Opposite effects of NO$_2$ on electrical injection in porous silicon gas sensors

TL;DR

This study investigates how NO$_2$ affects electrical injection in porous silicon sensors, revealing that the impact varies with layer thickness due to complex interactions involving carrier density and band bending.

Contribution

It demonstrates the opposite effects of NO$_2$ on electrical injection depending on the porous silicon layer thickness, highlighting the importance of structural parameters.

Findings

NO$_2$ detection sensitivity at 10 ppb via current monitoring

Injection decreases for thin porous layers (~few micrometers)

Effect explained by carrier density increase and band bending

Abstract

The electrical conductance of porous silicon fabricated with heavily doped p-type silicon is very sensitive to NO$_2$. A concentration of 10 ppb can be detected by monitoring the current injection at fixed voltage. However, we show that the sign of the injection variations depends on the porous layer thickness. If the thickness is sufficiently low -- of the order of few \micro\meter{} -- the injection decreases instead of increasing. We discuss the effect in terms of an already proposed twofold action of NO$_2$, according to which the free carrier density increases, and simultaneously the energy bands are bent at the porous silicon surface.