High-sensitivity of initial SrO growth on the residual resistivity in epitaxial thin films of SrRuO$_3$ on SrTiO$_3$ (001)

Abstract

The growth of SrRuO 3 (SRO) thin film with high-crystallinity and low residual resistivity (RR) is essential to explore its intrinsic properties. Here, utilizing the adsorption-controlled growth technique, the growth condition of initial SrO layer on TiO 2 -terminated SrTiO 3 (STO) (001) substrate was found to be crucial for achieving a low RR in the resulting SRO film grown afterward. The optimized initial SrO layer shows a c(2 × 2) superstructure that was characterized by electron diffraction, and a series of SRO films with different thicknesses (ts) were then grown. The resulting SRO films exhibit excellent crystallinity with orthorhombic-phase down to t ≈ 4.3 nm, which was confirmed by high resolution X-ray measurements. From X-ray azimuthal scan across SRO orthorhombic (02±1) reflection, we uncover four structural domains with a dominant domain of orthorhombic SRO [001] along cubic STO [010] direction. The dominant domain population depends on t, STO miscut angle (α), and miscut direction (β), giving a volume fraction of about 92 % for t ≈ 26.6 nm and (α, β) ≈ (0.14 o , 5 o ). On the other hand, metallic and ferromagnetic properties were well preserved down to t ≈ 1.2 nm. Residual resistivity ratio (RRR = ρ(300K)/ρ(5K)) reduces from 77.1 for t ≈ 28.5 nm to 2.5 for t ≈ 1.2 nm, while ρ(5K) increases from 2.5 µΩcm for t ≈ 28.5 nm to 131.0 µΩcm for t ≈ 1.2 nm. The ferromagnetic onset temperature (T ′ c ) of around 151 K remains nearly unchanged down to t ≈ 9.0 nm and decreases to 90 K for t ≈ 1.2 nm. Our finding thus provides a practical guideline to achieve high crystallinity and low RR in ultra-thin SRO films by simply adjusting the growth of initial SrO layer.