Optical Studies of a Nanoporous Array in Silicon

We have studied the effects of nanometer-scale texturing on the optical properties of silicon. Surfaces are textured using a non- lithographic template method combined with plasma etching. The process results in an array of cylindrical nanopores 60 nm in diameter and varying in depth up to 1 micron. We observe a significant reduction in near-normal specular optical reflectance from silicon textured this way. This reduction is across a broadband range of photon energies from 2.0 to 6.0 eV, and varies systematically with increasing pore depth. We develop a two-dimension Maxwell-Garnett effective medium approximation to model the specular reflectance. Agreement between the model and data is good provided we take into account the roughness of the nanoporous surface. Micro-Raman scatter is found to depend strongly on the texturing, exhibiting an increase in intensity with pore depth. These results support the notion that both insertion and extraction are enhanced by the nanopore surface treatment. We will summarize the nanotexturing approach, the optical properties, and the effective medium model.