Light emission in silicon tunnel diodes

We report general hot-carrier mechanisms for electroluminescence (EL) in metal-insulator-silicon tunnel diodes. We demonstrate these effects using various combinations of Si-oxide and Al-oxide tunnel-barrier insulators. In addition to an EL peak near the 1.1-eV Si band gap, we observe broad-spectrum EL that can span the detector-limited range from 0.7 eV to 2.6 eV (1780 nm to 480 nm). The maximum above-band-gap photon energy increases with the forward bias, consistent with hot-carrier recombination in Si. Below-band-gap EL is likely due to (i) hot-electron inter-conduction-band radiative transitions in Si and/or (ii) radiative recombination via localized interface states. Light emanates from specific sites with apparent size < 1 μm that appear during high-forward-current electrical stress. The number of sites can be in the hundreds, and is in direct proportion to the stress current, as anticipated for tunnel barrier dielectric breakdown. Current-voltage characteristics can be fit using a model appropriate to localized breakdown sites. Virtually all current is thought to cross the barrier at such sites, with local current densities as high as 108 A/cm². We also describe novel devices where tunnelling occurs at predetermined sub-micron sites formed in 18-nm-thick SiO₂ using electron-beam lithography and wet-chemical etching.