Network dimensionality of porous Si and Ge

A key issue in the elucidation of the mechanisms responsible for the enhanced and blue-shifted luminescence observed from nanostructures such as porous Si (PS) is the relevant dimensionality for the confinement of carriers. Our recent work from Si K-edge EXAFS analysis of red, yellow and green-emitting PS suggests that the strong blue shift is caused by quantum confinement, and that the nanocores of fresh PS are crystalline with limited disorder. Comparison of the calculated average sizes from the experimental 1st shell partial coordination numbers of two types of nanowire for both (100) and (111) orientations suggests that in fresh PS the structure is approximately but not exactly one-dimensional. The average core sizes are found to be 2.2, 1.9 and 1.3 nm respectively for red, yellow and green-emitting PS respectively. Recently porous Ge (PG) has been prepared by anodisation of Ge wafers. PG emits in the red, and the emission is similar to but weaker than that from PS. The etching process for PG produces a much rougher surface on a scale of microns however the nanostructure appears to be nanowire as for PS.