Effect of wetting layers on the strain and electronic structure of InAs self-assembled quantum dots

The effect of wetting layers on the strain and electronic structure of InAs self-assembled quantum dots grown on GaAs is investigated with an atomistic valence-force-field model and an empirical tight-binding model. By comparing a dot with and without a wetting layer, we find that the inclusion of the wetting layer weakens the strain inside the dot by only 1% relative change, while it reduces the energy gap between a confined electron and hole level by as much as 10%. The small change in the strain distribution indicates that strain relaxes only little through the thin wetting layer. The large reduction of the energy gap is attributed to the increase of the confining-potential width rather than the change of the potential height. First-order perturbation calculations or, alternatively, the addition of an InAs disk below the quantum dot confirm this conclusion. The effect of the wetting layer on the wave function is qualitatively different for the weakly confined electron state and the strongly confined hole state. The electron wave function shifts from the buffer to the wetting layer, while the hole shifts from the dot to the wetting layer.Comment: 14 pages, 3 figures, and 3 table

Carrier and Phonon Spectrum Modification in Quantum Dot Superlattices Designed for Thermoelectric Applications

Quantum dot superlattices have recently been proposed for thermoelectric applications. The predicted improvement of the thermoelectric figure of merit in such structures should come from the decreased lattice thermal conductivity due to additional boundary scattering and acoustic phonon spectrum modification, as well as change in the carrier transport and density of states. Here we outline a theoretical model to calculate carrier and phonon dispersion in such structures and present results for Ge/Si quantum dot superlattices. We argue that one can tune the mini-band carrier transport and phonon dispersion in such a way that electron-phonon coupling is suppressed. The latter may open up a novel way for the enhancement of the thermoelectric figure of merit

The effect of the strain relaxation in InAs/GaAs stacked quantum dots and multiple quantum wells on the Raman spectrum

Atomist,ic-level simulations of the Raman shift in InAs/GaAs multiple quantum well (MQW) and stxketl cjuantuni dot (SQD) structures as a function of interlayer separation axe reported. The simulations utilize an aupcmted Krating nxodcl which includes anharmonicity corrections. It is demonstrated that the iiiteract,iori 1,etwet:il ;~iigled ist or led bonds is responsible for the penetration of the strain into the GaAs barrier. This result is in contrast to a complete strain relaxatioll of t,he CaAs barrier which would be predicted by continuurn n~odels. Tension along the growth direction result in the red shift of the GaAs LO Rarnan peak as the bnrrirr thickwss decreases in both, MQW and SQD, structures

Effect of anharmonicity of the strain energy on band offsets in semiconductor nanostructures

Anharmonicity of the interatomic potential is taken into account for the quantitative simulation of the conduction and valence band offsets for strained semiconductor heterostructures. The anharmonicity leads to a weaker compressive hydrostatic strain than that obtained with the commonly used quasiharmonic approximation of the Keating model. Compared to experiment, inclusion of the anharmonicity in the simulation of strained InAs/GaAs anostructures results in an improvement of the electron band offset computed on an atomistic level by up to 100 meV

Effect of wetting layers on the strain and electronic structure of InAs self-assembled quantum dots

The effect of wetting layers on the strain and electronic structure of InAs self-assembled quantum dots grown on GaAs is investigated with an atomistic valence-force-field model and an empirical tight-binding model. By comparing a dot with and without a wetting layer, we find that the inclusion of the wetting layer weakens the strain inside the dot by only 1% relative change, while it reduces the energy gap between a confined electron and hole level by as much as 10%. The small change in the strain distribution indicates that strain relaxes only little through the thin wetting layer. The large reduction of the energy gap is attributed to the increase of the confining-potential width rather than the change of the potential height. First-order perturbation calculations or, alternatively, the addition of an InAs disk below the quantum dot confirm this conclusion. The effect of the wetting layer on the wave function is qualitatively different for the weakly confined electron state and the strongly confined hole state. The electron wave function shifts from the buffer to the wetting layer, while the hole shifts from the dot to the wetting layer

An atomistic model for the simulation of acoustic phonons, strain distribution, and Gruneisen coefficients in zinc-blende semiconductors

An accurate modeling of phonons, strain distributions, and Gr¨uneisen coefficients is essential for the qualitative and quantitative design of modern nanoelectronic and nanooptoelectronic devices. The challenge is the development of a model that fits within an atomistic representation of the overall crystal yet remains computationally tractable. A simple model for introducing the anharmonicity of the interatomic potential into the Keating two-parameter valence-force-field model is developed. The new method is used for the calculation of acoustic phonon and strain effects in zinc-blende semiconductors. The model is fitted to the Gr¨uneisen coefficients for long-wavelength acoustic phonons and reproduces the response to strain throughout the Brillouin zone in reasonable agreement with experiment