Project title: Theory and simulation of the dielectric properties of functional oxide thin films
Supervisors: Dr Arash Mostofi, Dr Paul Tangney and Prof Neil Alford
Thin films of paraelectric and ferroelectric materials such as barium strontium titanate (Ba1-xSrxO3) are hugely important technologically due to their remarkable, and tunable, dielectric properties. For example, they are used as resonators in mobile phones and integrated microwave tunable devices such as phase array antenna. There are suggestions that they might also be tunable at optical frequencies, making them useful in plasmonics.
For these reasons, the optical properties of thin films of barium strontium titanate (BSTO are being studied with the experimental technique of ellipsometry. The interpretation of data from this technique relies on models of the refractive index of the films and, until now, rather crude models have been used. Much more could be learned if better physically-motivated models were available. In reality, the refractive index n(z, λ) as a function of distance from the substrate (z) and wavelength (λ) is a complicated function which depends on many factors such as the strain profile in the film, the growth direction of the crystal, the composition (and its variation) within the film, the structure of the surface layer, and the distribution of defects such as grain boundaries.
In this project we aim to study all of these factors and to develop a multiscale model of the optical and dielectric properties of thin film functional oxides. We will help to interpret experiment and, from first principles, we will develop an understanding of the relationship between structure and function of these films. This will involve two strands: (i) calculating the electronic and optical properties of the bulk material as a function of strain using either density-functional theory (DFT) calculations augmented by many-body perturbation theory (in the form of the GW approximation), or time-dependent DFT, and (ii) developing a model for the structure and strain profile within the film, which will involve the use of interatomic potentials that have been fitted to DFT forces, coupled with detailed experimental characterization (to be performed in Prof Alford’s group).