Coupled donors in quantum dots: Quantum size and dielectric mismatch effects

Spatial and dielectric confinement modulations of the spontaneous emission rates, transition energies, and charge-density distributions of a singly ionized double donor system D2 + in a spherical quantum dot are calculated within the framework of the effective-mass envelope function approximation. Dipole moments, energy splittings, transition moments, electron-density distributions, and spontaneous emission rates involving bonding and antibonding lowest-lying molecular states are addressed for different dielectric environments, quantum dot radii, and relative locations of the coupled impurities inside the dot. The results indicate that the donor molecule behaves as heteropolar when the spatial confinement breaks the inversion symmetry, which is paralleled by a strong reduction in the excited-state radiative lifetime. Dielectric confinement, acting on a larger length scale than spatial confinement, may recover the bulklike homopolar character when the dot is embedded in a low dielectric constant medium. In the weak spatial confinement regime, dielectric effects can increase the corresponding bulk radiative lifetimes significantly and simultaneously modulate the charge-density distribution.