Fine-Tuning Oxygen Vacancies with Coherent Strain

Electron vacancies (i.e. holes) play a well-known role in the functioning of many semiconductor devices that make modern electronic devices possible. Now, scientists are finding that oxygen vacancies can lead to novel and potentially useful properties in transition-metal oxides—thin, layered materials that have become the focus of much attention recently for their material “tunability.” Because dramatic changes in electronic, magnetic, and structural properties can arise from very small changes in the concentrations of such defects, our ability to tailor and fine-tune defect densities and concentration profiles largely determines the full range of novel phenomena and functionalities accessible in such systems.

CaMnO₃ is a well-established example of a complex transition-metal oxide in which electronic and magnetic properties can be manipulated via tensile strain. A recent theoretical study suggested that coherent epitaxial strain (equal in all in-plane directions and caused by a lattice mismatch with a substrate) can facilitate the spontaneous formation of oxygen vacancies and even influence defect-site preference, opening the door to finer control of new properties and functionalities.

To gain a clearer understanding of this phenomenon, researchers from Temple University and Temple Materials Institute (Alexander Gray’s team) used a combination of bulk-sensitive x-ray spectroscopies (including high-resolution x-ray absorption spectroscopy at ALS Beamline 4.0.2 and hard x-ray photoemission spectroscopy), and several advanced theoretical methods to examine the relationship between applied coherent epitaxial strain and oxygen vacancy concentration in ultrathin single-crystalline CaMnO₃ films. The study establishes a direct link between coherent in-plane strain and oxygen-vacancy content, providing a new recipe for designing strongly correlated transition-metal oxides with tunable ionic-defect content.