Microstructural development and crystallization behavior of sol-gel derived barium titanate thin films

摘要

Barium titanate (BaTiO$sb3$) thin films were prepared by the sol-gel method using barium titanium methoxypropoxide-based precursor solutions. The effect of processing variables on the microstructure and crystallization behavior of the films was studied. Processing variables such as water of hydrolysis, type of heat treatment, solvent, and substrate were investigated. Sols containing between no added water of hydrolysis and 2 moles of water per mole of alkoxide were prepared. The barium titanium double alkoxide was dissolved in methoxypropanol, isopropanol, or butanol. Films were prepared by spin coating sols onto bare (100) Si and (100) Si with Pt or Ge barrier layers and heat treating using conventional furnace annealing or rapid thermal annealing. Microstructural characterization using transmission electron microscopy (TEM) revealed an almost process-independent microstructure. The amorphous gel films ($sim$300 nm thick) all crystallized into polycrystalline, randomly oriented BaTiO$sb3$ with a grain size of 20-60 nm and residual porosity on the scale of the grain size. Changing the precursor chemistry to an ethoxide-based sol resulted in the same film microstructure.;To understand this process-independent microstructure, the crystallization behavior of the amorphous gel films was characterized using in-situ TEM heating experiments. Gel films crystallized at approximately 600$spcirc$C into an intermediate nanoscale (5-10 nm) barium titanium carbonate phase, hypothesized to be BaTiO$rmsb2COsb3$, that subsequently transformed into nanocrystalline (20-60 nm) BaTiO$sb3$. Random nucleation in the bulk of the gel film was observed on (100) Si, Pt coated (100) Si, and MBE-grown (100) BaTiO$sb3$ on (100) Si substrates. In addition, epitaxial nucleation of BaTiO$sb3$ was concurrently observed on MBE-grown (100) BaTiO$sb3$ on (100) Si. High concentrations of water of hydrolysis could be used to prevent the formation of the intermediate carbonate phase, but the sols were not suitable for spin coating due to precipitation.;In addition to fabricating BaTiO$sb3$ films, barium strontium titanate ($rm Basb{x}Srsb{1-x}TiOsb3$) thin films with varying Sr concentration were prepared on Pt coated (100) Si substrates. Films with higher Sr concentrations had a larger average grain size, larger grain size distribution, and increased (111) orientation on (111) oriented Pt. All films had the cubic perovskite structure with a linear relationship between the Sr content and the lattice parameter. In-situ TEM heating experiments of amorphous $rm Basb{0.5}Srsb{0.5}TiOsb3$ films, along with results from thermochemical analysis, suggest that the gel films have a crystallization path similar to the crystallization of BaTiO$sb3$, with the formation of an intermediate $rm Basb{0.5}Srsb{0.5}TiOsb2COsb3$ phase prior to the formation of $rm Basb{0.5}Srsb{0.5}TiOsb3$.