Excitons in T-shaped quantum wires

Abstract

We calculate energies, oscillator strengths for radiative recombination, and two-particle wave functions for the ground-state exciton and around 100 excited states in a T-shaped quantum wire. We include the single-particle potential and the Coulomb interaction between the electron and hole on an equal footing, and perform exact diagonalization of the two-particle problem within a finite-basis set. We calculate spectra for all of the experimentally studied cases of T-shaped wires including symmetric and asymmetric ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ and ${\mathrm{In}}_{y}{\mathrm{Ga}}_{1\ensuremath{-}y}{\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ structures. We study in detail the shape of the wave functions to gain insight into the nature of the various states for selected symmetric and asymmetric wires in which laser emission has been experimentally observed. We also calculate the binding energy of the ground-state exciton and the confinement energy of the one-dimensional (1D) quantum-wire-exciton state with respect to the 2D quantum-well exciton for a wide range of structures, varying the well width and the Al molar fraction x. We find that the largest binding energy of any wire constructed to date is 16.5 meV. We also notice that in asymmetric structures, the confinement energy is enhanced with respect to the symmetric forms with comparable parameters but the binding energy of the exciton is then lower than in the symmetric structures. For ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ wires we obtain an upper limit for the binding energy of around 25 meV in a 10-\AA{} -wide GaAs/AlAs structure that suggests that other materials must be explored in order to achieve room-temperature applications. There are some indications that ${\mathrm{In}}_{y}{\mathrm{Ga}}_{1\ensuremath{-}y}{\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ might be a good candidate.