Epitaxial YHx switchable mirrors are characterized by micrometer-size domains delineated by a self-organized triangular network of ridges. On a macroscopic scale, the epitaxial films exhibit similar optical switching properties as the polycrystalline ones constituted of nanometer-size domains, although the microscopic domains switch independently. Their electrical transport properties are, however, different from those of polycrystalline films. Near the trihydride state, although the optical gaps of epitaxial and polycrystalline films are essentially the same, the electrical resistivity of epitaxial films is much lower. This is due to the metallic ridges in the epitaxial films, which are also responsible for a large in-plane anisotropy of the Hall voltage (i.e., up to 35% at 3 K in YH∼3). In the dihydride state, the temperature dependence of the optical transmission of both films is reminiscent of that exhibited by their electrical conductivity, σ(T). These features can be understood in a Drude model for free electrons, in which the only temperature-dependent parameter is the scattering time, τ.

We prepared epitaxial YH2 films on (111) CaF2 by pulsed laser deposition (PLD) from a metallic yttrium target. Without adding any reactive hydrogen, the dihydride is formed in situ due to the hydrogen evolving from the metallic target which contains ~7 at% H. Upon pulsed laser irradiation, the target acts as a pulsed source of both yttrium and hydrogen. The increased hydrogen content of the film as compared to the target is explained to be due to diffusion assisted preferential ablation of hydrogen. Due to this deposition process the hydrogen load on the deposition system is minimized, which is important in view of the fabrication of hydride/oxide stacks for all-solid-state switchable mirror devices. Furthermore, the preparation of both nanocrystalline and epitaxial YH2 films shows the versatility of the PLD process.

Rutherford backscattering (RBS) ion channeling measurements and X-ray diffraction experiments are performed to study the epitaxial nature of as-deposited yttrium on CaF2〈111〉 substrates and the effect of hydrogenation on the crystalline quality. The RBS and X-ray results clearly demonstrate the unique epitaxial relation between as-deposited films and the substrate, which is preserved upon loading with hydrogen. X-Ray diffraction reveals: (i) a remarkably large lattice expansion in the direction normal to the substrate, which decreases with increasing film thickness; and (ii) an in-plane compression of the lattice. This peculiar result is related to the difference in thermal expansion coefficients of film and substrate. RBS ion channeling measurements reveal a thickness dependence of the mismatch-induced stresses. As expected, the stresses relax with increasing distance from the film/substrate interface, but surprisingly, even with films as thick as 400 nm considerable dechanneling is still observed at the film surface. Film quality, i.e. the film/substrate mismatch as well as the induced stresses and their relaxation, are discussed in relation to atomic force microscopy (AFM) results on these epitaxial films.