釔鉍氧/釔鋇銅氧雙層薄膜之成長與超導特性之研究

Abstract

近年來拓樸絕緣體的討論度日益升高，拓樸絕緣體與超導體的界面研究更是近年來的研究重點，其主要原因是其非阿貝爾統計特性(Non-Abelian statistics)以及在量子計算和量子信息領域的應用潛力。本研究採用磁控濺鍍系統，將拓樸絕緣體材料釔鉍氧(YBiO3)成長於鈦酸鍶(100)基板上，並且經由X-ray繞射儀得到結構繞射，並利用原子力顯微鏡量測表面粗糙度以確認樣品品質。並且利用最佳化的釔鋇銅氧條件，將釔鉍氧成長於釔鋇銅氧上形成雙層薄膜。在製作過程中發現兩種材料會反應形成YBa2BiO6並且其反應速度會受溫度影響。最後我們將釔鋇銅氧鍍膜條件固定為Tg= 720 °C功率90 W，釔鉍氧鍍膜條件固定為Tg= 650 °C功率80 W，並且工作壓力固定為400 mtorr，製作出釔鋇銅氧厚度固定為100 nm，並在上面成長釔鉍氧厚度為10 nm、20 nm、50 nm、100 nm，利用四點量測系統測量得到臨界溫度(Tc)，單層釔鋇銅氧的Tc約為88 K，釔鉍氧厚度為10 nm、20 nm的Tc約為84 K、82 K，而厚度為50 nm的樣品則出現疑似半導體的特性，100 nm的電阻無窮大所以無法量測。之後我們針對釔鉍氧厚度為10 nm與單層釔鋇銅氧做磁性量測做比較，單層及雙層薄膜的Hc1分別為378 Oe、64.5 Oe，Hc2分別為15.67 T、9.748 T，並計算出相干長度(Coherence Lengh, ξ)與穿透深度(London Penetration Depth, λ) ，透過擬合λ-2的結果發現不論是單層的釔鋇銅氧或是雙層薄膜樣品的趨勢都不符合s-wave的超導體。最後利用磁滯曲線能計算臨界電流密度(Critical Current Density, Jc)以及釘扎力(Pinning Force, Fp)，經過計算得到單層釔鋇銅氧在0 K時Jc = 35.754 (106A/cm2)，雙層薄膜在0 K時為Jc = 11.177 (106A/cm2)。藉由擬合釘扎力的結果可以得到不同溫度下，可以推測釔鋇銅氧在77 K以下呈現一二維混和的磁通釘扎，77 K以及80 K時更接近一維磁通釘扎，而雙層薄膜在2 K及10 K時屬於一二維混和的磁通釘扎，在20 K到70 K的區間呈現二維的磁通釘扎，最後在77 K以及 80 K 時更接近三維的磁通釘扎。

In recent years, there has been a growing interest in topological insulators, with a particular focus on the study of interfaces between topological insulators and superconductors. This research direction has gained prominence due to the non-Abelian statistical properties exhibited by these materials and their potential applications in quantum computing and quantum information.In this study, a magnetron sputtering system was utilized to grow Yttrium Bismuth Oxide (YBiO3), a topological insulator material, on a Strontium Titanate (100) substrate. The structural characteristics were determined using X-ray diffraction, and atomic force microscopy was employed to measure the surface roughness and ensure the quality of the samples. By optimizing the growth conditions of Yttrium Barium Copper Oxide (YBCO), a bilayer thin film was fabricated with YBiO3 grown on top of YBCO. During the fabrication process, it was observed that the two materials reacted to form YBa2BiO6, and the reaction rate was found to be temperature-dependent.Subsequently, the growth conditions for the YBCO film were fixed at a growth temperature (Tg) of 720 °C with a power of 90 W, while the YBiO3 film was grown at Tg = 650 °C with a power of 80 W. The working pressure was maintained at 400 mtorr. The YBCO film thickness was fixed at 100 nm, and YBiO3 was grown on top with thicknesses of 10 nm, 20 nm, 50 nm, and 100 nm, respectively. The critical temperature (Tc) was measured using a four-point measurement system. The Tc of the single-layer YBCO was approximately 88 K, while for YBiO3 with thicknesses of 10 nm and 20 nm, the Tc was around 84 K and 82 K, respectively. However, the sample with a thickness of 50 nm exhibited characteristics resembling a semiconductor, and the resistance of the 100 nm sample was infinite, making it unmeasurable.Furthermore, a magnetic characterization was performed by comparing the magnetic properties of the 10 nm YBiO3 and the single-layer YBCO. The Hc1 values for the single-layer and bilayer films were measured as 378 Oe and 64.5 Oe, respectively, while the Hc2 values were found to be 15.67 T and 9.748 T, respectively. Coherence length (ξ) and London penetration depth (λ) were calculated. However, the fitting results of λ-2 indicated that neither the single-layer YBCO nor the bilayer films followed the expected trend for an s-wave superconductor.Finally, the critical current density (Jc) and pinning force (Fp) were determined by analyzing the hysteresis curve. The calculated Jc values at 0 K were 35.754 (106 A/cm2) for the single-layer YBCO and 11.177 (106 A/cm2) for the bilayer film. By fitting the pinning force results, it was inferred that YBCO exhibited a mixed one and two-dimensional flux pinning below 77 K, approaching one-dimensional pinning at 77 K and 80 K. For the bilayer film, a mixed one and two-dimensional flux pinning was observed at 2 K and 10 K, transitioning to two-dimensional pinning in the temperature range of 20 K to 70 K, and approaches three-dimensional pinning at 77 K and 80 K.