Preparation And Characterization Of Capacitance Structures Of BVO Ferroelectric Thin Films

Compared to the traditional non-volatile memories, the non-volatile memories based on ferroelectric thin films have many advantages such as low power consumption, high speed of writing and reading and good on-volatility. Ferroelectric thin films of PZT are the first material which is used for ferroelectric memory. However the drawbacks of PZT such as bad fatigue properties and the pollution to environment because of the exist of lead driving researchers to find new materials which have better prosperities to replace PZT. Recent years the ferroelectric thin films with Bismuth layer structures which have good dielectric prosperities and fatigue properties, attracts more interests. SBT and BIT are belong to this family.Bismuth vanadate (Bi2VO5.5, BVO) thin film is one of this kind of materials which can be formulated as (Bi2O2)2+ and (VO3.5□0.5)2-, where□represents oxide ion vacancies. It exhibits three main polymorphs with the temperature changed. The high temperature r-phase of BVO shows high ionic conductivity, because of which this material were investigated on its conductivity at first. BVO has great potential in the applications of fuel batteries and oxide sensor. Then with the development of the technique of thin film fabrications, BVO thin films have been prepared. Low dielectric constant and low crystallization temperature make this material a good candidate for FeRAM. The deposition technique of BVO thin films reported are most PLD technique. For PLD technique has some disadvantages such as high cost, low speed of film growth, and difficulties in the changing of amount of dopant. Finding a simple and low cost method to fabricate BVO thin films is good to the investigation on the film.In this thesis, BVO thin films have been prepared by sol-gel method which is a easy way to fabricate high-purity films with low cost. The microstructure, electrical and optical properties are investigated in details, and then the influences of the fabrication technique to the properties of BVO thin films are analyzed. The achievements of this thesis are as follows:1. High-quality busimuth vanadate thin films have been successfully prepared on Si (100) substrates and substrates with oxide electrodes by sol-gel spin-coating technique, using NH4VO3 and Bi(NO3)3·5H2O as the raw materials. In addition, Cu-doped and Fe-doped BVO thin films have been fabricated.2. Microstructure and surface morphology of BVO thin films are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). XRD results indicate that the films begin to show a high c-axis preferred orientation when the annealing temperature is 600℃. And with the increase of annealing temperature, the degree of c-axis preferred orientation increase remarkably, which can be compared to the film fabricated by PLD. The thin films show a uniform grain distribution, and the root mean roughness values of the film are small.3. MFS structure of Pt/BVO/Si and capacitance structure of Pt/BVO/LNO/Si are fabricated by preparing BVO on Si substrates and Si substrates with LNO electrode and then spattering Pt electrode on BVO thin films. The electric properties of the film including c-v properties, dielectric properties, ferroelectric properties and leakage properties were investigated. The results indicate that the electric properties are influenced by the annealing temperature and concentration of Fe dopant. The film show good properties when the annealing temperature is 650℃.4. Optical properties of BVO thin films are investigated for their potential application in optic devices. The reflective index and extinction coefficient of the BVO thin films are obtained by spectroscopic ellipsometric measurement. The differences of optical constants between BVO thin films annealing at various temperatures are analyzed and it can be found that the optical constants increase with annealing temperature due to the better crystallization at higher annealing temperature. Raman spectra measurements are carried out to study the lattice vibration modes of BVO thin films. Raman shifts reveal the influences of annealing temperature and Cu-doping to the microstructures of BVO films.