Microstructural characterization of sol–gel lead–zirconate–titanate thin films

Abstract

The techniques of x-ray diffraction, x-ray photoelectron spectroscopy (XPS), Auger analysis, and transmission and scanning transmission electron microscopy (TEM) have been applied to the analysis of thin films of Pb(Zr0.30Ti0.70)O3 (PZT30/70) deposited at low temperatures (510 °C) by a sol–gel process onto Pt/Ti electrodes on SiO2/Si 100 substrates. It is found that the platinum film is highly oriented with the [111] axis perpendicular to the substrate plane. The ferroelectric film tends to crystallize epitaxially upon this as columnar crystals. There are indications from the TEM of the existence of a second metallic phase at the interface between the platinum and the PZT30/70 film, which may be associated with its nucleation. The TEM shows the boundaries between the individual sol–gel layers, although the growing crystallites of the PZT30/70 propagate through these boundaries unhindered. The XPS and Auger analyses have shown that Pb penetrates through the Pt layer to the underlying Ti layer, even at the low crystallization temperatures used. There is also clear evidence for diffusion of the Zr and Ti prior to, or during the crystallization process, so that the Zr migrates to the surface of each sol–gel layer. The effects of using different crystallization processes on this compositional separation and the reasons for its occurrence are discussed, as are the possible effects upon macroscopically measured ferroelectric properties.

Compositional and Optical Gradient in Films of PbZrxTi1-xO3 (PZT) Family

“…The spontaneous polarization allows the cations to diffuse faster and is the reason why surface enrichment is so significant in ferroelectric films (Watts et al, 2005). The ferroelectric (FE) polarization induced electrochemically by this mechanism is in the direction observed experimentally by Impey et al, 1998, and. Pb 2+ diffusion may also lead to self-polarization, which causes the polarization inhomogeneity discussed above.…”

“…Surface enrichment of some elements has been reported by many authors (Impey et al, 1998, Watts et al, 2001, 2003Gusmano et al, 2002), and there are just few explanations for this phenomenon. An analogy may be drawn with the oxidation of metals such as Cu and Sn where the metals dif-fuse towards the reacting surface (Wagner, 1971;Cabrera and Mott, 1948).…”

“…8b that the Zr/Ti ratio decreases directionally from the substrate to the surface, which is opposite to the observations, e.g., of Ledermann by TEM. However, it is known that sol-gel thin films may have higher concentrations of Pb at the surface (Impey et al, 1998;Ledermann et al, 2003;Watts et al, 2005).…”

“…On the other hand, it was reported that the n increases with increasing Ti/Zr concentration (Tang et al, 2007;Yang et al, 2006). It is likely that the appearance of the depth profile for the LCI films is connected with the fact that PbTiO 3 (PTO) crystallizes before PbZrO 3 (PZO) (Impey et al, 1998), while crystallizing layers together may avoid preferential PTO and PZO crystallization. Better quality PZT 52/48 composition thin films can be made by annealing the films at higher temperatures using rapid thermal annealing (RTA) or oven, or to have a different Zr/Ti concentration ratio in each layer with the goal to anticipate the selection and diffusion processes (Calamea and Muralt, 2007).…”

“…• Thermodynamically driven diffusion and/or kinetic demixing (Cabrera, and Mott, 1948;Impey et al, 1998;Ohba et al, 1994;Wagner, 1971;Watts et al, 2001 and; • Stress (e.g., lattice mismatch and misfit stress with the substrate; stress dependence of film thickness), (Corkovic et al, 2008;Izyumskaya et al, 2007;Gkotsis et al, 2007); • Nucleation processes (Izyumskaya et al, 2007;Ohba et al, 1994;. Depending on deposition processes involved, some or even all of these factors can be incorporated and accountable for gradient formation in the films.…”