Fabrication Of Field-effect Transistors Research Articles

Wafer-scalable, aligned carbon nanotube transistors operating at frequencies of over 100 GHz

Wireless device technology operating in the millimetre-wave regime (30 to 300 GHz) increasingly needs to offer both high performance and a high level of integration with complementary metal–oxide–semiconductor (CMOS) technology. Aligned carbon nanotubes are proposed as an alternative to III–V technologies in such applications because of their highly linear signal amplification and compatibility with CMOS. Here we report the wafer-scalable fabrication of aligned carbon nanotube field-effect transistors operating at gigahertz frequencies. The devices have gate lengths of 110 nm and are capable, in distinct devices, of an extrinsic cutoff frequency and maximum frequency of oscillation of over 100 GHz, which surpasses the 90 GHz cutoff frequency of radio-frequency CMOS devices with gate lengths of 100 nm and is close to the performance of GaAs technology. Our devices also offer good linearity, with distinct devices capable of a peak output third-order intercept point of 26.5 dB when normalized to the 1 dB compression power, and 10.4 dB when normalized to d.c. power. Radio-frequency transistors made from aligned carbon nanotubes offer high linearity and an extrinsic cutoff frequency of over 100 GHz.

Material Design of New p-Type Tin Oxyselenide Semiconductor through Valence Band Engineering and Its Device Application.

This paper reports a new p-type tin oxyselenide (SnSeO), which was designed with the concept that the valence band edge from O 2p orbitals in the majority of metal oxides becomes delocalized by hybridizing Se 4p and Sn 5s orbitals. As the Se loading increased, the SnSeO film structures were transformed from tetragonal SnO to orthorhombic SnSe, which was accompanied by an increase in the amorphous phase portion and smooth morphologies. The SnSe0.56O0.44 film annealed at 300 °C exhibited the highest Hall mobility (μHall), 15.0 cm2 (V s)-1, and hole carrier density (nh), 1.2 × 1017 cm-3. The remarkable electrical performance was explained by the low hole effective mass, which was calculated by a first principle calculation. Indeed, the fabricated field-effect transistor (FET) with a p-channel SnSe0.56O0.44 film showed the high field-effect mobility of 5.9 cm2 (V s)-1 and an ION/OFF ratio of 3 × 102. This work demonstrates that anion alloy-based hybridization provides a facile route to the realization of a high-performance p-channel FET and complementary devices.

A photolithographic method for fabricating electron devices based on MOCVD-grown MoS2

In modern electronics, two-dimensional (2D) materials which possess atomically thin layers and periodic network structures have emerged as a new paradigm of materials with a lot of potentials. To fully realize the important commercial applications that 2D materials in modern electronics, one of the key issues, such as development of adequate lithographic processing for 2D materials, must be resolved. Here, we report an unprecedented and reliable photolithographic process for large-area patterning of molybdenum disulfide (MoS2) films on both SiO2/Si and polymer substrates, as well as a lift-off of deposited metals on MoS2 films using an Irgacure 651-doped poly(methyl methacrylate) resist and a water-free developer. To verify the feasibility of our process, the fabrication and device performance of MoS2 field-effect transistors (FETs) is also presented. We expect the proposed method to provide a reliable route to device fabrication with 2D materials and to be an important step toward their commercialization.

Solution-processed organic field-effect transistors using directed assembled carbon nanotubes and 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT)

Achieving low-cost fabrication of organic field-effect transistors (OFETs) has long been pursued in the semiconductor industry. Solution-based process allows the fabrication of OFETs cost-effective because of its merit of vacuum-free and room temperature operation. Here, we show a facile and scalable fabrication of solution-processed OFETs using carbon nanotube (CNT) as source/drain electrodes and 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) as semiconducting layer on silicon as well as on flexible and transparent polyethylene terephthalate (PET) substrates. The CNT electrodes and the C8-BTBT film are fabricated using a dip coating-based directed assembly process, and two dip coating parameters, the pulling speed and the solution concentration, are carefully chosen so that the thickness of the C8-BTBT film is close to that of the CNT electrodes. The fabricated OFET devices show typical p-channel behavior. Low-cost, ease of processing, wafer level scalability and good compatibility with various substrates make the fabrication process presented in this paper well suited for next-generation electronics and sensors.

Robust sandwiched fluorinated graphene for highly reliable flexible electronics

The high sensitivity of graphene to the surface condition of the gate dielectric layer and its poor van der Waals adhesion with a flexible substrate result in interfacial sliding and fracturing of graphene at low strains, making the successful utilization of pristine graphene (PG) in flexible electronics challenging. Here, we report a facile method for the fabrication of flexible graphene field effect transistors (F-GFETs) using sandwiched fluorinated graphene (FG). The “FG-PG-FG” sandwich structure shows a high optical transparency (>94%) with an average carrier mobility above 340 cm2/V·s, higher than that obtained when GO and Ion gel were used as gate dielectric materials on F-GFETs and a relatively low gate leakage current of ~160 pA. Furthermore, we observed a high mechanical stability, retaining >88% of the original current output against bending deformation of up to 6 mm and >77% after 200 bending cycles by applying a tensile strain of 1.56%, compared to the control sample. This improved performance is attributed to the fact that the sandwiched FG provides a good dielectric environment by tuning the C/F ratio, which tightly fixes the PG under strain. These findings provide a new route for the future development of graphene-based flexible electronics.

Atomic Layer Deposition of Al2O3 with Alcohol Oxidants for Impeding Substrate Oxidation

Atomic layer deposition (ALD) is a vapor phase thin film deposition technique based on self-limited surface reaction. ALD processes consist of two (or more than) half-reactions. The first half-reaction is the self-limiting adsorption of precursor molecules which contain core metal atoms; the second half reaction is the self-limiting reaction between surface adsorbed precursor molecules and reactants. Since ALD could deposit thin films with high quality, good uniformity, high conformality, and sub-nanometer thickness controllability, [1-3] ALD has been regarded as one of the most suitable deposition technologies for semiconductor device fabrication. Since thin films of alumina (Al2O3) have wide range of applications such as high-k dielectric material for electronic devices, mechanical and chemical protective coatings, diffusion barriers, and optical coatings, [4-7] ALD of Al2O3 process is the mostly and thoroughly studied. For the deposition of Al2O3, trimethylaluminum (TMA) have been the most widely used for Al precursor due to its high vapor pressure and reactivity. And water (H2O) is widely utilized as the oxygen source in ALD Al2O3 processes, because since H2O often shows facile ligand exchange reaction during ALD. However, ALD Al2O3 processes with H2O reactant showed undesirable substrate oxidation issue. For example, there was an unwanted interface oxide between ALD deposited Al2O3 film and Si substrate. The interface oxide could reduce the dielectric constant of the deposited thin films and increase leakage current density. [8-9] Especially, the oxidation of substrate is critical issue for 2 dimensional (2D) transition-metal dichalcogenides (TMDCs) based field effect transistors (FETs). From our previous results, the oxidation of MoS2 by H2O considerably degraded device performance. [10-11] To avoid the oxidation of substrates and improve device performance, it is necessary to develop a new ALD process by using oxidants with lower oxidation potential than that of H2O. Despite its technical importance, ALD Al2O3 processes with weaker oxidants such as alcohols have rarely been investigated. [8] For this reason, the chemical reaction mechanism between surface adsorbed precursor and reactant has not been clearly identified. In this work, we fundamentally investigated ALD process Al2O3 on Si substrate, using TMA and various alcohol oxidants (methanol (MeOH), ethanol (EtOH), and n-propanol (n-PrOH)). Furthermore, we investigate the reaction mechanism of various alcohol oxidants during ALD of Al2O3 with TMA. Density functional theory (DFT) calculations at B97D3 level of theory were performed using Gaussian 09 suite of programs. Our developed ALD processes showed typical ALD growth characteristics. The saturated growth rates with MeOH, EtOH and n-PrOH were 0.10, 0.96, and 0.74 Å/cycle, respectively. From the calculation results, we revealed that the beta-hydrogen transfer reaction of EtOH and n-PrOH could easily oxidize surface methyl group into surface hydroxyl. The results could be applicable to highly integrated semiconductor devices fabrication and 2D TMDC based FET fabrication processes.

Modulating the Charge Transport in 2D Semiconductors via Energy-Level Phototuning.

The controlled functionalization of semiconducting 2D materials (2DMs) with photoresponsive molecules enables the generation of novel hybrid structures as active components for the fabrication of high-performance multifunctional field-effect transistors (FETs) and memories. This study reports the realization of optically switchable FETs by decorating the surface of the semiconducting 2DMs such as WSe2 and black phosphorus with suitably designed diarylethene (DAE) molecules to modulate their electron and hole transport, respectively, without sacrificing their pristine electrical performance. The efficient and reversible photochemical isomerization of the DAEs between the open and the closed isomer, featuring different energy levels, makes it possible to generate photoswitchable charge trapping levels, resulting in the tuning of charge transport through the 2DMs by alternating illumination with UV and visible light. The device reveals excellent data-retention capacity combined with multiple and well-distinguished accessible current levels, paving the way for its use as an active element in multilevel memories.

Investigation of Electrode Electrochemical Reactions in CH3 NH3 PbBr3 Perovskite Single-Crystal Field-Effect Transistors.

Optoelectronic devices based on metal halide perovskites, including solar cells and light-emitting diodes, have attracted tremendous research attention globally in the last decade. Due to their potential to achieve high carrier mobilities, organic-inorganic hybrid perovskite materials can enable high-performance, solution-processed field-effect transistors (FETs) for next-generation, low-cost, flexible electronic circuits and displays. However, the performance of perovskite FETs is hampered predominantly by device instabilities, whose origin remains poorly understood. Here, perovskite single-crystal FETs based on methylammonium lead bromide are studied and device instabilities due to electrochemical reactions at the interface between the perovskite and gold source-drain top contacts are investigated. Despite forming the contacts by a gentle, soft lamination method, evidence is found that even at such "ideal" interfaces, a defective, intermixed layer is formed at the interface upon biasing of the device. Using a bottom-contact, bottom-gate architecture, it is shown that it is possible to minimize such a reaction through a chemical modification of the electrodes, and this enables fabrication of perovskite single-crystal FETs with high mobility of up to ≈15 cm2 V-1 s-1 at 80 K. This work addresses one of the key challenges toward the realization of high-performance solution-processed perovskite FETs.

Selective growth of uniform single-layer graphene on Cu foil and fabrication of damage-free field effect transistor combining with direct transfer

During selective growth of graphene by using silicon dioxide (SiO2) patterns on Cu foil (SOCF), multilayer graphene was grown on SOCF under the same conditions that are used to synthesize single-layer graphene (SLG) on blank Cu foil. The authors demonstrated that oxygen (O2) species that can be released from the SiO2 film did not affect the layer increase and that the SiO2 film of SOCF reduced the area of the exposed Cu surface and thereby increased the relative concentration of hydrogen (H2) to the Cu surface and initially grown graphene; as a result, extra graphene layers grew on SOCF. By adjusting the H2 supply and SiO2 coverage, uniformly-grown SLG patterns were obtained on SOCF. A damage-free graphene field effect transistor (GFET) was fabricated using selectively-grown SLG and direct transfer using parylene-C. The field effect mobility of the GFET was 7538.81 cm2/(V s), which is quite high compared to those of chemical vapor deposition based GFETs on flexible substrates that have been reported.

A Facile and Effective Method for Patching Sulfur Vacancies of WS2 via Nitrogen Plasma Treatment.

Although transition metal dichalcogenides (TMDs) are attractive for the next-generation nanoelectronic era due to their unique optoelectronic and electronic properties, carrier scattering during the transmission of electronic devices, and the distinct contact barrier between the metal and the semiconductors, which is caused by inevitable defects in TMDs, remain formidable challenges. To address these issues, a facile, effective, and universal patching defect approach that uses a nitrogen plasma doping protocol is developed, via which the intrinsic vacancies are repaired effectively. To reveal sulfur vacancies and the nature of the nitrogen doping effects, a high-resolution spherical aberration corrected scanning transmission electron microscopy is used, which confirms the N atoms doping in sulfur vacancies. In this study, a typical TMD material, namely tungsten disulfide, is employed to fabricate field-effect transistors (FETs) as a preliminary paradigm to demonstrate the patching defects method. This doping method endows FETs with high electrical performance and excellent contact interface properties. As a result, an electron mobility of up to 184.2 cm2 V-1 s-1 and a threshold voltage of as low as 3.8 V are realized. This study provides a valuable approach to improve the performance of electronic devices that are based on TMDs in practical electronic applications.

Boosting the ambipolar field-effect transistor performance of a DPP-based copolymer via electrohydrodynamic-jet direct writing

The electrohydrodynamic jet (EHD) printing technique allows for the direct printing of semiconducting polymer solutions to form desired patterns. Herein, an EHD-printed diketopyrrolopyrrole (DPP)-based copolymer, PTDPPSe-SiC5, was used as the active layer for the fabrication of ambipolar organic field-effect transistors (OFETs). Under optimized EHD conditions, the measured hole and electron mobilities of the PTDPPSe-SiC5 OFETs were 0.75 and 1.08cm2/(Vs), which led to a better electrical performance than that displayed by OFETs containing the same polymer but deposited using the spin-coating technique. Morphological analyses were carried out to help explain this difference in OFET performance, and showed the deployment of the EHD printing technique resulting in an improved polymer alignment and crystallinity, despite being performed without the use complex guided surface templates or additional pre- or post-deposition processing.

Molecular Beam Epitaxy Scalable Growth of Wafer-Scale Continuous Semiconducting Monolayer MoTe2 on Inert Amorphous Dielectrics.

Monolayer MoTe2 , with the narrowest direct bandgap of ≈1.1 eV among Mo- and W-based transition metal dichalcogenides, has attracted increasing attention as a promising candidate for applications in novel near-infrared electronics and optoelectronics. Realizing 2D lateral growth is an essential prerequisite for uniform thickness and property control over the large scale, while it is not successful yet. Here, layer-by-layer growth of 2 in. wafer-scale continuous monolayer 2H-MoTe2 films on inert SiO2 dielectrics by molecular beam epitaxy is reported. A single-step Mo-flux controlled nucleation and growth process is developed to suppress island growth. Atomically flat 2H-MoTe2 with 100% monolayer coverage is successfully grown on inert 2 in. SiO2 /Si wafer, which exhibits highly uniform in-plane structural continuity and excellent phonon-limited carrier transport behavior. The dynamics-controlled growth recipe is also extended to fabricate continuous monolayer 2H-MoTe2 on atomic-layer-deposited Al2 O3 dielectric. With the breakthrough in growth of wafer-scale continuous 2H-MoTe2 monolayers on device compatible dielectrics, batch fabrication of high-mobility monolayer 2H-MoTe2 field-effect transistors and the three-level integration of vertically stacked monolayer 2H-MoTe2 transistor arrays for 3D circuitry are successfully demonstrated. This work provides novel insights into the scalable synthesis of monolayer 2H-MoTe2 films on universal substrates and paves the way for the ultimate miniaturization of electronics.

Efficient combination of Roll-to-Roll compatible techniques towards the large area deposition of a polymer dielectric film and the solution-processing of an organic semiconductor for the field-effect transistors fabrication on plastic substrate

Abstract Flexible Organic Field-Effect Transistors (OFETs) constitute nowadays a highly promising field of the organic and printed electronics due to their multiple applications (flexible displays, sensors etc.). However, their cost-effective fabrication by large area Roll-to-Roll compatible printing methods still remains a challenge for their integration to commercial products. In this work, the moderate speed (1 m/min) process of the flexible cross-linked Poly(4-vinyl phenol) (PVP) polymer gate dielectric layer was carried out by integrating a slot-die method that mimics the Roll-to-Roll (R2R) coating conditions, in combination to an airbrush spray method for the solution-processing of the 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-PEN) organic semiconductor. Particularly, the PVP dielectric was slot-die-coated over a 90 × 15 cm2 substrate, while subsequently the TIPS-PEN semiconductor was sprayed onto the 15 × 20 mm2 cPVP-patterned plastic substrates. Α surface investigation study on the well-formed slot-die-coated cPVP strips was conducted, revealing desirable dielectric film topography as a result of the good control over the coating process. A detailed analysis of the cPVP thickness evolution along the 90 × 1.3 cm2 patterned stripe, was carried out. The morphological analysis of the sprayed TIPS-PEN layer over the cPVP film revealed well-organized large crystalline domains across the channel area, as a result of the sufficient crystallization time and the excellent cPVP surface morphology. The fabricated bottom gate/top contact flexible OFETs exhibited excellent I-V electrical characteristics with a maximum mobility of 0.21 cm2/V, negligible hysteresis, low threshold voltages (average value of −0.1 V) and on/off current ratios in the range of 103 to >104. These results demonstrate the potentiality of the proposed scalable methods for the large scale fabrication of high performance low-cost OFET devices.

Electric Double Layer Field-Effect Transistors Using Two-Dimensional (2D) Layers of Copper Indium Selenide (CuIn7Se11)

Innovations in the design of field-effect transistor (FET) devices will be the key to future application development related to ultrathin and low-power device technologies. In order to boost the current semiconductor device industry, new device architectures based on novel materials and system need to be envisioned. Here we report the fabrication of electric double layer field-effect transistors (EDL-FET) with two-dimensional (2D) layers of copper indium selenide (CuIn7Se11) as the channel material and an ionic liquid electrolyte (1-Butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6)) as the gate terminal. We found one order of magnitude improvement in the on-off ratio, a five- to six-times increase in the field-effect mobility, and two orders of magnitude in the improvement in the subthreshold swing for ionic liquid gated devices as compared to silicon dioxide (SiO2) back gates. We also show that the performance of EDL-FETs can be enhanced by operating them under dual (top and back) gate conditions. Our investigations suggest that the performance of CuIn7Se11 FETs can be significantly improved when BMIM-PF6 is used as a top gate material (in both single and dual gate geometry) instead of the conventional dielectric layer of the SiO2 gate. These investigations show the potential of 2D material-based EDL-FETs in developing active components of future electronics needed for low-power applications.

Field Effect Transistors Based on One-Dimensional, Metal-Oxide Semiconducting Nanofiber Mats

Metal-oxide semiconductors are very promising alternatives to amorphous and polycrystalline Si semiconductors. In addition to thin-film metal-oxide semiconductors, nanostructured metal-oxide semiconductors have also attracted considerable attention due to their interesting physical properties for versatile potential applications. Here, we report on metal-oxide semiconducting (InZnO and HfInZnO) nanofiber mats prepared by electrospinning. Thermal gravimetric analyses (TGA) and X-ray diffraction (XRD) measurements were carried out so as to investigate the phase transition from as-spun metal precursor/polymer matrix composite nanofibers mats to metal-oxide nanofiber mats. We fabricated field effect transistor (FETs) based on InZnO and HfInZnO nanofiber mats, and both FETs showed proper n-type transfer and output characteristics. FETs based on InZnO nanofiber mats-based FETs exhibited superior device performance: negligible hysteresis, a decent electron mobility of 5.49 cm2/Vs, and a good on/off current ratio of 107.

Fabrication of Top-Gate MoS2 FET with Transferred Al2O3 Gate Dielectric

This study describes the fabrication of top-gate molybdenum disulfide (MoS2) field-effect transistor (FET) using transfer printing of an Al2O3 gate dielectric. Semiconducting MoS2 has been considerable attention for applications as channel materials of FETs because a two-dimensional (2D) crystal is free from dangling bonds at the surface [1]. On the other hand, atomic layer deposition (ALD) of high-k gate dielectric is incompatible with MoS2 since an ALD precursor cannot be chemisorbed onto the surface of MoS2 due to the absence of dangling bonds [2]. Furthermore, ALD disrupts the surface structure and induces the oxidation of the channel layer [2], leading to the degradation of FET characteristics or the failure of devices. This study addresses these issues by utilizing the transfer printing of a high-k gate dielectric on a MoS2 layer [2], [3]. Transfer printing can avoid the deposition of a high-k gate dielectric directly on a fragile MoS2 surface. An abrupt and sharp interface can be anticipated by implementing the transfer printing of high-k gate dielectric.A key component for developing the transfer printing of the Al2O3 gate dielectric is to find out an appropriate sacrificial layer on the handling substrate. In this study, nickel (Ni) metal was adopted as the sacrificial layer for the transfer printing of the Al2O3 gate dielectric. It was revealed that the adhesion strength at Ni and thermally grown SiO2 interface is reduced by immersion in deionized water (DIW) [3]. The penetration of water into the interface results in the separation of Ni from the SiO2 surface. This Ni layer can be etched by using dilute nitric acid (HNO3). On the other hand, HNO3, which is an oxidizing agent, cannot etch an oxide gate dielectric.A 50-nm-thick Ni layer was deposited on the SiO2 (400 nm)/Si substrate by thermal evaporation (Fig. 1 (a)). The Al2O3 gate dielectric was deposited on the Ni layer by ALD with trimethylaluminum (TMA) and a H2O oxidant at a substrate temperature of 300 oC. Polymethyl methacrylate (PMMA) was prepared on Al2O3 by spin-coating as a protective layer. Next, a thermal release tape was attached to the PMMA layer. Then, the entire substrate was immersed in DIW for 5 min to weaken the adhesion strength between Ni and SiO2. The laminated structure was peeled off from the SiO2 surface in DIW. After the etching of Ni layer with dilute HNO3 (60% HNO3 : H2O = 1 : 10), the Al2O3 gate dielectric was transferred on MoS2 flakes. The Al2O3 gate dielectric and PMMA bilayer can be released from the thermal release tape by baking on a hot plate. After the removal of PMMA by oxygen plasma ashing, Al (40 nm) for the gate and Au (40 nm)/Ti (10 nm) for the source/drain were fabricated by the lift-off process. Finally, forming gas annealing (FGA) was performed in ambient (H2 : N2 = 3% : 97%) at 300 oC for 30 min to improve the electrical contacts at source/drain.The fabricated FET has small hysteresis, low leakage current, a subthreshold slope of 120 mV/dec, and a carrier mobility of 7.3 cm2/Vs (Fig. 1 (b)). The abrupt and sharp interface was demonstrated for transfer printing of the Al2O3 gate dielectric on the MoS2 channel layer. Process mismatches including temperature and chemical compatibility are avoidable by adopting the transfer printing method. The use of transferred high-k gate dielectrics is not restricted to MoS2 FETs. It is also applicable to various 2D layered materials. This study opens up the interesting direction for the research and development of 2D material-based functional devices. AcknowledgmentsThe authors would like to thank Professor Y. Kawano, Professor T. Hoshii, Professor I. Muneta, Professor K. Kakushima, Professor K. Tsutsui, and Professor H. Wakabayashi of Tokyo Institute of Technology for their continuous support in the experiments. This study was supported by JST CREST (Grant No. JPMJCR16F4) and a JSPS Grant-in-Aid for Young Scientists (B) (Grant No. 17K14662).

Towards pharmacological treatment screening of cardiomyocyte cells using Si nanowire FETs

Silicon nanowires (Si NWs) are the most promising candidates for recording biological signals due to improved interfacing properties with cells and the possibility of high-speed transduction of biochemical signals into detectable electrical responses. The recording of extracellular action potentials (APs) from cardiac cells is important for fundamental studies of AP propagation features reflecting cell activity and the influence of pharmacological substances on the signal. We applied a novel approach of using fabricated Si NW field-effect transistors (FETs) in combination with fluorescent marker techniques to evaluate the functional activity of cardiac cells. Extracellular AP signal recording from HL-1 cardiomyocytes was demonstrated. This method was supplemented by studies of the pharmacological effects of stimulations using noradrenaline (NorA) as a modulator of functional activity on a cellular and subcellular levels, which were also tested using fluorescent marker techniques. The role of calcium alteration and membrane potential were revealed using Fluo-4 AM and tetramethylrhodamine, methyl ester, perchlorate (TMRM) fluorescent dyes. In addition, chemical treatment with sodium dodecyl sulfate (SDS) solutions was tested. The results obtained demonstrate positive prospects for AP monitoring in different treatments for studies related to a wide range of myocardial diseases using lab-on-chip technology.

Two-dimensional pnictogens: A review of recent progresses and future research directions

Soon after the synthesis of two-dimensional (2D) ultrathin black phosphorus and fabrication of field effect transistors thereof, theoretical studies have predicted that other group-VA elements (or pnictogens), N, As, Sb, and Bi can also form stable, single-layer (SL) structures. These were nitrogene in a buckled honeycomb structure, arsenene, antimonene, and bismuthene in a buckled honeycomb, as well as washboard and square-octagon structures with unusual mechanical, electronic, and optical properties. Subsequently, theoretical studies are followed by experimental efforts that aim at synthesizing these novel 2D materials. Currently, research on 2D pnictogens has been a rapidly growing field revealing exciting properties, which offers diverse applications in flexible electronics, spintronics, thermoelectrics, and sensors. This review presents an evaluation of the previous experimental and theoretical studies until 2019, in order to provide input for further research attempts in this field. To this end, we first reviewed 2D, SL structures of group-VA elements predicted by theoretical studies with an emphasis placed on their dynamical and thermal stabilities, which are crucial for their use in a device. The mechanical, electronic, magnetic, and optical properties of the stable structures and their nanoribbons are analyzed by examining the effect of external factors, such as strain, electric field, and substrates. The effect of vacancy defects and functionalization by chemical doping through adatom adsorption on the fundamental properties of pnictogens has been a critical subject. Interlayer interactions in bilayer and multilayer structures, their stability, and tuning their physical properties by vertical stacking geometries are also discussed. Finally, our review is concluded by highlighting new research directions and future perspectives on the challenges in this emerging field.

Very large remanent polarization in ferroelectric Hf1-xZrxO2 grown on Ge substrates by plasma assisted atomic oxygen deposition

Plasma assisted atomic oxygen deposition was used to grow polycrystalline ferroelectric Hf1-xZrxO2 (x = 0.5–0.7) on technologically important (100) Germanium substrates showing sharp crystalline interfaces free of interfacial amorphous layers and strong evidence for the presence of a predominately orthorhombic phase. The electrical properties, evaluated using metal-ferroelectric-semiconductor (MFS) capacitors, show symmetric and robust ferroelectric hysteresis with weak or no wake-up effects. The MFS capacitors with x = 0.58 show very large remanent polarization up to 34.4 μC/cm2 or 30.6 μC/cm2 after correction for leakage and parasitics, combined with good endurance reaching 105 cycles at a cycling field of 2.3 MV/cm. The results show good prospects for the fabrication of Ge ferroelectric field effect transistors (FeFETs) for use in 1 T FeFET embedded nonvolatile memory cells with improved endurance.

Fabrication of organic field effect transistors using directed assembled and transfer printed carbon nanotube source/drain electrodes

Printing processes are increasingly used in the fabrication of organic field effect transistors (OFETs) owing to their merits of low cost, low processing temperature, vacuum-free nature, and compatibility with various substrates. Among various printing processes, transfer printing is preferable because of its high resolution and pattern fidelity. Here, we fabricate OFETs using directed assembled and transfer printed carbon nanotube (CNT) source/drain electrodes. The CNT electrodes are selectively assembled on a Damascene template using an electrophoretic directed assembly based inking process and then transferred onto a polyurethane film on a silicon substrate with a high pattern fidelity. Selective inking and direct transfer without sacrificial layers or intermediate transfer steps enable a high material efficiency and a low cost of our fabrication process. The fabricated OFETs with a spin-coated 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene semiconducting film show typical p-type behavior with an on/off ratio of approximately 105, implying great potential to use our method for practical OFET fabrication.