Light Emission in Metal-Semiconductor Tunnel Junctions: Direct Evidence for Electron Heating by Plasmon Decay

We study metal-insulator-semiconductor tunnel junctions where the metal electrode is a patterned gold layer, the insulator is a thin layer of Al₂O₃, and the semiconductor is p-type silicon. We observe light emission due to plasmon-assisted inelastic tunneling from the metal to the silicon valence band. The emission cutoff shifts to higher energies with increasing voltage, a clear signature of electrically driven plasmons. The cutoff energy exceeds the applied voltage, and a large fraction of the emission is above the threshold, ℏω > eV. We find that the emission spectrum manifests the Fermi-Dirac distribution of the electrons in the gold electrode. This distribution can be used to determine the effective electron temperature, T_e, which is shown to have a linear dependence on the applied voltage. The strong correlation of T_e with the plasmon energy serves as evidence that the mechanism for heating the electrons is plasmon decay at the source metal electrode.