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Title: | 單層石墨烯與石墨烯帶之電傳輸特性研究 Electronic Transport Properties of Monolayer Graphene and Graphene Ribbons |
Authors: | Bi-Yi Wu 吳稟弋 |
Advisor: | 梁啟德 |
Keyword: | 無序石墨烯,單層石墨烯,石墨烯帶,變程跳躍,共軛焦顯微鏡,邊界散射, disordered graphene,monolayer graphene,self-assembled graphene ribbons,variable range hopping,confocal scanning microscopy,boundary scattering, |
Publication Year : | 2019 |
Degree: | 博士 |
Abstract: | 石墨烯優異的電性質被期待在電子元件上有很多應用。然而,在元件製程過程中,石墨烯很容易被化學物質污染而形成無序石墨烯,以致影響石墨烯的電性。因此如何讓石墨烯在製程中不受污染是個很重要的課題。此篇論文將會分成三個主題去討論無序石墨烯與石墨烯帶對電傳輸特性之影響並提出改善製程的辦法。
第一個主題研究在無序石墨烯的電傳輸特性,此實驗使用化學氣相沈積石墨烯(chemical vapor deposition (CVD) graphene)和外延石墨烯(epitaxial graphene)的樣品,因為製程中污染或是石墨烯本身的缺陷而形成的無序石墨,其電傳輸機制可以用變程跳躍variable range hopping (VRH)去解釋,使用resistance curve derivative analysis (RCDA)方法可決定樣品的傳輸機制是屬於Mott VRH或Efros-Shklovskii (E-S) VRH。藉由分析樣品的電性,我們可以區分樣品是否有被污染或是本身具有缺陷。 第二個主題中,藉由改善製程和長晶方法,發現在單層外延石墨烯表面上的污染減少很多,因而可以在樣品中量測到優異的整數霍爾效應。而製程中有使用到王水對樣品做處理,王水的硝酸鍵雖然可以使單層外延石墨烯的載子濃度變低,有利於整數霍爾效應在低磁場被觀察到,但硝酸鍵在石墨烯表面上容易與水或是其他物質結合,所以在空氣中,樣品載子濃度會隨著時間改變,不利於樣品品質的穩定。為了解決這問題,我們利用Parylene C薄膜去封裝保護樣品表面不與空氣接觸,更進一步探討Parylene C封裝石墨烯的影響以及藉由恆溫恆溼機測試去評估維持樣品成效。 最後一個主題是研究樣品寬度會影響電傳輸特性。一般石墨烯的微小樣品是用電子束微影(electron-beam lithography)和傳統蝕刻去製作而成,而我們發現雖然改善的製程可以讓表面保持乾淨,但是邊緣卻會因為電子束微影的解析度或是光阻而變得不平整或是污染。當樣品寬度變足夠小的時候,邊緣特性是個對電傳輸特性很重要的因素,而邊緣的污染或不平整就會使樣品產生很多背向散射(back-scattering)去影響電傳輸。此實驗我們利用在碳化矽(SiC)高溫昇華技術去自然生長石墨烯帶來取代傳統蝕刻的石墨烯帶,再藉由共軛焦顯微鏡可以有效率去尋找和定位石墨烯帶去製作成樣品。實驗結果顯示,在自然生長石墨烯帶,因為在平滑和乾淨的邊緣中,背向散射機會減少,可以觀測到邊界散射。因此,我們的高品質自然生長石墨烯帶,保證邊緣更加乾淨和更平整,可以應用在許多微小電子元件上。 Graphene may find promising applications in electronics due to its extraordinary electrical properties. However, graphene generally becomes disordered and its electrical properties are affected by the chemical dopants and residues in the fabrication processes. Accordingly, it is highly desirable to avoid chemical dopants and residues on graphene during the fabrication. This thesis is divided into three parts to discuss the impact of electrical properties with disordered graphene and self-assembled graphene ribbons, and further study the amelioration methods for the fabrications. The first topic involves the electronic transport properties of disordered graphene. In these experiments, both chemical vapor deposition (CVD) graphene and epitaxial graphene (EG) samples are used. The mechanism of electric transport with disordered graphene from chemical dopants in fabrications or the defects is explicated by variable range hopping (VRH) model. By the resistance curve derivative analysis (RCDA), this behavior can be further referred to the Mott VRH or Efros-Shklovskii (E-S) VRH. Accordingly, we can determine whether the samples are doped or have defects with analyzing electronic transport properties of samples. In the second topic, we improve fabrication processes and the method of growth graphene, and the contamination or defects on monolayer EG are dramatically diminished. Thus, the remarkable integer quantum Hall effect (IQHE) in these samples can be observed. In our fabrication, dilute aqua regia (DAR) is used. In this case, the nitric acid of the DAR may dope graphene. Though the effect of the nitric acid can reduce the carrier density of the samples which allows us to observe the IQHE in the low-magnetic-field regime, the nitric acid may absorb H2O or other chemicals in ambient air makes graphene become p-doped with time. It is disadvantageous to keep the quality of IQHE. To solve this problem, we cover graphene samples with Parylene C to prevent the samples from exposing to ambient air. After that, we further study the effect of Parylene C encapsulation on graphene and the assessment of protecting ability with thermotron tests. The last topic presents that the electronic transport properties is dependent on the width of samples. In general processes, narrow graphene samples are fabricated by the electron-beam (e-beam) lithography and conventional reactive ion etching (RIE) processes. Though our fabrication process can keep the surface low-dopants, the edges of samples still become nonuniform and doped by e-beam lithography processes and the resist. It has been shown that the edge properties play an important role in transport properties in sufficiently narrow ribbons. The high possibility of back-scattering from nonuniform and doped edges influences electronic transport properties of graphene. In this topic, we use self-assembled graphene ribbons which are prepared by a high-temperature sublimation technique on SiC instead of conventional etched ribbons, and efficient confocal laser scanning microscopy (CLSM) characterization is used for choosing and locating graphene ribbons to make samples. The experimental results describe that the boundary scattering exists due to decreasing the possibility of back-scattering on the smooth and clean edges of a self-assembled graphene ribbon. Accordingly, our high-quality, self-assembled graphene ribbons which guarantee cleaner and more uniform edges may find promising applications in micro-electronics. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/77355 |
DOI: | 10.6342/NTU201901484 |
Fulltext Rights: | 未授權 |
Appears in Collections: | 應用物理研究所 |
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