| The discovery of ferroelectricity in HfO2-based materials has injected new vitality into the market of ferroelectric memory.Compared with the conventional perovskite ferroelectrics,HfO2-based ferroelectrics show advantages of contamination-free,large coercive electric field(Ec),perfect CMOS compatibility,and excellent ferroelectricity in sub-10 nm films,which makes it one of the most concerning materials in the last decade.Nowadays,HfO2-based ferroelectric films have been widely used as ferroelectric random access memory(FRAM)such as ferroelectric capacitor(FeCAP),ferroelectric field-effect transistor(FeFET),and ferroelectric tunnel junction(FTJ).In addition,FeFET and FTJ are also promising for neuromorphic computing and in-memory computing.The ferroelectricity of HfO2 is origin from the orthorhombic Pca21 phase,which can be stabilized by the combined effects of rapid thermal annealing(RTA),electrode stress,dopants,and film thickness.The polarization switching of HfO2 is dominated by the switching(movements)of the three-coordinated oxygens(O3)and the high switching barrier of O3 is the main reason for its large EC.Although the large EC can further open the memory window of FeFET,it is close to the dielectric breakdown field and thereby the endurance is worse than other ferroelectric films.How to decrease Ec or the switching barrier of O3 is now an important research topic.HfO2-based ferroelectric film generally consists of several kinds of crystal phases including the monoclinic phase at room temperature(RT),the tetragonal phase at high temperature,and the sub-stabilized orthorhombic phase.Such multi-phase-mixed films result in small grain sizes,high defect concentrations,and again bad endurance.While the endurance can be controlled by the RTA temperature,the underlying mechanisms and the reliability issues are still ambiguous.Besides,the polycrystal characteristics could have influenced the polarization switching mechanisms and the switching dynamics known as the nucleation limited switching(NLS)model,especially when HfO2-based device scales down and integrates into the high-density memory chips.The above issues have been systematically studied and the main conclusions are summarized below.First,based on the first-principles calculations,the effects of the lattice strain on the switching barrier of O3(Eb)and the spontaneous polarization(Ps)are studied in ferroelectric HfO2.Although the polarization switching is origin from the displacements of O3,the pathway of O3 is not unique.The pathways of O3 can be simply classified into Shift-Across(SA)and Shift-Inside(SI)pathways.Both Eb and Ps of the SA pathway are larger than that of the SI pathway due to the much larger displacement of O3 in the SA pathway.Besides,the two pathways depend on the interfacial details of HfO2/electrode,i.e.,O-terminated and Hfterminated interfaces could switch the polarization through the SA and SI pathways,respectively.By applying 3%lattice strains on HfO2,a ferroelectric-paraelectric phase transition from the polar Pca21 to the nonpolar Pbcn can be observed,which indicates the vanished ferroelectricity.Importantly,the variation trends of Eb and Ps under the same lattice strains are found to be highly different depending on the switching pathways.However,it is found that the compressive strains on the AB plane could effectively decrease Eb and keep high enough Ps,which verifies lattice strains as important methods for the ferroelectricity engineering of HfO2.The effects of oxygen defects on the ferroelectric properties of HfO2 are generally related to the field cycling behaviors.The three-coordinated oxygen vacancy(VO3),four-coordinated oxygen vacancy(VO4),oxygen interstitial(Oi),and oxygen Frenkel pair(OFP)are the possible defects considered in Hf0.5Zr0.5O2(HZO).Oi and OFP are the most stabilized and the second most stabilized defects among them.However,it is found that Oi could largely reduce Ps of HZO by 45%,which is attributed to the decreased ionic contribution.OFP and the oxygen vacancy could enhance Ps by an average of 15%due to the increased electronic contribution.The variations of Ps induced by oxygen defects are good reasons for different remanent polarization(Pr)observed in experiments as well as the enhanced Pr in the wake-up effect.However,the effects of oxygen vacancies on Ps should depend on the concentrations.High concentrations of oxygen vacancies will result in serious domain pinning and decreased remanent polarization.From experimental views,defects concentrations,ferroelectric properties,failure mechanisms,and endurance of HZO ferroelectric films are highly dependent on RTA temperature.The experimental results show that high RTA temperature could increase Pr and weaken the wake-up effects,but at the cost of high leakage current and poor endurance,which reveals the trade-off between Pr and endurance.The statistics of field-cycled capacitors from the fatigued state to the broken-down state present three failure behaviors related to the different changes of leakage current.The failed capacitors with abundant defects can be continually utilized as the resistive random memory,which indicates that the changes in leakage current may be induced by the redistribution and generation of defects at the grain boundaries and in the single grain.The time-dependent dielectric breakdown(TDDB)measurements show that the breakdown time(TBD)complies with the Weibull distribution.With lower RTA temperature,the initial defects concentration is lower and thus the device lifetime becomes longer.The Weibull slope of TBD becomes larger with decreased temperature,which implies that lowtemperature annealing could effectively improve the endurance and uniformity of HZO ferroelectric films.The polycrystal properties of HZO ferroelectric films restrict the domain propagation in the polarization switching process,which can be described by the nucleation limited switching(NLS)model.However,the feasibility of the NLS model and the variations in the average switching time and the concentration of pinning sites are not well addressed under different temperatures and device sizes.Considering the defects induced issues in HZO ferroelectric films,the intrinsic switching characteristics are measured accurately by using a novel pulse sequence eliminating the charge trapping,polarization unsaturation,and imprint effects.Although the average switching time decreases to a saturated value of 10-7s at low temperature,it decreases sharply with device size scaling and sub-ns switching behaviors are predicted in the sub-3.89μm device.The concentration of pinning sites increases with device size scaling and decreases to zero when the temperature is below 161 K.Besides,it is predicted that the concentration of pinning sites may have a sudden decrease to zero when the device size is approaching the grain size(10～30nm).The zero concentration of pinning sites implies that the NLS model will transform into the conventional Kolmogorov-Avrami-Ishibashi(KAI)model.This surprised but reasonable transition indicates that,as scaling the ferroelectric memory size or for operations under cryogenic temperatures,HZO ferroelectrics can switch much faster and more uniform.In conclusion,the polarization switching mechanisms and failure behaviors in HfO2-based ferroelectric thin films are investigated thoroughly by theory combined with experiments.It is found that the ferroelectric properties and reliability are highly dependent on the lattice strains,defect concentrations and RTA temperatures.Our researches provide scientific foundations for the preparations and developments of high-performance HfO2-based ferroelectric memory. |
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