Optical Properties of Amorphous Germanium Films

The optical constants of amorphous Ge films formed under well-defined conditions have been determined in the 0.1-25.0-eV spectral range. In the 0.1-1.8-eV range they were determined by analysis of precise reflectance and transmittance data ($\mathrm{RT}$), and in the 0.1-25.0-eV range by a Kramers-Kronig analysis of normal-incidence reflectance data. Both analyses gave the same results in the region of overlap. The absorption edge was found to be quite sharp (0.06 eV) and to occur usually at a photon energy of about 0.6 eV. The position of the edge was normally about 0.2 eV lower than the direct edge in crystalline Ge. No evidence was found for either the spin-orbit split valence band associated with crystalline Ge or a tailing and/or large number of states in the forbidden region. The smallest nonzero value of the absorption measured on the low-energy side of the absorption edge was about 10 ${\mathrm{cm}}^{-1\mathrm{}}$. There was no evidence for free-carrier absorption further in the infrared. At 0.1 eV, the index of refraction was 3.99 ± 0.04 as determined by the $\mathrm{RT}$ analysis. This value was in excellent agreement with the value of 4.00 derived from the zero-frequency dielectric constant, which has been calculated using the sum rule. This is also the value determined for crystalline Ge in the infrared. Reflectance data for amorphous Ge films deposited and measured in ultra-high vacuum (in situ) are reported for the region 2.0-11.8 eV. The density, determined by weighing films of known thickness, was 12-15% less than the density of crystalline Ge.