二碲化鎳單晶與鉍摻雜及薄膜之電磁傳輸特性研究

Abstract

本論文以第二型狄拉克半金屬二碲化鎳（NiTe₂）為研究主體，透過「單晶冷卻速率調控」、「鉍微量摻雜」與「Bi/NiTe₂ 薄膜異質介面」三個維度，系統性探討本徵缺陷與強自旋軌道耦合元素對其電磁傳輸性質之影響。實驗以固相熔融法分別製備爐冷與焠火之 NiTe₂ 單晶(NTC、NTQ)，以及摻雜濃度 0.005% 與0.01%之Ni₁₋ₓBiₓTe₂ 單晶（BNT005、BNT01），並以直流磁控濺鍍於 NiTe₂ 解理面成長 100 nm 之 Bi 薄膜後進行退火。XRD 與 EDX 分析顯示所有塊材樣品均存在顯著碲空缺（Ni:Te ≈ 1:1.17），惟藉由繞射系統性消光法則判定其仍維持1T-NiTe₂（P3̅m1）結構而非 NiTe 相；鉍摻雜則因巨大之離子半徑失配明顯破壞晶體成長，摻雜量愈高晶質劣化愈嚴重。電性量測方面，NTC、NTQ、BNT005 與BNT01 之殘餘電阻比（RRR）分別為 13.0、3.8、3.1 與 1.3，遠低於文獻完美單晶之 374，顯示低溫傳輸由靜態缺陷散射主導。霍爾分析指出各樣品以電子為主要載子，NTC 於約 40 K 出現載子符號反轉；鉍摻雜樣品則展現有效之受體摻雜作用，證實在排除熱處理變因下，微量 Bi 即可調控 NiTe₂ 之載子補償機制。磁阻量測顯示 10 K、60 kOe 下 NTC 具有達 23% 之線性不飽和磁阻，並隨焠火與摻雜逐步被壓制至 0.38%；NTC 系統性違反 Kohler's rule，而臨界磁場 B* 隨溫度之演化與磁阻來自 Abrikosov 量子極限效應。磁性量測顯示樣品磁化強度由Pauli 型順磁、抗磁背景與稀薄缺陷局域磁矩三者構成，NTC 於 25–50 K 之磁矩極大值可能對應缺陷磁矩之短程有序或凍結。最後，Bi/NiTe₂薄膜經 200 °C 退火後，於 M–T 量測中觀察到 Tc ≈ 3.89 K 之超導轉變，推測為介面原子相互擴散生成 NiBi₃ 超導相，初步展現於拓樸材料表面原位構築超導異質結構之可行性。

This thesis investigates the type-II Dirac semimetal nickel ditelluride (NiTe₂) along three dimensions: control of the single-crystal cooling rate, dilute bismuth doping, and Bi/NiTe₂ thin-film heterointerfaces, in order to systematically clarify how intrinsic defects and strong spin–orbit-coupled elements govern its electromagnetic transport properties. Furnace-cooled (NTC) and quenched (NTQ) NiTe₂ single crystals, together with Ni₁₋ₓBiₓTe₂ crystals at nominal doping levels of 0.005% and 0.01% (BNT005, BNT01), were synthesized by solid-state melt growth, while 100 nm Bi films were deposited on cleaved NiTe₂ surfaces by DC magnetron sputtering followed by annealing. XRD and EDX analyses reveal substantial tellurium vacancies in all bulk samples (Ni:Te ≈ 1:1.17); nevertheless, systematic-absence analysis confirms that the crystals retain the 1T-NiTe₂ (P3̅m1) structure rather than transforming into the NiTe phase. Bi doping severely degrades crystal growth due to the large ionic-size mismatch, with higher doping levels causing progressively worse crystallinity. The residual resistance ratios of NTC, NTQ, BNT005, and BNT01 are 13.0, 3.8, 3.1, and 1.3, respectively—far below the literature value of 374 for pristine crystals—indicating that low-temperature transport is dominated by static defect scattering. Hall measurements show electron-dominated conduction, with a carrier-sign reversal in NTC near 40 K; the Bi-doped samples exhibit effective acceptor doping, demonstrating that trace Bi alone can tune the carrier-compensation mechanism of NiTe₂ once thermal-processing variables are excluded. At 10 K and 60 kOe, NTC shows a linear, non-saturating magnetoresistance of 23%, which is progressively suppressed to 0.38% by quenching and doping. NTC systematically violates Kohler's rule, and the temperature evolution of its crossover field B* deviates from the Abrikosov quantum-magnetoresistance shows the magnetoresistance comes from quantum limits.Magnetization measurements resolve three contributions: a Pauli-type paramagnetic susceptibility, a diamagnetic background, and dilute defect-related local moments; the magnetization maximum of NTC at 25–50 K may signal short-range ordering or freezing of these defect moments. Finally, the annealed Bi/NiTe₂ film exhibits a superconducting transition at Tc ≈ 3.89 K in M–T measurements, attributed to interfacial interdiffusion forming the NiBi₃ superconducting phase, demonstrating the feasibility of constructing superconducting heterostructures in situ on a topological-material surface.