Ab initiothermodynamics of deposition growth: Surface terminations of TiC(111) and TiN(111) grown by chemical vapor deposition

Abstract

We present a calculational method to predict terminations of growing or as-deposited surfaces as a function of the deposition conditions. Such characterizations are valueable for understanding catalysis and growth phenomena. The method combines ab-initio density functional theory (DFT) calculations and experimental thermodynamical data with a rate-equations description of partial pressures in the reaction chamber. The use of rate equations enables a complete description of a complex gas environment in terms of a few, (experimentally accessible) parameters. The predictions are based on comparisons between free energies of reaction associated with the formation of surfaces with different terminations. The method has an intrinsic non-equilibrium character. In the limit of dynamic equilibrium (with equal chemical potential in the surface and the gas phase) we find that the predictions of the method coincide with those of standard ab-initio based equilibrium thermodynamics. We illustrate the method for chemical vapor deposition (CVD) of TiC (111) and TiN(111), and find that the emerging termination can be controlled both by the environment and the growth rate.

Computational scheme for ab-initio predictions of chemical compositions interfaces realized by deposition growth

“…3 can be divided into a stoichiometric part and an excess part. We assume that the deposition of the stoichiometric parts of the films is described by (8). Excess O can be deposited as…”

“…The free energy of the TiC/alumina systems is approximated by their DFT total energies, that is, G solid ≈ E solid . 3 Vibrational effects are not considered [3,8]. For gaseous constituents, we employ the ideal-gas approximation,…”

“…Here, p i = p i (t) is the pressure of chemical species i at time t inside the reaction chamber, p = p(t) = i p i (t) is the corresponding total pressure, c i is the concentration of the chemical species i in the supply gas, and ν H2O i and ν Al2O3 i are the stoichiometric coefficients 4 of the the chemical species i in reaction (7) and (8), respectively The rate at which the gas is supplied to (exhausted from) the chamber is R S (R E ) and the reaction rates for water production and alumina deposition are R H2O and R Al2O3 . to the right and corresponds to dynamic equilibrium in (7).…”

“…The key elements of our scheme are the use of Gibbs free energies of reaction as a predictor for the chemical composition and a modeling of the deposition conditions in terms of rate equations describing the deposition environment. Our approach extends the method of ab initio thermodynamics of deposition growth for surfaces [8] to interface modeling. Applying the scheme to the TiC/alumina interface in a chemical vapor deposition (CVD) environment we predict the formation of strongly adhering structures in agreement with the wearresistance [6] of CVD TiC/alumina multilayers.…”

On the change of preferential growth orientation in chemical vapor deposition of titanium carbide by aromatic hydrocarbon precursors

“…Combined DFT and thermodynamic concepts and rate equation simulations, denoted as an "ab initio thermodynamic method for deposition growth," have shown that the (111) surface of TiC is titanium-terminated under CVD conditions (1000 C and 5 kPa in an H 2 -TiCl 4 -CH 4 -HCl atmosphere). 11 Given that both titanium and carbon precursors are present during the deposition process since it is not an atomic layer deposition type process and given that the TiC surface is found to be Ti-terminated under these process conditions, areas of the (111) surface should rapidly become titanium terminated after being covered by carbon in the growth process. Thus, the carbon chemistry can be regarded as slower than the titanium chemistry in this process.…”