Band-edge biexciton in nanocrystals of semiconductors with a degenerate valence band

We present a theoretical analyses of the fine structure of the band-edge biexciton in nanometer-size crystallites [nanocrystal (NC) quantum dots] of direct semiconductors with a cubic lattice structure or a hexagonal lattice structure, which can be described within the framework of a quasicubic lattice model. The six ground biexciton states created from the two fourfold degenerate hole states and the two twofold degenerate electron states, according to the Pauli principle, are split into three levels by the crystal-shape asymmetry, the intrinsic crystal field (in hexagonal lattice structure), and the hole-hole exchange interaction. The size-dependent splitting and oscillator transition strength between the biexciton states and the ground exciton states were calculated in NCs with different types of spatial confinement: NCs surrounded by impenetrable barrier and NCs with a soft confinement created by gradually changing along the radius composition of the alloy forming the NC. The results of the calculations were compared with available experimental data on CdSe NCs.