- 學位論文
生長參數對PLD製備的GdBCO、Ti2O3、MnSe薄膜結構、傳輸、磁性能的影響
Studies on the effect of strain on the structure, transport and magnetic properties of GdBCO, Ti2O3 and MnSe Films prepared by the pulsed laser deposition method
摘要
由於薄膜材料呈現廣泛的功能特性，使得薄膜研究成為一個重要的研究課題。電、磁和超導性是一些最常被探究的主題。薄膜在構建具有良好工作效率的應用導向元件中扮演至關重要的角色，其中應變對於改變薄膜的電性和磁性更是關鍵。在本論文中，我們透過不同種類應變誘發的奈米結構、二次相奈米粒子、拉伸、壓縮和鬆弛外延來研究薄膜之晶體結構、電學和磁學性質，這開啟了新技術的研究大門。我們使用脈衝激光沉積（PLD）製備薄膜，PLD 以其高沉積速率和高動能物種而著稱，可以維持目標材料的化學計量，已被證明是一種用於獲得高性能磊晶薄膜的技術，且可實現功能性元件製備與特殊奈米結構薄膜等。本論文的主要探討四種不同材料系統的生長參數變化引起的結構、電傳輸及磁特性的變化。 我們研究了在離子束輔助沉積的 MgO 基板 (I-BAD) 上成長三維 (3D) BZO 納米結構網絡的 GdBa2Cu3O7-x (Gd123) 超導薄膜，Gd123 薄膜的超導轉變溫度 (Tc) 略微降低至 89 K。在77 K及 3 Tesla磁場下，我們將 JC（H//ab 平面）增強到 約為 H//c 軸的 76% 的0.38 MA/cm2。我們以渦流路徑模型 (vortex path model) 框架解釋了 JC 在高溫和高磁場 (77 K, 2 T) 和相應數據下的角度依賴性。另一方面，在 I-BAD 基板上成功生長了非晶 BaZrO3 (BZO) 摻雜的 Gd1Ba1Cu2.5 (Gd112.5) 薄膜。我們自主設計了一種低成本的家用模式爐，通過應用活化共蒸發沉積反應 (RCE-DR) 技術，在高溫和高氧壓下將非晶相快速轉化為超導相。我們使用穿透式電子顯微鏡 (TEM) 和二維 X 射線衍射 (2D XRD) 方法，分析主相和雜質相的晶粒尺寸分佈。 BZO 摻雜使 TC 從 92.53 K 略微降低到 89.18 K，然而，它在 3 T 和 65 K 時將 JC 從 0.115 提高到 0.31 MA/cm2。 我們成功地在 MgO (001) 和 (0001) 取向的 Al2O3 (C-cut) 單晶基板上製備了具有氧空位的外延 Ti2O3 薄膜，其 TC 分別約為 7.2 K 和 7 K。在 MgO 上生長的 Ti2O3 薄膜的上臨界場 HC2(0) 和相干長度 (coherence length) 約為 17.2 T 和 4.3 nm，而 C-cut Al2O3基板為 15.6T 和 4.54nm。通過 X 射線繞射 (XRD)， 研究Ti2O3 樣品的結構特性，確認其結構為正交相，這是Ti2O3 薄膜獲得超導性的主要原因。 此外，我們研究了在 SrTiO3 (STO) 基板上生長的超薄 MnSe 薄膜。它屬於四方晶系，其中a-b面拉伸，c軸縮短。發現四方MnSe薄膜呈現超順磁性及導電性的增加。透過光吸收光譜，瞭解這些MnSe薄膜的電子通過間接能隙到導電帶的躍遷顯著增強。而導電度明顯增加及超順磁特性，可能與基板的電荷轉移有關。。
並列摘要
Most of the films have prompted much research on their wide range of functional properties. Thin film plays a vital role in the construction of application-oriented devices with good working efficiency in which strain in thin films is often very important to modify the electrical and magnetic properties. In this thesis, we have studied the crystal structure, electrical, and magnetic properties by introducing different kinds of strain in the form of the nanostructure, secondary phase nanoparticles, tensile, compressive, and relaxed epitaxy in thin films which open the doors towards some new technologies. Pulsed laser deposition (PLD) method was used to deposit thin films, which is known for its high deposition rate with high kinetic energy species resulting in the stoichiometric removal of the target material to the thin films. It has proven to be an up-and-coming technique for obtaining high-performance epitaxial thin films, enabling the fabrication of functional devices. The thesis’s principal purpose is to focus on changes in structure, transport, and magnetic properties caused by the variation in growth parameters for four different kinds of systems. We studied the GdBa2Cu3O7–x (Gd123) films grown with a three-dimensional (3D) BZO nanostructure network on the ion-beam-assisted deposited MgO substrate (I-BAD). The GdBa2Cu3O7–x (Gd123) film’s superconducting transition temperature (TC) is slightly degraded to 89 K. A critical current density (JC ) of about 0.5 MA/cm2 was achieved along the (H//c-axis) under 3 Tesla of a magnetic field at 77 K. We enhanced the JC (H//ab-plane) to 0.38 MA/cm2, which is about 76% of H//c-axis case. The angular dependence of JC at 77 K and 2 Tesla data were interpreted within the vortex path model framework. On the other hand, amorphous BaZrO3 (BZO) doped Gd1Ba1Cu2.5 (Gd112.5) thin films were successfully grown on the I-BAD substrate. We indigenously designed a low-cost home mode furnace that rapidly converted the amorphous phase into a superconducting phase at high temperature and high oxygen pressure by implementing the reactive co-evaporation-deposition and reaction (RCE-DR) process. The grain size distribution of primary and impurity phases was analyzed using the transmission electron microscope (TEM) and two-dimensional X-ray diffraction (2D XRD) methods. The BZO-doping slightly degraded the TC from 92.53 K to 89.18 K. However, it improved the JC from 0.115 to 0.31 MA/cm2 at 3 T and 65 K. We successfully prepared epitaxial Ti2O3 thin films with oxygen vacancy on MgO (001) and (0001)-oriented Al2O3 single crystal substrates that show a TC about 7.2 K and 7 K, respectively. The upper critical field HC2(0) and coherence length of Ti2O3 film grown on MgO are about 17.2 T and 4.3 nm, whereas 15.6 T and 4.54 nm for C-cut Al2O3. The structural properties of Ti2O3 samples were studied by X-ray diffraction (XRD) and selected area electron diffraction (SAED) measurements. Its structure was confirmed as an orthorhombic phase, the essential rule for getting superconductivity in Ti2O3 thin films. Besides, we investigated ultra-thin MnSe films grown on SrTiO3 (STO) substrate. It belongs to the tetragonal crystal system in which the ab-plane is stretched, and the c-axis is shortened. It is found that the tetragonal MnSe thin-film shows interesting superparamagnetism and enhanced electrical conductivity. The optical absorption spectra indicate that the electron transition through the indirect bandgap to the conduction band is significantly enhanced in tetragonal MnSe thin film. The enhancement of conductivity and the emergence of superparamagnetism is likely attributed to the charge transfer from the substrate.