Superlattice Research Articles

A study of order-disorder phase transformation in Ag50Pd50 alloy

In this study, the structure of Ag50Pd50 alloy was investigated with temperature dependent in-situ X-ray diffraction, residual resistometry, and differential scanning calorimetry (DSC) as well as differential thermal analysis (DTA) techniques. Isochronal annealing experiment was performed to determine the impact of ordering on the residual electrical resistivity. The residual resistivity curve shows a sharp minimum near 250 °C indicating the on-set of ordering. The in-situ X-ray diffraction data taken for a well annealed sample shows no super lattice reflections over the investigated temperature range but the lattice parameter determined from this data shows an abrupt decrease in its value at 250 °C showing an order-disorder phase transformation (ODPT). The diffraction pattern of a sample annealed at 230 °C shows splitting of the high angle 511 fundamental reflection due to the tetragonal distortion. The peak intensity and c/a ratios are found to be 1:1.94 and 1.0028, respectively. A similar phase change was also observed during a DSC experiment around 250 °C on this alloy. The existence of these anomalies may be attributed to L1o - FCC type order-disorder phase transformation at this temperature.

Highly Transparent p‐AlGaN‐Based (326–341 nm)‐Band Ultraviolet‐A Light‐Emitting Diodes on AlN Templates: Recent Advances and Perspectives

The epitaxial growth of transparent p‐AlGaN‐based ultraviolet‐A (UVA), light‐emitting diodes (LEDs) may solve the problems of UVA light absorption through the GaN buffers and p‐GaN contact layers at (326–341 nm)‐band emission, respectively. Herein, first, an idea of conventional n‐AlGaN buffer layer (BL) and n‐AlGaN electron source layer (ESL) for the suppression of threading dislocation density (TDD) and enhancement of internal quantum efficiency (IQE) of UVA emitters, using low‐pressure metalorganic vapor‐phase epitaxy (LP‐MOVPE) is attempted. As a result, the total‐TDDs is reduced from ≈ 3 × 109 cm−2 to ≈ 1 × 109 cm−2 in the n‐AlGaN ESL of a 326 nm‐band UVA multiquantum‐wells (MQWs), and IQE is also improved from 30% to 52% at room temperature (RT). Second, an idea of Si‐doped n‐AlGaN Superlattices (SLs)‐based BL, using LP‐MOVPE is challenged. Subsequently, a record IQE of 56% at RT and high crystal quality in 341 nm‐band UVA MQWs are observed. Finally, using a well thickness ≈ 2 nm in SLs‐based UVA MQWs, the light power and external quantum efficiency (EQE), respectively, are remarkably enhanced from 3.5 mW and 0.5% to 7.5 mW and 1.4% on wafer in 341 nm‐Band UVA LED. The perspective for the improvements of UVA emitter's performances is also discussed.

HfO2-ZrO2 Superlattice Ferroelectric Capacitor With Improved Endurance Performance and Higher Fatigue Recovery Capability

HfO₂-ZrO₂ superlattice (SL) ferroelectric (FE) capacitor is demonstrated to have improved endurance performance and higher fatigue recovery capability compared to the HfZrO_<i>x</i> (HZO) device. During the cycling of polarization (<inline-formula> <tex-math notation="LaTeX">${P}$ </tex-math></inline-formula>) <i>vs.</i> voltage (<inline-formula> <tex-math notation="LaTeX">${V}$ </tex-math></inline-formula>) loops, the SL metal-FE-metal (MFM) capacitor exhibits the higher <inline-formula> <tex-math notation="LaTeX">${P}$ </tex-math></inline-formula> and the lower leakage current over the HZO device indicating the lower defect density in SL. The SL capacitor achieves an endurance of <inline-formula> <tex-math notation="LaTeX">${5}\times {10} ^{{12}}$ </tex-math></inline-formula> cycles, which is three orders of magnitude higher than the HZO device. The <inline-formula> <tex-math notation="LaTeX">${P}$ </tex-math></inline-formula> fatigue of the SL capacitor can be fully recovered through a &#x007E;30 s break, and that of HZO is only partially recovered utilizing the higher field cycling. This is because the trapping/detrapping process significantly decreases in HfO₂-ZrO₂ SL over HZO capacitor by the reduced defect density. These results prove that the HfO₂-ZrO₂ SL is a promising technology for endurance unlimited FE random access memory.

The Dependence of InAs/InAsSb Superlattice Detectors’ Spectral Response on Molecular Beam Epitaxy Growth Temperature

In this paper, we report on the influence of molecular beam epitaxial (MBE) growth temperature on the spectral response of the long-wavelength infrared radiation (LWIR), three-stage thermoelectrically (TE) cooled (T = 210, 230 K) InAs/InAsSb type-II superlattice (T2SL)-based detectors grown on the GaSb/GaAs buffer layers/substrates. Likewise, antimony (Sb) composition and the superlattice (SL) period could be used for spectral response selection. The presented results indicate that the growth temperature affects the 50% cut-off (λ50%cut-off) of the fabricated devices and could be used for operating wavelength tunning. Assuming constant Sb composition and T2SL period during MBE process, the growth temperature is presented to influence λ50%cut-off covering entire LWIR (e.g., temperature growth change within the range of 400–450 °C contributes to the λ50%cut-off ~ 11.6–8.3 μm estimated for operating temperature, T = 230 K). An increase in temperature growth makes a blueshift of the λ50%cut-off, and this is postulated to be a consequence of a modification of the SL interfaces. These results show an approach to the T2SL InAs/InAsSb deposition optimization by the growth temperature in terms of the spectral response, without influencing the T2SLs’ structural properties (Sb composition, SL period).

Analysis of electric-field domain formations and carrier transport phenomena in GaAs/AlAs asymmetric double-quantum-well superlattices

We evaluated the formation of electric-field domains (EFDs) in three kinds of asymmetric double-quantum-well (ADQW) superlattices (SLs) whose photoluminescence (PL) properties revealed various characteristics affected by EFD formation. In particular, anomalous comb-shaped PL branches were clearly observed. Each PL branch corresponds to one EFD, giving important information on EFD distribution in ADQW-SLs. We determined the strengths of all electric-fields in all the EFDs from their redshifted PL signals and plotted their reverse bias voltage dependence. We clearly identified the EFD distributions in the ADQW-SLs in which the resonant tunneling effect plays an important role to make carrier transport paths. These results demonstrate that the analysis of EFD distributions from PL properties is an effective tool to investigate the carrier transport phenomena in biased SLs.

Unexpected thermal conductivity enhancement in aperiodic superlattices discovered using active machine learning

While machine learning (ML) has shown increasing effectiveness in optimizing materials properties under known physics, its application in discovering new physics remains challenging due to its interpolative nature. In this work, we demonstrate a general-purpose adaptive ML-accelerated search process that can discover unexpected lattice thermal conductivity (κl) enhancement in aperiodic superlattices (SLs) as compared to periodic superlattices, with implications for thermal management of multilayer-based electronic devices. We use molecular dynamics simulations for high-fidelity calculations of κl, along with a convolutional neural network (CNN) which can rapidly predict κl for a large number of structures. To ensure accurate prediction for the target unknown SLs, we iteratively identify aperiodic SLs with structural features leading to locally enhanced thermal transport and include them as additional training data for the CNN. The identified structures exhibit increased coherent phonon transport owing to the presence of closely spaced interfaces.

Discovery of high thermoelectric performance of WS2-WSe2 nanoribbons with superlattice and Janus structures

Transition metal dichalcogenide monolayers have shown enormous potential in thermoelectric application in recent years. We now focus on the thermoelectric properties of WS2-WSe2 nanoribbons with superlattice (SL) and Janus (JA) structures using first-principles calculations. The WS2, WSe2, SL, and JA nanoribbons with the ribbon width from 5 to 7 have high structural stabilities. All nanoribbon electronic structures are semiconductors and the ribbon width will modify bandgaps. It can be also observed that WS2, SL, and JA nanoribbons with a ribbon width of 5 have the largest carrier mobilities (up to ~500–1400 cm2 V−1 s−1) and relaxation times (up to ~400–600 fs). We further calculate the electronic transport coefficients and discover that the SL and JA nanoribbons with a ribbon width of 5 exhibit the largest power factors as high as ~80 mW m−1 K−2. Afterwards, the minimum lattice thermal conductivities of SL and JA nanoribbons are 0.53 W m−1 K−1 and 0.61 W m−1 K−1, which are suppressed owing to the declining phonon group velocity and phonon lifetime. The maximum ZT values of SL and JA nanoribbons can reach 5.47 and 4.13. This investigation provides a solid evidence for the application of WS2-WSe2 nanoribbons as promising thermoelectric materials.

Growth of lattice matched InAs/AlSb superlattices by molecular beam epitaxy

The InAs/GaSb superlattices (SPLs) is an important component of quantum cascade laser (QCL) and interband cascade laser (ICL). In particular, the upper and lower SPL waveguide layers of the ICL are alternately grown from a large number of ultra-film epitaxial layers (nm) by molecular beam epitaxy(MBE). Subtle lattice mismatch may directly lead to the deterioration of material crystal quality, and the change of thicknessand the composition of each layer will strongly affect the structural performance of device material. The optimal growth temperature of InAs/GaSb SPLs is about 420 ℃. By growing GaSb/AlSb and InAs/GaSb SPL both with 40 short periods under the substrate rotating, the thickness of GaSb layer and AlSb layer are 5.448 nm and 3.921 nm, and the thickness of InAs layer and GaSb layer are 8.998 nm and 13.77 nm, respectively. The error is within about 10%, and the optimal growth conditions of InAs/AlSb SPLs are obtained. A lattice matched 40-period InAs/AlSb superlattice waveguide layer is grown on GaSb substrate. The influence of drifting As injection on the average lattice constant of InAs/AlSb superlattice is fully considered. Under the condition of fixed SOAK time of 3 s, the As pressure is changed to 1.7 × 10&lt;sup&gt;–6&lt;/sup&gt; mbar to adjust the average lattice constants of the superlattices and achieve their matching with the GaSb substrate lattice. The experimental results show that the 0 order satellite peak of the SPL coincides with the peak of the GaSb substrate, and has a perfect lattice matching, and that the sharp second order satellite peak and the periodic structure good repeatability also indicate that the superlattice material has the excellent structural quality of the SPLs structure.

Effect of interlayer coupling on the electron spectra of vertical superlattice

By use of the Green's function method analytical expressions for the dispersion lows of superlattice (SL) consists of two alternating 2D layers are obtained. Weak interlayer coupling regime is analized thoroughly. As the examples, the SLs graphene --- h-BN ( 1), AlN --- GaN ( 2), Gr --- Ni ( 3) and h-BN --- Ni ( 4) are considered and analytical electron spectrum characteristics for the corresponding 2D layers are given. It is shown that 1) electron effective masses become havier for the h-BN, AlN, GaN, and Ni layers in all SLs; 2) Fermi velocity becomes lower for the gapless graphene layer in the SL ( 1) and remains constant in the SL ( 3); 3) energy gaps become narrower for the h-BN, AlN and GaN layers in the SLs (1) and ( 2) and wider for h-BN layer in the SL (4). Keywords: dispersion low, Fermi velocity, effective mass, graphene-like compound, two-dimensional ferromagnetic metal.

First-principles calculations of vibrational spectra of CdSe/CdS superlattices

The vibrational spectra of CdSe/CdS superlattices (SLs) with different layer thicknesses are calculated from first principles within the density functional theory. It is shown that, along with folded acoustic and confined optical modes, a number of confined acoustic modes appear in SLs. In structures with a minimum thickness of one of the layers, microscopic interface modes similar to local and gap modes in crystals appear. An analysis of projections of the eigenvectors of vibrational modes in SLs onto the orthonormal basis of normal modes in binary compounds enables to establish the details of formation of these vibrational modes and, in particular, to determine the degree of intermixing of acoustic and optical modes. A comparison of the frequencies of vibrational modes in CdSe/CdS SLs and CdSe/CdS nanoplatelets enables to separate the influence of size quantization and surface relaxation on the vibrational frequencies in the nanoplatelets. Keywords: phonon spectra, semiconductor superlattices, cadmium selenide, cadmium sulfide, nanostructures.

Electronic property modulation in two-dimensional lateral superlattices of monolayer transition metal dichalcogenides.

Fabricating lateral heterostructures (HSs) and superlattices (SLs) provides a unique degree of freedom for modulating the physical properties of two-dimensional (2D) materials by varying the chemical component, geometric size and interface structure in the ultra-thin atomic thickness limit. While a variety of 2D lateral HSs/SLs have been synthesized, especially for transition metal dichalcogenides (TMDs), how such structures affect quantitatively the physical properties of 2D materials has not yet been established. We herein explore electronic property modulation in 2D lateral SLs of monolayer TMDs through first-principles high-throughput calculations. The dependence of the electronic structure, bandgap, carrier effective masses, charge density overlap on chemical components, interface type, and sub-lattice size of lateral TMD-SLs are investigated. We find that by comparison with their random alloy counterparts, the lateral TMD-SLs exhibit generally type-II band alignment, a wider range of bandgap tunability, larger carrier effective masses, and stronger electron-hole charge separation tendency. The bandgap variation with a sub-lattice size shows larger bowing parameters for the SLs with heterogeneous anions, by comparison with the homogeneous anion cases. A similar behavior is observed for the SLs with an armchair-type interface, by comparison with the zigzag-type interface cases. Further analyses reveal that the underlying physical mechanism can be attributed to the synergistic interplay among the band offset of sub-lattices, quantum confinement effect, and existing internal strain.

Electronic structure of short-period ZnSe/ZnTe superlattices based on DFT calculations

In the present study we discuss the effect of variation in the number of monolayers n on the electronic and optical properties of superlattices (SLs) (ZnSe)n/(ZnTe)n. The total energies were calculated by the full-potential linear muffin-tin orbital (FP-LMTO) method, and the exchange-correlation energy was applied in the local density approximation (LDA). First, the calculations show a decrease in the derivative of bulk modulus and electronic bandgap with an increase in the number of monolayers n. Second, the radiation energies up to 15 eV, the dielectric function ε(ω), the refractive index n(ω), and the reflectivity R(ω) are studied. These calculations may be beneficial to understand the properties of short-period superlattices (ZnSe)n/(ZnTe)n.

Synthesis, theoretical structural explication, super cell configuration, Hardness, tribological data and void space illustration of Creatininium hydrogen maleate - CHM crystal by softwares and by experimental techniques

Designing of new solids with desired physical and chemical properties highly depends on the understanding of intermolecular interactions and packing arrangement of atoms in the molecule. Now a days Organic complexes play a vital role in research and social applications. In this Context the title compound is synthesized from the aqueous solutions of creatinine and maleic acid taken in the proper stoichiometric ratio. Miniature sized Creatininium Hydrogen Maleate (CHM) crystals are obtained after 16 days. As synthesized compound is subjected to XRD analysis, which showed that it is crystallized in monoclinic structure with space group P21/n. Hirshfeld analysis is also carried on CHM compound to visualize and quantify various types of non-covalent interactions that stabilizes the crystalline packing. Hirshfeld surface and the associated plots are generated using crystal Explorer software. Negative value of dnorm showed that intermolecular contacts are shorter than the Vanderwall’s radii of atoms. Super cell lattice cell fabrication is performed for 3x3x3 methodology to enunciate the omnipresent way of periodic frontier states of the crystal systems by means of a larger cluster with quantum mechanical approximations by direct and with the coloured potential way.

Transmission in Through a Super Lattice of Magnetic Structure on Surface Oftopological Insulator

Transmission in Through a Super Lattice of Magnetic Structure on Surface Oftopological Insulator

Nanoscale Morphology of Short-Period {CdO/ZnO} Superlattices Grown by MBE

The nanoscale morphology of short-period {CdO/ZnO}n superlattices (SLs) grown by plasma-assisted molecular beam epitaxy on m-oriented sapphire substrates is studied. The SL structures consist of 25 repetitions of subsequently deposited ZnO and CdO layers. X-ray diffraction (XRD) and transmission electron microcopy (TEM) studies confirm the presence of 2D SL structures with the wurtzite phase of ZnO sublayers and the rocksalt structure of CdO sublayers. For thicker ZnO sublayers, the (11̅.3) twinning was observed. The SL periods are calculated based on high-resolution XRD and TEM analyses. By varying the thickness of the CdO and ZnO sublayers, it is possible to control the energy gap in these quasi-alloys. These results suggest the promising potential of their optical properties for light emission and/or applications in the detection of visible to ultra-violet light.

Comparative study of interfacial strain dependent phonon localization in the beryllium-zinc chalcogenide superlattices

A comprehensive study is reported for the novel (BeX)m/(ZnX)n (X = S, Se and Te) superlattices (SLs) to assess their acoustic and optical phonon characteristics. The use of an elastic continuum model has offered implicit equations for assessing folded acoustic modes (FAMs) and their splitting at the center and at the edge of mini-Brillouin zone. In (BeTe)m/(ZnSe)n SLs grown with Zn–Te interfaces, the Raman spectroscopy measurements revealed well resolved first order doublet and frequency shifts which compared favorably well with the simulated results. For the graded (BeX)10-Δ/(Be0.5Zn0.5X)Δ/(ZnX)10-Δ/(Be0.5Zn0.5X)Δ SLs, we have adopted a modified linear-chain model by meticulously integrating the effects of alloying and interfacial layer thickness Δ to study phonon dispersions (ωj(q→SL)) and Raman intensity profiles. By changing Δ from 1 to 3 monolayers, our study has distinctly revealed (a) a dramatic increase in the Raman scattering intensity of ZnX quasi-confined optical modes (Q-COMs) with slight upward shifts in frequencies, (b) an insignificant change in Raman intensity lines of BeX confined optical modes (COMs) with appreciable downward shifts (up to ∼ - 113 cm−1) in frequencies, and (c) trivial modifications in the FAMs. These atypical effects noticed in the optical modes are attributed to the localization of atomic displacements at the BeX-ZnX interfacial region. The variations of phonon mode frequencies and enhancement of Raman lines for the ZnX Q-COMs can be used as vital tools for establishing interfacial structures in graded SLs of technological importance.

Optical and structural properties of the GaAs heterostructures grown using AlGaAs superlattice buffer layer on compliant Si(100) substrates with the preformed porous-Si (por-Si) layer.

360 nm and 700 nm thick GaAs layers were grown by MO MOCVD growth technique directly on compliant Si (100) substrate and on supper-lattice (SL) AlGaAs buffer layer. The XRD study revealed better structural quality for the sample grown on SL / por-Si buffer. AFM study revealed a smoother sample surface with blocks of more regular rectangular shape and larger size as well. Photoluminescence spectra of the samples revealed an energy shift of PL maximum intensity for both samples. Sample grown on SL buffer also showed higher PL intensity corresponding to better crystalline perfection.

Observation of self-assembled InGaN/GaN superlattice structure grown on N-polar GaN by plasma-assisted molecular beam epitaxy

In this paper, we report on the observation of self-assembled InGaN/(In)GaN superlattice (SL) structure in a nominal “InGaN” film grown on N-polar GaN substrate. 350 nm thick InGaN films were grown at different temperatures ranging from 600 to 690 °C. Structural characterization was conducted via atomic force microscopy, scanning transmission electron microscopy, high-resolution x-ray diffraction, and XRD reciprocal space map. A SL structure was unexpectedly observed on all samples. However, the In content in each layer varied depending on growth temperature. By increasing the substrate temperature to 670 °C, a periodic structure composed of 3 nm In0.26Ga0.74N and 3 nm of GaN with a surface roughness of ∼0.7 nm was achieved. This work establishes a method for the growth of InGaN films with high structural quality on N-polar GaN and opens a new pathway for the design and fabrication of various electronic and optoelectronic devices with enhanced performance.

Reduction of in-plane and cross-plane thermal conductivities by polarization electric field induced in InxGa1-xN/GaN superlattice

In this paper, polarization electric field (PEF) induced in GaN/InxGa1-xN superlattice (SL) was studied and found it can be treated as a reductant of thermal conductivity (k). Elastic properties and phonon group velocity are modified by the impact of interfacial PEF as by inverse piezoelectric effect. It improves scattering and thermal resistance at the boundaries because of dissimilarity in specific heat and group velocity, that decreases transmission coefficient of phonon and more acoustic mismatches resulting in reduction of in-plane (kip) as well as cross-plane (kcp) thermal conductivities. kip of InxGa1-xN (5 nm)/ GaN (10 nm) SL with (without) interfacial PEF field are 7.807 (8.921), 7.350 (8.355) and 7.018 (8.090) Wm−1K−1 respectively, for x = 0.1, 0.3 and 0.5; whereas kcp for the similar compositions are respectively, 4.652 (5.710), 4.282 (5.221) and 4.081(5.185) Wm−1K−1 at 300 K demonstrating the reduction exceeds 20%. It shows that the optimal k can be accomplished with the adaptation of nitride SL's electric field for thermoelectric improvements.

Electronic and thermal properties of GeTe/Sb2Te3 superlattices by ab initio approach: Impact of Van der Waals gaps on vertical lattice thermal conductivity

In the last decade, several works have focused on exploring the material and electrical properties of GeTe/Sb2Te3 superlattices (SLs), in particular because of some first device implementations demonstrating interesting performances such as fast switching speed, low energy consumption, and non-volatility. However, the switching mechanism in such SL-based devices remains under debate. In this work, we investigate the prototype GeTe/Sb2Te3 SLs to analyze fundamentally their electronic and thermal properties by ab initio methods. We find that the resistive contrast is small among the different phases of GeTe/Sb2Te3 because of a small electronic gap (about 0.1 eV) and a consequent semi-metallic-like behavior. At the same time, the out-of-plane lattice thermal conductivity is rather small, while varying up to four times among the different phases, from 0.11 to 0.45 W m−1 K−1, intimately related to the number of Van der Waals (VdW) gaps in a unit block. Such findings confirm the importance of the thermal improvement achievable in GeTe/Sb2Te3 superlattices devices, highlighting the impact of the material stacking and the role of VdW gaps on the thermal engineering of the phase-change memory cell.