  Intervalley mixing between conduction-band states in low-dimensional Si/SiGe
heterostructures induces splitting between nominally degenerate energy levels.
The symmetric double-valley effective mass approximation and the empirical
pseudopotential method are used to find the electronic states in different
types of quantum wells. A reasonably good agreement between the two methods is
found, with the former being much faster computationally. Aside from being an
oscillatory function of well width, the splitting is found to be almost
independent of in-plane wave vector, and an increasing function of the
magnitude of interface gradient. While the model is defined for symmetric
envelope potentials, it is shown to remain reasonably accurate for slightly
asymmetric structures such as a double quantum well, making it acceptable for
simulation of multilayer intersubband optical devices. Intersubband optical
transitions are investigated under both approximations and it is shown that in
most cases valley splitting causes linewidth broadening, although under extreme
conditions, transition line doublets may result.

Mesoscale and Nanoscale Physics