Superlattice Band Research Articles

A designing principle for low dark-current strained layer superlattices

A systematic study of native point defect states is carried out to understand the origin of gap states in strained layer superlattices. Using a combination of first principles Hamiltonian, tight-binding Hamiltonian, and Green's function approach, we calculate the native point defect states in bulk InAs, bulk GaSb, and ten InAs-GaSb strained layer superlattices. We observe that the defects in strained layer superlattices located away from the interface create fewer mid gap states if the constituent bulk materials do not have defect states in that energy interval. This can be used as a design principle to shift the superlattice band gap and reduce or eliminate the mid-gap states.

Superlattice-Induced Insulating States and Valley-Protected Orbits in Twisted Bilayer Graphene

Twisted bilayer graphene (TBLG) is one of the simplest van der Waals heterostructures, yet it yields a complex electronic system with intricate interplay between moiré physics and interlayer hybridization effects. We report on electronic transport measurements of high mobility small angle TBLG devices showing clear evidence for insulating states at the superlattice band edges, with thermal activation gaps several times larger than theoretically predicted. Moreover, Shubnikov-de Haas oscillations and tight binding calculations reveal that the band structure consists of two intersecting Fermi contours whose crossing points are effectively unhybridized. We attribute this to exponentially suppressed interlayer hopping amplitudes for momentum transfers larger than the moiré wave vector.

Electronic structure and optical properties of (BeTe)n/(ZnSe)m superlattices

Abstract The structural, electronic and optical properties of (BeTe)n/(ZnSe)m superlattices have been computationally evaluated for different configurations with m = n and m≠n using the full-potential linear muffin-tin method. The exchange and correlation potentials are treated by the local density approximation (LDA). The ground state properties of (BeTe)n/(ZnSe)m binary compounds are determined and compared with the available data. It is found that the superlattice band gaps vary depending on the layers used. The optical constants, including the dielectric function ε(ω), the refractive index n(ω) and the refractivity R(ω), are calculated for radiation energies up to 35 eV.

Improvement of Quantum Efficiency in Transmission-Type Spin-Polarized Photocathode

We successfully developed a new transmission-type GaAs/GaAsP strained superlattice photocathode with an AlGaAs transparent inter-layer and Si3N4 anti-reflection coating. The electrons emitted from this photocathode showed a high spin polarization of 90% with a quantum efficiency as high as 0.4%. In the application for spin-polarized low energy electron microscope, a high emission current of over 1 µA was observed at 3.6 mW pump laser power. Transmission electron microscopy observation revealed that there were a small disorder and some dislocations in the GaAs/GaAsP superlattice layers. The disordered superlattice layers result in a fluctuation of the superlattice band structure and the dislocations trap the photo-electrons during the diffusion to the surface. Both of the defects influenced the performance of spin-polarized photocathode.

Strain-modulated Ge superlattices

We present a numerical study of the electronic and optical properties of a model single-element superlattice made of a periodic sequence of relaxed and strained regions of a germanium crystal, realized by means of an externally applied strain. We adopt the tight-binding model to evaluate the strain-driven modifications of the band structure and the optical properties. Superlattice band gaps, spatial confinement of near-gap valence and conduction states, and analysis of their symmetry character, have been obtained for different superlattice periodicities and strain intensities. Our results indicate that, for suitable choices of spatial periodicity and strain values, type-I and direct-gap superlattices, with strong dipole matrix elements, can be realized. Conceptually, we demonstrate that Ge single-element strained superlattices could be active materials for novel Si-compatible optical devices.

Topological Bloch bands in graphene superlattices

We outline a designer approach to endow widely available plain materials with topological properties by stacking them atop other nontopological materials. The approach is illustrated with a model system comprising graphene stacked atop hexagonal boron nitride. In this case, the Berry curvature of the electron Bloch bands is highly sensitive to the stacking configuration. As a result, electron topology can be controlled by crystal axes alignment, granting a practical route to designer topological materials. Berry curvature manifests itself in transport via the valley Hall effect and long-range chargeless valley currents. The nonlocal electrical response mediated by such currents provides diagnostics for band topology.

Superlattice band structure: New and simple energy quantification condition

Assuming an approximated effective mass and using Bastard׳s boundary conditions, a simple method is used to calculate the subband structure for periodic semiconducting heterostructures. Our method consists to derive and solve the energy quantification condition (EQC), this is a simple real equation, composed of trigonometric and hyperbolic functions, and does not need any programming effort or sophistic machine to solve it. For less than ten wells heterostructures, we have derived and simplified the energy quantification conditions. The subband is build point by point; each point presents an energy level. Our simple energy quantification condition is used to calculate the subband structure of the GaAs/Ga0.5Al0.5As heterostructures, and build its subband point by point for 4 and 20 wells. Our finding shows a good agreement with previously published results.

One-dimensional massless Dirac bands in semiconductor superlattices

Semiconductor superlattices may display dispersions that are degenerate either at the zone center or zone boundary [G. Bastard, Wave Mechanics Applied to Semiconductor Heterostructures, Monographies de Physique (Les \'Editions de Physique, Les Ulis, 1988); C. Sirtori, F. Capasso, D. L. Sivco, and A. Y. Cho, Appl. Phys. Lett. 64, 2982 (1994)]. We show that they are linear upon the wave vector in the vicinity of the crossing point. This establishes a realisation of massless Dirac bands within semiconductor materials. We show that the eigenstates and the corresponding Wannier functions of these superlattices have peculiar symmetry properties. We discuss the stability of the properties of such superlattices versus the electron in-plane motion. As a distinct fingerprint, the inter-sub-band magnetoabsorption spectrum for such superlattices is discussed.

Interplay between structure and electronic states in step arrays explored with curved surfaces

Atomic staircases in noble-metal surfaces are model one-dimensional superlattices, where free-electron-like surface states transform into superlattice bands with sizable quantum size shifts and gaps. At critical step spacings $d=n\ifmmode\times\else\texttimes\fi{}({\ensuremath{\lambda}}_{F}/2)$, such superlattice gaps lie at the Fermi energy, affecting the electronic energy and hence the structural stability of the step lattice, which is held by weak elastic interactions. We use Cu, Ag, and Au curved crystals to smoothly tune the superlattice constant $d$ in angle-resolved photoemission (ARPES) and scanning tunneling microscopy (STM) experiments. With ARPES we accurately quantify terrace-size effects and determine the superlattice potential, which increases from Ag to Cu and to Au. With STM we analyze the $d$-dependent terrace width distribution for Cu and Ag, and observe nonlinear variations in Cu. On the grounds of simple electronic and elastic models, we conclude that terrace width distribution instabilities and electronic energy variations at $d=n\ifmmode\times\else\texttimes\fi{}({\ensuremath{\lambda}}_{F}/2)$ have the same order of magnitude for Cu. In contrast, the weak superlattice potential in Ag, i.e., its smoother band-structure modulation, is not sufficient to alter the step lattice.

Formation of narrow spin-wave transmission bands in lateral magnetic superlattices

It is shown theoretically that an alternating magnetic field applied locally to a stripe periodic heterostructure with different saturation magnetizations, ${M}_{1}$ and ${M}_{2}$, excites dipolar spin waves, which are able to propagate along the periodicity direction within a narrow frequency band $(\ensuremath{\Delta}\ensuremath{\omega}/\ensuremath{\omega}<5\mathrm{%})$ in the absence of Bragg scattering. A mechanism is found for trapping the out-of-band modes inside a potential well whose formation is mediated by spin pinning at the ${M}_{1}/{M}_{2}$ interfaces.

Intersubband optoelectronics in the InGaAs/GaAsSb material system

In this article the authors report on a novel material system for optoelectronic intersubband devices. Superlattices of In0.53Ga0.47As/GaAs0.51Sb0.49 were grown by molecular beam epitaxy. Layer thickness and quality was investigated by high-resolution x-ray diffraction measurements and high-resolution transmission electron microscopy images. Intersubband absorption measurements on In0.53Ga0.47As/GaAs0.51Sb0.49 superlattices, revealed at room temperature transition energies from 213 to 107 meV for In0.53Ga0.47As well widths of 4.5–11.5 nm at room temperature. These results were used to fit parameters for self-consistent superlattice band structure calculations. Finally, quantum cascade lasers with an emission wavelength of 11.3 μm and quantum well infrared photodetectors with a peak response near 5.5 μm were realized in this material system.

Interface bands in carbon nanotube superlattices

AbstractWe study the electronic band structure of several carbon nanotube superlattices built of two kinds of intermolecular junctions: (12, 0)/(6, 6) and (8, 0)/(14, 0). In particular, we focus on the energy bands originating from interface states. We find that in case of the metallic (12, 0)/(6, 6) superlattices, the interface bands change periodically their character from bonding‐ to antibonding‐like vs. increasing length of the (6, 6) tube.We show that these changes are related to the decay of the charge density Friedel oscillations in the metallic (6, 6) tube. However, when we explore other chiralities without rotational symmetry, no changes in bondingantibonding character are observed for semiconductor superlattices, as exemplified in the case of (8, 0)/(14, 0) superlattices. Our results indicate that unless metallic tubes are employed in the junctions, the bondingantibonding crossings are not present (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

GaAs-based III–Ny–V1−y active regions based on short-period super-lattice structures

We present in this paper, based on conventional super-lattice band structure analysis, a theoretical model within the modified envelope function approximation and a propagation matrix algorithm for design and growth of optoelectronic devices based on III–N–V SPSL structures. The basic principle is to replace the GaInNAs random alloy used as the quantum well material with a SPSL. The design proposals given are based on growth of the individual binary and ternary configurations of 1.3 µm M(InAs)mN(GaNyAs)n SPSL structures, which should provide better compositional control as well as improved growth compared to the random alloy III–N–V-based GaxIn1−xNyAs1−y QW structures. We also show that there is very good agreement between the proposed numerical approach and experimental results.

Electronic superlattices and waveguides based on graphene: structures, properties and applications

AbstractThe new class of quasi‐2D superlattices based on graphene with periodically adsorbed hydrogen pairs was proposed. The ab initio DFT method was used for optimization of the atomic geometry and electronic structure of proposed structures. It was found that the superlattices band gap decreases nonmonotonically with distance between hydrogen pairs. Based on these results we hope that the graphene superlattices can be promising candidates for various nanotechnological applications especially as elements in nanoelectronic devices. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electronic superlattices in corrugated graphene

We theoretically investigate electron transport through corrugated graphene ribbons and show how the ribbon curvature leads to an electronic superlattice with a period set by the corrugation wavelength. Transport through the ribbon depends sensitively on the superlattice band structure which, in turn, strongly depends on the geometry of the deformed sheet. In particular, we find that for ribbon widths where the transverse level separation is comparable to the band edge energy, a strong current switching occurs as a function of an applied back gate voltage. Thus, artificially corrugated graphene sheets or ribbons can be used for the study of Dirac fermions in periodic potentials. Furthermore, this provides an additional design degree of freedom for graphene-based electronics.

Characterization of carriers in GaSb∕InAs superlattice grown on conductive GaSb substrate

We report on mobility spectrum analysis of electrical transport in a GaSb∕InAs superlattice (SL) grown on GaSb substrate. Despite domineering contribution to conduction from the substrate, it was possible to discern and characterize carriers from SL. A single electron specie with an ambient temperature mobility of ∼104cm2∕Vs was found to emanate from SL. We show that this carrier has an activation energy of 0.27eV and is associated with the SL band gap.

Quantitative mobility spectrum analysis of carriers in GaSb/InAs/GaSb superlattice

The authors have investigated electrical transport in a type II GaSb/InAs superlattice grown on GaSb using “quantitative mobility spectrum analysis.” Their results indicate that the superlattice contributes a lone electron specie with an ambient temperature mobility of ∼104 cm2/V s. Variable temperature studies in the range 50–300 K show that the carrier is associated with an activation energy of 0.27 eV, which is very close to the superlattice band gap.

Carbon nanotube superlattices in a magnetic field

AbstractThe influence of magnetic field on the band structure of carbon nanotube superlattices is investigated. In particular, we study superlattices built of finite sections of (6,6) and (12,0) tubes connected by pentagon/heptagon topological defects. Magnetic field is parallel to the axis of the superlattice. We demonstrate that the superlattice band structure does not show periodicity with the flux quantum, which is typical for pure carbon nanotubes. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008

Effect of interaction between periodic δ-doping in both well and barrier layers on modulation of superlattice band structure

The modulation of superlattice band structure via periodic delta-doping in both well and barrier layers have been theoretically investigated, and the importance of interaction between the delta-function potentials in the well layers and those in the barrier layers on SL band structure have been revealed. It is pointed out that the energy dispersion relation Eq. (3) given in [G. Ihm, S.K. Noh, J.I. Lee, J.-S. Hwang, T.W. Kim, Phys. Rev. B 44 (1991) 6266] is an incomplete one, as the interaction between periodic delta-doping in both well and barrier layers had been overlooked. Finally, we have shown numerically that the electron states of a GaAs/Ga0.7Al0.3As superlattice can be altered more efficiently by intelligent tuning the two delta-doping's positions and heights. (c) 2007 Elsevier B.V. All rights reserved.

Spontaneous emission of Bloch oscillation radiation from a single energy band

A theory for the spontaneous emission of radiation for a Bloch electron traversing a single energy band under the influence of a constant external electric field is presented. The constant external electric field is described in the vector potential gauge. The quantum radiation field is described by the free space quantized electromagnetic field in the Coulomb gauge. The instantaneous eigenstates of the Bloch Hamiltonian are introduced as basis states to analyze the Bloch dynamics to all orders in the constant external electric field. The radiation field described by the quantized electromagnetic field provides the vacuum fluctuations that are responsible for the spontaneous emission of radiation of the single electron from the upper regions of the energy band. It is shown that the spontaneous emission occurs with frequencies equal to integral multiples of the Bloch frequency without any ad hoc assumptions made concerning the existence of Wannier-Stark ladder levels. An explicit expression for the spontaneous emission transition probability is derived to first order in the quantized radiation field; results show the explicit dependence upon the electron energy band structure, photon polarization, and the directionality of the radiation output. As an illustration, spontaneous emission probabilities are developed and illustrated for nearest-neighbor tight-binding and more realistic superlattice band structure models. For the GaAs-based superlattices, the power radiated into free space from spontaneous emission due to Bloch oscillations in the terahertz frequency range is estimated to be about one-tenth of a microwatt.