Direct gap electroluminescence from Si/Ge_1-y Sn_y p-i-n heterostructure diodes

Electroluminescence spectra from Si/Ge_1-y Sn_y heterostructure diodes are reported. The observed emission is dominated by direct gap optical transitions and displays the expected compositional dependence of the peak energy. Weaker indirect gap emission is also observed, and their energies are consistent with a closing of the indirect-direct separation as the Sn concentration is increased. The intensity of the EL spectra shows a superlinear dependence on the injection current, which is modeled using a Van Roosbroeck-Shockley expression for the emission intensity. The model assumes quasiequilibrium conditions for the electrons populating the different valleys in the conduction band of Ge.