Band ordering of the pseudomorphic Si1-xGex/Si heterostructure:: the fundamental role of excitons

After years of controversial debate, evidence for an intrinsic type II band ordering at the Si/SiGe interface has been extracted from photoluminescence experiments under biaxial external strain [M.L.W. Thewalt, et al., Phys. Rev. Lett. 79 (1997) 269]. To quantify the conduction band offset, we numerically calculate the hitherto neglected exciton binding energies for that experimental situation. Our calculations within an effective mass model yield the two-particle groundstate wavefunctions for the excitons in the quantum well. It is demonstrated how binding energy and electron distribution vary with conduction band offset for the two types of excitons involving electrons in different conduction band valleys. Only due to the very different electron masses in growth direction it is possible that the Delta(2)-hh exciton forms the ground state, as was deduced from the strain dependent photoluminescence experiments. Including the exciton correction, those experiments can only be explained with a type II offset for the Delta(4) conduction band of more than about 40 meV (for x = 0.3), which is a much higher offset than has been assumed so far. (C) 2000 Elsevier Science S.A. All rights reserved.