Superlattice Materials Research Articles

STEM multiplication nano-moiré method with large field of view and high sensitivity

Strain is one of the important factors that determine the photoelectric and mechanical properties of semiconductor materials and devices. In this paper, the scanning transmission electron microscopy multiplication nano-moiré method is proposed to increase the measurement range and sensitivity for strain field. The formation principle, condition, and measurement range of positive and negative multiplication moiré fringes (PMMFs and NMMFs) are analysed in detail here. PMMF generally refers to the multiplication of field of view, NMMF generally refers to the multiplication of displacement measurement sensitivity. Based on the principle of multiplication nano-moiré, Theoretical formulas of the fringe spacing and strain field are derived. Compared with geometric phase analysis of deformation measurements based on high-resolution atom images, both the range of field of view and the sensitivity of displacement measurements of the multiplication moiré method are significantly improved. Most importantly, the area of field of view of the PMMF method is increased by about two orders of magnitude, which is close to micrometre-scale with strain measurement sensitivity of 2 × 10–5. In addition, In order to improve the quality of moiré fringe and the accuracy of strain measurement, the secondary moiré method is developed.The strain laws at the interface of the InP/InGaAs superlattice materials are characterised using the developed method.

Multicomponent perovskite superlattices

Multicomponent perovskite superlattices

Impact of conductivity type change in InAs/GaSb superlattice on low frequency noise of photoconductive long-wavelength infrared detectors

This Letter focuses on the 1/f noise properties of InAs/GaSb superlattice (SL), which is a promising material for infrared radiation detection and represents one of the alternatives to well-established bulk HgCdTe material. The InAs/GaSb SL material changes the conductivity type at temperature T ≈ 190 K, which has been correlated with measured 1/f noise. It was shown that 1/f noise comes from resistance fluctuations of linear noise sources. According to the electronic transport and 1/f noise models, the observed 1/f noise is connected with the hole conductivity component rather than the electron conductivity component, which is absent or at least immeasurable, even though electron conductivity governs the total conductivity of the InAs/GaSb SL. In the high-temperature region, the 1/f noise of InAs/GaSb SL is significantly smaller than that of InAs/InAsSb SL. The results favor InAs/GaSb SL material over InAs/InAsSb SL for photoconductive infrared detectors operating at room temperature.

Increasing the robustness and crack resistivity of high-performance carbon fiber composites for space applications

SummaryThe endeavors to develop manufacturing methods that can enhance polymer and composite structures in spacecraft have led to much research and innovation over many decades. However, the thermal stability, intrinsic material stress, and anisotropic substrate properties pose significant challenges and inhibit the use of previously proposed solutions under extreme space environment. Here, we overcome these issues by developing a custom-designed, plasma-enhanced cross-linked poly(p-xylylene):diamond-like carbon superlattice material that enables enhanced mechanical coupling with the soft polymeric and composite materials, which in turn can be applied to large 3D engineering structures. The superlattice structure developed forms an integral part with the substrate and results in a space qualifiable carbon-fiber-reinforced polymer featuring 10–20 times greater resistance to cracking without affecting the stiffness of dimensionally stable structures. This innovation paves the way for the next generation of advanced ultra-stable composites for upcoming optical and radar instrument space programs and advanced engineering applications.

The half metallic feature at high temperature of the novel half-Heusler alloys and their [100] oriented layered superlattices: A DFT investigations

Herein, we have investigated by means of the DFT calculations the magneto-electronic properties and thermoelectric responses of the novel half-Heusler NbFe(Mn)Sn compounds and their layered superlattices growing along [100] direction. The ground-state properties and magnetic stability revealed that the bulk compounds were ferromagnetic materials. The formation energy was determined meticulously and revealed that the studied materials could be synthesized experimentally. The half-metallic ferromagnetism with the presence of the strong p-d hybridization have been strongly confirmed from the magneto-electronic calculations for all the studied materials. The Curie temperature was estimated from the mean field approach, the obtained values exceed the ambient temperature. Furthermore, the thermoelectric response was evaluated and categorized the investigated compounds as a good thermoelectric materials due to the high figure of merit, large Seebeck coefficient and low electronic part of thermal conductivity. The curve of spin polarization (SP) as a function of temperature divulged that the 100% SP was conserved at high temperature for NbFeSn and superlattice materials. The phonon dispersion was calculated in order to examine the dynamic stabilities of the studied materials. The present detailed study made these materials as a potential candidates for spintronic thermoelectric devices.

High-yield exfoliation of 2D semiconductor monolayers and reassembly of organic/inorganic artificial superlattices

<h2>Summary</h2> The scalable preparation of high-purity monolayers is essential for practically integrating two-dimensional (2D) semiconductors in diverse technologies but remains a persistent challenge. Previous efforts to exfoliate 2D layered crystals by the organic ammonium intercalation usually produce few-layer nanosheets owing to a self-retarding effect that hinders the complete intercalation in neighboring layers. Herein, we report a unique "intercalation and separation" chemistry with a constant self-refreshing crystal surface that mitigates the self-retarding effect to ensure a complete intercalation of the bulk crystal, ultimately enabling high-yield solution-phase exfoliation of 2D semiconductor monolayers in excellent purity (e.g., monolayer purity of >95% for In₂Se₃ and InSe). Furthermore, we have assembled large-area organic/inorganic hybrid superlattices with diverse organic molecules and inorganic 2D monolayer crystals, thus creating a family of artificial superlattice materials with atomically modulated chemical compositions, widely tunable superlattice periodicities, and specifically tailorable electronic and thermal properties.

Mid-wavelength infrared avalanche photodetector with AlAsSb/GaSb superlattice

In this work, a mid-wavelength infrared separate absorption and multiplication avalanche photodiode (SAM-APD) with 100% cut-off wavelength of ~ 5.0 µm at 200 K grown by molecular beam epitaxy was demonstrated. The InAsSb-based SAM-APD device was designed to have electron dominated avalanche mechanism via the band structure engineered multi-quantum well structure based on AlAsSb/GaSb H-structure superlattice and InAsSb material in the multiplication region. The device exhibits a maximum multiplication gain of 29 at 200 K under -14.7 bias voltage. The maximum multiplication gain value for the MWIR SAM-APD increases from 29 at 200 K to 121 at 150 K. The electron and hole impact ionization coefficients were derived and the large difference between their value was observed. The carrier ionization ratio for the MWIR SAM-APD device was calculated to be ~ 0.097 at 200 K.

All-epitaxial long-range surface plasmon polariton structures with integrated active materials

We demonstrate all-epitaxial structures capable of supporting short- and long-range surface plasmon polariton (SRSPP and LRSPP) modes in the long-wave infrared region of the electromagnetic spectrum. The SRSPP and LRSPP modes are bound to the interfaces of a buried heavily doped (n++) semiconductor layer and surrounding quantum-engineered type-II superlattice (T2SL) materials. The surrounding T2SLs are designed to allow optical transitions across the frequency dispersion of the SPP modes. We map the SPP dispersion in our structure using grating-coupled angle- and polarization-dependent reflection and photoluminescence spectroscopy. The epitaxial structures are analytically described using a simplified three-layer system (T2SL/n++/T2SL) and modeled using rigorous coupled wave analysis with excellent agreement to our experimental results. The presented structures offer the potential to serve as long-range interconnects or waveguides in all-epitaxial plasmonic/optoelectronic systems operating in the long-wave infrared.

Highly Conductive Co-Doped Ga2O3:Si-In Grown by MOCVD

We report a highly conductive gallium oxide doped with both silicon and indium grown on c-plane sapphire substrate by MOCVD. From a superlattice structure of indium oxide and gallium oxide doped with silicon, we obtained a highly conductive material with an electron hall mobility up to 150 cm2/V·s with the carrier concentration near 2 × 1017 cm−3. However, if not doped with silicon, both Ga2O3:In and Ga2O3 are highly resistive. Optical and structural characterization techniques such as X-ray, transmission electron microscope, and photoluminescence, reveal no significant incorporation of indium into the superlattice materials, which suggests the indium plays a role of a surfactant passivating electron trapping defect levels.

Fabrication of a 1024 × 1024 format long wavelength infrared focal plane array based on type-II superlattice and barrier enhanced structure

1024 × 1024 format long-wavelength infrared focal plane arrays are realized using type-II superlattice material with a double-barrier enhanced structure grown on 3″ GaSb substrate. The detector structure material exhibits excellent crystalline quality and uniformity. A 100% cutoff wavelength of ~11.8 μm was obtained at 82 K. A dark current density of 1.4 × 10−6 A/cm2 and a differential resistance-area product RA of 4.7 × 104 Ωcm2 at 49 K, 3.6 × 10−5 A/cm2 and 2.8 × 103 Ωcm2 at 70 K, and 2.5 × 10−4 A/cm2 and 5.5 × 102 Ωcm2 at 82 K, at a reverse bias of ~ 50 mV, were obtained, respectively. At zero bias, the R0A was 5.1 × 104 Ωcm2, 290.6 Ωcm2, and 35.6 Ωcm2 at 49 K, 70 K, and 82 K, respectively. Noise equivalent temperature differences of 37.4 mK at 82 K and 31.9 mK at 70 K were achieved with an integration time of 0.12 ms, a 300 K background and f/2.0 optics. The large format FPA also shows excellent imaging.

Demonstration of MOCVD-Grown Long-Wavelength Infrared InAs/GaSb Superlattice Focal Plane Array

High-performance InAs/GaSb type-II superlattice infrared detectors and focal plane arrays (FPAs) are normally grown by molecular beam epitaxy (MBE). In this work, we demonstrate the first long-wavelength infrared InAs/GaSb superlattice FPA grown by metalorganic chemical vapor deposition (MOCVD) with clear image. High-quality superlattice material was obtained evidenced by sharp X-ray diffraction peaks and atomic flat surface. Electrical and optical measurements performed on single element detectors showed a 50% cut-off wavelength of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 10.1~\mu \text{m}$ </tex-math></inline-formula> , a dark current density of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.5\times 10^{-5}$ </tex-math></inline-formula> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , a peak responsivity of 0.88 A/W and a peak detectivity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.7\times 10^{11}$ </tex-math></inline-formula> cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\cdot $ </tex-math></inline-formula> Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> /W at 80 K. A <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$320\times256$ </tex-math></inline-formula> FPA with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$30~\mu \text{m}$ </tex-math></inline-formula> pixel pitch was then fabricated. With an integration time of 1.9 ms and an applied bias of -0.1 V, the FPA shows an average operability of 96.96%, a non-uniformity of 4.97%, a noise equivalent temperature difference of 51.1 mK and a peak detectivity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.3\times 10^{10}$ </tex-math></inline-formula> cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\cdot $ </tex-math></inline-formula> Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> /W at 80 K without thinning down the substrate.

Reversible multicolor chromism in layered formamidinium metal halide perovskites

Metal halide perovskites feature crystalline-like electronic band structures and liquid-like physical properties. The crystal–liquid duality enables optoelectronic devices with unprecedented performance and a unique opportunity to chemically manipulate the structure with low energy input. In this work, we leverage the low formation energy of metal halide perovskites to demonstrate multicolor reversible chromism. We synthesized layered Ruddlesden-Popper FAn+1PbnX3n+1 (FA = formamidinium, X = I, Br; n = number of layers = 1, 2, 3 … ∞) and reversibly tune the dimensionality (n) by modulating the strength and number of H-bonds in the system. H-bonding was controlled by exposure to solvent vapor (solvatochromism) or temperature change (thermochromism), which shuttles FAX salt pairs between the FAn+1PbnX3n+1 domains and adjacent FAX “reservoir” domains. Unlike traditional chromic materials that only offer a single-color transition, FAn+1PbnX3n+1 films reversibly switch between multiple colors including yellow, orange, red, brown, and white/colorless. Each colored phase exhibits distinct optoelectronic properties characteristic of 2D superlattice materials with tunable quantum well thickness.

Integrated thermoelectric sensors based on quantum dot superlattice for thermal management applications

We have developed high sensitivity integrated micro-thermoelectric sensors (µTESs), based on complementary metal-oxide-semiconductor (CMOS) technology. These sensors have been developed from SiGe and quantum dot superlattice (QDSL) materials. QDSLs consist of TiSi2 nanoparticles embedded into a SiGe matrix. This is the first time that thermoelectric devices integrate such silicon-based nanostructured materials. We have shown that all QDSL-based thermoelectric sensors present higher performances than SiGe-based ones. QDSL-based µTESs show high voltage factors equal to 6.92 V K−1 cm−2, higher than other CMOS compatible thermal sensors from literature. Their compactness and fast response time (<4 ms) make them very good candidates for thermal management applications. We have also developed a dedicated read-out interface to manage signals from several µTESs at the same time, offering a global solution for thermal mapping of CMOS circuits.

Excitonic Laughlin states in ideal topological insulator flat bands and their possible presence in moiré superlattice materials

We investigate few- and many-body states in half-filled maximally symmetric topological insulator flat bands realized by two degenerate Landau levels which experience opposite magnetic fields. This serves as a toy model of flat bands in moir\'e materials in which valleys have Chern numbers $C= \pm 1$. We argue that although the spontaneously polarized Ising Chern magnet is a natural ground state for repulsive Coulomb interactions, it can be in reasonable energetic competition with correlated states which can be viewed as Laughlin states of excitons when short distance corrections to the interaction are included. This is because charge neutral excitons in these bands behave effectively as charged particles in ordinary Landau levels. In particular, the Ising Chern magnet is no longer the ground state once the strength of a short range intra-valley repulsion is about $30\%$ larger than the inter-valley repulsion. Remarkably, these excitonic Laughlin states feature valley number fractionalization but no charge fractionalization and a quantized charge Hall conductivity identical to the Ising magnet, $\sigma_{xy}= \pm e^2/h$, and thus cannot be distinguished from it by ordinary charge transport measurements. The most compact excitonic Laughlin state that can be constructed in these bands is an analogue of $\nu=1/4$ bosonic Laughlin state and has no valley polarization even though it spontaneously breaks time reversal symmetry with a charge Hall conductivity $\sigma_{xy}= \pm e^2/h$.

Design and growth of short-period long wavelength infrared InAsSb/InAsSb superlattices on lattice engineered metamorphic buffers

There has been steady effort to advance Ga-free InAs1−xSbx/InAs1−ySby superlattice (SL) materials for long-wavelength infrared (LWIR) detector applications. Although Ga-free SLs grown pseudomorphically on GaSb wafers work well in mid-wavelength applications, most LWIR Ga-free SL designs suffer from relatively low optical absorption and poor hole transport. One of the main problems in using the Ga-free material system for LWIR applications is a tremendous constraint on band engineering imposed by matching the SL lattice parameter to the lattice constant of conventional GaSb substrates. In this work, we propose several lattice engineered short-period LWIR InAs1−xSbx/InAs1−ySby SL designs that can enhance optical absorption and vertical hole mobility at an 11 μm cutoff wavelength. We also performed test growths of the designed structures on lattice-matched homogeneous buffer layers on GaAs wafers.

Ultrahigh-strength and ductile superlattice alloys with nanoscale disordered interfaces.

Alloys that have high strengths at high temperatures are crucial for a variety of important industries including aerospace. Alloys with ordered superlattice structures are attractive for this purpose but generally suffer from poor ductility and rapid grain coarsening. We discovered that nanoscale disordered interfaces can effectively overcome these problems. Interfacial disordering is driven by multielement cosegregation that creates a distinctive nanolayer between adjacent micrometer-scale superlattice grains. This nanolayer acts as a sustainable ductilizing source, which prevents brittle intergranular fractures by enhancing dislocation mobilities. Our superlattice materials have ultrahigh strengths of 1.6 gigapascals with tensile ductilities of 25% at ambient temperature. Simultaneously, we achieved negligible grain coarsening with exceptional softening resistance at elevated temperatures. Designing similar nanolayers may open a pathway for further optimization of alloy properties.

Evaluation of lateral diffusion length in InAs/GaSb superlattice detectors grown by MOCVD

The lateral diffusion length ( L h ) of minority carriers in LWIR InAs/GaSb superlattice detectors grown by metalorganic chemical vapour deposition was evaluated. The deeply-etched PNn device exhibits a diffusion-limited behaviour at 80 K, with a dark current density as low as 9.1 × 10 −6 A/cm 2 at −0.1 V and a 50% cut-off of 10.1 μm. In shallow-etched pixels with a common absorber, both the photo-current and the dark current show a size-dependent behaviour. L h deduced from the two methods are 211 and 251 μm, respectively, which are longer than those in superlattice materials grown by molecular beam epitaxy.

Strain-induced the dark current characteristics in InAs/GaSb type-II superlattice for mid-wave detector

Type-II superlattice (T2SL) materials are the key element for infrared (IR) detectors. However, it is well known that the characteristics of the detectors with the T2SL layer are greatly affected by the strain developed during the growth process, which determines the performance of IR detectors. Therefore, great efforts have been made to properly control the strain effect and develop relevant analysis methods to evaluate the strain-induced dark current characteristics. In this work, we report the strain-induced dark current characteristics in InAs/GaSb T2SL MWIR photodetector. The overall strain of InAs/GaSb T2SL layer was analyzed by both high-resolution X-ray diffraction (HRXRD) and the dark current measured from the absorber layer at the elevated temperatures (≥ 110 K), where the major leakage current component is originated from the reduced minority carrier lifetime in the absorber layer. Our findings indicate that minority carrier lifetime increases as the tensile strain on the InAs/GaSb T2SL is more compensated by the compressive strain through ‘InSb-like’ interface, which reduces the dark current density of the device. Specifically, tensile strain compensated devices exhibited the dark current density of less than 2 ×10–5 A/cm2 at 120 K, which is more than one order of magnitude lower value compared to that of the device without tensile strain relaxation.

Vertical carrier transport in strain-balanced InAs/InAsSb type-II superlattice material

Anisotropic carrier transport properties of unintentionally doped InAs/InAs0.65Sb0.35 type-II strain-balanced superlattice material are evaluated using temperature- and field-dependent magnetotransport measurements performed in the vertical direction on a substrate-removed metal-semiconductor-metal device structure. To best isolate the measured transport to the superlattice, device fabrication entails flip-chip bonding and backside device processing to remove the substrate material and deposit contact metal directly to the bottom of an etched mesa. High-resolution mobility spectrum analysis is used to calculate the conductance contribution and corrected mixed vertical-lateral mobility of the two carrier species present. Combining the latter with lateral mobility results from in-plane magnetotransport measurements on identical superlattice material allows for the calculation of the true vertical majority electron and minority hole mobilities; amplitudes of 4.7 ×103 cm2/V s and 1.60 cm2/V s are determined at 77 K, respectively. The temperature-dependent results show that vertical hole mobility rapidly decreases with decreasing temperature due to trap-induced localization and then hopping transport, whereas vertical electron mobility appears phonon scattering-limited at high temperature, giving way to interface roughness scattering at low temperatures, analogous to the lateral electron mobility but with a lower overall magnitude.

Efficient interlayer charge release for high-performance layered thermoelectrics.

Many layered superlattice materials intrinsically possess large Seebeck coefficient and low lattice thermal conductivity, but poor electrical conductivity because of the interlayer transport barrier for charges, which has become a stumbling block for achieving high thermoelectric performance. Herein, taking BiCuSeO superlattice as an example, it is demonstrated that efficient interlayer charge release can increase carrier concentration, thereby activating multiple Fermi pockets through Bi/Cu dual vacancies and Pb codoping. Experimental results reveal that the extrinsic charges, which are introduced by Pb and initially trapped in the charge-reservoir [Bi2O2]2+ sublayers, are effectively released into [Cu2Se2]2− sublayers via the channels bridged by Bi/Cu dual vacancies. This efficient interlayer charge release endows dual-vacancy- and Pb-codoped BiCuSeO with increased carrier concentration and electrical conductivity. Moreover, with increasing carrier concentration, the Fermi level is pushed down, activating multiple converged valence bands, which helps to maintain a relatively high Seebeck coefficient and yield an enhanced power factor. As a result, a high ZT value of ∼1.4 is achieved at 823 K in codoped Bi0.90Pb0.06Cu0.96SeO, which is superior to that of pristine BiCuSeO and solely doped samples. The present findings provide prospective insights into the exploration of high-performance thermoelectric materials and the underlying transport physics.