  An efficient silicon based light source presents an unreached goal in the
field of photonics, due to Silicons indirect electronic band structure
preventing direct carrier recombination and subsequent photon emission. Here we
utilize inelastically tunneling electrons to demonstrate an electrically driven
light emitting silicon based tunnel junction operating at room temperature. We
show that such a junction is a source for plasmons driven by the electrical
tunnel current. We find that the emission spectrum is not given by the quantum
condition where the emission frequency would be proportional to the applied
voltage, but the spectrum is determined by the spectral overlap between the
energy dependent tunnel current and the modal dispersion of the plasmon.
Experimentally we find the highest light outcoupling efficiency corresponding
to the skin depth of the metallic contact of this metal insulator semiconductor
junction. Distinct from LEDs, the temporal response of this tunnel source is
not governed by nanosecond carrier lifetimes known to semiconductors, but
rather by the tunnel event itself and Heisenbergs uncertainty principle.
