Gerd Norga

Ultra-Thin PZT Films for Low-Voltage Ferroelectric Non-Volatile Memories

For the possible application of PZT thin films in ferroelectric non-volatile memories, the potential of low-voltage switching is a major requirement. By straightforward thickness scaling down to 75nm, operation voltage can be reduced to 1.5V without degradation of the hysteresis properties. However, interface effects increase the coercive field for thinner films, so that operation voltage does not scale linearly with thickness. In view of the large difference in coercive field between bulk and thin-films, on the other hand, material (and interface) optimization may constitute an interesting alternative route for further voltage scaling. By optimizing the composition and stoichiometry, less than 1.5V switching has indeed been obtained even for 150nm films. It is argued that an important material parameter affecting low-voltage switching behavior is the ferroelectric domain configuration and presence/absence of mobile domain walls in the PZT film.

Role of fluorite phase formation in the texture selection of sol-gel-prepared Pb(Zr 1− x , Ti x )O 3 films on Pt electrode layers

Transmission electron microscopy-selected area electron diffraction studies were performed on as-pyrolyzed sol-gel-derived Pb(Zr 1 - x , Ti x )O 3 films deposited on Pt electrode layers to elucidate the structural cause behind the large effects of pyrolysis conditions on orientation selection [L. Fe, G.J. Norga, H.E. Maes, and G. Maes, J. Mater. Res. 16, 2499 (2001)]. The crystallinity of the intermediate pyrochlore phase, which forms during pyrolysis, was found to strongly depend on pyrolysis conditions. Specifically, pyrolysis for 10 s at 350 °C was seen to result in conversion of the film to a well-crystallized, oxygen-deficient pyrochlore phase with the fluorite crystal structure (disordered pyrochlore). We speculate that formation of the metastable fluorite phase is favored by the reduced oxygen partial pressure, caused by burnoff of residual acetates. Longer pyrolysis times and/or higher pyrolysis temperatures result in a quasi-amorphous intermediate phase. The presence of the well-crystallized fluorite phase in the pyrolyzed film is seen to result in (11 1 )-oriented films after crystallization, while pyrolyzed films consisting of a quasi-amorphous intermediate phase turn out (100) or mixed (100)/(111) oriented after crystallization. An explanation for the observed orientation effects, on the basis of the different surface energetics of fluorite versus perovskite structure oxides, is proposed.

Effect of RuO2 growth temperature on ferroelectric properties of RuO2/Pb(Zr, Ti)O3/RuO2/Pt capacitors

We present a promising method for obtaining Pb(Zr, Ti)O3(PZT) layers with excellent endurance and pulse-switching properties on RuO2 electrodes using the sol–gel method. As the substrate temperature during reactive sputtering of the RuO2 bottom electrode layer is reduced, the (111) PZT texture component becomes more pronounced, an effect attributed to the change from columnar to granular RuO2 film morphology. Reducing the residual PZT (100) and (101) texture components was found to be a necessary condition for obtaining optimal pulse switching and endurance properties of the layers. Highly (111)-oriented PZT layers, obtained on RuO2 grown at 150 °C exhibit a net switched charge of >60 μC/cm2 during pulse measurement and <10% degradation after 1011 fatigue cycles.

Absorption-Reflection Infrared Spectroscopy Studies of Sol-Gel Prepared Ferroelectric Pb(Zr,Ti)O3 Thin Films on Pt Electrodes

Although the sol-gel method is ideally suited for the preparation of ferroelectric PZT thin films, poor reproducibility and the need to lower crystallization temperatures remain an issue. To address these problems, we have studied the mechanism of thin film formation using absorption-reflection infrared spectroscopy for a novel and less harmful precursor solution system based on butoxyethanol. By recording in situ infrared spectra we were able to monitor hydrolysis, condensation, decomposition and crystallization phenomena versus temperature. We speculate that the lower reactivity of butoxyethanol is responsible for the higher quality of Ti rich PZT based ferroelectric capacitors prepared by a butoxyethanol precursor solution compared with the more widely used methoxyethanol one. Finally, we observed that films decomposition kinetics is faster as compared to bulk samples and depends on film thickness and electrode layer.

Orientation selection in functional oxide thin films

Abstract Mechanisms for orientation selection in complex oxide thin films are discussed, focusing on two examples: (1) development of a preferential texture in sol-gel prepared polycrystalline PZT films on Pt electrode layers and (2) selection of oxides for epitaxial growth on Si (111) and (100). In sol-gel PZT, a direct link was found between the formation of a well crystallized transient fluorite phase, promoted by reducing pyrolysis conditions, and the emergence of a strongly preferred (111) texture after crystallization. Meanwhile, in the MBE growth of epitaxial oxides on silicon, matching of silicon and oxide surface lattices is not a sufficient condition to achieve layer-by-layer growth, needed for planar films. In fact, for planar growth on Si(111), fluorite-structure compounds are required, while rocksalt and perovskite structure oxides are suited for the growth of low-roughness films on Si(100). These examples illustrate the important role played by surface energy in orientation selection.

Bottom electrode and barrier materials issues in stacked capacitor type ferroelectric memories

Abstract A number of basic materials issues need to be solved to successfully realize the stacked ferroelectric capacitor cell concept, which is needed for FERAM with densities beyond about 1Mb. This paper discusses barrier materials selection and the role of bottom electrode microstructure for future PZT-based FERAM based on the stacked cell concept. Conventional methods for PZT texture control, employing Pt as a template layer, are less suited for stacked cell layouts, because of the poor diffusion barrier properties of Pt. Modification of the microstructure of RuO2 bottom electrodes has allowed us to eliminate the effect of underlying layers on the texture of PZT. The advantages of this approach in view of increasing circuit density of PZT-based FERAMs are discussed.

Explanation of the non-saturating behavior of the hysteresis loop based on the relaxation current

Abstract The hysteresis loop of ferroelectric thin film capacitors, shows sometimes non-saturation effects, i.e. the high electric field parts of the hysteresis loop do not close and the apparent remanent polarization increases with the amplitude of the measurement signal. Several models attributed this effect to the presence of leakage current, but the very high current levels predicted have not been always confirmed by the direct measurements of the DC leakage. Here, we show that the relaxation current, rather than the true DC leakage current can, in these cases, explains the non-saturating behavior. This effect would than not be caused by the movement of free charges, but rather to of the localized ones.

Effect of crystallisation on fatigue in sol-gel PZT ferroelectric capacitors with reactively sputtered RuO/sub 2/ electrode layers

Two new approaches for improving fatigue performance of sol-gel PZT based ferroelectric capacitors with RuO/sub 2/ electrodes are discussed. First, minimizing the time delay between RuO/sub 2/ sputtering and sol-gel PZT spinning increases the remanent polarisation of the crystallised PZT. Second, the use of thin (/spl sim/5 nm) high-Ti PZT seed layer was found to be effective for improving fatigue performance. For an optimized seedlayer thickness, FECAPs with excellent fatigue characteristics (less than 10% decrease in P/sub r/ after 10/sup 11/ cycles) were obtained.

Quantitative determination of the dielectric constant of the interfacial layer in PZT ferroelectric capacitors

The crystallization of PbZr/sub 0.2/Ti/sub 0.8/O/sub 3/ thin films was investigated by varying the crystallization temperature and time. Some of the films were not fully crystallized presenting amorphous top layers. The films have been structurally characterized by XTEM and XRD, and electrically characterized by hysteresis and CV measurements. The evolution of orientation and crystallization is observed for the different conditions and is correlated with hysteresis loops. The dielectric constant is determined for both the amorphous layers and the crystalline phase, and the CV behavior is explained.

PLZT: Precursor characterization and ferroelectric properties

Abstract PZT and PLZT precursor solutions were prepared by a sol-gel route, varying the Zr/Ti ratio and the La doping level. They were characterized by TGA coupled with FT-IR. Reactions taking place during heat treatments are discussed. Composition of PZT and PLZT thin films was measured by RBS and their orientation determined by XRD. Ferroelectric capacitors with Pt electrodes were characterized by hysteresis and fatigue measurements.