Electrons In Superlattices Research Articles

Visualizing band structure hybridization and superlattice effects in twisted MoS2/WS2 heterobilayers

A mismatch of atomic registries between single-layer transition metal dichalcogenides (TMDs) in a two-dimensional (2D) van der Waals heterostructure produces a moiré superlattice with a periodic potential, which can be fine-tuned by introducing a twist angle between the materials. This approach is promising both for controlling the interactions between the TMDs and for engineering their electronic band structures, yet direct observation of the changes to the electronic structure introduced with varying twist angle has so far been missing. Here, we probe heterobilayers comprised of single-layer MoS2 and WS2 with twist angles ranging from 2∘ to 20∘ and determine the twist angle-dependent evolution of the electronic band structure using micro-focused angle-resolved photoemission spectroscopy. We find strong interlayer hybridization between MoS2 and WS2 electronic states at the -point of the Brillouin zone, leading to a shift of the valence band maximum in the heterostructure. Replicas of the hybridized states are observed at the center of twist angle-dependent moiré mini Brillouin zones. We confirm that these replica features arise from the inherent moiré potential by comparing our experimental observations with density functional theory calculations of the superlattice dispersion. Our direct visualization of these features underscores the potential of using twisted heterobilayer semiconductors to engineer hybrid electronic states and superlattices that alter the electronic and optical properties of 2D heterostructures for a wide range of twist angles.

Interfacial Intermixing and Its Impact on the Energy Band Structure in Interband Cascade Infrared Photodetectors.

Multiple-stage interband cascade infrared photodetector (ICIP) is a new class of semiconductor infrared photodetector that exhibits improved device performance in terms of responsivity and detectivity. The design of the device structure and the electronic structure on superlattices and quantum wells assume abrupt interfaces. However, the emergence of possible interface segregation and atom exchange can only be determined experimentally, impacting the device performance. In this work, the interface atom intermixing and their effects on the energy band structure in a molecular beam epitaxy grown ICIP are studied. Scanning transmission electron microscopy (STEM) reveals atom diffusion and intermixing between the constituent layers of the cascade structure, causing a shift in the quantum state energy levels of the layers and the consequent misalignment of the cascade structures. Combining the STEM observation with high-resolution X-ray diffraction, the alloy composition profiles of the layers are determined. Using the "real" graded composition profiles, the effective band gap of the superlattice absorber and the energy levels of the relaxation region and the tunneling region are recalculated showing a cutoff wavelength of the superlattice absorber 4.93 μm, which is 0.78 μm smaller than that calculated using the nominal step composition profile. However, its agreement is greatly improved with the measured cutoff wavelength of 5.03 μm. The energy level of the narrowest quantum well in the relaxation region is 0.091 eV higher than the conduction miniband of the absorber, which is also consistent with the experiments that the pho-response exits a "turn on" voltage of 0.1 V. The results reported here will help optimize the energy structure design of future ICIP with improved device performance.

Reduction of efficiency droop by inserting superlattice quaternary-ternary electron blocking layer in GaN-based light-emitting diodes

We report a reduced the unwanted hole blocking of the electron blocking layer (EBL) by replacing the conventional EBL with a quaternary-ternary superlattice electron blocking layer. The effective barrier height for holes is reduced from ~ 348 meV to ~ 261 meV, thus, improving the concentration of holes by ~ 50% with respect to LED with conventional EBL. Similarly, the radiative recombination rate is also improved by ~ 79%. As a result, the overall droop ratio of internal quantum efficiency (IQE) of the proposed LED is reduced to ~ 23% at 100 A/cm2 in comparison to conventional LED.

Proposal for engineering 3D quantum Hall effect in an electronic superlattice

The three-dimensional (3D) quantum Hall effect (3DQHE) was initially proposed to be realized in systems with spontaneous charge-density-wave (CDW) or spin-density-wave (SDW), which has stimulated recent experimental progress in this direction. Here, instead of such intrinsic scenarios, we propose to realize the 3DQHE in a synthetic semiconductor superlattice. The superlattice is engineered along one direction, which is modeled by the Kronig-Penney type periodic potential. By applying a magnetic field along this direction, quantized 3D Hall conductivity can be achieved in certain parameter regimes, along with a vanishing transverse conductivity. We show that such results are robust against the disorder effect and can be hopefully realized by state-of-the-art fabrication techniques. Our work opens a new research avenue for exploring the 3DQHE in electronic superlattice structures.

Optimization of InGaAs/InAs photodetectors with superlattice electron barrier

The photodetector with superlattice electron barrier has received great attention in the past few years. In this paper, a type of photodetector with superlattice structure is simulated. InGaAs/InAs superlattices are inserted into the intrinsic layer of the PIN structure, and the positions of the superlattices are optimized. After changing the position of the superlattice, the dark current was reduced from 6.42 × 10 −5 A/cm 2 to 3.02 × 10 −5 A/cm 2 . The results show that placing the superlattice on top of the intrinsic layer can minimize the dark current of the photodetector. Similarly, after changing the number of superlattice layers, the dark current is reduced from 6.42 × 10 −5 A/cm 2 to 1.55 × 10 −5 A/cm 2 . In addition, the thickness of the superlattice was optimized, and the dark current was reduced from 6.42 × 10 −5 A/cm 2 to 3.1 × 10 −5 A/cm 2 . • We built a model of InGaAs/InAs photodetectors with superlattice electron barrier. • We changed the position、the number and the thickness of the inserted superlattice layer. • The dark current variation was analyzed by the energy band and internal electric field.

Improved polarization switching and piezoresponse in Nd and Mn co-doped BiFeO3 ceramics

Nano-to-micro polarization switching and electromechanical mechanisms in lead-free bismuth ferrite have shown opportunities in nonvolatile logic and optoelectronic applications. This report highlights improved ferroelectric polarization switching and electromechanical responses in (Nd, Mn) co-doped BiFeO3 ceramics via modulation of the B-site Mn substitution. The out-of-plane piezoresponse hysteresis loops reveals enhanced local polarization and electromechanical strain with increasing B-site Mn substitution. The B-site Mn substitution increases lattice distortion (or octahedral tilt) in rhombohedral R3c unit cell, confirmed by the enhanced ½(000)c superlattice electron diffraction. Average size of polar nano-regions (PNRs) and dielectric permittivity were also increased as Mn content increases. The synchrotron X-ray absorption spectra reveal a Mn3+ → Mn4+ valence shift and modulated orbital hybridizations between the O 2p and cation’s orbitals (Fe 3d and Bi 6 s(p)), which are critical factors for the improved polarization and dielectric responses.

Improved Performance of GaN-Based Ultraviolet LEDs With Electron Blocking Layers Composed of Double-Peak p-Type Al x Ga1−x N/GaN Superlattice Layers

Past studies have demonstrated the positive impact of step-graded $p$ -type Al x Ga1− x N/GaN superlattice (SL) electron blocking layer (EBL) structures on the efficiency performance of ultraviolet (UV) GaN-based light-emitting diodes (LEDs). However, the optimal Al-grading structure of these SL EBLs remains unclear owing to a lack of systematic investigation. The present work addresses this issue by applying EBL composed of alternating half-peak, single-peak, and double-peak $p$ -type Al x Ga1− x N/GaN SL structures with varying values of $x$ ranging from 0.05 to 0.15 in 0.05 step increments. Simulation analysis is employed to obtain the internal quantum efficiency (IQE), energy band diagrams, polarization compensation factor, hole concentration, and electron concentration of GaN-based UV LEDs with three different SL-EBL structures. The results obtained at an injection current of 200 mA demonstrate that UV LEDs with double-peak SL-EBL structures provide the maximum IQE, which is ~38% greater than that of devices employing the conventional EBL in simulation experiment. This SL-EBL is demonstrated to improve the hole injection and electron overflow performance of GaN-based UV LEDs owing to the polarization charge and lattice mismatch at the p- Al x Ga1− x N/GaN interfaces. The reduced Al composition on the p- GaN side reduces the potential barrier of hole injection. Moreover, the predicted increase in the IQE of GaN-based UV LEDs with the optimal SL-EBL is verified experimentally.

Nonmonotonic crossover in electronic phase separated manganite superlattices driven by the superlattice period

Studying manganite superlattices ${[{(\mathrm{LCMO})}_{2n}/{(\mathrm{PCMO})}_{n}]}_{t}$ made of ${\mathrm{La}}_{0.625}{\mathrm{Ca}}_{0.375}\mathrm{Mn}{\mathrm{O}}_{3}$ (LCMO) and ${\mathrm{Pr}}_{0.625}{\mathrm{Ca}}_{0.375}\mathrm{Mn}{\mathrm{O}}_{3}$ (PCMO), we found an unexpected behavior varying the period $n$. At small $n$, the ensemble is a three-dimensional ferromagnetic metal due to interfacial charge transfer. At large $n$, the LCMO layers dominate transport. However, rather than a smooth interpolation between these limits a sharp transport and magnetic anomaly is found at an intermediate critical PCMO thickness ${n}^{*}$. Magnetic force microscopy reveals that the phase-separation length scale also maximizes at ${n}^{*}$ where, unexpectedly, it becomes comparable to that of the ${({\mathrm{La}}_{1--y}{\mathrm{Pr}}_{y})}_{0.625}{\mathrm{Ca}}_{0.375}\mathrm{Mn}{\mathrm{O}}_{3}$ (LPCMO) alloy. We conjecture the phenomenon originates in a disorder-related length scale: Large charge-ordered clusters as in LPCMO can only nucleate when Pr-rich regions reach a critical size related to ${n}^{*}$.

Broadband wave plate based on one-dimensional photonic crystal containing negative-zero-positive index metamaterials

The spectra, the phase shifts, and the phase difference between TE and TM waves upon reflection from a one-dimensional photonic crystal (1D PC) containing negative-zero-positive index metamaterials (NZPIMs) have been investigated. It is found that there exist a zero-n̄ gap and a dirac point (DP) band gap, which can be adjusted by the incident angle, the thickness ratio, and the scaling factor. Furthermore, the reflection phase shifts of TE and TM waves within the two gaps change smoothly with increasing the frequency, and the phase differences could remain constant in rather wide frequency range, which can be conveniently tuned by the incident angles, the thickness ratio, and the scaling factor. Our results show that such a new 1D PC containing NZPIMs can serve as a wave plate using in wide spectral band, and suggest the analogous phenomena of valence electron in graphene superlattice.

Enhanced hole transport in AlGaN deep ultraviolet light-emitting diodes using a double-sided step graded superlattice electron blocking layer

In this paper, deep ultraviolet AlGaN light-emitting diodes (LEDs) with a novel double-sided step graded superlattice (DSGS) electron blocking layer (EBL) instead of a conventional EBL have been proposed for ∼ 254 n m wavelength emission. The enhanced carrier transport in the DSGS structure results in reduced electron leakage into the p -region, improved hole activation and hole injection, and enhanced output power and external quantum efficiency. The calculations show that output power of the DSGS structure is ∼ 3.56 times higher and electron leakage is ∼ 12 times lower, compared to the conventional structure. Moreover, the efficiency droop at 60 mA in the DSGS LED was found to be ∼ 9.1 % , which is ∼ 4.5 times lower than the regular LED structure.

Moiré Band Topology in Twisted Bilayer Graphene.

Recently twisted bilayer graphene (t-BLG) has emerged as a strongly correlated physical platform. Besides the apparent significance of band flatness, band topology may be another critical element in t-BLG and yet receives much less attention. Here we report the compelling evidence for nontrivial noninteracting Moiré band topology in t-BLG through a systematic nonlocal transport study and a K-theory examination. The nontrivial topology manifests itself as two pronounced nonlocal responses in the electron and hole superlattice gaps. We show that the nonlocal responses are robust to the twist angle and edge termination, exhibiting a universal scaling law. We elucidate that, although Berry curvature is symmetry-trivialized, two nontrivial Z2 invariants characterize the Moiré Dirac bands, validating the topological origin of the observed nonlocal responses. Our findings not only provide a new perspective for understanding the strongly correlated t-BLG but also suggest a potential strategy to achieve topological metamaterials from trivial vdW materials.

Photoresponse in a Strain-Induced Graphene Wrinkle Superlattice.

Applied strain introduces significant changes in the carbon-carbon bond of graphene and thereby forms electronic superlattices. The electron/phonon coupling and existence of pseudogauge fields within these superlattices render unique electronic and magnetism properties. However, the interfacial interactions between strained and pristine graphene have rarely been studied. Herein, we report a prominent increase in photocurrent at the interface between pristine graphene and the strain-induced superlattice (i.e., the graphene wrinkle). The photocurrent distribution indicates a large increase in the bending lattice of graphene. These results demonstrate that the photocurrent enhancement is due to the difference in the Seebeck coefficient between pristine graphene and deformed superlattices, resulting in a significant increase in the photothermoelectric effect at the interface.

Weak localization competes with the quantum oscillations in a natural electronic superlattice: The case of Na1.5(PO2)4(WO3)20

We report an investigation of the combined structural and electronic properties of the bronze Na$_{1.5}$(PO$_{2}$)$_{4}$(WO$_{3}$)$_{20}$. Its low dimensional structure and possible large reconstruction of the Fermi surface due to charge density wave instability makes this bulk material a natural superlattice with a reduced number of carriers and Fermi energy. Signatures of multilayered 2D electron weak localization are consequently reported, with an enhanced influence of quantum oscillations. A crossover between these two antagonistic entities previously observed only in genuine low dimensional materials and devices, is shown to occur in a bulk crystal due to its hidden 2D nature.

Strain Modulated Superlattices in Graphene.

Numerous theoretically proposed devices and novel phenomena have sought to take advantage of the intense pseudogauge fields that can arise in strained graphene. Many of these proposals, however, require fields to oscillate with a spatial frequency smaller than the magnetic length, while to date only the generation and effects of fields varying at a much larger length scale have been reported. Here, we describe the creation of short wavelength, periodic pseudogauge-fields using rippled graphene under extreme (>10%) strain and study of its effects on Dirac electrons. Combining scanning tunneling microscopy and atomistic calculations, we find that spatially oscillating strain generates a new quantization different from the familiar Landau quantization. Graphene ripples also cause large variations in carbon-carbon bond length, creating an effective electronic superlattice within a single graphene sheet. Our results thus also establish a novel approach of synthesizing effective 2D lateral heterostructures by periodically modulating lattice strain.

Achieving Out-of-Plane Thermoelectric Figure of Merit ZT = 1.44 in a p-Type Bi2Te3/Bi0.5Sb1.5Te3 Superlattice Film with Low Interfacial Resistance.

Recently, low-dimensional superlattice films have attracted significant attention because of their low dimensionality and anisotropic thermoelectric (TE) properties such as the Seebeck coefficient, electrical conductivity, and thermal conductivity. For these superlattice structures, both electrons and phonons show highly anisotropic behavior and exhibit much stronger interface scattering in the out-of-plane direction of the films compared to the in-plane direction. However, no detailed information is available in the literature for the out-of-plane TE properties of the superlattice-based films. In this report, we present the out-of-plane Seebeck coefficient, thermal conductivity, and electrical properties of p-type Bi2Te3/Bi0.5Sb1.5Te3 (bismuth telluride/bismuth antimony telluride, BT/BST) superlattice films in the temperature range of 77-500 K. Because of the synergistic combination of the energy filtering effect and low interfacial resistance of the superlattice structure, an impressively high ZT of 1.44 was achieved at 400 K for the 200 nm-thick p-type BT/BST superlattice film, corresponding to a 43% ZT enhancement compared to the pristine p-BST films with the same thickness.

Potential and limits of superlattice multipliers coupled to different input power sources

A theoretical study is presented to assess the performance of semiconductor superlattice multipliers as a function of the currently available input power sources. The prime devices which are considered as input power sources are Impatt diodes, InP Gunn devices, superlattice electron devices and Backward Wave Oscillator sources. These sources have been successfully designed to deliver input radiation frequencies in the range from 0.1 to 0.5 THz. We discuss the harmonic power generation of both odd and even harmonics by implementing an ansatz solution stemmed from a hybrid approach combining nonequilibrium Green's functions and the Boltzmann kinetic equation.

Band Offsets of Insulating & Semiconducting Oxides on (AlxGa1-x)O3

β-Ga2O3 is a wide bandgap semiconductor (~4.8 eV) with properties suited to power electronics, truly solar-blind UV detection, and extreme environment applications. Additional bandgap tunability can be achieved through incorporation of Al into β-Ga2O3, leading to β-(Al,Ga)2O3 monoclinic phase alloys with the bandgap tuned from 4.8- 6 eV. This enables heterostructure designs such as modulation-doped electron channels, quantum wells, and superlattices. Out of these possible structures, β-(AlxGa1-x)2O3/Ga2O3 heterostructure field-effect transistors (HFETs) have shown impressive and improving performance compared to Ga2O3 based metal semiconductor field effect transistor (MESFETs). For these heterostructures, other wide bandgap oxides can be used as gates dielectrics, and narrow energy bandgap conductive oxides can be utilized to reduce contact resistance between the ohmic contacts and semiconductor surface. For gate dielectrics on (AlxGa1-x)2O3, one must utilize oxide materials with a bandgap of 7 eV of higher to have adequate offsets in both the conduction and valence bands. In this study, we utilize x-ray photoelectron spectroscopy (XPS) to report on the determination of the band alignment of various insulating and semiconducting dielectrics onto (AlxGa1-x)2O3 at various aluminum concentrations. The dielectrics were deposited using atomic layer deposition (ALD) or sputtering. For SiO2 and Al2O3, there are differences of up to 1 eV in band alignments on single crystal (AlxGa1-x)2O3, depending on whether they are deposited by sputtering or ALD. For the conductive oxides, AZO/ β-(AlxGa1-x)2O3 and ITO/ β-(AlxGa1-x)2O3 heterojunctions have a nested gap (type I) band offset. The valence and conduction band offsets are negative in both cases and can be used to enhance carrier transport across the heterointerface.

Tunable Moiré Superlattice of Artificially Twisted Monolayers.

Twisting between two stacked monolayers modulates periodic potentials and forms the Moiré electronic superlattices, which offers an additional degree of freedom to alter material property. Considerable unique observations, including unconventional superconductivity, coupled spin-valley states, and quantized interlayer excitons are correlated to the electronic superlattices but further study requires reliable routes to study the Moiré in real space. Scanning tunneling microscopy (STM) is ideal to precisely probe the Moiré superlattice and correlate coupled parameters among local electronic structures, strains, defects, and band alignment at atomic scale. Here, a clean route is developed to construct twisted lattices using synthesized monolayers for fundamental studies. Diverse Moiré superlattices are predicted and successfully observed with STM at room temperature. Electrical tuning of the Moiré superlattice is achieved with stacked TMD on graphite.

Effect of electron blocking layer on the efficiency of AlGaN mid-ultraviolet light emitting diodes.

The performance of AlGaN-based mid and deep ultraviolet light emitting diodes (LEDs) is severely limited by electron overflow and by the poor hole injection into the device active region. We have studied the effect of various electron blocking layers on the performance of AlGaN LEDs operating at ~280 nm. It is observed that, compared to conventional p-type electron blocking layer, the incorporation of an n-type AlN/AlGaN superlattice electron blocking layer before the active region can significantly improve the device performance by reducing electron overflow without compromising hole injection. Direct on-wafer measurement showed an external quantum efficiency ~4.4% and wall-plug efficiency ~2.8% by optimizing the design of n-type AlN/AlGaN superlattice electron blocking layer, which is nearly a factor of five to ten times better than identical devices but with the incorporation of a conventional p-type electron blocking layer.

Optimization of In0.6Ga0.4As/InAs electron barrier for In0.74Ga0.26As detectors grown by molecular beam epitaxy

The effects of thickness and period number of In0.6Ga0.4As/InAs superlattice electron barrier on the performances of InP-based metamorphic In0.74Ga0.26As detectors have been investigated. The samples were grown by molecular beam epitaxy and characterized by high resolution X-ray diffraction reciprocal space maps, photoluminescence, atomic force microscope, plan view transmission electron microscopy and the device dark current measurements. Results show that the dark currents are reduced by insertion of an electron barrier with moderate thickness and period number in the absorption layer, but a superlattice electron barrier structure with excessive thickness and period number can lead to the deterioration of the material quality and device performances.