1. Introduction
Amorphous silicon germanium (a-SiGe) alloys have widely been
used as the absorption layer of the middle or/and bottom cells in
multi-junction thin film solar cells. Their optical band gap (Eg) shifts
to lower energies with increasing germanium content, which aims
to make a suitable and narrow band gap a-SiGe material of the solar
cells to enhance the light absorption in the long wavelength region
[1e4]. However, the increase of Ge content usually results in the
deteriorating of the performance of a-SiGe thin films and thus leads
to a low solar cell conversion efficiency [5