Binding energy and oscillator strength of excitonic molecules in type-II quantum wells

The binding energy and oscillator strength of excitonic molecules in type-II quantum wells are theoretically investigated by a variational method with the effective-mass approximation. We have studied two cases for an arrangement of electrons and holes in an excitonic molecule: One is that two holes in the molecule are confined within the same well, adjoined to the well for two electrons; the other is that each hole is confined within different wells, which sandwich the well for two electrons. We found that (i) the binding energy of the molecule depends on the electron-hole mass ratio and well widths, (ii) the bound state of the molecules exists for the limiting range of the well width, and (iii) in the case where each hole in the molecule is confined within a different well the binding energy of the molecule for the heavy-hole limit amounts to 10–30 % of the exciton binding energy for the well width comparable to the exciton Bohr radius. From consideration of the oscillator strength, it is found that, with appropriate parameters, the processes due to both the optical conversion of an exciton into an excitonic molecule and the molecule formation by a two-photon absorption have a similar order of magnitude for an absorption coefficient to that in the single exciton formation. A comparison with the experiment is also briefly discussed.