(103)與(110)軸向釔鋇銅氧超導薄膜之成長與特性研究

Abstract

摘要 本論文主要述及藉由本實驗室所發展的雷射蒸鍍大面積YBCO薄膜的獨特技術，詳細而有系統的探討YBCO薄膜的成長機制與基板表面狀態之關係。我們發現除了匹配的因素與基板表面起始狀態會對薄膜成長機制與最終的晶粒表面形貌有決定性的影響之外；在相同的基板上，成長溫度亦會影響YBCO薄膜的最終軸向。我們更進一步的對成長(103)與(110)方向YBCO薄膜的製程條件，及其結構與超導特性做較深入的研究。除了以掃描式電子顯微鏡及原子力顯微鏡詳細比較不同軸向薄膜成長機制與晶粒形貌演化過程作詳盡的研究之外，本文更首度提出以X-光繞射法配合控制氧含量來辨識成長之薄膜晶向結構的新穎方法，這種晶向結構的確認，對於後續薄膜物理特性的詮釋提供了關鍵性的支持。基於這些研究，我們將略述不同軸向薄膜在X-光吸收光譜所顯現的載子濃度與其相關的鍵結能帶的關係，以及載子在超快雷射的激發-探測研究中所顯現的載子鬆弛(relaxation)之各向異性。我們亦將述及這些薄膜在超導傳輸特性與高頻微波表面電阻所顯現的行為與晶粒結構之間的關係。

ABSTRACT This thesis is aimed to describe systematic studies of correlating the growth mechanisms and final grain morphologies of YBCO superconducting thin films with various processing parameters, such as the initial state of the substrate and growth temperatures. With the aid of the unique technology developed in our laboratory for depositing large area YBCO thin films by laser ablation, a detailed monitoring of grain evolution right from the first few monolayers is realized. It was found that even on the same substrate, the detailed substrate surface morphology as well as the growth temperatures are the keys in ultimately determining the final orientation and grain morphology of the laser ablated YBCO thin films. Based on the fundamental knowledge gained from the investigations, the growth and characterizations of (110)-and (103) oriented YBCO thin films growth on the same (110) SrTiO3 substrates are emphasized. Both the scanning electron microscopy (SEM) and atomic force microscopy (AFM) were employed to study the growth mechanism and evolution of grain morphology in films grown in different orientations. In addition to the detailed morphology characterizations, we proposed, for the first time, a novel method, combining conventional X-ray diffraction with film oxygen content controls, to distinguish the YBCO (110) and YBCO (103) films, which was virtually undiscernable by various techniques. The firm identification of exact film orientation has proven to be very important for supporting the interpretations of X-ray absorption spectra in determining the hole concentrations allocated in various bands associated with different Cu-O bonding. Furthermore, it also provides some new insights on the studies of femtosecond pump-probe measurements on carrier relaxation anisotropies in this material system. The prominent features of these studies together with the transport property and high-frequency (mm-wave band) microwave surface resistance anisotropies displayed by these films will also be described and briefly discussed.