In this paper, for the first time, a high performance hybrid silicon evanescent traveling wave electroabsorption modulator based on asymmetric intra-step-barrier coupled double strained quantum wells active layer is introduced which has double steps at III/V mesa structure. Through this active layer, hybrid silicon evanescent traveling wave electroabsorption modulator will be advantages such as very low insertion loss, zero chirp, high extinction ratio, and large Stark shift and better figures of merit as compared with multiquantum well and intra-step quantum well structures. Furthermore, traveling wave electroabsorption modulator with double steps III/V mesa structure results in a wider bandwidth as compared with one-step III/V mesa and mushroom structures. For the modulator with double steps III/V mesa structure with a 200 μm length, the 3 dB bandwidths are obtained as 132 and 52 GHz for 25 and 40 Ω characteristic impedances, respectively.
The single dot in magnetic field and double quantum dot with broken spin-orbital symmetry are discussed in the boson field environment. It is shown that the time dependent potential induces the Kondo effect, provided that the single boson energy compensates the spin or orbital splitting. The photon induced recovery of orbital degeneracy can occur within the same spin channel or with the spin mixing. In the former case the spin polarized photocurrent is expected.
CdS quantum dots were coated on TiO_2 layer by successive ionic layer adsorption and reaction method. An efficient photovoltaic energy conversion and significant quantum-size effect were observed. The magnitude of the short-circuit photocurrent density J_{SC} was found to be approximately 6.01 mA/cm^2 for graphene oxide-incorporated CdS/TiO_2 solar cell, while the J_{SC} of only CdS-sensitized solar cells was lower than 4.40 mA/cm^2. The efficiency of the CdS/TiO_2 solar cell with a graphene oxide layer containing CdS QDs was 60% higher than that of the CdS/TiO_2 solar cell. The cell efficiency was remarkably improved with the graphene oxide-incorporation. The carrier recombination of the QDs sensitized solar cells based on CdS-coated TiO_2 was significantly suppressed due to photogenerated charge carrier transports resulting from the presence of graphene oxide.
Temperature change in quantum cascade laser can be estimated by studying the device resistance change. Using this method we compared quantum cascade laser structure mounted on diamond heat spreader and without heat spreader. We have shown that the use of heat spreader reduces temperature increase even by 40%.
Room temperature, continuous wave operation of InGaN multi-quantum wells laser diodes made by rf plasma assisted molecular beam epitaxy at 411 nm wavelength is demonstrated. The threshold current density and voltage were 4.2 kA/cm^2 and 5.3 V, respectively. High optical power output of 60 mW was achieved. The lifetime of these laser diodes exceeds 5 h with 2 mW of optical output power. The laser diodes are fabricated on low dislocation density bulk GaN substrates, at growth conditions which resembles liquid phase epitaxy. We demonstrate that relatively low growth temperatures (600-700°C) pose no intrinsic limitations for fabrication of nitride optoelectronic components by plasma assisted molecular beam epitaxy.
Spin related phenomena in quantum nanostructures have attracted recently much interest due to fast growing field of spintronics. In particular complex nanostructures are important as they provide a versatile system to manipulate spin and the electronic states. Such systems can be used as spin memory devices or scalable quantum bits. We investigate the spin relaxation for an electron in a complex structure composed of a quantum dot surrounded by a quantum ring. We shown that modifications of the confinement potential result in the substantial increase of the spin relaxation time.
Spin transport in a semiconducting quantum wire connected to two spin-unpolarized electron reservoirs is investigated. The spin-orbit interaction is included via the Rashba Hamiltonian which together with the Zeeman Hamiltonian determines spin-filtering properties of the wire. The spin current as a function of the voltage was found to have an oscillatory or growing character.
The recent progress in growth of nitride based semiconductor structures made by plasma assisted MBE is presented. This technology is ammonia free and nitrogen for growth is activated in RF plasma source from nitrogen molecules. The new growth mechanism - adlayer enhanced lateral diffusion of adatoms on semiconductor surface is studied in plasma assisted MBE. This mechanism enables us to achieve high quality step-flow epitaxy at temperatures 600-750ºC, much lower than expected from classical estimates based on the melting point of GaN. We show that growth at low temperatures in metal rich (gallium or indium) regime, together with use of low dislocation bulk GaN substrates, results in high quality of (In, Al, Ga)N layers and sharp interfaces. We demonstrate record high mobility of two-dimensional electron gas at GaN/AlGaN interface (with mobility exceeding 100 000 cm^2/(V s) at 4.2 K and 2500 cm^2/(V s) at 300 K) and report on first blue-violet InGaN multiquantum well laser diodes, operating in 407-422 nm wavelengths range. In this paper, we discuss also properties of strain compensated InAlN/InGaN multiquantum wells grown by plasma assisted MBE which are very attractive for telecommunication applications at 1.5μm wavelengths like electro-optical modulators or all-optical switches.
We present theoretical studies of three-terminal ballistic junction in linear and non-linear regime. Various conductance and voltage dips and peaks are observed and their origin is explained as influence of the bend resistance and the threshold effect.
A model that explains the unusual characteristics of the AlGaAs/GaAs modulation-doped field-effect transistor (MODFET) with InAs quantum dots incorporated in the GaAs channel is presented. It is shown that the negative charge of electrons confined in quantum dots decreases the threshold gate-drain voltage at which the channel is fully depleted. This provides an impact ionization of quantum dots at a low drain voltage. Because of the quantum dot ionization, the quantum dot MODFET transconductance becomes large and negative. The increased transconductance, due to the additional doping of the GaAs and InAs channels by impurities, exceeds 10^3 mS/mm. It is shown that the insertion of InAs quantum well with quantum dots into the GaAs quantum well increases the electron maximum drift velocity up to 10^8 cm/s, and consequently, quantum dot MODFET current gain cut-off frequency up to few hundred gigahertz.
In this paper the electronic states in type-II superlattices are demonstrated. Band dispersions of InAs/GaSb periodic structure were calculated with the respect of the light and the heavy holes states mixing at InAs/GaSb interfaces. The effect of narrow energy band gap of InAs was taken into account and the wavelengths corresponding to optical transitions in the superlattice were presented.
We have studied theoretically the type-II GaAsSb capped InAs quantum dots for two structures differing in the composition of the capping layer, being either (i) constant or (ii) with Sb accumulation above the apex of the dot. We have found that the hole states are segmented and resemble the states in the quantum dot molecules. The two-hole states form singlet and triplet with the splitting energy of 4 μeV/325 μeV for the case (i)/(ii). We have also tested the possibility to tune the splitting by vertically applied magnetic field. Because the predicted tunability range was limited, we propose an approach for its enhancement.
We analyse the number field-theoretic properties of solutions of the eigenproblem of the Heisenberg Hamiltonian for the magnetic hexagon with the single-node spin 1/2 and isotropic exchange interactions. It follows that eigenenergies and eigenstates are expressible within an extension of the prime field ℚ of rationals of degree 2^3 and 2^4, respectively. In quantum information setting, each real extension of rank 2 represents an arithmetic qubit. We demonstrate in detail some actions of the Galois group on the eigenproblem.
In this paper we demonstrate how the tuning of the VECSEL heterostructure can be precisely determined. Since the VECSEL active region is embodied in a microcavity, the photoluminescence signal collected from the chip surface is modified by the resonance of this cavity. The angle resolved photoluminescence measurements combined with the temperature tuning of the structure allowed us to precisely determine VECSEL emission features. The investigated structure consists of GaAs cavity with six InGaAs quantum wells and is designed for lasing at 980 nm.
We present a review of theoretical methods used to study the electronic structure, optical and transport properties of intraband optoelectronic devices based on self-assembled quantum dots.
Thanks to their large conduction band offset (~1.8 eV for the GaN/AlN system) and subpicosecond intersubband scattering rates, III-nitride heterostructures in the form of quantum wells or quantum dots are excellent candidates for high-speed unipolar devices operating at optical-fiber telecommunication wavelengths, and relying on the quantum confinement of electrons. In this work, we present the plasma-assisted molecular-beam epitaxial growth of quantum well infrared photodetector structures. The growth of Si-doped GaN/AlN multiple quantum well structures is optimized by controlling substrate temperature, metal excess and growth interruptions. Structural characterization confirms a reduction of the interface roughness to the monolayer scale. P-polarized intersubband absorption peaks covering the 1.33-1.91μm wavelength range are measured on samples with quantum well thickness varying from 1 to 2.5 nm. Complete intersubband photodetectors have been grown on conductive AlGaN claddings, the Al mole fraction of the cladding matching the average Al content of the active region. Photovoltage measurements reveal a narrow (~90 meV) detection peak at 1.39μm at room temperature.
The fabrication technology of AlGaAs/GaAs based quantum cascade lasers is reported. The devices operated in pulsed mode at up to 260 K. The peak powers recorded at 77 K were over 1 W for the GaAs/Al_{0.45}Ga_{0.55}As laser without anti-reflection/high-reflection coatings.
We consider striking connections between the theory of homogenous isotropic Heisenberg ring (XXX-model) and algebraic number theory. We explain the nature of these connections especially applications of Galois theory for computation of the spectrum of the Heisenberg operators and Bethe parameters. The solutions of the Heisenberg eigenproblem and Bethe Ansatz generate interesting families of algebraic number fields. Galois theory yields additional symmetries which not only simplify the analysis of the model but may lead to new applications and horizons.
The fabrication of quantum cascade lasers emitting at 9 μm is reported. The devices operated in pulsed mode at up to 260 K. The peak powers recorded at 77 K were over 1 W and the slope efficiency η ≈ 0.5-0.6 W/A per uncoated facet. This has been achieved by the use of GaAs/Al_{0.45}Ga_{0.55}As heterostructure, with the "anticrossed-diagonal" design. Double plasmon planar confinement with Al-free waveguide has been used to minimize absorption losses. The double trench lasers were fabricated using standard processing technology, i.e., wet etching and Si_{3}N_{4} for electrical insulation. The quantum cascade laser structures have been grown by molecular beam epitaxy, with Riber Compact 21 T reactor. The stringent requirements - placed particularly on the epitaxial technology - and the influence of technological conditions on the device structure properties were presented and discussed in depth.
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