Amorphous bismuth-germanium thin films. II. Optical and photoelectrical properties

Thin films of bismuth-germanium have been fabricated in vacuum by the method of codeposit vapor quenching onto liquid nitrogen or water-cooled substrates. In this paper, Paper II of a two-paper series, the optical and photoelectrical characterization of these films are reported. Transmittance, reflectance, and absorptance data are reported in the spectral range 0.2–3.0 μ for BixGe1−x thin films in the composition range 0 ≤ x ≤ 0.25. Ultraviolet/visible near-normal reflectance spectra for films in the amorphous phase (0 ≤ x ≤ 0.2) closely resemble the optical reflectance spectra of amorphous germanium. The reflectance spectra of the amorphous films were static and insensitive to an increase in bismuth concentration. Additionally the spectra were devoid of any optical remnants of the crystalline spectra of either germanium or bismuth. The reflectance spectra of a Bi0.25Ge0.75 film which was x-ray polycrystalline revealed a broad local maximum near 2 eV, indicating crystalline bismuth transitions and the existence of a pure Bi phase. Near-infrared transmittance, reflectance, and absorptance data indicate that the optical energy gap, Egopt(x) of the a-BixGe1−x thin-film system is reduced with an increase in bismuth concentration. An expression for Egopt(x) as a function of bismuth concentration is developed using the formulation of Cardona from calculated absorptance data. ac photoconductivity studies at room temperature and low temperature (T = 162°K) have been accomplished for a-BixGe1−x thin films in the composition range 0 ≤ x ≤ 0.03. The binary films in this set were found to be photoconductive with a response of the same order of magnitude and spectral shape as that of amorphous germanium films. The observed photoconductivity was present at room temperature and increased one to two orders of magnitude upon cooling to T = 162°K. The photoconductive spectral edge occurred at a photon energy between 0.6 and 0.8 eV for all films studied. An observed shift in the spectral edge with bismuth concentration is tentatively attributed to the optical band-gap dependence on the alloy concentration.