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

This paper will review emerging non-volatile memory (NVM) technologies, with the focus on phase change memory (PCM), spin-transfer-torque random-access-memory (STTRAM), resistive random-access-memory (RRAM), and ferroelectric field-effect-transistor (FeFET) memory. These promising NVM devices are evaluated in terms of their advantages, challenges, and applications. Their performance is compared based on reported parameters of major industrial test chips. Memory selector devices and cell structures are discussed. Changing market trends toward low power ( e.g. , mobile, IoT) and data-centric applications create opportunities for emerging NVMs. High-performance and low-cost emerging NVMs may simplify memory hierarchy, introduce non-volatility in logic gates and circuits, reduce system power, and enable novel architectures. Storage-class memory (SCM) based on high-density NVMs could fill the performance and density gap between memory and storage. Some unique characteristics of emerging NVMs can be utilized for novel applications beyond the memory space, e.g. , neuromorphic computing, hardware security, etc . In the beyond-CMOS era, emerging NVMs have the potential to fulfill more important functions and enable more efficient, intelligent, and secure computing systems.

A review of emerging non-volatile memory (NVM) technologies and applications

One-dimensional (1D) metal-oxide nanostructures are ideal systems for exploring a large number of novel phenomena at the nanoscale and investigating size and dimensionality dependence of nanostructure properties for potential applications The construction and integration of photodetectors or optical switches based on such nanostructures with tailored geometries have rapidly advanced in recent years Active 1D nanostructure photodetector elements can be configured either as resistors whose conductions are altered by a charge-transfer process or as field-effect transistors (FET) whose properties can be controlled by applying appropriate potentials onto the gates Functionalizing the structure surfaces offers another avenue for expanding the sensor capabilities This article provides a comprehensive review on the state-of-the-art research activities in the photodetector field It mainly focuses on the metal oxide 1D nanostructures such as ZnO, SnO(2), Cu(2)O, Ga(2)O(3), Fe(2)O(3), In(2)O(3), CdO, CeO(2), and their photoresponses The review begins with a survey of quasi 1D metal-oxide semiconductor nanostructures and the photodetector principle, then shows the recent progresses on several kinds of important metal-oxide nanostructures and their photoresponses and briefly presents some additional prospective metal-oxide 1D nanomaterials Finally, the review is concluded with some perspectives and outlook on the future developments in this area

A comprehensive review of one-dimensional metal-oxide nanostructure photodetectors

Due to the unique set of properties possessed by ZnO, thin films of ZnO have received more and more interest in the last 20?years as a potential material for applications such as thin-film transistors, light-emitting diodes and gas sensors. At the same time, the increasingly stringent requirements of the microelectronics industry, among other factors, have led to a dramatic increase in the use of atomic layer deposition (ALD) technique in various thin-film applications. During this time, the research on ALD-grown ZnO thin films has developed from relatively simple deposition studies to the fabrication of increasingly intricate nanostructures and an understanding of the factors affecting the fundamental properties of the films. In this review, we give an overview of the current state of ZnO ALD research including the applications that are being considered for ZnO thin films.

https://iopscience.iop.org/article/10.1088/0268-1242/29/4/043001/pdf

Atomic layer deposition of ZnO: a review

Cu2O thin films were synthesized on Si (100) substrate with thermally grown 200-nm SiO2 by sol–gel spin coating method and postannealing under different oxygen partial pressure (0.04, 0.2, and 0.9 Torr). The morphology of Cu2O thin films was improved through N2 postannealing before O2 annealing. Under relatively high oxygen partial pressure of 0.9 Torr, the roughness of synthesized films was increased with the formation of CuO phase. Bottom-gated copper oxide (CuxO) thin film transistors (TFTs) were fabricated via conventional photolithography, and the electrical properties of the fabricated TFTs were measured. The resulting Cu2O TFTs exhibited p-channel operation, and field effect mobility of 0.16 cm2/(V s) and on-to-off drain current ratio of ∼1 × 102 were observed in the TFT device annealed at PO2 of 0.04 Torr. This study presented the potential of the solution-based process of the Cu2O TFT with p-channel characteristics for the first time.

p-Channel oxide thin film transistors using solution-processed copper oxide.

The bottle neck of ReRAM (Resistive RAM) for post-NAND storage application is high operational current [1,2]. Herein, we report a method to acquire low operational currents from a hetero structure ReRAM (AlOx/TiOx). The mechanism study of the hetero structure ReRAM reveals that the AlOx layer as a tunnel barrier is critical for switching, and thus switching parameters are governed by the properties of the AlOx layer. By tuning tunnel oxide properties along with adopting 5 nm sized “Dash BE” [3], operational currents of ≤ 10 µA have been achieved from this hetero structure device.

Low power operating bipolar TMO ReRAM for sub 10 nm era

Ultraviolet light emitting diode based on p-NiO/n-ZnO nanowire heterojunction

ZnO nanorods doped with In or Ga were synthesized by a facile hydrothermal process on sol–gel deposited ZnO seed layers. The effects of dopants on the morphology, chemical bonding structure, and optical property of the ZnO nanorods were discussed based on the field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) results. XRD and XPS results indicated that In or Ga ions were doped in the ZnO lattice. Undoped and In- or Ga-doped ZnO nanorods had a hexagonal pillar shape and a single crystal wurtzite structure with (001) growth direction. For the doped samples, the average diameter of the nanorods increased, while the number of the nanorods decreased, compared to the undoped sample. In the PL spectra, the near band edge (NBE) emission peaks of both doped samples shifted to a lower wavelength. On the other hand, the intensity of visible emission increased for the In-doped sample, but decreased for the Ga-doped sample. We considered that this variation was related to the different concentrations of the oxygen vacancies between the samples, which had bond strengths of Ga–O and In–O different from that of Zn–O.

Effects of In or Ga doping on the growth behavior and optical properties of ZnO nanorods fabricated by hydrothermal process

Highly sensible ultraviolet (UV) photodetectors were fabricated using vertically aligned dense ZnO nanowires based on mechanical Schottky contact with Pt coated polyimides. The effective mechanical contact was achieved by depositing transparent Al-doped ZnO (AZO) buffer layers prior to the growth of the nanowires, resulting in highly uniform nanowires with similar lengths. The AZO buffer and ZnO nanowires were prepared by atomic layer deposition (ALD) and metalorganic chemical vapor deposition, respectively. The electrical conductivity of the nanowires was varied by controlling the density of oxygen vacancies by using a controlled oxygen flow rate. The high conducting nanowire-based photodetector concurrently achieved a high photocurrent signal, fast response, and fast recovery, compared with high conducting film and low conducting nanowire photodetectors. A detailed discussion regarding the developed UV photo-detecting mechanism by the different conductivities and Schottky contact is presented in this work.

Yong Hun Kwon

Papers

p-Channel oxide thin film transistors using solution-processed copper oxide.

Low power operating bipolar TMO ReRAM for sub 10 nm era

Ultraviolet light emitting diode based on p-NiO/n-ZnO nanowire heterojunction

Effects of In or Ga doping on the growth behavior and optical properties of ZnO nanorods fabricated by hydrothermal process

Highly Sensible ZnO Nanowire Ultraviolet Photodetectors Based on Mechanical Schottky Contact