二維材料電漿摻雜及其薄膜電晶體應用

Abstract

本論文使用機械剝離法分離出擁有奈米級厚度的二硫化鉬並製作出薄膜電晶體，利用光學顯微鏡及原子力顯微鏡的搭配篩選出較佳厚度範圍的二硫化鉬，並利用低功函數金屬鉻當作金屬電極來達成歐姆接觸。其電晶體最好的電流開關比可以高達8個數量級，最好的場效電子遷移率可以達到約31 cm2/V-sec。 接著使用氣體電漿摻雜二硫化鉬，藉由改變其二硫化鉬的費米能階對其進行材料改質，利用離子反應蝕刻機，三氟甲烷氣體電漿對二硫化鉬表面進行改質，成功進行p型電洞摻雜並使其費米能階往價電帶下降0.7~0.8 eV，從電性圖中，在20分鐘後的摻雜下，其截止電壓往正電壓位移7.9 eV，電洞摻雜濃度為5.7×1011 cm-2，除此之外，使用電漿輔助化學沉積系統中的磷化氫電漿進行n型電子摻雜，從電性圖中，在15分鐘後的摻雜下，其截止電壓往負電壓位移2.1 eV，電子摻雜濃度為1.5×1011 cm-2，然而，在真空環境的材料分析下，由於二硫化鉬和含磷粒子間微弱的作用力，含磷粒子無法吸附在二硫化鉬表面上。 我們發現在空氣中二硫化鉬表面會物理吸附環境中的水氣和氧分子，造成電洞摻雜的效應並且產生遲滯現象，最後透過氧化鋁保護層去隔絕元件和外在環境中的粒子來減少遲滯效應的產生。

In this thesis, the mechanically exfoliated 2D material MoS2 nanosheet was successfully used to fabricate thin film transistor. Using optical microscopy and atomic force microscopy, the MoS2 flakes with appropriate thickness can be chosen. Ohmic contact of MoS2 TFT can be achieved by low work function metal Chromium. The highest on/off current ratio of MoS2 TFT was up to 8 order of magnitude and the mobility of 31 cm2/V-sec was achieved. The gas plasma treatment was used to dope the 2D material MoS2. It modified the material by tuning its Fermi-level. CHF3 plasma was used to treat MoS2 surface by RIE. It showed p-doping effect and the Fermi level shifted 0.7~0.8 eV toward the valence band. In I-V characteristics, the threshold voltage also showed p-doping effect and shifted 7.9 V to higher positive voltage after 20 minutes plasma treatment. The induced carrier charge density was 5.7×1011 cm-2. In addition, PH3 plasma was used by PECVD. In I-V characteristics, the threshold voltage showed n-doping effect and shifted 2.1 V to more negative voltage after 15 minutes plasma treatment. The induced carrier charge density was 1.5×1011 cm-2. However, the P-related dopants were desorbed under high vacuum material analysis because of their weak interactions between dopants and MoS2. Besides, it was found that the oxygen and water molecules were easily adsorbed at the MoS2 surface in air, which would lead to p-doping effect and hysteresis in devices. Finally, Al2O3 passivation layer was set to isolate MoS2 surface from ambient air to make a hysteresis free device.