利用大晶粒銅基材催化合成高品質石墨烯

High Quality Graphene Grown on Ultra-large grain size Copper Catalyst

林士煒(Shih-Wei Lin)

指導教授 ： 陳俊維

國立臺灣大學/工學院/材料科學與工程學研究所/碩士(2014年)

https://doi.org/10.6342/NTU.2014.11040

摘要

石墨烯是由碳原子以sp2鍵結方式所組成的二維奈米材料，由於其特殊的幾何結構使得石墨烯擁有特別的零能帶結構、高雙載子傳輸性質、高透光性、機械性質，因此在近年來已經吸引許多材料科學與凝聚態物理學者的注意以及研究。2004年Novoselov團隊首次利用膠帶反覆剝離石墨晶體得到單層石墨烯，此方法擁有極高的載子流動率>15000 cm2V-1s-1，但其面積小於100 um。 其中，利用銅為催化基材之化學氣相沉積法是最具前景的合成方法，可以降低生產成本，並合成出大面積、高品質及高載子流動率的石墨烯。然而，在目前化學氣相沉積法的研究中，合成單晶石墨烯在室溫下擁有最高的載子流動率>11000 cm2V-1s-1，但其目前最大面積為直徑2.3 mm且大小不均勻，仍然無法使用於大面積的積體電路中。因此合成大面積多晶結構之石墨烯為達到大面積且具備高載子流動率>1000 cm2V-1s-1之方法，其面積取決於銅基材之大小、載子流動率取決於石墨烯品質。
大面積多晶石墨烯由於多晶結構具有許多石墨烯晶界，在許多研究中已指出載子在流動過晶界時會大幅降低載子流動率之主因，因此石墨烯成長條件、銅基材表面形貌與銅晶面也已被報導指出能使石墨烯晶粒增大減少晶界以及改善石墨烯形貌增加載子流動率以及品質。
因此在論文的一部分中(第四章)，我們使用一般市售多晶銅箔成長石墨烯，一般市售銅箔由於冷壓加工表面有線條狀起伏以及加工痕跡，由於石墨烯易成核在銅箔表面起伏以及加工痕跡上使得石墨烯會複製銅箔形貌形成層數不均勻的石墨烯導致載子流動率低以及降低欲轉印基板接觸。為改善此問題我們以電化學拋光改善銅箔表面變為極光滑表面，使石墨烯成長不受銅箔表面形貌影響，使得載子流動率有大幅提升，此外在石墨烯-矽蕭特基太陽能電池得到高效率中證實以拋光後銅箔成長之石墨烯與矽基板接觸良好。
而在論文第二段(第五章)，我們參考文獻中升高銅箔成長前退火溫度使表面更加光滑以及使晶粒成長減少銅晶界、提高成長溫度(1000oC提高至1050oC)使石墨烯成核密度降低增加成長速度以增加石墨烯粒大小，在此研究中我們發現提高退火溫度除了表面光滑外銅的晶粒也有大幅變化，銅的晶粒大小成長至將近1 mm2，銅的晶面也從多晶取向的晶面重構成並形成銅(111)取向的晶面，近期文獻報導中指出銅(111)晶面相較於其他銅晶面更適合於石墨烯成長由於石墨烯與銅(111)晶面晶格匹配減少應力以及晶格不匹配時產生之晶界。而在高成長溫度(1050oC)中也發現石墨烯晶粒確實大於較低成長溫度(1000oC)以減少單位面積下晶界的數量。最後綜合兩部分結果我們成功的利用拋光銅箔藉由高溫退火得到大晶粒、銅(111)取向晶面之銅箔以及高溫成長得到大面積、高品質及高載子流動率之石墨烯。

關鍵字

石墨烯 ； 高載子流動率 ； 銅(111) ； 電化學拋光

並列摘要

Graphene is a single atom-thick two dimensional material which attracted a lot of attention in materials science in the current century. Chemical vapor deposition (CVD) is the most promising, inexpensive, high uniform and large-area for high quality graphene.
In recent year, grow single crystal graphene domain or grow large area graphene on a single crystal Cu (111) film had widely used for achieving high quality graphene. However the size of single crystal graphene still not achieve wafer-scale and single crystal Cu (111) film is expensive than commercial Cu foil. Therefore, we studied the Cu foil pretreatment and growth condition for creating an ultra-smooth and ultra-large grain size with Cu (111) crystal orientation.
In the first part, Cu foil was electropolished for smoothing the Cu surface. We used atomic force microscopy (AFM) to measure the roughness of Cu foil. After the pretreatment, the roughness of Cu foil reduced from 25.4nm to 0.704nm. We observed that graphene grown on such a smooth copper surface enable to grow smooth, high uniform, single layer without line or grain boundary replica. In electrical characteristic, we measure the Dirac point and carrier mobility with thin film transistor (TFT) device. Moreover, graphene covered on the substrate smoothly. Therefore the interface between graphene and silicon will contact batter, enhancing the performance of graphene silicon schottky-junction solar cell.
Secondly, following from first part we created ultra-large grain size Cu foil by annealing polished Cu foil in 1050oC. Copper grain size grown to 1 mm2. On the other hand, the polycrystalline copper foil undergoes a transformation from random crystal orientation to Cu (111) crystal orientation. In this part, we created copper foil with ultra-large grain size and polycrystalline Cu (111). Expect for favoring growth of graphene on Cu (111), we reported that low density of nucleation and faster growth rate at higher growth temperature (1050oC). With the condition of high growth temperature, larger graphene domain reduced the grain boundaries of graphene and enhancing the carrier transported.
As a result, combined with ultra-large grain size with Cu (111) crystal orientation and large graphene domain, we could grow high quality of graphene with highly uniform and high mobility ~ 5462 cm2V-1s-1.

並列關鍵字

graphene ； high mobility ； Cu(111) ； electropolish

參考文獻

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