臺灣博碩士論文加值系統

本論文研究經由化學還原的氧化石墨烯和銻磷化鎵在低溫下的磁性傳輸性質，實驗結果的統整簡述如下兩個部分。

1. 化學還原的氧化石墨烯之磁性傳輸行為
在近幾年關於化學還原的氧化石墨烯之電性傳輸行為已被廣泛研究，但是在磁場下的傳輸性質卻很少被探討。在第四章裡，我們研究經由化學方法高度還原的氧化石墨烯在磁場下的傳輸性質。本實驗外加的磁場為0~4 T，量測的溫度從最低溫4.5 K 到80 K。我們觀察到在三個不同厚度的氧化還原石墨烯在外加磁場下皆產生負磁阻的現象。從實驗結果發現，氧化還原石墨烯的負磁阻變化隨著厚度變厚和溫度升高皆變小。經由分析Sivan, Entin-Wohlman, and Imry (SE-WI) 所提出在變程躍遷區域下的負磁阻模型，我們得知我們的化學還原氧化石墨烯的電性傳輸符合Mott提出的變程躍遷模型。化學還原的氧化石墨烯呈現出來的磁性傳輸行為可以用在空隙產生的midgap態產生的自旋偶合效應解釋。此外，關於實驗中所觀察到的階梯狀負磁阻原因尚未明確，此現象也在氟化石墨烯被觀察到。此現象需要未來更深入的研究去探討、釐清它。
2. 銻磷化鎵之磁性傳輸行為
第二部份介紹銻磷化鎵在低溫垂直磁場下的傳輸行為。從霍爾量測實驗得知主要載子為電子且濃度為1016 cm-3。我們觀察到在溫度30 K至80 K之間，磁阻在低磁場的範圍為負磁阻，並隨著磁場加大而轉變為正磁阻。我們發現在溫度40 K至80 K 的傳輸行為同時符合二、三維的Mott和Efros Shklovskii (ES) 的變程躍遷模型。為了明瞭真正的傳輸行為，我們利用磁阻在變程躍遷的範圍下的理論去分析，其結果顯示厚度為720 nm的銻磷化鎵符合二維的Mott變程躍遷模型。

In this thesis, two materials, chemically reduced graphene oxide and GaPSb, were investigated for their magnetotransport behaviors under the influence of a magnetic fields up to 11.6 T at the temperatures between 4.5 K and 80 K. A brief summary of our discovery will be given in the following sections.
1. Magnetotransport in chemically reduced graphene oxide
In recent years, transport behavior in chemically reduced graphene oxide (R-GO) has been widely investigated. However similar research in the presence of a magnetic field was rare. In chapter 4, the transport behavior in fully R-GO samples with a magnetic field up to 4 T at selected temperatures between 4.5 K and 80 K was studied. Negative magnetoresistance (NMR) was observed in all our R-GO samples. The magnitude of NMR was found to decrease with the increasing temperature and the increasing thickness of the R-GO film. A Mott hopping behavior, concluded from the data analyzed by using Sivan, Entin-Wohlman, and Imry (SE-WI) model, was found to be the dominant electron conducting mechanism for the NMR phenomenon. The magnetotransport features can be explained by a model based on the spin-coupling effect from vacancy-induced midgap states that facilitate the Mott variable range hopping (VRH) conduction in the presence of an external magnetic field. The origin of the staircase-like NMR revealed on the 3-nm-thick sample is not clear although it was also observed at a lower temperature from the dilute fluorinated graphene. Further investigations are required to elucidate the nature of this unusual behavior.
2. Magnetotransport in gallium phosphide antimonide
The second part of this thesis was dedicated to the study of the transport behavior in a GaPSb sample in the presence of a perpendicular magnetic field at low temperatures. The Hall measurement indicated that the carrier concentration was about 1016 cm-3 and electrons were the dominant carriers in our sample. The NMR followed by PMR was observed along with the increase of a magnetic field up to 11.6 T. At the temperature regime between 40 K and 80 K, either three- or two-dimensional Mott VRH model or ES VRH model can be used to describe the electron transport behavior. To elucidate the ambiguity, the magnetoresistance theory in VRH regime was employed to analyze the transport behavior, showing that the transport behavior of the 720-nm-thick GaPSb is consistent with a two-dimensional Mott VRH conductance.

Contents
中文摘要 I
Abstract II
Publication: IV
List of Figures VIII
List of Tables XII
Chapter 1 1
Introduction 1
1.1 Graphene-based Materials 1
1.2 Two dimensional electron systems 3
1.3 Density of States 6
1.3.1 Density of states for three-dimensional systems 7
1.3.2 Density of states for other dimensional systems 8
Chapter 2 13
Background 13
2.1 Classical Hall Effect 13
2.1.1 Introduction 13
2.1.2 Derivation of the carrier concentration 14
2.2 Variable Range Hopping 15
2.2.1 Introduction 15
2.2.2 Mott’s model 17
2.2.3 Efros Shklovskii Model 20
2.3 Magnetoresistance in the variable range hopping regime 21
2.3.1 Introduction 21
2.3.2 NMR in the variable range hopping regime 23
2.4 van der Pauw Measurement 26
2.4.1 Introduction 26
2.4.2 Derivation of the theory in semi-infinite plane 28
2.4.3 Derivation of theory in an arbitrary shape 29
2.4.4 Practical application 32
Chapter 3 34
Experimental Techniques 34
3.1 Ohmic contacts 34
3.2 Four-terminal measurement 34
Chapter 4 36
Magnetotransport in chemically reduced graphene oxide 36
4.1 Introduction 36
4.2 Experimental details 37
4.2.1 Sample preparation 37
4.2.2 Experimental setup 38
4.3 Results and discussions 40
4.4 Summary 50
Chapter 5 53
Magnetotransport in gallium phosphide antimonide 53
5.1 Introduction 53
5.2 Experimental details 54
5.2.1 Sample structure 54
5.2.2 Experimental setup 55
5.3 Results and discussions 56
5.4 Summary 64
Chapter 6 66
Conclusions 66

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