Atomic Layer Deposition of Two-Dimensional Metal Dichalcogenides

Miika Mattinen
Abstract & Cover

Two-dimensional (2D) materials rank among the most scientifically exciting materials of the early 21st century. Transition metal dichalcogenides (TMDCs) have emerged into the spotlight due to the semiconducting nature of many TMDCs, which is in contrast to the most actively studied 2D material, semimetallic graphene. Research on the basic properties of TMDCs has been very active and fruitful, resulting in unveiling of many new phenomena and properties. Furthermore, there is a strong drive to realize the technological potential of TMDCs. For use in practical applications, TMDCs need to be synthesized as uniform films of controlled thickness on large and complex substrates. In order to realize cost-effective industrial production, the synthesis needs to be done at low temperatures using methods that are highly controllable, scalable, and repeatable. Atomic layer deposition (ALD) is an advanced gas-phase thin film deposition technique capable of fulfilling the requirements of many demanding applications. ALD has already proven its industrial applicability in fields ranging from electroluminescent displays to microelectronics, photovoltaics, and corrosion protection. To realize the potential of ALD in the deposition of TMDCs, suitable ALD precursors possessing adequate reactivity, volatility, and thermal stability have to be identified and evaluated. In this thesis, 29 precursor candidates were tested for seven metals. Successful ALD processes were developed for five 2D sulfides: MoS2, SnS2, WS2, HfS2, and ZrS2. In addition, ALD processes were developed for oxides of molybdenum and tungsten. The oxides may be converted into the respective 2D sulfides. Furthermore, α-MoO3 is a 2D material by itself. The sulfide processes varied in terms of their growth behavior and morphology of the films. All of the films crystallized in 2D structures. In the case of SnS2 and WS2, crystallization required mild post-deposition annealing, which preserves the smooth morphology of the as-deposited amorphous films and gives an additional degree of freedom in processing. Particular attention was paid to the role of the substrate in the growth of TMDCs in tuning the film growth, morphology, and crystallinity. HfS2, MoS2, SnS2, and ZrS2 films were observed to grow in a van der Waals epitaxial manner on mica, which is a promising approach to achieve high film quality under mild conditions. Once the deposition processes are developed, the produced films should be evaluated for the target applications. A major challenge is to improve the performance of large-area TMDC films grown under application-relevant conditions up to the level of TMDC flakes that have been manually exfoliated from bulk crystals. The possible applications of ALD TMDCs are comprehensively reviewed in the literature part of the thesis. In the experimental part, results on photodetector (HfS2, SnS2, and ZrS2), field-effect transistor (SnS2), and hydrogen gas sensor (MoOx) devices are shown. It is anticipated that the processes developed in this thesis can be used also for other applications. For example, the rough MoS2 and disordered WS2 films should be promising for energy storage and conversion applications. 

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University of Helsinki
(Helsinki, Finland)
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