Superlattice Heterostructures Research Articles

Construction of Multilayer Films and Superlattice- and Mosaic-like Heterostructures of 2D Metal Oxide Nanosheets via a Facile Spin-Coating Process.

This study reports a design of a variety of nanostructured films of 2D oxide nanosheets. We systematically examined the deposition of perovskite-type Ca2Nb3O10- nanosheets by spin-coating their dimethyl sulfoxide dispersion. Neat and homogeneous monolayer tiling was attained on various substrates by selecting an optimum rotation speed, which was dependent on the nanosheet concentration. Repeating the optimized spin-coating process allowed for layer-by-layer deposition of the nanosheets into multilayer films with a designed layer number. Vertical superlattice heterostructures could also be assembled by alternately spin-coating the suspensions of Ca2Nb3O10- and Ti0.87O20.52- nanosheets. Furthermore, spin-coating of a mixed suspension of Ca2Nb3O10- and Ti0.87O20.52- nanosheets led to a mixed mosaic-like monolayer of these two nanosheets. The present study thus demonstrated spin-coating as a facile and powerful route to construct various nanostructures based on 2D oxide nanosheets.

Interface engineered MBE grown InAs/GaSb based type-II superlattice heterostructures

Strain balanced InAs/GaSb type-II superlattice structures have been grown using molecular beam epitaxy. InSb like interfaces have been introduced at both InAs on GaSb and GaSb on InAs surfaces using migration enhanced epitaxy to compensate the tensile strain between the constituent binaries of the superlattice. The superlattice layers have been characterized using high resolution X-ray diffraction, atomic force microscopy, transmission electron microscopy and absorption spectroscopy. It is shown that the substrate temperature influences the InSb thickness at the interface and therefore the net strain in the superlattice. The dependence of the surface morphology on the strained or relaxed state of the superlattice is discussed. The optimized InAs(10 ML)/InSb(0.5 ML)/GaSb(12 ML)/InSb(0.5 ML) superlattice epilayers up to 200 periods have been grown at temperature of 450 °C and have a small net strain of 0.0013%. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) analysis in the strain compensated superlattice layers reveal InSb like interfaces. Excellent crystalline quality and morphology with an absorption edge at 5.4 µm in the mid-wave infra-red (MWIR) region are observed for the optimized epilayers.

Proposal for energy efficient spin transfer torque-magnetoresistive random access memory device

Utilizing the electronic analogs of optical phenomena such as anti-reflection coating and resonance for spintronic devices, we propose and theoretically analyze the design of a spin transfer torque-magnetoresistive random access memory (STT-MRAM) device. The proposed device consists of a superlattice heterostructure terminated with the anti-reflective regions sandwiched between the fixed and free ferromagnetic layers. Employing Green’s function spin-transport formalism coupled self-consistently with the stochastic Landau–Lifshitz–Gilbert–Slonczewski equation, we design an STT-MRAM based on the anti-reflective superlattice magnetic tunnel junction (AR-SLMTJ) device having an ultrahigh tunnel magnetoresistance (≈3.5×104%) and large spin current. We demonstrate that the STT-MRAM based on the AR-SLMTJ structure owing to the physics of bandpass spin filtering is nearly 1100% more energy efficient than trilayer magnetic tunnel junction (MTJ) based STT-MRAM. We also present detailed probabilistic switching and energy analysis to find out the optimal point of operation of a trilayer MTJ and AR-SLMTJ based STT-MRAM. Our predictions serve as a template to consider the heterostructures for next-generation spintronic device applications.

Hot carrier relaxation and inhibited thermalization in superlattice heterostructures: The potential for phonon management

One of the main loss mechanisms in photovoltaic solar cells is the thermalization of photogenerated hot carriers via phonon-mediated relaxation. By inhibiting these relaxation mechanisms and reducing thermalization losses, it may be possible to improve the power conversion efficiency of solar cells beyond the single gap limit. Here, type-II InAs/AlAsSb multi-quantum well (MQW) structures are investigated to study the impact of the phononic properties of the AlAsSb barrier material in hot carrier thermalization. Experimental and theoretical results show that by increasing the barrier thickness (increasing the relative contribution of AlAsSb content in the superlattices), the relaxation of hot carriers is reduced as observed in power-dependent photoluminescence and thermalization analysis. This is attributed to an increase in the phononic bandgap of the MQW with increasing AlAsSb composition reducing the efficiency of the dominant Klemens mechanism as the phononic properties shift toward a more AlSb-like behavior.

Synthesis of superlattice heterostructure of germanium quantum dots in silicon spacer layers and its application in photovoltaic solar cells

Superlattice heterostructure of dislocation-free Ge quantum dots buried in Si spacer layers has been synthesized using ultra high vacuum-chemical vapor deposition technique. The heterostructure has been used as active layer in Si solar cells based upon vertical p-i-n junction configuration. Experiment shows that such solar cells exhibit strong repose to infrared radiation in contrast with the negligible response of solar cells with active layers of pure Si having the same thickness. A solar cell with 300 nm thick superlattice is shown to have thermodynamic efficiency of about 11.2%. This is correlated with the rather high quality of the heterostructure used in the present study as judged from photoluminescence spectroscopy and electron microscopy as well as strain and dark current measurements. It is concluded that such type of heterostructure may provide a pathway to develop efficient and cost effective solar cells. • Superlattice heterostructure of Ge quantum dots on Si has been synthesized. • Various techniques have been used to characterize the nanostructures. • The heterostructure has been used as active layer in Si (p-i-n)-type solar cells. • The thermodynamic efficiency of respective solar cell is determined.

Фотопроводящий ТГц детектор на основе новых функциональных слоев в многослойных гетероструктурах

We report on the experimental study of the photoconductive antennas (PCAs) - detectors based on superlattice heterostructures (SLS) InGaAs/InAs/InAlAs with different types of elastic stresses in their functional layers. By using our laboratory time-domain THz spectrometer we measured and compared the detected THz signals, noise characteristics, and signal-to-noise ratios of the developed bow-tie PCA-detectors at different average probe power. We showed that the SLS-based PCA-detector with elastic stresses of both compression and tension in the layers demonstrates increased THz detection bandwidth compared to that for the SLS-based PCA-detector with only compression stresses in the SLS layers in the whole range of the optical probe power. We thus demonstrate that modification of the SLS via introduction of the elastic strain in the crystalline lattice of its layers could become an efficient approach to enhance the PCA-detectors performance leading to implementation of these PCAs to the spectroscopic THz setups.

Three-in-one cathode host based on Nb3O8/graphene superlattice heterostructures for high-performance Li–S batteries

Superlattice Nb3O8/rGO with alternately restacked Nb3O8 nanosheets and rGO improves Li–S batteries performance by maximizing synergistic effects of components to prevent “shuttle effect” and promote lithium polysulfide conversion and Li2S nucleation

Conversion efficiency of resonant cavity enhanced narrow bandgap interband cascade photovoltaic cells

By combining an interband cascade (IC) configuration with an optical cavity, a novel approach to achieve efficient narrow bandgap photovoltaic (PV) cells is proposed. The proposed resonant cavity enhanced (RCE) ICPV cells can significantly alleviate the challenging issues in narrow bandgap semiconductor materials, which include a small absorption coefficient, a short diffusion length, and a high dark current density. Through simulations with realistic material parameters based on InAs/GaSb superlattice heterostructures, RCE ICPV cells are capable of achieving a conversion efficiency that exceeds 60%, which is much higher than what can be achieved with any other approach, especially with materials of a bandgap smaller than 0.3 eV. By varying structure parameters such as the absorber thickness, number of cascade stages, and the top mirror reflectance, we demonstrate how an RCE ICPV cell can be constructed to achieve an optimized device performance with high conversion efficiency. Also, a design example of a practical RCE narrow bandgap ICPV cell is provided.

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.

A Photoconductive THz Detector Based on a Superlattice Heterostructure with Plasmonic Amplification

A highly sensitive terahertz (THz) detector based on a photoconductive antenna (PCA) with plasmonic amplification on the basis of an InGaAs/InAlAs superlattice heterostructure is proposed. A noticeable increase in the photocurrent recorded by a plasmonic PCA detector and the signal-to-noise ratio was experimentally detected compared to the same parameters for the PCA detector without plasmonic electrodes. The efficiency of plasmonic electrodes has been experimentally confirmed by pulsed THz spectroscopy by measuring the dependence of the detector’s THz signal amplitude on the polarization of the incident pump laser radiation.

Synthesis of Tunable SnS-TaS2 Nanoscale Superlattices

Nanoscale superlattices represent a compelling platform for designed materials as the specific identity and spatial arrangement of constituent layers can lead to tunable properties. A number of kinetically stabilized, nonepitaxial superlattices with almost limitless structural tunability have been reported in telluride and selenide chemistries but have not yet been extended to sulfides. Here, we present SnS-TaS2 nanoscale superlattices with tunable layer architecture. Layered amorphous precursors are prepared as thin films programmed to mimic the targeted superlattice; subsequent low temperature annealing activates self-assembly into crystalline nanocomposites. We investigate structure and composition of superlattices comprised of monolayers of TaS2 and 3-7 monolayers of SnS per repeating unit. Furthermore, a graded precursor preparation approach is introduced, allowing stabilization of superlattices with multiple stacking sequences in a single preparation. Controlled synthesis of the architecture of nanoscale superlattices is a critical path toward tuning their exotic properties and enabling integration with electronic, optical, or quantum devices.

Phononic bandgap and phonon anomalies in HfN and HfN/ScN metal/semiconductor superlattices measured with inelastic x-ray scattering

Epitaxial metal/semiconductor superlattice heterostructures with lattice-matched abrupt interfaces and suitable Schottky barrier heights are attractive for thermionic energy conversion, hot electron-based solar energy conversion, and optical hyperbolic metamaterials. HfN/ScN is one of the earliest demonstrations of epitaxial single-crystalline metal/semiconductor heterostructures and has attracted significant interest in recent years to harness its excellent properties in device applications. Although the understanding of the mechanism of thermal transport in HfN/ScN superlattices is extremely important for their practical applications, not much attention has been devoted to measuring their phonon dispersion and related properties. In this Letter, we employ non-resonant meV-resolution inelastic x-ray scattering to determine the momentum-dependent phonon modes in epitaxial metallic HfN and lattice-matched HfN/ScN metal/semiconductor superlattices. HfN exhibits a large phononic bandgap (∼40 meV) and Kohn anomaly in the longitudinal and transverse acoustic phonon modes at q ∼ 0.73 along the [100] and [110] directions of the Brillouin zone due to the nesting of the Fermi surface by the wave vector (q). The in-plane [100] acoustic phonon dispersion of the HfN/ScN superlattices is found to be dominated by the HfN phonons, while the optical phonons exhibit both ScN and HfN characteristics. First-principles density functional perturbation theory modeling is performed to explain the experimental phonon spectra, and temperature-dependent thermal conductivity is measured using a pump-probe spectroscopic technique. These results will help understand the phonons in HfN and HfN/ScN metal/semiconductor superlattices for thermionic energy conversion.

Growth, structural and optical properties of coherent κ-(AlxGa1−x)2O3/κ-Ga2O3 quantum well superlattice heterostructures

High quality heteroepitaxial (001)-oriented κ-(AlxGa1−x)2O3/κ-Ga2O3 quantum well superlattice heterostructures were deposited by tin-assisted pulsed laser deposition on c-sapphire substrates. Sharp superlattice fringes up to the ninth order in XRD patterns for Al-contents up to about 50 at. % confirm excellent structural quality and smooth interfaces in the multilayers on par with reports on homoepitaxial superlattices in the monoclinic modification. By employing elliptically segmented targets, the Al-content in the barrier layers of the superlattices was systematically varied in a range of 0.1 ≤ x ≤ 0.5 in a controlled and quasi-continuous manner. An in-depth investigation employing XRD 2θ-ω scans and reciprocal space map measurements on superlattices with different periods as well as single quantum well samples suggests coherent growth of the superlattices for application-relevant quantum well widths. The critical thickness for coherent growth of κ-Ga2O3 on κ-(AlxGa1−x)2O3 was further estimated to be at least 50 nm and 3 nm for x = 0.2 and x = 0.3, respectively. We determined absorption energies in optical transmission spectra for superlattices with x = 0.3 well below the bandgap of the barrier layers that decrease with increasing quantum well width suggesting transitions between localized states in the quantum wells as their origin. These results render superlattices in the metastable orthorhombic phase of Ga2O3 as a promising active layer for quantum well infrared photodetector applications.

An efficient tunable thermoelectric device based on n and p doped graphene superlattice heterostructures

Abstract This paper theoretically predicts a large Seebeck coefficient in p and n doped graphene superlattice heterostructures based thermoelectric device, by tuning the electrical and structural parameters. By introducing a single graphene layer doped to a different extent compared to other layers in between the symmetric p & n structure, a resonant tunneling state occurs inside the forbidden gap of the electron transmittance in respect of electron energy and this creates a giant thermoelectric effect. The tunneling state and the corresponding maximum value of the Seebeck coefficient can be modulated by changing the doping concentration and width of the graphene layers. The doping concentration and the type of graphene layer p or n are controlled by the magnitude and polarity of the electric potential applied to the electrodes placed over the graphene. The modulation of the Seebeck coefficient by doping concentration, period number, electron incidence angle and width of the graphene layers can lead to an electron wave filter and efficient tunable thermoelectric device for application in the field of non-conventional energy sources. This paper describes the effect of electrical and structural parameters on the Seebeck coefficient and assortment of these parameters for design an efficient graphene-based thermoelectric device.

Moir\xe9 patterns arising from bilayer graphone/graphene superlattice

Moiré patterns from two-dimensional (2D) graphene heterostructures assembled via van der Waals interactions have sparked considerable interests in physics with the purpose to tailor the electronic properties of graphene. Here we report for the first time the observation of moiré patterns arising from a bilayer graphone/graphene superlattice produced through direct single-sided hydrogenation of a bilayer graphene on substrate. Compared to pristine graphene, the bilayer superlattice exhibits a rippled surface and two types of moiré patterns are observed: triangular and linear moiré patterns with the periodicities of 11 nm and 8–9 nm, respectively. These moiré patterns are revealed from atomic force microscopy and further confirmed by following fast Fourier transform (FFT) analysis. Density functional theory (DFT) calculations are also performed and the optimized lattice constants of bilayer superlattice heterostructure are in line with our experimental analysis. These findings show that well-defined triangular and linear periodic potentials can be introduced into the graphene system through the single-sided hydrogenation and also open a route towards the tailoring of electronic properties of graphene by various moiré potentials.

Фотопроводящий THz-детектор на основе сверхрешеточной гетероструктуры с плазмонным усилением

We report on a high-sensitive THz plasmonic-enhanced photoconductive antenna (PCA)-detector based on an InGaAs/InAlAs superlattice heterostructure. A noticeable increase in the sensitivity and signal-to-noise ratio of the proposed PCA-detector is shown compared to a similar detector without plasmonic electrodes. The efficiency of plasmonic electrodes has been experimentally confirmed via pulsed THz time-domain spectroscopy by measuring the dependence of the THz amplitude of the detector’s signal on the polarization of laser excitation.

Observation of moiré excitons in WSe2/WS2 heterostructure superlattices.

Moiré superlattices enable the generation of new quantum phenomena in two-dimensional heterostructures, in which the interactions between the atomically thin layers qualitatively change the electronic band structure of the superlattice. For example, mini-Dirac points, tunable Mott insulator states and the Hofstadter butterfly pattern can emerge in different types of graphene/boron nitride moiré superlattices, whereas correlated insulating states and superconductivity have been reported in twisted bilayer graphene moiré superlattices1-12. In addition to their pronounced effects on single-particle states, moiré superlattices have recently been predicted to host excited states such as moiré exciton bands13-15. Here we report the observation of moiré superlattice exciton states in tungsten diselenide/tungsten disulfide (WSe2/WS2) heterostructures in which the layers are closely aligned. These moiré exciton states manifest as multiple emergent peaks around the original WSe2 A exciton resonance in the absorption spectra, and they exhibit gate dependences that are distinct from that of the A exciton in WSe2 monolayers and in WSe2/WS2 heterostructures with large twist angles. These phenomena can be described by a theoretical model in which the periodic moiré potential is much stronger than the exciton kinetic energy and generates multiple flat exciton minibands. The moiré exciton bands provide an attractive platform from which to explore and control excited states of matter, such as topological excitons and a correlated exciton Hubbard model, in transition-metal dichalcogenides.

Evidence of a gate-tunable Mott insulator in a trilayer graphene moiré superlattice

Mott insulator plays a central role in strongly correlated physics, where the repulsive Coulomb interaction dominates over the electron kinetic energy and leads to insulating states with one electron occupying each unit cell. Doped Mott insulator is often described by the Hubbard model3, which can give rise to other correlated phenomena such as unusual magnetism and even high-temperature superconductivity. A tunable Mott insulator, where the competition between the Coulomb interaction and the kinetic energy can be varied in situ, can provide an invaluable model system for the study of Mott physics. Here we report the realization of such a tunable Mott insulator in the ABC trilayer graphene (TLG) and hexagonal boron nitride (hBN) heterostructure with a moir\'e superlattice. Unlike massless Dirac electrons in monolayer graphene, electrons in pristine ABC TLG are characterized by quartic energy dispersion and large effective mass that are conducive for strongly correlated phenomena. The moir\'e superlattice in TLG/hBN heterostructures leads to narrow electronic minibands that are gate tunable. Each filled miniband contains 4 electrons in one moir\'e lattice site due to the spin and valley degeneracy of graphene. The Mott insulator states emerge at 1/4 and 1/2 fillings, corresponding to one electron and two electrons per site, respectively. Moreover, the Mott states in the ABC TLG/hBN heterostructure exhibit unprecedented tunability: the Mott gap can be modulated in situ by a vertical electrical field, and at the meantime, the electron doping can be gate-tuned to fill the band from one Mott insulating state to another. Our observation of a tunable Mott insulator opens up exciting opportunities to explore novel strongly correlated phenomena in two-dimensional moir\'e superlattice heterostructures.

Electronic band structure engineering in InAs/InSbAs and InSb/InSbAs superlattice heterostructures

We report a detailed ab initio study of two superlattice heterostructures, one component of which is a unit cell of CuPt ordered InSb0.5As0.5. This alloy part of the heterostructures is a topological semimetal. The other component of each system is a semiconductor, zincblende-InSb, and wurtzite-InAs. Both heterostructures are semiconductors. Our theoretical analysis predicts that the variation in the thickness of the InSb layer in InSb/InSb0.5As0.5 heterostructure renders altered bandgaps with different characteristics (i.e., direct or indirect). The study holds promise for fabricating heterostructures, in which the modulation of the thickness of the layers changes the number of carrier pockets in these systems.

First principles study of hBN-AlN short-period superlattice heterostructures

We report a theoretical study of the structural, electronic, and optical properties of hBN-AlN superlattice (SL) heterostructures using a first-principles approach based on standard and hybrid density functional theory. We consider a short-period (L < 10 nm) SL and find that its properties depend strongly on the AlN layer thickness LAlN. For LAlN≲1 nm, AlN stabilizes into the hexagonal phase and the SL displays insulating behavior with type II interface band alignment and optical gaps as small as 5.2 eV. The wurtzite phase is formed for thicker AlN layers. In these cases, built-in electric fields lead to the formation of polarization compensating charges as well as two-dimensional conductive behavior for electronic transport along interfaces. We also find defect-like states localized at interfaces which are optically active in the visible range.