<p>[Introduction] Rare earth hydrides have been extensively studied due to their potential applications for electronic and energy devices [1,2]. Thus, thin film growth methods are being developed [3]. Recently, we synthesized LaH₂(111) epitaxial thin films by reactive magnetron sputtering for the first time [4]. In contrast with a metallic conduction in LaH₂ bulk polycrystal, the thinner LaH₂(111) film showed semiconducting conduction [5]. This different conduction behavior could be affected by the film orientation. In this study, we grew LaH_x(001) epitaxial thin films with different thickness, and evaluated the electrical properties in order to clarify the relation between electrical conduction and the film orientation.</p><p></p><p>[Experiment] LaH_x epitaxial thin films (31−123 nm) were grown on CaF₂(001) substrate by reactive rf magnetron sputtering of La metal target in Ar/H₂ mixed atmosphere (H₂ concentration: 3.5%). The range of growth temperature (T_g) was 250–500 °C. After the film growth, the temperature was quenched to room temperature followed by the deposition of Si₃N₄ cap layer (~ 5 nm). The crystal structure was evaluated by X-ray diffraction method. The electrical properties were measured by van der Pauw method.</p><p></p><p>[Results and discussion] Above T_g =350 °C, impurity phases such as LaF₃ were formed in the films in addition to LaH₂ phase. However, LaH_x phase was obtained in the films without such impurity phases with T_g = 300 °C (Fig. 1(a)). For the 31 nm-thick film, single phase LaH₂(001) epitaxial thin film was obtained. For the larger thicknesses, LaH₃(001) epitaxial thin film was included in the LaH₂(001) epitaxial thin film. These results suggest that hydrogen was desorbed near the surface of the LaH₃(001) film, in contrast with the presence of only LaH₂(111) epitaxial thin film without LaH₃ phase irrespective of the thickness [4], indicating easier hydrogen desorption for the (111)-orientation. Concerning the temperature dependence of electrical resistivity, the 31 nm-thick LaH₂(001) film showed semiconducting conduction similar to the 33 nm-thick LaH₂(111) film, whereas the thicker LaH_x (001) films showed much higher electrical resistivity (Fig. 1(b)), attributed to the inclusion of LaH₃ phase [4]. Taking these results into account, it is plausible that the hydration process in LaH_x depends significantly on the crystal plane. The difference in hydrogen desorption speed would result in the formation of only LaH₂ phase in the (111)-orientation while LaH₂ and LaH₃ mixed phase in the (001)-orientation. Accordingly, the film orientation is an important factor to control the amount of hydrogen in rare earth hydride thin films, leading to the control of electrical conduction.</p><p></p><p>[References] [1] J. N. Huiberts et al., Nature 380, 231–234 (1996). [2] K. Fukui et al., J. Am. Chem. Soc. 144, 1523–1527 (2022). [3] R. Shimizu et al., J. Phys. Soc. Jpn. 89, 051012 (2020). [4] S. Miyazaki et al., JSAP spring meeting, 15a-PB1-15 (2020). [5] S. Uramoto et al., JSAP spring meeting, 25a-F307-8 (2022).</p>