We have investigated the multiple-scattering effect of surface-plasmon polaritons in the light-emission process from metal-insulator-metal tunnel junctions. The theory developed by Arya and Zeyher [Phys. Rev. B 24, 4048 (1983)] is extended to take into account the roughness at all interfaces of the junction. The light intensity due to multiple scattering from roughness at the metal-oxide interfaces first increases steeply, takes a maximum value, and then decreases slowly with the increase in the root-mean-squared amplitude of roughness . This behavior is very different from that due to roughness at the top surface of the junction investigated by Arya and Zeyher. In their case the predicted intensity due to multiple scattering increases monotonically with increasing . The origin of this difference is discussed.
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