Crystal structure prediction from bulk (oxynitrides) to interfaces (SnO₂/CdTe)

In recent discovery efforts for mixed-cation ternary nitrides and oxynitrides [JCP 154, 234706 (2021)], we successfully utilized the kinetically limited minimization (KLM) approach. Of increasing interest and importance is the prediction of interface structures, a rather multifaceted problem: Epitaxial interfaces between isostructural materials can often be readily constructed without structure prediction. Otherwise, interface structures are often obtained by splicing two freestanding surfaces with subsequent geometry optimization. However, the general interface problem remains very challenging. We adopted the KLM approach to slab geometry and applied it to SnO2/CdTe interfaces without and with the ubiquitous CdCl2 treatment in CdTe photovoltaics. We find that the lowest energy SnO2/CdTe interface is not a surface splice but has instead a fractional first CdTe atomic layer that forms the connection between the two rather dissimilar materials. This interface is highly defective, both structurally and electronically. The CdCl2 addition results in a thin interlayer which strongly reduces the interface energy, improves the atomic connectivity, and dramatically reduces the defect density of states in the CdTe band gap.