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.@en

In-situ deposited YH2 films ranging from epitaxial to nanocrystalline were prepared by pulsed laser deposition (PLD). PLD has the advantage that the yttrium target acts as a hydrogen source. Comparing the switching behavior to that of MBE-grown, ex-situ loaded films, the PLD-films show a reduced optical and electrical contrast in switching, which is probably due to the formation of Y(OH)3. On the other hand, we found that in the nanocrystalline PLD-films the hysteresis in switching is absent. Moreover, these films have a smoother surface morphology. Both effects are important for the implementation of switchable rare earth hydride films in all-solid-state devices.@en

The kinetics of the metal-insulator transition in polycrystalline, Pd-capped YHx switchable mirrors upon hydrogenation is investigated. Using the accompanying optical transition, we study switching of matrix-like samples with many (∼200) combinations of Pd and Y layer thicknesses. We find that: (i) With increasing Y thickness dY, the switching time τ increases for any constant Pd thickness dPd. (ii) With increasing dPd, there are three regimes. In regime I, it is impossible to switch a device. This can mainly be related to Pd-Y compound formation consuming all Pd within the UHV system, followed by surface oxidation in air. In regimes II and III switching is possible, but only in regime III does Pd form a closed cap layer. The Pd thickness needed for a closed cap layer depends on dY. (iii) An oxide buffer layer hinders Pd-Y interdiffusion, so that a thinner Pd cap layer is needed for switching than in the case without buffer layer. This is interesting for potential applications since it yields a higher optical transmission in the open state of the device.@en

Switchable mirrors^1-3 made of thin films of the hydrides of yttrium (YH(x)), lanthanum (LaH(x)) or rare-earth metals exhibit spectacular changes in their optical properties as x is varied from 0 to 3. For example, α- YH(x<0.23) is a shiny, hexagonally close-packed metal, β-YH(2±δ) is a face-centred cubic metal with a blue tint in reflection and a small transparency window at red wavelengths, whereas hexagonally close-packed γ- YH(x>2.85) is a yellowish transparent semiconductor. Here we show that this concentration dependence of the optical properties, coupled with the high mobility of hydrogen in metals, offers the possibility of real-time visual observation of hydrogen migration in solids. We explore changes in the optical properties of yttrium films in which hydrogen diffuses laterally owing to a large concentration gradient. The optical transmission profiles along the length of the film vary in such a way as to show that the formation of the various hydride phases is diffusion-controlled. We can also induce electromigration of hydrogen, which diffuses towards the anode when a current flows through the film. Consequently, hydrogen in insulating YH(3-δ) behaves as a negative ion, in agreement with recent strong-electron-correlation theories^4,5. This ability to manipulate the hydrogen distribution (and thus the optical properties) electrically might be useful for practical applications of these switchable mirrors.

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The surface morphology of pulsed-laser deposited YBa2Cu3O7-δ films is investigated by STM AFM. Instead of spiral growth, a 2D nucleation and growth behaviour is observed. As we find these 2D nuclei also on high-oxygen pressure DC sputtered films grown at a much lower growth rate, we conclude that the supersaturation is not a decisive parameter for the predominance of either growth mode. Instead, we attribute the absence of growth spirals to the non-steady state growth conditions inherent to the pulsed nature of the laser-ablation process. Growth spirals only develop, if a non-vanishing diffusional flow of adatoms towards the step edge is maintained. The number of growth spirals observed on a film is therefore not necessarily a measure for the number of screw dislocations. After wet-etching the films in Br-ethanol, we observe that etch pits are formed consisting of concentric steps. We conclude that these pits are due to repetitive nucleation around linear defects. The etchpit density identified in this way is of the order of 1 per μm2.@en