Influence of silicon dioxide capping layers on pore characteristics in nanocrystalline silicon membranes.

Porous nanocrystalline silicon (pnc-Si) membranes are a new class of membrane material with promising applications in biological separations. Pores are formed in a silicon film sandwiched between nm thick silicon dioxide layers during rapid thermal annealing. Controlling pore size is critical in the size-dependent separation applications. In this work, we systematically studied the influence of the silicon dioxide capping layers on pnc-Si membranes. Even a single nm thick top oxide layer is enough to switch from agglomeration to pore formation after annealing. Both the pore size and porosity increase with the thickness of the top oxide, but quickly reach a plateau after 10 nm of oxide. The bottom oxide layer acts as a barrier layer to prevent the a-Si film from undergoing homo-epitaxial growth during annealing. Both the pore size and porosity decrease as the thickness of the bottom oxide layer increases to 100 nm. The decrease of the pore size and porosity is correlated with the increased roughness of the bottom oxide layer, which hinders nanocrystal nucleation and nanopore formation.