At the first stage of the project the initial version of the algorithm for interfacial structure searching has been developed allowing to link the searching algorithm with an ab initio code. The procedure was designed to be automated through a generation of a set of sampling structures in the interfacial region using a set of parameters that controls the type of defect, distance to the interface, sites exchange. To test the applicability and performance of the developed algorithm two systems, which prone to develop 2D polar gases at the interfaces between constituting perovskites, have been studied: SrTiO3/PbTiO3 (STO/PTO), and, beyond the planned, LaAlO3/SrTiO3 (LAO/STO). For both systems we have found that the stability of defects is a subject of its proximity to the interface. This finding provides an insight into processing challenges for manufacturing industry.
Further development of the topological bias algorithm (TBA) involved an efficient treatment of the searching space within the interfacial region, as well as optimising the sampling procedure. Such reformulation requires a novel approach to deal with a reformulated basis of searching dimensions. This task is implemented in the TBA in terms of topological units that sample the interfacial area between two parent systems based on structural topological units (TU) frequently met in them. At this stage the TBA performed sampling of the interfacial area with imposed topological bias, send the constructed configurations to be optimised within the framework of the density functional theory (DFT), or reliable empirical potentials. Algorithm allows to rank the final energies allows to collect statistics of successful, i.e. stable and metastable, interfacial structures with an advantage of a statistical overview of defect population, stability, and possible propagation pathways.
To test the developed functionality, we have studied the SrTiO3/CaFeO3 systems in heterostructure and in open surface geometry. We have found that lattice relaxation in this system is closely related to its electronic and magnetic properties. As such, the geometry confinement leads to non-magnetic state of modelled heterostructure, while open-surface system shows pronounced relaxation related to the rotation of TiO6 and FeO6 octahedra that is accompanied with the formation of spin polarised layers apart from the surface and the interfacial layers.
Further development of the TBA was related the implementation of a variable stoichiometry based on the frequency of topological units met in parent materials, or, observed from the experiment. The methods as its current stage includes a possibility to apply imposed order of TUs. Further development of this technique will allow modelling of the interface growth process.
Using the developed functionality, we have performed the analysis of the formation mechanism of the interface between cubic perovskite SrTiO3 and disordered aluminium oxide Al2O3, i.e. SrTiO3/Al2O3 interface, which exhibits 2D electron gas (2DEG) formed at the interface. We found that this system exhibits a tilt of band edges in vicinity to the interface within a finite STO thickness. The origin of the tilt has entirely electronic nature and related to the charge disbalance of STO and alumina compounds which induces the interface polarity. Remarkably, the thickness, within which the tilted bands are observed, is affected by the presence of defects such as Oxygen vacancies. Thus, we assume that controlling the concentration of donor defects would be a perspective way to tune the density of the 2DEG in this system.
