Homogeneous versus Inhomogeneous Polarization Switching in PZT Thin Films: Impact of the Structural Quality and Correlation to the Negative Capacitance Effect

Lucian Pintilie,Cristina Florentina Chirila,Cristian Radu,Georgia Andra Boni,Cosmin Marian Istrate,Ioana Pintilie,Viorica Stancu,Lucian Trupina

doi:10.3390/nano11082124

Lucian Pintilie, Cristina Florentina Chirila + Show 6 more

Open Access

https://doi.org/10.3390/nano11082124

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Journal: Nanomaterials	Publication Date: Aug 20, 2021
Citations: 2	License type: CC BY 4.0

Affiliation: National Institute of Materials Physics

Abstract

Polarization switching in ferroelectric films is exploited in many applications, such as non-volatile memories and negative capacitance field affect transistors. This can be inhomogeneous or homogeneous, depending on if ferroelectric domains are forming or not during the switching process. The relation between the polarization switching, the structural quality of the films and the negative capacitance was not studied in depth. Here, Pb(Zr0.2Ti0.8)O3 (PZT) layers were deposited by pulse laser deposition (PLD) and sol-gel (SG) on single crystal SrTiO3 (STO) and Si substrates, respectively. The structural quality was analyzed by X-ray diffraction and transmission electron microscopy, while the electric properties were investigated by performing hysteresis, dynamic dielectric measurements, and piezo-electric force microscopy analysis. It was found that the PZT layers grown by PLD on SRO/STO substrates are epitaxial while the layers deposited by SG on Pt/Si are polycrystalline. The polarization value decreases as the structure changes from epitaxial to polycrystalline, as well as the magnitude of the leakage current and of the differential negative capacitance, while the switching changes from homogeneous to inhomogeneous. The results are explained by the compensation rate of the depolarization field during the switching process, which is much faster in epitaxial films than in polycrystalline ones.

Highlights

Polarization switching in ferroelectrics is exploited in applications as non-volatile memories or memristors [1,2]
The growth of the films was performed by pulse laser deposition (PLD) from targets with different purities, considering that the magnitude of the leakage current can be influenced by the amount of impurities acting as donors or acceptors
The following possible explanation emerges: the negative capacitance effect is more pronounced in the case of homogeneous polarization switching, that it is possible in high quality epitaxial monodomain films, and degrades in the case of the inhomogeneous polarization switching that occurs in polycrystalline layers

Summary

Introduction

Polarization switching in ferroelectrics is exploited in applications as non-volatile memories or memristors [1,2]. Many models were developed to simulate the kinetic of the switching, the most popular being Kolmogorov–Avrami–Ishibashi (KAI) and nucleation limited switching (NLS, known as the non-KAI model) [3,4,5,6] These models are based on the assumption that, starting from a certain orientation of polarization, if an electric field of opposite direction is applied on the sample, nuclei of polarization with orientation parallel with the applied field are formed first, followed by their growth until the polarization changes orientation in the entire volume of the sample. It is assumed that the nuclei form on structural defects that can induce local strain and internal electric fields favoring the new orientation of polarization This type of switching is known as being inhomogeneous, because it does not take place in the entire volume at the same time. Such switching is very fast and it was suggested that it can be induced by irradiation with infrared or THz light of suitable intensity [8,9]

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