The carrier ''antibinding'' in quantum dots: a charge separation effect

TL;DR

This paper explains the carrier 'antibinding' effect in quantum dots as a charge separation phenomenon caused by wavefunction spreading and barrier effects, providing a unified understanding of the energy behavior of electron-hole pairs.

Contribution

It offers a comprehensive analysis linking charge separation to antibinding, including the case of mixed carriers, and reveals a sum rule between different binding energies.

Findings

Antibinding results from charge separation due to wavefunction spreading.

Barrier height influences whether energy stays below or exceeds twice the one-pair energy.

A sum rule relates binding energies of two pairs and one pair plus one carrier.

Abstract

We show that the carrier ''antibinding'' observed recently in semiconductor quantum dots, i.e., the fact that the ground state energy of two electron-hole pairs goes above twice the ground-state energy of one pair, can entirely be assigned to a charge separation effect, whatever its origin. In the absence of external electric field, this charge separation comes from different ''spreading-out'' of the electron and hole wavefunctions linked to the finite height of the barriers. When the dot size shrinks, the two-pair energy always stays below when the barriers are infinite. On the opposite, because barriers are less efficient for small dots, the energy of two-pairs in a dot with finite barriers, ends by behaving like the one in bulk, i.e., by going above twice the one-pair energy when the pairs get too close. For a full understanding of this ''antibinding'' effect, we have also reconsidered the case of one pair plus one carrier. We find that, while the carriers just have to spread out of the dot differently for the ''antibinding'' of two-pairs to appear, this ''antibinding'' for one pair plus one carrier only appears if this carrier is the one which spreads out the less. In addition a remarkable sum rule exists between the "binding energies" of two pairs and of one pair plus one carrier.