探討還原氧化石墨烯於強電場下之電性傳輸機制

Abstract

本實驗使用Hummers method所製備的氧化石墨烯，利用凡德瓦力將氧化石墨烯附著於二氧化矽基板上，並透過原子力顯微鏡判斷氧化石墨烯的厚度，單層還原氧化石墨烯厚度約為1 nm，藉由高溫退火方式來去除氧化石墨烯上的氧官能基，形成還原氧化石墨烯，及掃描式電子顯微鏡及電子束微影系統對還原氧化石墨烯進行定位及電極微影，最後使用熱蒸鍍系統成長元件的金屬電極並用高溫加熱爐系統減少接點電阻產生，即完成還原氧化石墨烯元件。 本實驗為兩點探針量測，所測得元件的室溫電阻於零閘極偏壓下介於10P5P~10P7P Ω間，推得還原氧化石墨烯上氧覆蓋率於16~20%間，且電阻值隨著環境溫度降低而上升，其電性符合二維變程式跳躍傳輸特性，若還原氧化石墨烯於更高電場環境時，推測出其電性應該符合電場主導的二維變程式跳躍傳輸，從實驗發現還原氧化石墨烯在高場區段隨著環境溫度提高，其載子傳輸越偏離二維變程式跳躍傳輸，因此藉由scaling theory來修正跳躍傳輸方程式，並重新定義出其傳輸方程式需加入溫度項修正，藉以擬合出各溫度下的電性趨勢圖，其中觀察到強電場下的電流密度於高溫有不同級數的變化，推測由Schwinger mechanism所造成，當還原氧化石墨烯於中性點時，受到一高場影響而產生電子-電洞對，因此電流密度隨著電場提高從線性關係變成非線性關係，並定義出轉折電場ERCR，再藉由分析電流與電場間的次方關係，可推得Schwinger mechanism只存在於中性點上。本實驗除了探討還原氧化石墨烯的電性傳輸，還發展成元件上的應用，藉由高電場產生電子-電洞對的特性來改變元件的電性，也加以驗證Schwinger mechanism的效應。

In this study, electron transport of reduced graphene oxide (r-GO) have been investigated. Graphene oxide (GO) sheets are synthesized by the modified Hummers’ method. Graphene oxide sheets are deposited on SiO2 (300 nm)/Si substrates with pre-patterned alignment marks and micrometer scale electrodes. Prior to device fabrication, the r-GO sheets are subjected to thermal annealing at 400 oC for 24h to remove functional groups and to restore GO sheets back to conductive r-GO sheets. Using an atomic force microscope (AFM), the r-GO sheets was determined to be one layer thick. Our r-GO devices with two ohmic contacts are made by using standard electron-beam lithography and thermal evaporation. The r-GO devices show room-temperature (RT) resistances and the oxygen coverage varying from 10P5P~10P7P Ω and 16%~20%, respectively. Moreover, electron transport in r-GO devices can be well-described by Mott’s two-dimensional variable range hopping in low electric field. In addition, the low-temperature current-electric field (I-E) behavior can be well described by Shklovskii expression. However, the I-E curves of r-GO in high electric field do not follow the model prediction at room temperature. We found that the I-E curves show a transformation from linear to nonlinear in a high electric field. The I-E curves show power law behavior in high electric fields as well. We estimate the critical field (EC) for the transition to be EC ≈ 10P5 (V/m). According to Schwinger mechanism, the electron-hole pairs shall be generated near the charge neutrality point. Here, we learn the electric field behavior and compare experimental results with the theoretical model of Schwinger mechanism.