  We report electroluminescence originating from L-valley transitions in n-type
Ge/Si$_{0.15}$Ge$_{0.85}$ quantum cascade structures centered at 3.4 and 4.9
THz with a line broadening of $\Delta f/f \approx 0.2$. Three
strain-compensated heterostructures, grown on a Si substrate by ultrahigh
vacuum chemical vapor deposition, have been investigated. The design is based
on a single quantum well active region employing a vertical optical transition
and the observed spectral features are well described by non-equilibrium
Green's function calculations. The presence of two peaks highlights a
suboptimal injection in the upper state of the radiative transition. Comparison
of the electroluminescence spectra with similar GaAs/AlGaAs structure yields
one order of magnitude lower emission efficiency.
