製備超低接觸電阻石墨烯場效電晶體與金屬石墨烯介面特性研究

Abstract

接觸電阻的大小對於石墨烯元件的效能具有關鍵性的影響，本研究使用了一種無須高分子輔助的轉印手法將石墨烯轉印到所需要之基板，以確保石墨烯樣品表面完全清潔且沒有高分子殘留物，進而降低石墨烯與金屬電極間之接觸電阻以及提升元件效能。我們也使用了許多不同的結構與及方法對於接觸電阻進行進一步的優化。例如：使用真空退火，水溶液高分子摻雜，或是雙層金屬電極結構改良石墨烯元件與金屬電極之接觸特性。可進一步發現，使用無高分子轉印方法製備而成的元件，其退火溫度不需要如先前研究之高溫(大於2~300℃)，僅需100℃退火一小時便可達到最優良之接觸電阻(305 Ω•µm).而其中改良效果最為明顯之樣品為使用高分子摻雜過後，石墨烯之功函數具有大幅度提升之P型石墨烯。其接觸電阻可大幅降低至36 Ω•µm，而在經過低溫熱退火後則可降低至24 Ω•µm。本實驗以改良後之超低接觸電阻石墨烯場效電晶體進行電性量測後，可發現元件之載子遷移率具有大幅度的提升，搭配ODTS自我組裝層所修飾過之基板後便可得到電洞遷移率約為14400 cm2 V−1 s−1之 石墨烯電晶體。而經由水溶液P型摻雜修飾過之石墨烯雖然會因為庫倫散射之原因使其載子遷移率下降，但由於可大幅大改善石墨烯與金屬間之接觸電阻，因此載子遷移率亦可達到約5173 cm2 V−1 s−1 ，仍舊是非常具有競爭力之效能。另外本研究同時也對於金屬與石墨烯間之交互作用以光電子能譜術進行介面研究，發現金屬與石墨烯之間除了因為功函數的差異而影響載子流動外，亦會因為介面電荷的累積使得金屬與石墨烯之介面略呈N型摻雜，因此影響了金屬對石墨烯的摻雜效應，而使用高分子摻雜則無此現象發生。本研究對金屬以及石墨烯之介面特性，利用光電子能譜術做了完整的調查，也設計了降低石墨烯與金屬電極間之接觸電阻之結構與方法，除了提升石墨烯應用於各式元件之價值外，更可進一步搭配其他二維材料，使得二維材料的研究與應用能夠更廣更全面，以期對科學以及工業界的發展帶來更進一步的幫助。

Contact resistance is crucial for graphene base devices. We have developed a polymer-free (PF) transfer method to obtain a clean graphene surface to improve the graphene-metal contact. We also examined the optimum annealing temperature and time for the PF transfer of graphene, and observed that annealing at 100 °C for 1 h could reduce the contact resistance of graphene/Au from 833 to 305 Ω•µm. To further tune the work function of graphene to reduce contact resistance, we report a solution doping combine with PF method , and the contact resistance is dramatically reduced to 24 Ω•µm by P-doped graphene, which is the best result obtained in this study . We also developed a hydrophobic octadecyltrichlorosilane (ODTS) self-assembled monolayer (SAM) to prevent the substrate effect and combined this with the PF graphene and low contact resistance process. The mobility was enhanced by 200%, and the highest mobility value recorded was 14400 cm2 V−1 s−1for intrinsic graphene after annealing. And the mobility of P-doped graphene is 5173 cm2 V−1 s−1 which is similar to the intrinsic graphene on ODTS. Most importantly, our research also gives an investigation of graphene-metal interfaces to improve the metal contact resistance. And these results can be applied on many graphene metal base devices and significantly improving the performance.