- 學位論文
鉭薄膜的超導特性和鉭/氧化鎂/鉭異質結構在矽基板上之穿隧特性
Superconductivity of tantalum thin films and tunneling characteristics of Ta/MgO/Ta heterostructures on silicon substrates
摘要
近年來，鉭 (Ta)金屬於超導量子位元上的應用越來越受到關注。相較於傳統的超導體材料 (如鋁和鈮)，鉭的原生氧化層較為穩定且有較少的二能級缺陷，使得transmon量子位元有長去相干間。盡管已經製作出使用鉭薄膜當作共振腔材料的長去相干時間之 transmon量子位元，大多數使用藍寶石基板，與超大型積體電路 (VLSI)製程技術不相容。因此，本論文將研究鉭薄膜在矽基板(Si)上的超導特性以及鉭基異質結構的超導穿隧特性。 本論文第一個部分對鉭薄膜在矽基板上的超導特性進行研究。由磁控濺射沉積 鉭薄膜在不同材料上結構，並且在23 mK和300 K的溫度範圍於低溫系統中進行電阻值量測。使用低掠角 X光繞(GIXRD)技術對鉭薄膜的結晶相進行偵測。結果顯示，鉭薄膜在矽基板上是 β相位，超導臨界溫度約為0.71 K，而在鉬緩衝層上則是α相位，超導臨界溫度約為 2.8 K。至於 Ta/MgO/Ta/Si的結構，鉭薄膜在氧化鎂上是α相位混合β相位，而超導臨界溫度約為2.1 K，而Ta/MgO/Ta/Mo/Si的結構，鉭薄膜在氧化鎂上是α相位而超導臨界溫度約為3.8 K。顯示鉬緩衝層可有效的成長具有較高超導臨界溫度的α相位鉭薄膜在矽基板和氧化鎂上。 本論文第二個部分則探討Ta/MgO/Ta穿隧結在矽基板上於約瑟芬結之應用。上層及下層鉭薄膜的結晶相透過穿透式電子顯微鏡(TEM)和低掠角X光繞射(GIXRD)偵測，而超導臨界溫度則是由低溫電阻量測所取得，其中上層鉭薄膜為α相位而超導臨界溫度約為3.95 K，而下層鉭薄膜為β相位而超導臨界溫度約為0.58 K。接著Ta/MgO/Ta穿隧結的穿隧特性由鎖相技術量測於低溫23 mK至3.8 K，其中電導 -電壓曲線與金屬 -氧化層 -超導體穿隧特性相似，透過擬合電導-電壓曲線而提取出上下兩層鉭薄膜的超導能隙值，與理論值接近。由於穿隧結並沒有量測到約瑟芬電流，因此對於約瑟芬結的應用任需做更進一步的研究。
並列摘要
Tantalum (Ta) has attracted much attention recently for superconductor qubit applications. Compared to conventional superconducting materials, such as aluminum and niobium, the native oxide of tantalum is more stable and has fewer two-level defects, leading to a long decoherence time in transmon qubits. While high-performance transmon qubits using Ta films as a cavity material were demonstrated, most of them used sapphire substrates, which is not fully compatible with VLSI technology. In this thesis, the superconductivity of tantalum on silicon substrates and tunneling characteristics of Ta/MgO/Ta heterostructures are studied for future Ta-based superconducting qubit applications. In the first part of this thesis, the superconductivity of tantalum thin films on a silicon substrate is investigated. Ta films on different intermediate materials are deposited by a magnetron sputter and resistivity is characterized at 23 mK ~ 300 K in a dilution fridge. Grazing incidence X-ray diffraction (GIXRD) is used to characterize the crystalline phases of different layers in those structures. The results suggest that Ta thin films on silicon are β-phase with a critical temperature of ~ 0.71 K, while those on molybdenum (Mo) buffer layer are α-phase with a critical temperature of ~ 2.8 K. For the Ta/MgO/Ta, the Ta thin film on top of MgO is mixed of α- and β-phases and the critical temperature is ~2.1 K. The Ta/MgO/Ta/Mo structure shows that the tantalum thin film on top of MgO is α-phase and the critical temperature is ~ 3.8 K. The Mo buffer layer is effective in achieving α-phase Ta with a higher critical temperature both with and without MgO as an intermediate layer. The second part of the thesis is to investigate the Ta/MgO/Ta tunneling junction on silicon for Josephson junction applications. Transmission electron microscopy (TEM) and GIXRD are carried out to characterize the crystalline phases and electrical measurements are performed to extract the critical temperatures of the top and bottom Ta layers. The top Ta layers are α-phase with a critical temperature of ~ 3.95 K, while the bottom layers are β-phase with a critical temperature of ~ 0.58 K. The tunneling characteristics of the junction are carried out by a lock-in technique at 23 mK ~ 3.8 K. The conductance-voltage curves are similar to normal metal–insulator–superconductor tunneling characteristics (NIS) and fit to extract the superconducting energy gaps of the top and bottom Ta layers. The extracted superconducting energy gaps are close to the theoretical prediction. However, no Josephson current is observed in this tunneling junction, and further investigations are required.