摘要
作为可以在室温下稳定独立存在的平面材料,石墨烯是碳原子以sp2杂化形成的六角蜂窝状晶格结构的二维材料,也是构成石墨、碳纳米管和富勒烯等碳基材料的母体和基本单元。石墨烯独特的晶格结构赋予它丰富而新颖的物理、化学、机械、电学、热学和光学性质,被认为是在post-CMOS时代,突破Si技术所遇到的尺寸极限效应,延续Moore定律的候选材料之一。因此,制备高质量大面积低缺陷的石墨烯材料是一个亟待解决的问题。在众多石墨烯的制备方法中,由于易于与现有的半导体工艺技术相兼容,导致在SiC衬底上外延生长石墨烯的方法被认为是最有潜力制备石墨烯基电学器件而被寄予厚望。但是在该方法中,存在诸多影响石墨烯形成的因素,譬如,衬底类型(硅面和碳面)、生长环境(UHV和氩气压力)、温度和生长时间等。目前,学术界还没有完全理解外延石墨烯的形成机理。
本研究在理论上探索了SiC基石墨烯材料的生长机理,并通过工艺实验,最终获得了大面积外延石墨烯样品。重点研究了该方法制备石墨烯的工艺路线和表征技术,所涉及的研究内容和创新性成果如下：
(1)研究了用于生长石墨烯的SiC衬底的氢刻蚀工艺。在对衬底进行清洗的基础上,首先探索了硅面和碳面氢刻蚀的工艺方法,以去除SiC衬底表面的化学机械划痕,形成规则的台阶状条纹,其形貌特征表现为,台阶宽度约微米量级,高度约纳米量级,这样的结构有利于石墨烯在衬底的成核和生长。
(2)研究了不同的生长温度对形成石墨烯质量的影响。借助生长动力学和成核理论,结合Raman光谱,分析了在适宜的生长压力下,温度变化对于Si面和C面外延石墨烯的影响。结果表明,生长温度越高,石墨烯的晶格质量就越高,受衬底和杂质的影响也越小。进一步地,比较了Si面和C面外延石墨烯的填充结构,层厚等差异。
(3)外延大面积石墨烯的研究。针对目前在超高真空和大气压力氩气环境下制备石墨烯的优缺点,独辟蹊径的提出在较低的氩气压力环境下生长外延石墨烯的构想,探索出一整套工艺路径,最终制备出了大面积(厘米量级)高质量石墨烯样品,并且石墨烯的形貌结构和质量接近于现有的已经报道的研究成果。
全文共分为六章,阐述如下：
第一章为绪论部分,简要介绍石墨烯的晶格结构、基本性质、研究进展和四种主要制备方法(微机械剥离法、金属衬底的化学气相沉积法、氧化石墨还原法和在碳化硅衬底上外延生长石墨烯)的实验机理,工艺方法以及各自的优缺点。
第二章阐述了外延石墨烯生长机理和表征方法。阐述SiC单晶的物理性质,外延石墨烯形成的主要工艺流程(氢刻蚀、去除化合物和外延生长过程),以及外延石墨烯确认、形貌、尺寸和层厚等表征技术。
第三章包括氢刻蚀工艺探索。在对SiC衬底进行清洗的基础上,探索硅面和碳面氢刻蚀的工艺方法,即对4H/6H-SiC衬底的硅面(0001)和碳面(0001)分别使用不同的氢刻蚀工艺,使用AFM予以表征,得到硅面和碳面各自适宜的氢刻蚀工艺方法。
第四章从理论上分析了温度对外延石墨烯形成的影响。在适宜的生长压力不变的条件下,分别在4H-SiC衬底硅面(0001)改变生长温度(1200℃～1500℃),在6H-SiC碳面(000T)改变生长温度(1400℃～1600℃),查看石墨烯的生成尺寸和质量的变化并予以理论分析,尤其研究了在碳面形成的非AB填充型外延石墨烯的形成原因和Raman特征。
第五章大面积外延石墨烯的制备。确定了在较低的生长压力(4mbar)下,以4H-SiC衬底硅面(0001)制备厘米量级的外延石墨烯的适宜的工艺流程和SEM,AFM,Raman光谱和XPS对石墨烯的尺寸、表面形貌、层厚和衬底与石墨烯界面处碳原子的化学成分进行表征。
第六章结束语。总结全文并对后续研究予以展望。
As a flat planar material existed stably and independently at room temperature, graphene is a two-dimensional (2D) honeycomb lattice of sp2-hybridization carbon atoms, also a basic block and mother material of other carbon-based materials such as graphite, carbon nanotubes and Fullerenes and so on. The special lattice structure gives graphene the extraordinary physical, chemical, mechanical, electrical, thermal and optical properties, so graphene may be regarded as one of the promising candidates, aiming at breaking the physical limit effect in Si technology, extending Moore's law in the post-CMOS era. Therefore, how to prepare high-quality, large-area and low defect graphene is an urgent issue. Among the preparation methods, epitaxial graphene grown on single-crystalline silicon carbide (SiC) has been paid a high expectation in graphene-based electrical devices due to its compatibility with the current semiconductor technology. However, there still exist many factors to effect the formation of graphene, such as the type of substrate, Si-face or C-face, the conditions (UHV and argon pressure), growing temperature and time. At present, the formation mechanism of epitaxial graphene has not yet fully understood.
In this dissertation the formation mechanism of epitaxial graphene grown on SiC substrates is studied in theory and experiment. Especially the process method and characterization technology of epitaxial graphene are investigated, and the main contributions are as follows:
(1) The preparation of SiC subatrates used to grow high quality epitaxial graphene with hydrogen-etching process has been studied. Based on the pre-cleaning substrate, the hydrogen-etching process has been used to deal with the Si-face (0001) and C-face (0001)4H/6H-SiC substrates in order to remove the chemical mechanical scratches on as-received substrates and form a regular atomic stepped surface with the steps high several nanometers and terraces width microns. This morphography helps epitaxial graphene nuclearation and grow larger on the SiC subatrates.
(2) The effects of different growth temperature on the formation of epitaxial graphene have been studied. Using the growth kinetics, nuclearation theory and Raman spectroscopy, we have analyzed how the growth temperature affecting on the formation of epitaxial graphene grown on Si-or C-face SiC substrates under an appropriate pressure conditions. It shows that a higher growth temperature can help the formation of better quality epitaxial graphene samples and also can decrease the influence of the substrates and impurities. Moreover, the stacking types and the thickness of epitaxial graphene grown on Si-or C-face SiC substrates are compared.
(3) By analyzing the advantage and disadvantage of epitaxial graphene grown under ultra-high vacuum and atmospheric argon pressure conditions, a new approach has been proposed, which grows epitaxial graphene under lower-pressure argon condition and explores a full process routes to achieve centimeters order of magnitude of epitaxial graphene on Si-face of4H-SiC (0001) substrates at a pressure of4mbar. The measurement results demonstrate that the quality of epitaxial graphene is very similar to the current reported results in this field.
This dissertation is composed of six chapters as follows:
Chapter Ⅰ is Introduction to make a brief introduction of lattice structure, the basic properties, the current research progress of graphene, and the formation mechanism of four main preparation methods (Micro-mechanical exfoliation of graphite, Chemical Vapor Deposition on metal substrate, the reduction of graphite oxide and epitaxial graphene grown on SiC substrates) and their advantages and disadvantages.
In Chapter Ⅱ, the growth mechanism and characterization methods of epitaxial graphene are presented. It describes the physical nature of SiC single crystallines, the main growth process of epitaxial graphene (hydrogen-etching, removing oxides and epitaxial growth process), and the characterization techniques such as the confirmation, size, morphology and the thickness of epitaxial graphene.
In Chapter Ⅲ, the investigation of the hydrogen-etching process is presented. Based on pre-cleaning SiC substrates, different hydrogen-etching routes have been implemented on Si-and C-face4H/6H-SiC substrates. Using atomic force microscope, it is confirmed that Si-and C-face of SiC substrates have different hydrogen etching process.
In Chapter Ⅳ, the influence of growth temperatures on the formation of epitaxial graphene is analyzed. Under an appropriate pressure, by changing the growth temperatures (from1200to1500℃) for4H-SiC (0001) Si-face substrates, and by changing the growth temperatures (from1400to1600℃) for6H-SiC(0001) C-face substrates, the size and the quality of epitaxial graphene have been studied. Especially, the non-AB stacking type epitaxial graphene grown on the C-face substrates has been characterized using Raman spectroscopy in order to investigate its formation mechanism.
In Chapter Ⅴ, the growth of large-area epitaxial graphene is studied. At a lower pressure (4mbar), on the Si-face4H-SiC substrate, epitaxial graphene with size of centimeters order of magnitude has been successfully grown and then the characterizations, such as its size, thickness, morphological structure and chemical compositions of carbon atoms on the interface of epitaxial graphene and SiC substrates, have been achieved using scanning electron microscopy, atom force microscopy, Raman spectroscopy and X-ray Photoelectron Spectroscopy.
Chapter VI is the conclusion for this dissertation and gives some suggestions for the further research.
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