Alfred Kersch

The effect of dopants on the dielectric constant of HfO2 and ZrO2 from first principles

Based on first-principles calculations, we elucidate the influence of dopants on the dielectric properties of the high-k materials HfO2 and ZrO2. Our calculations demonstrate that by doping the tetragonal phase can become energetically more favorable than the monoclinic phase present at ambient conditions. The stabilization of the tetragonal phase increases the dielectric constant significantly. A series of dopants was investigated to understand the efficiency and the mechanism of the stabilization process. The calculations reveal that at a moderate doping level (∼12%) only some of the dopants stabilize the tetragonal phase and that Si is the most efficient stabilizer atom.

Stabilization of the high-k tetragonal phase in HfO2: The influence of dopants and temperature from ab initio simulations

By means of ab initio simulations we investigate the influence of dopants (Si, C, Ge, Sn, Ti, and Ce) on the transition from the monoclinic to the tetragonal phase in HfO2. In this study we focus first on the internal energy only, an approach common to ab initio simulations. In the second step we go beyond this approach in considering the Helmholtz free energy by additionally taking into account the contribution of the phonon density of states. Finally we discuss the change in transition temperature in the regime of thin films based on an empirical model. We find that both the contributions of the internal energy and phonons can be understood in terms of a model relying on the ionic radius of the dopants. Among the investigated dopants silicon is identified to promote the tetragonal phase most efficiently. The effectiveness of the various dopants is compared on the basis of a qualitative phase diagram for doping concentrations up to ∼12%.

Phase stability and dielectric constant of ABO3 perovskites from first principles

Using ab initio simulations we determine the stable phases of ABO3 perovskites (A=Ca,Sr,Ba;u2002B=Ti,Zr,Hf) at T=0u2002K by calculating the free energy. For these structures we calculate the dielectric constant and the bandgap. It turns out that for tolerance factors far from 1, the stable phase is always Pnma. For SrZrO3 and BaZrO3 with tolerance factors close to 1, we predict that the high temperature cubic phase is broken to I4/mcm like in case of SrTiO3 with a very small gain in free energy. The calculated dielectric constants are in agreement with the experimental values for the few cases known.