Tensile strain-induced disorder and weak localization in SrRuO3 thin films on (100) KTaO3 substrates

Abstract

SrRuO₃ (SRO) thin (~ 12 nm) films have been grown on KTaO₃ (001) substrates by RF magnetron sputtering. The as-prepared films are under enormous in-plane tensile strain, which corresponds to the elastic energy of ~ 1.5 MJ. Annealing in oxygen at 900 °C for 6 hr relaxes strain, partially lowering the elastic energy. The surface topography shows a transition from granularity as the Ar + O₂ pressure increases from 5 mTorr to 200 mTorr, with a simultaneous change in the average surface roughness from 4 nm to 0.8 nm. Annealing transforms the topography to island-type and enhances surface roughness. The films deposited at 5 mTorr are semiconducting, and annealing further enhances the resistivity, but the overall temperature dependence of resistivity (ρ-T) remains semiconducting. The ρ-T of films grown at 200 mTorr shows a metallic behavior with an inflection in the ρ-T at T_C~150 K, indicating the Curie transition. The resistivity upturn at lower temperatures shows the disordered nature of these films. Thus that large tensile strain causes strong disorder and hence is inimical to metallicity. The ρ-T behavior of the films grown at 5 mTorr follows the eq. \(\:\rho\:\left(T\right)=\frac{1}{{\sigma\:}_{0}+a{T}^{\frac{1}{2}}+{a}_{1}{T}^{\frac{p}{2}}}+b{T}^{\alpha\:}\) in the range 2K-300K with p=2 and α = 2. In the 200 mTorr deposited film, the above eq. is valid at T<95 K with p=2 and α = 1.5. At T_C<T≤300 K, the ρ-T follows eq. \(\:{\rho\:}\left(\text{T}\right)={{\rho\:}}_{0}+\:{{\rho\:}}_{1}{\text{T}}^{{\alpha\:}}\) with α = 1.3 and 1.5 for the as-grown and annealed films. The lower temperature ρ-T upturn appears to be due to either the disorder-enhanced renormalized e-e interaction (REEI) or weak localization (WL) effects. The temperature and magnetic field-dependent magnetoresistance evidence a substantial WL effect in the films grown at 200 mTorr. Our results establish a strong correlation between the nature of strain, surface topography, and carrier transport mechanisms.