Takehiro Noguchi

Analysis of the Dependence of Ferroelectric Properties of Strontium Bismuth Tantalate (SBT) Thin Films on the Composition and Process Temperature

Ferroelectric properties, crystal structure and microstructures were examined for various Sr/Bi/Ta atomic ratio strontium bismuth tantalate (SBT) films prepared by metalorganic decomposition at 700 and 800° C. The 20% Sr-deficient and 10% Bi-excess (0.8/2.2/2) composition showed maximum remanent polarization (P r) values for both 700 and 800° C crystallization temperatures. From TEM analysis, the P r dependence on composition variation around the stoichiometric 1/2/2 composition was related to grain size and volume of voids. The effect of postannealing after Pt top electrode fabrication was also studied. On the films prepared at 800° C, postannealing markedly reduced the capacitor shorting rate. This was attributed to recrystallization of the Pt top layer, based on SEM analysis of the Pt layer and the Pt/SBT interface.

Analysis of the degradation of PZT and SrBi2Ta2O9 thin films with a reductive process

Abstract Electrical properties, crystal structure and composition were analyzed about ferroelectric PZT and SBT thin films with a reductive heat treatment. PZT films annealed in the atmosphere containing H2 turned into paraelectric, while SBT films were shorted. Ferroelectricity of these degraded films recovered by a subsequent anneal in oxygen. Oxygen content in PZT and SBT films decreased by the reductive heat treatment and was compensated by the subsequent anneal in oxygen. Residual hydrogen was also detected in reduced SBT films. The loss of ferroelectricity or short of capacitor would be due to the oxygen defects and the residual hydrogen in ferroelectric thin films. A TiN layer covering ferroelectric capacitors acted as an H2 barrier. Pt top electrode also played a role in preventing the reduction. This effect was enhanced by using a Pt electrode thicker than 300 nm and by anneal it at 600 °C.

Dielectric anomaly in strontium bismuth tantalate thin films

The authors have succeeded in observing the dielectric anomaly in ferroelectric strontium bismuth tantalate (SBT) thin films, and studied how it varies with film composition. For a stoichiometric SBT film, the dielectric anomaly was diffused, and the Curie temperature (Tc) was approximately 260 °C, which is lower than that for the bulk ceramic. A SBT film with excess Bi composition showed Tc of approximately 300 °C, and Tc for Sr deficient SBT films was higher than 340 °C. The composition dependence of remanent polarization and coercive field near room temperature for the SBT films reflects the composition dependence of the Tc values.

Imprint Characteristics of SrBi2Ta2O9 Thin Films with Modified Sr Composition

Imprint characteristics of SrBi2Ta2O9 (SBT) thin films with modified Sr composition were investigated under elevated temperatures. A significant shift in readout charge was observed after applying unipolar pulses (dynamic imprint test) at above 100°C, while only a small shift was detected even when the capacitors were held at 150°C for up to 104 s without any electrical signals (static imprint test). The charge shift was due to the internal voltage formed by applying the unipolar pulses. Its increase, that is, degradation of dynamic imprint endurance with increasing temperature is attributed to both an increase of the internal voltage and a decrease of the coercive voltage (Vc). There was no significant difference in the internal voltage between Sr deficient and Sr non-deficient SBT. SBT with a Sr deficient composition had a relatively high dynamic imprint endurance compared to the Sr non-deficient films because of higher remanent polarization and larger Vc.

Sr content dependence of ferroelectric properties in srbi2ta2o9 thin films

Abstract The effects of Sr deficiency to the ferroelectric properties and microstructure of SBT thin films prepared at 700 or 800 °C were investigated. The maximum 2Pr was obtained at Sr deficiency of 20% for both prepared at 700 °C and 800 °C. We obtained 12.5 μC/cm2 of 2Pr for the films with 20% Sr deficiency prepared at 700 °C. Films with 20% Sr deficiency showed the most serious polarization fatigue for films prepared at 800°C while capacitors prepared at 700 °C were in breakdown at around 106–107 cycles. XRD analysis revealed that the film orientation changed with Sr deficiency and that formation of a 2nd phase was enhanced by Sr deficiency. Both the orientation change and the 2nd phase were thought to be the origin of 2Pr decrease in the range from 20% to 60% Sr deficiency. The grain growth suppression and the void generation of the stoichiometric films were remarkably improved with decrease in Sr content from 1.0 to 0.8. This would contribute to the 2Pr increase with decrease in Sr content in the r...

Temperature dependence of ferroelectric properties of SrBi2Ta2O9 thin films

Abstract Temperature dependence of ferroelectric properties was studied on SrBi2Ta2O9 thin films with stoichiometric composition and Sr deficient and/or Bi excess compositions. Decreasing rate of remanent polarization Pr with increasing temperature was fairly small for the 20% Sr deficient compositions compared to the stoichiometric Sr content compositions. The large Pr decrease for the stoichiometric compositions was attributed to the increase of fast polarization relaxation with increasing temperature. Fatigue-free property was confirmed even at 150 °C for all compositions.

Fatigue characteristics of PZT capacitors with Ir/IrOx electrodes

Fatigue characteristics of PZT capacitors with Ir/IrOx electrodes were investigated. The top and bottom Ir/IrOx electrodes prevented PZT capacitors from polarization fatigue up to 10/sup 11/ cycles. This fact was explained by the assumption that the Ir/IrOx electrodes suppress the formation of two kinds of domain pinning sites at the PZT/electrode interfaces. These pinning sites would originate from oxygen vacancies. The PZT with Ti-rich composition would enhance the formation of the pinning sites.