二維材料電學特性及超穎透鏡的光偏振態之研究

Two-dimensional Materials Based Electronics and Optical Polarization States on Metalens

張文豪(Wen-Hao Chang)

指導教授 ： 黃斯衍

共同指導教授 ： 藍彥文(Yann-Wen Lan)

國立臺灣大學/理學院/物理學研究所/博士(2021年)

https://doi.org/10.6342/NTU202101929

摘要

近年來，二維材料例如石墨烯（graphene）及過渡金屬硫化物（transition metal dichalcogenides, TMDs）已經被廣泛的研究，加上先進奈米科技也被發展用來製作新穎且微小化結構的電子或光學元件，因此二維材料與現今製程技術的整合將變得越來越重要。在此論文中，我們主要探討了缺陷的單層二硫化鉬（Molybdenum disulfide, MoS2）場效應電晶體（field effect transistors, FETs）及石墨烯基極熱電子電晶體（hot electron transistors, HETs）之電特性研究，此外，也特別針對介電質超透鏡（metalens）的光特性進行了實驗與討論。
負微分電阻（negative differential resistance, NDR）效應因其特殊的電子傳輸性質而被廣泛研究於許多應用。然而，有別於完美晶格結構的單層過渡金屬硫化物，此效應受晶格變形或硫原子缺陷所影響的效果更為明顯。因此在此研究中，我們提出了三種方式來製作具有缺陷的單層二硫化鉬場效應電晶體，包含氫氧化鉀（KOH）化學處理，電子束轟擊，與調整合成比例等方法。此外，我們也探討了硫缺陷與電子傳輸或光譜特性（包含Ｘ射線光光電子能譜，拉曼，光致發光）之間的關聯性。根據上述的結果，負微分電阻效應確實可以於缺陷的單層二硫化鉬場效應電晶體中被有效地觀測到，而經由原子的硫鉬比（S/Mo ratio）計算後可發現此效應將發生於硫缺陷的比例約為4.5 ~ 6.5%的組成當中。
另一方面，考量石墨烯與現有矽半導體技術的整合，我們也呈現了可操作於微波頻段的垂直式石墨烯基極熱電子電晶體。其垂直結構是由位於射極-基極（emitter-base）介面的雙層穿隧位能障，以及位於基極-集極（base-collector）介面的自然生成之二氧化矽（SiO2）所組成，其中此研究中所使用的雙層穿隧位能障可由二氧化鈦/二氧化鉿（TiO2/HfO2）或二硫化鉬/六方氮化硼（MoS2/h-BN）構成。我們的元件也呈現了室溫量測下幾個重要的特性，包含了約為65 GHz的本質電流增益截止頻率，相對高的電流密度（∼ 200 A/cm2），高的共基極電流增益（α* ∼ 99.2%）以及適當的共射極電流增益（β* ∼ 2.7）。
除了以上關於二維材料與電子元件整合的探討外，我們也研究了奈米結構超表面（metasurface）的光學特性，由於此結構具有尺度縮小化的優勢，因此對於超薄光學元件的應用也已經吸引了非常多的注意。此研究利用了超透鏡上的奈米結構作為微小相位板來產生徑向上具有不同偏振方向的同心圓偏振光束，並且使用了斯托克斯參數（Stokes parameters）來解析光在行進方向上鑲嵌於光強度中的偏振態。
我們相信在微小化的結構中呈現空間偏振態的多樣性將能夠提供更多關於光學應用的一個嶄新的自由度。此外，缺陷工程技術應用於過渡金屬硫化物及其他二維材料為基底的電子元件中也將展露出更多價值，並在未來的電性應用上開創出一條重要的道路。

關鍵字

負微分電阻 ； 缺陷 ； 二硫化鉬 ； 場效應電晶體 ； 石墨烯 ； 熱電子電晶體 ； Ｘ射線光光電子能譜 ； 拉曼 ； 光致發光 ； 超透鏡 ； 偏振

並列摘要

Two-dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMDs) are widely investigated in the recent decade. Owing to the advanced nanotechnologies which are developed to design novel and much smaller dimensional structures for either electrical or optical applications, integrating 2D-materials with nowadays device fabrications becomes more and more important. In this dissertation, we not only primarily demonstrate the electrical characterizations in both defective monolayer MoS2 field effect transistors (FETs) and graphene base hot electron transistors (HETs), but also specifically reveal the optical characterizations in dielectric metalens.
The negative differential resistance (NDR) effect is one of the special properties which has been extensively studied to design various applications. However, instead of defect free monolayer TMDs, the NDR effect is more profoundly affected by either deformations or sulfur (S) vacancies. Here, the defective monolayer MoS2 FETs are fabricated by means of three approaches such as chemical treatment with KOH, electron beam irradiation and intentional growth of MoS2. Besides, a correlation between the S-vacancies and either electron transport properties or spectroscopic characterizations, including X-ray photoelectron spectroscopy (XPS), Raman, and Photoluminescence (PL), is also presented. With the above knowledges, the NDR behavior can be effectively observed in the defective monolayer MoS2 FET with S-vacancies V_S~ 4.5 % to 6.5 %, which can be calculated from the atomic S/Mo ratio.
Furthermore, considering the integration of graphene with existing silicon semiconductor technologies, we also demonstrate a vertical graphene base hot electron transistor that performs in the radio frequency regime. The vertical structure is consisted of a double layered tunneling barrier (either TiO2/HfO2 or MoS2/h-BN) at emitter-base junction, and a native SiO2 at base-collector junction. Our device exhibits a relatively high current density (∼ 200 A/cm2), high common base current gain (α* ∼ 99.2%), and moderate common emitter current gain (β* ∼ 2.7) at room temperature with an intrinsic current gain cutoff frequency of ∼ 65 GHz.
In addition to the integration of 2D materials and electronics, we also present optical characterizations in nanostructured metasurfaces which have received extensive attention due to their reduced dimensionality for use with ultrathin optical components. We use nanostructures acting as tiny phase plates on a dielectric metalens to generate a concentric polarization beam with different orientations along the radial direction. Stokes parameters are utilized to investigate the comprehensive polarization states embedded in the optical intensity along the propagation direction.
We believe that the variety of spatial polarizations within a miniaturized configuration could provide a new degree of freedom for diverse optical applications. Furthermore, the defect engineered technologies in TMDs and 2D materials-based electronics may also pave the way for revealing more electrical applications in the future.

並列關鍵字

negative differential resistance (NDR) ； defect ； MoS2 ； field effect transistor (FET) ； graphene ； hot electron transistor (HET) ； X-ray photoelectron spectroscopy (XPS) ； Raman ； photoluminescence (PL) ； metalens ； polarization

參考文獻

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