Materials For Nanoelectronics Research Articles

Propose two-dimensional Sb2Te2X (X = S, Se) with isotropic electron mobility and remarkable visible-light response.

Two-dimensional (2D) crystals are emerging materials for nanoelectronics, and computationally identifying novel 2D materials with distinct electronic and optical properties furnishes a vital first step for future photovoltaic technology. Herein, based on the density functional theory and Keldysh nonequilibrium Green's function formalism, we reported new members of the family of 2D Group V-VI compounds, i.e., Sb2Te2X (X = S, Se) compounds, which exhibited excellent dynamic and thermal stabilities. It was found that 2D Sb2Te2S and Sb2Te2Se possess moderate band gaps of 0.87 and 0.76 eV, respectively, and they are advantageous over other frequently studied 2D materials. Most surprisingly, it was demonstrated that Sb2Te2X has two excellent characteristics, i.e., high isotropic electron mobility surpassing 103 cm2 V-1 s-1 and remarkable optical absorption over the entire visible region with a high photoresponse (∼0.044 A W-1). The exceptional electronic properties in combination with fascinating optical properties illustrate the great potential of Sb2Te2X, for example, in photovoltaic devices, boosting a new area in the research of Group V-VI 2D semiconductors.

Electronic properties of polymorphic two-dimensional layered chromium disulphide.

Two-dimensional (2D) Cr-based layered and non-layered materials such as CrI3, Cr2Ge2Te6, Cr2S3, CrSe, and CrOX (X = Cl and Br) have attracted considerable attention due to their potential application in spintronics. Despite few experimental studies, theoretical studies reported that 2D chromium dichalcogenide (CrS2) materials show unique properties such as valley polarization, piezoelectric coupling, and phase dependent intrinsic magnetic properties. Here, we report for the first time the synthesis of 2D layered CrS2 flakes down to the monolayer via the chemical vapor deposition (CVD) method, its phase structures and electronic properties. We observed the 2H, 1T, and 1T' phases coexisting in CVD grown monolayer CrS2. The formation of 1T' phases from 1T phases is described by dimerization of metal atoms at room temperature according to our molecular dynamics studies. The coexistence of 1T and 1T' phases with 2H phases is referred to as the 1T and 1T' puddling phenomenon. We theoretically showed that the monolayer 2H-CrS2 is a direct bandgap semiconductor with a gap of approximately 0.95 eV predicted by the PBE functional, while the 1T- and 1T'-CrS2 are metallic and semi-metallic with approximately 10 meV gap, respectively. Furthermore, 2H CrS2 exhibits nonmagnetic semiconducting properties while for ferromagnetic spin configuration, the 1T and 1T' CrS2 show magnetic characteristics with 0.531μB and 2.206μB magnetic moment per Cr atom respectively, for ferromagnetic spin configuration as predicted from DFT+U calculation. Importantly, CrS2-based field-effect transistors exhibit a p-type behavior. Our study would stimulate further exploration of 2D layered CrS2 with astonishing properties and open up a whole new avenue for the urgent need for developing multifunctional 2D materials for nanoelectronics, valleytronics, and spintronics.

Tuning the electronic properties and interfacial interactions of WS2/ZrO2(001) heterostructures by an external electric field, interlayer coupling and monolayer to few–layer of WS2 sheets

Abstract Understanding the connection between interfacial interactions and electronic properties are vital for the fabrication of ZrO2–based optoelectronic and nanoelectronics materials with desirable properties. In this study, the interfacial interactions, optical and electronic properties of WS2/ZrO2(001) heterostructures in the presence of an external electric field, interlayer coupling and monolayer to few–layer WS2 sheets is investigated for the first time using a first–principle calculation. The charge density distribution and Mulliken population charge analysis reveal that the charge carriers can effectively separate in the layered WS2/ZrO2 interface. The optical absorption spectra and band shape of these hybrid heterostructures are similar, with their bandgap decreases, as well as their charge transfer and redshift of the absorption edge increase upon increasing the number of WS2 sheets. Moreover, by varying the interlayer coupling or by applying a suitable external electric field with different strengths, the cohesion energy, work function and bandgap energy of WS2/ZrO2(001) heterostructure can be effectively tuned. These theoretical results are expected to offer useful insights into the design of future optoelectronic and nanoelectronic devices based on two–dimensional van der Waals heterostructures.

A New Method of the Pattern Storage and Recognition in Oscillatory Neural Networks Based on Resistive Switches

Development of neuromorphic systems based on new nanoelectronics materials and devices is of immediate interest for solving the problems of cognitive technology and cybernetics. Computational modeling of two- and three-oscillator schemes with thermally coupled VO2-switches is used to demonstrate a novel method of pattern storage and recognition in an impulse oscillator neural network (ONN), based on the high-order synchronization effect. The method allows storage of many patterns, and their number depends on the number of synchronous states Ns. The modeling demonstrates attainment of Ns of several orders both for a three-oscillator scheme Ns ~ 650 and for a two-oscillator scheme Ns ~ 260. A number of regularities are obtained, in particular, an optimal strength of oscillator coupling is revealed when Ns has a maximum. Algorithms of vector storage, network training, and test vector recognition are suggested, where the parameter of synchronization effectiveness is used as a degree of match. It is shown that, to reduce the ambiguity of recognition, the number coordinated in each vector should be at least one unit less than the number of oscillators. The demonstrated results are of a general character, and they may be applied in ONNs with various mechanisms and oscillator coupling topology.

Graphene Material and Perspectives for Nanoelectronics

The present paper investigates the electrical and physical characteristics of graphene material aiming its use as an active device base material for nanotransistors and nanosensors. The device structures namely nanotransistors are studied and compared to that of silicon NanoMOSFET. The transport mechanism model based essentially on the ballistic transport using the Fermi level variations has been discussed for both device structures SiNanoFET and GNanoFET. The switching mechanisms and the threshold voltage limits are discussed aiming reduction of the switching time and ON/OFF ratio for better device reliability. Finally a perspective on device integration is introduced.

3D Dirac Plasmons in the Type-II Dirac Semimetal PtTe_{2}.

Transition-metal dichalcogenides showing type-II Dirac fermions are emerging as innovative materials for nanoelectronics. However, their excitation spectrum is mostly unexplored yet. By means of high-resolution electron energy loss spectroscopy and density functional theory, here, we identify the collective excitations of type-II Dirac fermions (3D Dirac plasmons) in PtTe_{2} single crystals. The observed plasmon energy in the long-wavelength limit is ∼0.5 eV, which makes PtTe_{2} suitable for near-infrared optoelectronic applications. We also demonstrate that interband transitions between the two Dirac bands in PtTe_{2} give rise to additional excitations at ∼1 and ∼1.4 eV. Our results are crucial to bringing to fruition type-II Dirac semimetals in optoelectronics.

Black phosphorus: an efficient co-catalyst for charge separation and enhanced photocatalytic hydrogen evolution

Black phosphorus (BP), a phosphorus analog of graphene, is a two-dimensional material with the thickness-dependent bandgap and a high carrier mobility, which emerges as a promising material in nanoelectronics and optoelectronics. In this study, BP loaded with surface hydroxyl has been prepared by liquid ultrasonic exfoliation and used as the co-catalyst, which results in 341% photocatalytic hydrogen evolution enhancement to the anatase TiO2 under the simulated sunlight irradiation and a quantum efficiency as high as 18.23% at 365 nm. The improved photocatalytic activity has been ascribed to the Z-scheme mechanism. These results indicate that 2D surface hydroxyl-loaded BP can be used as a broadband photocatalyst by forming type II heterostructures with other wide-bandgap semiconductors.

(Invited) Atomic Layer Deposition for the Synthesis and Integration of 2D Materials for Nanoelectronics

Graphene and other layered 2D materials have been the focus of intense research in the last decade due to their unique physical and chemical properties. This presentation will highlight our recent progress on the synthesis and integration of 2D materials for nanoelectronics applications using atomic layer deposition (ALD). ALD is a chemical process that is based on self-limiting surface reactions and results in ultrathin films, with sub-nm control over the thickness and wafer-scale uniformity. In the first part of this presentation I will focus on the fabrication of low resistance contacts and ultrathin dielectrics to graphene using atomic layer deposition. In the second part I will show how we use plasma enhanced-ALD to synthesize large-area 2D transistion metal dichalcogenides for nanoelectronics applications.

High performance self-gating graphene/MoS2 diode enabled by asymmetric contacts

A graphene-MoS2 (GM) heterostructure based diode is fabricated using asymmetric contacts to MoS2, as well as an asymmetric top gate (ATG). The GM diode exhibits a rectification ratio of 5 from asymmetric contacts, which is improved to 105 after the incorporation of an ATG. This improvement is attributed to the asymmetric modulation of carrier concentration and effective Schottky barrier height (SBH) by the ATG during forward and reverse bias. This is further confirmed from the temperature dependent measurement, where a difference of 0.22 eV is observed between the effective SBH for forward and reverse bias. Moreover, the rectification ratio also depends on carrier concentration in MoS2 and can be varied with the change in temperature as well as back gate voltage. Under laser light illumination, the device demonstrates strong opto-electric response with 100 times improvement in the relative photo current, as well as a responsivity of 1.9 A W−1 and a specific detectivity of 2.4 × 1010 Jones. These devices can also be implemented using other two dimensional (2D) materials and suggest a promising approach to incorporate diverse 2D materials for future nano-electronics and optoelectronics applications.

Dirac states from px,y orbitals in the buckled honeycomb structures: A tight-binding model and first-principles combined study**Project supported by the National Key Research and Development Projects of China (Grant No. 2016YFA0202300), the National Natural Science Foundation of China (Grant No. 61390501), the Science Fund from the Chinese Academy of Sciences (Grant No. XDPB0601), and the US Army Research Office.

Dirac states composed of px,y orbitals have been reported in many two-dimensional (2D) systems with honeycomb lattices recently. Their potential importance has aroused strong interest in a comprehensive understanding of such states. Here, we construct a four-band tight-binding model for the px,y-orbital Dirac states considering both the nearest neighbor hopping interactions and the lattice-buckling effect. We find that px,y-orbital Dirac states are accompanied with two additional narrow bands that are flat in the limit of vanishing π bonding, which is in agreement with previous studies. Most importantly, we analytically obtain the linear dispersion relationship between energy and momentum vector near the Dirac cone. We find that the Fermi velocity is determined not only by the hopping through π bonding but also by the hopping through σ bonding of px,y orbitals, which is in contrast to the case of pz-orbital Dirac states. Consequently, px,y-orbital Dirac states offer more flexible engineering, with the Fermi velocity being more sensitive to the changes of lattice constants and buckling angles, if strain is exerted. We further validate our tight-binding scheme by direct first-principles calculations of model-materials including hydrogenated monolayer Bi and Sb honeycomb lattices. Our work provides a more in-depth understanding of px,y-orbital Dirac states in honeycomb lattices, which is useful for the applications of this family of materials in nanoelectronics.

In Situ Observation of Resistive Switching in an Asymmetric Graphene Oxide Bilayer Structure.

Graphene oxide decorated with oxygen functional groups is a promising candidate as an active layer in resistive switching devices due to its controllable physical-chemical properties, high flexibility, and transparency. However, the origin of conductive channels and their growth dynamics remain a major challenge. We use in situ transmission electron microscopy techniques to demonstrate that nanoscale graphene oxide sheets bonded with oxygen dynamically change their physical and chemical structures upon an applied electric field. Artificially engineered bilayer reduced graphene oxide films with asymmetric oxygen content exhibit nonvolatile write-once-read-many memory behaviors without experiencing the bubble destruction due to the efficient migration of oxygen ions. We clearly observe that a conductive graphitic channel with a conical shape evolves from the upper oxygen-rich region to the lower oxygen-poor region. These findings provide fundamental guidance for understanding the oxygen motions of oxygen-containing carbon materials for future carbon-based nanoelectronics.

Bandgap Engineering of Graphene Nanoribbons by Control over Structural Distortion.

Among organic electronic materials, graphene nanoribbons (GNRs) offer extraordinary versatility as next-generation semiconducting materials for nanoelectronics and optoelectronics due to their tunable properties, including charge-carrier mobility, optical absorption, and electronic bandgap, which are uniquely defined by their chemical structures. Although planar GNRs have been predominantly considered until now, nonplanarity can be an additional parameter to modulate their properties without changing the aromatic core. Herein, we report theoretical and experimental studies on two GNR structures with "cove"-type edges, having an identical aromatic core but with alkyl side chains at different peripheral positions. The theoretical results indicate that installment of alkyl chains at the innermost positions of the "cove"-type edges can "bend" the peripheral rings of the GNR through steric repulsion between aromatic protons and the introduced alkyl chains. This structural distortion is theoretically predicted to reduce the bandgap by up to 0.27 eV, which is corroborated by experimental comparison of thus synthesized planar and nonplanar GNRs through UV-vis-near-infrared absorption and photoluminescence excitation spectroscopy. Our results extend the possibility of engineering GNR properties, adding subtle structural distortion as a distinct and potentially highly versatile parameter.

Direct Evidence of Lithium Ion Migration in Resistive Switching of Lithium Cobalt Oxide Nanobatteries.

Lithium cobalt oxide nanobatteries offer exciting prospects in the field of nonvolatile memories and neuromorphic circuits. However, the precise underlying resistive switching (RS) mechanism remains a matter of debate in two-terminal cells. Herein, intriguing results, obtained by secondary ion mass spectroscopy (SIMS) 3D imaging, clearly demonstrate that the RS mechanism corresponds to lithium migration toward the outside of the Lix CoO2 layer. These observations are very well correlated with the observed insulator-to-metal transition of the oxide. Besides, smaller device area experimentally yields much faster switching kinetics, which is qualitatively well accounted for by a simple numerical simulation. Write/erase endurance is also highly improved with downscaling - much further than the present cycling life of usual lithium-ion batteries. Hence very attractive possibilities can be envisaged for this class of materials in nanoelectronics.

Modulation of magnetism in transition-metal-doped two-dimensional GeS

Two-dimensional (2D) germanium monosulfide (GeS) is a promising nanoelectronic material with a desirable band gap, high carrier mobility, and anisotropic structures. In this work, we present a density functional theory study on the magnetism of 3d TM (TM = Fe, Co and Ni)-doped 2D GeS. We find that the TM atoms strongly bond to the GeS sheet with quite sizable binding energies due to the sp3-like hybridization of 2D GeS. The Fe- and Co-doped GeS show nonzero magnetic ground states. Hubbard parameter U hardly affects the magnetic moment when U is no more than 6 eV. In particular, substitutional Fe (Fe@GeS) and substitutional Co (Co@GeS) present high-spin states with 4 μB and 3 μB. The magnetism of TM-doped 2D GeS mainly arises from the crystal field splitting and spin exchange splitting of TM-3d orbitals. The magnetic and electronic properties of the Fe@GeS and Co@GeS systems can be easily controlled in a small vertical external electric field (Eext). The underlying mechanism of spin crossover is that Eext affects the crystal field splitting and then shifts the relative positions of 3d orbitals, which tunes the spin configurations. These results render monolayer GeS a promising 2D material for applications in future spintronics.

Study of local currents in low dimension materials using complex injecting potentials

A complex potential is constructed to inject electrons into the conduction band, mimicking electron currents in nanoscale systems. The injected electrons are time propagated until a steady state is reached. The local current density can then be calculated to show the path of the conducting electrons on an atomistic level. The method allows for the calculation of the current density vectors within the medium as a function of energy of the conducting electron. Using this method, we investigate the electron pathway of graphene nanoribbons in various structures, molecular junctions, and black phosphorus nanoribbons. By analyzing the current flow through the structures, we find strong dependence on the structural geometry and the energy of the injected electrons. This method may be of general use in the study of nano-electronic materials and interfaces.

Investigations on the structural, mechanical, thermal, and electrical properties of Ce-doped TiO2/poly(n-butyl methacrylate) nanocomposites

This study focused on the fabrication of poly(n-butyl methacrylate) (PBMA) nanocomposites with various concentrations of cerium-doped titanium dioxide (Ce–TiO2) nanoparticles via in situ polymerization technique. The structural characterization and the material properties of all the composites were analyzed by UV–visible, FTIR, XRD, SEM, DSC, TG, and tensile strength measurements. The UV–visible and FTIR studies confirmed the effective inclusion of Ce–TiO2 nanoparticles into the PBMA matrix. The change in amorphous morphology of PBMA to a crystalline structure was observed from the XRD pattern. The SEM morphology revealed the attachment of nanoparticles in the polymer matrix. The inclusion of Ce–TiO2 nanoparticles enhanced the glass transition temperature, and thermal stability of the PBMA matrix was revealed from DSC and TG, respectively. The tensile strength of PBMA was greatly enhanced by the addition of Ce–TiO2 nanoparticles. The AC conductivity, dielectric constant, and dielectric loss studies were also performed in the frequency range 102–106 Hz, and it was observed that addition of Ce–TiO2 nanoparticles greatly enhanced the electrical properties of PBMA. The change in dielectric constant with the addition of nanoparticles was correlated with a theoretical modeling study. This work also extended to study the role of Ce–TiO2 nanoparticles in the reinforcing mechanism of the nanocomposite by comparing the actual tensile strength of the composite with different theoretical modeling. The high dielectric constant and tensile strength of composite are beneficial in designing lightweight and highly efficient nanoelectronic materials based on the family of polybutyl acrylates.

(Invited) Self-Aligned Short-Channel Heterojunction Diodes and Transistors Based on Carbon Nanotubes and Related Nanoelectronic Materials

Parallel self-aligned fabrication methods in modern silicon-based microelectronics have enabled sub-lithographic registration between processing steps, ultimately facilitating substantial advances in circuit complexity over the past few decades. In contrast, while carbon nanotubes and related nanoelectronic materials have shown significant potential for digital and analog electronics due to their high mobilities, ultrathin geometry, and broad range of permutations in van der Waals (vdW) heterojunctions, devices based on vdW nanoelectronic materials have not fully exploited parallel self-aligned fabrication. Here, introduce a self-aligned processing methodology that enables the realization of field-effect transistors with channel lengths below 150 nm with minimal short-channel effects and improved current saturation, as demonstrated with semiconducting carbon nanotubes and monolayer MoS2. In vdW heterojunctions, this self-aligned approach allows dual-gate electrostatic control of the carrier density in both of the constituent semiconductors while concurrently achieving independent gate control of the short-channel series transistors. Since this self-aligned methodology is compatible with a diverse range of nanoelectronic materials and can be implemented in parallel via large-area lithographic processes without sacrificing lateral spatial resolution, it is likely to impact a wide range of vdW heterojunction devices.

(Invited) Beyond the Highs and Lows: A Selectively Colorful Yet Chilly Perspective on the Future of Dielectrics in Nanoelectronic Devices

High-dielectric constant (high-k) gate oxides and low-dielectric constant (low-k) interlayer dielectrics (ILD) have dominated the nanoelectronic materials research scene over the past decade, but have recently reached a state of maturity and perhaps the limits of their scaling. Based on this, one may ponder the direction of nanoelectronic dielectric research and the future of the field. However, as we will clarify, the well has fortunately not yet run dry! In this address, we seek to illustrate that high-k gate oxides and low-k ILDs represent just a small fraction of the numerous dielectric materials utilized in nanoelectronic products and that numerous new applications for dielectric materials are emerging as the industry transistions to novel patterning schemes and prepares for life post CMOS scaling. We will begin by briefly exploring the entirety of dielectric materials utilized in advanced nanoelectronic products with a particular focus on often ignored etch stopping, diffusion barrier, and passivation dielectrics and their more specialized requirements relative to high-k and low-k dielectrics. We will then transition to examining the various novel pitch division schemes being developed to cope with the delay of EUV lithography and the abundant new dielectric material applications these schemes create. We will specifically focus on the various new spacer and hard mask dielectrics that pitch division requires, their colorful selectivity requirements, and suggest future research needed to enable even more exotic patterning schemes. We will further examine how what was once old can become new again and specifically explore how both high-k and low-k dielectrics are being utilized to fill the needs of future beyond CMOS devices. In particular, the application of high-k and low-k dielectrics in novel resistive switching, magnetic, and nano-electro-mechanical device and sensor applications will be surveyed along with the research needed to extend these materials into even more diverse applications. We will conclude by reviewing the unique obstacles that dielectric materials present for the realization of advanced cryogenic quantum computing architectures and the necessary associated research vectors to prevent dielectrics from becoming quantum computing’s Achilles heel.

ОСОБЛИВОСТІ ПРОВЕДЕННЯ ДОСЛІДЖЕНЬ МІКРОГЕОМЕТРІЇ ТА ПОВЕРХНЕВИХ ВЛАСТИВОСТЕЙ ДІЕЛЕКТРИЧНИХ МАТЕРІАЛІВ ДЛЯ МІКРО- І НАНОЕЛЕКТРОНІКИ

The article briefly describes the principle and operating modes of atomic-force microscope (AFM), as well as the features of microgeometry and surface properties of dielectric materials for micro and nanoelectronics using AFM method. The purpose of this article is to determine the features of microgeometry research and surface properties of dielectric materials by atomic force microscopy, as well as the advantages and limitations of the use of this method in the investigation of dielectric materials. The advantages and limitations of the application of this method for surface and surface properties of dielectrics are shown. The main directions of study of topographies, mechanical and electromagnetic properties of surfaces of dielectrics, which are carried out with the help of atomicforce microscopy method, as well as limitations and drawbacks in the use of this method in the investigation of individual properties of dielectric surfaces are also considered. The further development of this method consists in the implementation of a number of techniques and devices based on equipment for atomic-force microscopy. Perspective instrumental methods with the use of AFM show the implementation of nanodrill and nanorethene instruments, allowing the creation of micro and nanoelectric dielectrics in surfaces, forming structural formations, obtaining threads from carbon nanotubes, conducting mechanoactivation of surface layers of dielectrics by nanoparticle.

EVOLUTION OF MODELS AND ALGORITHMS FOR PARAMETER CALCULATION IN MICRO– AND NANOELECTRONICS MATERIALS TECHNOLOGY

Results of developing a system of models and algorithms for parameter calculation in micro and nanoelectronics materials processes and equipment design have been considered. A distinctive feature of the teaching methods for special technological courses on electronics materials is that the courses are designed by analogy with electronics materials technologies: from a bulk single crystal to device structures the typical dimensions of which are within several tens of nanometers. A scientific model approach to the solution of technological problems has been developed during the study of heat and mass transfer processes which, along with the interaction processes in liquids and gas and with account of the heterogeneous reactions, are the theoretical basis of the electronics materials technology. The possibilities of physical and mathematical modeling have been compared. Approaches to the creation of mathematical models for the single crystals growth processes of semiconductors, epitaxial layers and heterostructures have been considered and their possible practical applications have been outlined. We show that the ideas put forward by V.V. Krapukhin at early stages of training specialists in electronics materials technology and further developed by his students have formed the basis for the training of several generations of highly skilled specialists