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

Abstract

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 O2‐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.