There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

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

Kesterite-based solar cells are attracting considerable attention in recent years, owing to the reduced toxicity and greater abundance of their constituent elements. In this brief review, we discuss the current status of this important technology by focusing on three key aspects of the device: (i) the interface between the kesterite absorber and the Mo back contact, (ii) the kesterite absorber bulk defects and grain boundaries and (iii) the interface between the kesterite absorber and the buffer layer. By identifying key issues to be addressed, we provide suggestions for their potential improvement and future research. Copyright © 2016 John Wiley & Sons, Ltd.

The current status and future prospects of kesterite solar cells: a brief review

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.

Molybdenum thin film deposited by in-line DC magnetron sputtering as a back contact for Cu(In,Ga)Se2 solar cells

Effect of the duty ratio on the indium tin oxide (ITO) film deposited by in-line pulsed DC magnetron sputtering method for resistive touch panel

Simulation study on effect of drain underlap in gate-all-around tunneling field-effect transistors

We have investigated the effects of vacuum annealing on the optical and electrical properties of the p-type copper-oxide thin-film transistors (TFTs). The vacuum annealing of the copper-oxide thin-film was performed using the RF magnetron sputter at various temperatures. From the x-ray diffraction and UV-vis spectroscopy, it is demonstrated that the high-temperature vacuum annealing reduces the copper-oxide phase from CuO to Cu2O, and increases the optical transmittance in the visible part of the spectrum. The fabricated copper-oxide TFT does not exhibit the switching behavior under low-temperature vacuum annealing conditions. However, as the annealing temperature increases, the drain current begins to be modulated by a gate voltage, and the TFT exhibits a high current on?off ratio over 104?as the vacuum annealing temperature increases over 450??C. These results show that the vacuum annealing process can be an effective method of simultaneously improving the optical and electrical performances in p-type copper-oxide TFTs.

https://iopscience.iop.org/article/10.1088/0268-1242/28/1/015005/pdf

Effects of vacuum annealing on the optical and electrical properties of p-type copper-oxide thin-film transistors

We investigate the charge transport mechanism and subgap density of states (DOS) in p-type Cu2O thin-film transistors (TFTs) using the bias and temperature dependence of the drain currents. Among several charge transport mechanisms, the experimental data are well matched with a multiple trapping and release model, which suggests that the charge transport in the Cu2O TFT is mainly limited by trap states at grain boundaries or dielectric/semiconductor interface. The subgap DOS is extracted based on the Meyer-Neldel rule. Large density of subgap states is extracted, which is considered to be the reason of low mobility in fabricated Cu2O TFTs.

Eou-Sik Cho

Papers

Molybdenum thin film deposited by in-line DC magnetron sputtering as a back contact for Cu(In,Ga)Se2 solar cells

Effect of the duty ratio on the indium tin oxide (ITO) film deposited by in-line pulsed DC magnetron sputtering method for resistive touch panel

Simulation study on effect of drain underlap in gate-all-around tunneling field-effect transistors

Effects of vacuum annealing on the optical and electrical properties of p-type copper-oxide thin-film transistors

Investigation of the charge transport mechanism and subgap density of states in p-type Cu2O thin-film transistors