Atomic Layer Deposition of Crystalline MoS₂ Thin Films: New Molybdenum Precursor for Low‐Temperature Film Growth

Abstract

Compared to the most well-known 2D material, graphene, which is a semi-metal, the semiconducting 2H phase of MoS 2 is advantageous in having a band gap suitable for electronic applications. In bulk form, MoS 2 has an indirect band gap of 1.3 eV, which increases as a function of decreasing film thickness. In monolayer MoS 2 (thickness ≈0.6 nm), the band gap becomes direct with a width of 1.8 eV. [1] Importantly, to meet the requirements of different applications, properties of MoS 2 and other TMDCs can be tuned by controlling the thickness, [1] doping and alloying, [5][6][7][8] surface modification and functionalization, [9][10][11] strain, [12,13] and by creating heterostructures with other 2D materials. [6,[14][15][16] The appealing properties of TMDCs have led to a wide range of proposed applications. MoS 2 has been extensively studied as a channel material in conventional field-effect transistors, [17][18][19][20][21] as well as phototransistors and other optoelectronic devices. [16,21,22] The 2D structure of TMDCs plays a crucial role in possible applications relying on more exotic quantum phenomena, such as valleytronics. [23,24] MoS 2 has also shown promise in, for example, catalysis, [25] batteries, [26] photovoltaics, [27] sensors, [28] and medicine. [29] The production of high-quality, large-area MoS 2 films with a thickness controllable down to a monolayer, as required in many of the aforementioned applications, still remains a major challenge. Additionally, in many cases, the processing temperature should be kept as low as possible in order to avoid damaging sensitive substrates, such as polymers or nanostructures. Initially, flakes of monolayer MoS 2 were produced from natural MoS 2 crystals using micromechanical exfoliation, a topdown method capable of producing high-quality monolayers, albeit with poor throughput as well as limited control over flake thickness and dimensions. [4,30,31] Liquid-phase exfoliation of bulk crystals, on the other hand, offers good scalability, but often suffers from limited flake size, poor crystallinity, or contamination. [4,31,32] Bottom-up methods offer a more controllable way to produce MoS 2 films. High-quality MoS 2 thin films are most commonly deposited by chemical vapor deposition (CVD) or sulfurization of metal or metal oxide thin films. The most common Molybdenum disulfide (MoS 2 ) is a semiconducting 2D material, which has evoked wide interest due to its unique properties. However, the lack of controlled and scalable methods for the production of MoS 2 films at low temperatures remains a major hindrance on its way to applications. In this work, atomic layer deposition (ALD) is used to deposit crystalline MoS 2 thin films at a relatively low temperature of 300 °C. A new molybdenum precursor, Mo(thd) 3 (thd = 2,2,6,6-tetramethylheptane-3,5-dionato), is synthesized, characterized, and used for film deposition with H 2 S as the sulfur precursor. Self-limiting growth with a low growth rate of ≈0.025 Å cycle −1 , straightforward thickness control, and large-area uni...

2D MoSe2 Structures Prepared by Atomic Layer Deposition

“…Recently, the possibility to employ ALD as a technique to grow MoS 2 has been reported. In these works (CH 3 ) 2 S 2 or H 2 S were used as the S precursor and Mo(CO) 6 , MoCl 5 or Mo(thd) 3 as the Mo precursors. From the practical point of view, MoSe 2 is even more interesting than MoS 2 since MoSe 2 possesses a higher electrical conductivity than MoS 2 which is extremely important in electrochemical applications such as solar cells, batteries, supercapacitors, and electrocatalysis .…”

Nucleation mechanism during WS2 plasma enhanced atomic layer deposition on amorphous Al2O3 and sapphire substrates

“…This avoids the need for a material transfer from a growth substrate to the target substrate. Nonetheless, only few ALD processes of MoS2 and WS2 are reported, using dihydrogen sulfide (H2S) in combination with either metalhalide precursors such as molybdenum pentachloride (MoCl5) or tungsten hexafluoride (WF6), or metal-organic precursors such as molybdenum hexacarbonyl (Mo(CO)6), tris(2,2,6,6-tetramethylheptane-3,5-dionato) molybdenum(III) (Mothd3) or tetrakis(dimethylamido) molybdenum Mo(NMe2)4 [13][14][15][16][17] . In the absence of a template for epitaxial seeding, the layers are either amorphous or polycrystalline, depending on the deposition temperature (typically below 450C) and precursor chemistry.…”

“…The majority of the reports discusses the ALD process conditions and the physical properties of the grown layers [13][14][15][16][17] . In general, the crystal domain size is smaller for ALD than for CVD (< 100 nm).…”

Polarized Raman spectroscopy to elucidate the texture of synthesized MoS2

“…4,[9][10][11][12][13] However, in order to adopt these materials in the field of nanoelectronics, the development of scalable synthesis methods for 2D TMDs such as MoS 2 and WS 2 are essential. [14][15][16][17] This is closely intertwined with the development of reliable characterization techniques for synthesized TMDs, in which Raman spectroscopy is of particular interest. Raman spectroscopy has already emerged as the go-to technique for characterization of exfoliated single-crystalline TMDs yielding insight into e.g.…”

“…atomic-layer deposition (ALD) and chemical vapour deposition (CVD). 12,15,17,[27][28][29][30][31] Synthetic MoS 2 films can be amorphous or crystalline with a texture of the crystalline material ranging from in-plane oriented grains (i.e. horizontally aligned layers) to complex 3D structures with out-of-plane oriented (OoPO) grains (i.e.…”