Understanding the role of oxygen-vacancy defects in Cu₂O(111) from first-principle calculations

The presence of defects, such as copper and oxygen vacancies, in cuprous oxide films determines their characteristic carrier conductivity and consequently their application as semiconducting systems. There are still open questions on the induced electronic re-distribution, including the formation of polarons. Indeed, to accurately reproduce the structural and electronic properties at the cuprous oxide surface, very large slab models and theoretical approaches that go beyond the standard generalized gradient corrected density functional theory are needed. In this work we investigate oxygen vacancies formed in proximity of a reconstructed Cu₂O(111) surface, where the outermost unsaturated copper atoms are removed, thus forming non-stoichiometric surface layers with copper vacancies. We address simultaneously surface and bulk properties by modelling a thick and symmetric slab, to find that hybrid exchange-correlation functionals are needed to describe the oxygen vacancy in this system. Our simulations show that the formation of oxygen vacancies is favoured in the sub-surface layer. Moreover, the oxygen vacancy leads to a splitting and left-shift of the shallow hole states in the gap, which are associated with the deficiency of copper at the surface. These findings suggest that surface electronic structure and reactivity are sensitive to the presence of oxygen vacancies, also when the latter are formed deeper within the film.

铜氧化物薄膜中缺陷的存在，如铜和氧空位，决定了其特有的载流子电导率，从而影响其作为半导体系统的应用。关于由此引起的电子重新分布，包括极化子的形成，仍存在未解之谜。实际上，为了精确地再现铜氧化物表面的结构和电子性质，需要采用超出标准广义梯度校正密度泛函理论的理论方法和大块模型。在本研究中，我们探讨了在重构的Cu₂O(111)表面附近形成的氧空位，其中移除了最外层的未饱和铜原子，从而形成了含有铜空位的非化学计量表面层。通过模拟一个厚而对称的大块模型，我们同时研究了表面和体相性质，发现需要使用混合交换关联泛函来描述该系统中的氧空位。我们的模拟结果表明，氧空位的形成在亚表面层中更为有利。此外，氧空位导致能隙中浅能级空态的分裂和左移，这与表面铜的缺乏有关。这些发现表明，表面电子结构和反应性对氧空位的存在极为敏感，即使后者在薄膜内部形成得较深。