Heterostructure Layers Research Articles

DNA Translocation through Vertically Stacked 2D Layers of Graphene and Hexagonal Boron Nitride Heterostructure Nanopore.

Cost-effective, fast, and reliable DNA sequencing can be enabled by advances in nanopore-based methods, such as the use of atomically thin graphene membranes. However, strong interaction of DNA bases with graphene leads to undesirable effects such as sticking of DNA strands to the membrane surface. While surface functionalization is one way to counter this problem, here, we present another solution based on a heterostructure nanopore system, consisting of a monolayer of graphene and hexagonal boron nitride (hBN) each. Molecular dynamics studies of DNA translocation through this heterostructure nanopore revealed a surprising and crucial influence of the heterostructure layer order in controlling the base specific signal variability. Specifically, the heterostructure with graphene on top of hBN had nearly 3-10× lower signal variability than the one with hBN on top of graphene. Simulations point to the role of differential underside sticking of DNA bases as a possible reason for the observed influence of the layer order. Our studies can guide the development of experimental systems to study and exploit DNA translocation through two-dimensional heterostructure nanopores for single molecule sequencing and sensing applications.

Nonlocal scattering matrix description of anisotropic polar heterostructures

Polar dielectrics are a promising platform for mid-infrared nanophotonics, allowing for nanoscale electromagnetic energy confinement in oscillations of the crystal lattice. We recently demonstrated that in nanoscopic polar systems a local description of the optical response fails, leading to erroneous predictions of modal frequencies and electromagnetic field enhancements. In this Paper we extend our previous work providing a scattering matrix theory of the nonlocal optical response of planar, anisotropic, layered polar dielectric heterostructures. The formalism we employ allows for the calculation of both reflection and transmission coefficients, and of the guided mode spectrum. We apply our theory to complex AlN/GaN superlattices, demonstrating the strong nonlocal tuneability of the optical response arising from hybridisation between photon and phonon modes. The numerical code underlying these calculations is provided in an online repository to serve as a tool for the design of phonon-based mid-infrared optoelectronic devices.

Strain determination in heterostructures by TEM in selected area electron diffraction mode

The technique for determination of strain and elemental composition distribution across the graded layer in heterostructures by means of selected area electron diffraction in transmission electron microscope (TEM) is proposed. The approach accounts for the modification in the residual elastic strain due to sample thinning during TEM specimen preparation. The technique is approved using In0,7Ga0,3As/InxAl1-xAs/GaAs(001) and Al0,75Ga0,25N/AlN/Al2O3 heterostructures, and the results are in a good agreement with those obtained by alternative ways.

Плазмохимическое и жидкостное травление в постростовой технологии каскадных солнечных элементов на основе гетероструктуры GaInP/GaInAs/Ge

Investigation and development of a separating mesa-structure creating technology for fabricating multi-junction solar cells based on the GaInP/GaInAs/Ge heterostructure has been carried out. Studied were methods of etching of heterostructure layers: liquid chemical etching in the etchants based on HBr, H2O2, K2Cr2O7 and plasma-chemical etching in the stream of operating gas BCl3. The comparative analysis of etching methods was studied. The protective masks based on photoresist layer and TiOx/SiO2 were developed. Multi-junction solar cells with low parameters of leakage current less than 10-7 A at voltage 0,5-1 V were created.

Double-Channel Electron Transport in Suspended Quantum Point Contacts with in-Plane Side Gates

The conductance of a suspended quantum point contact fabricated on the basis of GaAs/AlGaAs heterostructures with a two-dimensional electron gas and equipped with side gates separated from the constriction using lithographical trenches is investigated. The gate dependences of the conductance of such structures correspond to the unusual double-channel mode with independent quantization of the conductance of two channels, and the conductance of separate channels can be independently controlled using two side gates. The electrostatic-formation mechanism of a two-channel structure inside a single constriction associated with the lateral redistribution of low-mobility X-valley electrons containing superlattice heterostructure layers, which leads to the formation of a potential barrier separating the conductivity electrons into two channels symmetrically shifted towards the lithographic trenches, specifying the nanostructure geometry, is considered.

Contact and Bulk Resistivity Screening for Advanced Crystalline Silicon Solar Cell Concepts by an Economical and Reliable Transfer Length Measurement Method Based on Laser Micro‐Patterning

Herein, test structures for transfer length measurements (TLMs) are prepared by means of a femtosecond laser micro‐machining process for quick and easy measurements of contact resistance and bulk resistivity in thin‐film multilayer stacks for advanced high‐efficiency silicon solar cells. In particular, silicon heterostructure layer systems composed of tin‐doped indium oxide (ITO)/n‐doped amorphous silicon (a‐Si) on crystalline silicon (c‐Si) substrates are used. To study the reliability of this novel laser‐based TLM test structure preparation method, measured data from laser structured TLM patterns are compared with conventional structures based on micro‐lithography. For small TLM test structures with contact distances between 100 and 1000 μm, the results suggest a significant dependence of the measured contact resistance values on the size of the laser prepared structures. This may be attributed to redeposited material during the laser ablation procedure at the flanks of the contact pads and insufficient mesa structuring. However, the laser and lithography‐based preparation methods yield comparable results when larger structures (contact distances between 200 and 2200 μm) are implemented. Herein, size effects can be neglected. For both methods, the specific contact resistance values of ITO/n‐doped a‐Si/c‐Si heterostructures are calculated from 8 to 45 mΩ cm2 for various samples from the same batch.

Operational Limits and Failure Mechanisms in All-2D van der Waals Vertical Heterostructure Devices with Long-Lived Persistent Electroluminescence.

Various 2D materials can be assembled into vertical heterostructure stacks that emit strong electroluminescence. However, to date, most work is done using mechanical exfoliated materials, with little insights gained into the operation limits and failure mechanisms due to the limited number of devices produced and the device-to-device variances. However, when using chemical vapor deposition (CVD) grown 2D crystals, it is possible to construct dozens of devices to generate statistics and ensemble insights, providing a viable way toward scalable industrialization of 2D optoelectronics. In particular, the operation lifetime/duration of electroluminescence and subsequent failure mechanisms are poorly understood. Here, we demonstrate that all-2D vertical layered heterostructure (VLH) devices made using CVD-grown materials (Gr:h-BN:WS2:h-BN:Gr) can generate strong red electroluminescence (EL) with continuous operation for more than 2 h in ambient atmospheric conditions under constant bias. Layer-by-layer controlled assembly is used to achieve graphene top and bottom electrodes in a crossbar geometry, with few layered h-BN continuous films as tunnel barriers for direct carrier injection into semiconducting monolayer WS2 single crystals with direct band gap recombination. Tens of the devices were fabricated in a single chip, with strong EL routinely measured under both positive and negative graphene electrode bias. The success rate for EL emission in devices is over 90%. EL starts to be detected at bias values of ∼5 V, with bright red emission located at the crossbar intersection site, with intensity increasing with applied bias. Long-lived persistent EL is demonstrated for more than 2 h without significant degradation of WS2 under high bias conditions of 20 V. In cycling tests, the EL signal peak position and intensity stay almost the same after several ON/OFF cycles with high bias, which proves that our device has good stability and durability when pulsed. Breakdown of the device is shown to occur at a bias value of ∼35 V, whereby current reduces to zero and EL abruptly stops, due to breakdown of the top graphene electrode, associated with local heating accumulation. This study provides a viable way for wafer-scale fabrication of high-performance 2D EL arrays for ultrathin optoelectronic devices and sheds light on the mechanisms of failure and operation limits of EL devices in ambient conditions.

Influence of a piezoelectric ZnO intermediate layer on Rayleigh waves propagating in Sc43%AlN57%/ZnO/diamond hetero-structures subjected to uniaxial stress

Surface acoustic waves (SAWs) are of practical importance across several fields. Particularly, a wide range of SAW devices is available, which are based on the stacking piezoelectric layered hetero-structures. As such, efficient and stable algorithm of analyzing electromechanical coupling factor (K2) should be of specific interest, inspiring a variety of piezoelectric layered structures. Motivated by the above considerations, this paper exploits the performance of Sc43% AlN57%/ZnO/diamond piezoelectric hetero-structures using matrix and polynomial methods. Consequently, dispersion curves, the electromechanical coupling factor and profiles of Rayleigh waves are calculated, in which the properties of hexagonal intermediate ZnO layer are varied. Here, $$ (0001) - [10\bar{1}0]{\text{ZnO}} $$ or $$ (11\bar{2}0) - [0001]{\text{ZnO}} $$ directions of the middle layer were integrated with Sc43% AlN57%/diamond to be a composite hetero-structure with a larger K2 of SAW. Accordingly, a commercial ZnO specimen was characterized by scanning electron microscopy (SEM) as well as Raman and XRD spectroscopy to determine its available crystallographic directions and the C-axis hexagonal character. Comparing with conventional AlN/diamond hetero-structures, the inclusion of ZnO as the intermediate layer provides with an excellent electromechanical coupling factor which significantly improves the $$ (11\bar{2}0) - [0001] $$ structure. However, comparing with the ZnO/diamond hetero-structure, a high phase velocity (Vph) was observed. Therefore, the development of a well-balanced structure that maximizes the values of Vph and K2 is systematically challenging. Moreover, it was found that initial stresses have a significant effect on the Rayleigh wave velocities. Fundamental A0 Lamb mode can be generated in piezoelectric hetero-structures when the properties of substrate remain very low relative to the film. Numerical results obtained in this paper can provide a deeper insight into the nature of the Rayleigh wave behaviors in piezoelectric hetero-structures, involving a ZnO intermediate layer.

Enhanced spectral splitting in a novel solar spectrum optical splitter based on one dimensional photonic crystal heterostructure

Controlling the distribution of solar spectrum in different bands would boost the energy harvesting efficiency and optimize the energy dispatchability. 1D photonic crystal with intrinsic optical band gap can be used to split the solar spectrum for hybrid photovoltaic/thermal solar applications. Here, we designed an efficient solar spectrum optical filter based on a cermet layer, Si/SiO2 1D photonic crystal, and top heterostructure layer. Compared with 1D photonic crystal structure, the 1D photonic crystal heterostructure with top YSZ layer can realize the reflectance of greater than 92% in PV band and the low average reflectance in two thermal bands by tuning the effective impedance of multilayer films. The enhanced reflectance in PV band results from the huge mismatching of impedance between free space and the heterostructure structure. The top dielectric layer can also be extended to other oxides.

Synthesis of layered silicon-graphene hetero-structures by wet jet milling for high capacity anodes in Li-ion batteries

While silicon-based negative electrode materials have been extensively studied, to develop high capacity lithium-ion batteries (LIBs), implementing a large-scale production method that can be easily transferred to industry, has been a crucial challenge. Here, a scalable wet-jet milling method was developed to prepare a silicon-graphene hybrid material to be used as negative electrode in LIBs. This synthesized composite, when used as an anode in lithium cells, demonstrated high Li ion storage capacity, long cycling stability and high-rate capability. In particular, the electrode exhibited a reversible discharge capacity exceeding 1763 mAh g−1 after 450 cycles with a capacity retention of 98% and a coulombic efficiency of 99.85% (with a current density of 358 mA g−1). This significantly supersedes the performance of a Si-dominant electrode structures. The capacity fade rate after 450 cycles was only 0.005% per cycle in the 0.05–1 V range. This superior electrochemical performance is ascribed to the highly layered, silicon-graphene porous structure, as investigated via focused ion beam in conjunction with scanning electron microscopy tomography. The hybrid electrode could retain 89% of its porosity (under a current density of 358 mA g−1) after 200 cycles compared with only 35% in a Si-dominant electrode. Moreover, this morphology can not only accommodate the large volume strains from active silicon particles, but also maintains robust electrical connectivity. This confers faster transportation of electrons and ions with significant permeation of electrolyte within the electrode. Physicochemical characterisations were performed to further correlate the electrochemical performance with the microstructural dynamics. The excellent performance of the hybrid material along with the scalability of the synthesizing process is a step forward to realize high capacity/energy density LIBs for multiple device applications.

Magnetic order in 3D topological insulators—Wishful thinking or gateway to emergent quantum effects?

Three-dimensional topological insulators (TIs) are a perfectly tuned quantum-mechanical machinery in which counterpropagating and oppositely spin-polarized conduction channels balance each other on the surface of the material. This topological surface state crosses the bandgap of the TI and lives at the interface between the topological and a trivial material, such as vacuum. Despite its balanced perfection, it is rather useless for any practical applications. Instead, it takes the breaking of time-reversal symmetry (TRS) and the appearance of an exchange gap to unlock hidden quantum states. The quantum anomalous Hall effect, which has first been observed in Cr-doped (Sb,Bi)2Te3, is an example of such a state in which two edge channels are formed at zero field, crossing the magnetic exchange gap. The breaking of TRS can be achieved by magnetic doping of the TI with transition metal or rare earth ions, modulation doping to keep the electronically active channel impurity free, or proximity coupling to a magnetically ordered layer or substrate in heterostructures or superlattices. We review the challenges these approaches are facing in the famous 3D TI (Sb,Bi)2(Se,Te)3 family and try to answer the question whether these materials can live up to the hype surrounding them.

α -(AlxGa1−x)2O3 single-layer and heterostructure buffers for the growth of conductive Sn-doped α-Ga2O3 thin films via mist chemical vapor deposition

A third generation mist chemical vapor deposition (3rd G mist CVD) system was used to grow six single-layer and two heterostructure α-(AlxGa1−x)2O3 buffers on c-plane sapphire substrates for the subsequent deposition of conductive Sn-doped α-Ga2O3 (Sn:α-Ga2O3) thin films. In the six single-layer buffers, the Al contents x increased from 0 to 0.66. The two heterostructure buffers consisted of six ∼20-nm- and ∼100-nm-thick layers laying on top of each other. The 3rd G mist CVD system enabled the growth of these complicated multi-layer heterostructures in a single run, while mono-crystallinity was still maintained in all grown layers. Strain was observed in the 20-nm heterostructure, while the layers in the 100-nm heterostructure almost fully relaxed and the Vegard’s law was followed even when the α-(AlxGa1−x)2O3 layers were stacked on each other. Transmission electron microscopy analyses show that the dislocation densities remained high in the order of 1010 cm−2 despite the employment of the buffers. PtOx and AgOx Schottky diodes (SDs) were fabricated on the Sn:α-Ga2O3 films. The barrier height vs ideality factor plots could be fitted by linear dependences, indicating that the large ideality factors observed in α-Ga2O3 SDs could be explained by the inhomogeneity of the SDs. The extrapolation of the dependences for the PtOx and AgOx SDs yielded homogeneous Schottky barrier heights of ∼1.60 eV and 1.62 eV, respectively, suggesting that the Fermi level was pinned at the Ec − 1.6 eV level. The Sn:α-Ga2O3 film grown on the strained 20-nm heterostructure buffer showed best characteristics overall.

Investigation of the Effect of Doping on Transition Layers of Anisotype GaInAsP and InP Heterostructures Obtained by the Method of MOCVD

When studying doped anisotypic heterostructures with Ga1 – xInxAsyP1 – y layers grown on InP substrates with an InP buffer layer by metal organic chemical vapor deposition, the presence of transition regions is revealed in the Ga1 – xInxAsyP1 – y layer from the side of the substrate in some of the samples, along which the arsenic content (y) increases from the interface with the InP layer to the surface of the structure by a Δy value of up to 0.15, while the content of elements of the third group (x) remains constant.

Elastic Dipole Mechanism of the Formation and Collapse of Fano Resonances at the Transmission of Transverse Phonons through Layered Magnetic Heterostructures

At the resonant passage of a plane elastic shear wave through an acoustically continuous heterostructure consisting of magnetic and nonmagnetic layers in a symmetric nonmagnetic environment, the excitation of propagating exchangeless and exchange spin waves can lead to the formation of elastodipole analog of the Fano resonance and accompanying dynamic effects, including the collapse of the Fano resonance and the appearance of coupled magnon states in the continuum of transverse phonons.

Comparison of structure and multiferroic performances of bilayer and trilayer multiferroic heterostructures

This paper reports on the comparison of structure and multiferroic performances for Ba0.85Ca0.15Zr0.1Ti0.9O3/La0.7Sr0.3MnO3 (BCZT/LSMO) bilayer and BCZT/LSMO/BCZT trilayer multiferroic heterostructures. The epitaxial growth and the strain states of the multiferroic heterostructures are confirmed by the XRD RSM, which demonstrates that both the BCZT layer and LSMO layer in the trilayer heterostructure suffer larger residual strain than the bilayer one. The room temperature multiferroic nature of the multiferroic heterostructures is proved by the simultaneous observed ferroelectric and ferromagnetic properties as well as the magnetoelectric effect, which shows strong dependence of the residual strain. As compared with the trilayer heterostructure, the bilayer heterostructure shows better electrical and ferromagnetic properties as well as the ME effect. The largest piezoelectric coefficient of 40.5 p.m./V, the highest remnant polarization of 23.3 μC/cm2 and the biggest ferromagnetic Curie temperature of 316.7 K are obtained in the bilayer heterostructure because of the smaller residual strain. And the bilayer heterostructure also shows the strongest magnetoelectric effect with the αE31 value of 215.5 mV/cm·Oe.

Study on the transport properties of borophene/phosphorene heterojunctions

The electronic and transport properties of borophene/phosphorene heterojunctions are systematically investigated by first-principles calculations. The geometrical structures and electronic band structures of monolayer borophene, monolayer phosphorene and heterojunctions with different distances between layers are analysed. The calculated results show that the band gap can be adjusted by tuning the distance between heterostructure layers. Then the transport properties of the heterojunction with different electrodes are also studied. It can be found that the selected different electrodes have significant effect on the transport property of the heterojunction. Interestingly, when the heterostructure is selected as the electrode, the heterostructure with an interlayer distance of 2.65 Å showed the best transport performance. It is hoped that this paper may provide useful insights for the design of nanoelectronic devices based on borophene/phosphorene heterojunctions.

Robust In-Zn-O Thin-Film Transistors with a Bilayer Heterostructure Design and a Low-Temperature Fabrication Process Using Vacuum and Solution Deposited Layers.

We report on the design, fabrication, and characterization of heterostructure In-Zn-O (IZO) thin-film transistors (TFTs) with improved performance characteristics and robust operation. The heterostructure layer is fabricated by stacking a solution-processed IZO film on top of a buffer layer, which is deposited previously using an electron beam (e-beam) evaporator. A thin buffer layer at the dielectric interface can help to template the structure of the channel. The control of the precursors and of the solvent used during the sol–gel process can help lower the temperature needed for the sol–gel condensation reaction to proceed cleanly. This boosts the overall performance of the device with a significantly reduced subthreshold swing, a four-fold mobility increase, and a two-order of magnitude larger on/off ratio. Atomistic simulations of the a-IZO structure using molecular dynamics (both classical and ab initio) and hybrid density functional theory (DFT) calculations of the electronic structure reveal the potential atomic origin of these effects.

Graphene/WS2 Nanodisk Van der Waals Heterostructures on Plasmonic Ag Nanoparticle-Embedded Silica Metafilms for High-Performance Photodetectors

Coupling plasmons and excitons provide a promising approach to enhance the performance of photodetectors based on two-dimensional (2D) atomic layer heterostructures. Herein, we report a nanohybrid ...

Large Switchable Photoconduction within 2D Potential Well of a Layered Ferroelectric Heterostructure.

The coexistence of large conductivity and robust ferroelectricity is promising for high-performance ferroelectric devices based on polarization-controllable highly efficient carrier transport. Distinct from traditional perovskite ferroelectrics, Bi2 WO6 with a layered structure shows a great potential to preserve its ferroelectricity under substantial electron doping. Herein, by artificial design of photosensitive heterostructures with desired band alignment, three orders of magnitude enhancement of the short-circuit photocurrent is achieved in Bi2 WO6 /SrTiO3 at room temperature. The microscopic mechanism of this large photocurrent originates from separated transport of electrons and holes in [WO4 ]-2 and [Bi2 O2 ]+2 layers respectively with a large in-plane conductivity, which is understood by a combination of ab initio calculations and spectroscopic measurements. The layered electronic structure and appropriately designed band alignment in this layered ferroelectric heterostructure provide an opportunity to achieve high-performance and nonvolatile switchable electronic devices.

Effects of structural distortions on the electronic structure of T -type transition metal dichalcogenides

Single-layer transition metal dichalcogenides (TMDCs) can adopt two distinct structures corresponding to different coordination of the metal atoms. TMDCs adopting the T-type structure exhibit a rich and diverse set of phenomena, including charge density waves (CDW) in a $\sqrt{13}\times\sqrt{13}$ supercell pattern in TaS$_2$ and TaSe$_2$, and a possible excitonic insulating phase in TiSe$_2$. These properties make the T-TMDCs desirable components of layered heterostructure devices. In order to predict the emergent properties of combinations of different layered materials, one needs simple and accurate models for the constituent layers which can take into account potential effects of lattice mismatch, relaxation, strain, and structural distortion. Previous studies have developed ab initio tight-binding Hamiltonians for H-type TMDCs [arXiv:1709.07510]. Here we extend this work to include T-type TMDCs. We demonstrate the capabilities of our model using three example systems: a 1-dimensional sinusoidal ripple, the 2$\times$2 CDW in TiSe$_2$, and the $\sqrt{13}\times\sqrt{13}$ CDW in TaS$_2$. Using the technique of band unfolding we compare the electronic structure of the distorted crystals to the pristine band structure and find excellent agreement with direct DFT calculations, provided the magnitude of the distortions remains in the linear regime.