The Fabrication And Microstructure Studies Of Bi-based Oxide Thin Films By Molecular Beam Epitaxy Method

In this dissertation, Bi-based oxide thin films were prepared by the molecular beam epitaxy (MBE) method. A homemade ozone condensing device was adopted as the oxide source. X-ray reflectivity (XRR) and surface profile testing instrument technique were used to obtain the evaporation rates of Bi, Sr, Ca, and Cu elements. The experimental procession to prepare Bi2Sr2CaCu2O8+δfilms was investigated in detail. In addition, the preparation, properties and microstructure of Bi2.1-yCaySr1.9_yCuO6+δfilms were also investigated based on the obtaining of Bi2Sr2CuO6+δfilms. Moreover, MgO (100) substrates were treated via photolithography and Ar reactive ion etching, and Bi2.1CaySr1.9-yCuO6+δ(y=0.8) thin films were deposited on such substrates to investigate the epitaxy growth modes of the films. The detail contents of the dissertation are as follows:1. A homemade ozone condensing device was adopted as the oxide source to prepare BSCCO thin films by MBE. The influences of temperature of silica gel and concentrated time on the concentration of ozone were discussed. Results reveal that the concentration of ozone is higher than 95mol% when the temperature of silica gel is -85℃and concentrated time is 6h; The high concentration(>95mol%) can be maintained to more than 5h if the pressure of the condensed device is 2.7×103Pa, which facilitates the preparation of Bi-based oxide thin films.2. XRR and surface profile testing instrument technique were used to obtain the evaporation rates of Bi, Sr, Ca and Cu elements. Cu films with fixed thickness were deposited on Sr and Ca films to avoid the oxidation of the two kinds of metal elements. Results reveal that there is a liner relationship between the evaporation rates of Bi, Sr, Ca and Cu elements and the temperature of every effusion cell, which fulfill the expression of Clausius-Clapeyron equation. 3. The influences of composition, substrate temperature and the ozone partial pressure on the formation of Bi-2212 phase in the film were investigated in detail. In addition, the effects of growth rate, substrates and buffer layers on the epitaxial properties of the thin films were also researched. Results reveal that the region to form Bi-2212 phase can be obtained through the BiO-(Sr+Ca)O-CuO ternary phase diagrams; The purity of Bi-2212 phase and the morphology of the films are desirable when the substrate temperature is 720℃and the ozone partial pressure is 2.7×10-3Pa; The substrate temperature and ozone partial pressure have a remarkable influence on the formation of Bi-2212 phase, and the influences were studied by enthalpy change in detail; The decrease in the growth rate favors the mixed growth of the films, which can obtain smooth surface; Compared with MgO(100) substrate, SrTiO3(100) is more suitable as the substrate to deposited Bi-2212 films with high properties because of the lower lattice mismatch; Bi-2201 buffer layers with different thickness were deposited between the MgO(100) substrates and Bi-2212 thin films. The morphology becomes smoother and the Tc is increased.4. The experimental conditions to prepare Bi2.1CaySr1.9-yCuO6+δthin films were investigated based on the obtaining of Bi-2201 thin films, and the crystallinty, surface morphology, physical properties and microstructure of the Bi2.1CaySri.9-yCuO6+δthin films were also investigated. Results reveal that substrate temperature region to form Bi2.1CaySr1.9-yCu06+δphase is wide, and crystallinty is good when the temperature is 680℃; The length of c-axis decrease with the increase of y value, and the conductivity of the film is different with different y values; The microstructure of the Bi2.1CaySr1.9-yCuO6+δ(y=0.8) film and the interface between film and substrate were observed with high resolution electron microscope. The film is epitaxial growth along c-axis, and the length of c-axis is shortened with the doping of Ca element; The stress derived from mismatch of thin film and substrate can be released in half unit cell or result in crystal lattice distortion; The first layer is BiO-layer in the process of thin film growth. 5. Grooves with 2μm in width and 40nm in height were obtained on MgO (100) substrate via photolithography and Ar reactive ion etching, and Bi2.1Ca0.8Sr1.1CuO6+δthin films were deposited on such substrates. Results reveal that Bi2.1Cao.8Sr1.1CuO6+δthin films with different orientations were obtained on such substrates, and the etching treatment is favorable to deposit Bi-based thin films with epitaxial growth along a, b-axis orientation.