Nanostructured bismuth strontium tantalate sol-gel derived xerogel
Ferroelectric perovskites have diverse applications in opto- and microelectronics, optical and photonic devices due to wide bandgaps, large electronoptical and nonlinear optical coefficients, high dielectric constants and spontaneous polarization. Less attention has been drawn to synthesis of nanotextured perovskites. One of the methods of fabrication of nanoteхtured materials is sol-gel synthesis in porous anodic alumina (PAA). The porous anodic alumina matrix reduces the grain size of the embedded composites, but the morphology and structure of many of them have not yet been investigated. Several perovskites, such as strontium titanate, bismuth strontium tantalate as well as nanoteхtured alumina, are radiative resistant that stimulate additional interest in sol-gel synthesis of perovskites in porous anodic alumina. There are several structural modifications of bismuth strontium tantalate (BST) perovskite for diverse methods of synthesis. In this work, we report on examinations of BST xerogel on monocrystalline silicon and PAA by scanning electron microscopy (SEM) and X-ray diffraction (XRD).
The XRD spectrum indicates the crystalline structure of the 7-layered xerogel film of90 nm thickness generated on monocrystalline silicon that results from thermal treatment at 700 °C and 800 °C (Fig. 1).
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Figure 1. XRD spectra of the BST xerogel formed on monocrystalline silicon annealed for 30 min at 700 °C (a) and 800 °C (b).
The spectra of the xerogels with different bismuth excess amounts and thermally treated atthe same temperatures are identical. The xerogel spectra reveal the presence of the dominant phase of Bi2SrTa2O9 (according to PDF 00‑049-0609) (Fig. 1a,b). The side phase of Bi2SiO5 (PDF 00-036-0288) is prominent for samples thermally treated at 700 °C(Fig. 1a) and it is not evident for the sample thermally treated at 800 °C(Fig. 1b). The Bi2SiO5 phase (PDF 00-036-0288) may be formed at the BST film/substrate interface as a result of interaction of the bismuth excess in the xerogel with oxidized silicon. Thus, this effect is not the result of the bismuth excess amount and is influenced only by the temperature of thermal treatment.
According to the SEM examination, the resultant PAA films consist of hexagonal cells of approximately 200 nm diameter with pore sizes of 90‑100 nm (Fig. 2). The brightlayer between PAA and the silicon substrate is the anodizedtantalum, with tantala protrusions slightly penetrating thepores at their bases due to expansion of the material volume duringanodizing. A thin anodic alumina barrier layer of approximately50 nm thickness covering the tantala protrusions is evidentin the micrographs. Deposition of five xerogel layers results in a uniform distribution of the sol-gel within the pore volume (Fig. 2a,b). The scanning electron micrographs recorded in BSD mode at 1 kV allow ready observation of the distribution of the xerogel due to Z-contrast with the PAA matrix. The xerogel species of brighter appearance form aggregates rather than fill uniformly the pore volume; the xerogel covers the pore walls and bases uniformly leaving the pore mouths open. Deposition of ten xerogel layers results in filling of the majority of the pore volume (Fig. 2c,d).
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Figure 2. Scanning electron images of porous anodic alumina film generated on monocrystalline silicon after annealing at 700 °C for 40 min: (a), (b) images after deposition of 5 xerogel layers; (c), (d) images after deposition of 10 xerogel layers; (a), (c) secondary and (b), (d) backscattered electron images.
Nanostructured SBT xerogels have been fabricated by the sol-gel route in PAA supported on monocrystalline silicon. The phase composition consists of the main phase of Bi2SrTa2O9 generated after annealing at 800 °C and the secondary phase of Bi2SiO5 observed after annealing at the relatively low temperature of 700 °C. The BST xerogel fills the entire pore volume of the porous anodic alumina with pores of 90-100 nmdiameter afterspinning of ten layers. Comparison of the phase composition of the BST xerogel generated on the monocrystalline silicon substrate and within nanosized pores of anodic alumina needs further investigation.