Time-resolved electroluminescence of porous silicon

Abstract

The demonstration of electroluminescence (EL) from porous silicon devices has raised the hope of silicon-based optoelectronics. Since it is known that the blue photoluminescence (PL) decay time of porous silicon is in the nanosecond range there is also some possibility of finding fast blue electroluminescence. We have compared the EL decay and rise times of samples prepared by different technologies. Samples with yellow/red spectra show a non-exponential EL decay with typical times of 20–30 μs, similar to the behaviour of the photoluminescence. We think that for these samples the decay time is limited by the same physical mechanism for EL and PL. For samples that show blue/green EL we find exponential decay times in the range of 10–25 μs. We attribute this behaviour to internal RC constants that sustain an internal current through the light-emitting structures.For samples, which consist of a metal contact that is directly evaporated onto the luminescent material, we find delay times and rise times in the range of 100 μs before EL starts after turning on the voltage. For the pn-type samples the delay and rise times are in the range of 20 μs, that means that they are comparable with the decay time. In the latter case the metal contact is applied to the top layer of the sample (p-implanted layer) which does not contribute to the EL. This observation leads to the conclusion that for devices where metal contacts are in direct contact with the luminescent porous silicon parasitic capacitors are built up that have to be charged before EL can start.