金屬/石墨烯介面傳輸機制研究

Abstract

石墨烯具高載子遷移率與低電阻率等優異特性，其電性引起學者與產業界高度興趣，由於電性上常以兩點方式進行測量，因此石墨烯與金屬電極間的接觸電阻對元件的影響應該加以考慮，才可使得兩點量測之石墨烯奈米元件的電性傳輸更加清楚。 本實驗我們採用膠帶剝離的方式從石墨中取得厚度約3-5 nm的石墨烯，利用電子束微影與熱蒸鍍技術製成兩點量測的奈米元件，量測其電學特性。元件室溫電阻值為103-105 Ω，並由閘極效應量測結果得到石墨烯的特殊性質-雙極性特性，其顯示之狄拉克點證明我們所製作的元件存在石墨烯特性，但過大的電阻率說明了接觸電阻的存在，此來源之一可能是鈦在蒸鍍過程轉變為鈦氧化物無序半導體，另一則是製程過程中所殘留之光阻劑，無論是鈦氧化物或光阻劑，等效而言，電極與石墨烯接面皆產生了一位能障。對於金屬與石墨烯接面之傳輸機制，可藉由電阻與溫度的變化關係，利用熱擾動引致穿隧效應模型擬合來做探討，其結果顯示電子在低溫下是以穿隧方式通過接面之位障，隨著溫度的增加，穿隧機率逐漸受熱擾動影響而變大，電阻值逐漸降低，在高溫時，電子由熱活化效應所主導，並發現樣品電阻值與穿隧機率成反比，因為在固定電壓下，穿隧電流的增加反應出電阻值的下降，這項結果更驗證了熱擾動引致穿隧效應模型在本次實驗定性上成功地做了合理的解釋。

Graphene has recently been the most shining star in the carbon family due to its significant potential for fundamental studies and applications in future electronics. Because most electronics have a two-probe configuration with source and drain electrodes for electrical operations, the contact problem shall be encountered inevitably. Identification and determination of the contact effect, therefore, become very important. In this study, our graphene films are prepared and positioned on silicon substrates capped with a 300 15 nm thick SiO2 layer by mechanical exfoliation from the highly oriented pyrolytic graphite. Graphene films are made with a thickness down to 3-5 nm and an area up to several μm2. Prior to electron beam lithography process, the as-prepared graphene films are annealed in a high vacuum at 400 0C for 6 h in order to remove any contamination on its surfaces. Electron beam lithography technique is then employed to fabricate electrodes in connection with the graphene. The separation distance between two nearest-neighbor electrodes is kept constant to be ~2 μm for all devices. After the fabrication of two-probe graphene devices, their temperature dependence of electrical properties is measured. The room-temperature (RT) resistance of all graphene devices ranges from 103 to 105 Ω, higher than thoses reported in literatures. This large RT resistance significantly hinders the outstanding performance of original graphene channels and causes a quite low mobility (< 102 cm2/V s) in our graphene devices. We conjecture that the large RT resistance comes from the nanocontact resistance at electrode/graphene interface. In addition, the resistances as function of temperature of these graphene devices show a temperature independent behavior, like tunneling resistance, at low temperatures. On the other hand, the resistance depends comparably on temperature at a high temperature range. Moreover, electron transport in graphene devices is well-fitted with the fluctuation-induced tunneling model in the whole temperature range between 300 K and 5 K.