Two-dimensional (2D) transition metal dichalcogenides, such as WS₂, MoS₂, etc., have attracted tremendous interest as a new channel material that could facilitate continued transistor scaling since atomically thin TMDs do not suffer from channel scaling issues that limit Si transistors [1, 2]. Most literature describes work done on exfoliated flakes, but metal-organic CVD is the preferred method for growing wafer-scale WS₂ films on substrates compatible with contemporary CMOS process technology.

Attention areas of conventional WS₂ MOCVD relate to the high supersaturation conditions during deposition and consequently, the high WS₂ nucleation density on the growth substrate leading inevitably to crystallite sizes well below 1 µm. Crystal grain boundaries hamper charge transport and therefore need to be minimized.

In this work, we demonstrate improved control over precursor adsorption, desorption, and diffusion by working under low supersaturation conditions, which ultimately results in WS₂ layers with larger crystal size and layered crystalline structure.

By using alternative tungsten precursors such as tungsten chlorides, reaction mixture supersaturation reduces significantly. A technically easier option in view of comparatively higher vapor pressure for metal-organic precursors such as W(CO)₆, is to keep using convenient W(CO)₆ precursor and convert it into tungsten chlorides inside the reactor by co-injecting HCl. This resembles approaches of III-nitride MOCVD [3-5] and silane-base silicon CVD [6] communities, where volatile chlorine-containing compounds of Ga, Al, or Si were produced in situ by reaction with HCl.

A series of WS₂ films were deposited on 300 mm SiO₂-coated Si substrates using W(CO)₆ and H₂S precursors with additional HCl flow (HCl-assisted MOCVD) and without additional HCl flow (“conventional” MOCVD) using an industry-standard, modified 300 mm epitaxial reactor connected to an ASM Epsilon3200 platform.

By thermodynamic analysis of WS₂ deposition from the W(CO)₆ and H₂S precursor pair, the equilibrium vapor pressure of tungsten precursor (P_W < 10^-28 atm) is at least twenty orders-of-magnitude lower than the initial tungsten precursor partial pressure (P_W≈10^-6...10^-8 atm). This has two detrimental consequences. First, a high nucleation rate results in massive secondary nucleation on the surface and parasitic nucleation in the gas phase. Second, the desorption flux of metal precursor, which is directly proportional to the equilibrium vapor pressure, is negligible compared with the adsorption flux, meaning that the tungsten precursor adsorption is irreversible. In this case, the only mechanism for lateral mobility of tungsten adatoms is the surface diffusion – a relatively slow process, the rate of which decreases exponentially with decreasing temperature.

In contrast, near-equilibrium conditions are achieved at a temperature range of 800-1000°C by adding 10% of HCl to the carrier gas flow. Such conditions promote a reduced nucleation rate, an efficient lateral precursor transport by gas-phase diffusion, and therefore, an improved crystal quality.

Films deposited with HCl-MOCVD demonstrate more than a 1000x reduction in the nucleation density and a 10x increase in the crystal size, compared to films grown by a conventional MOCVD process at the same temperature. Besides that, the HCl-MOCVD process allows lateral selective growth initiated by pre-deposited WS₂ seeds: while no WS₂ nucleation and growth occurred on the bare SiO₂ surface, mass-transport-limited WS₂ growth was observed on SiO₂ surface with WS₂ seed crystals prefabricated in conventional MOCVD. This preliminary insight into the growth behavior during the HCl-assisted MOCVD opens opportunities to increase the WS₂ crystallite size even further beyond 1 µm.

[1] R. Chau, IEDM, 2019, p. 1.1.1
[2] Y. Shi et al., IEDM, 2021, p. 37.1.1
[3] O. Kryliouk et al., Materials Science and Engineering: B. 66 (1999), 26
[4] Y. Kumagai et. al., Journal of Crystal Growth 246 (2002) 215
[5] D. Fahle et al., Journal of Crystal Growth 370 (2013) 30
[6] J. Bloem et al., Journal of crystal growth 49 (1980), 435