Modeling of gas-phase and surface reactions in liquid-source chemical-vapor deposition of (Ba,Sr)TiO3 films

Abstract

Reaction mechanisms for chemical-vapor deposition (CVD) of (Ba,Sr)TiO3 (BST) films have been studied by modeling reactions that should occur in the gas phase and on the film surfaces. We considered a conventional model and two other kinds of models: a copolymerization and a surface reflection model. The conventional model consisted of thermal decomposition of dipivaloylmethanato (DPM) source materials in the gas phase, followed by deposition of the decomposition products on the film surfaces. In the copolymerization model, Ba(DPM)2 and Sr(DPM)2 were assumed to preferentially copolymerize with TiO(DPM)2 in the gas phase, and the resulting products behave as dominant film precursors. In the surface reflection model, moreover, each source material was assumed to thermally decompose in the gas phase, and the products from TiO(DPM)2 adsorbed on film surfaces prevent the other Ba and Sr precursors from sticking onto the surfaces. Numerical simulations were performed for CVD of SrTiO3 films, and the atomic incorporation rates of Sr and Ti into SrTiO3 films were calculated as a function of the titanium source flow rate. The conventional model gave numerical results that the Sr incorporation rate was independent of the titanium flow rate, while the experiments showed a decrease with increasing flow rate. This implies that the conventional model cannot explain the CVD-SrTiO3 reaction mechanisms. On the other hand, the incorporation rates of Sr obtained from the two experimental models decreased with increasing titanium flow rate, being in agreement with the experiments. Moreover, the overall sticking probabilities for SrTiO3 film deposition calculated using the experimental models also fitted the experiments, where the surface reflection model gave a better agreement. These results indicate that the experimental models, especially the surface reflection model is suitable to explain the CVD reaction mechanisms for SrTiO3, and thus, BST film deposition.