Optical probing of polycrystalline laser ablated lead free ferroelectric CaBi₄Ti₄O₁₅ thin films

High quality CaBi₄Ti₄O₁₅ (CBTi) ferroelectric thin films were fabricated on fused silica at 650°C using pulsed laser deposition (PLD) method. These CBTi thin films were analyzed for their structural, microstructural and optical properties. The structural properties show the films were in orthorhombic structure with crystallite size in 11-14 nm range. The increment in the thickness of the films leads to enhancement of grain size and increase in surface roughness as seen in FE-SEM and AFM images. Ultraviolet-Visible-NIR spectrophotometer (190-2500 nm) was used to study the room temperature thickness dependent optical properties of CBTi thin films. Transmission spectra show that the refractive index and optical band gap changes with thickness variation. The band gap of the PLD deposited thin films was found to be in the range of 3.6-3.7 eV. As thickness increases from 150 nm to 910 nm, the crystallite size and surface roughness increases and band gap decreases. The phase formation and the role of stress and strain were also discussed with the help of Raman modes. (author)