Surface and interface effects in oxygen deficient SrMnO3 thin films grown on SrTiO3

Complex oxide functionality, such as ferroelectricity, magnetism or superconductivity is often achieved in epitaxial thin-film geometries. Oxygen vacancies tend to be the dominant type of defect in these materials but a fundamental understanding of their stability and electronic structure has so far mostly been established in the bulk or strained bulk, neglecting interfaces and surfaces present in a thin-film geometry. We investigate here, via density functional theory calculations, oxygen vacancies in the model system of a SrMnO3 (SMO) thin film grown on a SrTiO3 (STO) (001) substrate. Structural and electronic differences compared to bulk SMO result mainly from undercoordination at the film surface. The changed crystal field leads to a depletion of subsurface valence-band states and transfer of this charge to surface Mn atoms, both of which strongly affect the defect chemistry in the film. The result is a strong preference of oxygen vacancies in the surface region compared to deeper layers. Finally, for metastable oxygen vacancies in the substrate, we predict a spatial separation of the defect from its excess charge, the latter being accommodated in the film but close to the substrate boundary. These results show that surface and interface effects lead to significant differences in stability and electronic structure of oxygen vacancies in thin-film geometries compared to the (strained) bulk.

诸如铁电性 (ferroelectricity)、磁性 (magnetism) 或超导性 (superconductivity) 之类的复杂氧化物功能，通常可在外延薄膜结构 (epitaxial thin-film geometries) 中实现。氧空位 (oxygen vacancies) 往往是这类材料中占主导地位的缺陷类型，但迄今为止，对其稳定性与电子结构的基础认知大多建立在块体或应变块体材料体系中，却忽略了薄膜结构中存在的界面与表面效应。本研究通过密度泛函理论 (density functional theory) 计算，对以钛酸锶 (SrTiO₃, STO)（001）为衬底生长的锰酸锶 (SrMnO₃, SMO) 薄膜模型体系中的氧空位展开了探究。相较于块体锰酸锶，薄膜的结构与电子性质差异主要源于薄膜表面处的低配位 (undercoordination) 效应。改变后的晶体场会导致亚表面价带态 (subsurface valence-band states) 发生耗散，并将该电荷转移至表面Mn原子，这两种效应均会显著影响薄膜中的缺陷化学 (defect chemistry) 行为。最终表现为，相较于薄膜深层区域，氧空位在表面区域的形成具有更强的热力学偏好性。最后，针对衬底中的亚稳态氧空位 (metastable oxygen vacancies)，我们预测其缺陷与过剩电荷 (excess charge) 会发生空间分离 (spatial separation)：过剩电荷将被容纳于薄膜中但紧邻衬底界面处。上述结果表明，相较于（应变）块体材料，薄膜结构中的表面与界面效应会使得氧空位的稳定性与电子结构出现显著差异。