Electron beam lithography and reactive ion etching were used to fabricate diffraction gratings with periods between 70 and 110 nm. The gratings were formed in a thermally grown SiO2 layer and used as substrates for aluminum-aluminum oxide-gold tunnel junctions. The light emission from the tunnel junctions under direct-current bias conditions is due to the scattering of the surface plasmon polaritons (SPPs). In particular, by isolating the scattering of the slow mode SPP off the gratings the dispersion relation of the slow mode was determined.