Prediction of an Excitonic Ground State in $\mathrm{I}\mathrm{n}\mathrm{A}\mathrm{s}/\mathrm{I}\mathrm{n}\mathrm{S}\mathrm{b}$ Quantum Dots

Using atomistic pseudopotential and configuration-interaction many-body calculations, we predict an excitonic ground state in the $\mathrm{I}\mathrm{n}\mathrm{A}\mathrm{s}/\mathrm{I}\mathrm{n}\mathrm{S}\mathrm{b}$ quantum-dot system. For large dots, the conduction band minimum of the InAs dot lies below the valence band maximum of the InSb matrix. Due to quantum confinement, at a critical size calculated here for various shapes, the gap $E_g$ between InAs conduction states and InSb valence states vanishes. Strong electron-hole correlation effects are induced by the spatial proximity of the electron and hole wave functions, and by the lack of strong (exciton unbinding) screening, afforded by the existence of discrete 0D confined energy levels. These correlation effects overcome $E_g$, leading to the formation of a biexcitonic ground state (two electrons in InAs and two holes in InSb) being energetically more favorable (by $\sim 15\mathrm{m}\mathrm{e}\mathrm{V}$) than the dot without excitons.

Figure 1

(a) The isotropic and biaxial strain profiles along the [001] direction; (b) Strain-modified band offset along the [001] direction calculated from the Pikus-Bir model. The unstrained InAs and InSb band edges are also indicated as “InAs CBM,” “InAs VBM,” “InSb CBM,” and “InSb VBM”. Here, $e1$, hh, 1h, and s.o. denote the band characters of the confining potentials. Note the reversal of the $\mathrm{h}\mathrm{h}/\mathrm{l}\mathrm{h}$ characters inside vs outside the QDs.

Figure 2

Single-particle spectrum of D1 ($104\AA \times 26\AA$), D2 ($142\AA \times 36\AA$), and D3 ($156\AA \times 39\AA$). $e1,e2,e3,e4$ are the InAs-confined electron states, while h1, h2, h3, and h4 are the interface localized hole states.

Figure 3

Wave functions of the InAs-confined electron states ($e1–e4$) and the first two hole states (h1–h2). The transparent lenses indicate the positions of the InAs dots. The isosurfaces enclose 50% of the state density except for $e4$, which is only weakly confined. For $e4$, the isosurface enclose only about 10% of the state density. The contour plot are slices of the density taken from chosen planes.

Figure 4

Exciton and biexciton energies of (a) lens-shaped and (b) spherical InAs dots embedded in the InSb matrix.