Final PhD Seminar-Porosity Induced by Ion Irradiation in Ge and Si1-xGex Alloys
When heavy ions with energies in the range of hundreds of keV penetrate c-Ge, they lose their energy through elastic interactions with the target nuclei and can lead to the formation of porosity at moderate ion fluences above about 1015 ions/cm2 at room temperature (RT). Porosity is typically a structure of holes of a few nanometres in diameter separated by thin walls and the porous thickness can be up to hundred nanometres long. The formation of amorphous porous layers in Ge is associated with extensive swelling. The advantage of such large surface area opens the door for a wide range of applications for example, in lithium ion batteries as an anode, in gas sensors, in thermoelectric applications and in optoelectronic applications. Ion bombardment of c-Si, on the other hand, under comparable implantation conditions does not show pore formation.
In my project, we aim to investigate the porous structure in Si1-xGex alloys (x= 0.83, 0.77, and 0.65) under 140 keV Ge ion irradiation at different ion fluence and temperatures regimes. The formation of a porous layer on the Ge surface is considered sometimes as a significant issue and must be avoided or minimized such as in Ge-Sn photonics. Therefore we also focus on the effect of a cap layer (SiO2) on pore formation in Ge and its alloys prior to irradiation. Multi-characterization techniques were used to investigate the surface morphology after irradiation, such as scanning and transmission electron microscopy (SEM) and (TEM), step height measurements and small angle x-ray scattering (SAXS). In particular, to measure the pore size and the sidewall thickness, we used (SAXS) as a complementary technique to TEM. SAXS provides a better statistical average of the radius of features as it gives the average of entire bulk porous structure compared with surface-sensitive SEM. We found that using an appropriate core shell cylinder model fits the data and is consistent with TEM.