Thickness-dependent Insulator-to-metal Transition in Epitaxial RuO2 Films

Epitaxially grown RuO2 films on TiO2 (110) exhibit significant in-plane strain anisotropy, with a compressive strain of-4.7% along the [001] crystalline direction and a tensile strain of +2.3% along [110]. As the film thickness increases, anisotropic strain relaxation is expected. By fabricating Hall bar devices with current channels along two in-plane directions, (001) and (110), we reveal anisotropic in-plane transport in RuO2/TiO2 (110) films grown via the solid-source metal-organic molecular beam epitaxy approach. For film thicknesses (tfilm) 3.6 nm, the resistivity along (001) exceeds that along the (110) direction at all temperatures. With further decrease in film thickness, we uncover a transition from metallic to insulating behavior at tfilm 2.1 nm. Our combined temperatureand magnetic field-dependent electrical transport measurements reveal that this transition from metallic to insulating behavior is driven by electron-electron interactions.