Correlation between transport, dielectric, and optical properties of oxidized and nonoxidized porous silicon

摘要

Using the correlation between transport, dielectric relaxation phenomena, and the state of oxidation of porous silicon, we have found that there are two major transport routes in the porous silicon medium. The first route of conduction via the disordered tissue that surrounds the silicon nanocrystals dominates for nonoxidized porous silicon and gradually disappears during the first stage of oxidation, where the disordered tissue is oxidized. The second route of tunneling and hopping in between the nanocrystals persists up to the second stage of oxidation where the nanocrystals are oxidized. This dual transport route model is consistent with the presence of two Meyer-Neldel rules and two power-law ac conductivities at mid and high temperatures. In addition, we have found that dc conduction is limited by narrow geometrical constrictions along the transport path that give rise to a Coulomb blockade type activation process. At lower temperatures the transport is characterized by a Cole-Cole relaxation associated with thermally activated intercrystallites hopping due to the shorter distances needed to walk by the carriers. Finally, for oxidized porous silicon we have found a new relaxation mechanism of interfacial polarization that is characterized by long relaxation times at high temperatures.