Analyzing BaTiO3 for ferroelectric tuning of the LaAlO3/SrTiO3 interface

This master thesis describes a study on barium titanate (BaTiO3, BTO) for the ferroelectric tuning of the two-dimensional electron gas (2DEG) at the interface between lanthanum aluminate (LaAlO3, LAO) and strontium titanate (SrTiO3, STO). This research is initiated from an interest in ferroelectrically enhanced strong spin-orbit coupling in the LAO-STO 2DEG for the research on Majorana fermions. The main goal of this research project was realizing and characterizing ferroelectric tuning of the LAO-STO 2DEG. The conducting interface between the insulators LAO and STO can have many remarkable properties, such as 2D superconductivity and strong spin-orbit coupling. This spin-orbit coupling can be enhanced by a ferroelectric top layer. Under the right conditions such a materials system is predicted to host Majorana fermions. The spontaneous polarization of the ferroelectric top layer establishes an electric field towards the LAO-STO interface which influences the electronic properties of the 2DEG. When this field is strong enough and has the right polarity it might deplete the LAO-STO 2DEG, resulting in an insulating interface. Since the spontaneous polarization direction of the ferroelectric can, in principle, be switched in a temporary externally applied electric field, the system could act as an electronic switch. The BTO-LAO-STO system, in which BTO is employed as the ferroelectric, can therefore be used to ferroelectrically tune the LAO-STO interface 2DEG. In this research this materials system has been manufactured, characterized and analyzed using different techniques. The main tool used is piezoresponse force microscopy (PFM), that allows imaging and manipulation of the polarization in ferroelectric materials. The LAOSTO interface in BTO-LAO-STO was electronically characterized and turned out to exhibit qualitatively the same behaviour as LAO-STO samples without a BTO top layer. In all samples analyzed local ferroelectricity was repeatedly observed in the out-of-plane direction. The PFM measurements revealed that the materials system has a preferred (spontaneous) polarization direction, which can only be switched as long as an external electric field is present. This is due to internal electric field biases in the system and is highly disadvantageous for the pursued goal of this project. To explain this preferred (spontaneous) polarization direction a band bending model was developed. The presence of internal biases is in this model ascribed to band bending at the different interfaces in the BTO-LAO-STO materials system, which are expected to pin the polarization. This model turned out to be only in partial agreement with the examined materials systems. Therefore, the model was extended by considering additional sources of internal bias. Among these are internal biases due to oxygen vacancies and strain in the materials system. The relative contribution of all possible sources of internal bias could not be determined from the current research. Gating experiments on BTO-LAO-STO have been performed on a gate bar device to find any contribution from ferroelectric tuning of the 2DEG. However, the sought-after effect has not been clearly observed. This might be due to the inhomogeneity of the ferroelectricity in all samples. At crucial spots on the gate bar no ferroelectricity was found, which is detrimental for the functioning of the device. The main goal of this research has therefore only been partly reached. Recommendations are presented to reach this goal in the near future.