Porous SiGe nanostructures formed by electrochemical etching of thin poly-SiGe films

We have studied the effect of the current density (J) and etching time in the formation of porous poly-SiGe (pp-SiGe) layers, with Ge fractions in the 0 to 0.55 range, by electrochemical etching. The starting material was unintentionally doped polycrystalline SiGe thin films, deposited in amorphous state by low-pressure chemical vapor deposition and subsequently crystallized. pp-Si structures were found to be homogeneous at large scale and show nanocrystalline of around 5 nm for low J values. The pp-SiGe structures show nonuniformities in their structure; the nanocrystal size is around 10 nm for low J and decreases for high J values and increasing etching times, reaching values of 2-4 nm. The composition of the pp-SiGe films, for low to moderate J values, shows a slight Ge enrichment. For high J values, in samples with x = 0.38, all the Si was etched and the composition of the porous layer is pure Ge. This effect is not so strong for samples with high Ge fractions (x = 0.55). The dominant radiative recombination mechanism is not excitonic; we associate it with recombination via defects in the crystalline structure of the porous film or states at the interface nanocrystals/native SiO 2 .