Photo-sensitive sol–gel solutions for the low-temperature UV-assisted processing of PbTiO3 based ferroelectric thin films

Abstract

Photo-sensitive PbTiO₃ solutions were synthesised by a sol–gel process that combines the use of a diol solvent, HO(CH₂)₃OH, and a titanium alkoxide modified with a β-diketonate, Ti(OC₃H₇)₂(CH₃COCHCOCH₂)₂. O-Nitrobenzyl alcohol (NBA), 1-hydroxy-cyclohexyl-phenyl-ketone (HPK) and acetylacetone (ACACH) were added to the synthesised sols as UV-activators. Solutions with nominal compositions of Pb_0.76Ca_0.24TiO₃ were prepared. These solutions have a maximum of absorption in UV between 200 and 300 nm. Pyrolysis of the solutions was studied by means of thermogravimetry and differential thermal analysis (TGA/DTA). Films were deposited by spin-coating onto TiO₂/Pt/TiO₂/Ti/SiO₂/(100)Si substrates. They were irradiated at 250 °C with light of λ ∼ 225 nm and then treated at 550 or 450 °C for 420 s with Rapid Thermal Processing (RTP) in air, using a heating rate of ∼30 °C s⁻¹. Crystalline phases and structures developed in the films were studied by X-ray diffraction with the Bragg–Brentano geometry and with grazing incidence (XRD and GIXRD). Films showed the formation of a single perovskite phase. These films have spontaneous pyroelectricity with a maximum value of the pyroelectric coefficient of γ_s ∼ +12 × 10⁻⁹ C cm⁻² K⁻¹. Non-irradiated films prepared at 550 °C did not show a defined ferroelectric response. The ferroelectric properties measured in the films processed at 550 °C under UV were similar to those reported for films prepared at higher temperatures without irradiation. Values of the remanent polarisation of P_r ∼ 22 µC cm⁻² and of the coercive field of E_c ∼ 170 kV cm⁻¹ were obtained in these irradiated films prepared at 550 °C.