Preparation And Properties Of Ce₁Y₂Fe₅O₁₂ And Hf_xZn_1-xO Films

Ce-substituted yttrium iron garnet (Ce_xY_3-xFesO₁₂ Ce:YIG) thin films have been widely used in many fields such as high performance non-reciprocal wave-guide devices and integrated optical devices for its large magneto-optic effect and low propagation loss. Doped ZnO has been under an extensive research since appropriate impurity doping can further improve the properties of ZnO. ZnO-based films deposited on flexible polymer substrates have recently gained tremendous interests because they are light and flexible to be easily deformed, and can be used for certain applications, such as smart cards, electronic paper and flexible display where flexibility and lightweight are needed. Pulsed laser deposition is one of the most potential deposition technologies, especially for multi-element oxides thin films who desire film stoichiometry and lower substrate temperature.In this dissertation, The Ce₁Fe₅O₁₂ thin films and Hf-doped ZnO thin films with different Hf contents (Hf_xZn_1-xO) were deposited on various substrates using pulsed laser deposition. The influences of deposition parameters, post-annealing and doping concenstrations on the structure, magnetic, electrical and optical properties of these two multi-element thin films were investigated. The main results are as follows:1. Ce₁Y₂Fe₅O₁₂ films were prepared on GGG (111) and Si (100) substrates respectively using PLD, the microstructures and magnetic properties of films were studied. It was found that crystalline Ce₁Y₂Fe₅O₁₂ films with YIG (444) preferred orientation can be obtained under an optimum condition with pulse frequency of 5 Hz and substrate temperature of 800℃, the films show mostly paramagnetic. Crystalline Ce₁Y₂Fe₅O₁₂ thin films can not be achieved directly on Si substrates; the as-depositited films show amorphous structure and weak magnetion. After a post-annealing under temperature of 700℃, one can found that the films on Si become polycrystalline and show strong ferromagnetic with easy axis of magnetization lying in the plane of the film.2. The effects of annealing temperature and SiO₂ buffer layer on the microstructure and magnetion of Ce₁Y₂Fe₅O₁₂ thin films on Si were studied. The results show that two steps of phase segregation occur with the post-annealing temperature increasing: at first, Ce₁Y₂Fe₅O₁₂ is decomposed into YIG and non-magnetic CeO₂ when annealed at 800 ℃; then YIG continues to be decomposed forming Fe₂O₃ when the temperature is increased up to 900 ℃. Consequently, the saturation magnetization of Ce₁Y₂Fe5O₁₂ films decreases first and then increases with the post-annealing temperature going up, which indicates that the saturation magnetization of Ce₁Y₂Fe₅O₁₂ films is mainly related to the microstructure. A SiO₂ buffer layer is found to be able to improve the magnetic properties of Ce₁Y₂Fe₅O₁₂ films by reducing the oxygen vacances existing in the films.3. Hf_xZn_1-xO films with different Hf concentrations were deposited on Si (100) and PET substrates respectively by PLD, the microstructures and optical properties of films were studied, the results show that Hf ions can be effectively doped into ZnO even on the flexible PET substrates via PLD. All the as-deposited Hf_xZn_1-xO films on Si with Hf contents 0≤x≤15 at % crystallize in a ZnO hexagonal wurtzite structure with a highly preferred c-axis orientation. However, for the films prepared on PET, the films with Hf contents more than 5 at % begin to show amorphous structure. All the films show a hingh transtrmittance; two ultraviolet peaks centered at about 364 and 380 nm co-exist as observed in the fluorescent spectra. The influences of Hf doping concentrations on the films were also studied, one can find that with increasing Hf contents, the lattice constants of Hf_xZn_1-xO films increase, the morphology of the films deteriorates and the intensity of fluorescent peaks enhances remarkably. At the same time, energy gaps increase while the positions of ultraviolet peaks remain unchanged. A possible mechanism of increased luminescence was then discussed.4. The transparent and conductive properities of Hf_xZn_1-xO films deposited under O₂ pressure of 5 Pa and with Hf contents 0≤x≤2 at % were studied. It is found that the films also show a hingh transtrmittance, the conductivity of films decreases first and then increases with increasing Hf contents. The films with Hf contents x=0.5 at % has a minimum of sheet resistance of 16 Ω/□ (With resistivity about 1.2 ×10^-3 Ω·cm), while the transtrmittance maintain at about 85 %.