Ab initio structure modelling of complex thin-film oxides: thermodynamical stability of TiC/thin-film alumina (original) (raw)

We present an efficient and general method to identify promising candidate configurations for thin-film oxides and to determine structural characteristics of (metastable) thin-film structures using ab initio calculations. At the heart of this method is the complexity of the oxide bulk structure, from which a large number of thin films with structural building blocks, that is motifs, from metastable bulk oxide systems can be extracted. These span a coarse but well-defined network of initial configurations for which density functional theory (DFT) calculations predict and implement dramatic atomic relaxations in the corresponding, resulting thin-film candidates. The network of thin-film candidates (for various film thicknesses and stoichiometries) can be ordered according to their variation in ab initio total energy or in ab initio equilibrium Gibbs free energy. Analysis of the relaxed atomic structures for the most favored structures gives insight into the nature of stable and metastable thin-film oxides. We investigate ultrathin alumina nucleated on TiC as a model system to illustrate this method. The stable α-and metastable κ-Al2O3 bulk structures lead to an alumina-film candidate-space that consists of 38 configurations for a given film thickness, including three different stoichiometries. We identify the stoichiometries that are relevant in equilibrium with an O environment from ab initio thermodynamics calculations of the relaxed configurations. These relevant stoichiometries are Al4n−4O6n and Al4n−2O6n (only in equilibrium at extremely low O chemical potentials), with n = 2, 3, 4 identifying the number of oxygen layers. The films with Al4nO6n stoichiometry are not stable for any allowed value of the O chemical potential. Our analysis of the atomic structure shows that the favorable structural motifs of the relaxed films heavily differ from those in the bulk. In particular the number of tetrahedrally coordinated Al ions is much higher in the films and the corresponding tetrahedra are oriented differently than in the bulk. This finding of additional or novel favorable motifs documents that the method is capable of catching thin-film candidates with a structural nature that is not explicitly included in the network of initial thin-film configurations. Our analysis also shows that the thermodynamically most stable TiC/Al4n−4O6n systems decay into a partly decoupled TiC/O/Al4n−4O6n−6 system, with only a weak binding of the Al4n−4O6n−6 film on the TiC/O substrate.