  Although most work toward the realization of group IV quantum cascade lasers
(QCLs) has focused on valence-band transitions, there are many desirable
properties associated with the conduction band. We show that the commonly cited
shortcomings of n-type Si/SiGe heterostructures can be overcome by moving to
the (111) growth direction. Specifically, a large band offset and low effective
mass are achievable and subband degeneracy is preserved. We predict net gain up
to lattice temperatures of 90 K in a bound-to-continuum QCL with a double-metal
waveguide, and show that a Ge interdiffusion length of at least 8 angstroms
across interfaces is tolerable.
