The ferroelectric and antiferroelectric properties of ZrO2 ultrathin films (~12 nm in thickness) prepared by atomic layer deposition (ALD) were tailored by introducing sub-nanometer interfacial layers between the ZrO2 ultrathin film and top and bottom Pt electrodes. In terms of polarization switching ability, the ferroelectricity of ZrO2 ultrathin films was significantly enhanced by an HfO2 interfacial layer (i.e., a Pt/HfO2/ZrO2/HfO2/Pt layered arrangement). While, a TiO2 interfacial layer (i.e., a Pt/TiO2/ZrO2/TiO2/Pt layered arrangement) led to a transition from ferroelectricity to antiferroelectricity. The modulation of ferroelectricity and antiferroelectricity of ZrO2 ultrathin films by the interfacial layers can be achieved without post-annealing.
Wurtzite aluminum nitride (AlN) of space group P63mc has long been recognized as a non-ferroelectric material, lacking the polarization switching ability. This paper reports the induction of ferroelectricity in a single crystalline epitaxial AlN ultrathin film with a thickness of 8−10 nm. The ferroelectric AlN epilayer was grown on a single crystalline GaN layer, forming a [0001]-oriented AlN/GaN epitaxial heterostructure with two reversible polar variants: [000-1] and [0001]. The AlN epilayer exhibited soft ferroelectricity with large switching currents and a polarization value of ~3.0 μCcm-2 during a 180° polarization switch. The AlN epilayer was prepared by the atomic layer deposition technique at 300°C in conjunction with in-situ atomic layer annealing. The two-dimensional electron gas (2DEG) at the AlN/GaN interface could be manipulated by the ferroelectric switching in the AlN epilayer. Strain engineering via lattice mismatch at the AlN/GaN interface was the key to creating a ferroelectric AlN/GaN heterojunction. Based on the reciprocal space mapping analysis, the AlN ferroelectricity is believed to be stemming from the out-of-plane compressive strain and inplane tensile strain present in the [0001]-oriented AlN epilayer. The discovery of low-temperature prepared, CMOScompatible AlN ultrathin films with soft ferroelectric characteristics will undoubtedly spur new fundamental and applied research in low-dimensional ferroelectric systems based on the AlN/GaN heterojunction.
This study investigates the microstructure, optical absorption and photoelectric properties of nanostructured composites of TiO2 nanotube arrays and SrTiO3 or CeO2 nanoparticles. The composites were fabricated by anodization and hydrothermal methods and their UV-visible and ultraviolet photoelectron spectra (UV-Vis and UPS) were measured to determine the band structures of the TiO2-SrTiO3 and TiO2-CeO2 heterojunctions. The heterojunctions are designed to promote the separation of photo-induced electron and hole (e-/h+) pairs when the nanostructured composites are adopted in photocatalytic or photoelectrode applications. Approximately 1.0 and 0.8 eV shifts in conduction band position were determined for the TiO2-SrTiO3 and TiO2-CeO2 heterojunctions, respectively. The photocurrent densities of the TiO2- SrTiO3 and TiO2-CeO2 composites were about 20 to 40% larger than that of the TiO2 nanotube arrays under identical irradiation conditions. The size of the SrTiO3 and CeO2 nanoparticles, which could be controlled by the hydrothermal temperature and time, and the concentration of oxygen vacancies within the TiO2 nanotubes were identified to be the key factors governing the photocurrent densities of the nanostructured composites.
The polarization switching characteristics of lead-free a(Bi1/2Na1/2)TiO3−bBaTiO3−c(Bi1/2K1/2)TiO3 (abbreviated as
BNBK 100a/100b/100c) ferroelectric ceramics are investigated. This is achieved through examining their polarization
and strain hystereses inside and outside the morphotropic phase boundary (MPB). The total induced electrostrain (&egr;33,total)
and apparent piezoelectric charge coefficient (d33) first increase dramatically and then decrease gradually as the BNBK
composition moves from the tetragonal phase to the MPB and then to the rhombohedral phase. The measured
polarization hystereses indicate that the BNBK compositions situated near the rhombohedral side of the MPB typically
possess higher coercive field (Ec) and remanent polarization (Pr), while the compositions situated near the tetragonal side
of the MPB possess higher apparent permittivity. Adverse effects on the ferroelectric properties are observed when
BNBK is doped with donor dopants such as La and Nb. On the contrary, intricate hysteresis behaviors are observed when
acceptor dopant Mn is introduced into BNBK. Under an alternating electric field of ±5.0 MVm-1, BNBK 85.4/2.6/12, a
composition well within the MPB, exhibits an &egr;33,total of ~0.14%, an apparent d33 of 295 pCN-1, an Ec of 2.5 MVm-1 and a
Pr of 22.5 &mgr;Ccm-2. These notable ferroelectric property values suggest a candidate material for lead-free actuator
applications. The present study provides a systematic set of hysteresis measurements which can be used to characterize
the switching behaviors of BNBK-based lead-free ferroelectrics.
Samples of soft PZT-5H, hard PZT-4D and Barium Titanate were subjected to multi-axial loading in stress and electric field space. The loading paths were: (1) Poling with electric field, followed by repolarizing with electric field at an angle to the original poling direction. (2) Proportional loading with electric field and coaxial compressive stress, the proportions of stress and electric field being varied between tests. In case (1) the poled material was cut to produce faces angled to the original poling direction. The measured material responses are reported and initial switching surfaces are calculated based on an offset from linear response in electric displacement. The measurements are used to assess the features required in micromechanical or phenomenological models of switching.
Fatigue crack growth is studied in PZT-5H and PLZT 8/65/35 ferroelectric ceramics under purely electrical cyclic loading. The growing cracks are intergranular and exhibit features such as bifurcation, tunneling, arrest and bursts of rapid growth. Two ferroelectrics show different cracking behaviors once the crack growth commences - the crack growth rate in PZT-5H decreases with the number of cycles, whereas for PLZT 8/65/35, a period of rapid steady-state crack growth is typical. A band of damaged material forms around the crack and propagates through the ceramic. The thickness of this band of damaged material is directly proportional to the strength of the applied electric field. Crack growth measurements are presented for two ferroelectric compositions under varying load amplitude and with varying frequency and test geometry. Potential mechanisms underlying the fatigue behavior include cracking at local stress concentrations due to inhomogeneity at the grain length scale, a wedging mechanism in the crack wake, the action of the crack as a field intensifier, and the degradation of a actuating mechanism.
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