Graphene interface engineering: surface/substrate modifications cum metal contact exploration.

石墨烯具有獨特的電學,熱力學及機械性能,在科學研究和技術領域受到廣泛的關注。特別是,以石墨烯為基礎的石墨烯場效應電晶體近年來得到了快速發展,使其成為後矽基時代的可選用材料之一。不同于傳統的體半導體材料,石墨烯具有獨特的二維結構;它與周圍環境的介面相互作用對石墨烯器件有決定性的影響。研究石墨烯的介面特性在石墨烯應用中具有重要的意義。因此研究者對於發掘在納米尺度上的石墨烯介面規律及由此獨特的介面特性所導致的電子結構、載流子輸運性質和其他相關現象具有濃厚的興趣。在本論文中,我們從實驗和理論兩個方面對石墨烯與不同基底的介面耦合機制,由金屬電極到石墨烯的電荷注入以及其表面的吸附物對石墨烯的摻雜作用進行了深入细致的研究。首先,通過對薄層石墨烯的表面功能化,可以對其電子結構進行有效的控制和調整。在石墨烯薄片表面吸附不同的自組裝有機分子,可以實現對石墨烯的電子和空穴摻雜。另外,我們對由電子束放射產生的摻雜效應也進行了研究。我們發現當利用電子束處理包含不同層數的石墨烯薄片時,可以形成石墨烯pn結。 其次,我們對石墨烯基底對於石墨烯的重要作用進行了深入的探究。由於商用矽片中存在的帶電雜質及石墨烯褶皺對放置於其上的石墨烯樣品產生了極大的影響,使得石墨烯的遷移率遠小於其理論值。為消弱由基底產生的不利影響,我們利用自組裝單分子膜對二氧化矽/矽襯底的表面進行鈍化處理,從而減少不必要的散射。通過鈍化處理,載流子遷移率上升了近一個數量級(達到 47,000 cm²/Vs)。此外,我們對石墨烯與不同金屬電極接觸的介面電學性質也進行了系統研究。我們發現較低的電阻及線性的電流電壓關係對於石墨烯場效應電晶體並非始終成立。對於本征石墨烯,我們發現石墨烯和金屬電極的接觸具有‘空間電荷區限制’和‘歐姆接觸’兩種接觸模式。並且在偏置電壓控制下,接觸電阻可以可逆的在兩種接觸模式中切換。我們發現該現象可以歸結于石墨烯獨特的錐型能帶色散關係。該現象提供了新的製備高密度非易失性石墨烯記憶體的方法。Graphene is an appealing material in both science and technology. Its distinct electronic, thermal and mechanical properties have stimulated enormous scientific interest. In particular, graphene-based field-effect transistors (GFET) have been developed rapidly and are now considered an option for post-silicon electronics. In contrast to traditional semiconductors, the unique two dimensional structure of graphene offers the possibility of studying the interface characteristics for its proximity to the top surface and interface between graphene and the outside environment. We are thus interested in understanding graphene surface and interfacial issues associated with electronic structure, carrier transport and related phenomena on a nano-scale. In this thesis, we investigate both experimentally and theoretically the mechanisms of graphene interfacial couplings to different substrates, charge injection from metal electrodes and its interplay with inert adsorbates.At first, few layer graphene’s (FLG) electronic properties are adjusted efficiently and controllably through functionalizing its top surface. Both n-type and p-type doped exfoliated graphene sheets are present by virtue of adsorbing organic molecules. Additionally, the doping effects induced by electron beam (EB) irradiation are also studied. We find that by irradiating graphene with EB, graphene p-n junctions can be formed if EB irradiation is applied across a single graphene sheet containing regions with different layers.Secondly, the crucial roles played by the supported substrate in graphene applications are meticulously interrogated. The existence of charge impurities and ripples adversely affects the mobility of high quality mechanically exfoliated graphene on commercially available SiO₂/Si wafers inferior to its theoretical limit. To suppress the deleterious substrate effect, we utilize self-assembled monolayers to passivate the SiO₂/Si substrate surface. After diminishing the unwanted scattering origins by this method, an increase in carrier mobility by nearly one order of magnitude (up to 47,000 cm²/Vs) is obtained.Furthermore, the electronic properties of the interfaces between graphene and various metal electrodes are systematically investigated. Our study unambiguously reveals that a low electrical resistance as well as a linear current-voltage relation is not always granted for GFETs. Interestingly, for graphene on SiO₂/Si passivated with highly-ordered OTMS, both ‘space charge region limited’ and ‘ohmic’ contacts can be obtained with a single metal electrode. We also find that by utilizing voltage bias, the contact can be reversibly altered between high resistance and low resistance. We ascribe the phenomenon to graphene’s cone energy dispersion relationship as well as the vanishing density of states at the Dirac points. Our results herald a new avenue for achieving high density non-volatile graphene memory devices.Detailed summary in vernacular field only.Detailed summary in vernacular field only.Detailed summary in vernacular field only.Wang, Xiaomu.Thesis (Ph.D.)--Chinese University of Hong Kong, 2012.Includes bibliographical references.Abstract also in Chinese.Abstracts in English and Chinese.Chapter Chapter 1 --- Introduction --- p.1Chapter 1.1 --- Electronic Properties of Graphene --- p.1Chapter 1.1.1 --- Graphene Band Structure --- p.1Chapter 1.1.2 --- Physical Properties of Graphene --- p.4Chapter 1.1.3 --- Carrier Transport in Graphene --- p.7Chapter 1.1.4 --- Optical Properties of Graphene --- p.9Chapter 1.2 --- Motivation and Outline of the Thesis --- p.10Chapter 1.2.1 --- Graphene Field-Effect Transistors --- p.10Chapter 1.2.2 --- Interface Engineering --- p.15Chapter Chapter 2 --- Sample Preparation Details and Characterization Techniques --- p.26Chapter 2.1 --- Graphene Preparation --- p.26Chapter 2.1.1 --- Mechanical Exfoliation --- p.26Chapter 2.1.2 --- Reduced Graphite Oxide --- p.28Chapter 2.1.3 --- Graphene Synthesis by CVD on Copper Substrates --- p.28Chapter 2.2 --- Characterization of Graphene --- p.29Chapter 2.2.1 --- Optical Microscopy --- p.30Chapter 2.2.2 --- Raman Spectroscopy --- p.30Chapter 2.2.3 --- Scanning Probe Microscopic Techniques --- p.32Chapter 2.3 --- GFET Fabrication --- p.33Chapter 2.3.1 --- Photolithography Process --- p.34Chapter 2.3.2 --- Shadow Mask Method Process --- p.34Chapter 2.3.3 --- Lithography-Free Process --- p.35Chapter Chapter 3 --- Top Surface Modification of Graphene --- p.37Chapter 3.1 --- Charge Transfer by Organic Molecules in Doping of Graphene --- p.37Chapter 3.1.1 --- Overview --- p.37Chapter 3.1.2 --- Kelvin Probe Force Microscopy --- p.41Chapter 3.1.3 --- Experimental Details --- p.45Chapter 3.1.4 --- P-type Doping of Graphene by F4-TCNQ --- p.47Chapter 3.1.5 --- N-tpye Doping of Graphene by VOPc --- p.48Chapter 3.1.6 --- Mechanism of Charge Transfer: A Quantitative Analysis --- p.55Chapter 3.2 --- Asymmetric Doping of Graphene by Electron Beam Irradiation --- p.66Chapter 3.2.1 --- Overview --- p.66Chapter 3.2.2 --- Experimental Details --- p.67Chapter 3.2.3 --- Transport Measurements --- p.70Chapter 3.3 --- Summary --- p.73Chapter Chapter 4 --- Substrate Modification for Graphene --- p.81Chapter 4.1 --- Substrate Effects Adjusted by Thermal Annealing --- p.81Chapter 4.1.1 --- Overview --- p.81Chapter 4.1.2 --- Experimental Details --- p.82Chapter 4.1.3 --- Mechanism of Graphene/Substrate Interaction --- p.84Chapter 4.2 --- Modified Substrate by Highly Ordered OTMS SAMs --- p.85Chapter 4.2.1 --- Overview --- p.85Chapter 4.2.2 --- Experimental Details --- p.87Chapter 4.2.3 --- Transport Measurements --- p.94Chapter 4.2.4 --- Summary --- p.111Chapter Chapter 5 --- Graphene/Metal Contacts --- p.116Chapter 5.1 --- Graphene/Metal Contacts --- p.116Chapter 5.1.1 --- Overview --- p.116Chapter 5.1.2 --- Experimental Details --- p.118Chapter 5.2 --- Contact Modes and Related Memory Devices --- p.125Chapter 5.2.1 --- Bistable Contact Modes --- p.125Chapter 5.2.2 --- Related Memory Devices --- p.132Chapter 5.2.3 --- Contact Mechanism --- p.136Chapter 5.3 --- Transport Mechanism for OFF States --- p.147Chapter 5.3.1 --- Temperature-Dependent Transport Measurements --- p.148Chapter 5.3.2 --- WKB Approximations --- p.151Chapter 5.3.3 --- Tunneling between Fermi Liquid and Luttinger Liquid --- p.154Chapter 5.4 --- Summary --- p.156Chapter Chapter 6 --- Conclusions and Outlook --- p.161Chapter 6.1 --- Major Findings and Summary --- p.161Chapter 6.2 --- Outlook for Future Research --- p.165Chapter Appendix A --- Transport Model of GFET --- p.169Chapter A.1 --- Transport Models --- p.169Chapter A.2 --- Drift Current Model --- p.171Chapter A.3 --- Quantum Transport Theory of GFET --- p.175Chapter A.4 --- Brief Outline of NEGF --- p.17