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We report on a new design of terahertz quantum cascade laser based on a single, potential-inserted quantum well active region. The quantum well properties are engineered through single monolayer InAs inserts. The modeling is based on atomistic, spds* tight-binding calculations, and performances are compared to that of the classical three-well design. We obtain a 100% increase of the oscillator strength per unit length, while maintaining a high, nearly temperature-independent contrast between phonon-induced relaxation times of the upper and lower lasing states. The improved performances are expected to allow THz lasing at room temperature. [ABSTRACT FROM AUTHOR]

Crystal phase heterostructures, consisting of homogeneous composition zinc-blende inclusions in wurtzite InP and GaAs nanowires are investigated theoretically in the frame of the extended-basis tight-binding approach. Increased band gap for the wurtzite phase and staggered type II band alignment are predicted for both materials. Comparison of theoretical results with microphotoluminescence measurements on single InP nanowires yields fair semiquantitative agreement. [ABSTRACT FROM AUTHOR]

Abstract: A quantitative comparison between different model calculations of valence band states in GaAs/AlGaAs heterostructures is presented. We demonstrate that a 14-band Hamiltonian using a completely new parameterization based on fits of the tight-binding band structure leads to energy dispersion relations in excellent agreement with experiment, whereas previous parameterizations result in significant deviations. The relevance of the present results to the calculation of spin-related phenomena is discussed. [Copyright &y& Elsevier]

We present accurate tight-binding (TB) parametrizations of the quasi-particle band structures of AlN, GaN, and InN, both for the zincblende and wurtzite phases. For this purpose, an empirical sp³d[sup 5]s[sup *] nearest-neighbor TB model including the spin-orbit interaction is used, ensuring the transferabiliy of the Slater-Koster parameters to changes in the structural environment. The present approach results in excellent agreement with experimental band positions and ab initio calculations for the lowest conduction band and the valence-band splittings. As a first application, we investigate the electronic band structure of an ordered A[sub 0.5]Ga[sub 0.5]N alloy in the wurtzite phase. [ABSTRACT FROM AUTHOR]

III-V/Si heterostructures are currently investigated for silicon photonics and solar energy conversion. In particular, dilute nitride alloy GaAsPN grown on a GaP/Si platform exhibits lattice match with Si and an optimal band gap configuration for tandem solar cell devices. However, monolithic "coherent" growth of the GaP thin layer on Si suffers from the nucleation of extended structural defects, which can hamper device operation as well as the GaP/Si interface level and through their propagation inside the overall heterostructure. However, the effect of such structural defects on optical and transport properties is actually not well understood in details. In this letter, we investigate the anti phase domains defect (also called inversion domains) by means of ab initio calculations giving insights into the alteration of optical and transport properties of GaP due to the defective GaP/Si interface. [ABSTRACT FROM AUTHOR]

We compare the potentiality of bulk InGaPN and GaAsPN materials quasi-lattice-matched to silicon (Si), for multi-junction solar cells application. Bandgaps of both bulk alloys are first studied by a tight-binding model modified for nitrogen incorporation in diluted regimes. The critical thicknesses of those alloys are then calculated for various compositions. For the same lattice-mismatch and nitrogen amount, the bandgap of bulk GaAsPN is found to be closer to the targeted gap value of 1.7 eV for high efficiency tandem solar cell. GaPN and GaAsPN epilayers are then grown by molecular beam epitaxy on GaP substrate and studied by photoluminescence and X-ray diffraction. A GaAsPN bulk alloy emitting light at 1.77 eV at room temperature is obtained, demonstrating promising properties for further use in III-V/Si photovoltaic multijunction solar cells. [ABSTRACT FROM AUTHOR]

We propose an engineering of the optical properties of GaAs/AlGaAs quantum wells using AlAs and InAs monolayer insertions. A quantitative study of the effects of the monolayer position and the well thickness on the interband and intersubband transitions, based on the extended-basis sp 3 d 5 s ⁎ tight-binding model, is presented. The effect of insertion on the interband transitions is compared with existing experimental data. As for intersubband transitions, we show that in a GaAs/AlGaAs quantum well including two AlAs and one InAs insertions, a three level { e 1 , e 2 , e 3 } system where the transition energy e 3 − e 2 is lower and the transition energy e 2 − e 1 larger than the longitudinal optical phonon energy (36 meV) can be engineered together with a e 3 − e 2 transition energy widely tunable through the TeraHertz range. [ABSTRACT FROM AUTHOR]

We report first-principle density functional calculations of the spontaneous polarization, piezoelectric stress constants, and elastic constants for the III-V wurtzite structure semiconductors InAs and InP. Using the density functional theory implemented in the VASP code, we obtain polarization values-0.011 and-0.013 C/m, and piezoelectric constants e ( e) equal to 0.091 (-0.026) and 0.012 (-0.081) C/m for structurally relaxed InP and InAs respectively. These values are consistently smaller than those of nitrides. Therefore, we predict a smaller built-in electric field in such structures. [ABSTRACT FROM AUTHOR]

A procedure to obtain single-electron wave functions within the tight-binding formalism is proposed. It is based on linear combinations of Slater-type orbitals whose screening coefficients are extracted from the optical matrix elements of the tight-binding Hamiltonian. Bloch functions obtained for zinc-blende semiconductors in the extended-basis spds* tight-binding model demonstrate very good agreement with first-principles wave functions. We apply this method to the calculation of the electron-hole exchange interaction, and obtain the dispersion of excitonic fine structure in bulk GaAs. Beyond semiconductor nanostructures, this work is a fundamental step toward modeling many-body effects from post-processing single-particle wave functions within the tight-binding theory. [ABSTRACT FROM AUTHOR]

CdSe nanoplatelets show perfectly quantized thicknesses of a few monolayers. They present a situation of extreme, yet well defined quantum confinement. Due to a large dielectric contrast between the semiconductor and its ligand environment, interaction between carriers and their dielectric images strongly renormalize bare single particle states. We discuss the electronic properties of this original system in an advanced tight-binding model, and show that Coulomb interactions, including self-energy corrections and enhanced electron-hole interaction, lead to exciton binding energies up to several hundred meV. [ABSTRACT FROM AUTHOR]

The nature of the ground optical transition in an (In,Ga)As/GaP quantum dot is thoroughly investigated through a million atoms supercell tight-binding simulation. Precise quantum dot morphology is deduced from previously reported scanning-tunneling-microscopy images. The strain field is calculated with the valence force field method and has a strong influence on the confinement potentials, principally, for the conduction band states. Indeed, the wavefunction of the ground electron state is spatially confined in the GaP matrix, close to the dot apex, in a large tensile strain region, having mainly Xz character. Photoluminescence experiments under hydrostatic pressure strongly support the theoretical conclusions. [ABSTRACT FROM AUTHOR]

We report on semilocal and hybrid density functional theory study of strained wurtzite crystals of InAs and InP. The crystal-field splitting has a large and nonlinear dependence on strain for both crystals. Moreover, the study of the electronic deformation potentials reveals that the well-known quasi-cubic approximation fails to reproduce the electronic features of the non-ideal c/a ratio. This theoretical study is of crucial importance for the simulation of self-assembled InAs/InP nanowires. [ABSTRACT FROM AUTHOR]

Spin splittings in quantum wells have attracted considerable attention over the past decade due to potential application of semiconductor spin properties to "spintronic devices." Recent experimental results stimulate theoretical investigations of new physical situations like unconventional growth directions. Here we focus on electron spin properties in (110)-oriented quantum wells that are of particular interest because qualitative symmetry analysis shows that spin relaxation by the D'yakonov-Perel' mechanism should be strongly suppressed in this geometry. We combine symmetry analysis, envelope function theory, and tight-binding calculation and obtain quantitative description of the in-plane wave vector, well width, and applied electric field dependence of the spin structure of electron subbands in (110) quantum wells. [ABSTRACT FROM AUTHOR]

We study in detail self-assembled (In,Ga)As quantum dots grown on GaP substrate from the structural, theoretical, and optical points of view. Single quantum dot morphology is first determined at the atomic level using plane-view scanning tunneling microscopy. Unusual C2 symmetry properties with high-index {136} facets are demonstrated for small quantum dots, whereas the apparent shape of the quantum dots approximately exhibits C2vsymmetry, with the appearance of low-index {111} facets when the quantum dot ripens. This is interpreted as a consequence of the competition between strain and surface energy during quantum dot formation. Electronic properties are then simulated using both k?p and tight-binding models. The indium content and geometry of the quantum dots are found to have a strong influence on the transition type (direct-indirect). Finally, temperature-dependent optical properties of quantum dots are analyzed between 10 and 375 K. Photoluminescence and time-resolved photoluminescence studies show a clear proximity of two different types of optical transitions. Supported by the theoretical calculations, these transitions are interpreted as a competition between conduction band states in the χ and χ valleys. [ABSTRACT FROM AUTHOR]

Based on density functional theory, the electronic and optical properties of hybrid organic/perovskite crystals are thoroughly investigated. We consider the monocrystalline 4F-PEPI as material model and demonstrate that the optical process is governed by three active Bloch states at the Γ point of the reduced Brillouin zone with a reverse ordering compared to tetrahedrally bonded semiconductors. Giant spin-orbit coupling effects and optical activities are subsequently inferred from symmetry analysis. [ABSTRACT FROM AUTHOR]

Band structure calculations of strained Ga(NAsP) quantum wells are performed within the framework of the extended-basis sp3d5s* tight-binding model. The nitrogen contribution is taken into account by introducing an additional sN orbital into the tight-binding basis. Biaxial strain effects on the band alignment of bulk Ga(NAsP) is studied for the ultra-diluted regime. We demonstrate a good agreement with experimental data both for transition energies and optical gain in Ga(NAsP) quantum wells. The effect of N incorporation in the laser active areas is simulated. [ABSTRACT FROM AUTHOR]

Abstract: Density Functional Theory is used to study the vibrational properties of 2H-PbI2 semiconductor. The Born charge tensors are determined. Calculated phonon frequencies at the Brillouin zone center are compared to Raman scattering and IR absorption measurements. The computed Raman spectra show a good agreement with available experimental data. The simulated phonon dispersion curves are compared with triple-axis neutron scattering measurements. [Copyright &y& Elsevier]

Using an advanced tight-binding approach, we estimate the anisotropy of the tunnel transmission associated with the rotation of the 5/2 spin of a single Mn atom forming an acceptor state in GaAs and located near an AlGaAs tunnel barrier. Significant anisotropies in both in-plane and out-of-plane geometries are found, resulting from the combination of the large spin-orbit coupling associated with the p-d exchange interaction, cubic anisotropy of heavy-hole dispersion and the low C2v symmetry of the chemical bonds. [ABSTRACT FROM AUTHOR]

A correction to the article "Evaluation of InGaPN and GaAsPN Materials Lattice-Matched to Si for Multi-Junction Solar Cells," published in the previous issue, is presented.

We report on the achievement of high density (In,Ga)As self-assembled quantum dots on GaP substrate with a good homogeneity. Good structural and electronic properties have been achieved, as revealed by room temperature photoluminescence measurements and by comparison to both InAs/GaAs and InAs/InP materials reference systems. This is supported by atomistic calculations where the indium incorporation in InGaAs/GaP quantum structures is found to enhance both the type-I bandlineup and direct bandgap properties. The photoluminescence temperature dependence of the bandgap evidences the quantum confinement effects. Our results provide a valid framework to implement silicon optical devices based on InGaAs/GaP nanostructures. [ABSTRACT FROM AUTHOR]

We illustrate how the linear combination of zone center bulk bands combined with the full-zone k·p method can be used to accurately compute the electronic states in semiconductor nanostructures. To this end we consider a recently developed 30-band model which carefully reproduces atomistic calculations and experimental results of bulk semiconductors. The present approach is particularly suited both for short-period superlattices and large nanostructures where a three-dimensional electronic structure is required. This is illustrated by investigating ultrathin GaAs/AlAs superlattices. [ABSTRACT FROM AUTHOR]

This letter deals with the electroluminescence emission at room temperature of two light-emitting diodes on GaP substrate, based on ternary GaAsP/GaP and quaternary GaAsPN/GaPN multiple quantum wells. In agreement with tight-binding calculations, an indirect band gap is demonstrated from the temperature-dependent photoluminescence for the first structure. High efficiency photoluminescence is then observed for the second structure. Strong electroluminescence and photocurrent are measured from the diode structures at room temperature at wavelengths of 660 nm (GaAsP/GaP) and 730 nm (GaAsPN/GaPN). The role of the incorporation of nitrogen on the optical band gap and on the nature of interband transitions is discussed. [ABSTRACT FROM AUTHOR]

We study the complex electronic band structure of low In content InGaAs/GaP quantum dots. A supercell extended-basis tight-binding model is used to simulate the electronic and the optical properties of a pure GaAs/GaP quantum dot modeled at the atomic level. Transitions between hole states confined into the dots and several XZ-like electronic states confined by the strain field in the GaP barrier are found to play the main role on the optical properties. Especially, the calculated radiative lifetime for such indirect transitions is in good agreement with the photoluminescence decay time measured in time-resolved photoluminescence in the µs range. Photoluminescence experiments under hydrostatic pressure are also presented. The redshift of the photoluminescence spectrum with pressure is also in good agreement with the nature of the electronic confined states simulated with the tight-binding model. [ABSTRACT FROM AUTHOR]

Band structure calculations of complete InAs monolayer in AlGaAs/GaAs quantum wells are performed within the framework of the extended-basis sp3d5s* tight-binding model. We show that the optical properties can be tuned from the quantum well energy below the GaAs band-gap depending on the well thickness and the position of the probe. The results are supported by differential reflectivity measurements and represent a concept for optoelectronic devices with an operation wavelength widely tuneable around 850 nm employing GaAs process technology. [ABSTRACT FROM AUTHOR]