A novel secular equation for the coupled-band envelope-function approximation for superlattices and application to InAs/InxGa1 − xSb superlattices

This paper derives and demonstrates a new secular equation for the coupled-band envelope-function approximation (EFA) formalism for superlattices in order to overcome the difficulty of handling rapidly growing or decaying wavefunction components, in particular, the ‘wing solutions’. In a second development, the generally nonHermitian secular equation is made Hermitian, making it easier to locate multiply degenerate roots. In the process, the simple Kronig–Penney model is recast into a form closely related to that for the underlying quantum well (QW) problem. The present method is applicable to Burt's EFA formalism. An InAs/InGaSb type-II superlattice is used to demonstrate the method.     

InAs/GaSb superlattices grown by LP-MOCVD for ∼10 μm wavelength infrared range

A type-II InAs/GaSb superlattice (SL) was grown on Te-doped (1 0 0) GaSb substrate by low pressure metal organic chemical vapor deposition (LP-MOCVD). The samples were obtained at different growth temperatures and with different interface layers. By introducing an InAsSb interface layer between InAs and GaSb, a good surface morphology of the superlattice was achieved when the sample growth temperature was around 500–520 °C. The photoluminescence (PL) peak wavelength of the sample was 10.7 μm at 77 K, with FWHM of ∼30 meV.     

LP-MOCVD两步生长法制备的InAs0.9Sb0.1/GaAs

利用LP-MOCVD技术,采用两步生长法,在(100)GaAs单晶衬底上生长高质量的InAs0.9Sb0.1。用扫描电镜、X射线单晶衍射、Hall测量以及Raman光谱等方法对材料进行了表征。分析了缓冲层和外延层生长温度对外延层表面形貌的影响。获得了表面光亮,室温载流子浓度为1.9×1017cm-3和迁移率为6214cm2/V.s的InAs0.9Sb0.1。在室温Raman光谱中观察到InAs0.9Sb0.1中InAs(LO)的233cm-1和InSb(LO)的187cm-1两种光学声子行为。     

Magnetoabsorption of Dirac Fermions in InAs/GaSb/InAs “Three-Layer” Gapless Quantum Wells

Subablative exposure of tightly focused visible-range femtosecond laser pulses on a thin translucent nanocrystalline copper(I) oxide on a silica glass substrate results not only in its annealing (resolidification), but apparently also in reduction of copper ions to the metallic state via single-photon absorption and the following thermal decomposition (disproportioning). Partial or complete ablation of the film within the laser focal spot and also its subablative optically contrast modification through formation of colloidal nanoparticles or annealing (resolidification) make it possible to consider this material in the thin-film form as a novel optical platform for direct laser writing of vis-IR metasurfaces and thin-film sensing plasmonic and all-dielectric nanostructures.     

Comparison of the calculated electronic structures of [111] and [001] GaAs/AℓxGa1-xAs semiconductor superlattices

A theoretical comparison of the electronic structures of long-period (about 300 Å) N_GaAs×N_AℓGaAs GaAs/Aℓ_0.3Ga_0.7As superlattices grown along the [111] direction and superlattices grown along the [001] direction is presented. Almost all qualitative features of the theoretical results are in good agreement with experiments by Hayakawa et al. The observed optical transition enhancement in the [111]-oriented quantum-well structures is caused by the topological difference in the superlattices, and is only partly due to the fact that the heavy holes in the [111] superlattices have larger transverse effective masses, and therefore have larger two-dimensional valence band densities of states.     

π-phase magnetism in ferromagnet-superconductor superlattices

New 0π and ππ Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) states with antiferromagnetic orientation of magnetizations in the neighboring layers of a ferromagnetic metal (FM) are predicted for FM/superconductor (FM/S) superlattices. Under certain conditions, the critical temperature T _c of these states is higher than for the known 00 and π0 LOFF states with ferromagnetic ordering of the FM layers. It is shown that the nonmonotonic behavior of T _c in the FM/S superlattices with S-layer thickness d _s less than the threshold value d _s ^π is due to the phase transition cascade 0π-ππ-0π At d _s >d _s ^π , the T _c oscillations are caused by the 00-π0-00 transitions. New logic elements based on the FM/S structures and combining the advantages of the superconducting and magnetic data-record channels in a single sample are proposed.     

Radiative corrections in K→π ℓ + ℓ − decays

We calculate radiative corrections to the flavor-changing neutral current process K→π ? ⁺ ? ^?, both for charged and neutral kaon decays. While the soft-photon approximation is shown to work well for the muon channels, we discuss the necessity of further phase space cuts with electrons in the final state. It is also shown how to transfer our results to other decays such as η→γ ? ⁺ ? ^? or ω→π ⁰ ? ⁺ ? ^?.     

Time-resolved spin dynamics in strained Zn1−xMnxSe/ZnSe spin superlattices

We use time-resolved spectroscopy to directly compare the spin dynamics in strained ZnMnSe epilayers and strained ZnMnSe/ZnSe spin superlattices. The spin-relaxation in these materials is observed to depend strongly on the concentration of the magnetic impurities. Surprisingly, however, no difference is observed between the spin-relaxation of excitons in the quasi-three-dimensional epilayers or the two-dimensional spin superlattices. Indeed, the spin-relaxation of excitons is also seen to be independent of the applied magnetic field or magnetization of the Mn-impurities.     

Geometrically induced π-band splitting in graphene superlattices

According to band folding analyses, the graphene superlattices can be differed by whether the Dirac points are folded to Γ point or not. In previous studies, the inversion symmetry preserved defects open bandgap in the former superlattices while they cannot in the latter ones. In this paper, by using density functional theory with generalized gradient approximation, we have carefully studied the electronic properties of the latter graphene superlattices, in which the defects would induce π-band splitting to get the π_a1–π_a2 and π_z1–π_z2 band sets. Based on our detailed studies, such splitting could be attributed to the geometrically induced bond-symmetry breaking. In addition, these band sets could be shifted toward each other by the methodology of strain engineering. A bandgap would be opened once the band sets start to overlap. Then,its gap width could be continuously enlarged by enhancing strain until reaching the maximum value determined by the defect density. These studies contribute to the bandstructure engineering of graphene-based nanomaterials, which would be interesting to call for further investigations on both theory and experiment.     

A 10Gb/s Directly-Modulated 1.3μm InAs/GaAs Quantum-Dot Laser

We demonstrate 10 Gb/s directly-modulated 1.3 μm InAs quantum-dot （QD） lasers grown on GaAs substrates by molecular beam epitaxy. The active region of the QD lasers consists of five-stacked InAs QD layers. Ridge-waveguide lasers with a ridge width of 4 μm and a cavity length of 600 μm are fabricated with standard lithography and wet etching techniques. It is found that the lasers emit at 1293 nm with a very low threshold current of 5 mA at room temperature. Furthermore, clear eye-opening patterns under 10 Gb/s modulation rate at temperatures of up to 50oC are achieved by the QD lasers. The results presented here have important implications for realizing low-cost, low-power-consumption, and high-speed light sources for next-generation communication systems.     

Evidence for interlayer donor–acceptor recombination in type II GaAs/AlAs superlattices

Steady-state and time-resolved photoluminescence of (GaAs)₇(AlAs)₉type II superlattices grown simultaneously by molecular beam epitaxy on (311)A and (100) GaAs substrates, intentionally undoped or uniformly doped with silicon, has been studied. It is shown that at temperatures T >  30 K, the dominant line in the photoluminescence spectra of superlattices is caused by donor–acceptor recombination between the donors located in the AlAs layers and the acceptors in the GaAs layers. The sum of the binding energies of the donors and acceptors in the pairs has been determined. A spectrally-dependent linear polarization of the donor–acceptor line along the direction of the interface corrugation of the superlattice has been discovered in the spectra of (311)A-oriented superlattices.     

Broadening processes in GaAsδ-doped quantum wire superlattices

We use both Quantum Hall and Shubnikov de Haas experiments at high magnetic field and low temperature to analyse broadening processes of Landau levels in a δ -doped 2D quantum well superlattice and a 1D quantum wire superlattice generated from the first one by controlled dislocation slips. We deduce first the origin of the broadening from the damping factor in the Shubnikov de Haas curves in various configurations of the magnetic field and the measured current for both kinds of superlattice. Then, we write a general formula for the resistivity in the Quantum Hall effect introducing a dephasing factor we link to the process of localization.     

Dynamic dc voltage band in vertical transport of superlattices

By the numerical method, we show a transition process from static to dynamic electric-field domain formation in semiconductor superlattices. During this transition, there can be noticed a sawtooth-like zone in which static and dynamic electric-field domain zones appear alternatively with increasing voltage. Therefore, a dynamic dc voltage band emerges from each sawtooth-like branch of the current－voltage characteristic. These results are qualitatively in agreement with experiment.     

InAs和InAs/GaSb异质结的MOCVD生长和表征

以三甲基铟(TMIn)、砷烷(AsH3)、三甲基镓(TMGa)和三甲基锑(TMSb)为源,用水平常压MOCVD技术,在较低的Ⅴ/Ⅲ比的条件下(1.5～4)于GaAs和GaSb衬底上成功地生长了InAs合金和InAs/GaSb异质结。实验表明,生长温度在500℃～620℃范围内,InAs外延生长是扩散控制的。在Ⅴ/Ⅲ比为2.5时,生长效率(相对Ⅲ族源)为3×103μm/mol.不掺杂InAs外延层为n型的,室温迁移率为2000cm2/V.s.InAs/GaSb异质结的12KPL谱为一个在375meV处较宽的与杂质相关的跃迁峰,和一个在417meV附近的几乎被杂质峰湮没的带边峰.     

A coupled Raman-Brillouin study of direct and folded acoustic modes in long-period GaAsAlAs superlattices

The frequency and intensity of direct acoustic modes (Brillouin lines) and folded acoustic modes are investigated in superlattices of period ≅ 500 Å as a function of the scattering wave vector. Both LA and TA polarizations are observed. The existence of folded acoustic modes at frequency lower than the Brillouin line is pointed out for scattering wave vectors larger than the first Brillouin-zone edge. We also emphasize the “anomalous” intensity behaviour of the different modes which is explained by an improved theory which takes into account the modulation of the acoustical and optical layer properties in the superlattice.     

Fano-like electron–phonon interference in δ-doping GaAs superlattices

The interaction between electron excitations and LO phonons is studied by Raman scattering inδ-doping GaAs superlattices. The Raman spectra measured close to the E₀ +  Δ₀resonance of GaAs present Fano-like coupling of the LO phonons with the quasicontinuum single-particle electron excitations. Due to the self-consistent origin of the electron-energy spectrum in δ-doping superlattices the resonance of the Fano interference was found to be strongly dependent on the electron density as well as the excitation energy.