Superlattice Research Articles

Self-Assembly of Atomically Aligned Nanoparticle Superlattices from Pt-Fe3O4 Heterodimer Nanoparticles.

Multicomponent nanoparticle superlattices (SLs) promise the integration of nanoparticles (NPs) with remarkable electronic, magnetic, and optical properties into a single structure. Here, we demonstrate that heterodimers consisting of two conjoined NPs can self-assemble into novel multicomponent SLs with a high degree of alignment between the atomic lattices of individual NPs, which has been theorized to lead to a wide variety of remarkable properties. Specifically, by using simulations and experiments, we show that heterodimers composed of larger Fe3O4 domains decorated with a Pt domain at one vertex can self-assemble into an SL with long-range atomic alignment between the Fe3O4 domains of different NPs across the SL. The SLs show an unanticipated decreased coercivity relative to nonassembled NPs. In situ scattering of the self-assembly reveals a two-stage mechanism of self-assembly: translational ordering between NPs develops before atomic alignment. Our experiments and simulation indicate that atomic alignment requires selective epitaxial growth of the smaller domain during heterodimer synthesis and specific size ratios of the heterodimer domains as opposed to specific chemical composition. This composition independence makes the self-assembly principles elucidated here applicable to the future preparation of multicomponent materials with fine structural control.

Ferromagnetic order controlled by the magnetic interface of LaNiO3/La2/3Ca1/3MnO3 superlattices

Interface engineering in complex oxide superlattices is a growing field, enabling manipulation of the exceptional properties of these materials, and also providing access to new phases and emergent physical phenomena. Here we demonstrate how interfacial interactions can induce a complex charge and spin structure in a bulk paramagnetic material. We investigate a superlattice (SLs) consisting of paramagnetic LaNiO3 (LNO) and highly spin-polarized ferromagnetic La2/3Ca1/3MnO3 (LCMO), grown on SrTiO3 (001) substrate. We observed emerging magnetism in LNO through an exchange bias mechanism at the interfaces in X-ray resonant magnetic reflectivity. We find non-symmetric interface induced magnetization profiles in LNO and LCMO which we relate to a periodic complex charge and spin superstructure. High resolution scanning transmission electron microscopy images reveal that the upper and lower interfaces exhibit no significant structural variations. The different long range magnetic order emerging in LNO layers demonstrates the enormous potential of interfacial reconstruction as a tool for tailored electronic properties.

Effects on RF Performance for AlGaN/GaN HEMT on Si Substrate with AlGaN Buffer Engineering

In this paper, we demonstrate the AlGaN/GaN high electron mobility transistors (HEMTs) on Si substrate using an AlGaN back-barrier (BB) and super-lattice (SL) buffer to achieve high breakdown and low current collapse (CC) properties for Radio-frequency (RF) applications. The HEMTs also demonstrated low initial vertical leakage current, high thermal stability, and smaller drain-lag with reduced leakage currents as compared to the devices using step-graded (SG) AlGaN buffer. Also, the device showed improved RF performances with higher f T and f max as compared to the devices with SG buffer.

Investigation of the phonon dispersion associated with superlattice reflections in the BiS2 -based superconductor LaBiS2O0.5F0.5

The phonon dispersion of a $\mathrm{Bi}{\mathrm{S}}_{2}$-based superconductor $\mathrm{La}\mathrm{Bi}{\mathrm{S}}_{2}{\mathrm{O}}_{0.5}{\mathrm{F}}_{0.5}$ is investigated by first-principles calculations and inelastic x-ray scattering experiments. The origin of superlattice (SL) reflections arising from transverse-type lattice modulation, which were recently reported in [J. Kajitani et al., J. Phys. Soc. Jpn. 90, 103601 (2021)], is discussed in terms of lattice dynamics. Our first-principles calculations of phonon dispersion and the Fermi surfaces (FSs) demonstrate that the phonon mode corresponding to the transverse-type lattice modulation is unstable, and the propagation vector corresponding to the SL reflections is close to the FS nesting vector, which suggests that the phonon softening originates from the FS nesting. Against these calculated expectations, measured phonon dispersion in $\mathrm{La}\mathrm{Bi}{\mathrm{S}}_{2}{\mathrm{O}}_{0.5}{\mathrm{F}}_{0.5}$ along the Z-A direction, where the SL point is located, shows no remarkable temperature dependence, and there are no steeply declining branches accompanied with a softening around the SL point. Based on these results, we discuss the two possibilities for the transverse lattice modulation in $\mathrm{La}\mathrm{Bi}{\mathrm{S}}_{2}{\mathrm{O}}_{0.5}{\mathrm{F}}_{0.5}$: the order-disorder-type structural transition and the displacive structural transition with an overdamped mode, for both of which the local structure distortion or the short-range correlation within the $\mathrm{Bi}{\mathrm{S}}_{2}$ plane would be essential.

High-quality AlGaN epitaxial structures and realization of UVC vertical-cavity surface-emitting lasers

AlGaN-based vertical-cavity surface-emitting lasers (VCSELs) have garnered recent interest due to their superior material properties and device benefits. Nevertheless, AlGaN-based VCSELs are extremely difficult to realize due to numerous technical limitations associated with both material epitaxial growth and chip fabrication. This study fabricated a high-quality AlGaN multiple quantum wells (MQWs) structure using epitaxial lateral overgrowth and analyzed it using X-ray diffraction (XRD) and photoluminescence (PL) measurements. With an edge dislocation density (DD) of 109 cm−2, XRD measurements reveal that the AlN template is nearly fully relaxed. The subsequent AlGaN/AlN superlattice (SL) layer is introduced to decrease the edge DD, and the edge DD in the MQWs is ∼108 cm−2. According to PL measurements, the internal quantum efficiency of the MQWs is as high as 62%, and radiative recombination dominated the emission of the MQWs at room temperature. Using these epitaxial wafers, ultraviolet radiation C (UVC) VCSELs were fabricated using various techniques, including laser lift-off (LLO) and chemical mechanical polishing (CMP). The crystallinity of the MQWs was unaffected by sapphire substrate removal using LLO. After removing the sapphire substrate using LLO and CMP, UVC surface-stimulated emission was observed in MQWs. AlGaN-based UVC VCSELs with lasing wavelengths of 275.91, 276.28, and 277.64 nm have been fabricated. The minimum threshold for UVC VCSELs is 0.79 MW cm−2, which is a record low.

Study of bulk carrier properties in largely hybridized InAsSb/GaSb superlattice materials

The prospect of implementing quantum device architectures with technologically mature III–V semiconductors requires precisely controlled topologically protected edge states and bulk insulation. However, experimentally reaching this regime with III–V heterostructure epitaxy has been difficult due to charged bulk defects. Charged defects prevent bulk insulation and disturb the stability of edge states. Hence, we study carrier properties in a hybridized III–V InAs0.9Sb0.1/GaSb superlattice (SL) structure. We realize that an electron density (mobility) is limited to approximately 1012 cm−2 (104 cm2/V s). In order to understand these limits, the authors investigate the in-plane mobility of hybridized SLs as a function of current-carrying layer thickness, L, to determine scattering mechanisms that restrict carrier mobility. Although theory predicts the in-plane mobility is proportional to L6 dominated by interface roughness scattering (IRS) at low temperatures, we report that mobility follows the fourth power dependence, which is a weaker than expected from IRS theory. We attribute the discrepancy between experiment and the model to interface intermixing and wave function penetration into barrier regions. We use this understanding to develop a strategy for realizing high-performance topological materials.

Detection of hot electrons originating from an upper valley at ∼1.7eV above the Γ valley in wurtzite GaN using electron emission spectroscopy

Using electron emission spectroscopy, measurement and analysis were conducted on the energy distribution of vacuum emitted electrons from electrically driven InGaN/GaN green (peak wavelengths $\ensuremath{\lambda}\ensuremath{\approx}515\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$) light-emitting diodes (LEDs) with and without a prewell superlattice (SL). We report on the detection of a high-energy upper valley at $\ensuremath{\sim}1.7\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ above the $\mathrm{\ensuremath{\Gamma}}$ valley from samples with no prewell SL. We propose that these upper valley electrons originate predominantly from trap-assisted Auger recombination (TAAR) in green LEDs, as the intensity of these peaks is found to have quadratic dependence on the carrier density $n$ [see Espenlaub et al., J. Appl. Phys. 126, 184502 (2019)]. The high-energy upper valley peak was not observed in the sample with a prewell SL which is attributed to gettering by the prewell SL of still unidentified impurities that act as TAAR centers.

Insight into refractive index modulation as route to enhanced light coupling in semiconductor nanowires.

Recent developments in chemical processes to prepare single-crystalline nanowire (NW) superlattices (SLs) have discovered a range of unique nanophotonic properties. In particular, diameter-modulated silicon NW geometric SLs (GSLs) have shown their ability to produce complex interference effects through which enhanced light manipulation is achieved. Here, we re-imagine the origin of the complex interference effects occurring in shallow-modulated GSLs and present a refractive index modulation as a key deciding factor. We introduce the design of a NW refractive index SL (ISL), a hypothetical uniform-diameter NW in which the refractive index is periodically modulated, and explain the coupling effect between Mie resonance and bound guided state. We apply the ISL concept to other NW SL systems and suggest potential routes to bring substantial enhancements in lasing activities.

Design of high polarization low switching barrier hybrid improper ferroelectric perovskite oxide superlattices.

Hybrid improper ferroelectricity is a useful tool to design ABO3/A'BO3 polar superlattices from non polar building blocks. In this study, we have designed high polarization-low switching barrier hybrid improper ferroelectric superlattices with efficient polarization, and polarization-magnetization switching properties above room temperature, using density functional theory and ab initio molecular dynamics simulations. Superlattices with a chemical formula of (AAlO3)m/(A'AlO3)n, where m/n = 1/1, 1/3, 3/1, 1/5 and 5/1, A, A' = Lanthanide and Y cations are considered to outline the design principles behind polarization switching and (LaFeO3)3/(CeFeO3)1 is investigated for polarization-magnetization switching. We find that the unconventional switching paths via out-of-phase rotation QR- (a0a0c-) and tilt precession QTP always yield lower switching barrier compared to those via in-phase rotation QR+ (a0a0c+) and tilt QT (a-a-c0) of BO6 octahedra. Results from ab initio molecular dynamics simulations estimate the temperature at which the lowest energy barrier can be overcome. It is possible to tune the polarization switching barrier by tuning the tolerance factor, A,A' cation radius mismatch and super lattice periodicity. For switching via QR-, the switching barrier varies exponentially with rotation angle, indicating how high switching barrier is expected for systems, away from cubic symmetry. We provide a recipe to overcome such a bottleneck by tuning superlattice periodicity. Finally, we have proposed the multiferroic device application concept through a proposed polarization-temperature hysteresis loop and magnetization switching.

Энергетический спектр и волновые функции дырок в 2D-сверхрешетках из квантовых проволок (GaAs/AlAs)

A numerical analysis of the hole energy spectrum and wave functions in a 2D (GaAs/AlAs) quantum wire (QWr) superlattice (SL) with arbitrary cross-section is presented. The intricate fourfold degenerate nature of valence band Г8 of the given materials is taken into account. The presented approach is based on a modified effective mass method, wherein the microscopic structure of the heterostructure and hole kp-interaction in the valence band are considered. It is shown that the solution of this problem in a k-representation is reduced to an eigenvalue problem for a matrix Hamiltonian. Superlattices of quantum wires with different cross-sections (square, circle, ellipse, and ring) are considered as an example. The influence of superlattice parameters on the hole energy spectrum is studied. Anti-crossing effects in the hole dispersion law are observed. For the hole ground and excited states, the wave function spatial distribution in the plane perpendicular to the QWr axis is also studied.

A physics rule to design aperiodic width-modulated waveguides for minimum phonon transmission with Bayesian optimization.

Aperiodic nano-waveguides (nWVGs) and superlattices (SLs) limit phonon transmission and heat conduction much more efficiently than periodic ones. They could block parasitic heat conduction that restricts heat management and energy conversion at the nanoscale. Aperiodicity can be realized in multiple ways and with variable degrees of complexity. Machine learning optimization of hetero-SLs for minimum coherent phonon conduction showed optimal aperiodicity for moderate disorder against physics intuition. Here, we report on optimal aperiodicity in width-modulated nWVGS for maximum disorder as expected by physics. Optimizing aperiodic geometry modulation is particularly challenging due to the enormous possible configurations. We set up a feasible optimization problem removing unnecessary complexity and we demonstrate efficient Bayesian optimization. Our results confirm the predicted physics rule that minimum thermal conductance occurs for the most disordered arrays of modulation units; the degree of disorder being quantified by the number of non-identical modulation units. Our work opens a route to design geometrical aperiodicity and control transmission across metamaterials.

Прогнозная оценка технологических показателей при обогащении железистых кварцитов

Прогнозная оценка технологических показателей при обогащении железистых кварцитов

Effects of AlN/GaN superlattice buffer layer on performances of AlGaN/GaN HEMT grown on silicon for sub-6 GHz applications

In this study, the effects of AlN/GaN superlattice (SL) thickness on performances of AlGaN/GaN high electron mobility transistor (HEMT) heterostructure grown by metal-organic chemical vapor deposition on silicon is investigated. Stress in GaN is controlled by varying the total thickness of the AlN/GaN SL. Improved crystal quality and surface roughness accomplished with 2200 nm-thick AlN/GaN SL, leads to an increase in high electron mobility (1760 cm2 (V s)−1) as well as two-dimensional electron gas concentration (1.04 × 1013 cm−2). AlGaN/GaN metal–insulator-semiconductor HEMT (MIS-HEMT) fabricated on the heterostructure with SL buffer layer exhibits a significant improvement in maximum saturation current of 1100 ± 29 mA mm−1 at V GS = 0 V and a low on-resistance of 4.3 ± 0.15 Ω mm for the optimized AlN/GaN SL. The 2200 nm-thick AlN/GaN SL supports the growth of stress-free GaN heterostructure, which can reduce the insertion loss for sub-6 GHz radio frequency (RF) applications. This GaN HEMT structure based on SL buffer layer is suitable for low-frequency RF power applications.

Effects of stacking periodicity on the structural, electronic, optical and thermoelectric properties of GaSb/InSb superlattices

In this paper, we show the effect of monolayers (m and n) on the structural, electronic, optical and thermoelectric properties of (GaSb)m/(InSb)n (m-n: 1-1, 2-2, 3-3 and 3–1) superlattices (SLs) within Density Functional Theory (DFT) calculations. The formation energy (Efor) validates the stability of the vertically stacked SLs. The results reveal that the (GaSb)m/(InSb)n semiconductor nature has a direct bandgap (ΓV-ΓC). However, the bandgap is constant when m = n, and increases as m > n. In addition, the optical properties calculations, such as the real and imaginary parts of the dielectric function, reflectivity, refractive index, optical conductivity and absorption coefficient, are up to 13.00 eV. Furthermore, the thermoelectric parameters are computed over a broad temperature range of 300 K–800 K to explore the potential of (GaSb)m/(InSb)n for high-performance technological applications. According to the current research, (GaSb)m/(InSb)n SLs are promising candidates for novel infrared (IR) detector devices.

Photothermal investigation for optimizing a lattice strain relaxation condition of InGaAs/GaAsP superlattice photovoltaic structures from a nonradiative transition point of view

The carrier collection efficiencies of InGaAs/GaAsP superlattice (SL) photovoltaic structures were optimized by choosing adequate manufacturing parameters, such as the composition and thickness of the quantum wells (QWs) and barrier layers. However, no insights have been observed from the viewpoint of the nonradiative transition of photoexcited carriers. In this study, piezoelectric photothermal (PPT) and photoluminescence (PL) measurements were performed as a function of temperature from 100 to 340 K. Using a piezoelectric transducer, the PPT signal detected the heat generated by nonradiative recombination (NR). The indium composition of the QW layer was fixed at 0.3, and the phosphorus composition x[P] in the barrier layer was changed from 0.4 to 0.6. The observed temperature dependences of the PPT and PL signal intensities were analyzed using a rate equation for the photoexcited carriers in e1 and hh1 quantized levels. Four carrier dissipating processes, namely, radiative recombination, NR, thermal escape from the QW thermal excitation (TE), and tunneling after thermal excitation (TATE), were considered for both electrons and holes. Thermal activation energies were included in the NR, TE, and TATE processes. Because nonradiative and radiative transition components cause PPT and PL signals, curve fitting of the temperature behavior enabled us to determine the activation energies. We then found that the activation energy of the NR process reached a maximum at x[P] = 0.45. No such maxima were observed for the TE and TATE process. This result was explained by a trade-off between the strain valance condition over the entire range of the SL structure and the local residual strain at the interfaces between the QW, interlayer, and barrier layer when x[P] increased. Because no software can theoretically calculate the activation energy of the NR process, we demonstrated the usefulness of the present PPT experimental methodology for investigating carrier transport properties.

Physical origin of the endurance improvement for HfO2-ZrO2 superlattice ferroelectric film

HfO2-ZrO2 superlattice (SL) ferroelectric (FE) ultrathin films exhibit significant improvement in endurance performance compared with solid-solution HfxZr1−xO2 (HZO). Despite the experimental evidence, the underlying microscopic mechanisms of the enhanced reliability of SL remain elusive. This Letter explores the mechanism by performing first-principle calculations on SL and HZO systems. The enhanced endurance in the SL can be well explained by higher oxygen vacancy (Vo) migration energy barriers along the FE polarization direction, which slow down the increase in Vo. The suppression of Vo increase will potentially help maintain the stability of the FE phase and alleviate the fatigue. Based on this mechanism, we suggest that doping materials with higher Vo migration barriers can further improve the endurance of HfO2-based FE devices. This work facilitates the future development of HfO2-based FE devices with enhanced endurance and reliability.

High-Throughput Screening of Aperiodic Superlattices Based on Atomistic Simulation-Informed Effective Medium Theory and Genetic Algorithm

Superlattices (SLs) have received great attention as thermoelectric materials because phonon transport in them can be tailored independently without changing the electronic properties to enhance the thermoelectric figure of merit and conversion efficiency. Heterogeneous aperiodic SLs (ap-SLs) have lower thermal conductivity than their periodic counterparts (p-SL) due to Anderson localization of coherent phonons. Due to enormous amounts of layered structure combinations in ap-SLs, it is challenging to efficiently screen through all the combinations for the structure with the minimum thermal conductivity. In this work, a modified effective medium theory (m-EMT) is established to predict the thermal conductivity of Si/Ge p-SLs and ap-SLs. The effects of Anderson localization and layer thickness distribution are considered using a correction function for the degree of randomization (DOR) in layer thickness. The combination of the m-EMT and the genetic algorithm enables high-throughput screening of enormous amounts (∼ 2n, where n is the number of atomic layers) of SL structures with micron-scale total SL thickness. The thermal conductivities of ap-SLs can be constantly reduced to 1.5 W/(m·K) at average periodic layer thicknesses of 2.0 nm and a DOR of 0.8, regardless of the total SL thicknesses. Phonon spatial localization energy distribution further reflects the intense Anderson localization at interfaces, especially for those sandwiched between the highly asymmetric thick and thin layers. This work provides a high-throughput screening tool to theoretically design micron-scale and heterogeneous SLs, with detailed structural guidance for experimentally growing ap-SL with minimum thermal conductivities.

The Influence of the Growth Temperature on the Structural Properties of {CdO/ZnO}30 Superlattices

The {ZnO/CdO}30 superlattices (SLs) series were grown at different temperatures (350–550 °C) on m-plane Al2O3 substrates by molecular beam epitaxy. The structural properties of SLs were investigated using high resolution XRD and TEM methods. The periodic structure of the obtained {ZnO/CdO}30 SLs was confirmed by cross-sectional TEM images. Satellite peaks (S1, S2, S3...) surrounding the main zero-order peak (S0) and characteristic of the periodic superlattice structure were found in the X-ray analysis. X’Pert Epitaxy software was used to simulate the XRD data to determine the ZnO and CdO sublayers thicknesses and the SLs period. It was found that growth at relatively low temperatures is favorable for the quality of the superlattices. The thicknesses of the ZnO and CdO sublayers change with growth temperature. Some deterioration of the {ZnO/CdO}30 structure quality and changes in the period of the superlattices were also confirmed by TEM analysis.

Tailoring Interfacial Charge Transfer for Optimizing Thermoelectric Performances of MnTe‐Sb2Te3 Superlattice‐Like Films

AbstractInterfacial charge transfer has a vital role in tailoring the thermoelectric performance of superlattices (SLs), which, however, is rarely clarified by experiments. Herein, based on epitaxially grown p‐type (MnTe)x(Sb2Te3)y superlattice‐like films, synergistically optimized thermoelectric parameters of carrier density, carrier mobility, and Seebeck coefficient are achieved by introducing interfacial charge transfer, in which effects of hole injection, modulation doping, and energy filtering are involved. Carrier transport measurements and angle‐resolved photoemission spectroscopy (ARPES) characterizations reveal a strong hole injection from the MnTe layer to the Sb2Te3 layer in the SLs, originating from the work function difference between MnTe and Sb2Te3. By reducing the thickness of MnTe less than one monolayer, all electronic transport parameters are synergistically optimized in the quantum‐dots (MnTe)x(Sb2Te3)12 superlattice‐like films, leading to much improved thermoelectric power factors (PFs). The (MnTe)0.1(Sb2Te3)12 obtains the highest room‐temperature PF of 2.50 mWm−1K−2, while the (MnTe)0.25(Sb2Te3)12 possesses the highest PF of 2.79 mWm−1K−2 at 381 K, remarkably superior to the values acquired in binary MnTe and Sb2Te3 films. This research provides valuable guidance on understanding and rationally tailoring the interfacial charge transfer of thermoelectric SLs to further enhance thermoelectric performances.

Interfacial strain-dependent localization of phonons in (CdS)m/(CdTe)n superlattices

Comprehensive results of the acoustic and optical phonon traits are reported for the strained (CdS)m/(CdTe)n superlattices (SLs) using microscopic and macroscopic methods. Simulations by realistic rigid-ion-model provided atypical vibrational characteristics in the growth [001] as well as in-plane ([100], [110]) directions of SLs exhibiting anisotropy of zone-center optical-, acoustic-anti-crossing phonons, mini-gap formation, confinement, and gap modes. In (CdTe)n/(CdS)m and (CdS)n/(ZnS)m, the use of elastic continuum techniques offered accurate values of folded acoustic modes with proper shifts of the longitudinal-optical (ΔωLO) and transverse-optical (ΔωTO) phonons. Changes in ΔωLO have instigated strong motives for calculating the Raman intensity profiles in the graded (CdTe)10-Δ/(CdTe0·5S0.5)Δ/(CdS)10-Δ/(CdTe0·5S0.5)Δ SLs by meticulously including the interfacial layer thickness Δ (≡ 1 to 3 monolayers (MLs)). With Δ (≡ 3 ML), the Raman intensity features have exhibited downwards (upwards) shifts of CdS (CdTe)-like ωLO modes by ∼ - 20 cm−1 (∼+18 cm−1) in good agreement with the strain-induced results. Appraisals of Raman intensity shifts in the confined optical mode regions (see: Supplementary material) are ascribed to the localization/delocalization of atomic amplitudes at the CdS–CdTe interface. We strongly believe that the reported phonon aspects in CdTe/CdS SLs will encourage Raman spectroscopists to validate our theoretical conjectures and engineers to evaluate potentials of the CdTe-based SLs to improve/design electronic devices of many contemporary applications.