Superlattice

We foresee applications and interesting possibilities of incorporating the photonic crystals concept into superconducting electronics. In this paper, we present interesting features of the computed lower band structure of a nondissipative superconductor-dielectric superlattice using the two-fluid model and the transcendental equation [Pochi Yeh, Optical Waves in Layered Media, Wiley Series in Purl and Applied Optics (Wiley, New York, 1988)]. The necessary conditions for approximating the complex conductivity by an imaginary conductivity is derived and the feasibility of achieving the conditions are discussed. The superlattice dispersion obtained is similar to that of the phonon-polariton dispersion in ionic crystal. We found a nonlinear temperature-dependent "polariton gap" and a low-frequency (plasma) gap, and suggested the existence of a photon-superelectron hybrid around the polariton gap. The polariton gap may be observed in an infrared-microwave regime using a high-T-c, superconductor with sufficiently low normal-fluid relaxation time (approximate to 10(-15) s), and in an optical regime using lower penetration depth (approximate to 50 nm) and extremely low relaxation time (approximate to 10(-17) s).

A new type of photovoltaic device where GaAs/GaInNAs multiple quantum wells (MQW) or superlattice (SL) are inserted in the i-region of a GaAs p-i-n solar cell (SC) is presented. The results suggest the device can reach record efficiencies for single-junction solar cells. A theoretical model is developed to study the performance of this device. The conversion efficiency as a function of wells width and depth is modeled for MQW solar cells. It is shown that the MQW solar cells reach high conversion efficiency values. A study of the SL solar cell viability is also presented. The conditions for resonant tunneling are established by the matrix transfer method for a superlattice with variable quantum wells width. The effective density of states and the absorption coefficient for SL structure are calculated in order to determinate the J-V characteristic. The influence of superlattice length on the conversion efficiency is researched, showing a better performance when width and cluster numbers are increased. The SL solar cell conversion efficiency is compared with the maximum conversion efficiency obtained for the MQW solar cell and shows an efficiency enhancement.

Title: Stark quantization in superlattices. Authors: Tsu, Raphael; Esaki, L. Affiliation: AA(University of North Carolina at Charlotte, Charlotte, North Carolina 28223) AB(IBM Thomas J. Watson Research Center, Yorktown Heights, New York 10598). ...

A ferroelectric superlattice with an antiferroelectric interfacial coupling is considered; the same model describes a bilayer with antiferroelectric coupling. By mapping minimum points in the Landau free energy expression and plotting them against the applied electric field, a triple hysteresis loop pattern is obtained. The loop patterns vary between typically ferroelectric and typically antiferroelectric depending on the layer thicknesses and the magnitude of the interfacial-coupling constant. This work suggests the possibility of designing multilayer elements for computer memories with four or more different storage states. (C) 2000 American Institute of Physics. [S0003-6951(00)03843-2].

We report on the effect of strain on the optical and structural properties of 5-, 10-, and 20- period GaN/AlN superlattices (SLs) deposited by plasma-assisted molecular beam epitaxy. The deformation state in SLs has been studied by high resolution transmission electron microscopy (HRTEM), X-ray diffraction, and micro-Raman, Fourier transform infrared (FTIR), and photoluminescence spectroscopy. HRTEM images showed that the structural quality of the SL layers is significantly improved and the interfaces become very sharp on the atomic level with an increase of the SL periods. A combined analysis through XRD, Raman, and FTIR reflectance spectroscopy found that with increasing number of SL periods, the strain in the GaN quantum wells (QWs) increases and the AlN barrier is relaxed. Based on the dependence of the frequency shift of the E2High and E1TO Raman and IR modes on the deformation in the layers, the values of the biaxial stress coefficients as well as the phonon deformation potentials of these modes in both GaN and AlN were determined. With increasing number of SL periods, the QW emission considerably redshifted in the range lower than the GaN band gap due to the quantum confined Stark effect. The influence of strain obtained by the XRD, Raman, and FTIR spectra on the structural parameters and QW emission of GaN/AlN SLs with different numbers of periods is discussed.

Coherently grown nanolayered TiN/CrN thin films exhibit a superlattice effect in fracture toughness, similar to the reported effect in indentation hardness. We found-by employing in-situ micromechanical cantilever bending tests on free-standing TiN/CrN superlattice films-that the fracture toughness increases with decreasing bilayer period (Λ), reaching a maximum at Λ ~ 6 nm. For ultrathin layers (Λ ~ 2 nm), the fracture toughness drops to the lowest value due to intermixing and loss of superlattice structure. Both, fracture toughness and hardness peak for similar bilayer periods of TiN/CrN superlattices. Hard coatings are used to protect engineering components, e.g. cutting tools, from severe external loads and harsh environments [1]. Thereby, the coatings should ideally be strong and tough. Multilayer coatings composed of two coherently stacked, alternating materials with a periodicity length in the nanometer range, referred to as superlattice films, have been reported to possess exceptional high hardness values exceeding that of their single layered constituents by some hundred percent. In the 1980s, Helmersson et al. [2] reported on a hardness enhancement of up to ~250% compared to single-layered materials for the single-crystalline coherent TiN/VN superlattice (SL) structure grown by physical vapor deposition on single crystalline MgO (100) substrates. Thereby, the peak hardness was found for a periodicity length of ~ 5 nm. Later, an hardness enhancement was observed for a row of other SL film systems grown on MgO (100), but also on (native oxide) of Si (100) and polycrystalline steel substrates [3]. Besides high hardness values, a sufficiently high fracture toughness is needed to ensure the integrity of bulk and coated engineering components. Unfortunately, these material properties are commonly mutually influential (especially for materials showing plastic behavior), as a high strength often implies a low fracture toughness and vice versa [4]. In the last decades various strategies have successfully shown how to break down this relationship, spanning from grain refinement toughening based on the classical Hall-Petch relation used in a variety of steels [5,6]-to recently found nanoscaled twinning mechanisms being operative in high-entropy alloys-enabling exceptional high fracture-resistance even at cryogenic temperatures [7]-and several other mechanisms presented in Ref. [4]. Strategies for enhancing the (fracture) toughness of ceramic coatings (see review by Zhang et al. [8]) include: incorporating a ductile phase; toughening through a nanocrystalline microstructure, composition, or structure grading; multilayer structur-ing; phase transformation toughening; or apparent toughening by implementation of compressive stresses, most of them being already effectively applied in industrial products. However, the exceptional effect of a superlattice structure on the fracture toughness has yet not been reported. Here, we study the influence of the superlattice structure on the fracture toughness. Therefore, we have conducted micromechanical experiments on freestanding superlattice coatings with different bilayer periods (Λ). The isomorphous face-centered cubic (B1) TiN/CrN superlattice grown on Si (100) substrates served as a model system. The constituents TiN and CrN represent one of the most widely used nitrogen based hard coating materials and their shear moduli (~180 GPa [9] and ~ 135 GPa [10], respectively) are significantly different, which promotes the superlattice effect [11]. TiN/CrN multilayer films with equal thick layers-bilayer periods ranging from ~2 to 200 nm, and total film thicknesses of ~2 μm-were synthesized by dc unbalanced reactive magnetron sputtering. All films were grown on Si (100) substrates (7 × 20 × 0.38 mm 3) in an AJA International Orion 5 magnetron sputtering system equipped with one two-inch Cr and one three-inch Ti target (both from Plansee Composite Materials GmbH, 99.6 at.% purity). Prior to the deposition, the substrates Scripta Materialia 124 (2016) 67-70

Using SQUID magnetometry and both x-ray- and neutron-diffraction techniques, we have studied the structural and magnetic ordering of a series of Fe3O4/NiO superlattices grown by MBE. X-ray diffraction reveals that the superlattices are coherent, single phase crystals with narrow interfaces. Symmetry differences between the Fe3O4 spinel and NiO rocksalt structures lead to interfacial stacking faults, manifested in some diffraction intensities.

A theoretical study of GaAs/GaInNAs solar cells based on multiple-quantum well solar cells (MQWSCs) and superlattice solar cell (SLSC) configuration is presented. The conversion efficiency as a function of the quantum well width and depth is modeled for MQWSC, reaching high values. A study of the SLSC viability is also presented. The influence of the cluster width on the conversion efficiency is researched showing a better performance when width and the cluster number are increased. The SLSC conversion efficiency is compared with the maximum conversion efficiency obtained for the MQWSC showing that it is reached an amazing increment of 4%.

Graphene's electronic structure can be fundamentally altered when a substrate- or adatom-induced Kekulé superlattice couples the valley and isospin degrees of freedom. Here, we show that the band structure of Kekulé-textured graphene can be re-engineered through layer stacking. We predict a family of Kekulé graphene bilayers that exhibit band structures with up to six valleys, and room-temperature Dirac quasiparticles whose masses can be tuned electrostatically. Fermi velocities half as large as in pristine graphene put this system in the strongly coupled regime, where correlated ground states can be expected.

In this paper, we present more general, exact, and concise expressions for calculating the electromagnetic density of modes (EDOM) in one dimension photonic crystal (superlattice) for E and H polarizations. The expression is used for numerical computation of the EDOM in the lower-index (dielectric constant) layer. We discuss the difference between the EDOM in high- and low-index layers as due to the presence of waveguiding modes and evanescent-excited Bloch modes in the higher-index layer. Two methods of computation are presented to compute the EDOM in the lower-index layer. We suggest the possibility of using the EDOM to establish population inversion, which may be useful for higher-frequency lasers (e.g., x rays) and control any radiative processes. We also elaborate on the limitations of the results of Alvarado Rodriguez ct al. as due to the approximation used in the evaluation of partial derivative omega/partial derivativek(y,z) for del (k)omega and comment on the Limitations of the one-dimensional EDOM expression of Bendickson et al.

This thesis examines the quantum dynamics of electrons in periodic semiconductor superlattices in the presence of electric fields, especially uniform static fields. Chapter 1 is an introduction to this vast and active field of research, with an analysis and suggested solutions to the fundamental theoretical difficulties. Chapter 2 is a detailed historical review of relevant theories, and Chapter 3 is a historical review of experiments. Chapter 4 is devoted to the time-independent quantum mechanical study of the electric-field-induced changes in the transmission properties of ballistic electrons, using the transfer matrix method. In Chapter 5, a new time-dependent quantum mechanical model free from the fundamental theoretical difficulties is introduced, with its validity tested at various limiting cases. A simplified method for calculating field-free bands of various potential models is designed. In Chapter 6, the general features of "Shifting Periodicity", a distinctive fe...

We use a standard 8 × 8 envelope-function approximation (EFA) formalism taking into
account the effect of anisotropic and other interface (IF) interactions to investigate the
electronic and optical properties of short-period superlattice laser structures
(InAs/GaSb/InSb)×N grown on a GaSb substrate. We find that the band gaps numerically
calculated at different temperatures give a good fitting with the experimental data confirming
the model used. The calculated modal gain demonstrates that it is possible to achieve lasing
operation at room temperature forN >12 and for a reasonable total optical loss αt =25 cm−1. Therefore, the 8 × 8 EFA formalism with IF design serves as a tool to model the
optoelectronic properties of InAs/GaSb/InSb SPSLs.

—AlN/CrN superlattices with a B1 cubic crystal structure and a bilayer period of 4 nm were deposited by reactive  radiofrequency magnetron sputtering. The coatings were investigated with respect to their thermal stability and changes in microstructure and chemical composition at 900 °C. The AlN layers show high chemical stability but undergo dissolution by pinching off at grain boundaries. A transformation from cubic to hexagonal AlN with subsequent coarsening at grain boundary triple junctions is observed. In contrast to AlN, the CrN layers show poor chemical stability and their compositions are shifted towards Cr2N upon annealing in a protective argon atmosphere due to nitrogen loss. However, even after establishing Cr2N stoichiometry the crystal structure of the layers remains cubic.

ABSTRACT This paper presents the simulation of mid-infrared InAs/GaSb superlattice (SL) detectors by using a finite element FEMLAB code, which is a practical and useful tool to characterize and to define microelectronic devices with appropriate geometry. In the framework of this FEMLAB code, the electro-optic performances of the SL photodetectors for room temperature (RT) operation are theoretical evaluated in terms of zero-bias resistance-area product R0A and of external quantum efficiency η in order to estimate the specific detectivity D*. In the case of realistic SL active zone, calculated results show an optimal active zone thickness of about 1.3μm and a RT specific detectivity Dλ*=2×109cmW-1Hz1/2 at λ=4μm. This expected value demonstrates that InAs/GaSb SL detector is a competitive technology suitable for third generation detectors.

We present a review of our works in quantum well and superlattice solar cells. In this work, the quantum efficiency for AlGaAs/GaAs multi-quantum well solar cell has been calculated and compared with available data from the group at the Imperial College, London. Quantum efficiency calculations is presented and compared with experimental data for several AlGaAs/GaAs multi-quantum well solar cell, obtaining good agreement. The photocurrent is calculated from the quantum
efficiency calculations and included in the J(V) relation to optimize the efficiency. It also shows that for a range of quantum well widths and barrier bandgaps the conversion
efficiencies of the quantum well solar cell are higher than the corresponding homogeneous p-i-n solar cell. Our results give a broad representation of quantum well solar cell operation, and provide a profitable guide for designing and interpreting
the performance characteristics of AlGaAs/GaAs QWSCs. Also, a theoretical model is performed to study the viability of the AlGaAs/GaAs superlattice solar cell. Using the Transfer Matrix Method, the conditions for resonant tunneling are established for particular superlattice geometry with variably spaced quantum wells. The effective density of states and the absorption coefficients are calculated to determinate the J-V characteristic. Radiative, non radiative and interface recombination were evaluated from a modeled superlattice solar cell and their values are compared with a multi-quantum well solar cell of a same aluminum composition. A discussion about the conditions where superlattice solar cell performance overcomes that of a multi-quantum well solar cell is addressed.

In this paper, we have demonstrated the electronic resonant tunneling effect in graphene superlattice heterostructures with an electric potential controlled defect layer. This layer is created by a single irregular electrode inserted in between two different superlattices. We have numerically investigated the tunable giant thermoelectric effect in the above-mentioned structure which is caused by complete electronic tunneling using a transfer matrix approach. The magnitude of the maximum Seebeck coefficient generated in the above structures is several times more than that in an individual superlattice as well as in superlattice heterostructures. This structure can be tuned to give a maximum Seebeck coefficient by varying the applied voltage on the defect layer. By this method, the magnitude of the Seebeck coefficient is found to be 384.9 mV/K, which is the largest reported ever. The tunneling state and the maximum value of the Seebeck coefficient are found to be located in the small overlapped forbidden ga...

The assembling of metal phthalocyanines on the rippled moiré superlattice of graphene/Ir(111) intercalated with one Co layer is driven by the site-dependent polarization field induced by the incommensurate graphene-Co interface. We have performed an X-ray absorption and photoemission study to unveil the role of the metallic centers and of the organic ligands in the molecule-Co interaction process mediated by graphene. Notably, we consider different electronic molecular orbitals, i.e. phthalocyanines with Cu and Mn metallic ions. The spectroscopic response suggests almost unaltered CuPc molecular states upon adsorption, and the rippled graphene carpet decouples completely the electronic interaction between the molecules and the Co layer, while a slight hybridization is present for MnPcs. MnPc molecules, trapped in the valleys of the moiré graphene superlattice, slightly intermix, through the orbitals protruding out of the molecular plane, with the underlying Co, while the organic ligands are almost unaltered. Graphene acts as an interlayer and mediates the interaction between metal phthalocyanines and the metallic substrate, preventing a strong chemical intermixing and enabling the assembly of almost unaltered molecules, preserving their electronic/magnetic state.

A new kind of superlattice system is proposed where wells of two different widths are closely associated through thin barriers.The narrow wells are used as the optically active part of the device where the excitonic absorption can be bleached very rapidly by an external light source. The wider wells trap the excess carriers in a very short time so that the absorption is quickly restored. All optical switches with a time response shorter than 1 ps can be made with such a system.

Title: Stark quantization in superlattices. Authors: Tsu, Raphael; Esaki, L. Affiliation: AA(University of North Carolina at Charlotte, Charlotte, North Carolina 28223) AB(IBM Thomas J. Watson Research Center, Yorktown Heights, New York 10598). ...

Pierre Lefebvre, Philippe Christol, and Henry Mathieu Groupe d'Etudes des Semiconducteurs, Université de Montpellier II: Sciences et Techniques du Languedoc, Case Courrier 074, 34095 Montpellier CEDEX 5, France. ... 25H. Mathieu, P. Lefebvre, and P. Christol, J. Appl. Phys. ...

We describe a general method for producing ultrahigh-density arrays of aligned metal and semiconductor nanowires and nanowire circuits. The technique is based on translating thin film growth thickness control into planar wire arrays. Nanowires were fabricated with diameters and pitches ( ...