Effect of composition in Si_1-xGe_x seed layer on the solid phase crystallization of ultrathin amorphous silicon layer

The effect of top silicon-germanium (Si_1-xGe_x) layers on the crystallization of underlying amorphous Si (a-Si) films was investigated for various Ge compositions in Si_1-xGe_x. Si and Si_1-xGe_x films were sequentially deposited on thermal SiO₂ by using SiH₄ and GeH₄ source gases in reduced pressure chemical vapor deposition. The fraction and the microstructure of underlying crystallized Si were characterized by plan-view transmission electron microscopy. Scanning nanobeam diffraction was performed to investigate the grain size of crystallized Si films. The crystallization of the underlying a-Si thin film in a-Si_1-xGe_x/Si bilayer stack gradually increased with the increasing Ge composition of the top Si_1-xGe_x layer. The grain size of underlying crystallized Si films showed a dependence on the Ge composition of top Si_1-xGe_x. For Si_1-xGe_x with 22% Ge, microtwins were observed to be the dominant defect, while stacking faults were dominant for Si_1-xGe_x with 47% Ge. The Ge composition and the thickness of Si_1-xGe_x are important factors to obtain the optimized crystallization time and grain size of ultrathin a-Si with a reduced density of intragrain defects.