六方氮化硼基板應用於二硫化鉬薄膜電晶體與二硫化鉬-黑磷異質接面特性探討

Study of MoS2 TFT with Hexagonal Boron Nitride Substrate and MoS2-Black Phosphorus Heterostructures

洪彬超(Pin-Chao Hung)

指導教授 ： 李嗣涔

國立臺灣大學/電機資訊學院/電子工程學研究所/碩士(2018年)

https://doi.org/10.6342/NTU201800956

摘要

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利用二氧化矽基板製作出薄膜電晶體，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。其電晶體最好的場效電子遷移率可以達到44.7 cm2/V-s與最低遲滯為46.2 V。相較於二氧化矽，六方氮化硼具有較平滑的表面且無懸浮鍵與電荷缺陷，故以其取代二氧化矽作為二硫化鉬薄膜電晶體的基板以改善場效電子遷移率與降低遲滯效應。藉由導入六方氮化硼基板，二硫化鉬薄膜電晶體的場效電子遷移率能提升至117 cm2/V-s，遲滯降低至5 V。此外，藉由比較二硫化鉬薄膜電晶體在兩種不同基板且不同大氣環境下，顯示遲滯效應主要是由二硫化鉬與氧化層之間接面特性所影響，而吸附在二硫化鉬表面的水氣與氧氣並不會造成遲滯效應。然而吸附在表面的水氣與氧氣會使二硫化鉬薄膜電晶體的特性隨時間而下降，因此六方氮化硼被用來當作二硫化鉬薄膜電晶體的保護層並且能使其有更好的穩定度。
接著使用三氟甲烷電漿使二硫化鉬薄膜有p型電洞參雜，且藉由X光光電子能譜儀可得知參雜後其費米能階往價電帶下降0.4 eV。因此藉由三氟甲烷電漿參雜二硫化鉬形成p區並結合未參雜的二硫化鉬(n區)成功的製作出二硫化鉬整流二極體，其開關電流比值大約為100倍、理想因子為2.44。然而為了有更好的理想因子(介於1與2 之間)，p型二維材料黑磷被用來取代三氟甲烷電漿參雜的二硫化鉬並與未參雜二硫化鉬結合製作出二硫化鉬-黑磷異質接面，其開關電流比值大約為30倍、理想因子為1.84。最後利用寬頻氙燈量測二硫化鉬整流二極體與二硫化鉬-黑磷異質接面的光學特性，可以發現雖然二硫化鉬-黑磷異質接面的光響應比二硫化鉬整流二極體還要小，但二硫化鉬-黑磷異質接面有著比二硫化鉬整流二極體還要好的光偵測能力。

關鍵字

二硫化鉬薄膜二極體 ； 六方氮化硼 ； 遲滯效應 ； p型電洞參雜 ； 二硫化鉬-黑磷異質接面

並列摘要

In this thesis, the mechanically exfoliated 2D material MoS2 nanosheet was successfully used to fabricate thin film transistor (TFT) on SiO2 substrate. By 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 mobility of MoS2 TFT on SiO2 substrate is 44.7 cm2/V-s and the lowest hysteresis is 46.2 V. In order to further improve the mobility and reduce the hysteresis, the SiO2 substrate was replaced by h-BN substrate because h-BN has an atomically smooth surface that is relatively free of dangling bonds, charge traps and is naturally flat which can improve the interface property between MoS2 and oxide layer. By introducing h-BN substrate, the mobility of MoS2 TFT can be improved to 117 cm2/V-s and its hysteresis can be reduced to 5 V. Furthermore, by comparing the performance of MoS2 TFTs among SiO2 and h-BN substrates with different atmospheric environment, it shows that interface property between MoS2 and oxide layer would influence hysteresis seriously and the molecules such as H2O and O2 adsorbed on MoS2 surface would not cause hysteresis. However, the molecules such as H2O and O2 adsorbed on MoS2 surface would degrade the characteristics of MoS2 TFT. Therefore, the h-BN passivation layer is used to passivated the MoS2 TFT and it shows it has better stability by using h-BN passivation layer.
Besides, the CHF3 plasma was used to dope the MoS2 film to p-type material. By using XPS analysis, it shows that the Fermi level of MoS2 film would shift about 0.4 eV toward the valence band. Therefore, CHF3 plasma doping is used to generate p-region and combine with pristine MoS2 (n-region) to fabricate MoS2 rectifying diode. The current rectification ratio is about 2 order and its ideality factor is 2.44. In order to have better ideality factor (in the interval between 1 and 2), black phosphorus(BP) which is p-type 2D material is used to combine with MoS2 to fabricate MoS2-BP heterostructure n-p junction. The current rectification ratio is 30 and its ideality factor is 1.84 which is better than MoS2 rectifying diode. Finally, the Xe lamp which is broadband light source is used to measure optical properties of MoS2 rectifying diode and MoS2-BP heterostructure n-p junction. It shows that although MoS2-BP heterostructure n-p junction has lower responsivity than MoS2 rectifying diode, it has better photodetecting ability than MoS2 rectifying diode.

並列關鍵字

molybdenum disulfide (MoS2) thin film transistor (TFT) ； hexagonal boron nitride (h-BN) ； hysteresis effect ； p-type doping ； MoS2-Black Phosphorus heterostructure

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