Thickness-induced metal–semiconductor transition in LaH2 epitaxial thin films grown by reactive rf magnetron sputtering†

Rare-earth hydrides have been extensively studied for their metal–insulator transition, high-temperature superconductivity and high hydride ionic conduction. Hence, research on their thin films is of great interest for exploring future-/next-generation device applications. In this study, (111)-oriented LaH₂ epitaxial thin films with varying thicknesses were grown for the first time via reactive rf magnetron sputtering. In the thicker films, the out-of-plane and in-plane lattice spacings were almost similar to those of bulk LaH₂. As the thickness decreased, the out-of-plane lattice spacing increased significantly, probably due to lattice strain, while the in-plane lattice spacing increased slightly. The thicker films exhibited metallic behavior similar to bulk LaH₂, whereas the thinner films were narrow band-gap semiconductors with a direct transition, indicating a thickness-induced metal–semiconductor transition without altering the hydrogen composition. These results suggest that strain engineering of rare-earth hydrides could enable the control of their physical properties even under ambient conditions.