六方氮化硼基板應用於二維材料二硫化鎢薄膜電晶體特性探討

本論文使用機械剝離法分離出擁有奈米級厚度的二硫化鎢，並製作出薄膜電晶體，在機械剝離法中，搭配聚二甲基矽氧烷去除二硫化鎢表面的殘膠，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鎢，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7個數量級，最好的場效電子遷移率可以達到約38 cm2/V-sec。由於二硫化鎢的特性隨著厚度不同而有不同的變化，加上機械剝離法分離出隨機厚度的二硫化鎢，因此利用離子反應蝕刻機，四氟甲烷電漿進行二硫化鎢的蝕刻，得到適當厚度範圍的二硫化鎢薄膜電晶體。在電性圖中，經過4秒鐘的蝕刻，其臨界電壓往正電壓位移27 V，電洞摻雜濃度為1.94×1012 cm-2，此外，由於蝕刻對二硫化鎢表面的破壞,電晶體的電流開關比及場效電子遷移率皆下降。傳統上，使用二氧化矽作為基板，但其場效電子遷移率遠小於理論預測結果，原因為二氧化矽與二硫化鎢間的缺陷導致，因此，選擇結構相近且無懸浮鍵的六方氮化棚作為基板，在電性圖中，電流開關比達到7個數量級，場效電子遷移率高達106 cm2/V-sec，相比二氧化矽基板，提升179% 在空氣中二硫化鎢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，最後使用氮化棚基板和退火可完全消除遲滯效應。

關鍵字

二硫化鎢薄膜電晶體；聚二甲基矽氧烷；六方氮化棚；電洞摻雜效應；退火；遲滯效應

並列摘要

In this thesis, the mechanically exfoliated 2D material WS2 nanosheet was successfully used to fabricate thin film transistor. Using 3M scotch tape method and PDMS stamp can avoid the residues of 3M scotch tape being left on the surface of WS2. Using optical microscopy and atomic force microscopy, the WS2 flakes with appropriate thickness can be chosen. Ohmic contact of WS2 TFT can be achieved by low work function metal Chromium. The highest on/off current ratio of MoS2 TFT was up to 7 order of magnitude and the mobility of 38 cm2/V-sec was achieved. The electronic and physical properties of WS2 are greatly dependent on the layer thickness and owing to the scotch tape method , the thickness of the WS2 flakes is random. Therefore , in order to get the appropriate thickness of WS2 , CF4 plasma is used to control the thickness of WS2 flakes by reactive ion etching (RIE). From XPS spectra after etching, the Fermi level of the WS2 moves downward valence band which represents the p-doping effect. In I-V characteristics, the threshold voltage shifts to higher positive voltage about 27 V and 1.94×1012 cm-2 induced carrier charge density are achieved after 4 sec CF4 gas plasma etching. In addition, owing to the damage of the WS2 surface after etching, the on/off current ratio and mobility both decreased. In general, the mobility of WS2 TFT on the SiO2 substrate measured in experiments is much lower than the theoretical calculation and it can be attributed to the defects existing in the WS2/SiO2 interface. Hence, the hexagonal boron nitride (h-BN) is an ideal substrate because it provides an atomically flat surface without dangling bonds and charged impurity. In I-V characteristics, the on/off current ratio is up to 107 and the field-effect mobility is 106 cm2/V-sec. The mobility is enhanced by 179%. Besides, it is found that the oxygen and water molecules are easily adsorbed at the WS2 surface in air, which would lead to p-doping effect and hysteresis in devices. The hysteresis effect and the humidity issue can be reduced by annealing. Further, the hysteresis effect can be completely eliminated with h-BN substrate because it has few charge impurities.

並列關鍵字

tungsten disulfide (WS2) thin film transistor ； polydimethylsiloxane (PDMS) ； hexagonal boron nitride (H-BN) ； p-doping effect ； annealing ； hysteresis effect.

參考文獻

51. Cui, Y.; Xin, R.; Yu, Z.; Pan, Y.; Ong, Z. Y.; Wei, X.; Wang, J.; Nan, H.; Ni, Z.; Wu, Y.; Chen, T.; Shi, Y.; Wang, B.; Zhang, G.; Zhang, Y. W.; Wang, X., High-Performance Monolayer WS2 Field-Effect Transistors on High-kappa Dielectrics. Adv Mater 2015, 27 (35), 5230-4.

2. Schwierz, F., Graphene transistors. Nat Nanotechnol 2010, 5 (7), 487-96.

4. Pacilé, D.; Meyer, J. C.; Girit, Ç. Ö.; Zettl, A., The two-dimensional phase of boron nitride: Few-atomic-layer sheets and suspended membranes. Applied Physics Letters 2008, 92 (13), 133107.

5. Jo, S.; Ubrig, N.; Berger, H.; Kuzmenko, A. B.; Morpurgo, A. F., Mono- and bilayer WS2 light-emitting transistors. Nano Lett 2014, 14 (4), 2019-25.

6. Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A., Single-layer MoS2 transistors. Nat Nanotechnol 2011, 6 (3), 147-50.

國際替代計量

六方氮化硼基板應用於二維材料二硫化鎢薄膜電晶體特性探討

全文下載

主題瀏覽