Crystal and Electronic Structure Predictions in Oxide-Nitrides and Interfaces with Hetero-Anionic Interlayers

The kinetically limited minimization (KLM) approach to crystal structure prediction combines aspects of random sampling and basin hopping approaches and reduces the sampling space by enforcing minimal pairwise interatomic distances [1]. It is well suited for metastable materials and has been successfully applied in discovery efforts for mixed-cation ternary nitrides [2]. Motivated by intriguing features of oxide-nitride alloys, such as the “perfect short rang order” phase in ZnSnN2:ZnO [3], we recently extended the structure search to ternary oxynitrides [1], identifying 34 phases that are either on the convex hull or within a potentially viable energy window [1]. All 6 known oxynitrides in the search space were recovered in the correct crystal structure as thermodynamically stable phases.<br/><br/>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 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 hetero-anionic interlayer which strongly reduces the interface energy, improves the atomic connectivity, and dramatically reduces the defect density of states in the CdTe band gap.<br/>[1] <i>Computational Discovery of Stable and Metastable Ternary Oxynitrides</i>, A. Sharan, S. Lany, J Phys Chem 154, 234706 (2021).<br/>[2] <i>A Map of the Inorganic Ternary Metal Nitrides</i>, W. Sun <i>et al</i>, Nat Mater 18, 732 (2019).<br/>[3] <i>Perfect short-range ordered alloy with line-compound-like properties in the ZnSnN2:ZnO system</i>, J. Pan, J.J. Cordell, G.J. Tucker, A. Zakutayev, A.C. Tamboli, S. Lany, NPJ Comp Mater 6, 63 (2020).