Structure of thin-film oxides: an ab initio study of TiC/Alumina

Licentiatavhandling, 2008

Oxides and oxide films play a major role in present-day technologies. Identification and analysis of their atomic and electronic structure are important to develop new functional materials. At the same time, the ionic character, complexity, and structural flexibility make accurate atomic structure determinations difficult, both from the experimental and the theoretical point of view. This licentiate thesis presents an efficient and general method to identify promising candidates for oxide thin films and to study their structural elements using ab initio density functional theory calculations. Thin films generated from building blocks of the complex bulk structure of (meta-)stable oxides form a well-defined network of initial configurations. Strong atomic relaxations are predicted and characterized by ab initio calculations. We order the resulting, relaxed thin-film candidates according to variations in ab initio total energy and ab initio thermodynamic Gibbs free energy. The relaxed structures for the most favorable films provide insight on the atomic configuration of the truly stable and metastable films. The method is illustrated and tested for thin-film alumina (Al2O3) on a titanium carbide substrate (TiC). The TiC/alumina system plays an important role for wear-resistant coatings grown by chemical-vapor deposition (CVD). The bulk structures of the stable alpha- and metastable kappa-Al2O3 lead to the identification of 38 initial thin-film configurations for a given film thickness n, including three different stoichiometric compositions, Al_{4n-4}O_{6n}, Al_{4n-2}O_{6n}, and Al_{4n}O_{6n}. The results of extensive density functional theory studies show that the energetically most favorable thin films differ heavily from their initial structures and possess up to 60 % tetrahedrally coordinated Al ions. This is considerably more than in the main bulk-alumina crystal structures. The method is capable of catching thin film candidates with structural building blocks that are not explicitly included in the network of initial configurations. The results of this licentiate thesis can have implications for the theory description of CVD growth of wear-resistant TiC/alumina multilayer coatings. We find that the thermodynamically favored TiC/Al_{4n-4}O_{6n} systems, stable in the physically relevant range of the oxygen chemical potential, separate into a tightly bound O-monolayer on TiC (TiC/O) and a weakly bound Al_{4(n-1)}O_{6(n-1)} overlayer. Strongly binding films are predicted to be stabilized only at extreme UHV conditions or by Ti defects between the tightly bound O-monolayer and the alumina overlayer. The thesis suggests that the nonequilibrium nature of the CVD growth environment plays an important role in securing the necessary strength of the TiC/alumina binding and the optimal alumina nucleation.