Si-SiO₂-Si₃N₄ heterostructures obtained by depositing silicon nitride onto a silicon substrate oxidized in dry oxygen were studied by measuring electroluminescence (EL) in the electrolyte-insulator-semiconductor system. The EL spectra display the emission bands typical of a silicon oxide layer and an intense band at 2.7 eV characteristic of the radiative relaxation of excited silylene centers. Since these centers are typical of silicon oxynitride layers, it is concluded that such a layer is formed at the boundary.