Mechanism of polarization fatigue in BiFeO₃: The role of Schottky barrier

Abstract

By using piezoelectric force microscopy and scanning Kelvin probe microscopy, we have investigated the domain evolution and space charge distribution in planar BiFeO 3 capacitors with different electrodes. It is observed that charge injection at the film/electrode interface leads to domain pinning and polarization fatigue in BiFeO 3 . Furthermore, the Schottky barrier at the interface is crucial for the charge injection process. Lowering the Schottky barrier by using low work function metals as the electrodes can also improve the fatigue property of the device, similar to what oxide electrodes can achieve. a) Electronic mail: yuanguoliang@mail.njust.edu.cn b) Electronic mail: jlwang@ntu.edu.sg 2 Ferroelectric fatigue refers to the decrease of switchable polarization in a ferroelectric material after repetitive electrical cycling. 1 It is detrimental to ferroelectric based devices and should be minimized. There have been a large number of reports on the mechanism of the polarization fatigue. [2][3][4][5] The models proposed can generally be classified into three categories, namely charge injection, 6-9 defects, i.e. oxygen vacancies, redistribution 10-14 and local phase decomposition. [15][16][17] In our previous report, we have demonstrated that charge injection is likely the cause of fatigue in BiFeO 3 . 18 By using a planar capacitor (inset of Fig. 1 (a)), we have conducted piezoelectric force microscopy (PFM) and scanning Kelvin probe microscopy (SKPM) studies to investigate the domain evolution and space charge activities in BiFeO 3 during fatigue measurement. A clear correlation between injected electrons at the electrode/BiFeO 3 interface and domain pinning is established as shown in Figs. 1(a)-1(c) (For details, please refer to Ref. 18). However, this is not to say that defects are irrelevant to polarization fatigue. On the contrary, these injected electrons must be trapped in gap states which can be associated with existing defects or created by the high energy injected electrons. We emphasize that it is not the redistribution/accumulation of defects, but rather the charging/discharging of defects, leads to polarization fatigue eventually. To clarify the difference between metal and oxide electrodes, we have prepared and investigated BiFeO 3 planar capacitors using (La 0.7 ,Sr 0.3 )MnO 3 as electrodes. All the parameters for BiFeO 3 films deposition are the same as reported in our previous study. 18 Since (La 0.7 ,Sr 0.3 )MnO 3 requires higher deposition temperature than BiFeO 3 , the electrodes are prepared (by standard lithography and etching) first followed by BiFeO 3 (40 nm) deposition. Macroscopic polarizationelectric field measurement reveals no fatigue after 10 10 cycles (data not shown). In the in-plane (IP) PFM images taken after opposite electric field is applied to the film, no domain pinning is 4 observed up to 10 10 cycles (Figs. 1(d) and 1(e)). Furthermore, SKPM image reveals negligible electron injection at the (La 0.7 ,Sr 0.3 )MnO 3 /BiFeO 3 interfaces ( Fig. 1(f)), whereas signi...

Integration of amorphous ferromagnetic oxides with multiferroic materials for room temperature magnetoelectric spintronics

“…In addition, bipolar polarization fatigue measurements showed at least ~10 10 switching cycles, as seen in Fig. 4(g), a performance that is better than that for commonly used metallic electrodes such as platinum and comparable to the best crystalline oxide electrodes for BiFeO 3 [33][34][35] .…”

“…However, one concern with such an approach is the formation of strong Schottky barriers at the interface between the metal and the oxide ferroelectric; this invariably leads to degradation issues such as polarization fatigue [33][34][35] , imprint and in the case of ferromagnets such as CoFe, the possibility for interfacial oxidation of the Co 36 . In contrast, oxide metals such as La-Sr-Co-O3, SrRuO3 have been demonstrated to show a strong resistance to such interfacial degradation, mainly because they form Ohmic (or almost Ohmic) contacts to the ferroelectrics [33][34][35] . Furthermore, such oxide metals are able to accommodate the transport of charged oxygen vacancies and thus prevent the formation of interfacial charged layers that have been identified as a possible cause for the degradation.…”

Non-volatile memory based on the ferroelectric photovoltaic effect

“…Furthermore, our recent study has revealed that the Schottky barrier at the interface is critical for the charge injection process. By using a low work function metal to reduce the barrier height, charge injection can be greatly reduced and fatigue performance significantly improved22. The transmission coefficient of the Pt/Fe layer is about 35% within the visible spectrum (Supplementary Fig.…”

Switchable photovoltaic response from polarization modulated interfaces in BiFeO3 thin films

“…13,14 Moreover, since the polarization can be partially screened by the ionic displacement at the interface, 15,16 the BFO/LSMO interface shows nearly flat-band condition, while Pt/BFO interface has a Schottky barrier. 17 This allows us to identify the E De contribution to the photovoltaic response using LSMO/BFO/ LSMO samples. Afterward, we can then clarify the effect of different types of interface by replacing the top LSMO with Pt.…”