Intrinsic Optical Absorption in Single-Crystal Germanium and Silicon at 77°K and 300°K

The intrinsic absorption spectra of high-purity single-crystal germanium and silicon have been measured at 77°K and 300°K. The spectral regions studied encompassed a range of absorption coefficient from 0.1 ${\mathrm{cm}}^{-1\mathrm{}}$ to ${10}^5$ ${\mathrm{cm}}^{-1\mathrm{}}$ for each material. The germanium data may be interpreted as indicating a threshold for direct transitions at 0.81 ev at 300°K and at 0.88 ev at 77°K. The threshold for indirect transitions was placed at 0.62 ev and 0.72 ev for 300°K and 77°K, respectively. For silicon the data were not as readily interpreted However, there is an indication that the threshold for direct transitions should be placed at about 2.5 ev and the threshold for indirect transitions at 1.06 ev and 1.16 ev at 300°K and 77°K, respectively.