PhD Defence Alim Solmaz

Alim Solmaz is PhD student in the MESA+ research group Inorganic Materials Science.

His promoter is Guus Rijnders.

Interface and domain engineering in ferroelectric bifeo₃ thin films 

There has been a tremendous interest in multiferroic materials in the last decades. Coupling between the electrical, magnetic and structural order parameters has been at the core of multiferroic device applications. These coupling effects were aimed to create novel functionalities in nanoelectronics devices. One of the most striking and desired applications of multiferroic materials is in memory devices for data storage. Current hard disks are based on storing data bits in magnetic domains where the up and down oriented domains correspond to fundamental data storage bits of 0 and 1. Reading and writing of magnetic domains are done by application of a magnetic field. Although magnetic data reading is a convenient method, due to its non-destructive nature, data writing by magnetic fields requires the use of a magnetic coil and high currents. This hampers the miniaturization of magnetic domains which is fundamental for creating bits with smaller sizes. In addition, use of high currents to create large enough magnetic fields to switch the state of a data bit causes high heat waste which is also often a detrimental factor to the device performance and lifetime. To overcome these issues, multiferroic materials offer a promising direction to write data bits using an electric field whereas read-out can be done by a magnetic field. This has been the most powerful driving force for research into multiferroic materials, especially BiFeO₃ (BFO), which shows coupling between magnetic and ferroelectric order parameters at room temperature. Realization of BFO device applications depends on overcoming the challenges regarding BFO thin film growth and its functional properties. Therefore, this thesis focused on unraveling the scientific issues with BFO thin film growth by engineering the interface with the underlying substrate, enabling control over the specific domain formation. Moreover, it studies the functional properties with a focus on the variation in conducting behavior.

In chapter 2, the surface termination of SrTiO₃ (STO) substrates is shown to change the growth kinetics of BFO thin films. A B-site terminated (TiO₂ atomic plane) STO surface leads to BFO thin film growth with a high nucleation density, resulting in formation of all structural domain variants of BFO with some defects visible on the thin film surface. On the other hand, an A- site terminated (SrO atomic plane) STO surface facilitates step-flow like BFO thin film growth due to the enhanced surface diffusivity. This results in an increased influence of the substrate surface step edges on the BFO thin film growth and leads to a selectivity among the structural domain variants. BFO thin films on A- site terminated STO consist of two structural variants with domains ordered over a longer range as compared to BFO thin films on B-site terminated STO substrates.

The study of substrate interface effects is extended from surface termination to surface symmetry in chapter 3. BFO thin films grown on TbScO₃ (TSO) (110)_o and (001)_o substrates show existence of different structural variants with significant variations in formation of domains and domain walls. On both substrates, two structural variants of different pairs are predominantly selected due to different symmetry breaking parameters. TSO (110)_o substrates mainly promote the formation of neutral 109° domain walls with very narrow domain widths. Selectivity in structural variants arises due to the non-90° angle which is positioned along the out-of-plane direction of the TSO (110)_o substrate. On the other hand, TSO (001)_o substrates cause formation of charged 109° domain walls in a meandering shape. In this case, the selectivity in structural variants arises from the fact that the non-90° angle lies in the substrate surface plane. Differences between the BFO samples on TSO (110)_o and (001)_o substrates clearly show that substrate effects are not only limited to strain, as often considered to be the biggest effect, but that substrate symmetry plays a crucial role in the growth of BFO thin films.

Successful device applications of BFO depend strongly on its electrical properties. Therefore, chapter 4 is devoted to the study of the conducting properties in BFO thin film stacks. Two aspects are considered as influential parameters, namely the electrode layers and the surface termination. As electrodes, La_2/3Sr_1/3MnO₃ (LSMO) and Nb doped STO (Nb:STO) are used. It is shown that a lower Schottky barrier exists at the LSMO-BFO interface, as compared to the Nb:STO-BFO interface. Therefore, a higher conduction is observed in BFO stacks with LSMO electrode layers. With regard to the surface termination, the difference between Schottky barrier values at A- site LSMO-BFO and B- site LSMO-BFO interfaces is very low. In contrast, Schottky barrier at A-site Nb:STO-BFO is nearly twice as at B-site Nb:STO-BFO interface. Interface engineering for BFO thin film stacks enables control of the Schottky barrier at the interface so that device leakage problems can be overcome.

One of the most controversial topics in BFO thin film functional properties is the domain wall conductivity. Chapter 5 shows the results of domain wall conductivity measurements in BFO thin films on Nb:STO and TSO (001)_o substrates. In BFO/Nb:STO samples non-stochastic changes in current are observed during current-voltage measurements. These are claimed to arise from the oxygen vacancies that lie at random levels in the energy band. Moreover, positive polarization charges are held accountable to gather such oxygen vacancies in the vicinity of head-to-head 109° domain walls in BFO thin films on TSO (001)_o substrates. This leads to the formation of preferential domain wall conductivity in BFO thin films.