Amorphous Ge alloy films. III. Optical properties

The optical absorption coefficient and refractive index of amorphous films of Ge co-deposited with different concentrations of Cu, Fe, and Al has been studied in the spectral range of 0.7 to 3.0 μm. Amorphous Ge films exhibit a sharp absorption edge which first shifts to higher energies and then shifts to lower energies. Whereas the sharpness of the absorption edge of a-Ge is retained on alloying with Cu and Fe, the addition of Al causes the edge to become diffuse. The refractive index at 2.50 μm increases with increasing concentration of the alloying element. The results can be understood in terms of the removal of voids and associated dangling bonds and the formation of impurity states and are consistent with the asymetrical density-of-states model proposed for these amorphous Ge alloys (see Part II)