Structural, electrical, and optical properties of amorphous germanium films

Abstract

Measurements of the ac and dc resistivity in the temperature range of 77-750°K and of the optical transmission and reflectance have been made on amorphous Ge films of 1000-Å to 7-μ thickness prepared by thermal evaporation. The resistivity of amorphous Ge films at room temperature is ∼100Ω cm, and increases with decreasing temperature. The conduction is thermally activated, with activation energy decreasing continuously from 0.5 eV at 600°K down to 0.069 eV at 77°K. At 450°C, the amorphous → crystalline transformation is observed and is accompanied by an irreversible decrease of resistivity by a factor of ≥20. The ac resistivity of amorphous films is found to decrease with frequency ω. At high frequencies, the resistivity varies as ω^-n, where n lies between 0.5 and 1, depending on temperature. The resistivity decreases with applied field beyond the Ohmic regime. At 77°K, the current increases exponentially with the square root of the field for fields >∼ 2.5× 10³ V/cm. The optical absorption coefficient is found to vary as exp (hv/0.14 eV), in the energy range of 0.6-1.24 eV. Below 0.6 eV, the absorption coefficient falls rapidly down to a value of ∼60 cm^-1 at 0.53 eV. The dc and ac electrical properties may be understood in terms of a mechanism of conduction by hopping of the carriers in the localized states (near the "fuzzy" band edges) caused by the fluctuation potential in the disordered lattice. The exponential decay of the opticalabsorption coefficient with energy near the absorption edge and the presence of a red-shifted absorption edge are consistent with the crystalline energy-band diagram modified by the presence of acceptor states due to vacancies, by tailing, and by localization of states near the band edges due to disorder. The sharpness (which is comparable to that in single-crystal Ge) of the amorphous absorption edge is, however, not compatible with the "tailing" concept.