Journal of Alloys and Compounds 397 (2005) 9–16 Electrical and optical properties of epitaxial YH x switchable mirrors S. Enache ∗ , T. Leeuwerink, A.F.Th. Hoekstra, A. Remhof, N.J. Koeman, B. Dam, R. Griessen Faculty of Sciences, Department of Physics and Astronomy, Condensed Matter Physics, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands Received 20 November 2004; accepted 7 December 2004 Available online 13 March 2005 Abstract Epitaxial YH x 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 . © 2005 Elsevier B.V. All rights reserved. Keywords: Electronic transport; Hydrogen storage materials; Thin films 1. Introduction In 1996, Huiberts et al. [1] discovered that polycrystalline Y and La films, covered with a thin Pd layer, switch reversibly from shiny metallic to transparent semiconductors upon H absorption. Polycrystalline Y films consist of ∼200-nm size domains which are largely c-axis-oriented and do not show in- plane ordering [2]. The epitaxial Y films, grown on CaF 2 [3] or Nb buffer layers on Al 2 O 3 [4] or on W substrates [5], con- sist of ∼10-m size domains, which are highly oriented along the c-axis and also, exhibit in-plane ordering. These domains are separated by a triangular network of ridges (∼200 nm in width) [6]. The ridges develop during growth as a result of the mismatch of the film with the substrate [7]. More ridges are formed during the – and – transformations upon H ab- sorption [8]. Using transmission electron microscopy (TEM), Kooi et al. showed that the ridges are crystalline with the c- axis almost in-plane [9]. They result from {10 12} deforma- tion twinning [9]. Remarkably, the ridges act as a barrier for ∗ Corresponding author. E-mail address: enchul@yahoo.com (S. Enache). lateral H diffusion and as a sort of microscopic lubricant for the domain switching. On a microscopic scale, the epitaxial domains switch es- sentially independently from each other [6], while the ridges switch collectively at a different rate; upon H absorption (or desorption), the H concentration within the ridges is smaller than that of the adjacent domains. This is completely different from polycrystalline films which switch homogeneously. On a macroscopic scale, both films exhibit astonishingly similar features upon H absorption, i.e., a large increase in electrical resistivity and a transition from shiny metallic to transparent yellowish semiconducting. Despite that, the epitaxial YH 3 films have a much lower electrical resistivity and a slightly larger optical gap than the polycrystalline ones loaded under similar conditions (e.g., 1 bar H 2 at room temperature). Much is known about the fabrication [2], optimization [10], structural quality [2], electrical and optical properties [11] of polycrystalline Y films upon H absorption at room temperature. Also, the temperature dependence of the electrical resistivity and Hall effect of polycrystalline YH x films (with 0 <x< 3) are well documented in literature [12,13]. However, the temperature dependence of the optical 0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.01.039