The chemistry of post transition metals is dominated by the group oxidation state N and a lower N-2 oxidation state, which is associated with occupation of a metal s2 lone pair, as found in compounds of Tl(I), Pb(II) and Bi(III). The preference of these cations for non-centrosymmetric coordination environments has previously been rationalised in terms of direct hybridisation of metal s and p valence orbitals, thus lowering the internal electronic energy of the N-2 ion. This explanation in terms of an on-site second-order Jahn–Teller effect remains the contemporary textbook explanation. In this tutorial review, we review recent progress in this area, based on quantum chemical calculations and X-ray spectroscopic measurements. This recent work has led to a revised model, which highlights the important role of covalent interaction with oxygen in mediating lone pair formation for metal oxides. The role of the anion p atomic orbital in chemical bonding is key to explaining why chalcogenides display a weaker preference for structural distortions in comparison to oxides and halides. The underlying chemical interactions are responsible for the unique physicochemical properties of oxides containing lone pairs and, in particular, to their application as photocatalysts (BiVO4), ferroelectrics (PbTiO3), multi-ferroics (BiFeO3) and p-type semiconductors (SnO). The exploration of lone pair systems remains a viable a venue for the design of functional multi-component oxide compounds.

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Stereochemistry of post-transition metal oxides: revision of the classical lone pair model.

The development of transparent p-type oxide semiconductors with good performance may be a true enabler for a variety of applications where transparency, power efficiency, and greater circuit complexity are needed. Such applications include transparent electronics, displays, sensors, photovoltaics, memristors, and electrochromics. Hence, here, recent developments in materials and devices based on p-type oxide semiconductors are reviewed, including ternary Cu-bearing oxides, binary copper oxides, tin monoxide, spinel oxides, and nickel oxides. The crystal and electronic structures of these materials are discussed, along with approaches to enhance valence-band dispersion to reduce effective mass and increase mobility. Strategies to reduce interfacial defects, off-state current, and material instability are suggested. Furthermore, it is shown that promising progress has been made in the performance of various types of devices based on p-type oxides. Several innovative approaches exist to fabricate transparent complementary metal oxide semiconductor (CMOS) devices, including novel device fabrication schemes and utilization of surface chemistry effects, resulting in good inverter gains. However, despite recent developments, p-type oxides still lag in performance behind their n-type counterparts, which have entered volume production in the display market. Recent successes along with the hurdles that stand in the way of commercial success of p-type oxide semiconductors are presented.

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Recent Developments in p-Type Oxide Semiconductor Materials and Devices.

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 tomorrow's 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.

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Metal oxide semiconductor thin-film transistors for flexible electronics

Transparent conducting oxides constitute a unique class of materials combining properties of electrical conductivity and optical transparency in a single material. They are needed for a wide range of applications including solar cells, flat panel displays, touch screens, light emitting diodes and transparent electronics. Most of the commercially available TCOs are n-type, such as Sn doped In2O3, Al doped ZnO, and F doped SnO2. However, the development of efficient p-type TCOs remains an outstanding challenge. This challenge is thought to be due to the localized nature of the O 2p derived valence band which leads to difficulty in introducing shallow acceptors and large hole effective masses. In 1997 Hosono and co-workers (1997 Nature 389 939) proposed the concept of 'chemical modulation of the valence band' to mitigate this problem using hybridization of O 2p orbitals with close-shell Cu 3d (10) orbitals. This work has sparked tremendous interest in designing p-TCO materials together with deep understanding the underlying materials physics. In this article, we will provide a comprehensive review on traditional and recently emergent p-TCOs, including Cu(+)-based delafossites, layered oxychalcogenides, nd (6) spinel oxides, Cr(3+)-based oxides (3d (3)) and post-transition metal oxides with lone pair state (ns (2)). We will focus our discussions on the basic materials physics of these materials in terms of electronic structures, doping and defect properties for p-type conductivity and optical properties. Device applications based on p-TCOs for transparent p-n junctions will also be briefly discussed.

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P-type transparent conducting oxides.

Here, we report the fabrication of nanoscale (15 nm) fully transparent p-type SnO thin film transistors (TFT) at temperatures as low as 180 °C with record device performance. Specifically, by carefully controlling the process conditions, we have developed SnO thin films with a Hall mobility of 18.71 cm(2) V(-1) s(-1) and fabricated TFT devices with a linear field-effect mobility of 6.75 cm(2) V(-1) s(-1) and 5.87 cm(2) V(-1) s(-1) on transparent rigid and translucent flexible substrates, respectively. These values of mobility are the highest reported to date for any p-type oxide processed at this low temperature. We further demonstrate that this high mobility is realized by careful phase engineering. Specifically, we show that phase-pure SnO is not necessarily the highest mobility phase; instead, well-controlled amounts of residual metallic tin are shown to substantially increase the hole mobility. A detailed phase stability map for physical vapor deposition of nanoscale SnO is constructed for the first time for this p-type oxide.

Record mobility in transparent p-type tin monoxide films and devices by phase engineering.

Tin monoxide (SnO) is a stable p-type oxide semiconductor. This paper reports electrical properties, electronic structures, and thin-film transistors (TFTs) of SnO. Epitaxial films were fabricated by pulsed laser deposition. The Hall mobility and the hole density of the epitaxial films were 2.4 cm 2 V -1 S -1 and 2.5 x 10 17 , respectively. X-ray photoelectron spectroscopy (PES) indicated that the closed-shell 5s 2 orbitals of Sn 2+ ions heavily contribute to the hole conduction path in SnO. Top gate type TFTs (W/L=300/50 μm) employing 20 nm thick SnO channels exhibited field-effect mobilities μ sat = 0.7 cm 2 V -1 s -1 and μ lin = 1.3 cm 2 V -1 s -1 , which are larger by two orders of magnitude than those reported for p-channel oxide TFrs to date. On/off current ratios were ~10 2 and subthreshold voltage swings (S) ~7 V/decade. The parameters required for TFT simulations were estimated by ultraviolet PES and first-principles calculations. The TFT simulations indicated that subgap hole trap density in the SnO channel is > 10 19 cm -3 , which limits the TFT mobilities and the S values.

Tin monoxide as an s-orbital-based p-type oxide semiconductor: Electronic structures and TFT application

We have analyzed the mechanism of off-leakage current in an lightly doped drain (LDD) poly-Si thin-film transistor by investigating the activation energy Ea. It is found that Ea decreases as the gate and drain voltages increase. We have also discussed the mechanism using a device simulator. It is found that a hole channel is lightly formed in the LDD region, and a pseudo p-n junction appears at the junction between the LDD and drain regions. The aforementioned experimental behavior of Ea is because the electric field increases there. These results suggest that the off-leakage current is caused by the phonon-assisted tunneling with Poole-Frenkel effect at the junction between the LDD and drain regions.

Mechanism Analysis of Off-Leakage Current in an LDD Poly-Si TFT Using Activation Energy

We propose a thermal sensor using a poly-Si thin-film transistor (TFT) with widened detectable temperature range. The cell circuit is composed of one transistor and one capacitor. The cell capacitor is charged during initializing periods and discharged during holding periods, and the dropped voltages are measured during detecting periods. The temperature is detected because the discharge current is subject to the temperature dependence of the transistor characteristic. The detectable temperature is expanded to -10°C-140°C by adding multiple holding periods. We think that it is promising to integrate this thermal sensor in some applications using TFTs.

Thermal Sensor Using Poly-Si Thin-Film Transistor With Widened Detectable Temperature Range

In this letter, a magnetic-field area sensor using poly-Si micro Hall devices is reported. A matrix array of Hall devices is formed on a glass substrate using fabrication processes compatible with poly-Si thin-film transistors. Here, 3 × 3 Hall devices are arrayed every 1 × 1 mm, and the dimension of the principal part is less than 50 × 50 μm. A compensation technique of the characteristic variation is used, and it is confirmed that the area sensing is correctly conducted. This sensor presents a novel application using thin-film technologies for giant-micro flexible electronics.

Magnetic-Field Area Sensor Using Poly-Si Micro Hall Devices

We have developed a soft actuator using an ionic polymer-metal composite (IPMC) composed of gold electrodes deposited using vacuum evaporation. In comparison with the conventional plating deposition, the vacuum evaporation both is a simple fabrication process and is environmentally friendly. We observe how the gold electrodes adhere to the ionic polymer using a cross-sectional scanning electron microscope and confirm the working as a soft actuator by applying voltage. Although the displacement characteristic is not comparable to that of the conventional IPMC, this work is worthwhile because many researchers become able to easily enter into the research field of soft actuators.

M. Kimura

Papers

Tin monoxide as an s-orbital-based p-type oxide semiconductor: Electronic structures and TFT application

Mechanism Analysis of Off-Leakage Current in an LDD Poly-Si TFT Using Activation Energy

Thermal Sensor Using Poly-Si Thin-Film Transistor With Widened Detectable Temperature Range

Magnetic-Field Area Sensor Using Poly-Si Micro Hall Devices

Soft Actuator Using Ionic Polymer–Metal Composite Composed of Gold Electrodes Deposited Using Vacuum Evaporation