以濺鍍方式成長鐵－硒－碲超導薄膜之製程與特性研究

Abstract

鐵基超導發展至今已十來年，雖熱潮稍退，但近年由於拓撲超導相關研究群起，鐵基超導材料再度引發關注，包含與拓撲材料結合形成二維介面拓撲超導、於厚度極小之鐵基超導表面觀察到拓撲特性等。 本實驗利用射頻磁控濺鍍成長鐵－硒－碲超導薄膜於氧化鎂和氟化鈣基板上，透過成長溫度、電流源功率、腔體氣壓、靶材與基板間距等參數之調整，以及X射線繞射分析（XRD）、能量色散光譜分析（EDS）等數據判斷薄膜品質，試圖穩定並優化製程，其中發現靶材之成份比例與薄膜樣品有一定程度之落差，在此我們亦探討其背後機制，包含元素之熔點、蒸氣壓等，並以元素之間鍵結能力之強弱其影響尤甚。經XRD數據得知晶相為多晶tetragonal超導相，而於較低溫成長時會部分混雜hexagonal非超導相，甚至使tetragonal相消失。依據EDS數據判定薄膜成分，發現其元素比例和濺鍍靶材有很大落差。由於Fe-Se的鍵結能力遠比Fe-Te大，若靶材中提供過量的Se元素，成長薄膜時，所有Fe元素會被迫和Se進行鍵結，使Te元素無法順利與Fe鍵結而揮發逸散，無法留在薄膜結構內。 研究後期亦利用商業化SQUID量測系統，針對較佳樣品進行電性與磁性的測量。我們無觀察到超導態特有之抗磁性現象，顯示該樣品其超導相的佔比不高。透過電性量測OP45樣品，發現於低溫時有明顯轉折，其Tc-onset約為12.5 K，接近鐵－硒－碲塊材之臨界溫度，但直至2 K仍未抵達零電阻，顯示該樣品超導相並不完全。透過Werthamer-Helfand-Hohenberg理論與Ginzburg-Landau理論計算樣品之超導相干長度ξab(0) = 21.1 nm，與參考文獻相比大了約一個數量級。透過Kim – Anderson熱激發磁通蠕動理論計算釘扎能與外加磁場的關係，發現不符三維超導體系應有之數值。

It has been a decade since the discovery of the iron-based superconductivity. Although the related popularity has slightly gone down a couple years ago, it rises up again due to the research of the topological superconductivity. For example, topological superconductivity can be made from the combination of an iron-based superconducting material and a topological material, two-dimensional topological superconductivity can also be observed on the surface of an ultra-thin iron-based superconductor thin film. In this research we fabricated Fe-Se-Te (FST) superconducting thin films on MgO and CaF2 single crystal substrates by radio-frequency magnetron sputtering. By the adjustments of the related parameters such as substrate temperature, sputtering power, chamber pressure, the distance between the target and the substrate, and the judgements from the X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS) data, we tried to stabilize and optimize the process of the fabrication. We found that the stoichiometry of the thin film samples was different from that of the targets, therefore we discussed the probable mechanism or parameters behind this, including the melting point and vapor pressure of the related elements, and the bond energy of different elements. According to XRD data, the crystalline phase was a polycrystalline tetragonal superconducting phase for samples grown at a higher temperature, while for samples grown at a lower temperature, the crystalline phase was partially mixed with a hexagonal non-superconducting phase and the tetragonal phase even disappeared. The composition of the FST films was judged based on the EDS data, and it was found that there was a big difference of element ratio between the FST thin films and the sputtering target. Since the bonding ability of Fe-Se is much greater than that of Fe-Te, if excessive Se elements are provided in the target material, most Fe elements will be forced to bond with Se, making Te elements unable to bond with Fe smoothly. Thus the Te atoms escape and cannot stay in the structure of FST thin films. We also made some electric and magnetic measurements on an optimum sample using a commercial SQUID system. We did not observed the diamagnetic phenomenon for the superconducting state, indicating that the sample has a low proportion of superconducting phase. Through electrical measurement, a significant resistive transition low temperatures was observed. The Tc-onset is about 12.5 K, being close to the critical temperature of FST bulk material, but it did not reach zero resistance until 2 K, indicating that the superconducting phase in this sample was not complete. The superconducting coherence length ξab(0) of the sample was calculated by Werthamer-Helfand-Hohenberg theory and Ginzburg-Landau theory, and obtained that ξab(0) = 21.1 nm, which was about an order of magnitude larger than those reported in the references. The relationship between the pinning energy and the applied magnetic field was calculated based on the Kim-Anderson thermal excitation magnetic flux creep theory, and it was found that it did not match the relationship that a three-dimensional superconducting system should have.