Helium ion penetration in sputtering cathode materials: A crucial process for the helium treatment of oxide thin films

ABSTRACT We previously reported that out-of-plane lattice of in-plane tensile strained (001) La0.7Ca0.3MnO3 thin films can be stretched by He treatment of the films via adding helium gas in an Ar/O2 mixed sputtering atmosphere. To gain more insight into the He treatment process, in this study, we carried out He plasma treatment on a target and He atom adsorption on a substrate surface respectively before/between sputtering deposition of La0.7Ca0.3MnO3 films. The films deposited after the target pre-treatment show remarkable changes in the out-of-plane lattice and magnetotransport properties, while the films deposited through the latter method changes only slightly, suggesting that the He+ ion penetration in the cathode target plays a key role in the He treatment of the manganite films. Furthermore, we placed the as-grown films at the cathode position and exposed them directly to He discharge plasma. After the He plasma post-treatment, the out-of-plane lattice parameter increases from 3.823 to 3.851A, accompanied by an increase in the metal-insulator transition temperature from 260 to 290 K. The results demonstrate that the He plasma treatment is a simple strategy for the single-axis control of epitaxial thin films.