雙層石墨烯之載子傳輸之研究

Abstract

我們利用機械剝離法來製造雙層石墨烯至一片上面有300奈米厚的二氧化矽的高參雜濃度的矽基版上。然後我們利用光學顯微鏡測量其對比度並與拉曼光譜做比對來決定其層數。利用一套標準的電子束微影與物理氣化沉積法來製作鉻/金電極。 我們測量了在1.5 K到300 K之中，不同溫度下電阻隨著閘極電壓的變化。以及研究了在遠離載子中性點(CNP)時電阻隨著溫度改變的變化。 我們發現在低溫的區域中在遠離CNP時電阻隨著溫度改變的變化是由近藤效應主導。載子濃度越低，近藤溫度就越低。在高溫的區域中是由變程躍遷主導。加總這兩個導電途徑(channel)可以描述我們的數據。我們找到有文獻用計算的方式討論出這種金屬態與絕緣體態共存的傳導行為。在我們的樣品之中的金屬傳導行為是近藤效應，是由閘極引發的導電載子引起的。而絕緣體傳導行為是變程躍遷，是由電子電洞低漥區(electron-hole puddles)引起的

We use mechanical exfoliation to prepare bilayer layer graphene and substrate is heavy doping Si chip with 300nm SiO2 film. After that, we measure contrast by optical microscope and Raman spectra for determining the number of layers. Contact electrodes are made of Cr/Au by a stander procedure of electron beam lithography and physical vapor deposit (PVD). . We measure the gate voltage dependence of the resistance at several temperatures from 1.5K to 300 K, and study the temperature dependence of resistance.

We find that the temperature dependence of resistance R (T) faraway from CNP is dominated by Kondo effect at low temperature. The lower the carrier density, the lower the Kondo temperature. At high temperature, it is dominated by variable range hopping (VRH). The summation of the two conduction channel describes the whole R (T) behavior.

We find that there is a literature discussing this coexistence of metallic and insulating transport behavior by calculation. For our sample, metallic transport behavior is Kondo effect from gate-induce charges and insulating transport behavior is VRH from electron-hole puddles.