雙層石墨烯之載子傳輸與低頻雜訊之研究

Abstract

本實驗以膠帶沾黏石墨塊後，透過不斷重複撕黏的方式，將石墨的厚度不斷削薄，最後壓印在基板上，基板表面有一層厚度300 nm的二氧化矽，經由拉曼光譜與光學的對比度做互相對照，確認石墨烯層數，並集中在雙層石墨烯，再透過電子束微影製程製作鉻金電極。經由四點量測，來研究低溫下雙層石墨烯的載子傳輸性質與低頻雜訊。 由低頻雜訊的量測中，可以發現雙層石墨烯的電阻可以受到閘極電壓所調控，經由固定在不同的閘極電壓並量測其低頻雜訊，可以發現不同溫度下雜訊強度A隨著閘極電壓改變而變化，而且在電中性點(CNP)的A值都是最小的。 在載子傳輸的測量中，因為電阻與閘極電壓的關係圖中無法判斷CNP的位置，而研究其造成的原因，透過研究不同閘極電壓下的電阻與溫度關係圖發現在溫度較低時可以用近藤效應(Kondo effect)來描述電阻的行為，而在30 K以上則可以再加上變程跳躍(VRH)機制來說明，最後由外加垂直磁場發現樣品是一正磁電阻，其近藤效應並非由磁性雜質所產生，推測此樣品可能存在著雜質或碳缺陷，造成CNP附近的電阻因能隙之間的局域態(localized states)較多而下降。

In this experiment, tapes were used to stick a part of graphite from bulk and then tear repeatedly on the substrates for thinning the thickness of the graphite on the tapes. Finally, very thin graphite (graphene) was imprinted on the SiO2/Si substrates, where the SiO2 layer is 300 nm thick. Through Raman spectroscopy, we can find the relation between the layer number of graphene and the contrast of optical image. Here we focused on bilayer graphene. Graphene devices were made by the technique of electron beam lithography and thermal evaporation of chrome/gold electrodes. We used the four-probe measurement to study carrier transport properties and low frequency noise from room temperature to low temperatures. The resistance of bi-layer graphene can be tuned by back-gate voltages (that is the R(Vg)). From the measurement of low frequency noise, we have found 1) the noise amplitude A is minimized at carrier neutrality point (CNP), 2) at fixed gate voltages noise amplitude A varies with temperatures. The R(Vg) of some samples showed double-peak structures, therefore the positions of CNP cannot be well determined. This phenomena may be due to the existence of impurities or carbon defects which induce some localized states in the gap of band structure of bi-layer graphene. The higher density of states at mid-gap can cause the dip of R(Vg) and also double-peak structure near the CNP. By studying the relationship between resistances and temperatures (R(T)) at different gate voltages, we have found the R(T) behavior at liquid helium temperatures can be well described by the model of Kondo effect. Moreover, at temperatures higher than 30 K, variable range hopping is gradually coupled into the carrier transport and dominate R(T) behavior at higher temperatures. While applying a perpendicular magnetic field on the sample, the sample showed a positive magnetoresistance, which suggesting that the Kondo effect in these bi-layer graphene is not induced by magnetic impurities.