) are created. The mechanical stability down to a tensile radius of 6 mm is demonstrated for the TFTs and photodetectors, simultaneously realized on the same flexible plastic substrate. These results open the way for flexible low-power integrated (opto-)electronic systems based on metal-halide perovskites.

Charge Trapping Mechanism Leading to Sub-60-mV/decade-Swing FETs

In this paper, we present a novel method to reduce the subthreshold swing (SS) of FETs below 60 mV/decade. Through modeling, we directly relate trap charge movement between the gate electrode and the gate dielectric to SS reduction. We experimentally investigate the impact of charge exchange between a Cu gate electrode and a 5-nm-thick amorphous Al2O3 gate dielectric in an InGaZnO4 thin-film transistor. Positive trap charges are generated inside the gate dielectric while the semiconductor is in accumulation. During the subsequent detrapping, the SS diminishes to a minimum value of 46 mV/decade at room temperature. Furthermore, we relate the charge trapping/detrapping effects to a negative capacitance behavior of the Cu/Al2O3 metal–insulator structure.

Buckled Thin-Film Transistors and Circuits on Soft Elastomers for Stretchable Electronics

Although recent progress in the field of flexible electronics has allowed the realization of biocompatible and conformable electronics, systematic approaches which combine high bendability (<3 mm bending radius), high stretchability (>3-4%), and low complexity in the fabrication process are still missing. Here, we show a technique to induce randomly oriented and customized wrinkles on the surface of a biocompatible elastomeric substrate, where Thin-Film Transistors (TFTs) and circuits (inverter and logic NAND gates) based on amorphous-IGZO are fabricated. By tuning the wavelength and the amplitude of the wrinkles, the devices are fully operational while bent to 13 μm bending radii as well as while stretched up to 5%, keeping unchanged electrical properties. Moreover, a flexible rectifier is also realized, showing no degradation in the performances while flat or wrapped on an artificial human wrist. As proof of concept, transparent TFTs are also fabricated, presenting comparable electrical performances to the nontransparent ones. The extension of the buckling approach from our TFTs to circuits demonstrates the scalability of the process, prospecting applications in wireless stretchable electronics to be worn or implanted.

Flexible Quasi-Vertical In-Ga-Zn-O Thin-Film Transistor With 300-nm Channel Length

In this letter, we report a flexible Indium-Gallium-Zinc-Oxide quasi-vertical thin-film transistor (QVTFT) with 300-nm channel length, fabricated on a free-standing polyimide foil, using a low-temperature process <;150 °C. A bilayer lift-off process is used to structure a spacing layer with a tilted sidewall and the drain contact on top of the source electrode. The resulting quasi-vertical profile ensures a good coverage of the successive device layers. The fabricated flexible QVTFT exhibits an ON/OFF current ratio of 104, a threshold voltage of 1.5 V, a maximum transconductance of 0.73 μS μm-1, and a total gate capacitance of 76 nF μm-1. From S-parameter measurements, we extracted a transit frequency of 1.5 MHz. Furthermore, the flexible QVTFT is fully operational when bent to a tensile radius of 5 mm.

Gain-Tunable Complementary Common-Source Amplifier Based on a Flexible Hybrid Thin-Film Transistor Technology

In this letter, we report a flexible complementary common-source (CS) amplifier comprising one p-type spray-coated single walled carbon nanotube and one n-type sputtered InGaZnO4 thin-film transistor (TFT). Bottom-gate TFTs were realized on a free-standing flexible polyimide foil using a maximum process temperature of 150 °C. The resulting CS amplifier operates at 10 V supply voltage and exhibits a gain bandwidth product of 60 kHz. Thanks to the use of a p-type TFT acting as a tunable current source load, the amplifier gain can be programmed from 3.5 up to 27.2 V/V (28.7 dB). To the best of our knowledge, this is the highest gain ever obtained for a flexible single-stage CS amplifier.

Positive charge trapping phenomenon in n-channel thin-film transistors with amorphous alumina gate insulators

In this work, we investigate the charge trapping behavior in InGaZnO4 (IGZO) thin-film transistors with amorphous Al2O3 (alumina) gate insulators. For thicknesses ≤10 nm, we observe a positive charge generation at intrinsic defects inside the Al2O3, which is initiated by quantum-mechanical tunneling of electrons from the semiconductor through the Al2O3 layer. Consequently, the drain current shows a counter-clockwise hysteresis. Furthermore, the de-trapping through resonant tunneling causes a drastic subthreshold swing reduction. We report a minimum value of 19 mV/dec at room temperature, which is far below the fundamental limit of standard field-effect transistors. Additionally, we study the thickness dependence for Al2O3 layers with thicknesses of 5, 10, and 20 nm. The comparison of two different gate metals shows an enhanced tunneling current and an enhanced positive charge generation for Cu compared to Cr.