Optical conductivity of amorphous Ta and β-Ta films

Thin films of tantalum evaporated in high vacuum onto fused silica substrates at liquid-nitrogen temperature are amorphous, even after heating above room temperature. On room-temperature substrates, β-Ta, with a 30-atom tetragonal unit cell, is deposited. The electron-diffraction interference function of the amorphous a-Ta agreed best with dense random packing models, although a β-Ta microcrystalline model also gave fairly good agreement. The complex optical conductivity was determined from reflection and transmission measurements in the spectral range 0.5–6.5 eV at room temperature. The results for a-Ta and β-Ta were very similar to each other, differing significantly from bcc α-Ta (deposited onto heated substrates or crystallized from amorphous films), and they could be extrapolated to the measured dc conductivity at zero energy. After subtraction of a Drude contribution with an electron mean-free-path on the order of the interatomic distance, the remainder showed some features of the interband absorption in α-Ta, though somewhat smoothed and weakened. We conclude that the local arrangement of near neighbors is more important than whether long-range order exists.