Quantum transport in two-dimensional electron system: 1.Transition dynamics in the electrical breakdown of the integer quantum Hall effect 2.Disorder induced scattering in chemical vapor deposited graphene

二維電子系統的量子傳輸現象: 1. 整數霍爾效應中電性崩壞現象的動態機制 2. 化學氣相沉積石墨烯中的缺陷散射之研究

李明洋

指導教授 ： 陳正中

國立清華大學/理學院/物理學系/博士(2013年)

https://doi.org/10.6843/NTHU.2013.00559

摘要

我們研究兩種不同二維電子系統(two-dimensional electron system)的電性傳輸，包含石墨烯(graphene)和異質結構介面中的二維電子氣(two-dimensional electron gas)。異質結構介面中的二維電子氣由於有很高的載子遷移率(mobility)以及可以局限至更低維度，使得其在學術領域以及工業方面有廣泛的應用。石墨烯是單一層碳原子形成蜂窩狀的二維電子系統。此晶格結構有著許多特殊的電子傳輸與光學特性，成為極受矚目的材料。
本論文討論兩個研究主題：整數霍爾效應(integer quantum Hall effect)中電性崩壞(electrical breakdown)現象的動態機制以及化學氣相沉積石墨烯(chemical vapor deposited graphene)中缺陷散射對電性傳輸的影響。我們由研究整數霍爾態(quantum Hall state)的崩壞現象的動態機制來了解整數霍爾效應的機制。藉由探討不同外加霍爾電場(Hall field)的掃描速率與臨界電場(critical field)的關係，我們觀察到電性崩壞現象的雙穩態特性(bistable nature)以及量測出兩個平衡態之間的躍遷率(escape rate)與霍爾電場的關係。此結果與基於bootstrap electron heating model的模擬相互吻合，進而表示電性崩壞現象的展現是由本身的躍遷率以及實驗架設的條件來決定。
石墨烯是一個嶄新的材料且有相當高的發展潛力，但是石墨烯本身的特性對於缺陷與環境相當敏感。我們主要探討缺陷對於化學氣相沉積石墨烯在電性傳輸上的影響。我們研究電導(conductivity)與磁導(magneto-conductivity)與載子濃度的關係。研究的範圍跨越電中性點(charge neutral point)，並將實驗結果與探討結構缺陷(structural disorder)和帶電雜質(charged disorder)的理論研究做比較。磁導展現出弱局域現象(weak localization)；且在電中性點附近，由於電子電洞坑(electron hole puddle)的出現增加inter-valley的散射，進而增強弱局域現象的表現。我們發現載子遷移率與帶電雜質的密度有很強的關聯性，而且帶電雜質引發的不均勻電位擾動(inhomogeneous potential fluctuation)是影響石墨烯在電中性點附近電性傳輸特性的主要原因。在電導與溫度以及載子濃度的探討更進一步證實結構缺陷會主導在高載子濃度時的電性傳輸，但在低載子濃度時，由帶電雜質主導電性傳輸特性。

關鍵字

石墨烯 ； 二維電子氣 ； 霍爾效應 ； 電性崩壞 ； 磁導 ； 帶電雜質 ； 弱局域現象 ； 電子電洞坑 ； 結構缺陷

並列摘要

We study the electric transport of two different two-dimensional electron systems: graphene, and two-dimensional electron gas (2DEG) formed between the interface of semiconductor heterostructure. 2DEG has been widely applied to both academic researches and industrial uses due to its high mobility and constructability to lower dimensional electronic system. Graphene is an intrinsic two-dimensional electronic system consisting of a monolayer carbon atoms hybridized in honeycomb lattice. This lattice structure gives rise to many special electronic and optical properties, which raise general interest in the field.
This thesis presents two research interests which are the transition dynamics in the electrical breakdown of the integer quantum Hall effect (QHE) in 2DEG system, and the disorder induced scattering in chemical vapor deposited graphene. QHE is an important feature in two-dimensional electronic system, we investigate the dynamic properties of the breakdown of integer quantum Hall states (QHSs) to study the mechanism of the QHE. With systematically study on the critical field of the breakdown with different scan rates of the applied Hall field, we observe bistable nature of the breakdown phenomena. We find the Hall field dependent escape rate between the low-dissipation QHS and the dissipation state ranges from a few seconds to 10 μs in bistable regime. The results are consistent with the simulation based the bootstrap electron heating model. This suggests that the dynamic behaviors are governed by the lifetime and the applied experimental setup condition.
Graphene is a novel material and has great potential on application owing to its special transport properties. Nevertheless, graphene is sensitive to the disorder and the environment, which might limit or change its properties. We investigate the effects of random disorders in electric transport on chemical vapor deposited (CVD) graphene. We study the carrier density dependence of conductivity and magneto-conductivity crossing the charge neutral point and compare our data with pervious theories concerning sharp structural disorder and charged disorder. The magneto-conductivity exhibits weak localization behavior and weak localization is enhanced by inter-valley scattering with the presence of the electron hole puddle near charge neutral point (CNP). The electric mobility shows strong correlation with the charged impurity density. We find that the inhomogeneous potential fluctuation induced by charged impurities plays a dominated role in electronic transport especially near the CNP. The study on temperature and carrier density dependent conductivity further suggests that the sharp defect dominate the transport in high carrier density regime, but the charge impurity becomes important in lower density regime.

並列關鍵字

graphene ； two dimensional electron gas ； quantum Hall effect ； electrical breakdown ； electron heating model ； magneto-conductivity ； structural disorder ； weak localization ； charged disorder ； electron hole puddle

參考文獻

[66] Y. Shao, J. Wang, H. Hu, J. Liu, I. A. Aksay, and Y. Lin, Electroanalysis 22, 1027 (2010).

[96] C. C. Tang, M. Y. Li, L. J. Li, C. C. Chi, and J. C. Chen, Appl. Phys. Lett. 99, 112107 (2011).

[91] Y.-W. Tan, Y. Zhang, H. L. Stormer, and P. Kim, Eur. Phys. J. Spec. Top. 148, 15 (2007).

[59] Q. Li, E. H. Hwang, and S. Das Sarma, Phys. Rev. B 84, 115442 (2011).

[12] C. Y. Su, Y. P. Xu, W. J. Zhang, J. W. Zhao, X. H. Tang, C. H. Tsai, and L. J. Li, Chem. Mater. 21, 5674 (2009)