Observation of phonon confinement in SiGe nanocrystals and preferential etching of Si in porous Si_1–xGe_x films

The structure of porous Si_1–xGe_x films was characterized using Raman spectroscopy. As the film porosity increases the sizes of Si_1–xGe_x nanocrystals decrease, and also the film composition slightly modifies in favor of Ge. The Si_1–xGe_x nanocrystals were estimated to be 12.0 nm, 16.1 nm and 19.3 nm average diameter in three porous samples prepared from undoped Si_0.87Ge_0.13 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 Si_1–xGe_x alloys, a very useful technique for characterizing the structure of porous Si_1–xGe_xfilms.