Salt-assisted chemical vapor deposition of two-dimensional transition metal disulfides
preface
The rapid development of science and technology in recent years, coupled with the emergence of artificial intelligence, the Internet of Things and big data analysis, has led to an extremely high demand for processors with high computing power and speed, and traditional three-dimensional (3D) silicon semiconductor manufacturing technology is rapidly approaching the physical and process limits of the 2nm technology node, which will hinder the further development of current silicon semiconductor manufacturing technology and prevent further shrinkage, therefore, new materials with nano- or sub-nanometer scale feature sizes must be explored.
Halogen salt-assisted growth mechanism of CVD (HSA-CVD) in two-dimensional TMDC
Conventional chemical vapor deposition
A conventional CVD setup for growing 2D TMDCs (e.g., MoS2) is shown in Figure 1A, where crucibles containing transition metal oxide (TMO) powders and growth substrates are loaded in a high-temperature center, chalcogenide powders are loaded upstream in the low temperature zone, CVD-grown MoS2 was first demonstrated in 2012 by decomposition of (NH4)2 MoS4 and vulcanization of Mo membranes and MoO3, respectively.
Role of halogens in HSA-CVD
For CVD growth of two-dimensional TMDCs under mild conditions, the growth of large WSe2 and WS2 monolayers (by comparing tungsten oxide (WO 2.9, 1473 °C) with halide salts (A X, A = Na, K; X = Cl,Br,I) mixed, WSe2 and WS2 monolayers can grow at atmospheric pressure and significantly lower temperatures at 700°C, more than 100°C lower than the same process using only tungsten oxide.
The role of alkali metals in HSA-CVD
Alkali metals (Li, Na, K) are an important component of alkali metal halide salts and behave quite differently from halogens in SA-CVD growth of 2D TMDCs, therefore, 2D based on Mo and W can be confirmed by simple vulcanization, selenization, and tellurization of molten salts (Na2MoO4 and Na2WO4, respectively) in comparison between the internal and external regions of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). VLS growth of TMDCs.
Versatility for two-dimensional material growth
The most impressive achievement of HSA-CVD is that it can be used to grow 47 different types of two-dimensional materials, including 32 binary metal chalcogenides, MX2, and MX (M = Mo, W, Re, Ti, Zr, Hf, V, Nb, Ta, Pt, Pd, Fe; X=S, Se, Te), 13 ternary and quaternary alloys, 2 heterojunction pure metals such as Ta and Re can also be mixed directly with NaCl or KI to grow Ta and Re-based two-dimensional TMDCs.
HSA-CVD upgrade (Salt 1.0+ technology)
Despite its many advantages, HSA-CVD (Salt 1.0 technology) also has many disadvantages, a serious problem is that there is too much vapor from the transition metal (TM) precursors in the growth chamber, so the growth of 2D TMDC is strongly dependent on the position of the substrate.
Application of molten salt CVD (Salt 2.0 technology).
In contrast to Salt 1.0 technology with complex VSS and VLS growth patterns, Salt 2.0 technology uses molten salt to grow 2D TMDC in VLS mode, and to guarantee the success of Salt 2.0 technology, molten salt must meet three main criteria: (1) a fairly high melting point; (2) Ability to form a stable melt with low vapor pressure at CVD growth temperature; (3) Soluble in H2O, the first two criteria ensure VLS growth of 2D TMDCs.
Alkali metal – two sides of a coin
Regardless of the technology used, alkali metals are an integral part of promoting the lateral growth of 2D TMDCs, and many alkali-based chemicals such as NaOH, KOH, Na2SO4, KH2PO4, Na2CO3, KNO3, CH3 COONa, C6H5O7Na3, PTAS, EDTA-4Na, etc., can be used to enhance CVD growth of 2D TMDCs.
conclusion
Due to the rapid development of SA-CVD, a wide variety of two-dimensional materials have been synthesized in recent years, and the role of halogens and alkali metals in promoting the growth of 2D TMDCs, in general, halogens increase the volatility of metal precursors with high melting points, thus triggering the rapid growth of VSS.
