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Description, Role of charged defects in resistance fatigue of ferroelectric tunnel junctions has been revealed at atomic scale., Ferroelectric tunnel junctions (FTJs) are promising candidates for next-generation memories due to fast read/write speeds and low-power consumptions. Here, we investigate resistance fatigue of FTJs, which is performed on Pt/BaTiO3/Nb:SrTiO3 devices. By direct observations of the 5–unit cell–thick BaTiO3 barrier with high-angle annular dark-field imaging and electron energy loss spectroscopy, oxygen vacancies are found to aggregate at the Pt/BaTiO3 interface during repetitive switching, leading to a ferroelectric dead layer preventing domain nucleation and growth. Severe oxygen deficiency also makes BaTiO3 lattices energetically unfavorable and lastly induces a destruction of local perovskite structure of the barrier. Ferroelectric properties are thus degraded, which reduces barrier contrast between ON and OFF states and smears electroresistance characteristics of Pt/BaTiO3/Nb:SrTiO3 FTJs. These results reveal an atomic-scale fatigue mechanism of ultrathin ferroelectric barriers associated with the aggregation of charged defects, facilitating the design of reliable FTJs and ferroelectric nanoelectronic devices for practical applications.

Recently, flexoelectric effect has attracted considerable attention owing to ubiquitous existence in all dielectrics, regardless of the symmetry. It promises intriguingly physical phenomena, such as strain gradient-induced electric polarizations, photocurrents, and interfacial transports, as well as their electromechanical coupling with external force loading, in diverse materials for multifunctional applications in electronics. In this work, we report the flexoelectric-modulation on surface potential of LaFeO3 (LFO) thin-film heterostructures. The LFO thin film with or without the flexoelectric effect has been achieved by controlling epitaxial misfit against a substrate. Lattice structures and strain behaviors are observed by atomic-resolution high-angle annular dark-field imaging. Grown on a LaAlO3 substrate, a giant strain gradient of ∼3 × 106 m−1 is generated in the LFO thin film due to the gradual relaxation of large misfit strain with increasing thickness, yielding a robust flexoelectric polarization pointing to the heterostructure surface. In contrast, the LFO is almost fully strained on a SrTiO3 substrate due to the small lattice mismatch. The flexoelectric polarization results in an increase in surface potential in the LFO heterostructure due to the incomplete screening of positive polarization bound charges, as observed by scanning kelvin probe microscopy. Furthermore, x-ray photoelectron spectroscopy reveals that the flexoelectric polarization can downward bend the band alignment of the LFO layer and modulate the interfacial potential barriers. These results provide the way for experimental observations of the flexoelectric effect and deliver physical insight into deep understanding of interfacial electronic structures of flexoelectric-based devices.

Recently, relaxor ferroelectric thin-film capacitors have attracted considerable attention for energy storage applications since their slim-type polarization–electric field hysteresis loops can yield large recoverable energy density ( Wrec) and high efficiency ( η). In this work, we study the effects of buffer layers on energy storage properties of 0.93Pb(Mg1/3Nb2/3)O3-0.07PbTiO3 (PMN-PT) thin-film capacitors with a 5 nm-thick SrTiO3 (STO) and LaAlO3 (LAO) films. The energy storage properties of Pt/PMN-PT/SrRuO3 (SRO) capacitors are found to be significantly changed by incorporating the STO or LAO buffer layer at the top Pt/PMN-PT interface, while inserting the buffer layer at bottom PMN-PT/SRO interface shows negligible effects on the electrical properties. Specifically, with the STO buffering, the breakdown field is dramatically increased in the Pt/STO/PMN-PT/SRO capacitor due to the existence of an internal field in the STO, which prevents the growth of electrical trees from the bottom SRO to the top Pt electrode, and a large Wrec of ∼48.91 J/cm3, more than three times of that of the PMN-PT capacitor, is achieved. However, buffered by the LAO, the Pt/LAO/PMN-PT/SRO capacitor exhibits a reduced relaxor character, which may be ascribed to a pinning effect of nanodomains associated with the charged LAO/PMN-PT interface. As a result, both Wrec and η are significantly lowered, compared to the non-buffered PMN-PT capacitor. These results provide physical insights into the modulation of relaxor and dielectric behaviors by designing the characteristics of buffer layers, demonstrating a way for enhancing energy storage properties in thin-film capacitors.

Ferroelectric tunnel junctions (FTJs) are promising candidates for the next-generation memory technologies. The electroresistance mechanism, however, has been reported not only from the polarization-modulation of barrier profiles. Electrical migration of charged defects has also been observed as a possible origin for the resistive switching. Here, we achieve two kinds of electroresistance behaviors in Pt/Pb(Zr,Ti)O3/(La,Sr)MnO3 tunnel junctions by introducing oxygen vacancies in the Pb(Zr,Ti)O3 barrier. The oxygen vacancies are observed by x-ray photoelectron spectroscopy, and their effects on the widely adopted piezoresponse force microscopy characterizations of ultrathin ferroelectric films have been analyzed by AC voltage-dependent hysteresis loops. For the Pt/Pb(Zr,Ti)O3/(La,Sr)MnO3 device that is modulated by the polarization reversal, a counterclockwise resistance–voltage ( R– V) relationship is observed due to the tunneling between high and low barriers, whereas the R– V hysteresis loop is changed to clockwise with the existence of oxygen vacancies, in which conductive filaments are formed in the Pb(Zr,Ti)O3 barrier. However, such an ionic electroresistance is not stable during repetitive switching. Further investigation on memristive behaviors is, thus, performed on the ferroelectric-controlled Pt/Pb(Zr,Ti)O3/(La,Sr)MnO3 tunnel junctions. An excellent linearity is achieved in continuous resistance change owing to the nucleation-limited-switching mode of domain switching in the Pb(Zr,Ti)O3 barrier, giving rise to spike-timing-dependent plasticity behaviors for the Hebbian rule of learning and memory. These results provide insight into the distinguishing of ferroelectric and ionic contributions in electroresistance of FTJ devices, facilitating deep understanding of nonvolatile resistive memories.

Recently, ion-doped HfO2 thin films are highly desirable for the next-generation nonvolatile memories due to excellent compatibility with current complementary metal-oxide-semiconductor processes and robust ferroelectricity persisted down to the nanoscale. In this work, we study conduction mechanisms of 4 and 8 nm-thick La:HfO2 ultrathin films sandwiched between Pt and (La0.67,Sr0.33)MnO3 (LSMO) electrodes based on band alignments of the Pt/La:HfO2/LSMO, measured by x-ray photoelectron spectroscopy, and temperature-dependent current-voltage curves from 50 to 300 K. In a 4 nm-thick La:HfO2 thin-film capacitor, the conduction mechanism is found to be governed by direct tunneling at 50–100 K and phonon-assisted indirect tunneling when the temperature is further increased to 300 K in which the [Formula: see text] acceptors are served as localized states, facilitating hole hopping through the La:HfO2 barrier. When the thickness is increased to 8 nm, the tunneling through a La:HfO2 layer is suppressed, and the current-voltage character becomes rectifying, which is regulated by the dominated La:HfO2/LSMO interfacial barrier. The transport for a forward bias of the La:HfO2/LSMO barrier is found to be governed by thermionic-field emission, exhibiting a temperature-independent build-in potential of ∼2.77 V. For the reverse bias, the Fowler–Nordheim tunneling is observed. The revealing of conduction mechanisms in terms of band alignments sheds light on leakage problems and facilitates the design of HfO2-based ferroelectric devices with excellent insulating character for high-performance memory applications.

To explore the impact of ideal health behaviors on the newly developed carotid plaques.A total of 5 852 employees (including retired employees from Tangshan Kailuan company) aged over 40 years were included in this study through stratified random sampling. Subjects with previous stroke, transient ischemic attack, myocardial infarction were excluded. Results from the unified questionnaire, blood biochemistry measurements and ultrasonography carotid artery measurements were analyzed. Present study analyzed the data from 2 372 participants without carotid plaques in 2010-2011 examinations. The newly developed carotid plaques in 2012-2013 health examinations were observed and multiple logistic regression analysis was used to explore the impact of ideal health behaviors and factors on the newly developed carotid plaques.(1) There were 359 subjects with newly developed carotid artery plaques among the 2 372 subjects (15.1%), prevalence rate was 23.1% (43/186), 17.5% (186/1 065), 12.4% (122/986), 5.9% (8/135) in the groups with 0-1, 2-3, 4-5 and 6-7 components of ideal cardiovascular health behaviors and factors, respectively (P0.001). (2) Multiple logistic regression analysis showed that after adjusting for age, gender, high density lipoprotein cholesterol, heart rate, and high sensitive c-reactive protein, compared to with 0-1 components of ideal cardiovascular health behaviors and factors, participants with 4-5 and 6-7 components of ideal cardiovascular health behaviors and factors were associated with reduced risk of the newly developed carotid plaques, the OR (95% CI) values were 0.52(0.34-0.80) and 0.28(0.12-0.64), respectively.Higher number of ideal cardiovascular health behaviors and factors is associated with lower incidence of newly developed carotid plaques.