The Optical Constants of Germanium in the Infra-Red and Visible

Abstract

Thin films of germanium have been evaporated on glass, quartz, and calcium fluoride slides. The thickness of these films ranged from 4×${10}^{-6\mathrm{}}$ to 1×${10}^{-4\mathrm{}}$ cm. Transmission of infra-red light through the films exhibited the usual interference phenomena, showing alternate maxima and minima as the wave-length was varied. From these maxima and minima the index of refraction of the films was determined. This index was 4.3 for $\lambda$ greater than 6×${10}^{-4\mathrm{}}$ cm wave-length. It increased to 5.2 at 8.0×${10}^{-5\mathrm{}}$ cm and then fell off rapidly to 2.3 at 4.0×${10}^{-5\mathrm{}}$ cm. To determine the values of the index and extinction coefficients in the visible region where the absorption is large, a wedge of germanium was made. Transmission was then determined as a function of film thickness for several wave-lengths. The values of the optical constants in this region were then obtained by comparing these results with the theoretical transmission equations. The extinction coefficient increased from 0.4 at $\lambda =1\mathrm{}\times {10}^{-4\mathrm{}}$ cm to 2.8 at $\lambda =4\mathrm{}\times {10}^{-5\mathrm{}}$ cm. The density of the films was determined by an interferometric method and was found to be the same as bulk germanium. The real and imaginary parts of the dielectric constant for germanium are calculated and compared with the known results for silicon. The absorption bands for both elements are shown to be consistent with the electron band structure deduced from their semiconducting properties.