本論文探討二維過渡金屬硫族化合物（two-dimensional transition-metal dichalcogenides, TMDs）薄膜之可規模化製備、材料分析與元件應用，並以二硫化鉬（MoS2）與二硫化鎢（WS2）為主要研究材料。本研究建立無轉移之氧化物硫化轉換製程，先利用射頻磁控濺鍍沉積過渡金屬氧化物前驅層，再於 750 °C 下以 H2S 進行硫化，將其轉換為 TMD 薄膜。最終層數可藉由前驅層沉積時間調整。由於藍寶石基板具有良好的熱穩定性，本製程可避免薄膜轉移常引入的污染、皺褶、裂痕與不均勻性。
本研究系統性分析薄膜之層數相依材料特性。拉曼光譜確認 MoS2 具有 E2g1 與 A1g 特徵振動模態，且峰值間距會隨層數改變。穿透式電子顯微鏡分析顯示薄膜具有規則層狀結構，層間距約為 0.65 nm；能量散射光譜亦確認其元素組成具有均勻性。光學顯微鏡與二次諧波產生映射結果顯示，相較於單層 MoS2，少層 MoS2 具有較佳的大面積連續性。
以所製備薄膜為基礎，本研究完成 pH 感測器、NO2 氣體感測器與光偵測器，並比較其元件特性。由於通道對表面電位調變高度敏感，單層 MoS2 具有良好的 pH 響應，但在強酸環境下穩定性較差。在 NO2 暴露下，單層 MoS2 呈現 n 型感測行為，而雙層 MoS2 則呈現類 p 型響應。在 370 nm 光照下，雙層 MoS2 光偵測器可達約 3.28 μA 光電流與約 7.6 × 10-2 A/W 響應度。與 MoS2 相較，WS2 雖具有較低電流，卻展現較佳的化學穩定性與恢復穩定度。本研究亦在經環境暴露的雙層 MoS2 中觀察到負光導效應，並將其歸因於氧氣與水氣相關的可逆表面捕捉機制。
接著，本研究利用接觸工程改善載子注入。由於費米能階釘扎效應限制了金屬功函數的影響，Ti 與 Ni 接觸在初始狀態下呈現相近行為。對雙層 MoS2 光偵測器而言，Ni 接觸在真空中以 200 °C 退火 30 分鐘，可在維持半導體特性的同時得到最明顯且有效的提升。過度退火則會使響應劣化，顯示熱製程窗口必須受到控制，才能改善金屬/MoS2 介面而不使金屬化區域過度延伸至通道。
最後，本研究在藍寶石基板上實現無轉移上閘極 MoS2 薄膜電晶體，整合直接成長之雙層通道、Ni/Au 源汲極接觸、原子層沉積 Al2O3 閘極介電層，以及以無光罩微影定義 5 μm 通道。接觸退火前，雙層元件之開關電流比約為 1.6 × 105，次臨界擺幅約為 1.40 V/dec。經 200 °C 退火 30 分鐘後，輸出電流由約 8.60 × 10-6 A 提升至 2.68 × 10-4 A，增強約 31 倍。最佳化之無轉移元件可達約 4.1 × 106 開關電流比、約 39.3 cm2/Vs 表觀場效遷移率，以及約 3.4 × 10-11 A 低關態電流。雖然轉移式基準元件可提供約 0.65 V/dec 的較佳次臨界擺幅與約 3.0 × 107 的開關電流比，無轉移結構仍為可規模化 TMD 元件整合提供更實際的製程路徑。

This dissertation investigates the scalable fabrication, material characterization, and device applications of two-dimensional transition-metal dichalcogenide (TMD) thin films, with a primary focus on molybdenum disulfide (MoS2) and tungsten disulfide (WS2). A transfer-free oxide-to-sulfide conversion route was developed by depositing transition-metal oxide precursors through radio-frequency magnetron sputtering and converting them into TMD films by H2S sulfurization at 750 °C. The final layer number can be tuned through the precursor deposition time. Sapphire was selected as a thermally stable growth substrate, allowing the process to avoid the contamination, wrinkles, cracks, and non-uniformity commonly introduced by film transfer.
The layer-dependent material properties were systematically evaluated. Raman spectroscopy confirmed the characteristic E2g1 and A1g vibration modes of MoS2, and their peak separation changed with layer number. Transmission electron microscopy verified an ordered layered structure with an interlayer spacing of approximately 0.65 nm, while energy-dispersive spectroscopy confirmed compositional uniformity. Optical microscopy and second-harmonic-generation mapping showed that few-layer MoS2 provides better large-area continuity than monolayer MoS2.
Based on these films, pH sensors, NO2 gas sensors, and photodetectors were fabricated and compared. Monolayer MoS2 exhibited a strong pH response because its channel is highly sensitive to surface-potential modulation, although it was less stable under strong acid exposure. Under NO2 exposure, monolayer MoS2 showed n-type sensing behavior, whereas bilayer MoS2 showed a p-type-like response. Under 370 nm illumination, the bilayer MoS2 photodetector achieved a photocurrent of approximately 3.28 μA and a responsivity of about 7.6 × 10-2 A/W. Compared with MoS2, WS2 provided lower current but better chemical robustness and recovery stability. Negative photoconductivity was also identified in ambient-exposed bilayer MoS2 and was attributed to reversible oxygen- and moisture-related surface trapping.
Contact engineering was then used to improve carrier injection. Ti and Ni contacts showed similar initial behavior because Fermi-level pinning limited the influence of metal work function alone. For bilayer MoS2 photodetectors, vacuum annealing of the Ni contact at 200 °C for 30 min produced the strongest useful enhancement while preserving semiconducting behavior. Excessive annealing degraded the response, indicating that the thermal process window must be controlled to improve the metal/MoS2 interface without extending a metallic region too far into the channel.
Finally, a transfer-free top-gate MoS2 thin-film transistor (TFT) was demonstrated on sapphire by combining a directly grown bilayer channel, Ni/Au source-drain contacts, an atomic-layer-deposited Al2O3 gate dielectric, and a 5 μm channel defined by maskless photolithography. Before contact annealing, the bilayer device exhibited an on/off current ratio of approximately 1.6 × 105 and a subthreshold swing of about 1.40 V/dec. After annealing at 200 °C for 30 min, the output current increased from approximately 8.60 × 10-6 A to 2.68 × 10-4 A, corresponding to an enhancement of about 31 times. The optimized transfer-free device achieved an on/off ratio of approximately 4.1 × 106, an apparent field-effect mobility of about 39.3 cm2/Vs, and a low off current of about 3.4 × 10-11 A. Although a transferred benchmark device provided a sharper subthreshold swing of approximately 0.65 V/dec and an on/off ratio of about 3.0 × 107, the transfer-free structure offers a more practical route toward scalable TMD device integration.

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Chapter 4
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[3] Tian, X.; Wang, S.; Li, H.; Li, M.; Chen, T.; Xiao, X.; Wang, Y., “Recent advances in MoS2-based nanomaterial sensors for room-temperature gas detection: a review”, Sensors & Diagnostics, vol. 2, no. 2, pp. 361-381, 2023.
[4] Liu, D.; Guo, Y.; Fang, L.; Robertson, J., “Sulfur vacancies in monolayer MoS2 and its electrical contacts”, Applied Physics Letters, vol. 103, no. 18, Article 183113, 2013.
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Chapter 5
[1] Kuan, C. Y.; Chang, S. P.; Liou, G. Y.; Chang, S. H.; Chang, S. J., “Fabrication and application of two-dimensional tungsten disulfide thin film”, International Journal of Modern Physics B, vol. 39, no. 6, Article 2540013, 2025.
[2] Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S., “Electronics and optoelectronics of two-dimensional transition metal dichalcogenides”, Nature Nanotechnology, vol. 7, no. 11, pp. 699-712, 2012.
[3] Sebastian, A.; Pendurthi, R.; Choudhury, T. H.; Redwing, J. M.; Das, S., “Benchmarking monolayer MoS2 and WS2 field-effect transistors”, Nature Communications, vol. 12, no. 1, Article 693, 2021.
[4] Kuan, C. Y.; Chang, S. P.; Chang, S. J.; Chen, J. F.; Lai, W. C., “Tailorable 2D MoS2 via Oxide Sulfidation for Photodetection and Contact Engineering”, Sensors, vol. 26, no. 11, Article 3523, 2026.
[5] Tian, X.; Wang, S.; Li, H.; Li, M.; Chen, T.; Xiao, X.; Wang, Y., “Recent advances in MoS2-based nanomaterial sensors for room-temperature gas detection: a review”, Sensors & Diagnostics, vol. 2, no. 2, pp. 361-381, 2023.
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[8] Yin, Z.; Li, H.; Li, H.; Jiang, L.; Shi, Y.; Sun, Y.; Lu, G.; Zhang, Q.; Chen, X.; Zhang, H., “Single-Layer MoS2 Phototransistors”, ACS Nano, vol. 6, no. 1, pp. 74-80, 2012.
[9] Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F., “Atomically Thin MoS2: A New Direct-Gap Semiconductor”, Physical Review Letters, vol. 105, no. 13, Article 136805, 2010.
[10] Gustafson, J. K.; Wines, D.; Gulian, E.; Ataca, C.; Hayden, L. M., “Positive and Negative Photoconductivity in Monolayer MoS2 as a Function of Physisorbed Oxygen”, The Journal of Physical Chemistry C, vol. 125, no. 16, pp. 8712-8718, 2021.
[11] Tailor, N. K.; Aranda, C. A.; Saliba, M.; Satapathi, S., “Negative Photoconductivity: Bizarre Physics in Semiconductors”, ACS Materials Letters, vol. 4, no. 11, pp. 2298-2320, 2022.
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Chapter 6
[1] Liu, D.; Guo, Y.; Fang, L.; Robertson, J., “Sulfur vacancies in monolayer MoS2 and its electrical contacts”, Applied Physics Letters, vol. 103, no. 18, Article 183113, 2013.
[2] Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S., “Electronics and optoelectronics of two-dimensional transition metal dichalcogenides”, Nature Nanotechnology, vol. 7, no. 11, pp. 699-712, 2012.
[3] Sebastian, A.; Pendurthi, R.; Choudhury, T. H.; Redwing, J. M.; Das, S., “Benchmarking monolayer MoS2 and WS2 field-effect transistors”, Nature Communications, vol. 12, no. 1, Article 693, 2021.
[4] Kappera, R.; Voiry, D.; Yalcin, S. E.; Branch, B.; Gupta, G.; Mohite, A. D.; Chhowalla, M., “Phase-engineered low-resistance contacts for ultrathin MoS2 transistors”, Nature Materials, vol. 13, no. 12, pp. 1128-1134, 2014.
[5] Jin, L.; Wen, J.; Odlyzko, M.; Seaton, N.; Li, R.; Haratipour, N.; Koester, S. J., “High-Performance WS2 MOSFETs with Bilayer WS2 Contacts”, ACS Omega, vol. 9, no. 29, pp. 32159-32166, 2024.
Chapter 7
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