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

Abstract

• La 0.7 Ca 0.3 MnO 3 films sputtered from He-treated target surface. • He + ion penetration in the cathode target is the crucial step for the He treatment. • He treatment increased the films’ transition temperature and c-axis lattice parameter. • Direct film exposition to He plasma was more effective than He target treatment. We previously reported that out-of-plane lattice of in-plane tensile strained (001) La 0.7 Ca 0.3 MnO 3 thin films can be stretched by He treatment of the films via adding helium gas in an Ar/O 2 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 La 0.7 Ca 0.3 MnO 3 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 3.851Å, 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.