Enhanced leakage current properties of HfO2/GaN gate dielectric stack by introducing an ultrathin buffer layer

The band alignments of HfO₂/GaN, HfO₂/SiO₂/GaN and HfO₂/Al₂O₃/GaN gate dielectric stacks were comparatively investigated by using X-ray photoelectron spectroscopy. It was observed that the introduction of an ultrathin buffer layer film (SiO₂ or Al₂O₃) in HfO₂/GaN stack can make the band alignments more symmetrical with larger barrier height as identified by the valence band offsets and electron energy loss spectrum measurements. At room temperature, the leakage current density as function of temperature is 4.1 × 10^?6, 3 × 10^?7 and 9.8 × 10^?8 A cm^?2 at the bias of 1 V for the HfO₂/GaN, HfO₂/Al₂O₃/GaN and HfO₂/SiO₂/GaN gate dielectric stacks, correspondingly. With temperature increase from room temperature to 300 °C, the HfO₂/SiO₂/GaN gate dielectric stack exhibits lowest lower leakage current density than that of others. The HfO₂/GaN high-k gate dielectric stack with an ultrathin SiO₂ buffer layer appears to be a promising candidate for future GaN based high temperature metal-oxide-semiconductor (MOS) devices applications.     

Fabrication of large area hexagonal boron nitride thin films for bendable capacitors

Highly reliable and bendable dielectrics are desired in flexible or bendable electronic devices for future applications. Hexagonal boron nitride （h-BN） can be used as bendable dielectric due to its wide band gap. Here, we fabricate high quality h-BN films with controllable thickness by a low pressure chemical vapor deposition method. We demonstrate a parallel-plate capacitor using h-BN film as the dielectric. The h-BN capacitors are reliable with a high breakdown field strength of -9.0 MV/cm. Tunneling current across the h-BN film is inversely exponential to the thickness of dielectric, which makes the capacitance drop significantly. The h-BN capacitor shows a best specific capacitance of 6.8 F/cm^2, which is one order of magnitude higher than the calculated value.     

Carrier Modulation in 2D Transistors by Inserting Interfacial Dielectric Layer for Area-Efficient Computation

2D materials with atomic thickness display strong gate controllability and emerge as promising materials to build area-efficient electronic circuits. However, achieving the effective and nondestructive modulation of carrier density/type in 2D materials is still challenging because the introduction of dopants will greatly degrade the carrier transport via Coulomb scattering. Here, a strategy to control the polarity of tungsten diselenide (WSe₂) field-effect transistors (FETs) via introducing hexagonal boron nitride (h-BN) as the interfacial dielectric layer is devised. By modulating the h-BN thickness, the carrier type of WSe₂ FETs has been switched from hole to electron. The ultrathin body of WSe₂, combined with the effective polarity control, together contribute to the versatile single-transistor logic gates, including NOR, AND, and XNOR gates, and the operation of only two transistors as a half adder in logic circuits. Compared with the use of 12 transistors based on static Si CMOS technology, the transistor number of the half adder is reduced by 83.3%. The unique carrier modulation approach has general applicability toward 2D logic gates and circuits for the improvement of area efficiency in logic computation.     

Electron tunneling through ultrathin boron nitride crystalline barriers

We investigate the electronic properties of ultrathin hexagonal boron nitride (h-BN) crystalline layers with different conducting materials (graphite, graphene, and gold) on either side of the barrier layer. The tunnel current depends exponentially on the number of h-BN atomic layers, down to a monolayer thickness. Conductive atomic force microscopy scans across h-BN terraces of different thickness reveal a high level of uniformity in the tunnel current. Our results demonstrate that atomically thin h-BN acts as a defect-free dielectric with a high breakdown field. It offers great potential for applications in tunnel devices and in field-effect transistors with a high carrier density in the conducting channel.     

Gate-Coupling-Enabled Robust Hysteresis for Nonvolatile Memory and Programmable Rectifier in Van der Waals Ferroelectric Heterojunctions

Ferroelectric field-effect transistors (FeFETs) are one of the most interesting ferroelectric devices; however, they, usually suffer from low interface quality. The recently discovered 2D layered ferroelectric materials, combining with the advantages of van der Waals heterostructures (vdWHs), may be promising to fabricate high-quality FeFETs with atomically thin thickness. Here, dual-gated 2D ferroelectric vdWHs are constructed using MoS₂, hexagonal boron nitride (h-BN), and CuInP₂S₆ (CIPS), which act as a high-performance nonvolatile memory and programmable rectifier. It is first noted that the insertion of h-BN and dual-gated coupling device configuration can significantly stabilize and effectively polarize ferroelectric CIPS. Through this design, the device shows a record-high performance with a large memory window, large on/off ratio (10⁷), ultralow programming state current (10⁻¹³ A), and long-time endurance (10⁴ s) as nonvolatile memory. As for programmable rectifier, a wide range of gate-tunable rectification behavior is observed. Moreover, the device exhibits a large rectification ratio (3 × 10⁵) with stable retention under the programming state. This demonstrates the promising potential of ferroelectric vdWHs for new multifunctional ferroelectric devices.     

On the Working Mechanisms of Molecules-Based Van der Waals Dielectrics

Sb₂O₃ molecules offer unprecedented opportunities for the integration of a van der Waals (vdW) dielectric and a 2D vdW semiconductor. However, the working mechanisms underlying molecules-based vdW dielectrics remain unclear. Here, the working mechanisms of Sb₂O₃ and two Sb₂O₃-like molecules (As₂O₃ and Bi₂O₃) as dielectrics are systematically investigated by combining first-principles calculations and gate leakage current theories. It is revealed that molecules-based vdW dielectrics have a considerable advantage over conventional dielectric materials: defects hardly affect their insulating properties. This shows that it is unnecessary to synthesize high-quality crystals in practical applications, which has been a long-standing challenge for conventional dielectric materials. Further analysis reveals that a large thermionic-emission current renders Sb₂O₃ difficult to simultaneously satisfy the requirements of dielectric layers in p-MOS and n-MOS, which hinders its application for complementary metal-oxide-semiconductor (CMOS) devices. Remarkably, it is found that As₂O₃ can serve as a dielectric for both p-MOS and n-MOS. This work not only lays a theoretical foundation for the application of molecules-based vdW dielectrics, but also offers an unprecedentedly competitive dielectric (i.e., As₂O₃) for 2D vdW semiconductors-based CMOS devices, thus having profound implications for future semiconductor industry.     

Wafer-Scale Single Crystal Hexagonal Boron Nitride Layers Grown by Submicron-Spacing Vapor Deposition

The direct growth of wafer-scale single crystal two-dimensional (2D) hexagonal boron nitride (h-BN) layer with a controllable thickness is highly desirable for 2D-material-based device applications. Here, for the first time, a facile submicron-spacing vapor deposition (SSVD) method is reported to achieve 2-inch single crystal h-BN layers with controllable thickness from monolayer to tens of nanometers on the dielectric sapphire substrates using a boron film as the solid source. In the SSVD growth, the boron film is fully covered by the same-sized sapphire substrate with a submicron spacing, leading to an efficient vapor diffusion transport. The epitaxial h-BN layer exhibits extremely high crystalline quality, as demonstrated by both a sharp Raman E_2g vibration mode (12 cm⁻¹) and a narrow X-ray rocking curve (0.10°). Furthermore, a deep ultraviolet photodetector and a ZrS₂/h-BN heterostructure fabricated from the h-BN layer demonstrate its fascinating properties and potential applications. This facile method to synthesize wafer-scale single crystal h-BN layers with controllable thickness paves the way to future 2D semiconductor-based electronics and optoelectronics.     

介质衬底上生长h-BN二维原子晶体的研究进展

六方氮化硼(h-BN)二维原子晶体以其独特的结构、优异的性质以及广泛的应用前景引起了人们的普遍关注。高质量h-BN材料的制备是其性质研究与实际应用的前提。机械剥离的h-BN尺寸有限, 普遍采用的化学气相沉积(CVD)技术通常以过渡金属为衬底, 器件应用时需要将h-BN转移到其它衬底上。因此, 在介质衬底上直接生长h-BN成为二维材料研究领域的一个重要发展方向。本文总结了近年来介质衬底(包括: Si基衬底、蓝宝石衬底和石英衬底等)上直接生长h-BN二维原子晶体的主要进展。人们采用CVD、金属有机气相外延法(MOVPE)、物理气相沉积法(PVD)等方法, 通过提高生长温度、衬底表面处理、两步生长等工艺实现了介质衬底上直接生长h-BN。此外, 还介绍了介质衬底上h-BN二维原子晶体的主要应用。     

Glue-assisted grinding exfoliation of large-size 2D materials for insulating thermal conduction and large-current-density hydrogen evolution

Two-dimensional (2D) materials have many promising applications, but their scalable production remains challenging. Herein, we develop a glue-assisted grinding exfoliation (GAGE) method in which the adhesive polymer acts as a glue to massively produce 2D materials with large lateral sizes, high quality, and high yield. Density functional theory simulation shows that the exfoliation mechanism involves the competition between the binding energy of selected polymers and the 2D materials which is larger than the exfoliation energy of the layered materials. Taking h-BN as an example, the GAGE produces 2D h-BN with an average lateral size of 2.18 μm and thickness of 3.91 nm. The method is also extended to produce various other 2D materials, including graphene, MoS₂, WS₂, Bi₂O₂Se, mica, vermiculite, and montmorillonite. Two representative applications of thus-produced 2D materials have been demonstrated, including 2D h-BN/polymer composites for insulating thermal conduction and 2D MoS₂-based electrocatalysts for large-current-density hydrogen evolution, indicating the great potential of massively produced 2D materials.     

Degradation and breakdown characteristics of Al/HfYOx/GaAs capacitors

Effects of surface passivation and the interfacial layer on the reliability characteristics of Al/HfYO_x/GaAs metal-oxide-semiconductor capacitor structures are reported. Stress-induced leakage current mechanism, critical for understanding the degradation and breakdown in Al/HfYO_x/GaAs capacitors, has been studied in detail. While the devices fabricated with (NH₄)₂S-passivated GaAs substrates show both the soft and hard breakdown failure modes, capacitors with ultrathin interfacial layer (Ge or Si) show only hard breakdown. It is shown that the degradation dynamics follows more closely the logistic power-law relationship rather than the conventional power-law model, frequently used to describe leakage current conduction in high-k gate dielectrics.     

Controlled deposition of new organic ultrathin film as a gate dielectric layer for advanced flexible capacitor devices

This letter describes a new organic (1-bromoadamantane) ultrathin film as gate dielectric, which was successfully deposited by sol–gel spin-coating process on a flexible polyimide substrate at room temperature. The metal–insulator-metal (MIM) device with organic (1-bromoadamantane) ultrathin (10 nm) film as gate dielectric layer operated at gate voltage of 5.0 V, showing a low leakage current density (5.63 × 10^?10 A cm^?2 at 5 V) and good capacitance (2.01 fF μm^?2 at 1 MHz). The chemical structure of the 1-bromoadamantane layer was investigated by Fourier transform infrared spectrometer. The excellent leakage current density and better capacitance, probably due to the presence of polar, non-polar, low-polar groups, and bromine atoms in ultrathin film. Practical properties of the film in MIM capacitor such as dielectric constant as well as bending result of leakage current density and breakdown voltage have been better related to such fundamental adhesion nature over flexible substrate. This permits estimation of the properties of new dielectric in thin film form and short lists of the best materials for low loss and good capacitance flexible capacitors could be drawn up in future.     

Electrical properties of pulsed laser deposited Bi<Subscript>2</Subscript>Zn<Subscript>2/3</Subscript>Nb<Subscript>4/3</Subscript>O<Subscript>7</Subscript> thin films for high K gate dielectric application

Metal Insulator Semiconductor (MIS) capacitors with monoclinic bismuth zinc niobate pyrocholre having the composition Bi2Zn2_/3Nb_4/3O₇ (m-BZN) dielectric layer were fabricated and characterized. Capacitance voltage (C–V) and current voltage measurements were utilized to obtain the dielectric properties, leakage current density and interface quality. The results shows that the obtained m-BZN thin films presents a high dielectric constant in between 30 and 70, a good interface quality with silicon and a leakage current density of 10 μA/cm² for a field strength of 100 kV/cm which is acceptable for high performance logic circuits. The equilent oxide thickness for the films annealed at 200 °C was 10 nm. These results suggest that m-BZN thin films can be potentially integrated as gate dielectric materials in CMOS technology.     

Study of electrical and microstructure properties of high dielectric hafnium oxide thin film for MOS devices

Hafnium oxide (HfO₂) has emerged as the most promising highkdielectric for MOS devices. As-deposited sputtered HfO₂ thin films have large number of defects resulting in increased oxide charge and leakage current. In this paper the effect of sputtering voltage, bias sputtering and post deposition thermal annealing is investigated. The I–V and C–V characteristics of the dielectric film are studied employing Al–HfO₂–Si MOS capacitor structure. It is found that oxide charge increases with increasing sputtering voltage. Thermal annealing in oxygen reduces the interface/oxide charges and leakage current. It is shown that applying substrate bias during film deposition leakage current is further reduced by an order of magnitude. The microstructure of thin film is examined by AFM. The reduction in surface roughness with bias sputtering is shown. The experimental results are presented and discussed for device application.     

Electrostatic doping of graphene through ultrathin hexagonal boron nitride films

When combined with graphene, hexagonal boron nitride (h-BN) is an ideal substrate and gate dielectric with which to build metal|h-BN|graphene field-effect devices. We use first-principles density functional theory (DFT) calculations for Cu|h-BN|graphene stacks to study how the graphene doping depends on the thickness of the h-BN layer and on a potential difference applied between Cu and graphene. We develop an analytical model that describes the doping very well, allowing us to identify the key parameters that govern the device behavior. A predicted intrinsic doping of graphene is particularly prominent for ultrathin h-BN layers and should be observable in experiment. It is dominated by novel interface terms that we evaluate from DFT calculations for the individual materials and for interfaces between h-BN and Cu or graphene.     

Recent Advances in Rolling 2D TMDs Nanosheets into 1D TMDs Nanotubes/Nanoscrolls

Transition metal dichalcogenides (TMDs) van der Waals (vdW) 1D heterostructures are recently synthesized from 2D nanosheets, which open up new opportunities for potential applications in electronic and optoelectronic devices. The most recent and promising strategies in regards to forming 1D TMDs nanotubes (NTs) or nanoscrolls (NSs) in this review article as well as their heterostructures that are produced from 2D TMDs are summarized. In order to improve the functionality of ultrathin 1D TMDs that are coaxially combined with boron nitride nanotubes and single-walled carbon nanotubes. 1D heterostructured devices perform better than 2D TMD nanosheets when the two devices are compared. The photovoltaic effect in WS₂ or MoS₂ NTs without a junction may exceed the Shockley–Queisser limit for the above-band-gap photovoltage generation. Photoelectrochemical hydrogen evolution is accelerated when monolayer WS₂ or MoS₂ NSs are incorporated into a heterojunction. In addition, the photovoltaic performance of the WSe₂/MoS₂ NSs junction is superior to that of the performance of MoS₂ NSs. The summary of the current research about 1D TMDs can be used in a variety of ways, which assists in the development of new types of nanoscale optoelectronic devices. Finally, it also summarizes the current challenges and prospects.     

Tunneling currents through ultra thin HfO2/Al2O3/HfO2 triple layer gate dielectrics for advanced MIS devices

The dramatic scaling down of silicon integrated circuits has led to an intensive study of high dielectric constant materials as an alternative to the conventional insulators currently employed in microelectronics, i.e., silicon dioxide, silicon nitride, or oxynitride, which seem to have reached their physical limit in terms of reduction of thickness due to large leakage gate current. Introducing a physically thicker high-K material can reduce the leakage current to the acceptable limit. There are many potential candidates for high-K gate dielectrics with the K-valves ranging from 9 to 80. These are Al₂O₃, Y₂O₃, La₂O₃, Ta₂O₅, TiO₂, ZrO₂ and HfO₂. It is important to study the various leakage mechanisms in these films with the aim of improving their leakage current characteristics for use in advanced microelectronics devices. A procedure for calculating the tunneling current for stacked dielectrics is developed and subsequently applied to ultra thin films with equivalent oxide thickness (EOT) of 3.0 nm. Tunneling currents have been calculated as a function of gate voltage for different structures. Direct and Fowler-Nordheim tunneling currents through triple layer dielectrics are investigated for substrate injection. Using exact tunneling transmission calculations, current density–gate voltage (J _g?V _g) characteristics for ultra thin single layer gate dielectrics with different thicknesses have been shown to agree well with recently reported experiments. Extensions of this approach demonstrate that tunneling currents in HfO₂/Al₂O₃/HfO₂ structure with equivalent oxide thickness of 3.0 nm can be significantly lower than that through single layer oxides of the same thickness.     

Ultrasonic‐Ball Milling: A Novel Strategy to Prepare Large‐Size Ultrathin 2D Materials

Large‐size ultrathin 2D materials, with extensive applications in optics, medicine, biology, and semiconductor fields, can be prepared through an existing common physical and chemical process. However, the current exfoliation technologies still need to be improved upon with urgency. Herein, a novel and simple “ultrasonic‐ball milling” strategy is reported to effectively obtain high quality and large size ultrathin 2D materials with complete lattice structure through the introduction of moderate sapphire (Al₂O₃) abrasives in a liquid phase system. Ultimately numerous high‐quality ultrathin h‐BN, graphene, MoS₂, WS₂, and BCN nanosheets are obtained with large sizes ranging from 1–20 µm, small thickness of ≈1–3 nm and a high yield of over 20%. Utilizing shear and friction force synergistically, this strategy provides a new method and alternative for preparing and optimizing large size ultrathin 2D materials.     

Layer-Controlled Perovskite 2D Nanosheet Interlayer for the Energy Storage Performance of Nanocomposites

Polymer-based nanocomposites are desirable materials for next-generation dielectric capacitors. 2D dielectric nanosheets have received significant attention as a filler. However, randomly spreading the 2D filler causes residual stresses and agglomerated defect sites in the polymer matrix, which leads to the growth of an electric tree, resulting in a more premature breakdown than expected. Therefore, realizing a well-aligned 2D nanosheet layer with a small amount is a key challenge; it can inhibit the growth of conduction paths without degrading the performance of the material. Here, an ultrathin Sr_1.8Bi_0.2Nb₃O₁₀ (SBNO) nanosheet filler is added as a layer into poly(vinylidene fluoride) (PVDF) films via the Langmuir–Blodgett method. The structural properties, breakdown strength, and energy storage capacity of a PVDF and multilayer PVDF/SBNO/PVDF composites as a function of the thickness-controlled SBNO layer are examined. The seven-layered (only 14 nm) SBNO nanosheets thin film can sufficiently prevent the electrical path in the PVDF/SBNO/PVDF composite and shows a high energy density of 12.8 J cm⁻³ at 508 MV m⁻¹, which is significantly higher than that of the bare PVDF film (9.2 J cm⁻³ at 439 MV m⁻¹). At present, this composite has the highest energy density among the polymer-based nanocomposites under the filler of thin thickness.     

Reduced leakage current and enhanced ferroelectric properties in Mn-doped Bi0.5Na0.5TiO3-based thin films

Bi_0.5(Na_0.76K_0.2Li_0.04)_0.5TiO₃ thin films were deposited on SrRuO₃-coated (001)-SrTiO₃ substrates by pulsed laser deposition. The effects of oxygen pressure and Mn doping on the leakage current and ferroelectric and dielectric properties were investigated. The remnant polarization and dielectric constant (at 10 kHz) of Mn-doped film deposited at 400 mtorr were measured to be 23 μC cm^?2 and 660, respectively. The leakage current density of Mn-doped films was suppressed by more than two orders of magnitude and the polarization was considerably enhanced. The XPS results showed coexistence of Mn²⁺, Mn³⁺, and Mn⁴⁺ in doped films. Oxidation of Mn²⁺ to higher valence states by absorbing holes along with occupation of A-site vacancies was suggested as the possible reason for a reduced leakage current and dielectric loss in Mn-doped films.     

Dielectric properties and resistance to fatigue failure of different barrier layers prepared on flexible stainless-steel foils by ion-beam assisted deposition

A stainless-steel foil is an attractive candidate for the substrate of flexible display devices and integrated solar modules. For electrical insulation and ion diffusion reduction, a barrier layer should be coated on the stainless-steel foil surface. In this study, different barrier layers such as SiO_x, TaO_x, TiO_x and TaO_x/SiO_x were prepared on the flexible stainless-steel foils (SUS 304) by ion-beam assisted deposition. The dielectric properties of the barrier layers, including resistance, reactance, leakage current density, breakdown field strength and performance index, were investigated. The resistance to fatigue failure of the barrier layers was evaluated by insulating tests after the specimen foils were flattened. The results show that the dielectric properties and the resistance to fatigue failure of the TaO_x/SiO_x composite barrier layer are better than those of the SiO_x or the TaO_x barrier layer. The best dielectric properties and resistance to fatigue failure are achieved with the 4-μm thick TaO_x/SiO_x composite barrier layer.