The Study on Pressure-Dependent Dual Growth Modes for Large Scale Growth of Monolayer MoS2

Abstract

2차원 나노물질인 MoS2는 나노미터 수준의 두께에서도 반도체 성질을 갖기 때문에, 차세대 반도체 물질로 많은 관심을 받는 물질이다. 따라서, 원자층의 MoS2를 합성하기 위해 많은 연구가 진행되고 있으며, 특히 chemical vapor deposition 방법은 높은 품질의 MoS2를 기판 위에 균일하게 합성할 수 있다는 점에서 적절한 합성 방법으로 평가받고 있다. 하지만 MoS2가 어떠한 성장 메커니즘을 통해 성장하는지에 대해서는 알려진 바가 거의 없다. 본 연구에서는 탄소를 포함하지 않는 무기 증기상 전구체를 이용하여 MoS2가 어떠한 성장 메커니즘을 통해 성장하는지에 초점을 맞추었다. 우선, 대면적으로 합성된 MoS2를 분광학, 전자 현미경 분석을 통해 결함 없이 높은 결정성을 갖는다는 것을 확인하였다. 이후 전구체의 압력을 조절하여 진행한 대조실험에서 압력에 따라 다르게 성장하는 경향을 확인하였다. 저압 조건에서는 반응시간이 길어져도 단층으로 성장이 끝나는 반면, 고압 조건에서는 반응시간에 따라 지속해서 성장하는 경향이 관찰되었다. 원인 분석을 위해 진행된 Density functional theory 계산을 통해 저압에서는 기판과 전구체의 화학적 흡착만 발생하지만, 고압에서는 Dangling bonds가 없는 MoS2층과 전구체의 물리적 흡착 또한 발생할 수 있음을 확인하였으며, 이러한 결과들을 통해 무기 증기상 전구체에서 압력에 따라 어떤 성장 메커니즘을 통해 MoS2가 성장하는지 성공적으로 규명하였다.|As MoS2, two-dimensional material as well as semiconductor, has received tremendous attention as next-generation material which can replace conventional semiconductor, many attempts have been carried out to synthesize MoS2 thin film for various application areas. Although various methods for growth of monolayer MoS2 using chemical vapor deposition have been attempted, homogeneous monolayer MoS2 film with high crystallinity for short growth time is still challenging. Recent advanced synthetic methods can solve these drawbacks, but the growth mechanism of MoS2 is still unveiled clearly. Because understanding the growth mechanism is very important for synthesizing high quality MoS2, there are many attempts to understand the mechanism. Recently, some papers provide meaningful evidence that how MoS2 grows on the substrate through growth mechanism. When NaCl is added with precursor, it forms intermediates which have low melting points facilitating the growth of MoS2. Furthermore, when organic precursors are used for synthesizing MoS2, whole precursors can be flowed into furnace as gas-state during reaction and result in homogeneous MoS2 film. Herein, I developed another synthetic method, which can achieve previously mentioned both strengths, using MoOCl4 of pentacoordinated inorganic complex as Mo precursor and H2S as S precursor. Compared to the previous reported methods, our method shows that MoS2 can be grown on the substrate with high growth-rate and quality, which is confirmed through several analytical techniques. Additionally, since I use inorganic precursors that can be converted into gas-state easily due to low melting points, I can control pressure of precursor precisely during reaction. As a result, I observed two different growth modes according to pressure of precursors, i) chemisorption-driven growth ii) physisorption-driven growth. At the low-pressure condition, I can synthesize monolayer MoS2 at even 50 min growth time because the growth self-terminate until monolayer resulting from only chemisorption-driven growth between MoOCl4 and substrate. At the high-pressure condition, in contrast, both chemisorption- and physisorption-driven growth can occur, which results in thicker MoS2 film as growth time increases. To support the growth mechanisms I suggest, the density functional theory calculation is carried out and reveals that MoOCl4 has stronger bonding energy and shorter distance with quartz surface than with MoS2 surface. Therefore, I believe that our demonstrations do not only provide the fundamental insight to understand the precise growth mechanism of MoS2, but also enable various two-dimensional materials including heterostructure to be grown as the application demands.