可調性化學修飾於高性能石墨烯電晶體之研究

Abstract

石墨烯是一種單層碳原子以sp2鍵結方式排列而成的二維奈米材料，由於此特殊的幾何結構使得石墨烯有著非常特別的能帶結構、機械性質及熱導性質，因此近年來許多學者在石墨烯領域已投入相當多的研究，特別在電性部分，由於石墨烯在室溫下具有極高之載子遷移率>15000 cm2V-1s-1 使得它被認為是次世代高速元件的核心材料。 然而在一般的邏輯電路中，互補式金屬氧化物半導體(CMOS)是電路中最重要組成之一，此元件必須同時由N型和P型金屬氧化物半導體所構成。由於純石墨烯本質上具有雙載子傳輸特性，為了製成N型和P型金屬氧化物半導體元件需要分別對石墨烯摻雜額外載子。因此如何藉由摻雜來調變石墨烯之電性將成為未來是否能把石墨烯應用在電路上的一個重要關鍵。 在石墨烯的結構中，由於整層碳原子都直接暴露在大氣環境中，除了石墨烯本身表面上部容易吸附一些摻雜物(如水氣或氧氣分子)外，石墨烯與下層基板接觸之介面亦容易對其造成載子摻雜現象，使得石墨烯元件在大氣環境下非常容易因為外部環境的摻雜物影響而變得不穩定。 因此在本研究中，我們藉由改變石墨烯的上下層介面來調變其電子傳輸特性。在文中的第一個部分(第四章)，我們先利用有機自組層來鈍化下層基板表面，進一步減少基板表面的極性物質與石墨烯之交互作用，使得石墨烯的載子遷移率大幅提升。 而在文章的第二部分(第五章)，我們發現到TiOx對石墨烯而言是一種新穎的N型摻雜物，藉由TiOx在上層摻雜過程中同時對元件形成一自組封裝層，使得元件在被摻雜後還能隔絕大氣環境中的其他外部摻雜物。因此在最後我們將石墨烯的上下兩層皆用此材料覆蓋後可以得到相當程度的N-型參雜濃度且同時具有相當好的空氣穩定性

Graphene, which consists of a single atom-thick plane of carbon atoms arranged in a honeycomb lattice, has attracted a large amount of research because of its novel electronic, mechanical, and thermal properties arising from its unique 2D energy dispersion. For its electronic property, ultrahigh mobility > 15000cm2V-1S-1 can be reached in ambient which is higher than carbon nanotube, silicon, which has potential to fabricate next generation of high-speed electronic device or transistor. However, Modern logic circuits are based on silicon complementary metal oxide semiconductor (CMOS) technology. Both p-type and n-type conductions are necessary to construct complex digital circuits. Since intrinsic graphene demonstrates ambipolar transport behavior, it will be important to control the electrical properties of graphene. Therefore, doping graphene becomes a crucial technique for making graphene based circuits. Due to the fact that the entire layer of carbon atoms has an immediate exposure to the surroundings, not only dopants in air (such as oxygen, water.) adsorbed on top of graphene but also the substrate under graphene would contribute to considerable doping effect on graphene, making graphene devices very unstable in surroundings. Therefore, controllable doping level with excellent air stability is necessary for fabricating either n-type of p-type transistor based on graphene. In first part of this work (chapter 4), we focused on the bottom interface between graphene and substrate. Via using an organic self-assembling monolayer to passivate polar groups on SiO2 substrate, the mobility of graphene on this substrate is significant enhanced due to reduced interaction between graphene and dopants on substrate. Next, in the second part of this article, we induced titanium sub-oxide as an “active” n-type doping material and simultaneously a capping layer for graphene transistor, which prevented graphene from the outer dopants in air. A novel structure is induced via fully coverage for both side of graphene by the hydrophobic layer composed of titanium sub-oxide. The device exhibit strong n-type behavior with good stability, which makes this technique promising for graphene based nonoelectronics in the future.