Free carriers reactivation in mesoporous p+-type silicon by ammonia condensation: an FTIR study

The influence of ammonia, in the range of temperatures from ≈100 K to ambient, on the IR spectrum of p+-type silicon (60% porosity) has been investigated. In contrast with the case of NO2 [1], interaction with NH3 at low pressures and room temperature does not cause any reactivation of carriers (as monitored by the loss of transparency in the IR). Extensive reactivation is instead observed at low temperatures and high NH3 pressures, the sample becoming strongly opaque below 2000 cm–1. When the temperature rises, ammonia is outgassed and the original transparency is restored. Ammonia is an electron-donor molecule, so in principle chemisorption could occur, but plays no role in reactivation, in contrast with the NO2 case. The findings, resembling those reported for polar liquids wetting p+-type PS, are explained on the basis of dielectric effects [2]: the observed loss of transparency is related to the presence of a high-density gas filling the pores, possibly in a sort of supercritical state or at the most physisorbed on the walls.