SrBi2(TaxNb1−x)2O9 (SBTN) layered structure oxides have been investigated for possible fatigue free ferroelectric thin film applications. High quality stoichiometric thin films of these oxides have been fabricated, across the solid solution range, using the technique of pulsed laser ablation SBTN thin films with a composition close to x=0.4 showed excellent ferroelectric properties and no fatigue up to 1010 switching cycles. Typically, these films showed a 2Pr value of 22 μC/cm2, an Ec of 60 kV/cm and a resistivity close to 5 × 1012 Ω-cm. The ferroelectric properties of these films were found to depend on the orientation. Predominantly c-axis oriented films have been fabricated on MgO (100)/Pt (100) substrates. By comparison, the c-axis oriented films were found to exhibit similar Pr values but significantly lower Ec values (by 15 kV/cm) and a higher resistivity. The fatigue free behavior of these layered structure oxides has been ascribed to the absence of any volatile species in their sublattice exhibiting polarization (i.e., the perovskite-like units in the layered structure).

Crystallization and texture formation of a number of layered perovskite thin films prepared by the MOD method has been investigated. The crystallization temperature increases with increasing complexity of the structure, rendering it more difficult to obtain c-axis oriented films. Texture formation can be facilitated by the introduction of a seeding layer, such as PbBi2Nb2O9, which has a somewhat lower crystallization temperature, and by multiple coatings, which allow successive crystallization. Grain size is also reduced by the above practice. These results are compared with similar findings in PZT thin films to conclude that texture development in layered perovskites is nucleation controlled.

Structural and electrical properties were investigated for chemically prepared SrBi2Ta2O9(SBT) thin films on Pt/Ti/SiO2/Si substrates. Good ferroelectric properties were obtained with a Pt top electrode: Pr=10.0μC/cm2 and Ec-34kV/cm. Au top electrodes resulted in smaller Pr. However, no fatigue was observed up to 109 switching cycles regardless of the top electrode material. Grains were spherical, not columnar, and the average grain size was 200nm. A marked structural change took place in the bottom Pt/Ti electrode during film preparation. The SIMS analysis indicates the reaction between Bi and Pt

Using pulsed laser deposition, Bi4Ti3O12 thin films were grown on (0001) and (1102) surfaces of Al2O3. Substrate temperature from 700 to 800 °C and oxygen pressure from 50 to 1000 mtorr were varied, and their effects on Bi4Ti3O12 film growth behavior was investigated. Only for a narrow range of deposition parameters, can highly oriented Bi4Ti3O12(104) films be grown on Al2O3(0001). Further, epitaxial BTO(004) films can be grown on Al2O3(1102). The growth behavior of preferential BTO film orientations can be explained in terms of atomic arrangements in the Bi4Ti3O12 and the Al2O3 planes.

Using La0.5Sro.5CoO3 bottom electrode layers, Bi4Ti3O12 thin films were grown on LaAlO3(001), Al2O3(0001), and Si(001) substrates. Crystalline orientation of the Bi4Ti3O12 thin films was examined by x-ray diffraction techniques. The cross-sectional microstructures of Bi4Ti3O12/La0.5Sr0.5CoO3 heterostructures are investigated. It is found that the crystalline orientation and the microstructure affect leakage current behavior of the Bi4Ti3O12 layers.

Ion beam sputter deposition using a Kaufman source is a well-established technique for fabricating thin films. This method has the capability for yielding smooth surfaces and depositing uniform films over a large area. We have used this technique, in the past, with a 5″ diameter target to deposit smooth ferroelectric bismuth titanate (Bi4Ti3O12) films. One of the major obstacles was fabricating the large target with the desired composition. To reduce cost by simplifying the target preparation process we developed a ‘mosaic’ target to deposit ferroelectric Bi4Ti3O12 films. Films were deposited at different substrate temperatures. Measurements show that the post-annealing of the deposited films results in measurable ferroelectric properties and enhanced low field dielectric behavior in the films. The discussion will include the target fabrication, the observed properties of the resultant films, and the effect of the deposition and post-annealing temperatures on the film properties.

Bismuth titanate thin films have been grown on (100) silicon substrate by atmospheric pressure metalorganic chemical vapour deposition (APMOCVD). The phase formation of Bi-Ti-0 films are dependent upon the composition, growth temperature and RTA post-annealing process. The film grown at 550°C, under optimized growth condition, is single crystal film with (100) orientation, while the (001) preffered oriented films were obtained after rapid thermal annealing. The orientation of Bi4Ti3O12 films mainly depends upon the lattice match with the substrate. At 550°C, the best lattice match is (100) of Bi4Ti3O12 with (100) of silicon. At higher temperature (>675°C), the Bi4Ti3O12 has a tetragonal structure and the best lattice match is (001) Bi4Ti3O12 with (100) of silicon due to the different thermal expansion coefficient of two materials.

Switchable polarization can be significantly suppressed in ferroelectric (FE) materials by optical, thermal, and electrical processes. The thermal process can occur by either annealing the FE in a reducing environment or by heating it in air to 100°C while impressing a bias near the switching threshold. The optical process occurs while biasing the FE near the switching threshold and illuminating with bandgap light. And the electrical suppression effect occurs by subjecting the FE to repeated polarization reversals. Using electron paramagnetic resonance, polarization-vol tage measurements, and charge injection scenarios, we have been able to elucidate both electronic and ionic trapping effects that lead to a suppression in the amount of switchable polarization in FE materials. The relative roles of electronic and ionic effects in the same material can depend on the stress condition. For instance, in oxidized BaTiO3 crystals, optical and thermal suppressions occur by electronic domain pinning; electrical fatigue in the BaTiO3 crystals also appears to involve electronic charge trapping, however, it is suggested that these electronic traps are further stabilized by nearby ionic defects. In sol-gel PZT thin films with either Pt, RuO2, or La-Sr-Co-O electrodes it appears that the polarization suppression induced by electrical fatigue, a temperature/bias combination, or a light/bias combination are all primarily due to the trapping of electronic charge carriers to first order.

La-Sr-Co-O(LSCO)/Pb-La-Zr-Ti-O(PLZT)/La-Sr-Co-O heterostructures with different crystalline quality have been grown on LaAlO3 and Si with a bismuth titanate template layer. Imprint of the heterostructures are investigated in comparison with fatigue and retention. Oxygen ambient during cooling the heterostructures was also varied to investigate the imprint. With consideration of the switching behavior caused by imprint, imprint behavior was deconvoluted from fatigue and retention. In epitaxial PLZT capacitors grown on LaAlO3, imprint behavior was found to be significantly dependent on the oxygen ambient. As the oxygen ambient became more reducing, the capacitors developed more voltage asymmetry in hysteresis loops and a more preferred polarization state directed towards the top electrode. On the other hand, oriented PLZT capacitors exhibited less pronounced effect of the oxygen ambient on the imprint behavior.

Imprint was observed for fatigued PZT (Zr/Ti = 53/47) capacitors which were subjected to a thermal cycle (at 80°C for a period of time). It was also observed for capacitors whose top electrode was defined by ion milling. In addition, capacitors with an asymmetric multilayered structure showed severe imprint problems. Based on the above observations, imprinting was categorized as:

1) Intrinsic imprint which is caused by inherent conditions such as device structure, configuration, and material composition.

2) Extrinsic imprint which is generated by external sources such as electrical stress, thermal stress, time, and processing.

Imprint mechanisms were discussed for each imprinting category. Most importantly, based on partial switching, a new memory logic was suggested. It is shown that stable imprinting may be useful for low density FRAM applications when the proposed new memory logic is applied. Reliability of the new logic was also shown to be significantly better than conventional operation.

The effects of electron beam irradiation on the electrical characteristics of Pt-PZT-Pt and Pt-PLZT-Pt (with La concentration of 5% and 10%) thin film capacitors were studied. The top electrode of each capacitor was bombarded by localized energetic electrons with incident energies varying from 5 keV to 25 ke.V. Large distortions in the hysteresis loops were observed when the incident electrons had enough energy to penetrate the 1000Å Pt top electrode. The damage effects are strongly dependent on the original domain orientation when the film is irradiated, and an applied a.c. field after the irradiation seems to induce some recovery effects. The damage mechanism is explained by a simple model which considers the effect of space charge fields generated by the charge carriers trapped at the domain boundaries.

Reactive ion etching damage to sputtered Pt/PZT/Pt ferroelectric capacitors was studied using Ar and CHCIFCF3 etch gases. Electrical properties, hysteresis, fatigue, and leakage current of PZT capacitors, before and after etching, were compared to examine the etching damage. It is found that the damage effects depend on etching time and are mainly due to the physical bombardment effect. The PZT capacitors etched with CHCIFCF3 plasma showed less damage than those etched in Ar plasma. The electric properties of etched Pt/PZT/Pt capacitors are recovered by annealing at 400 °C for 30min.

We demonstrate an epitaxial heterostructure of Ba0.5Sr0.5 TiO3/SrRuO3/YSZ on Si for potential charge storage applications. The dielectric Ba0.5Sr0.5TiO3 (BST) and conductive oxide SrRuO3 are both grown (110) oriented on YSZ (100) buffered Si by 90° off-axis sputtering. These films showed a high degree of crystallinity with minimal interdiffusion at the interfaces as examined by X-ray diffraction, Rutherford backscattering spectroscopy, and cross-section transmission electron microscopy. The in-plane epitaxial alignment of the films is BST/SRO 〈111〉 // YSZ 〈110〉 with a four-fold degeneracy. The dielectric constant and loss tangent of the epi-BST films are 360 and 0.01 at 10 kHz. The leakage current density is < 4×10∼−7 A/cm2 at 1 V. The room temperature dielectric constant (ε) of the BST films shows a roll-off in the 1–10 MHz range. This is attributed to the existence of a series resistance in the measurement circuit, which likely arises from the SrRuO3 electrode.

Ultrathin films of perovskite PbTiO3, 10–100nm thick, were epitaxially grown on miscut (001)SrTiO3 substrate by rf-magnetron sputtering at 600°C. The electron microscope and high resolution x-ray diffraction analysis suggested the evidence of epitaxial growth of (001)PbTiO3/(001)SrTiO3 with three dimensional crystal orientation. The stoichiometric film shows extremely smooth surface with the surface roughness less than 3nm. Deposition on a miscut substrate under stoichiometric conditions is essential to make continuous thin films of single crystal perovskite PbTiO3.

Pb(Zr,Ti)O3 (PZT) thin films are being developed for use in optical and electronic memory devices. To study ferroelectric switching behavior, we have produced relatively untextured PZT thin films on Si substrates. We have developed a method for using x-ray diffraction to observe domain switching in situ. Our study involved the use of a micro-diffractometer to monitor the switching behavior in relatively small (0.7mm diameter) electroded areas. Diffraction analyses were done while DC voltages were applied and removed, representing several places in the hysteresis loop. In particular, we were looking for relative intensity changes in the [h 00],[00l] diffraction peaks as a function of position in the hysteresis loop. Our study indicates that the 90° domain switching exhibited by bulk ferroelectrics, is very limited in films on Si when grain sizes are less than about 1 μm.

The migration of oxygen vacancies and their entrapment near film-electrode interfaces has been proposed as a cause of fatigue (i.e., polarization weakening) in ferroelectric thin film capacitors. To test this idea, lead zirconate titanate (PZT) thin films were epitaxially deposited by laser ablation on LaAlO3 substrates with yttrium barium cuprate (YBCO) base electrodes. Thin film capacitors were formed by deposition of noble metal (Pt) cap electrodes; half of them were then electrically fatigued by repeated polarization reversals (108 cycles). The distributions of oxygen in the two halves were then compared by means of accelerator-based nuclear backscattering (using the narrow elastic resonance at 3.045 MeV in the scattering of 4He from 16O) throughout the bulk of the PZT films and especially right under the Pt electrodes. We were unable to detect any difference in the oxygen profiles to within the accuracy of measurement, which was about 1 % of the oxygen concentration. Compositional changes, at least involving oxygen, do not seem to be responsible for the striking electrical alterations seen in fatigued PZT.

The band structure and energy levels of defects in BaTi3, PbTiO3 and PbZrO3 are calculated and compared. The band gap of Pb(Zr,Ti)O3 varies little because the band edges are formed of Pb 6s and 6p states. The charged Ti and O vacancies are found to be shallow. Transition metal impurities at the B site are found to give rise to numerous charge states because of their high intra-atomic electron repulsion. The levels of Cr4+, Fe4+ and Co4+ lie near midgap. Ti3+ trapped electron centers become deep at higher Zr contents in PZT.

Positron annihilation techniques have been applied to characterize vacancy-related defects in ferroelectric thin film structures. Variable energy positron beam measurements were carried out on doped and undoped Pb(Zr,Ti)O3 (PZT) samples subjected to different post-deposition cool down and anneal conditions. The PZT was deposited by sol-gel with either with platinum or RuO2 electrodes, or by laser ablation with La0.5Sr0.5CoO3 electrodes. The RuO2 and La0.5Sr0.5CoO3 electrode samples showed a smaller S-parameter compared to those deposited with Pt electrodes consistent with an improved PZT layer quality. For laser ablated samples cooled in a reducing ambient an increase in S-parameter for both the PZT and La0.5Sr0.5CoO3 layers was observed indicating an increase in neutral or negatively charged open-volume defects.

An impact of different factors on the shape of dielectric hysteresis loop in ferroelectric thin films is discussed. They include: polycrystallinity and disorientation of polarization axis, polarization nonuniformity and depolarization fields, interface contact layer and series capacity, contact potential difference and interaction of volume impurity centers with polarization field. A comparison between models and experiment show that compromise of interface layer and space charge permits to explain all main distortions of dielectric hysteresis loop in thin films at realistic parameters of the layers. The obtained results permit to explaine the distortions of real hysteresis loop and also to obtain quality parameters of samples, which are important for improving of film preparation techniques.

In the present paper three mechanisms of self-polarization of ferroelectric crystals are discussed: polarization by the crystal bending, that by the “organized” Phase transition and that by the internal field. It is shown that these mechanisms operate also in the case of films. However, the using of cathode sputtering leads to the appearance of two new mechanisms: polarization by the field biasing the substrate with respect to the gas discharge at the film deposition temperature Ts below the Curie point temperature Tc (Ts < TO; and polarization by the field captured by the traps of the negative charge having no time to relax for the time of cooling the film to Tc (Ts > Tc).