Enhancement of ferroelectricity and orientation in solution-derived hafnia thin films through heterogeneous grain nucleation

Abstract

In this work, we demonstrate an approach for improving ferroelectric properties of La:HfO2 thin films by shifting the grain growth regime toward heterogeneous nucleation. A dilute 0.083 M instead of a 0.25 M solution together with an annealing step after every spin-coating cycle film gives rise to a significant improvement of ferroelectric properties. While a remanent polarization of 7 μC/cm2 was found for randomly oriented conventional films, the value of 15 μC/cm2 for the dilute solution is a result of a mixed 111 and 002 preferential orientation. A more than 50% improved breakdown voltage stems from a global density improvement from 8.0 to 8.4 g/cm3 as obtained from x-ray reflectivity (XRR). We also find superstructure peaks in XRR hinting on periodic alternations of the local density throughout the film thickness. Scanning transmission electron microscopy and secondary ion mass spectrometry confirm this periodicity. The sensitivity of XRR for this periodicity was leveraged to gain further insights in the origin of this superstructure with additional experiments. We conclude that both orientation and the superstructure are caused by a “layered structure” according to Schuler's microstructural zone model. However, our data also provide evidence for parallel chemical effects of cap formation in each stacked sub-layer. While this work shows a significant enhancement of ferroelectric properties, it also provides insights into further optimization potential of solution deposition of HfO2/ZrO2 thin films. Our XRR-based approach supplemented with suitable additional analysis can be of great value for the optimization of other solution-derived thin films beyond the material class studied here.

Impact of La Concentration on Ferroelectricity of La-Doped HfO2 Epitaxial Thin Films

“…Data of series of polycrystalline films with varied La concentration are shown with open gray circles (t = 10 nm), 20 open up blue triangles (t = 10 nm), 18 open pink right triangles (t = 45 nm), 17 and open gold left triangles (t = 10 nm). 19 Other reported data of polycrystalline films with fixed La concentration are indicated by open pink pentagon (t = 45 nm) 44 and open violet diamond (t = 50 nm). 45 45 and open down green triangles.…”

Low-toxicity Chemical Solution Deposition of Ferroelectric Ca:HfO2

“…Finally, future and extensive research can be done on the proposed precursor solution by studying different parameters such as drying temperatures, 15 heating ramps, 53 diverse dopants, layer-by-layer annealing, 67 thickness constraint, 56 wake-up free synthesis, 53,67 etc. Therefore, there is plenty of promising research to improve this low-toxicity chemical solution route for ferroelectric HfO2 synthesis.…”

“…The study of these thicker samples can help to highlight the following points: i) Although CSD methods have been more successful in producing thick, ferroelectric, and piezoelectric HfO2 films, a layer-by-layer annealing process often has had to be adopted. 49 Our current CSD method could benefit from such an approach but, for the moment, we consider that good results can be attained with fewer processing steps. ii) The annealing step is one of the most important steps towards achieving control of ferroelectric HfO2.…”

Low-toxicity Chemical Solution Deposition of Ferroelectric Ca:HfO2

“…These effects are due to the propensity of HfO2 to crystallize into the monoclinic phase when its vertical direction growth is not restricted, which favors the growth of the crystallites. 57 This is important for piezoelectric applications, where the control of ferroelectricity in thick HfO2 films is a key point. 4 Fig.…”

“…Finally, future and extensive research can be done on the proposed precursor solution by studying different parameters such as drying temperatures, 15 heating ramps, 54 diverse dopants, layer-by-layer annealing, 68 thickness constraint, 57 wake-up free synthesis, 54,68 etc. Therefore, there is plenty of promising research to improve this low-toxicity chemical solution route for ferroelectric HfO2 synthesis.…”