The Effect of Film Structure on the Optical Conductivity of Thin Tantalum and Manganese Films.

Optical constants have been determined from measurements of reflection and transmission on thin films of beta tantalum, amorphous tantalum, and alpha manganese. The metals were deposited by electron-beam evaporation onto fused silica substrates in base pressures of about 5 x 10('-7) Torr. The film structure deposited was strongly dependent on substrate temperature. We have observed and compared the optical conductivity results for three film structure types: (1) Long-range disorder and short-range disorder in amorphous Ta. (2) Long-range order and short-range disorder in tetragonal beta Ta and complex body centered cubic alpha manganese. (3) Long-range order and short-range order in body centered cubic alpha Ta. The optical conductivity of the metals with short -range disorder is distinguished by a lack of low energy absorption and a nearly flat extrapolation to the dc conductivity. The low energy conductivity of these metals can be modeled with the Drude free-electron theory, yielding a relaxation time with an order of magnitude shorter than that for a material with short-range order, i.e. alpha Ta. We believe the reason for the lack of low energy optical absorption to be due to a reduction in hybridization of the s and d bands. This reduction, a consequence of the short-range disorder, raises the importance of the dipole selection rule forbidding d-d band transitions. A tight-binding model for the d bands of the metals with many atoms in their unit cells, as for beta Ta (30 atoms) and alpha Mn (29 atoms), would produce a large number of bands. This would cause a broadening of the d band absorption. Thus, the optical absorption is dominated by short-range disorder.