Application of numerical exciton-wave-function calculations to the question of band alignment in Si/Si1-xGex quantum wells

We numerically calculate the two-particle ground-state wave functions for e xcitons in Si0.7Ge0.3 quantum wells in an effective mass model and demonstr ate how oscillator strength, binding energy, and electron distribution vary with conduction-band offset and well width. Recombination energies for the two types of excitons involving electrons in different conduction-band val leys are compared. We point out that only due to the very different electro n masses in growth direction for Delta(4) and Delta(2) valleys is it possib le that the Delta(2)-heavy-hole exciton forms the ground state, as was repo rted in recent photoluminescence experiments at extremely low excitation po wers [M. L. W. Thewalt et al., Phys. Rev. Lett. 79, 269 (1997)]. It is show n that those experiments can only be explained with a type-II offset for th e Delta(4) conduction band of about 40 meV, in contrast to the common assum ption that those data would have proven an offset of at most 10 meV for x = 0.3. [S0163-1829(99)05019-5].