| 研究実績の概要 |
Wafer-Scale and Deterministic Patterned Growth of Monolayer MoS2 via Vapor-Liquid-Solid Method (Salt 2.0 technique)
Two-dimensional (2D) atomically thin transition metal dichalcogenides (TMDCs), e.g., MoX2 and WX2 (X = S, Se, Te), have attracted worldwide interest since its experimental discovery. The widely used chemical vapor deposition (CVD) method using volatile oxide precursors, e.g., MoO3, WO3, etc. suffers from poor reproducibility and scalable production. Furthermore, the lack of site-controlled growth has limited its direct integration in electronic devices. The important achievement in this project is the wafer-scale and deterministic patterned growth of monolayer MoS2 monolayers with high quality and high reproducibility. When using the non-volatile salts, e.g., Na2MoO4, it shows a solid to liquid transition at the high growth temperature (700-800℃). And thus, monolayer MoS2 crystals precipitate at the interface of “liquid” and substrate surface when sulfur was incorporated. In sharp contrast to the conventional CVD method, the main advantages of using non-volatile salts are: 1) capable of uniform growth of monolayer MoS2 flakes on 4-inch-scale substrates; 2) capable of growing 2-inch-wafer-scale continuous MoS2 film with a grain size exceeding 100 μm; 3) capable of patterned (site-controlled) growth of MoS2 monolayers without using catalyst seeds; 4) high growth reproducibility; 5) high electrical quality of as-grown MoS2 monolayers with an average electron mobility of more than 20 cm2/Vs. This achievement was published in Nanoscale, 2019, 11, 16122-16129.
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| 現在までの達成度 (区分) |
現在までの達成度 (区分)
2: おおむね順調に進展している
理由
Synthesis of large-area, high-quality 2D TMDCs using non-volatile salt precursors.
The Slat 2.0 technique developed in this project is a general method for growing a large group of 2D TMDCs (MoX2 and WX2 (X = S, Se, Te)). The unique of Salt 2.0 technique is the use of non-volatile and water-soluble salt precursors, e.g., Na2MoO4 and Na2WO4. The salts precursors can be uniformly deposited on large wafers via spin-coating method. After reacting with chalcogens (S, Se, Te) in CVD process, the corresponding 2D TMDC monolayers can be grown in large-area, high-quality, and high-reproducibility as we demonstrated in the growth of MoS2 monolayer.
To date, 2-inch-wafer-scale MoSe2 film with a grain size of more than 200 μm and large WS2 single crystals with a crystal size of ~ 500 μm had been achieved. These are the biggest monolayer MoSe2 and WS2 single crystals ever grown with CVD method. These achievements also manifest the Salt 2.0 technique is a promising method to achieve the final goal of this project: wafer-scale growth of monolayer 2D TMDC single crystals.
Currently, most of the efforts are focused on the CVD, optimizing the growth conditions, understanding the chemical reactions, and improving reproducibility. Based on the gained knowledge on CVD growth of 2D TMDCs, the size of 2D TMDC single crystals increased substantially.
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| 今後の研究の推進方策 |
Synthesis of wafer-scale 2D TMDC single crystals.
Almost all the wafer-scale CVD-grown 2D TMDC films are polycrystals. The films contain high density of grain boundaries. These grain boundaries (line defects) in the 2D TMDC films cause the non-uniform electrical quality and greatly hinder the application of 2D TMDCs in electronic devices.
Recent development in CVD growth of 2D materials had already demonstrated the wafer-scale growth of graphene and h-BN single crystals using metal substrates with specific surface steps. The surface step is a unique feature which can guide the nucleation and orientation of the CVD-grown graphene/h-BN domains. Because of the same orientation of all the small domains, they finally merged as large graphene/h-BN single crystals.
Take advantage of using substrates with surface steps and the well-developed Salt 2.0 technique, it is very promising to achieve the wafer-scale growth of 2D TMDC single crystals. Beside the wafer-scale growth 2D TMDC single crystals, it is also important to evaluate the quality and uniformity of the 2D TMDC films. More characterization will be employed to make sure there is no grain boundary in the single crystals. More field effect transistors using 2D TMDC single crystals will be fabricated and test. With the electrical quality and uniformity information, CVD growth conditions will be further optimized.
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