Asymmetric Heterostructures Research Articles

Resonant transport in a double-barrier heterostructure AlxGa1−xAs/GaAs and mass discontinuity: a semiclassical path integral approach

In this paper, we investigate the dependence of the resonant carrier transport in an AlGaAs/GaAs double-barrier heterostructure on the electron’s effective mass discontinuity between the well and the barrier layer. We use a one-dimensional model for the effective potential and consider the activation of the resonant tunneling mechanism when a bias is applied between the emitter and the collector. We calculate the system’s Green’s function through semiclassical path integration to obtain the complex spectrum. Electron’s effective mass discontinuity affects our calculations through the contributions of the various propagations inside the heterostructure and the multiple reflections from the turning points. Our findings show that the lowest resonance energy decreases linearly with increasing bias, while the real energy shift of the resonance decreases in a non-linear manner with the mass discontinuity. We also observe that the resonant transport time increases nearly linearly with the mass discontinuity. These results demonstrate the impact of the carrier’s inertia enhancement on the resonance characteristics and are consistent with established concepts in quantum mechanics and previously published works. Furthermore, we identify a critical bias value that distinguishes the behavior of the symmetric and asymmetric heterostructure. Additionally, the derivative of the transport time with respect to mass discontinuity is found to be independent of the applied bias. Our results provide concise analytical relationships involving parameters such as barrier and well thickness and shape, enabling the prediction and optimization of various nanoscale devices’ performance.

Brain‐inspired Multimodal Synaptic Memory via Mechano‐photonic Plasticized Asymmetric Ferroelectric Heterostructure

AbstractNeuromorphic devices capable of emulating biological synaptic behaviors are crucial for implementing brain‐like information processing and computing. Emerging 2D ferroelectric neuromorphic devices provide an effective means of updating synaptic weight aside from conventional electrical/optical modulations. Here, by further synergizing with an energy‐efficient synaptic plasticity strategy, a multimodal mechano‐photonic synaptic memory device based on 2D asymmetric ferroelectric heterostructure is presented, which can be modulated by external mechanical behavior and light illumination. By integrating the asymmetric ferroelectric heterostructured field‐effect transistor and a triboelectric nanogenerator, the mechanical displacement‐derived triboelectric potential is ready for gating, programming, and plasticizing the synaptic device, resulting in superior electrical properties of high on/off ratios (> 107), large storage windows (equivalent to ≈95 V), excellent charge retention capability (> 104 s), good endurance (> 103 cycles), and primary synaptic behaviors. Besides, optical illumination can effectively synergize with mechanoplasticity to implement multimodal spatiotemporally correlated dynamic logic. The demonstrated multimodal memory synapse provides a facile and promising strategy for multifunctional sensory memory, interactive neuromorphic devices, and future brain‐like electronics embodying artificial intelligence.

Light-Triggered Anti-ambipolar Transistor Based on an In-Plane Lateral Homojunction.

Currently, the construction of anti-ambipolar transistors (AATs) is primarily based on asymmetric heterostructures, which are challenging to fabricate. AATs used for photodetection are accompanied by dark currents that prove difficult to suppress, resulting in reduced sensitivity. This work presents light-triggered AATs based on an in-plane lateral WSe2 homojunction without van der Waals heterostructures. In this device, the WSe2 channel is partially electrically controlled by the back gate due to the screening effect of the bottom electrode, resulting in a homojunction that is dynamically modulated with gate voltage, exhibiting electrostatically reconfigurable and light-triggered anti-ambipolar behaviors. It exhibits high responsivity (188 A/W) and detectivity (8.94 × 1014 Jones) under 635 nm illumination with a low power density of 0.23 μW/cm2, promising a new approach to low-power, high-performance photodetectors. Moreover, the device demonstrates efficient self-driven photodetection. Furthermore, ternary inverters are realized using monolithic WSe2, simplifying the manufacturing of multivalued logic devices.

Performance-enhanced tribovoltaic nanogenerator by regulating carrier transportation with an asymmetric heterostructure

The tribovoltaic nanogenerator (TVNG) is emerging as a promising circuit-integrative energy harvester, with notable advantages like direct current output and large current density. Nevertheless, the research on optimizing charge excitation and carrier transportation remains deficient. In this work, we report an alternative approach for practically promoting the output performance of silicon (Si)-based TVNG. A well-designed asymmetric heterostructure is achieved by inserting an appropriate interlayer between the friction layer and the bottom electrode. Carrier extraction efficiency has been promoted effectively, while carrier recombination has been restrained owing to the built-in electric field excited by p-Si/ZnO heterojunction. The coupling mechanism of the built-in electric field and the interfacial electric field has been revealed explicitly with a comprehensive theoretical model, which is based on the capacitance feature of the PN junction. The designed multilayer TVNG (MTVNG) has shown 20 times higher output compared to normal Si-based TVNGs. Apart from presenting fundamental insights into the tribovoltaic effect, we have developed a dual-mode analysis method for vibration monitoring. This work expands the path to improve TVNG output through multi-electric field coupling and provides new inspiration for miniaturized vibration sensors in real-time deployments.

Janus Hydrophobic Structural Gel with Asymmetric Adhesion in Air/Underwater for Reliable Mechanosensing

AbstractReliable interfacial bonding is an essential guarantee that flexible electronics can output realistic signals, especially for underwater scenes. However, conventional self‐adhesive materials usually suffer from underwater adhesion failure, conflict between adhesion and cohesion, as well as adverse effects of isotropous adhesion and residue, greatly limiting their applications in flexible electronics. Herein, a Janus hydrophobic structural gel (HSG) with asymmetric adhesion is fabricated by a “grafting one twig on another” approach (in situ constructing a hydrophobic anti‐adhesive gel on the top of a hydrophobic self‐adhesive gel). The hydrophobic adhesion layer with long C18 aliphatic chains achieves a reliable underwater bonding (interfacial toughness exceeds 80 J m−2) with the assistance of high mobility of polymer chains, multiple interfacial interactions, and effective removal of interfacial liquid. The hydrophobic anti‐adhesion layer containing poly(ionic liquid) is more robust due to electrostatic and ion‐dipole interactions, ensuring the mechanical strength of the integral HSG. Such asymmetric heterostructure avoids common nonessential adhesion and residue, facilitating operation. The intrinsic hydrophobicity of HSG also prevents water erosion, achieving reliable underwater adhesion and sensing. As a result, the assembled sensor based on HSG can stably monitor human motions and wirelessly transmit underwater information, exhibiting an enormous potential in wearable electronics.

Hydrostatic pressure and temperature effects on nonlinear optical properties in harmonic-Gaussian asymmetric double quantum wells

The paper examines the linear and non-linear optical characteristics of an electron in harmonic Gaussian asymmetrical double quantum wells, taking into account thermodynamic variables such as temperature and hydrostatic pressure. Numerical calculations by considering the effective mass and parabolic band approximation are performed. The electron contained within an asymmetric double well generated by the sum of a parabolic and Gaussian potential has its eigenvalues and eigenfunctions determined using the diagonalization approach. For nonlinear optical coefficients, the density matrix expansion is used. Wavefunctions and energy levels vary as an effect of the applied fields. In harmonic Gaussian asymmetrical double quantum wells, the total optical absorption coefficient (TOAC), the relative refractive index changes (RRIC), and second harmonic generation (SHG) have all been theoretically investigated. The magnitude and position of the resonant peaks are significantly influenced by the hydrostatic pressure and temperature effects. With controllable coupling and externally applied hydrostatic pressure and temperature, the potential model presented in this study can be used to simulate and manipulate the optical and electronic properties of the asymmetric double-quantum heterostructures, such as double quantum dots and wells.

Self-powered AlGaN-based MSM solar-blind ultraviolet photodetectors with high Al-content AlxGa1−xN/AlyGa1−yN asymmetrical heterostructure

An Al0.4Ga0.6N-based metal–semiconductor–metal (MSM) ultraviolet (UV) photodetector (PD) based on intentionally asymmetrical polarized Al0.55Ga0.45N/Al0.4Ga0.6N/Al0.65Ga0.35N heterostructure has been fabricated and investigated, which achieves the self-powered capabilities. Operated at zero bias, the device presents an ultralow dark current of 7.8 × 10−13 A, a high peak responsivity of 0.04 A/W at ∼275 nm, a cut-off wavelength at ∼285 nm, and a corresponding detectivity of 3.1 × 1012 Jones. In comparison, the device with symmetrical heterostructure has no response at 0 V, confirming the effect of the proposed asymmetrical MSM structure. Furthermore, it is expressly demonstrated that the structure provides an asymmetrical energy band due to different barrier heights at the metal/Al0.4Ga0.6N and metal/Al0.55Ga0.45N/Al0.4Ga0.6N Schottky contacts and enhances the built-in electric field in the Al0.4Ga0.6N active layer owing to its polarization effect through simulations theoretically. Therefore, the improvement of photogenerated carrier transport can be obtained at 0 V, contributing to the high-performance self-powered UV PD.

High-Power Multimode Laser Diodes (λ = 976 nm) Based on Asymmetric Heterostructures with a Broadened Waveguide and Reduced Vertical Divergence

High-Power Multimode Laser Diodes (λ = 976 nm) Based on Asymmetric Heterostructures with a Broadened Waveguide and Reduced Vertical Divergence

Interfacial engineering-induced Janus heterostructures with enhanced electronic regulation for efficient oxygen electrocatalysis in rechargeable Zn-air batteries

Construction of efficient non-noble metal electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are extremely important for zinc-air batteries (ZABs). Herein, a novel Janus heterostructure, coupling CeO2 with bimetallic metal sulfides (ZnCoS) with precise heterointerfaces, was designed through an interfacial engineering-induced strategy. Benefiting from the interfacial synergistic effect, the asymmetric Janus heterostructure of CeO2/ZnCoS (J−CeO2/ZCS) exhibits satisfactory ORR and OER activity, in terms of E1/2 of 0.87 V for ORR and overpotential of 290 mV for OER. Theoretical calculation further demonstrates that the heterointerface effectively modulates the electronic structure and charge redistribution, thus improving redox reactivity of active sites and the absorption/desorption of reaction intermediates. When the J-CeO2/ZCS applied in liquid and flexible ZABs as air cathodes, the devices deliver excellent power density and cycling stability. This Janus structure of composite catalysts affords a new opportunity for designing efficient oxygen electrocatalysts with heterostructures.

Quasi-CW, High-Power, Extended-Cavity Laser Diode Microarrays (λ = 976 nm) Based on Asymmetric Heterostructures with an Ultrawide Waveguide

Quasi-CW, High-Power, Extended-Cavity Laser Diode Microarrays (λ = 976 nm) Based on Asymmetric Heterostructures with an Ultrawide Waveguide

Thermal rectification through the topological states of asymmetrical length armchair graphene nanoribbons heterostructures with vacancies

We present a theoretical investigation of electron heat current in asymmetrical length armchair graphene nanoribbon (AGNR) heterostructures with vacancies, focusing on the topological states (TSs). In particular, we examine the 9-7-9 AGNR heterostructures where the TSs are well-isolated from the conduction and valence subbands. This isolation effectively mitigates thermal noise of subbands arising from temperature fluctuations during charge transport. Moreover, when the TSs exhibit an orbital off-set, intriguing electron heat rectification phenomena are observed, primarily attributed to inter-TS electron Coulomb interactions. To enhance the heat rectification ratio (η Q ), we manipulate the coupling strengths between the heat sources and the TSs by introducing asymmetrical lengths in the 9-AGNRs. This approach offers control over the rectification properties, enabling significant enhancements. Additionally, we introduce vacancies strategically positioned between the heat sources and the TSs to suppress phonon heat current. This arrangement effectively reduces the overall phonon heat current, while leaving the TSs unaffected. Our findings provide valuable insights into the behavior of electron heat current in AGNR heterostructures, highlighting the role of topological states, inter-TS electron Coulomb interactions, and the impact of structural modifications such as asymmetrical lengths and vacancy positioning. These results pave the way for the design and optimization of graphene-based devices with improved thermal management and efficient control of electron heat transport.

Rate equations description of the asymmetric double barrier electronic cooler

Recent experimental results showed that an electron gas in an asymmetrical double barrier heterostructure can be effectively cooled down under resonant tunneling condition, thus leading to the realization of an electronic cooler. The cooling process is a multi-parameters phenomenon and it is desirable to handle this problem through a reasonably simple approach, in order to understand the role of each parameter. To this end, we present a rate equation modeling of the electron cooling. We model the resonant tunnel injection of the electrons in the well and their thermionic emission assisted by Longitudinal Optical (LO) phonons absorption and emission. The influence of several parameters on the electronic temperature is discussed. This simple model compares rather well to the predictions of non-equilibrium Green function approach and to the experiments.

Implementation of energy barrier layers for 1550 nm high-power laser diodes

The influence of a thin AlInAs energy barrier on the efficiency of 1550 nm high-power semiconductor lasers efficiency has been experimentally studied. It was shown that the position and number of barriers in an asymmetric laser heterostructure based on a 1.8–1.9 μm thick waveguide has a significant effect on the output optical power. It is shown that in a barrierless structure, the main reason for radiative efficiency decrease is internal quantum yield drop due to the absence of an energy barrier for the type-II heterojunction at the waveguide-p-cladding interface and electron leakage to the p-emitter layer. It is demonstrated that the implementation of single AlInAs energy barrier layer on waveguide-p-cladding heterojunction allows significantly increase laser diode maximum output power. 2 W maximum CW optical power has been achieved from 40 μm aperture laser diode at heatsink temperature 25 °C.

Temperature-Induced Revolving Effect of Electronic Flow in Asymmetric Double-Barrier Semiconductor Heterostructures

Temperature-Induced Revolving Effect of Electronic Flow in Asymmetric Double-Barrier Semiconductor Heterostructures

PbTiO3 Based Single-Domain Ferroelectric Photocatalysts for Water Splitting

ConspectusSolar-driven photocatalytic water splitting paves a way to produce green hydrogen for building a sustainable clean energy system, particularly within the framework of the carbon-neutral initiative. However, to date, the solar-to-hydrogen (STH) conversion efficiency of particulate photocatalysts falls far short of the demand of over 10% for industrial applications. Single-domain ferroelectric semiconductor materials with amazing unidirectional spontaneous polarization penetrating the bulk are promising candidates as photocatalysts for water splitting. Their existing inherent internal field set up by polarization can cause both two oppositely charged surfaces (namely, polar surfaces) and spatial separation of photogenerated charge carriers between them upon light excitation. These unique properties provide sufficient new room for flexibly engineering of bulk and surface/interface structures to enhance photocatalytic water splitting.In this Account, we offer a systematic overview of the exploration of PbTiO3 based single-domain ferroelectric photocatalysts for efficient solar water splitting. In detail, the controlled growth, surface modification, heterostructures, and Z-scheme system of single-domain PbTiO3 ferroelectrics are discussed by considering polar surfaces, spatial separation of the photoexcited charges, selective distribution of defects, and selective adsorption of reactive species. We start with the controlled synthesis of single-domain ferroelectric PbTiO3 nanoplates to realize the opposite directional transport of photoexcited charges driven by the polarized internal electric field, which can be verified by the fact that photochemical reduction/oxidation deposits occur on opposite polar surfaces. Then, we move to the selective modification of the specific polar surface to promote the transfer of photoexcited charges by applying the spatial separation effect of photoexcited charges and defects related internal screening mechanism. Furthermore, asymmetric heterostructures and Z-scheme systems are rationally designed by selective adsorption of reactive species on charged surfaces to promote charge separation and suppress redox mediator side reactions, respectively. Although significant advances have been achieved, many efforts remain needed toward the industrial production target. Based on single-domain ferroelectric photocatalysts, we present the following perspectives on the utilization of a wide solar spectrum for improving solar energy conversion efficiency. First, proper dopants can be incorporated homogeneously in the bulk to reduce the bandgap without obviously weakening ferroelectricity. Second, the epitaxial growth of narrow bandgap semiconductors on the ferroelectrics is feasible to induce additional polarization in the region close to their interface. These two aspects are anticipated to enable strong visible light absorption and charge separation efficiency. Third but not least, the assembly of Z-scheme systems with two narrow bandgap ferroelectrics is desirable to mimic natural photosynthetic systems with quantum yield approaching one unit.

Harmonic-Gaussian Symmetric and Asymmetric Double Quantum Wells: Magnetic Field Effects

In this study, we considered the linear and non-linear optical properties of an electron in both symmetrical and asymmetrical double quantum wells, which consist of the sum of an internal Gaussian barrier and a harmonic potential under an applied magnetic field. Calculations are in the effective mass and parabolic band approximations. We have used the diagonalization method to find eigenvalues and eigenfunctions of the electron confined within the symmetric and asymmetric double well formed by the sum of a parabolic and Gaussian potential. A two-level approach is used in the density matrix expansion to calculate the linear and third-order non-linear optical absorption and refractive index coefficients. The potential model proposed in this study is useful for simulating and manipulating the optical and electronic properties of symmetric and asymmetric double quantum heterostructures, such as double quantum wells and double quantum dots, with controllable coupling and subjected to externally applied magnetic fields.

Rashba spin-orbit coupling induced modulation of magnetic anisotropy in canted antiferromagnetic heterostructures

The combined effect of charge transfer, broken symmetries, and spin-orbit coupling (SOC) yields interfacial magnetism and transport in spatially asymmetric quantum oxide heterostructures. The polar nature of such interfaces activates the Rashba effect which provides a platform to modify the electronic band structure, whereas the presence of competing magnetic anisotropies is essential for deriving the topologically nontrivial spin textures. Entanglement of these two effects wherein the Rashba physics can be utilized to manipulate magnetic anisotropy (MA) is contemplated as a pathway to alternative electronic states. Here, using the $\mathrm{CaMn}{\mathrm{O}}_{3}/\mathrm{CaIr}{\mathrm{O}}_{3}$ heterostructure as a template, we demonstrate that Rashba SOC modulates the MA and controls the sign and magnitude of anomalous Hall conductivity consequent to the modification in the electronic band structure via the reconstruction of Berry curvature. This approach to tune and control the multipronged magnetotransport phenomena via Rashba SOC (without external bias voltage) is potentially relevant for spin-orbitronics functionalities.

Bridge role of weak chemical bonding in photocatalytic performance of asymmetric 2H-MoS2/BiOCl Janus heterostructure

The geometric and electronic structure, partial (band decomposed) charge density, charge transfer, electron localization function and photocatalytic mechanism of the asymmetric 2H-MoS2/BiOCl Janus heterostructure were systematically studied with first-principles density functional theory. Our calculations showed that there exist several newly formed weak Bi-S bonds with shorter bond lengths between BiOCl and 2H-MoS2 which act as an electron transport bridge along the direction perpendicular to the heterojunction interface. This newly weak bonds lead to the formation of occupied shallow defect levels approximately 0.0–0.9 eV below the bottom of the conduction band. Electrons located at these defect levels can easily jump into the conduction band as a donor energy level under thermal fluctuations and simultaneously further promote the effective separation of photo-generated electron-hole pairs in the BiOCl. The photogenerated electrons located around Bi-atom layer in the conduction band of BiOCl transfer to the valence band of 2H-MoS2 around the S-atom layer through the interface of the asymmetric 2H-MoS2/BiOCl Janus heterostructure, which significantly reduce photo-generated holes in the 2H-MoS2 and electrons in the BiOCl. The large numbers of photogenerated electrons from the 2H-MoS2 cannot recombine with holes owing to lack of sufficient holes. They will move to the surface and greatly improve the hydrogen production activity in the 2H-MoS2. While the photogenerated holes from the BiOCl will significantly improve the ability of BiOCl to oxidize pollutant in the water owing to the absence of sufficient electrons. Our studies provide new way for the design of asymmetric Janus double-layer heterostructures with newly formed weak chemical bonding.

Стимулированное излучение в гетероструктурах InAs/InAsSb/InAsSbP с асимметричным электронным ограничением

The electroluminescent characteristics of the InAs/InAs1−ySby /InAsSbP asymmetric light-emitting diode heterostructures with high InSb mole fraction in the active region (y > 0.09) in the temperature range 4.2−300 K have been studied. Stimulated emission was achieved in the wavelength range 4.1−4.2 μm at low temperatures (T < 30 K). It was found that the electroluminescence spectra were formed as a result of the superposition of contributions from different channels of radiative recombination of charge carriers near the type II heterointerface. The effect of the quality of the type II InAsSb/InAsSbP heterojunction on the radiative interface transitions with an increase in the content of InSb in the ternary solid solution is considered.

Stimulated emission in the InAs/InAsSb/InAsSbP heterostructures with asymmetric electronic confinement

The electroluminescent characteristics of the InAs/InAs1-ySby/InAsSbP asymmetric light-emitting diode heterostructures with high InSb mole fraction in the active region (y>0.09) in the temperature range 4.2-300 K have been studied. Stimulated emission was achieved in the wavelength range 4.1-4.2 μm at low temperatures (T<30 K). It was found that the electroluminescence spectra were formed as a result of the superposition of contributions from different channels of radiative recombination of charge carriers near the type II heterointerface. The effect of the quality of the type II InAsSb/InAsSbP heterojunction on the radiative interface transitions with an increase in the content of InSb in the ternary solid solution is considered. Keywords: heterojunctions, InAs, antimonides, electroluminescence, light-emitting diodes.