Formation of crystalline silicon-germanium thin films on silicon substrates by solid phase crystallization

We have researched on the formation of crystalline silicon-germanium (c-SiGe) films as a new material for bottom cells of silicon-based multi-junction solar cells. We conducted solid phase crystallization (SPC) from amorphous SiGe (a-SiGe) precursors with 80-70% Ge fraction deposited on n-type [100] single crystalline silicon (c-Si) substrates. Preferential crystal growth following the orientation of c-Si substrates is successfully done by the SPC at temperatures from 500 to 950 °C, although X-ray diffraction indicates that the c-SiGe films have the mosaic structures consisting of crystalline domains with several tens of nanometers. Lower SPC temperature is more appropriate to obtain better crystallinity SiGe with larger domain size and lower mosaicity as long as the crystallization occurs. The inter-diffusion between the a-SiGe precursors and Si substrates occurs at relatively high SPC temperatures (≥ 850 °C), and the Ge fraction in the c-SiGe films becomes lower than that in the a-SiGe precursors.