Prism-coupled light emission from tunnel junctions containing interface roughness: Theory

We have developed a theory of prism-coupled light emission from tunnel junctions that incorporates the effect of roughness at all interfaces and have carried out a comprehensive numerical study of theoretical predictions. We assume a known current-fluctuation source due to tunneling electrons and calculate the radiated power using electromagnetic Green's functions for the multilayered tunnel-junction structure. The effect of roughness is incorporated in the Green's functions by a first-order perturbation method. We calculated the angle and energy dependence of p- and s-polarized emissions from both sides of a prototype tunnel junction that consists of a glass prism, an Al film, an Al oxide barrier, a Au film, and vacuum. The numerical results show that the p-polarized emission through the coupling prism occurs from the fast surface-plasmon mode, which is mainly generated by scattering of the slow surface-plasmon mode via interface roughness. The p-polarized emission from the vacuum side and the s-polarized emission from both sides arise from the direct conversion of the slow mode into external free photons via scattering by interface roughness. The emissions caused by roughness at different interfaces are calculated separately, and we conclude that the roughness at the oxide interfaces is most effective in causing light emission. This is because the slow mode which is most effectively excited by the tunneling current is localized at the oxide interfaces.