| 摘要: | 矽基電晶體近年來隨著尺寸微縮，伴隨而來的是散熱、漏電流、高功耗及高接觸阻抗等問題，特別是當體厚度小於5 nm時，矽的電傳輸特性發生劣化，如載子遷移率(Carrier mobility)會急遽的降低。而二維(Two-dimensional, 2D)材料由於表面沒有懸鍵、厚度僅有幾個原子層，且遷移率不隨厚度變化而影響等優點，被視為下世代的電子元件材料，其中又以過渡金屬硫屬化合物(Transition metal dichalcogenides, TMDCs)因具有可調控的能隙及優異的電性而備受關注。然而二維電晶體的應用上，受到費米能階釘扎(Fermi-level pinning)的影響，造成調整不同功函數之接觸金屬，皆無法有效降低其接觸阻抗，因而出現如凡德瓦接觸(Van der Waals contacts)、邊接觸(Edge contacts)、摻雜(Doping)、相位工程(Phase engineering)以及嵌入緩衝層(Inserting buffer layer)等方式以改善，然而，這些方式大多都具有不穩定及無法工業製造、相容現有製程等問題。
因此，本研究提出了一種透過氫電漿剝除二硫化鉬(MoS2)表面硫(S)原子層，使鉬(Mo)原子層裸露出來，接著沉積金屬電極，以形成金屬-金屬接觸(Metal-metal contact)以改善接觸阻抗。透過拉曼(Raman)、光致發光(Photoluminescene, PL)光譜，可以觀察到MoS2兩個特徵峰E2g與A1g消失，且不是相轉變，而從X射線光電子能譜儀(X-ray photoelectron spectroscope, XPS)的定量分析，可以觀察到S/Mo由原先1.69下降到1.25，證實S原子有被剝除。
元件的製備則是先比較了濕式與乾式轉印方式的差異，藉由光學影像以及原子力顯微鏡(Atomic force microscope, AFM)形貌分析可以得知表面完整性大約是一樣(98.5%)，而表面潔淨度的比較則是乾式轉印(95.7%)較濕式轉印(93.2%)佳，因此選擇轉印品質較優秀的乾式轉印作為後續元件應用上的製備手法。電性表現方面，電漿處理後的元件與未改質的相比，其開電流與遷移率分別有10及6倍以上的提升，且在經過150C的退火後，金屬電極與金屬Mo原子的接觸會更好，其開電流與遷移率可以有4倍以上的提升，驗證了本方法可改善二維材料金屬接觸而提升電晶體整體電傳輸特性。;The size of silicon-based transistors has been scaling down in recent years, causing problems such as heat dissipation, leakage current, high power consumption, and high contact resistance, especially, when the thickness of silicon is less than 5 nm, the carrier mobility of silicon will decrease rapidly. Two-dimensional (2D) materials are regarded as the next-generation electronic device materials due to their advantages such as no dangling bonds on the surface, only a few atomic layers in thickness, and the mobility is not affected by changes in thickness. Among 2D materials, transition metal dichalcogenides (TMDCs) have attracted much attention due to their tunable energy gap and excellent electrical properties. However, in the application of 2D transistors, due to the effect of Fermi-level pinning, the contact resistance can not decrease by adjusting the contact metals of different work functions. In order to improve contact resistance, there are some methods have been reported, such as van der Waals contacts, edge contacts, doping, phase engineering, and inserting buffer layer et al., but most of these methods are unstable and can not be directly applied in IC industry.
Therefore, this study proposes a method by using hydrogen plasma to strip sulfur (S) atoms on the surface of molybdenum disulfide (MoS2) in order to expose the molybdenum (Mo) atoms and then deposit metal electrodes to form metal-metal atomic conjugation for improving the contact resistance. The characterizations through the Raman and photoluminescence (PL) spectrum, it could be observed that the two characteristic peaks of E2g and A1g of MoS2 are disappeared, and no phase transition. With the quantitative analysis of the X-ray photoelectron spectroscope (XPS), it could be observed that S/Mo decreased from 1.69 to 1.25, confirming that the S atoms have been successfully stripped.
The devices are compared to the differences between wet and dry transfer methods, through the morphology analysis of the optical image and atomic force microscope (AFM), it could be known that the surface integrity is the same (98.5%), and the surface cleanliness is shown that dry transfer (95.7%) is better than wet transfer (93.2%). Therefore, the dry transfer method with better transfer quality is selected as the preparation method for subsequent device applications. In terms of the electrical performance, compared with the non-plasma treated device, the on-current and mobility of the plasma-treated device are increased by more than 10 and 6 times, respectively. After annealing at 150C, the contact between the metal electrodes and Mo atoms would be better, and its on-current and mobility could be improved by more than 4 times, which confirms the feasibility of this method. |
