G. D. Hu

Structure control and characterization of SrBi2Ta2O9 thin films by a modified annealing method

SrBi2Ta2O9 (SBT) ferroelectric thin films were prepared by metalorganic decomposition on Pt/Ti/SiO2/Si substrates at annealing temperatures ranging from 600 to 750 °C. The SBT thin films were annealed layer by layer during the spin-coating process using a rapid thermal annealing (RTA) furnace. The relative intensity of (200) peak in x-ray diffraction increased with the increase of the annealing temperature. A (200)-predominant film can be formed at 700 and 750 °C. For the film annealed by RTA furnace at 650 °C, the remanent polarization (2Pr) and coercive field (2Ec) were 19.8 μC/cm2 and 116 kV/cm, respectively.

Low-temperature preparation and characterization of SrBi2Ta2O9 thin films on (100)-oriented LaNiO3 electrodes

SrBi2Ta2O9 (SBT) ferroelectric thin films were prepared by the metalorganic decomposition technique at annealing temperatures of 600 and 650 °C on Pt/Ti/SiO2/Si substrates coated by (100)-oriented LaNiO3 (LNO) metal oxide thin films, which were fabricated by the sol–gel technique combined with a layer-by-layer annealing method at 600 °C. A (200)-predominant SBT thin film can be formed on LaNiO3(100)/Pt/Ti/SiO2/Si substrate at 600 °C. The effect of the LNO oxide electrode on the dielectric and ferroelectric properties of SBT thin film annealed at 600 °C was studied. Although the remanent polarization of the (200)-predominant SBT thin film is not as large as expected, the film can be uniformly polarized and imaged using an atomic force microscope in the piezoelectric mode.

Domain imaging and local piezoelectric properties of the (200)-predominant SrBi2Ta2O9 thin film

The domain structure of the (200)-predominant SrBi2Ta2O9 (SBT) thin film was detected by an atomic force microscope in the piezoelectric mode. It was found that the content of the grains split by single domain walls is less than 5%. The types of domain walls formed in individual grains were identified by analyzing the dependence of piezoelectric coefficient (d33) on the alternating current driving electric field. Several grains larger than 300 nm were found to be split by non-180° domain walls. To study the switching properties, the (200)-predominant SBT thin film was polarized and imaged over a large area. Unswitchable grains cannot be observed both in the area polarized using +8 V and in the region polarized using −8 V.

Effects of a Bi4Ti3O12 buffer layer on SrBi2Ta2O9 thin films prepared by the metalorganic decomposition

Ferroelectric SrBi2Ta2O9 thin films have been deposited on the Bi4Ti3O12 buffered Pt/Ti/SiO2/Si substrates using the metalorganic decomposition technique at annealing temperatures ranging from 600 to 750 °C. No pyrochlore phase was found in the SrBi2Ta2O9 thin films although the Bi2Ti2O7 phase appeared in the Bi4Ti3O12 buffer layers. A SrBi2Ta2O9 film with (200) predominant orientation was formed at 650 °C. The effects of the Bi4Ti3O12 buffer layer and post-annealing temperature on the structure, surface morphology, and electrical properties of SrBi2Ta2O9 thin films were analyzed.

Tip Effects of Piezoelectric-Mode Atomic Force Microscope for Local Piezoelectric Measurements of an SrBi2Ta2O9 Thin Film

Piezoelectric-mode atomic force microscope was utilized to characterize a polycrystalline SrBi2Ta2O9 thin film. It was found that a hysteresis loop can be obtained using a stiff tip with force constant of 48 N/m. On the other hand, the detected signal using a soft conducting tip with force constant of 3 N/m increases linearly with the increase of the applied dc electric field. In order to identify the types of the measured signals, we simulated a case, in which an ideal piezoelectric hysteresis loop is overlapped by a linear electrostatic signal. It was found that the coercive voltage of the electrostatic-force-coupled hysteresis loops decreases with the increase of the electrostatic signal. Furthermore, the electrostatic-force-coupled piezoresponse can not reach a saturated value. This suggests that the signal measured using the soft tip is an electrostatic predominant signal, while the signal detected using the stiff tip is a piezoelectric predominant response.

Preparation of (100)-Oriented LaNiO3 Oxide Electrodes for SrBi2Ta2O9-Based Ferroelectric Capacitors

LaNiO3 thin films were prepared on SiO2/Si and Pt/Ti/SiO2/Si substrates using a modified sol-gel technique. To decrease the annealing temperature, LaNiO3 thin films were annealed layer-by-layer in a rapid thermal annealing furnace. The effects of preheating temperature and annealing temperature on the crystallographic orientation and resistivity of LaNiO3 thin films were studied respectively. (100)-oriented LaNiO3 thin films can be formed on SiO2/Si and Pt/Ti/SiO2/Si substrates at an annealing temperature of 600°C. The typical room-temperature resistivity of the (100)-oriented LaNiO3 thin films deposited on SiO2/Si substrates is about 4.7×10-4 Ωcm. To verify the function of LaNiO3 thin films as the bottom electrodes of ferroelectric capacitors, SrBi2Ta2O9 thin films were fabricated on LaNiO3-coated Pt/Ti/SiO2/Si substrates by the metal organic decomposition technique. No evident fatigue was observed for an SrBi2Ta2O9 thin film annealed at 600°C, although its remanent polarization is not as large as that for the SrBi2Ta2O9 thin films deposited directly on Pt/Ti/SiO2/Si substrates.

Ellipsometric Spectra and Optical Properties of Anisotropic SrBi2Ta2O9 Films

Ellipsometric spectra of the SrBi2Ta2O9 (SBT) films of (200) and (0010) predominant orientation are measured and analysed in the range of photon energy from 2 to 5?eV. The results show that the oriented SBT films appear to be strongly anisotropic. The ellipsometric spectra of the (200)-predominant SBT films are different from those of the (0010)-predominant films. We suggest an analysis model for these oriented films and perform the fitting of the anisotropic ellipsometric spectra. The refractive index and the extinction coefficient of the ordinary ray and the extraordinary ray for these oriented SBT films are obtained. The ordinary refractive index is larger than the extraordinary one.

Synthesis and characterization of SrTiO/sub 3/ thin films by a modified metalorganic decomposition technique

High quality strontium titanate thin films on platinized Si(100) have been synthesized using non-poisonous inorganic solvent and a layer-by-layer annealing technique at different temperatures. The films have shown good structural and electrical properties. The dielectric constant and dissipation factor at a frequency of 100 MHz are 230 and 0.05, respectively, for a 80 nm thick film annealed at 650/spl deg/C. The capacitance versus applied voltage characteristic shows that the capacitance is almost independent of the applied voltage. The leakage current density is found to be in the order of 10/sup -7/ A/cm/sup 2/ for the film in an applied electric field of about 100 kV/cm.

Ellipsometric study of optical properties of oriented SBT thin films

Abstract The SrBi2Ta2O9 films of (200) and (0010) predominant orientation were prepared by metalorganic decomposition. Ellipsometric spectra of these oriented films were measured and analyzed in the range of photon energy from 2 to 5.5eV. The ellipsometric spectra of the (200)-predominant SBT films are different from those of the (0010)-predominant films. Basing on a simplified model, the effective refractive index n and the effective extinction coefficient k of both oriented SBT films were obtained. In the photon energy range of 2–5.5eV, the n value of the (0010)-predominant film is bigger than that of the (200)-predominant film (δn / 0.2 at 500nm). The band gap of the SBT thin films is found to be about 3.8eV.

Effects of a Bi/sub 4/Ti/sub 3/O/sub 12/ buffer layer on SrBi/sub 2/Ta/sub 2/O/sub 9/ thin films prepared by the metal organic solution deposition technique

Ferroelectric SrBi/sub 2/Ta/sub 2/O/sub 9/ (SBT) thin films with 20 mol% excess Bi content have been deposited on the Bi/sub 4/Ti/sub 3/O/sub 12/ (BTO) buffered Pt/Ti/SiO/sub 2//Si substrates using the metal organic solution deposition (MOSD) technique at annealing temperatures ranging from 650/spl deg/C to 750/spl deg/C. The BTO single buffer layer with different excess Bi content (0 mol%, 20 mol% and 40 mol%) was prepared by the same deposition method. No pyrochlore phase was found in the SBT thin films although the Bi/sub 2/Ti/sub 2/O/sub 7/ phase appeared in the BTO buffer layer. The effects of the BTO buffer layer and post annealing temperature on the dielectric and ferroelectric properties of SBT thin films were analyzed.