Microstructure And Electrical Properties Of PST And BNT Ferroelectric Films Influenced By TiO₂ Buffer Layers

Recently, inserting a buffer layer between the Si substrates and ferroelectric films has been an effective way to improve the properties of the films. Nowadays, a lot of materials can be used as the buffer layer. In this article, we choosed the TiO2 as the buffer layer, and fabricated the TiO2 layers with different thicknesses (5 nm,10 nm and 20 nm) on Pt/Ti/SiO2/Si substrate by Sol-Gel method. Then we deposited PST (Pb1-xSrxTiO3, x=0.4,0.6) and BNT (Bi3.54Nd0.46Ti3O12) films on the buffer layers and investigated their microstructure and electrical properties.Firstly, we investigated the influence of TiO2 buffer layers on the microstructure and electrical properties of PST films. It was found that the microstructure of the films changed from random orientation to (l00) preferred orientation and the surface roughness was reduced to a large extent, after inserting the buffer layers between the Pt substrates and PST films. Meanwhile, the electrical properties of the PST films with TiO2 buffer layer were also improved, including the increase of dielectric constant and its tunability by direct current voltage, as well as the decrease of the dielectric loss and leakage current density. Since the strained rutile layer copies the Pt (111) lattice, the activation energy for nucleation of PST thin film is reduced and the nucleation density is increased after inserting the TiO2 buffer layers. Thus the PST films become denser and smoother, and accordingly the electrical properties of them are also improved largely. Among the different thicknesses of the TiO2 layers between 0 and 20 nm, it was observed that the 5 nm thick TiO2 buffer layer had the most significant effect on the microstructure and electrical properties of PST films.Secondly, we studied the influence of TiO2 buffer layers on the microstructure and electrical properties of BNT ferroelectric films. The crystallized TiO2 buffer layers can also improve the microstructure of BNT thin films. When inserting the buffer layers between the Pt substrates and BNT films, the microstructure of the films changed from random orientation to (117) preferred orientation and the crystal grains became smaller and denser, leading to a smoother and evener surface of the films. On the aspect of the electrical properties, the dielectric constant and remanent polarization value (2Pr) of BNT thin films with TiO2 buffer layers were increased while its dielectric loss and leakage current density were decreased to a certain extent. Their voltage endurance was also enhanced after inserting the buffer layer. These results are mostly due to the (117) preferred orientation and the improved microstructure of BNT thin films. Among the different thicknesses of the TiO2 layers between 0 and 20 nm, it was found that the 20 nm thick TiO2 buffer layer has a relatively large effect on the microstructure and electrical properties of BNT films.Based on above research, we suggested that the crystallized TiO2 buffer layers can improve the microstructure and electrical properties of both the PST and BNT ferroelectric films.