MnSe/SrTiO3 薄膜之結構與磁性質研究

Abstract

本論文主要研究分子束磊晶成長之厚 MnSe/SrTiO₃(001) 薄膜的結構與磁性質，並探討成長條件、局部結構差異與異常磁性行為之間的關聯。藉由 X 光繞射、穿透式電子顯微鏡、掃描穿透式電子顯微鏡能量散射 X 光分析、位置解析選區電子繞射／快速傅立葉轉換，以及零場冷卻與場冷卻磁量測，比較不同基板熱處理條件與沉積時間下薄膜的結構與磁性演化。

室溫 X 光繞射結果顯示，各樣品皆以岩鹽型 MnSe 結構為主，未觀察到可明確歸屬於六方 NiAs 型 MnSe 的主要繞射訊號。然而，MnSe (002) 與 (004) 反射所呈現的多重峰成分，以及穿透式電子顯微鏡與位置解析繞射所顯示的局部晶格尺度與取向差異，說明薄膜並非單一且完全均勻的晶體結構，而具有局部結構非均勻性。不同成長與熱處理條件亦會改變界面特徵及局部結構差異，顯示相近的平均岩鹽結構並不代表其微觀結構完全等價。

磁量測顯示兩個主要溫度尺度。首先，多數樣品在約 4–100 K 範圍內具有低溫 ZFC 磁化率上升及 ZFC/FC 分離，顯示系統中存在具有能障與磁不可逆性的低溫磁性成分。其次，部分樣品在約 100–220 K 出現寬廣且具有製程依賴性的磁性異常，其特徵溫度與曲線形狀亦隨外加磁場改變。以 AG4-530 為例，低磁場下可觀察到非單調的 ZFC 行為與明顯的 ZFC/FC 分離，而在 5000 Oe 下兩條曲線趨近重合，並轉為單調的 FM-like 溫度依賴。

綜合結構與磁性結果，約 4–100 K 的低溫磁不可逆行為顯示存在具有能障分布的磁性自由度，但僅根據目前的磁量測與結構分析，尚不足以唯一判定其微觀起源。相較之下，約 100–220 K 的寬廣磁性異常及其磁場依賴性，較符合有限範圍 ferromagnetic correlation 的物理圖像。這些結果指出，MnSe 薄膜的磁性不僅由平均晶體結構決定，局部晶格差異、界面、缺陷，以及不同結構區域間的磁性交互作用亦可能扮演重要角色。

本論文另於附錄中探討兩項先進半導體元件與互連問題。第一項建立 HZO 負電容 MOS 閘極堆疊的高頻小訊號模型，將 Landau–Devonshire 能量曲率、Landau–Khalatnikov 動態、損耗、半導體表面節點與電晶體負載共同納入 RF-admissibility 評估。結果顯示，最低的本徵擺幅並不必然對應最佳的 GHz 操作條件；只有同時滿足穩定性、被動性、頻率一致性與實際電晶體負載限制的狀態，才能形成有限且可使用的偏壓–頻率操作視窗。

第二項建立拓撲受限的 backside power-delivery network 電熱與可靠度設計方法，以固定導孔直徑下的 p_via⁻² 密度尺度，同時考慮 IR 壓降、線路與導孔／接點電遷移、熱限制及實作預算。在 36,288 個設計狀態中，僅以 IR drop 篩選會錯誤保留 1,218 個不符合導孔／接點可靠度限制的 false-safe states，並顯示 GAA、CFET-P 與 CFET-S 拓撲在不同背面供電架構下具有顯著不同的可行設計視窗。

整體而言，本研究顯示，對 MnSe/SrTiO₃ 薄膜而言，相似的平均晶體結構並不足以描述其完整磁性行為；局部結構非均勻性與不同磁性交互作用的共同演化，是理解其製程與磁場相依磁性異常的重要線索。附錄中的兩項研究亦顯示，在先進元件與互連設計中，只有同時納入相關物理與可靠度限制，才能得到具有實際意義的可行操作視窗。

This thesis primarily investigates the structural and magnetic properties of thick MnSe films grown on SrTiO₃(001) by molecular beam epitaxy, with particular emphasis on the relationship among growth conditions, local structural variations, and anomalous magnetic behavior. X-ray diffraction, transmission electron microscopy, scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy, position-resolved selected-area electron diffraction/fast Fourier transform analysis, and zero-field-cooled/field-cooled magnetometry are combined to compare films prepared with different substrate thermal treatments and deposition durations.

Room-temperature X-ray diffraction shows that all investigated films are predominantly rocksalt MnSe, without a major diffraction feature that can be unambiguously assigned to the hexagonal NiAs-type MnSe phase. Nevertheless, the multiple components observed in the MnSe (002) and (004) reflections, together with local variations in lattice metrics and crystallographic orientation revealed by TEM and position-resolved diffraction, indicate that the films cannot be regarded as structurally uniform single crystals. Growth and thermal-treatment conditions further modify the interfacial characteristics and the degree of local structural variation, demonstrating that a similar average rocksalt structure does not necessarily imply equivalent microscopic structures.

Magnetometry reveals two principal temperature scales. First, most samples exhibit an increase in the ZFC susceptibility and a separation between the ZFC and FC curves over approximately 4–100 K, indicating a low-temperature magnetic component associated with energy barriers and magnetic irreversibility. Second, selected samples exhibit a broad, process-dependent magnetic anomaly at approximately 100–220 K, whose characteristic temperature and shape evolve with applied magnetic field. In AG4-530, for example, a nonmonotonic ZFC behavior and pronounced ZFC/FC separation are observed at low fields, whereas at 5000 Oe the two curves become nearly coincident and evolve into a monotonic FM-like temperature dependence.

Taken together, the structural and magnetic results indicate that the magnetic irreversibility observed at approximately 4–100 K involves magnetic degrees of freedom associated with an energy-barrier distribution; however, the present structural and magnetic measurements are insufficient to uniquely determine their microscopic origin. In contrast, the broad magnetic anomaly at approximately 100–220 K and its field dependence are more consistent with a finite-range ferromagnetic-correlation picture. These observations indicate that the magnetic behavior of MnSe films cannot be determined solely from their average crystallographic structure. Local lattice variations, interfaces, defects, and magnetic coupling among structurally distinct regions may also play important roles.

Two additional studies on advanced semiconductor devices and interconnects are presented in the appendices. The first develops a high-frequency small-signal model for an HZO negative-capacitance MOS gate stack by combining Landau–Devonshire curvature, Landau–Khalatnikov dynamics, loss, a semiconductor surface node, and transistor loading within an RF-admissibility framework. The results show that the minimum intrinsic swing does not necessarily correspond to the most useful GHz operating condition. A finite bias–frequency window is retained only when stability, passivity, frequency consistency, and realistic transistor-loading constraints are simultaneously satisfied.

The second study develops a topology-constrained electrothermal-reliability design methodology for backside power-delivery networks. Using the p_via⁻² density scaling associated with a fixed via diameter, IR drop, wire and via/contact electromigration, thermal constraints, and implementation budget are evaluated simultaneously. Among 36,288 design states, IR-only screening incorrectly retains 1,218 false-safe states that violate via/contact reliability, while GAA-, CFET-P-, and CFET-S-like topologies exhibit distinctly different feasible windows under different backside power-delivery architectures.

Overall, the MnSe/SrTiO₃ results demonstrate that a similar average crystal structure is insufficient to describe the complete magnetic behavior of the films. The coupled evolution of local structural heterogeneity and magnetic interactions provides an important route toward understanding their process- and field-dependent magnetic anomalies. The two appendix studies further demonstrate that physically meaningful operating windows in advanced electronic systems emerge only when the relevant dynamic, thermal, reliability, and implementation constraints are evaluated simultaneously.