Since the first finding on the switching mirror of yttrium hydride, 1) metal-insulator transition in hydrogenation of metals has been a popular issue of research and development. Among various candidate metals and alloys, yttrium is still an attractive material to investigate the optical behavior of hydrides. 2, 3) Its optical property change during hydrogenation has strong dependence on the hydrogen concentration. In the early stage,-YHx< 0: 21 is metallic with reflection, and, metallic hydride (-YH2þ ) is very weakly transparent. In the final stage, a semi-conducting phase of-YH3À becomes completely transparent. The transparent window starts to open when x ¼ 1: 8 but closes off at x ¼ 2: 09 before opening again gradually to its full transparency at x ¼ 2: 9 across a broader spectrum. As had been reported, this optical change is reversible between YH2 and YH3; however, the window is not fully shut after deloading and dehydrogenation process. To be further complicated, this switching process has a significant large hysteresis. 4, 5) Band structure models were used in the theoretical approach to describe the transparent state or the opening of bandgap in the hydrogenated rare-earth thin films. 6, 7) There is also a mess in theoretical modeling. The dramatic change in the optical property is explained by the structural distortion or Peierls distortion. 8) The strong electron correlation model is effective to describe the windowing behavior. In addition, neither strong electron correlation nor the distortion mechanism is needed in the recent GW approximation. 9) In order to loosen this tangled issue, more data and information must be experimentally obtained with respect to the change of electrical structure and optical properties during hydrogenation and dehydrogenation.

Ellipsometry has grown to be one of the most effective methods to make characterization on various thin films with respect to electrical and optical property transients in oxidation and evaporation. 10, 11) In order to make full use of ellipsometric data, theoretical modeling has been developed for measurement of thin film thickness and refractive index12) and for quantitative description of optical and electric properties. 13) Without rationally designed modeling, accurate properties cannot be estimated in practice. In particular, reliable modeling tool with sufficient robustness is needed to deal with variable thin film structures during hydrogenation and de-hydrogenation processes. 14) In the present paper, an ellipsometric characterization method is developed to quantitatively evaluate on the optical and electric properties of multi-layered thin film system even during hydrogenation. Four types of dielectric dispersive functions with two auxiliary functions are available to deduce the parameters in electrical structure by rationally curvefitting the ellipsometrically measured data. Two-step procedure is proposed to construct a rational multi-layered model corresponding to microstructure of hydrogenated thin films. At first, an yttrium oxide film deposited on SiO2 substrate is prepared to determine the dielectric functions of yttrium oxide or Y2O3 layer as the first step in the present multilayered model. These obtained functions are further utilized as the second step to quantitatively describe the optical and electric properties of yttrium-base multi-layers. This model represents the dielectric response of multi-layered system, which is composed of the metallic yttrium layer, the composite layer of yttrium and Y2O3, and the Y2O3 top layer with the surface roughness. In the second, a palladium film deposited on SiO2 substrate is prepared in Step-I to characterize the palladium hydride or PdHx formation during …