Jinzhao Wang

Preparation and dielectric properties of compositionally graded (Ba,Sr)TiO3 thin film by sol-gel technique

Abstract Compositional graded Ba x Sr 1- x TiO 3 ( x =0.6, 0.7, 0.8, 0.9, 1.0) (BST) thin films (less than 400 nm) were fabricated on Si and Pt/Ti/SiO 2 Si substrates by sol-gel technique. A special heating treatment was employed to form the uniform composition gradients at 700 °C. The microstructures of the films were studied by means of X-ray diffraction, atomic force microscope and field emission scanning electron microscopy. The results show that the films have uniform and crack-free surface morphology with perovskite structure phase. The small signal dielectric constant (ɛ r ) and dielectric loss (tanδ) are found to be 335 and 0.045 at room temperature and 200 kHz. The dielectric properties change significantly with applied dc bias, and the graded thin film show high tunability of 42.3% at an applied field of 250 kV/cm. All the results indicate that the graded BST thin films prepared by sol-gel technique have a promising candidate for microelectronic device.

Effect of substrate temperature on microstructures and dielectric properties of compositionally graded BST thin films

Abstract Compositionally graded Ba 1- x Sr x TiO 3 (BST) ( x = 0–0.3) thin films were prepared on Pt/Ti/SiO 2 /Si substrate at different substrate temperatures ranging from 550 °C to 650 °C by radio-frequency (rf) magnetron sputtering. The effect of substrate temperature on the preferential orientation, microstructures and dielectric properties of compositionally graded BST thin films was investigated by X-ray diffraction, scanning electron microscopy and dielectric frequency spectra, respectively. As the temperature increases, the preferential orientation evolves in the order: randomly orientation→ (111)→ highly oriented (111) (α (111) = 60.2%). The surface roughness of the graded BST thin films varies with the substrate temperatures. No visible internal interface in the compositionally graded thin films can be observed in the cross-sectional SEM images. The graded BST thin films deposited at 650 °C possess the highest dielectric constant and dielectric loss, which are 408 and 0.013, respectively.

Tunneling electroresistance effect in Pt/MgO/Pt/PbTiO3/Pt ferroelectric tunnel junctions

Tunneling electroresistance (TER) effect was investigated in Pt/MgO/Pt/PbTiO3/Pt ferroelectric tunnel junctions (FTJs) theoretically. Compared to Pt/MgO/PbTiO3/Pt FTJs with the same composite barrier thickness (unit cells), FTJs with a thicker Pt interlayer (2 ∼ 3 unit cells) could provide 1 ∼ 6 orders of magnitude improvement in the TER ratio in a wide range of polarization. Resonant tunneling effect and/or enhanced asymmetry of the potential energy profile induced by the resonant-tunneling structure of MgO/Pt/PbTiO3 in Pt/MgO/Pt/PbTiO3/Pt FTJs is responsible for the TER improvement.

Influences of deposition parameters on microstructure and dielectric properties of BST thin film deposited by R.F. magnetron sputtering

Down graded barium strontium tianate(BST) thin films were prepared on Pt/Ti/SiO2/Si(100) substrate by radio frequency magnetron sputtering. The BST thin films were deposited at various substrate temperatures, then annealed at 700 °C for an hour. The influences of the deposition parameters on the crystallization behavior, microstructure and dielectric properties of BST thin films were investigated by X-ray diffraction, field emission scanning electron microscopy and dielectric frequency spectra. XRD results indicate that the BST thin films deposited at higher temperature have improved crystallization structure.The SEM observations show that the surface of the films is smooth with homogeneous grains.The dielectric properties of the films have been examined and discussed. The experiments show that BST thin films deposited at 650oc, 3.0Pa working pressure exhibits superior dielectric properties: the highest dielectric constant is 448 and lowest dielectric loss is 0.013 at 100 kHz, respectively. These results make BST thin films be a promising candidate for microelectronic device application.

Ferroelectric tunnel junctions with multi-quantum well structures

Ferroelectric tunnel junctions (FTJs) with multi-quantum well structures are proposed and the tunneling electroresistance (TER) effect is investigated theoretically. Compared with conventional FTJs with monolayer ferroelectric barriers, FTJs with single-well structures provide TER ratio improvements of one order of magnitude, while FTJs with optimized multi-well structures can enhance this improvement by another order of magnitude. It is believed that the increased resonant tunneling strength combined with appropriate asymmetry in these FTJs contributes to the improvement. These studies may help to fabricate FTJs with large TER ratio experimentally and put them into practice.