Unusual Phase Formation in Reactively Sputter-Deposited La-Co-O Thin-Film Libraries

La-based perovskites are versatile materials that are of interest for solid oxide fuel cells and electrocatalytic water splitting. During fabrication of composition spread thin-film libraries of La-Co-based oxide systems, an unusual phase formation phenomenon is observed: instead of the expected continuous composition gradient, single-phase regions with homogeneous composition form (La2O3 or stoichiometric La-perovskite). This phenomenon, which occurs during reactive cosputtering, is independent of the used substrate. However, a dependency on the O-2-portion in the process gas and the substrate temperature is observed. It can be described as a self-organized growth, where excess transition metal cannot be incorporated into the lattices of the forming single-phase regions, and therefore, not into the growing film. It is hypothesized that due to the high reactivity of La and the significantly low formation energies of La2O3 and La-perovskites, the reactive sputter deposition of La-based oxide films, which is a physical vapor deposition process, can turn partially-regarding film growth-into a chemical vapor deposition-like process. The described single-phase regions form and lead to a discontinuous composition spread, with preferred growth of the thermodynamically most stable phases. This phenomenon can be leveraged for the exploration of multinary perovskite thin-film libraries, where the B-site atoms of La-perovskites are systematically substituted.