Optical and electrical properties of terbium films as a function of hydrogen concentration

The absorption of hydrogen in the rare earth metals produces interesting changes in their optical and electrical properties. In this work we study these properties on Tb films, 50 nm thick, as a function of hydrogen concentration. The Tb films are covered with a 15 nm thick Pd overcoat which allows hydrogenation while protecting the highly reactive Tb film from contamination. Electrical resistivity is measured using the van der Pauw method, the transmission spectra are measured in the visible region and the concentration using a quartz crystal microbalance. The maximum concentration reached was 2.95 H/Tb at a hydrogen pressure of 450 Torr. From the measured transmission spectra, using a Spectral Projected Gradient Method, we obtain the Tb film's complex refractive index. From this we calculate the real and imaginary parts of the dielectric function, epsilon(1) and epsilon(2), and follow their evolution as a function of hydrogen concentration. epsilon(1) changes from negative values, characteristic of metals, to positive values characteristic of semiconductors and dielectrics. epsilon(2) decreases indicating a significant decrease in light absorption. These results are similar to those observed on Dy, Gd, La and Y films.