Pulsed Laser Deposition of Ferroelectric Gradient Films
Avila Romar is a PhD student in the Department of Inorganic Materials Science. (Co)Promotors are prof.dr.ir. G. Koster and prof.dr.ing. A.J.H.M. Rijnders from the faculty of Science & Technology (TNW), University of Twente.
This thesis investigates the use of pulsed laser deposition (PLD) to fabricate ferroelectric thin films with property gradients along the substrate length. By intentionally offsetting the substrate alignment with respect to the plasma plume, off-center PLD produces films in which thickness, stoichiometry, and strain vary laterally in a controlled manner. This approach allows the systematic study of position-dependent properties within a single sample, enabling a more efficient exploration of ferroelectric behavior than traditional sample-by-sample studies. The work focuses on two ferroelectric oxides, BaTiO3 (BTO) and Hf0.4Zr0.6O2 (HZO). Across these systems, the role of growth parameters is examined in relation to film morphology, stoichiometry, strain state, and ferroelectric switching.
Chapter 3 explored parameter sweeps of laser fluence and process pressure when growing BTO on SrTiO3 (STO) substrates. Off-center PLD was used to examine how the two growth parameters influence property gradients. At a fixed pressure, changes in laser fluence strongly impacted surface morphology and stoichiometry. Low fluence produced Ba-rich islands localized beneath the plume center, yielding highly uneven films when grown off-center. Above a threshold fluence (1.9 J/cm2) the films transitioned to uniform stepped surfaces without islands, though oxygen vacancy concentration increased with fluence, as reflected in the out-of-plane lattice parameter cBTO. Changes in cBTO were also observed and measured across the sample, and this was attributed to plume-induced variations in Ti oxidation state. Meanwhile, varying oxygen pressure at fixed fluence revealed that pressure plays a more dominant role than fluence in governing film properties. At low pressures (0.005-0.02 mbar), the films exhibited smooth, stepped surfaces, with surface termination evolving from TiO₂-rich to mixed. At intermediate pressures (0.05-0.1 mbar), Ba-rich islands appeared uniformly across the sample. Beyond 0.1 mbar, a sharp stoichiometric shift occurred, with the films becoming Ti-rich at 0.2 mbar, accompanied by the formation of a low-crystallinity interfacial layer and significant lattice relaxation. Overall, these results show that the process pressure most strongly determines morphology, stoichiometry, and unit cell volume gradients, while the laser fluence is critical for avoiding localized Ba-rich phases. The interplay between these parameters establishes clear thresholds for controlling oxygen vacancy gradients and cation balance in BTO films.
Chapter 4 demonstrated the step-wise off-center deposition of a homogeneous La0.7Sr0.3MnO3 (LSMO) layer followed by a single HZO layer with a thickness gradient. This asymmetric heterostructure allowed for the systematic study of ferroelectric scaling behavior within a single sample. Structural and chemical analysis confirmed that stoichiometry and film quality remained uniform laterally, with thickness as the primary variable. Under constant voltage poling, the coercive field decreased with thickness according to the JKD scaling law (n ≈ –2/3). Under constant electric field, however, the coercive field remained nearly thickness-invariant (n ≈ 0). While both trends had been reported separately in prior studies, this work represents the first demonstration of both behaviors within a single film, highlighting the importance of measurement conditions. Furthermore, constant-field cycling improved polarization retention and reduced imprint compared to constant-voltage cycling, pointing to the significant influence of extrinsic effects such as leakage, imprint, and oxygen vacancy-driven resistive switching. These results validate off-center PLD as a powerful method for probing intrinsic and extrinsic switching behaviors in ferroelectric HZO, with direct implications for device reliability.
Chapter 5 investigated BTO grown on STO-buffered silicon, with either LSMO or La0.07Ba0.93SnO3 (LBSO) as the bottom electrode. This work focused on strain engineering and out-of-plane ferroelectric measurements with ferroelectric films grown on silicon. Thermal strain from silicon was found to be negligible under the slow cooling and high oxygen pressures employed, suggesting that plume asymmetry was the dominant source of lateral strain variations. BTO on LBSO electrodes exhibited in-plane tensile strain, with BTO in-plane lattice parameters close to 4.08 Å. However, strain relaxation occurred via the formation of a second, relaxed BTO layer, accompanied by domain reorientation and polarizability variations linked to plume asymmetry during growth. In contrast, the BTO/LSMO film had a smaller lattice mismatch between LSMO and BTO and exhibited stable ferroelectric switching and scaling behavior consistent with thin-film ferroelectrics; however, asymmetric electrode configurations led to large imprint values. For BTO/LBSO, out-of-plane ferroelectric switching could not be confirmed, likely due to the ferroelectric domain orientation being perpendicular to the direction of the electrodes. Nevertheless, LBSO remains a promising pathway for strain engineering in silicon-integrated devices, if the proper device geometry can be implemented.
By exploiting gradients in composition, thickness, and strain, off-center PLD provides a high-throughput framework for exploring growth-structure-property relationships in ferroelectric oxides. The results not only deepen understanding of ferroelectric materials but also point to strategies for optimizing films in applications ranging from nonvolatile memory to silicon-integrated ferroelectric devices.
