非線性光學檢測單層過渡金屬二硫屬化物缺陷之研究

Abstract

隨著半導體產業在微縮元件上面臨瓶頸，各大廠商越發重視新型材料的研發。原子級厚度的二維材料(two-dimensional materials, 2D materials)憑藉卓越的機械強度、光電性質與能帶調制性，成為眾所矚目的焦點。其中，單層過渡金屬二硫屬化物(monolayer transition metal dichalcogenide, 1L-TMDC)擁有適宜的能隙與高度穩定性，再加上基底面(basal plane)與邊緣(edge)擁有迥異的導電能力，被譽為是新型半導體材料的明日之星，具有多重應用價值。由於化學氣相沉積(chemical vapor deposition, CVD)技術日趨成熟，1L-TMDC的大規模工業製造指日可待。然而，1L-TMDCs缺陷與應變經常減損元件性能，使得生產線上的非破壞性快速檢測變得至關重要。因此，本研究全面整合了一般拉曼光譜(Raman spectroscopy)、旋光解析拉曼光譜 (helicity-resolved Raman spectroscopy, HRRS)、室溫光致發光(photoluminescence, PL)光譜及二次諧波產生(second harmonic generation, SHG)量測，對富含應用價值的CVD 單層二硫化鉬(monolayer molybdenum disulfide, 1L-MoS2)與CVD 單層二硒化鎢(monolayer tungsten diselenide, 1L-WSe2)進行周密的探索。 我們首次觀察到CVD 1L-MoS2鋸齒型(zigzag)邊緣在垂直方向存在單軸壓縮應變梯度，並可透過鄰近邊緣處拉曼光譜的E'模式先行硬化，以及平行偏振解析SHG強度(I∥)極座標圖的六瓣對稱變形得以驗證；拉曼A'1峰值與PL光譜擬合分析則揭示了邊緣處的P型摻雜。我們主張1L-MoS2為了沿邊緣釋放CVD製程導入的拉伸應變，促使點缺陷排列成平行邊緣的線缺陷，從而造就了垂直邊緣方向的單軸壓縮應變場；此外，由於空氣分子易吸附於缺陷位置，導致電子轉移至吸附分子上，引起了邊緣處的P型摻雜。我們還發覺當激發光偏振垂直於扶手椅(armchair)方向的壓縮應變時，I∥擁有最高應變對比度，可不經擬合便鑑定出應變方向。 我們還初次發現了連續波(continuous wave, cw)雷射退火除了在CVD 1L-MoS2基底面引入熱致破壞之外，也可釋放CVD製程引入的拉伸應變；該拉伸應變的釋放能藉由E'模式軟化趨勢反彈以及A0峰值藍移印證。在拉曼強度與光學顯微鏡下無法識別的5秒cw雷射處理區，可經由SHG映射影像直接觀測；且SHG單點數據只需拉曼光譜百分之一的時間即可獲取，突顯了SHG檢測作為即時快速檢測工具的潛能。我們還定量了破壞造成的SHG去極化(depolarization)現象，並發現一分鐘的雷射退火可使SHG去極化從6.92%上升至7.53%。 由於1L-WSe2的拉曼光譜受限於高度重疊的聲子峰值，對晶體品質進行準確檢測極具挑戰性；因此，我們利用自行搭建的HRRS系統探討1L-WSe2在cw雷射下的熱致破壞性質，同時評估其他非線性光學方法的缺陷檢測能力。這是首篇研究以HRRS技術解析出熱破壞1L-WSe2的E'模式硬化與A'1模式軟化，分別指出了壓縮應變與N型摻雜之存在。相比之下，一般拉曼光譜以及PL光譜僅能取得1L-WSe2的訊號強度改變。我們還確認了熱致破壞可明顯降低1L-WSe2的SHG訊號強度，並導致SHG的去極化現象；此外，SHG極化非均向度也被證實在共極化(co-polarized)與正交極化(cross-polarized)的收光幾何條件下皆可計算。 本論文通過綜合應用非線性光學檢測技術，詳盡揭示了CVD 1L-MoS2的固有邊緣缺陷與熱致破壞特性，同時展示了HRRS在評估1L-WSe2晶體品質方面的強大潛能。我們還驗證了SHG技術在快速品質檢測中的實用性，並通過SHG極化非均向度揭示缺陷引起的去極化現象。這些結果不僅有助於推動1L-MoS2與1L-WSe2的應用與研究，也突顯了非線性光學技術在二維材料品質檢測中的不凡可能。

As the semiconductor industry faces challenges in scaling down devices, there is an increasing emphasis on developing novel materials. Two-dimensional materials (2D materials) with atomic-scale thickness have garnered significant attention due to their exceptional mechanical strength, optoelectronic properties, and bandgap tunability. Among them, monolayer transition metal dichalcogenides (1L-TMDCs) are characterized by appropriate bandgap, high stability, and distinct conductive properties between the basal plane and edges. 1L-TMDCs are thus hailed as the future of new semiconductor materials with versatile applications. With the maturation of chemical vapor deposition (CVD) techniques, large-scale industrial production of 1L-TMDCs is imminent. However, defects, strains, and thermal degradation in 1L-TMDCs often compromise device performance, underscoring the critical need for non-destructive, rapid inspection technology on production lines. Therefore, this thesis integrates conventional Raman spectroscopy, helicity-resolved Raman spectroscopy (HRRS), room-temperature photoluminescence (PL) spectroscopy, and second harmonic generation (SHG) measurements to thoroughly explore the characteristics of CVD monolayer molybdenum disulfide (1L-MoS2) and monolayer tungsten diselenide (1L-WSe2), which are both rich in applications. For the first time, we have uncovered a uniaxial compressive strain field perpendicular to the CVD 1L-MoS2 zigzag edges, evidenced by the early hardening of the Raman E' mode and the six-petal deformation in the parallel-polarized SHG (I∥) polar plot. Analysis of the Raman A'1 peak and deconvoluted PL spectroscopy revealed the presence of p-type doping at the edges. We propose that point defects aggregate into line defects parallel to the edges to release the tensile strain from the CVD process, thereby creating uniaxial compressive strain fields orthogonal to the edges. Moreover, the air molecules readily adsorbing at defect sites leads to electron transfer from CVD 1L-MoS2 to molecules, resulting in p-type doping at the edges. Our findings also unveil that I∥ has the highest intensity contrast when polarized excitation is perpendicular to the armchair-direction compressive strain, enabling the determination of strain direction without fitting. Furthermore, we were the first to identify that continuous wave (cw) laser annealing not only generates oxidative etching on the 1L-MoS2 basal plane but also releases the tensile strain induced by the CVD process. Tensile strain release via cw laser annealing is confirmed by the reversed trend of E' mode softening and the blue-shifted A0 peak. SHG mapping allows for the direct observation of areas annealed with a 5-second cw laser, which are indiscernible in optical microscopy and Raman intensity measurements. Additionally, SHG spectroscopy requires only 1/100th of the time required for Raman spectroscopy, highlighting its potential as a real-time rapid inspection tool. We also quantified the SHG depolarization caused by thermal damage and found that only 1 minute of laser annealing can elevate SHG depolarization from 6.92% to 7.53%. Due to the highly overlapping phonon peaks in the Raman spectrum of 1L-WSe2, accurately assessing crystal quality is extremely challenging. Therefore, we utilized a custom-built HRRS system to investigate the thermal degradation properties of 1L-WSe2 under cw laser exposure, while contrasting the detection capabilities of other nonlinear optical methods. This study is the first to resolve the stiffened E' mode and the softened A'1 mode of 1L-WSe2 through HRRS, indicating the presence of compressive strain and n-type doping introduced by thermal degradation, respectively. In comparison, conventional Raman spectroscopy and PL spectroscopy only captured alterations in signal intensity. We also confirmed that thermal damage notably reduces the SHG intensity of 1L-WSe2 and induces SHG depolarization. SHG polarization anisotropy was corroborated to be calculable under both co-polarized and cross-polarized detection geometries. This thesis employs nonlinear optical detection techniques to comprehensively unearth intrinsic edge defects and thermal damage characteristics of CVD 1L-MoS2, while demonstrating the HRRS’s potential in evaluating the crystal quality of 1L-WSe2. Furthermore, we validated the practicality of SHG detection for rapid quality assessment and gauged the degree of SHG depolarization induced by defects. These findings advance applications and research in 1L-MoS2 and 1L-WSe2, accentuating the remarkable possibilities of nonlinear optical detection in assessing the quality of 2D materials.