Improved optical, dielectric, and nonlinear properties of CaCu3Ti4O12 thin films by chromium doping

In the present work, the optical and dielectric properties, microstructures, and nonlinear electrical performances were systematically investigated for Ca1-xCrxCu3Ti4O12 (x = 0.00 - 0.40) thin films prepared on Pt/Ti/SiO2/Si substrates using the sol-gel method. X-ray photoelectron spectroscopy (XPS) verified that Cr3+ and Cr6+ were present in the thin films. Additionally, the content of secondary phase of titanium oxide (TiO2) increased, the grain size decreased, and the surface roughness improved with the increase in the doped content of Cr. Due to the reduced grain size and oxygen vacancy concentration, the optical energy band gap increased from 2.92 to 3.54 eV. The change in the dielectric constants (Ɛr) of the thin films followed the internal barrier layer capacitor model. Doping was found to decrease the dielectric loss (tan δ) and enhance the nonlinear properties, which was due to the presence of TiO2 secondary phase, high density, reduced grain size, incremental potential barrier height and grain boundary resistance. The Ca0.8Cr0.2Cu3Ti4O12 thin film had a high Ɛr of around 9340, low tan δ of around 0.012 at 1 kHz, nonlinear coefficient of about 4.64, and breakdown field of ca. 89 kV/mm. These results indicated that the doping of an appropriate amount of Cr could effectively modify the microstructure of CaCu3Ti4O12 thin films and improve their optical and electrical properties.