Study on characterization method of optical constants of germanium thin films from absorption to transparent region

In this paper, the physical dispersion model of the optical constants of the amorphous germanium thin films was studied. Based on the Cody-Lorentz model, the optical constants of the films are characterized from the visible region to the long-wave infrared region. The optical constants are calculated from the inverse calculation of the reflection spectrum and the transmission spectrum in visible region. The calculated optical constants are extended to the infrared transparent region of the germanium thin films, which is consistent with the actual measurement results. The calculated optical constants are verified by the Cauchy model in the transparent region. According to the properties of band gap and tail width, the physical mechanism why the refractive index and extinction coefficient of films is greater than that of bulk material was determined. The decay rate of the germanium film in the long wave direction is lower than that of the germanium crystal due to the presence of the tail state in the germanium film material. Therefore, the refractive index and extinction coefficient of the film are greater than those of the bulk material.