| 摘要: | 延續摩爾定律(Moore′s Law)的發展，半導體尺寸不斷萎縮下，開始產生許多靜電效應、短通道效應、高功耗及高接觸阻抗等等問題，尤其是在矽的電傳輸特性上，隨著尺寸不斷縮小，載子遷移率(Carrier mobility)也會急遽下降，為了改善其問題，其中在二維(Two-dimensional, 2D)材料的優點為表面沒有懸鍵，因此在尺寸不斷萎縮下，並不會對導致遷移率下降，其中二維材料以過渡金屬硫化物(Transition metal dichalcogenides, TMDCs)擁有適當能隙及良好傳輸特性等而受到關注。
然而在電晶體上由於金屬與半導體材料接觸時會產生MIS(Metal−insulator−
semiconductor)費米能階高度差，當電子要穿隧時會產生蕭特基能障(Schottky barrier)，為了改善其接觸阻抗，因此出現了相位工程(Phase engineering)、邊角接觸(Edge contact)、嵌入緩衝層(Inserting buffer layer)、凡德瓦接觸(Van der waals)、間隙狀態飽和(Gap-state saturation, GSS)、分子或化學摻雜(Molecular/chemical doping)，雖然以上方法大多具有不穩定性或金屬熔點太低會對後段半導體造成擴散等問題。
本研究首先研究MoS2的合成與轉印參數的優化並以基礎元件作為驗證，接著聚焦於利用氫電漿對MoS2表面進行低損傷轟擊，並對上層表面的硫剝除，此時讓裸露出來的Mo直接與後續沉積的金屬直接接觸，讓金屬與金屬接觸以改善接觸阻抗，其中使用拉曼(Raman)、光致發光(Photoluminescene, PL)光譜進參數測試，在MoS2主要峰值為E2g及A1g，在電漿處理20分鐘時，其特徵峰會消失，並且未發現J1、J2、J3峰值，因此證明不是相轉變，而從X射線光電子能譜儀(X-ray photoelectron spectroscope, XPS)中可以發現從原本S/Mo比從2降至1.02，並利用原子力顯微鏡(Atomic force microscope, AFM)進行厚度量測發現從原本1.264nm降至0.654 nm，證明此實驗成功地達到選擇性上層硫原子的剝除。
此外，也針對SnS2進行討論，氫電漿剝除上層硫再加熱硒粉進行硒化，使用Raman進行參數測驗及分析，當SnS2的A1g與SnSe2的A1g在等高狀態為我們所要的SnSSe的材料，後續使用XPS證明其表面元素比例變化，Sn:S從1:1.74到進行硒化後Sn:S:Se為1:0.88:0.79，在開爾文探針力顯微鏡(Kelvin probe force microscope, KPFM)上兩者分別為5.269 eV、5.145 eV，最終以AFM驗證上此方法可成功將多層SnS2減薄成雙層厚度並進行硒化，而獲得SnSSe，並研究其電傳輸特性特性，可提升載子遷移率與N型摻雜。
;With the continuous development of Moore′s Law, the shrinking size of semiconductors has brought about various challenges such as electrostatic effects, short-channel effects, high power consumption, and high contact impedance. Particularly in silicon, the electrical transport properties are affected as the size decreases, leading to a significant drop in carrier mobility. To address these issues, the advantages of two-dimensional (2D) materials come into play. One of the key advantages of 2D materials is the absence of dangling bonds on their surfaces, which allows for better preservation of carrier mobility as the size decreases. Among 2D materials, transition metal dichalcogenides (TMDCs) have gained significant attention due to their suitable bandgaps and excellent transmission characteristics.
However, in transistors, when metal comes into contact with semiconductor materials, there is a Fermi level offset in the MIS (Metal-Insulator-Semiconductor) structure, resulting in a Schottky barrier when electrons tunnel across. To improve the contact impedance, various methods have emerged, including phase engineering, edge contact, inserting buffer layers, Van der Waals contacts, gap-state saturation (GSS), and molecular or chemical doping. However, most of these methods suffer from instability issues or low metal melting points that can lead to diffusion problems in the subsequent semiconductor layers.
In this research, the synthesis and transfer parameters of MoS2 were first investigated and verified using basic devices. The focus then shifted to the use of hydrogen plasma for low-damage bombardment of the MoS2 surface and the removal of the sulfur layer on the surface. This exposed the Mo surface, which directly contacts the subsequently deposited metal, improving the contact impedance. Raman spectroscopy and photoluminescence (PL) spectroscopy were used for parameter testing. The main peaks in MoS2 were identified as E2g and A1g. After 20 minutes of plasma treatment, these characteristic peaks disappeared, and no J1, J2, or J3 peaks were observed, indicating that no phase transition occurred. X-ray photoelectron spectroscopy (XPS) revealed a decrease in the S/Mo ratio from the original 2 to 1.02, and atomic force microscopy (AFM) measurements showed a reduction in thickness from 1.264 nm to 0.654 nm. These results confirm the successful selective removal of the upper layer of sulfur atoms in this experiment.
In addition, SnS2 was also discussed in this study. The upper layer of sulfur was removed using hydrogen plasma, followed by annealing with selenium powder for selenization. Raman spectroscopy was used for parameter testing and analysis. When the A1g peaks of SnS2 and SnSe2 overlapped, it indicated the formation of the desired material, SnSSe. X-ray photoelectron spectroscopy (XPS) was used to confirm the changes in surface element ratios. The Sn:S ratio changed from 1:1.74 to 1:0.88:0.79 (Sn:S:Se) after selenization. Kelvin probe force microscopy (KPFM) measurements showed the respective work function values of 5.269 eV and 5.145 eV for SnS2 and SnSSe. Finally, atomic force microscopy (AFM) was employed to verify that this method successfully reduced the thickness of multilayer SnS2 to bilayer and achieved selenization, resulting in SnSSe. The electrical transport characteristics of SnSSex were studied, showing improved carrier mobility and N-type doping. |
