以幫浦油氣作為碳源生長大面積石墨烯及其電性研究

Abstract

石墨烯是一種單層原子厚度，以sp2鍵結成六角晶格結構的碳材料，具有許多優異的特性，如高電子遷移率、高透光性、導熱性好、機械強度強、可撓性佳，是極有潛力的前瞻性材料。在各界的熱烈研究之下，石墨烯已經有許多的製備方式，如微機械剝離法、碳化矽磊晶法、化學氣相沉積法等等，其中以化學氣相沉積法最有潛力。一般化學氣相沉積法使用銅箔作為基板，通入氫氣、甲烷(作為碳源)生長石墨烯，可以生長出均勻的單層石墨烯(佔總面積的95%)，有少部分區域為兩層或三層石墨烯(佔5%)，但無法控制層數、也無法長出均勻之多層石墨烯。因此，為了研究少數層石墨烯(兩層以上)的有趣特性，研發出控制不同層數的石墨烯成長方式，是需要突破的課題。 本研究發現了新式的化學氣相沉積法，以迴旋油墊幫浦的油氣回流作為碳源，不通氫氣與烷類氣體，在真空度3mTorr的情況下在銅箔基板生長出不同層數的石墨烯。我們以清洗過全新的石英管、通高純度氮氣以及更換油墊幫浦為乾式幫浦等三種方式，證明碳的來源就是幫浦油氣的回流。經過拉曼光譜分析樣品，得知當成長的溫度為1050°C，時間為30分鐘，基板是使用0.01mm厚度的銅箔時，可以穩定成長大面積的單層石墨烯，但使用0.1mm厚度的銅箔時會生長出不同層數石墨烯及石墨。 生長後的石墨烯可利用聚甲基丙烯酸甲酯(PMMA)轉移至任意的目標基板。在霍爾量測中，石墨烯之電子遷移率可達440cm2/Vs，與普通化學氣相沉積方式生長的石墨烯電子遷移率526 cm2/Vs相距不遠。我們將石墨烯轉移到二氧化矽基板上製作成石墨烯電晶體，在石墨烯電晶體的場效電性量測中，可知石墨烯為高的p型摻雜，主要由於大氣中的水氣、氧氣吸附與殘留的光阻。 雖然品質仍需改善，但以油氣為碳源提供了一種新式簡便的方式成長不同層數之大面積石墨烯，不需通氫氣也使得製程更安全。不僅如此，此新方法可在銅箔基板上長出多層石墨烯，突破了已往在銅箔上只能成長單層石墨烯的情形，若是能穩定的控制石墨烯的層數，將會是石墨烯成長的一大進展。

Graphene, an atomically thin sheet of sp2-bonded carbon atoms arranged in a hexagonal lattice, possesses many exceptional properties, such as high mobility, high transparency, great thermal conductivity, strong mechanical property and high elasticity. Owing to the much study all over the world, there are already many ways to produce graphene, such as micromechanical exfoliation, epitaxial growth on silicon carbide, and chemical vapor deposition (CVD). Among them, CVD is the most promising method. In a typical CVD method, copper foil substrate is chosen to grow graphene in hydrogen and methane (as carbon source) flowing. It can grow uniform monolayer graphene (95% of total area) while some regions are 2 or 3 layers graphene (5% of total area). But the number of graphene layers cannot be controlled and uniform multi-layer graphene cannot be grown either. Therefore, in order to study the interesting properties of few-layer graphene, developing a graphene production method to control different numbers of graphene layers is an issue to overcome. In this work, we found a new kind of CVD method, using oil backstreaming from the oil seal rotary vane pump as carbon source to grow different numbers of graphene layers in 3 mTorr on copper foil without hydrogen and hydrocarbon gas flow. We use three ways of cleaned new quartz tube, high purity nitrogen flow and replacing oil seal rotary vane pump with dry pump to prove that the carbon source is exactly the backstreaming of oil vapor. In Raman spectrum analysis, we discovered that graphene can grow on 0.01mm thick (99.9% purity) copper foil in 1050°C and 30 minutes growth condition while different numbers of graphene layers and graphite grow on 0.1mm thick (99.96% purity) ones randomly. After growth, graphene can be transferred to arbitrary target substrate with Polymethylmethacrylate (PMMA). In Hall effect measurement, graphene mobility can reach 440cm2/Vs and it is comparable to normal CVD graphene’s mobility 526 cm2/Vs. We transferred graphene to SiO2 substrate to fabricate graphene field effect transistor (FET). In graphene FET electricity measurement, we found that graphene are highly p-doped mainly due to the absorption of water and oxygen in air or photoresist residue. Although the quality still needs to be improved, using oil vapor as carbon source provided a new and easy way to grow different numbers of graphene layers. It also makes the production safer without hydrogen flow. Moreover, this new method enables multi-layer graphene to grow on copper foil, overcoming the situation of only monolayer graphene formation on copper foil in the past. If we can control the numbers of graphene layers stably, it will be a milestone in graphene growth.