Effect of valence band splitting on the biexciton binding energy in wurtzite‐type semiconductors

A lower bound for the biexciton binding energy in wurtzite‐type semiconductors is calculated using a 64 × 64 matrix Hamiltonian obtained by the k · p method which takes into account the interband coupling‐induced nonparabolicity of the two upper valence bands A and B as well as their different anisotropies and symmetries. A properly symmetric variational ansatz for the ground state vector of the Γ 1 biexciton (with AB and BB biexciton states admixed) yields an analytic expression for the binding energy. Its numerical evaluation for CdS as a function of the nonparabolicity parameters U and W shows a sensitive dependence on U , but only a weak dependence on W . It turns out that a strong valence band coupling ( U = – 2 h 2 /2 m 0 as proposed earlier) would result in a marked increase of biexciton binding energy due to the strong nonparabolicity of the A valence band in this case.