鉭金雙層薄膜之電磁傳輸特性研究

Abstract

近年，如何提升超導薄膜的超導相變溫度Tc為許多實驗的研究重點。其中部分實驗指出在超導金屬薄膜上非超導金屬，在非金屬層不超過一定厚度的前提下會使系統Tc上升，稱為反鄰近效應。本研究中，主要討論於傳統超導材料鉭薄膜上鍍金薄膜，分析其對雙層膜系統之電磁傳輸特性的影響。 首先，本實驗首先著重於以直流濺鍍技術製備Tc接近塊材鉭（Tc ≈ 4.483 K）的薄膜樣品。由於鉭具有常見於塊材鉭的α態與常見於濺鍍薄膜的β態，其中β態的Tc低於1 K，故尋找方法將使薄膜中的鉭偏好α態為本實驗的重點。多次調整參數後發現，提高濺鍍時的基板溫度（高於293 ⁰C）及增加薄膜厚度（約800奈米）可以有效的提升樣品的Tc至3.55~3.65 K，而X光繞射分析結果也顯示基板溫度較高及厚度較厚的樣品具有α態的晶格結構。 進一步，我們固定薄膜鉭的厚度於約800奈米，並在其上方鍍厚度10奈米至100奈米不等的金薄膜，以探討其對樣品的Tc影響。傳統鄰近效應中，雙層膜的Tc隨上層膜的厚度增加而降低。然而，本實驗發現鉭金雙層膜的Tc與金薄膜的厚度dAu間的關係並非單調遞增或遞減，而是呈現先遞增後遞減的變化趨勢，出現反鄰近效應。此非單調關係的可能解釋為，在厚度不高時上層膜可抑制系統的漲落因而改善樣品的Tc，然而當上層膜的厚度持續增加，傳統鄰近效應的影響仍會使樣品Tc降低。 除了樣品Tc的研究，我們亦分析樣品在外加磁場下的電磁傳輸行為，以及驗證其是否有BKT相變，進一步發現部分樣品可能具有二維超導的性質。透過此研究，我們發現了鉭薄膜在金薄膜的影響下展現出非單調的Tc與dAu關係，可協助未來研究者改善超導系統的超導相變溫度。此外，若未來可在不降低Tc的前提下，有效降低樣品厚度，雙層膜系統或可作為二維超導行為的研究對象。

An overlayer of normal metal on a superconducting metal film could enhance the Tc of bilayer film. This phenomenon is known as the inverse proximity effect (IPE). In this work, we investigated the effect of the gold (Au) overlayer on the superconducting tantalum (Ta) film and discussed the electromagnetic transport properties of the Ta/Au bilayer film. First, our work focused on the direct current (DC) sputtering parameter of the Ta film. By adjusting substrate temperature and film thickness, we found that a higher temperature (above 295 ⁰C) and a greater film thickness could enhance the Tc of Ta films to about 3.6 K. From the X-ray diffraction patterns, we could also see that the above conditions facilitated the formation of α-Ta. For the Ta film thinner than 133 nm, we could not observe Tc above 2 K. Next, with the thickness of the Ta film fixed at 800 nm, we added an Au overlayer of 10-100 nm thick to investigate its effects on Ta films. With the increase in the thickness of the Au overlayer, the Tc of the bilayer film first increased and then decreased, exhibiting the sign of IPE. A possible explanation for this nonmonotonic relation is that the Au overlayer first mitigated fluctuations in the Ta film and thus enhanced the Tc. However, the proximity effect dominated and thus reduced the Tc as the Au overlayer continued to increase. Furthermore, we checked the existence of 2-D superconductivity by examining the 2-D Tinkham model, the U ∝ -lnH relationship between the activation energy U and the applied field H, and the occurrence of the Berezinskii-Kosterlitz-Thouless transition. Among the seven Ta/Au bilayer samples, the two samples grown on the new silicon substrates exhibited 2-D superconducting behaviors. This result suggests that the normal-superconducting bilayer thin films could be a potential candidate for research on how fluctuations affect 2-D superconducting behavior.