Polar Heterostructures Research Articles

Synergizing internal electric field and quantum-confined stark effect for customized carrier dynamics in two-dimensional heterostructures

The customized transport of carriers within two-dimensional (2D) heterostructures (HSs) is instrumental in enhancing the electronic and optoelectronic functionalities. Internal electric field (IEF) has opened potential pathways for manipulating carrier migration behaviors, unlocking diverse applications such as photocatalysts, solar cells, photodetectors, and other photoelectrochemical (PEC) devices. While it is intuitively understood that the orientation of IEF governs the direction of carrier transport, there are notable discrepancies between theoretical predictions and experimental results concerning the dynamics of photogenerated carriers. In this work, based on several polar HSs with tunable IEF, we unveiled the role of IEF in the carrier dynamics within these HSs. Our findings show that IEF not only regulates the band alignment of the HSs but also manipulates the electron–phonon (e-ph) coupling. The synergistic interplay of IEF and the quantum-confined Stark effect determines the dynamics of photoexcited carriers, facilitating the transition between type-Ⅱ and Z-scheme. These insights offer prospective strategies for tailoring interlayer charge carrier migration pathways and the photocatalytic activity of HSs.

The coupling effect of polarization and lattice strain on charge transfer between quintuple-layer Al2X3/Al2Y3 (X, Y = O, S, Se, Te; X ≠ Y) interfaces

The electronic properties of two-dimensional (2D) polar semiconductor heterostructures are closely related to the degree of interlayer charge transfer. Taking 2D quintuple-layer (QL)-Al2S3 and QL-Al2O3 semiconductor as examples, we study the electronic properties and interlayer charge transfer of QL-Al2S3/Al2O3 interfaces by first-principles calculations. The driving force of the directional interlayer charge transfer is the polarization electric field. The kinetic process of interface charge transfer is related to the charge (strain) distribution of QL-Al2S3 and QL-Al2O3. By changing the strain distribution of QL-Al2S3/Al2O3 interfaces with same polarization arrangement, the electronic properties and interfacial charge transfer of heterojunctions can be adjusted. Our work is an important indicator for understanding the dynamic process of interlayer charge transfer between 2D polar semiconductors. In addition, we propose one method for regulating the electronic properties of heterojunctions by coupling polarization and lattice strain.

Tunable infrared surface phonon–plasmon coupling in graphene-integrated polar semiconductor heterostructure

Within Reststrahlen bands of polar semiconductors, surface phonon–plasmon coupling is of great interest in infrared nanophotonics. Here, we demonstrate an active long-wavelength infrared device of graphene integrated with an AlN/SiC polar heterostructure. As a low-loss dielectric design, the subwavelength structure device takes advantage of interfacial photogating effect on electrostatic doping of the graphene and the interfaced SiC, and the tunable spectral behavior is originated from the hybridization of the doping-dependent surface phonon–plasmon resonances. This finding provides a steady-state manipulating method to the surface modes for the low-loss nanophotonic devices on SiC platform, and the graphene Fermi level tunable to cross the Dirac point in a steady response even makes the intrinsic graphene photodetectors feasible.

Room temperature valley polarization via spin selective charge transfer

The two degenerate valleys in transition metal dichalcogenides can be used to store and process information for quantum information science and technology. A major challenge is maintaining valley polarization at room temperature where phonon-induced intervalley scattering is prominent. Here we demonstrate room temperature valley polarization in heterostructures of monolayer MoS2 and naphthylethylammine based one-dimensional chiral lead halide perovskite. By optically exciting the heterostructures with linearly polarized light close to resonance and measuring the helicity resolved photoluminescence, we obtain a degree of polarization of up to −7% and 8% in MoS2/right-handed (R-(+)-) and left-handed (S-(-)-) 1-(1-naphthyl)ethylammonium lead iodide perovskite, respectively. We attribute this to spin selective charge transfer from MoS2 to the chiral perovskites, where the perovskites act as a spin filter due to their chiral nature. Our study provides a simple, yet robust route to obtain room temperature valley polarization, paving the way for practical valleytronics devices.

Defeating broken symmetry with doping: Symmetric resonant tunneling in noncentrosymetric heterostructures

Resonant tunneling transport in polar heterostructures is intimately connected to the polarization fields emerging from the geometric Berry-phase. In these structures, quantum confinement results not only in a discrete electronic spectrum, but also in built-in polarization charges exhibiting a broken inversion symmetry along the transport direction. Thus, electrons undergo highly asymmetric quantum interference effects with respect to the direction of current flow. By employing doping to counter the broken symmetry, we deterministically control the resonant transmission through GaN/AlN resonant tunneling diodes and experimentally demonstrate the recovery of symmetric resonant tunneling injection across the noncentrosymmetric double-barrier potential.

Inversion of the Internal Electric Field due to Inhomogeneous Incorporation of Ge Dopants in GaN/AlN Heterostructures Studied by Off-Axis Electron Holography.

The engineering of the internal electric field inside III-nitride devices opens up interesting perspectives in terms of device design to boost the radiative efficiency, which is a pressing need in the ultraviolet and green-to-red spectral windows. In this context, it is of paramount importance to have access to a tool like off-axis electron holography which can accurately characterize the electrostatic potentials in semiconductor heterostructures with nanometer-scale resolution. Here, we investigate the distribution of the electrostatic potential and chemical composition in two 10-period AlN/GaN (20 nm/20 nm) multilayer samples, one of these being non-intentionally doped and the other with its GaN layers heavily doped with Ge at a nominal concentration ([Ge] = 2.0 ± 0.2 × 1021 cm-3) which is close to the solubility limit. The electron holography experiments demonstrate the effects of free carrier screening in the case of Ge doping. Furthermore, in the doped sample, an inversion of the internal electric field is observed in some of the AlN layers. A correlated study involving holography, electron dispersive X-ray spectroscopy, and theoretical calculations of the band diagram demonstrates that the perturbation of the potential can be attributed to Ge accumulation at the heterointerfaces, which paves the way to the use of Ge delta doping as a design tool to tune the electric fields in polar heterostructures.

Phonon polaritons in van der Waals polar heterostructures for broadband strong light-matter interactions.

Phonon polaritons in polar crystals have recently gained significant attention due to their remarkable confinement and enhancement of electromagnetic fields, low group velocities, and low losses. However, these unique properties, resulting from the coupling between photons and lattice vibrations, exhibit limited spectral responses that may hinder their practical applications. Here, we propose and experimentally demonstrate that polar van der Waals heterostructures can integrate their polar constituents to enable broadband phonon polariton responses. A polar heterostructure is created by simply transferring thin flakes of two polar van der Waals materials, hexagonal boron nitride (hBN) and α-phase molybdenum trioxide (α-MoO3), onto a polar quartz substrate. Direct infrared nanoimaging experiments show that this integrated heterostructure supports phonon polaritons in a broadband infrared spectral range from 800 to 1700 cm-1. Further, numerical calculations predict vibrational strong coupling for a few molecule monolayers with multiple molecular absorption modes and phonon polaritons in the heterostructure. Our findings suggest that broadband phonon polariton responses in van der Waals integrated heterostructures have the potential to pave the way for the development of broadband and integrated infrared devices of molecular sensing, signal processing, and energy control.

The tunable interface charge transfer by polarization in two dimensional polar Al2O3/MoSO heterostructures

The evolution of band alignment, surface charge redistribution and interface charge transfer is simultaneous in 2D vdWs polar heterostructures with different polarization arrangements.

A rational design of titanium-based heterostructures as electrocatalyst for boosted conversion kinetics of polysulfides in Li-S batteries

Lithium–sulfur batteries have great potential for next-generation electrochemical storage systems owing to their high theoretical specific energy and cost-effectiveness. However, the shuttle effect of soluble polysulfides and sluggish multi-electron sulfur redox reactions has severely impeded the implementation of lithium–sulfur batteries. Herein, we prepared a new type of Ti3C2-TiO2 heterostructure sandwich nanosheet confined within polydopamine derived N-doped porous carbon. The highly polar heterostructures sandwich nanosheet with a high specific surface area can strongly absorb polysulfides, restraining their outward diffusion into the electrolyte. Abundant boundary defects constructed by new types of heterostructures reduce the overpotential of nucleation and improve the nucleation/conversion redox kinetics of Li2S. The Ti3C2–TiO2@NC/S cathode exhibited discharge capacities of 1363, and 801 mAh g−1 at the first and 100th cycles at 0.5C, respectively, and retained an ultralow capacity fade rate of 0.076% per cycle over 500cycles at 1.0C. This study provides a potential avenue for constructing heterostructure materials for electrochemical energy storage and catalysis.

The first-principles study of structural and electronic properties of two-dimensional SiC/GeC lateral polar heterostructures

Two-dimensional (2D) lateral polar heterostructures, constructed by seamlessly stitching 2D polar materials, exhibit unique properties triggered by the in-plane charge transfer between different elements in each domain. Our first-principles study of 2D SiC/GeC lateral polar heterostructures has unraveled their interesting characteristics. The local strain induced by a lattice mismatch leads to an artificial uniaxial strain along the interface. The synergistic effect of such uniaxial strain, the microstructure of interface, and the width of domains modulates the feature of the bandgap with an indirect bandgap nature in armchair lateral heterostructures and a direct bandgap nature in zigzag lateral heterostructures. The bandgap monotonically decreases with increasing the width of domains, showing its tunability. Furthermore, the valence band maximum is found to be mainly contributed from C-2p orbitals located at both GeC and SiC domains, and the conduction band minimum is mainly contributed from Ge-4p orbitals located at the GeC domain, implying that most excited electrons prefer to stay at the GeC domain of the SiC/GeC lateral polar heterostructures. Interestingly, a net charge transfer from the SiC domain to the GeC domain was found, resulting in a spontaneous lateral p–n junction, and there is a net charge redistribution at the interfacial region leading to a built-in electric field which is expected to reduce the carrier recombination losses, implying the promising application for visible light photocatalyst, photovoltaics, and water splitting to achieve clean and renewable energy.

Transport properties of polarization-induced 2D electron gases in epitaxial AlScN/GaN heterojunctions

AlScN is attractive as a lattice-matched epitaxial barrier layer for incorporation in GaN high electron mobility transistors due to its large dielectric constant and polarization. The transport properties of polarization-induced two-dimensional (2D) electron gas of densities of ∼2×1013/cm2 formed at the AlScN–GaN interface is studied by Hall-effect measurements down to cryogenic temperatures. The 2D electron gas densities exhibit mobilities limited to ∼300 cm2/V s down to 10 K at AlScN/GaN heterojunctions. The insertion of a ∼2 nm AlN interlayer boosts the room temperature mobility by more than five times from ∼300 cm2/V s to ∼1573 cm2/V s, and the 10 K mobility by more than 20 times to ∼6980 cm2/V s at 10 K. These measurements provide guidelines to the limits of electron conductivities of these highly polar heterostructures.

<italic>κ</italic>-Ga<sub>2</sub>O<sub>3</sub>/GaN ferroelectric/polar semiconductor heterojunction for high electron mobility transistors

<p indent="0mm">Polar semiconductor heterostructures offer an alternative strategy to manipulate polarization towards advanced devices with engineered functionality and improved performance. Recently, Prof. Jiandong Ye’s group from Nanjing University reported on a novel <italic>κ</italic>-Ga₂O₃/GaN ferroelectric/polar heterojunction with switchable polarization under external electric field. A type-II band alignment is determined at the <italic>κ</italic>-Ga₂O₃/GaN polar hetero-interface, with charge dipoles induced by spontaneous polarization leading to the observed band bending. Benefiting from the large band discontinuity and ferroelectric manipulation, it is expected to realize the switching of the interfacial two-dimensional electron gas between accumulation and depletion states, which allows the rational design of <italic>κ</italic>-Ga₂O₃ ferroelectric/polar heterojunctions for the application of high electron mobility transistors, ferroelectric non-volatile memories and even ultra-low loss negative capacitance transistors.

Role of low temperature Al(Ga)N interlayers on the polarity and quality control of GaN epitaxy

Polarity control is essential for lateral polarity heterostructures. N-polar GaN films were directly grown on high temperature AlN buffer layer. A thin low temperature AlN interlayer could invert the polarity of subsequent GaN layer to Ga-polarity but a low temperature GaN interlayer with the same condition could not do it rather deteriorate the quality of GaN film. In addition, a relationship between the yellow luminescence band and the surface roughness was discovered. The blue luminescence band was found only on the inverted Ga-polar GaN film and was connected to screw dislocation density.

An Epitaxial Ferroelectric ScAlN/GaN Heterostructure Memory

AbstractElectrically switchable bistable conductance that occurs in ferroelectric materials has attracted growing interest due to its promising applications in data storage and in‐memory computing. Sc‐alloyed III‐nitrides have emerged as a new class of ferroelectrics, which not only enable seamless integration with III‐nitride technology but also provide an alternative solution for CMOS back end of line integration. In this paper, the resistive switching behavior and memory effect in an ultrawide‐bandgap, high Curie temperature, fully epitaxial ferroelectric ScAlN/GaN heterostructure is reported for the first time. The structure exhibits robust ON and OFF states that last for months at room temperature with rectifying ratios of 60–210, and further shows stable operation at high temperatures (≈670 K) that are close to or even above the Curie temperature of most conventional ferroelectrics. Detailed studies suggest that the underlying mechanism is directly related to a ferroelectric field effect induced charge reconstruction at the hetero‐interface. The robust resistive switching landscape and the electrical polarization engineering capability in the polar heterostructure, together with the promise to integrate with both silicon and GaN technologies, can pave the way for next‐generation memristors and further enable a broad range of multifunctional and cross‐field applications.

Atomic-Level Structure Determines Electron-Phonon Scattering Rates in 2-D Polar Metal Heterostructures.

The electron dynamics of atomically thin 2-D polar metal heterostructures, which consisted of a few crystalline metal atomic layers intercalated between hexagonal silicon carbide and graphene grown from the silicon carbide, were studied using nearly degenerate transient absorption spectroscopy. Optical pumping created charge carriers in both the 2-D metals and graphene components. Wavelength-dependent probing suggests that graphene-to-metal carrier transfer occurred on a sub-picosecond time scale. Following rapid (<300 fs) carrier-carrier scattering, charge carriers monitored through the metal interband transition relaxed through several consecutive cooling mechanisms that included sub-picosecond carrier-phonon scattering and dissipation to the silicon carbide substrate over tens of picoseconds. By studying 2-D In, 2-D Ga, and a Ga/In alloy, we resolved accelerated electron-phonon scattering rates upon alloy formation as well as structural influences on the excitation of in-plane phonon shear modes. More rapid cooling in alloys is attributed to increased lattice disorder, which was observed through correlative polarization-resolved second harmonic generation and electron microscopy. This connection between the electronic relaxation rates, far-field optical responses, and metal lattice disorder is made possible by the intimate relation between nonlinear optical properties and atomic-level structure in these materials. These studies provided insights into electronic carrier dynamics in 2-D crystalline elemental metals, including resolving contributions from specific components of a 2-D metal-containing heterojunction. The correlative ultrafast spectroscopy and nonlinear microscopy results suggest that the energy dissipation rates can be tuned through atomic-level structures.

Time-resolved cathodoluminescence investigations of AlN:Ge/GaN nanowire structures

Light emitting diodes represent a key technology that can be found in many areas of everydays life. Therefore, the improvement of the efficiency of such structures offers a high economic and ecological potential. One approach is electrostatic screening of the quantum-confined Stark effect (QCSE) in polar III-V heterostructures by n-type doping in order to increase the oscillator strength of electronic transitions in quantum structures. In this study, we analyzed the cathodoluminescene (CL) spectra of different functional parts of individual AlN/GaN nanowire superlattices and studied their decay characteristics with sub-nanosecond time resolution. This allows us to extract information about strain and electric fields in such heterostructures with an overall spatial resolution <100 nm. The samples, which were investigated in a temperature range from 10 to 300 K by using time-integrated cathodoluminescence spectroscopy (TICL) and time-resolved cathodoluminescence spectroscopy (TRCL) consist of GaN bottom and top layer and a 40-fold stack of GaN nanodiscs, embedded in AlN barriers that were doped with Ge. We show, that the QCSE is reduced with increasing doping concentration due to a screening of the internal electric fields inside GaN nanodiscs, resulting in a reduction of the carrier lifetimes and a blue shift of the emitted light. Due to the small diameter of the electron excitation beam CL offers the possibility to individually analyze the different functional parts of the nanowires.

Using Light for Better Programming of Ferroelectric Devices: Optoelectronic MoS2‐Pb(Zr,Ti)O3 Memories with Improved On–Off Ratios

AbstractFerroelectric (FE) devices are conventionally switched by an application of an electric field. However, the recent discoveries of light–matter interactions in heterostructures based on 2D semiconductors and FE materials open new opportunities for using light as an additional tool for device programming. Recently, a purely optical switching of FE polarization in heterostructures comprising 2D MoS2 and FE oxide perovskites, such as BaTiO3 and Pb(Zr,Ti)O3 (PZT), was demonstrated. In this work, it is investigated whether this optical switching has a practical value and can be used to improve functional characteristics of MoS2‐PZT FE field‐effect transistors for nonvolatile memory applications. It is demonstrated that the combined use of an electrical field and visible light improves the nonvolatile ON/OFF ratios in MoS2‐PZT memories by several orders of magnitude compared to their purely electrical operation. The memories are read at zero gate voltage (VG) in darkness, but their ON and OFF currents, which routinely varied for different devices by over 105, are achieved by programming at the same VG = −6 V with (ON state) and without (OFF state) light illumination, demonstrating its crucial importance. The light can likely serve as an important tool for better programming of a large variety of other semiconductor‐FE devices.

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.

Electric-field assisted optimal quantum transport of photo-excitations in polar heterostructures

Transition-metal-oxide (TMO) heterostructures are promising candidates for building photon-harvesting devices which can exploit optimal quantum transport of charge excitations generated by light absorption. Here we address the explicit role of an electric field on the quantum transport properties of photo-excitations subject to dephasing in one-dimensional chains coupled to a continuum of states acting as a sink. We show that the average transfer time to the sink is optimized for suitable values of both the coupling strength to the sink and the electric field, thus fully exploiting the coherence-enhanced efficiency in the quantum transport regime achievable in few monolayers TMO heterostructures. The optimal coupling to the continuum remains approximately the same as that in absence of electric field and is characterizing the Superradiant Transition. On the other hand, the optimal electric field for which we provide estimates using an analytical expression is dependent on the initial state.

Low-barrier Mott diodes with near-surface polarization-induced δ -doping

The possibility of a controlled decrease in the effective height of the Schottky (Mott) barrier to the AlGaN/GaN (Ga-face polarity) heterostructure due to the modification of the shape of the barrier by the electric field of the polarization charge arising in the plane of the heterojunction because of the jump in electric polarization is experimentally shown. A decrease in the effective barrier height is related to an increase in the role of electron tunneling through the barrier. The effective barrier height can be controlled by varying the thickness and chemical composition of the AlGaN layer and choosing the metal of the barrier contact. Test low-barrier Mott Ti/AlGaN/GaN diodes demonstrating high values of the ampere-watt sensitivity (9 A/W) for a low specific differential resistance (4 × 10–4 Ω⋅cm2) at zero bias have been manufactured.