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

THz intersubband electroluminescence from n-type Ge/SiGe quantum cascade structures