Lens-shaped Dots Research Articles

Phonon impact on optical control schemes of quantum dots: Role of quantum dot geometry and symmetry

Phonons strongly influence the optical control of semiconductor quantum dots. When modeling the electron-phonon interaction in several theoretical approaches the quantum dot geometry is approximated by a spherical structure, though typical self-assembled quantum dots are strongly lens-shaped. By explicitly comparing simulations of a spherical and a lens-shaped dot using a well-established correlation expansion approach we show that indeed lens-shaped dots can be exactly mapped to a spherical geometry when studying the phonon influence on the electronic system. We also give a recipe to reproduce spectral densities from more involved dots by rather simple spherical models. On the other hand, breaking the spherical symmetry has a pronounced impact on the spatio-temporal properties of the phonon dynamics. As an example we show that for a lens-shaped quantum dot the phonon emission is strongly concentrated along the direction of the smallest axis of the dot which is important for the use of phonons for the communication between different dots.

Optical properties of self-organized wurtzite InN∕GaN quantum dots: A combined atomistic tight-binding and full configuration interaction calculation

In this work, we investigate the electronic and optical properties of self-assembled InN∕GaN quantum dots. The one-particle states of the low-dimensional heterostructures are provided by a tight-binding model that fully includes the wurtzite crystal structure on an atomistic level. Optical dipole and Coulomb matrix elements are calculated from these one-particle wave functions and serve as an input for full configuration interaction calculations. We present multiexciton emission spectra and discuss in detail how Coulomb correlations and oscillator strengths are changed by the piezoelectric fields present in the structure. Vanishing exciton and biexciton ground state emission for small lens-shaped dots is predicted.

Strain-induced interfacial hole localization in self-assembled quantum dots: CompressiveInAs∕GaAsversus tensileInAs∕InSb

Using an atomistic pseudopotential approach, we study how the shape of the dot (spherical vs lens shaped) affects the position-dependent strain and the electronic properties of tensile $(\mathrm{In}\mathrm{As}∕\mathrm{In}\mathrm{Sb})$ and compressive $(\mathrm{In}\mathrm{As}∕\mathrm{Ga}\mathrm{As})$ quantum dots. We compare the strain profiles, strained modified band offsets, confined levels, and atomistic wave functions of these dots. We show (i) how the existence of position-dependent strain in nonflat heterostructures can control the electronic properties, leading, for example, to interfacial localization of hole states on the interface of matrix-embedded dots and (ii) how the dots shape can control the level sequence and degeneracy. For example in spherical dots, one finds degenerate light-hole (LH) and heavy-hole (HH) states, whereas in lens-shaped dots one can have as the highest-occupied hole state either (a) a LH state inside the dot, becoming a HH state outside the dot $(\mathrm{In}\mathrm{As}∕\mathrm{In}\mathrm{Sb}$ tensile case) or (b) a HH state inside the dot, becoming a LH states outside the dot ($\mathrm{In}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ compressive case).

Polarization-resolved electro-absorption in InAs/GaAs quantum dots in waveguide structures—modeling of size, shape and In-content

Integrating InAs self-assembled quantum dots into p–i–n (and n–i–p) waveguide structures allows for polarization resolved electro-absorption measurements of quantum dot single excitonic transitions. In this geometry, the field dependence of three electron heavy-hole and three electron light-hole transitions has been investigated. The light-hole ground state exciton is separated from the heavy-hole ground state transition by 160 meV . A zero field polarization of the heavy-hole excitons is found, which compensates at −75 kV/cm . The light hole excitons, however, do not indicate any dipole moment. In contrast to the heavy-hole transitions they show a strongly pronounced field dependence. Modeling these extensive results, the parameter space with respect to shape, size and In-content of the quantum dot turns out to be very narrow. As a key issue, the light-/heavy-hole splitting is strongly influenced by the biaxial strain, which in turn depends on the width/height ratio of the dot. The best agreement with the experimental data has been achieved by assuming a lens-shaped dot with a trumpet-shaped In-distribution.

Electron-hole alignment in InAs/GaAs self-assembled quantum dots: Effects of chemical composition and dot shape

We investigate theoretically the effects of chemical composition and shape on the electronic states in InAs/GaAs self-assembled quantum dots, by using an eight-band strain-dependent $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ Hamiltonian. For a number of InAs dots with different shapes, and especially with different gallium concentration profiles, we found various ranges of separation between electrons and holes. We show that gallium diffusion changes the confining potential for both electrons and holes through the strain profile in the dots, leading to totally different hole states from those in pure InAs dots. We also compute the electron-hole separation as a function of electron and hole energy levels. For the same gallium concentration profile, pyramidal dots exhibit the inverted alignment with the largest electron-hole separation compared with other two types, truncated-pyramidal and lens-shaped dots. Our calculations agree well with recent experiments [P.W. Fry et al., Phys. Rev. Lett. 84, 733 (2000)].