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Register a new userMechanism of Polarization Fatigue in BiFeO3: the Role of Schottky Barrier
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By using piezoelectric force microscopy and scanning Kelvin probe microscopy, we have investigated the domain evolution and space charge distribution in planar BiFeO3 capacitors with different electrodes. It is observed that charge injection at the film/electrode interface leads to domain pinning and polarization fatigue in BiFeO3. 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.
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As a room-temperature multiferroic, BiFeO3 has been intensively investigated for both magnetoelectric devices and non-volatile ferroelectric memory applications. Both magnetoelectric and ferroelectric memory devices have the same control knob: polarization switching by an applied electric field. Due to the rhombohedral symmetry of BiFeO3, there are four ferroelastic variances and three different polarization switching events: (1) 71{deg} switching from r1- to r3+, (2) 109{deg} switching from r1- to r2+ (or r4+), and (3) 180o switching from r1- to r1+ (the superscript + and - stand for up and down polarization, respectively). Each switching path is coupled to a different reorientation of the BiFeO3 unit cell, and hence different coupling to the magnetic order as well as different magnitudes of switchable polarization. A degradation of the ferroelectric properties of BiFeO3 will result in losing controllability of magnetic order switching in magnetoelectric devices and capacity for information storage in ferroelectric memory devices. Especially, polarization fatigue will directly restrict the reliability of the actual devices. Hence it is important to understand the intrinsic fatigue behavior of each polarization switching path in BiFeO3 thin films. In this communication, we report polarization fatigue in BiFeO3 depending on switching path, and propose a fatigue model which will broaden our understanding of the fatigue phenomenon in low-symmetry materials.
We have studied the polarization fatigue of La and Mg co-substituted BiFeO3 thin film, where a polarization peak is observed during the fatigue process. The origin of such anomalous behavior is analyzed on the basis of the defect evolution using temperature-dependent impedance spectroscopy. It shows that the motion of oxygen vacancies (VO..) is associated with a lower energy barrier, accompanied by the injection of electrons into the film during the fatigue process. A qualitative model is proposed to explain the fatigue behavior, which involves the modification of the Schottky barrier upon the accumulation of VO.. at the metal-dielectric interface.
Fatigue failure in ferroelectrics has been intensively investigated in the past few decades. Most of the mechanisms discussed for ferroelectric fatigue have been built on the hypothesis of variation in charged defects, which however are rarely evidenced by experimental observation. Here, using a combination of complex impedance spectra techniques, piezoresponse force microscopy and first-principles theory, we examine the microscopic evolution and redistribution of charged defects during the electrical cycling in BiFeO3 thin films. The dynamic formation and melting behaviors of oxygen vacancy (VO) order are identified during the fatigue process. It reveals that the isolated VO tend to self-order along grain boundaries to form a planar-aligned structure, which blocks the domain reversals. Upon further electrical cycling, migration of VO within vacancy clusters is accommodated with a lower energy barrier (~0.2 eV) and facilitates the formation of nearby-electrode layer incorporated with highly concentrated VO. The interplay between the macroscopic fatigue and microscopic evolution of charged defects clearly demonstrates the role of ordered VO cluster in the fatigue failure of BiFeO3 thin films.
Statistical distribution of switching times is a key information necessary to describe the dynamic response of a polycrystalline bulk ferroelectric to an applied electric field. The Inhomogeneous Field Mechanism (IFM) model offers a useful tool which allows extraction of this information from polarization switching measurements over a large time window. In this paper, the model was further developed to account for the presence of non-switchable regions in fatigued materials. Application of the IFM- analysis to bipolar electric cycling induced fatigue process of various lead-based and lead-free ferroelectric ceramics reveals different scenarios of property degradation. Insight is gained into different underlying fatigue mechanisms inherent to the investigated systems.
Electron spin polarizations of 32% are obtained in a GaAs quantum well via electrical injection through a reverse-biased Fe/AlGaAs Schottky contact. An analysis of the transport data using the Rowell criteria demonstrates that single step tunneling is the dominant transport mechanism. The current-voltage data show a clear zero-bias anomaly and phonon signatures corresponding to the GaAs-like and AlAs-like longitudinal-optical phonon modes of the AlGaAs barrier, providing further evidence for tunneling. These results provide experimental confirmation of several theoretical analyses indicating that tunneling enables significant spin injection from a metal into a semiconductor.
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