TiberCAD: A new multiscale simulator for electronic and optoelectronic devices

In the last years GaN-based heterostructures have attracted much attention for their application as optoelectronic devices. The strain due to lattice mismatch of the constituent materials plays a crucial role in the behaviour of these structures, especially if they are of reduced dimensions, as e.g. nanocolumns. We show an implementation of a new device simulator which accounts for strain-related effects and quantum mechanical properties and couples them with the transport of the quasi-particles in the system. Simulations of an AlGaN/GaN nanocolumn LED are reported as an example.     

WSe2/GeS 异质结的增强光致发光

二维半导体材料为纳米尺度的光学性质研究提供了良好的支持. 当将其构筑成异质结时, 界面间的相互作用可以改变原光电性质或产生新的性质, 是二维材料光电子器件功能控制的重要手段. 利用机械剥离法制备WSe2/GeS 异质结, 通过发光光谱研究异质结层间激子的光学性质. 结果表明:p 型 GeS 与弱 n 型 WSe2 构筑成异质结时会产生新的层间激子. 与 GeS 和 WSe2 的荧光发射强度相比, 异质结的层间激子发光强度显著增加. 此研究为设计具有先进光电性能的二维半导体器件提供了思路.     

Structure and peculiarities of nanodeformation in Ti–Zr–Ni quasi-crystals

Ti–Zr–Ni samples with a substantial predominance of icosahedral quasicrystalline phase were produced by the melt-spinning technique. Their structure and mechanical properties were studied by X-ray diffraction and nanoindentation methods. The quasicrystalline phase was found to have a primitive lattice with the quasicrystallinity parameter a _q = 0.5200–0.5210?nm. Quasicrystalline deformation behaviour under nanoindentation versus phase composition and structure is discussed in comparison with single crystal W–12?wt%?Ta. The estimated elastic modulus E of the quasicrystalline phase shows no correlation with the element composition. The nanohardness was shown to increase with increasing quasicrystalline-phase perfection. Load–displacement curves of Ti–Zr–Ni quasicrystals (QCs) show stepwise character with alternation of elastic and plastic sections. Such non-uniform plastic flow in QCs might be caused by the localization of plastic deformation in shear bands. The non-uniformity of the plastic deformation increases with the increasing quasicrystalline phase perfection.     

Ab initio exploration of d<Superscript>0</Superscript> digital magnetic heterostructures: the case of MgO and CaO <Emphasis Type="Italic">δ</Emphasis>-doped with potassium

Previous studies of digital magnetic heterostructures have mainly focused on 3d transition metal δ-doped semiconductors. In this work, sp-electron digital magnetic heterostructures without magnetic ions are proposed. Based on a theoretical density functional investigation, electronic structures and magnetic properties of MgO and CaO δ-doped with K were reported. The results show that these heterostructures are half-metallic ferromagnetic materials having semiconducting up spins and metallic down spins, with an exchange interaction much stronger than that of a random alloy with similar K concentration. Our first-principles calculations show that the carriers at the Fermi level are strongly confined within a few monolayers around the KO plane. This strong confinement is responsible for the large exchange coupling and the two-dimensional half-metallic behavior. The thickness of the host semiconducting spacer does not significantly change the global electric and magnetic features.     

Piezoelectric excitation of semiconductor plates

The microelectronics revolution is due largely to the electronic properties of semiconducting crystals. Almost all binary semiconductors are piezoelectric, and can be fashioned with high resistivity, permitting incorporation of mechanical motion as an additional variable, leading to the possibility of mixed-effect devices with novel properties. In order to further develop these new structures, this paper provides newly derived expressions for piezoelectric excitation of simple thickness plate modes of binary semiconductors, by both thickness and lateral electric fields.     

Edge-and strain-induced band bending in bilayer-monolayer Pb_2Se_3 heterostructures

By using scanning tunneling microscope/microscopy(STM/STS), we reveal the detailed electronic structures around the sharp edges and strained terraces of lateral monolayer-bilayer Pd₂Se₃ heterostructures. We find that the edges of such heterostructures are well-defined zigzag type. Band bending and alignment are observed across the zigzag edge, forming a monolayer-bilayer heterojunction. In addition, an n-type band bending is induced by strain on a confined bilayer Pd₂Se₃ terrace. These results provide effective toolsets to tune the band structures in Pd₂Se₃-based heterostructures and devices.     

金属纳米棒弯曲力学行为的分子动力学模拟

纳米结构的力学性能是纳米超微型器件设计的基础,分子动力学是研究纳米结构力学行为的有效方法.本文采用镶嵌原子方法模拟金属铜纳米棒的弯曲力学行为.计算结果表明由于尺寸效应和表面效应的影响,在纳观尺度下纳米结构表现出与宏观尺度下完全不同的力学特征.金属纳米棒弯曲力学过程分为初始变形迟滞阶段、线弹性变形阶段和塑性变形阶段.塑性变形阶段表现出“刚化”、“台阶”和较强的延性等特征. 关键词： 纳米结构 纳米棒 弯曲性能 分子动力学     

Graphene based heterostructures

The two dimensional charge carriers in monolayer and bilayer graphene are described by massless and massive chiral Dirac Hamiltonians, respectively. These two-dimensional materials are predicted to exhibit a wide range of behavior, etc. However, graphene devices on a typical three-dimensional insulating substrates such as SiO₂ are highly disordered, exhibiting characteristics that are far inferior to the expected intrinsic properties of graphene. We have developed a novel technique for substrate engineering of graphene devices using layered dielectric materials to build graphene based vertical heterostructures. We employ hBN, an insulating isomorph of graphite, as a substrate and gate dielectric for graphene electronics. In this review, we describe the fabrication and characterization of high-quality exfoliated mono- and bilayer graphene devices on single-crystal hBN substrates, using a mechanical transfer process. Graphene devices on hBN substrates have mobilities and carrier inhomogeneities that are almost an order of magnitude better than devices on SiO₂. We use the enhanced mobility of electrons in hBN supported graphene to investigate the effects of electronic interactions. We find that interactions drive spontaneous breaking of the emergent SU(4) symmetry of the graphene Landau levels, leading to a variety of non-trivial integer and fractional quantum Hall states. The ability to assemble crystalline layered materials in a controlled way permits the fabrication of graphene devices on other promising dielectrics and allows for the realization of more complex graphene heterostructures.     

Recent Advances in the Study of Phosphorene and its Nanostructures

Phosphorene, a single layer of black phosphorus, has attracted considerable attention recently due to its intriguing structures and fascinating electronic properties. In particular, its remarkable properties, such as high charge carrier mobility, direct band-gap semiconducting characteristics, and strong anisotropies in electro-optical and thermo-mechanical properties, etc., are opening up brand-new opportunities for its applications in nanoelectronics, optoelectronics, sensors, energy conversion, and advanced engineering materials, etc. In this article, we present recent advances in the study of phosphorene and its derivatives (nanoribbons, nanotubes, fullerenes, and heterostructures) with special emphasis on structures, morphologies, properties (electronic, optical, magnetic, thermal, mechanical), and applications (transistors, phonon detectors, digital circuits, sensors, thermoelectric materials, Li-ion batteries). In addition, routes for modifying these properties by physical and chemical functionalization, defect engineering, strain engineering, and electric fields are discussed. Our intent is to present a state-of-the-art view in this fast-evolving field, with a balanced theoretical and experimental perspective.     

Terahertz oscillations in semiconducting carbon nanotube resonant-tunneling diodes

The paper presents the simulation and possible physical implementation of a resonant tunneling diode based on a semiconducting single-walled carbon nanotube, which exceeds the performance of similar resonant tunneling devices based on semiconductor heterostructures. In this respect, the oscillation frequency and the output power are predicted to be greater by one order of magnitude, attaining 16 THz and 2.5 μW, respectively. The generated THz signal is directly radiated into free-space through the injection contacts of the resonant tunneling diode, which have the shape of a bowtie antenna.     

Improving the device performances of two-dimensional semiconducting transition metal dichalcogenides: Three strategies

Two-dimensional (2D) semiconductors are emerging as promising candidates for the next-generation nanoelectronics. As a type of unique channel materials, 2D semiconducting transition metal dichalcogenides (TMDCs), such as MoS₂ and WS₂, exhibit great potential for the state-of-the-art field-effect transistors owing to their atomically thin thicknesses, dangling-band free surfaces, and abundant band structures. Even so, the device performances of 2D semiconducting TMDCs are still failing to reach the theoretical values so far, which is attributed to the intrinsic defects, excessive doping, and daunting contacts between electrodes and channels. In this article, we review the up-to-date three strategies for improving the device performances of 2D semiconducting TMDCs: (i) the controllable synthesis of wafer-scale 2D semiconducting TMDCs single crystals to reduce the evolution of grain boundaries, (ii) the ingenious doping of 2D semiconducting TMDCs to modulate the band structures and suppress the impurity scatterings, and (iii) the optimization design of interfacial contacts between electrodes and channels to reduce the Schottky barrier heights and contact resistances. In the end, the challenges regarding the improvement of device performances of 2D semiconducting TMDCs are highlighted, and the further research directions are also proposed. We believe that this review is comprehensive and insightful for downscaling the electronic devices and extending the Moore’s law.     

Si-Ge-GaAs nanoheterostructures for photovoltaic cells

Synthesis from molecular beams in an ultrahigh vacuum is a promising method for producing multilayer semiconducting thin-film structures for high-efficiency conversion of heat and solar energies into electricity, where cascade converters with complex optimized chemical compositions and alloying profiles are necessary. Until recently, nanotechnologies of heterostructures, such as quantum wells, superlattices, and quantum dots, were not applied for photovoltaic conversion. The state of the art of technologies in this field is analyzed.     

Effect of sample tilt on nanoindentation behaviour of materials

Analysis of nanoindentation is based on the elastic solution of a rigid indenter perpendicularly penetrating a flat contact surface. In reality, nanoindentation is often performed on a tilt sample surface due to sample tilt mounting or the existing roughness of a polished or raw surface. In this study, finite element simulations as well as nanoindentation experiments on a fused-quartz sample with different tilt angles were carried out to investigate the influence of sample tilt on nanoindentation behaviour of materials. It was found that sample tilt results in increases in the indentation load, contact area and contact stiffness at the same penetration depth. The contact area increase caused by sample tilt cannot be accounted for by Sneddon's equation, commonly used in nanoindentation analysis. This results in a significant underestimation of indentation projected contact area, which in turn leads to an overestimation of the mechanical properties measured by nanoindentation.     

Measurement of nanomechanical properties of biomolecules using atomic force microscopy

The capabilities of atomic force microscopy (AFM) have been rapidly expanding beyond topographical imaging to now allow for the analysis of a wide range of properties of diverse materials. The technique of nanoindentation, traditionally performed via dedicated indenters can now be reliably achieved using AFM instrumentation, enabling mechanical property determination at the nanoscale using the high spatial and force resolutions of the AFM. In the study of biological systems, from biomolecules to complexes, this technique provides insight into how mesoscale properties and functions may arise from a myriad of single biomolecules. In vivo and in situ analyses of native structures under physiological conditions as well as the rapid analysis of molecular species under a variety of experimental treatments are made possible with this technique. As a result, AFM nanoindentation has emerged as a critical tool for the study of biological systems in their natural state, further contributing to both biomaterial design and pharmacological research. In this review, we detail the theory and progression of AFM-based nanoindentation, and present several applications of this technique as it has been used to probe biomolecules and biological nanostructures from single proteins to complex assemblies. We further detail the many challenges associated with mechanical models and required assumptions for model validity. AFM nanoindentation capabilities have provided an excellent improvement over conventional nanomechanical tools and by integration of topographical data from imaging, enabled the rapid extraction and presentation of mechanical data for biological samples.     

N、P掺杂硼烯/石墨烯异质结的密度泛函理论研究

采用基于密度泛函理论的第一性原理计算方法系统研究了氮、磷掺杂对硼烯/石墨烯异质结的几何结构和电子性质的影响.结果表明,相较完整硼烯/石墨烯异质结的金属特性,氮、磷掺杂的硼烯/石墨烯异质结均表现为半导体特性.室温下的分子动力学模拟进一步论证了相关体系的动力学稳定性.研究结果能够为硼烯/石墨烯异质结在新型二维半导体材料中的应用提供参考价值.     

Berkovich indentation-induced deformation behaviors of GaN thin films observed using cathodoluminescence and cross-sectional transmission electron microscopy

Presented in this study are the Berkovich nanoindentation-induced mechanical deformation mechanisms of metal-organic chemical-vapor deposition (MOCVD) derived GaN thin films, investigated by using the cathodoluminescence (CL) and the cross-sectional transmission electron microscopy (XTEM) techniques. The multiple “pop-in” events were observed in the load-displacement (P-h) curves and appeared to occur randomly with increasing the indentation load. These instabilities are attributed to the dislocation nucleation and propagation. CL images of nanoindentation show a very well-defined rosette structures with the hexagonal system, and clearly display the distribution of deformation-induced extended defects/dislocations which affect CL emission. By using focused ion beam (FIB) milling to accurately position the cross-section of an indented area, XTEM results demonstrate that the major plastic deformation is taking place through the propagation of dislocations. The present observations are in support to the massive dislocations activities occurring underneath the indenter during the loading cycle. No evidence of either phase transformation or formation of micro-cracking was observed by using XTEM observations. We also discuss how these features correlate with Berkovich nanoindentation-induced defects/dislocations microstructures. Finally, this study has significant implications for the extent of contact-induced damage during fabrication of GaN-based optoelectronic devices.     

Spin transport properties in Fe-doped graphene/hexagonal boron-nitride nanoribbons heterostructures

By combining non-equilibrium Green's function (NEGF) with density functional theory (DFT), we systematically study the spin-related transport properties of the heterostructures composed of graphene and hexagonal boron-nitride (h-BN) when the metal Fe is doped different positions of the heterostructures interface. The results show that the heterostructures exhibit obvious spin-filtering effect (SFE) and negative differential resistance (NDR) due to the different absorbing positions of the metal Fe. And the spin filtering ratio can reach more than 90% in a specific bias voltage range. Moreover, spin-rectifying behaviors are detected in the heterostructures. Whether it is for the design of multifunctional devices or the synthesis of spintronic devices, these findings will have some reference value.     

MBE-grown MCT hetero- and nanostructures for IR and THz detectors

We present an overview of our technological achievements in the implementation of detector structures based on mercury cadmium telluride (MCT) heterostructures and nanostructures for IR and THz spectral ranges. We use a special MBE design set for the epitaxial layer growth on (013) GaAs substrates with ZnTe and CdTe buffer layers up to 3” in diameter with the precise ellipsometric monitoring in situ. The growth of MCT alloy heterostructures with the optimal composition distribution throughout the thickness allows for the realization of different types of many-layered heterostructures and quantum wells to prepare the material for fabricating single- or dual-band IR and THz detectors.We also present the two-color broad-band bolometric detectors based on the epitaxial MCT layers that are sensitive in 150–300-GHz subterahertz and infrared ranges from 3 to 10 μm, which operate at the ambient or liquid nitrogen temperatures as photoconductors, as well as the detectors based on planar HgTe quantum wells. The design and dimensions of THz detector antennas are optimized for reasonable detector sensitivity values. A special diffraction limited optical system for the detector testing was designed and manufactured. We represent here the THz images of objects hidden behind a plasterboard or foam plastic packaging, obtained at the radiation frequencies of 70, 140, and 275 GHz, respectively.     

半导体量子器件物理讲座 第一讲 异质结构和量子结构

随着半导体材料超薄层外延生长和微细加工技术的进展,人们已研制成功多种多样的半导体量子器件,以量子理论为基础,以半导体量子器件为研究对象,形成了一门新的学科－半导体量子电子学和量子光电子学,文章着重介绍半导体异质结构和量子结构,包括其能带结构、态密度分布等性质。     

Effect of the primary particle morphology on the micromechanical properties of nanostructured alumina agglomerates

Depending on the application of nanoparticles, certain characteristics of the product quality such as size, morphology, abrasion resistance, specific surface, dispersibility and tendency to agglomeration are important. These characteristics are a function of the physicochemical properties, i.e. the micromechanical properties of the nanostructured material. The micromechanical properties of these nanostructured agglomerates such as the maximum indentation force, the plastic and elastic deformation energy and the strength give information on the product properties, e.g. the efficiency of a dispersion process of the agglomerates, and can be measured by nanoindentation. In this study a Berkovich indenter tip was used for the characterisation of model aggregates out of sol–gel produced silica and precipitated alumina agglomerates with different primary particle morphologies (dimension of 15–40 nm). In general, the effect of the primary particle morphology and the presence or absence of solid bonds can be characterised by the measurement of the micromechanical properties via nanoindentation. The micromechanical behaviour of aggregates containing solid bonds is strongly affected by the elastic–plastic deformation behaviour of the solid bonds and the breakage of solid bonds. Moreover, varying the primary particle morphology for similar particle material and approximately isotropic agglomerate behaviour the particle–particle interactions within the agglomerates can be described by the elementar breaking stress according to the formula of Rumpf.