Short-range compositional randomness of hydrogenated amorphous silicon-germanium films

The optical absorption spectrum of hydrogenated amorphous silicon-germanium (a-Si_1-xGe_x:H) films can be tuned by the Ge content. As a result, there is considerable interest in applications of this photovoltaic alloy in solar cells. However, some aspects of the relationship between microstructure and optoelectronic properties are not yet fully understood. We report here a study of the local Ge environment in a-Si_1-xGe _x:H films having demonstrated high photovoltaic efficiency. We present GeK-edge extended x-ray absorption fine structure (EXAFS) measurements for samples with Ge content ranging from x=0.10 to 0.45. A structural analysis of the EXAFS reveals compositional randomness in the local Ge environment for x<0.4, indicating short-range random miscibility of Ge and Si in the alloying process. This is consistent with measurements of earlier generations of Si-rich a-Si_1-xGe_x:H films, and suggests that one must look beyond the first coordination shell to understand the relationship between microstructure and optoelectronic properties. We also find no compositional dependence in the Ge environment nearest-neighbor bond lengths, indicating that there is little topological rigidity in the amorphous phase. Together, these results exclude any substantial Ge nanocrystallization or segregation into regions of concentrated Ge hydrides, but do not prohibit more complicated compositional inhomogeneity caused by partial phase separation on longer length scales.