First principles molecular dynamics of electrified silicon/water interfaces*

Si-based materials have been used in a myriad of devices, including as cathodes in photoelectrochemical cells and p-i-n junctions in optoelectronic bio-modulators. In both cases, the surfaces are in contact with water, and the interface is under the effect of an electric field. We built an atomistic model of the hydrogenated Si(100) surface in contact with water and we carried out first principles simulations in the presence of an electric field aimed at understanding faradic and capacitive processes at the interface. Simulations were carried out with the Qbox code (http://qboxcode.org). Our calculations of flat-band potentials reveal how doping or applied electric fields affect capacitive processes. Specifically, we find that highly doped p-i-n Si junctions are desired to obtain larger capacitive currents. In addition, our calculations of 2D scanning tunneling current images at the interface show that small faradic currents may originate from the oxidation of Si-H bonds by water molecules, and we suggest these currents should be detectable by patch clamp measurements.