寬度引發的不均勻性對石墨烯傳導的影響

Abstract

自 2004 年以來，石墨烯材料被機械式剝離法取得之後，引發學界的高度興趣。由於石墨烯具有高導電、導熱以及高載子遷移率的特性，很多的研究專注於研究高載子遷移率下的費米子傳輸行為。然而,低遷移率的石墨烯仍具有高度的潛力。在很多研究中指出，由於雜質摻雜所造成的低載子遷移率的石墨烯樣品上可以觀察到能隙被打開的現象。在本篇論文中,我著重於導電通道的寬度對於無序性石墨烯樣品的影響。 藉由光蝕刻技術，我們把樣品製作成標準的四點量測形狀(Hall bar pattern)。其中導電通道的寬度分別為 1.25、5.90 以及 25.09 μm。我們逐步量測從 2 K 到 30 K 電流對電壓的關係圖，其中樣品在溫度低的區間中展現出高度的非線性，而此非線性區域與 Variable range hopping 的電子傳輸機制有高度的關聯性。在不同寬度的樣品中，我們發現導電通道較寬的樣品上適用這個模型。因此我們推測寬度較窄的樣品可能存在某種機制導致電性傳輸比寬的樣品好。此現象也可以藉由電流密度對電壓的關係圖中展現出來，較窄的樣品中面電流遠大於較寬的樣品。透過拉曼分析表面的雜質強度，我們發現到雖然摻雜的濃度相同，但是在窄的樣品上不均勻性較高。此不均勻性反而使整體的導電 性增加。 此份研究嘗試了不同寬度的導電通道在無序性石墨烯樣品上的研究，在未來的展望中，可以測量高載子遷移率的樣品下傳導的差異性以及不同寬度的石墨烯樣品在磁場下載子傳輸的影響。

Ever since graphene, a single layer of graphite, was realized experimentally by the mechanically exfoliated method in 2004, it has become a major field of interest in the condensed matter physics. It has many fascinating properties, such as high electric conductivity, thermal conductivity, and mobility, etc. Also, with the features of massless fermion as carriers, many researches have been focused on the investigation in high-mobility graphene devices. On the other hand, low-mobility ones also show great potential with an opened band gap and other impurities-scattering effects. In this thesis, we focus on the width effect in a disordered graphene system to its electronic transport properties. By using processes involving a photolithography method, we patterned our chemical vapor deposited (CVD) graphene into devices with different channel width Hall bars. The channel widths of our devices are 1.25, 5.90, and 25.09 μm respectively. Current-voltage (I-V ) measurements were performed from 2 K to 30 K with a 2 K temperature increment. At the low temperature region, the nonlinear I-V curve at the low field region was observed. The resistance-temperature (R-T ) diagrams, which reveals the hopping mechanism within each channel, was derived through differentiating the inversed slope of the I-V diagrams. By comparing the R-T diagrams among three devices, the one with narrowest channel width was found to have the highest current density. Combing these results with the standard deviation of impurity strengths acquired from Raman spectroscopy, we conclude that the width induced inhomogeneity in a graphene system can act as a major impact on its electronic transport behavior. Our work opens the door to the study of graphene samples with different channel widths. More works could be investigated in the near future, including subjects regarding high-mobility samples and the transport under a vertical magnetic field. Other intrinsic effects may come out to complete the whole picture on the transport behavior of this fascinating material.