On the Importance of the Work Function and Electron Carrier Density of Oxide Electrodes for the Functional Properties of Ferroelectric Capacitors

Jun Wang,Minh Duc Nguyen,Minh Thanh Do,Guus Rijnders,Gertjan Koster,Nicolas Gauquelin,Evert Houwman,Johan Verbeeck

doi:10.1002/pssr.201900520

Jun Wang, Minh Duc Nguyen + Show 6 more

Open Access

https://doi.org/10.1002/pssr.201900520

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Abstract

It is important to understand the effect of the interfaces between the oxide electrode layers and the ferroelectric layer on the polarization response for optimizing the device performance of all‐oxide ferroelectric devices. Herein, the effects of the oxide La0.07Ba0.93SnO3 (LBSO) as an electrode material in an PbZr0.52Ti0.48O3 (PZT) ferroelectric capacitor are compared with those of the more commonly used SrRuO3 (SRO) electrode. SRO (top)/PZT/SRO (bottom), SRO/PZT/LBSO, and SRO/PZT/2 nm SRO/LBSO devices are fabricated. Only marginal differences in crystalline properties, determined by X‐ray diffraction and scanning transmission electron microscopy, are found. High‐quality polarization loops are obtained, but with a much larger coercive field for the SRO/PZT/LBSO device. In contrast to the SRO/PZT/SRO device, the polarization decreases strongly with increasing field cycling. This fatigue problem can be remedied by inserting a 2 nm SRO layer between PZT and LBSO. It is argued that strongly increased charge injection into the PZT occurs at the bottom interface, because of the low PZT/LBSO interfacial barrier and the much lower carrier density in LBSO, as compared with that in SRO, causing a low dielectric constant, depleted layer in LBSO. The charge injection creates a trapped space charge in the PZT, causing the difference in fatigue behavior.

Highlights

Materials on the polarization properties is important and has been discussed extensively
A commonly used explanation for the fatigue resistance is that oxide electrodes act as oxygen vacancy sinks that suppress oxygen vacancy accumulation.[16,17,18]
We show that there is a static field at the LBSO/

Summary

The intrinsic coercive fields

Àci at the PZT–electrode interfaces (index i for intrinsic), which are the values of a locally applied electrical field to create reverse domain nucleation at that interface, can be written as. The fatigue cycling changes the properties of the LBSO/PZT interface We explain this in terms of the electron injection model described in the literature.[12,44] During switching, reverse domains are nucleating at the interface, leading to an extremely large temporary local electrical field in the reversal domains of the value Eloc $ Ps=ε0εi, where εi is the dielectric constant of a low-dielectric-constant layer at the electrode/PZT interface.[12,44]. With the number of switching cycles, the amount of trapped charge (per unit area), σtr, increases and creates an electric field Eσ directed from the top electrode to the bottom electrode in the PZT (opposite to the self-bias field induced by the work function difference) This increasing electric field Eσ can pin the polarization, causing the observed fatigue. This work clearly shows that the work function and carrier density of oxide electrodes in ferroelectric devices play an important role in the polarization switching and fatigue properties of the ferroelectric layer

Experimental Section

Conflict of Interest