Jingsong Liu

Domain evolution in ferroelectric thin films during fatigue process

A combination of piezoresponse force microscopy and x-ray diffraction sin2Ψ stress measurement have been used to investigate domain evolutions in (111)-oriented Pb(Zr0.55Ti0.45)O3 (PZT) and (115)-oriented SrBi2Ta2O9 (SBT) thin films with the repeated switching cycles (fatigue process). In the PZT film, it has been found that 90° a-c domain walls form at the beginning of fatigue process and increase in amount with accumulation of switching cycles, accompanied by increase of stress. However, domain pattern and stress in the SBT film seem to be unchanged as the fatigue process is going on. The increase in stress with the repeated switching cycles in the PZT film can be attributed to the lattice distortion caused by polarization rotation. On the contrary, polarization switching in SBT involves little lattice distortion and lets it in a nearly stress-free status during the fatigue process. The difference in domain evolution during the fatigue process suggests the important role of stress in fatigue behaviors o...

The Study of Ferroelectric La-Doped PbTiO3 Thin Films Prepared by RF Magnetron Sputtering

Ferroelectric La-modified lead titanate (PLT) thin film were grown on Pt/Ti/SiO2/Si by sputtering the Pb0.93La0.07TiO3 targets containing an amounts of excess 8% PbO. The effects of sputtering and annealing conditions on the crystalline structures and the surface morphologies of the PLT thin films have been investigated. The remanent polarization (Pr) and the coercive field of the PLT film through rapid thermal annealing (RTA) was 10.2 μC/cm2 and 45 kV/cm respectively. The maximum pyroelectric coefficient reached 19 nC/cm2.K at 20°C.

GROWTH AND CHARACTERIZATION OF PZT FILMS WITH DIFFERENT GRAIN SIZES

ABSTRACT Pb(Zr0.55Ti0.45)O3 (PZT) thin films with different grain size have been fabricated by controlling the post-annealing process. PZT thin films nucleated at 550°C and the grains began growing above 650°C. Long nucleation process time but relatively short growth process time resulted in small grain. In contrast, long growth process time but short nucleation process time resulted in large grain size. PZT thin films with different grain size show different electrical properties. The polarization increases with the increase of grain size. However, the coercive field exhibites a paraboliclike relationship with grain size. The coercive field increases with the increase of grain size below 70 nm, whereas decreases as the grain size increases above 70 nm. The grain size of 70 nm is thought to be the critical size for the domain structure transition. While increasing the grain size, PZT thin films experience a transition from single domain state to multi domain state, which is related to the polarization switching. It is well known that the polarization of ferroelectric thin films with multi domain structure switches much easier than that with single domain structure. Obviously, the coercive field dependence on the grain size is interpreted.

Investigation of the crystal structure and electrical properties of La3+-doped SrBi2Ta2O9 ceramics

The layered-perovskite ferroelectric ceramics of La3+-doped SrBi2Ta2O2 (SBT), with the chemical formula of SrBi(2 - x)LaxTa2O9 (SBLT), have been prepared by the conventional mixed-oxide method. The effect of substitution of La3+ for Bi3+ in the crystal structure and electrical properties of SBT ceramics was explored with the aid of X-ray diffraction, ε(T) curve and ferroelectric hysteresis loop measurements. The electrical properties such as dielectric constant (ε) and remanent polarization (Pr) showed a dependence on the crystal structure, and both reached maxima of 243 and 25 μC cm−2, respectively, with 6 at % La3+ substitution, accompanying the greatest structure change. Theoretical considerations were presented to suggest that the atomic displacements and the crystal deformation implied by the crystal structure change are responsible for the improvement of electrical properties. On the other hand, degradation of fatigue resistance was observed in SBLT ceramics, which is believed to be caused by the chemical environment change of the perovskite layers arising from La3+ substitution on Bi3+ sites.

90° switching of polarization in La3+-doped SrBi2Ta2O9 thin films

The crystal structure and polarization switching behavior of SrBi1.4La0.6Ta2O9 (SBLT) thin films have been studied by x-ray diffraction and piezoresponse force microscopy (PFM), respectively. Compared with SrBi2Ta2O9 (SBT), SBLT thin films show a reduced orthorhombic distortion. The polarization rotation of SBLT thin film, which is driven by negative and positive direct current (dc) biases, has been investigated by a combination of vertical and lateral PFM (VPFM and LPFM, respectively). After dc bias applications, the VPFM image is hardly changed, whereas the LPFM image experiences an obvious variation. It is believed that such difference is caused by 90° polarization switching. However, this kind of switching can be only realized by the exchange of a axis and b axis. By virtue of the reduced orthorhombic distortion, the a-b exchange in SBLT is easier than that in SBT. Unfortunately, stress is created due to the 90° polarization switching in SBLT thin films. The internal stress is found to increase with t...

Ferroelectric Property Dependence on the Texture of SrBi2Ta2O9 Thin Films

ABSTRACT SrBi2Ta2O9 (SBT) thin films with different textures were fabricated by pulsed laser deposition under different deposition conditions. The influence of the deposition condition on the texture of the SBT films can be explained by the interactions between the plume and the substrate. With the aid of x-ray diffractometry, piezoreponse scanning probe microscopy, and ferroelectric-property measurements, a correlation between the film texture, as well as the domain structure, and the ferroelectric properties was established. It was found that the ferroelectrics properties, such as the polarization and the coercive field, had a strong dependence on the texture of the SBT films. Whats more, 90° domains were observed in the (115)-textured SBT films. These results are of interest for the fabrication of ferroelectric memories with fatigue free SBT thin films.

FABRICATION AND ELECTRICAL CHARACTERISTICS OF A METAL-FERROELECTRIC-POLYSILICON-INSULATOR-Si FIELD EFFECT TRANSISTOR

ABSTRACT A p-channel metal/ferroelectric/polycrystalline silicon/insulator/silicon structure field effect transistor (FET) with a Pb(Zr0.52Ti0.48)O3 ferroelectric layer has been proposed and demonstrated. The Pb(Zr0.52Ti0.48)O3 ferroelectric layer (200 nm) was deposited by radio frequency magnetron sputtering. The counterclockwise capacitance-voltage (C-V) characteristics of the capacitor and the clockwise drain current-gate voltage (I d -V g ) characteristics of the p-channel ferroelectric FET demonstrate that the device can realize a memory effect due to the ferroelectric polarization of the Pb(Zr0.52Ti0.48)O3 thin film. The size of the memory windows has been investigated. The memory widow measured from C-V and I d -V g curves of the device are consistent.

CONTRIBUTION OF DOMAIN MOTION TO THE DIELECTRIC PROPERTIES OF FERROELECTRIC THIN FILMS

ABSTRACT The variations of polarization and dielectric properties with the grain size of PZT films have been investigated experimentally and theoretically. It is revealed that polarization increases much more quickly with the grain size than dielectric constant does. The polarization increases as the grain size increases and is saturated when the grain size is about 60 nm, whereas the dielectric constant is saturated when the grain size is about 70 nm. Because the dielectric response of ferroelectrics is affected by the domain switching and exhibits hysteresis, the contribution of domain dynamics to the dielectric properties must be considered. Varying grain size will change the domain dynamics due to the close relationship of domain width and grain size. Based on the Landau-Ginzburg (LG) model, the variations of remnant polarization and dielectric constant against grain sizes have been investigated, considering the contribution of domain motion. The calculation result is consistent with the experimental result.

Fabrication of Ferroelectric Thin Films With Alternate Deposition of Multilayers

Pb(Zr0.52Ti0.48)O3 (PZT) and SrBi2Ta2O9 (SBT) films have been fabricated onto Pt/Ti/SiO2/Si substrates with a new chemical solution deposition (CSD) technique, in which PbZrO3(PZ)/PbTiO3(PT) and SrTa2O6(ST)/Bi2O3(B) precursors were spin-coated and heated individually. Polycrystalline PZT and SBT films were formed through the reactions between the multilayers at relative low temperatures, without post-annealing. With the aid of AFM, an island-column hybrid growth mode and a nucleus growth mode were suggested for the PZT and SBT film growths, respectively. However, it was found that the deposition orders affected substantially the microstructure of SBT films. This study suggests that the processing temperatures for the ferroelectric film fabrications can be reduced with alternate deposition of multilayers.