化學氣相沉積二硫化鉬薄膜

Abstract

二硫化鉬為結構與石墨烯相似的二維材料，六角平面的鉬，上下被硫原子平面夾住形成三角柱結構，雖然結構相似，性質卻是截然不同。二硫化鉬由塊材減薄為單層時，會展現出量子侷限效應，材料能隙由非直接轉換為1.8 eV的直接能隙，使單層二硫化鉬具有良好的發光特性，因而成為眾多科學家的研究目標。起初，單層二硫化鉬能夠用膠帶撕取出來，不過尺寸有限且不能大量輸出單層材料，因此能夠合成大面積薄膜的化學氣相沉積法成為大家熱切運用的製程，並且期待量產於業界。然而，眾所皆知化學氣相反應製程當中，能夠影響薄膜成果的因素相當多，例如溫度、壓力、成長基板與前驅物的距離以及成長基板的擺放方式等實驗參數皆能使薄膜形貌與厚度產生變化。為了明確知道調整的參數對成長結果的影響，首先需要了解二硫化鉬的成長機制，在本論文中，我們以實驗結果推測二硫化鉬薄膜的形成機制是由三氧化鉬蒸氣被初步還原為二氧化鉬蒸氣，接著二氧化鉬蒸氣再與硫一起落在基板上反應形成二硫化鉬分子，經過擴散而成長為連續薄膜。最後，我們在700˚C的溫度之下，將試片直立擺放，成長出3.5×2.2 cm2的雙層二硫化鉬連續薄膜，並且發現此薄膜具有極強的發光效率，約為機械剝離法製備的二硫化鉬單層之發光效率的十三倍。此外，雙層二硫化鉬場效電晶體展現了10 cm2/V-s的載子遷移率以及104的電流開關比，顯示我們成長的薄膜具有作為良好光電元件的材料之潛力。

Molybdenum disulfide (MoS2) is a two-dimensional hexagonal lattice. In contrast to graphene, which has no bandgap by nature, MoS2 monolayer has a direct bandgap of 1.8 eV due to the quantum confinement effect. It exhibits a high on/off current ratio and strong luminescence. Therefore, MoS2 monolayers have attracted much attention in experimental and theoretical researches for its potential applications in optoelectronic devices, field effect transistors, low power switches, valleytronics, etc. For practical applications, it is of importance to develop a reliable growth process to synthesize large-area, uniform, and continuous MoS2 monolayers. Here we use the chemical vapor deposition method for the growth of MoS2 atomic layers, using the precursors, MoO3 and sulfur and high-purity argon carrier gas, onto the silicon oxide substrates. In this study, we investigate the influences of growth temperature, pressure, substrate positions and the substrate orientation on the morphology and thickness of the MoS¬2 thin films and infer the mechanism of MoS2 growth in our system from the experimental results. Continuous MoS2 bilayers in a size of 3.5×2.2 cm2 can be obtained. They exhibit an on/off ratio of 104, mobility of ~10 cm2/V-s, and high luminescence, which is ~13 times stronger than that of the exfoliated MoS2 monolayers.