Semiconductor nanowires have attracted extensive interest as one of the best-defined classes of nanoscale building blocks for the bottom-up assembly of functional electronic and optoelectronic devices over the past two decades. The article provides a comprehensive review of the continuing efforts in exploring semiconductor nanowires for the assembly of functional nanoscale electronics and macroelectronics. Specifically, we start with a brief overview of the synthetic control of various semiconductor nanowires and nanowire heterostructures with precisely controlled physical dimension, chemical composition, heterostructure interface, and electronic properties to define the material foundation for nanowire electronics. We then summarize a series of assembly strategies developed for creating well-ordered nanowire arrays with controlled spatial position, orientation, and density, which are essential for constructing increasingly complex electronic devices and circuits from synthetic semiconductor nanowires. Next, we review the fundamental electronic properties and various single nanowire transistor concepts. Combining the designable electronic properties and controllable assembly approaches, we then discuss a series of nanoscale devices and integrated circuits assembled from nanowire building blocks, as well as a unique design of solution-processable nanowire thin-film transistors for high-performance large-area flexible electronics. Last, we conclude with a brief perspective on the standing challenges and future opportunities.

Nanowire Electronics: From Nanoscale to Macroscale

Recent years have witnessed GaN-based devices delivering their promise of unprecedented power and frequency levels and demonstrating their capability as an able replacement for Si-based devices. High-electron-mobility transistors (HEMTs), a key representative architecture of GaN-based devices, are well-suited for high-power and high frequency device applications, owing to highly desirable III-nitride physical properties. However, these devices are still hounded by issues not previously encountered in their more established Siand GaAs-based devices counterparts. Metal–insulator–semiconductor (MIS) structures are usually employed with varying degrees of success in sidestepping the major problematic issues such as excessive leakage current and current instability. While different insulator materials have been applied to GaN-based transistors, the properties of insulator/III-N interfaces are still not fully understood. This is mainly due to the difficulty of characterizing insulator/AlGaN interfaces in a MIS HEMT because of the two resulting interfaces: insulator/AlGaN and AlGaN/GaN, making the potential modulation rather complicated. Although there have been many reports of low interface-trap densities in HEMT MIS capacitors, several papers have incorrectly evaluated their capacitance–voltage (C–V) characteristics. A HEMT MIS structure typically shows a 2-step C–V behavior. However, several groups reported C–V curves without the characteristic step at the forward bias regime, which is likely to the high-density states at the insulator/ AlGaN interface impeding the potential control of the AlGaN surface by the gate bias. In this review paper, first we describe critical issues and problems including leakage current, current collapse and threshold voltage instability in AlGaN/GaN HEMTs. Then we present interface properties, focusing on interface states, of GaN MIS systems using oxides, nitrides and high-κ dielectrics. Next, the properties of a variety of AlGaN/GaN MIS structures as well as different characterization methods, including our own photo-assisted C–V technique, essential for understanding and developing successful surface passivation and interface control schemes, are given in the subsequent section. Finally we highlight the important progress in GaN MIS interfaces that have recently pushed the frontier of nitride-based device technology.

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Insulated gate and surface passivation structures for GaN-based power transistors

Artificial photosynthesis via solar water splitting provides a promising approach to storing solar energy in the formof hydrogen on a global scale.However, an efficient and cost-effective solar hydrogen production system that can compete with traditionalmethods using fossil fuels is yet to be developed. A photoelectrochemical (PEC) tandem cell consisting of a p-type photocathode and an n-type photoanode, with the photovoltage provided by the two photoelectrodes, is an attractive route to achieve highly efficient unassistedwater splitting at a low cost. In this article, we provide an overview of recent developments of semiconductormaterials, includingmetal oxides, nitrides, chalcogenides, Si, III–Vcompounds and organics, either as photocathodes or photoanodes for water reduction and oxidation, respectively. In addition, recent efforts in constructing a PEC tandem system for unassisted water splitting are outlined. The importance of developing a single-photon photocathode and photoanode that can deliver high photocurrent in the lowbias region for efficient PEC tandem system is highlighted. Finally, we discuss the future development of photoelectrodematerials, and viable solutions to realize highly efficient PECwater splitting device for practical applications.

https://iopscience.iop.org/article/10.1088/2399-1984/aa88a1/pdf

Roadmap on solar water splitting: current status and future prospects

Within its rich phase diagram titanium dioxide is a truly multifunctional material with a property palette that has been shown to span from dielectric to transparent-conducting characteristics, in addition to the well-known catalytic properties. At the same time down-scaling of microelectronic devices has led to an explosive growth in research on atomic layer deposition (ALD) of a wide variety of frontier thin-film materials, among which TiO2 is one of the most popular ones. In this topical review we summarize the advances in research of ALD of titanium dioxide starting from the chemistries of the over 50 different deposition routes developed for TiO2 and the resultant structural characteristics of the films. We then continue with the doped ALD-TiO2 thin films from the perspective of dielectric, transparent-conductor and photocatalytic applications. Moreover, in order to cover the latest trends in the research field, both the variously constructed TiO2 nanostructures enabled by ALD and the Ti-based hybrid inorganic-organic films grown by the emerging ALD/MLD (combined atomic/molecular layer deposition) technique are discussed. CONTENTS

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Titanium dioxide thin films by atomic layer deposition: a review

A controlled field-effect passivation by a well-defined density of fixed charges is crucial for modern solar cell surface passivation schemes Al2O3 nanolayers grown by atomic layer deposition contain negative fixed charges Electrical measurements on slant-etched layers reveal that these charges are located within a 1 nm distance to the interface with the Si substrate When inserting additional interface layers, the fixed charge density can be continuously adjusted from 35 × 1012 cm–2 (negative polarity) to 00 and up to 40 × 1012 cm–2 (positive polarity) A HfO2 interface layer of one or more monolayers reduces the negative fixed charges in Al2O3 to zero The role of HfO2 is described as an inert spacer controlling the distance between Al2O3 and the Si substrate It is suggested that this spacer alters the nonstoichiometric initial Al2O3 growth regime, which is responsible for the charge formation On the basis of this charge-free HfO2/Al2O3 stack, negative or positive fixed charges can be formed by i

On the Control of the Fixed Charge Densities in Al2O3-Based Silicon Surface Passivation Schemes

Germanium is a promising material for future very large scale integration transistors, due to its superior hole mobility. However, germanium-based devices typically suffer from high reverse junction leakage due to the low band-gap energy of 0.66 eV and therefore are characterized by high static power dissipation. In this paper, we experimentally demonstrate a solution to suppress the off-state leakage in germanium nanowire Schottky barrier transistors. Thereto, a device layout with two independent gates is used to induce an additional energy barrier to the channel that blocks the undesired carrier type. In addition, the polarity of the same doping-free device can be dynamically switched between p- and n-type. The shown germanium nanowire approach is able to outperform previous polarity-controllable device concepts on other material systems in terms of threshold voltages and normalized on-currents. The dielectric and Schottky barrier interface properties of the device are analyzed in detail. Finite-element...

Enabling Energy Efficiency and Polarity Control in Germanium Nanowire Transistors by Individually Gated Nanojunctions

Symmetrical Al2O3-based passivation layers for p- and n-type silicon

Atomic layer deposition-grown Al2O3 thin films are grown on n-type GaN and annealed at 300 or 500 °C in various atmospheres. Metal–insulator–semiconductor capacitors (MISCAPs) are used as simplified test structures for AlGaN/GaN heterostructure field effect transistors with an Al2O3 gate dielectric. Electrical characterization of the unannealed MISCAPs reveals a low leakage current density of ∼1.4 × 10−9 A/cm2 at −2 MV/cm. Annealing at 500 °C in N2 or a forming gas results in a degradation of this leakage level by more than one order of magnitude, whereas the leakage current of the Al2O3 films annealed at 500 °C in O2 is increased to ∼5.2 × 10−9 A/cm2 at −2 MV/cm. The photoassisted capacitance–voltage technique, the conductance method, and border trap analysis are used to study the influence of the annealing ambient atmosphere upon the Al2O3/GaN interface. For all atmospheres, thermal treatments at 500 °C marginally affects the border oxide trap density, but the forming gas anneal at 500 °C passivates the...

Impact of postdeposition annealing upon film properties of atomic layer deposition-grown Al2O3 on GaN

Reconfigurable nanowire transistors provide the operation of unipolar p-type and n-type FETs freely selectable within a single device. The enhanced functionality is enabled by controlling the currents through two individually gated Schottky junctions. Here we analyze the impact of the Schottky barrier height on the symmetry of Silicon nanowire RFET transfer characteristics and their performance within circuits. Prospective simulations are carried out, indicating that germanium nanowire based RFETs of the same dimensions will show a distinctly increased performance, making them a promising material solution for future reconfigurable electronics.

Paul M. Jordan

Papers

On the Control of the Fixed Charge Densities in Al2O3-Based Silicon Surface Passivation Schemes

Enabling Energy Efficiency and Polarity Control in Germanium Nanowire Transistors by Individually Gated Nanojunctions

Symmetrical Al2O3-based passivation layers for p- and n-type silicon

Impact of postdeposition annealing upon film properties of atomic layer deposition-grown Al2O3 on GaN

Material Prospects of Reconfigurable Transistor (RFETs)–From Silicon to Germanium Nanowires