Observation of phonon confinement in SiGe nanocrystals and preferential etching of Si in porous Si1-xGex films

The structure of porous Si1–xGex films was characterized using Raman spectroscopy. As the film porosity increases the sizes of Si1–xGex nanocrystals decrease, and also the film composition slightly modifies in favor of Ge. The Si1–xGex nanocrystals were estimated to be 12.0 nm, 16.1 nm and 19.3 nm average diameter in three porous samples prepared from undoped Si0.87Ge0.13 films with 20 min, 10 min and 2 min etch times, respectively, at the same current density 22 mA cm–2. The final Ge fractions were determined for the samples as 17.4%, 14.5% and 13.0%, respectively. It is seen by this study that Raman spectroscopy is, as it has always been for bulk Si1–xGex alloys, a very useful technique for characterizing the structure of porous Si1–xGexfilms.