In semiconductor and insulating materials, point defects and impurities are universally present, significantly influencing the materials' electrical, optical, mechanical, and ion transport properties. Understanding these point defects is vital for developing advanced materials. With improvements in computational methods and computing power, first-principles calculations focusing on point defects have become increasingly prevalent. These calculations now encompass not only defect formation energies and thermodynamic transition levels but also optical transitions mediated by defects and carriers' non-radiative recombination velocities.

Calculations of point defects are usually performed under periodic boundary conditions, which introduce errors due to the finite size of the models, affecting energy calculations. Over the years, various correction methods have been developed, with the Freysoldt et al.'s method (FNV) standing out for its high-precision corrections. We expanded the FNV method to apply to any material system and enabled fully automated corrections. This method is now widely implemented and used in many computational codes.

Additionally, we have proposed methods for accurately calculating energy surfaces in configurational coordinate space necessary for studying optical transitions mediated by defects and the non-radiative recombination velocities of carriers. We've also developed techniques for automating corrections for point defects in two-dimensional materials.

This presentation will discuss the theories behind these correction methods and their applications, including large-scale calculations of oxygen vacancies and the exploration of solar cell materials. By addressing the challenges of calculating and correcting point defects, we pave the way for innovative material development, highlighting the intersection of computational physics and material science in advancing technology and energy solutions.

Ferroelectrics have recently gained interest as candidates to electrically modulate the phonon transport via domain wall engineering and domain wall reconfiguration. However, additional phonon scattering sources such as point defects and grain boundaries can severely limit the performance in such ferroelectric-based phononic devices. Here, we report the thermal conductivity of a series of PbTiO3 and BaTiO3 thin films with intentionally different cationic concentrations to introduce a controllable source of point defects. An analysis of the data based on the Debye model allows deconvoluting the effect of point defects from that of the boundary scattering (arising from grain boundaries and domain walls) on the thermal conductivity and, thus, understanding the phonon scattering mechanism that governs the thermal transport in these ferroelectric films. Our results show that even small concentrations of atomic defects may have a dominant effect on the thermal conductivity, and therefore they must be carefully controlled in the fabrication of thermal-regulation devices based on tunable ferroelectric domain walls.

Oxygen vacancies are considered as the most important point defects responsible for degradation, aging and pinning of domain walls in oxide ferroelectrics, but are also very elusive. Controlled amounts of O vacancies up to 1.5% have been introduced in ceramic BT and BCTZ and their anelastic and dielectric spectra have been studied. Oxygen vacancies strongly depress the Curie temperature, may induce a reversed thermal hysteresis and dependence on the thermal history. It is argued that the major effect on Tc is from electron doping, in turn dependent on the aggregation state of the vacancies. The latter can be probed through elastic energy loss peaks in the paraelectric phase, which allow the barriers for hopping of isolated and reorientation of paired O vacancies to be measured. These barriers are found to decrease with increasing concentration of O vacancies. To explain the reversed thermal hysteresis it is assumed that O vacancies dope electrons more effectively when isolated than aggregated, and in the ferroelectric phase they disperse to decorate the domain walls as isolated point defects. In this manner, after aging in the ferroelectric phase, the pairs formed in the paraelectric phase dissolve, electron doping increases and Tc is reduced.

Ferroelectric domain walls can exhibit fundamentally different electronic properties than the surrounding bulk material, making them interesting for the application in next-generation electronic devices. After comprehensive studies of the fundamental physics of domain walls, the community is now more and more shifting the focus towards their integration and performance in different device architectures.
Here, we investigate the electrical conductance in ferroelectric domains and domain walls in proximity to metallic contacts, using the semiconducting ferroelectric ErMnO₃ as model system. To study the local effects of electronic reconstruction phenomena that naturally arise at semiconductor-metal interfaces, we deposit metal electrodes contacting to domains and domain wall and map the local transport behavior near the electrode by conductive atomic force microscopy. We observe a region of reduced domain conductance in the proximity of the electrode, which we explain by a depletion region forming at the metal-semiconductor interface. The conductance of the domain walls, however, remains largely constant also in this contact region, leading to a strongly increased domain wall to bulk current ratio.
Our results provide new insight into how electric currents flow through ferroelectric domain walls that are in contact with metallic electrodes, which is essential for the design of domain wall-based electronic devices.