The Tunable Dielectric Of Epitaxial Strontium Titanate Thin Films

SrTiO3 (STO) thin films have excellent nonlinear dielectric property at the liquid nitrogen temperature. Due to the variation of dielectric constant under the applied electric field, STO thin films can be widely used in electrically tunable microwave devices. Although the dielectric materials have been widely studied, it is still difficult to obtain the high tunability and the low loss tangent at the same time. Multi-layer epitaxial thin films can achieve the coordination between the tunability and the dielectric loss according to the recent studies. LaNiO3 (LNO), a typical perovskite structure with good electrical conductivity and well lattice matching with STO, can be used as electrode marerials in epitaxial STO thin films. In the thesis, the effects of the grown temperature and the electrode material on the tunable dielectric of STO thin films were studied, the primary conclusions are as follows:Firstly, the epitaxial STO thin films with LNO bottom electrode were fabricated on the(00l) LAO substrate by sol-gel method. The detection method of the heterogeneous films with the same crystal was studied, and it was found that the crystal plan with high indices of crystal face could be used to determine the in-plane orientation of the expitaxial thin films. In addition,the influence of heat treatment temperature on the tunable dielectric of STO/LNO composite films was studied. With the grown temperature increasing, the tunability of Pt/STO/LNO structure improved to 49.8%, the loss tangent decreased to 0.0115 and the figure of merit was 43.3 at the optimal heat treatment temperature.Secondly, the LNO patterned film was prepared by photosensitive sol-gel method, and then used as top electrode on STO/LNO composite thin films to fabricate LNO/STO/LNO capacitive structure. The LNO/STO/LNO structure showed more excellent dielectric properties compared with the Pt/STO/LNO heterostructure and the figure of merit could reach 82.4.Finally, the influence mechanism of different top electrodes on the dielectric properties of STO/LNO composite films was analyzed by J-V characteristic curves. With the temperature decreasing (within the temperature range of 80-300 K), the leakage current density of Pt/STO/LNO and LNO/STO/LNO heterostructures decreased. Schottky barrier existed in the Pt/STO interface of Pt/STO/LNO heterostructures, otherwise the apparent barrier was not observed in the LNO/STO/LNO structure. It was speculated that the polarization process of the STO thin films was influenced by the presence of the Schottky barrier, resulting in higher loss tangent in Pt/STO/LNO heterogeneous structure.