Study On The Properties Of Strontium Doped Lead Titanate Ferroelectric Thin Films

Strontium doped lead titanate (Pb_xSr_1-x)TiO₃ (x=0.50, 0.25, respectively abbreviated as PST50, PST25) thin films were prepared on platinized silicon substrates by sol-gel technique and rapid thermal annealing process. And these thin films were characterized for their phase structure, surface morphology, dielectric properties and leakage current mechanism. All the measurements were carried at room temperature.Firstly, we fabricated PST50 thin films on Pt/Ti/SiO₂/Si substrates without and with LaNiO₃ (LNO) buffer layer. The structural and electrical properties of these two kinds of films were examined. The PST50 films grown on the Pt/Ti/SiO₂/Si substrates without LNO buffer layer, showed the random orientation. However, PST50 thin films grown on Pt/Ti/SiO₂/Si substrate with the LNO bufferlayer exhibited highly (100) orientation. Compared with randomly oriented films, the highly (100)-oriented PST50 thin films have higher dielectric constant, the dielectric constant and loss tangent are 850 and 0.032, respectively at 100 kHz.Secondly, PST25 thin films were grown directly on Pt/Ti/SiO₂/Si substrates. The X-ray diffraction results showed a pure perovskite with polycrystalline structure. PST25 thin films with relatively high dielectric constant and low dielectric loss were about 350 and 0.01, respectively at 100 kHz. And dielectric tunability of 22.7% at 100 kHz and 150 kV/cm were obtained for the films. The phase temperature of PST25 film was -107℃, compared with the PST25 bulk ceramic (-25℃), the phase transition shift to the lower temperature up to -80℃. A low leakage current density was observed about 10^-7 A/cm² over the applied voltage of 0 to±4 V and the leakage current mechanism was discussed.We found that strontium doped lead titanate thin films own excellent dielectric and ferroelectric properties. The introduction of LNO buffer layer improved the match between films and substrate and enhanced the dielectric and fierroelectric properties. The results showed that PST thin film is an attractive candidate for dynamic random access memories and microwave tunable devices applications.