Chemical vapor deposition of monolayer-thin WS2 crystals from the WF6 and H2S precursors at low deposition temperature

Abstract

Monolayer-thin WS2 with (0002) texture grows by chemical vapor deposition (CVD) from gas-phase precursors WF6 and H2S at a deposition temperature of 450 °C on 300 mm Si wafers covered with an amorphous Al2O3 starting surface. We investigate the growth and nucleation mechanism during the CVD process by analyzing the morphology of the WS2 crystals. The CVD process consists of two distinct growth regimes. During (i) the initial growth regime, a fast and self-limiting reaction of the CVD precursors with the Al2O3 starting surface forms predominantly monolayer-thin WS2 crystals and AlF3 crystals that completely cover the starting surface. During (ii) the steady-state growth regime, a much slower, anisotropic reaction on the bottom, first WS2 layer proceeds with the next WS2 layer growing preferentially in the lateral dimensions. We propose that the precursor adsorption reaction rate strongly diminishes when the precursors have no more access to the Al2O3 surface as soon as the WS2 layer completely covers the Al2O3 surface, and that the WS2 crystal basal planes and AlF3 crystals have a low reactivity for WF6 adsorption at 450 °C. Nonetheless, a second layer of WS2 starts to form before the first WS2 layer completely covers the starting surface, albeit the surface coverage of the second layer is low (< 20%, after 25 minutes of CVD reaction).During the steady-state growth regime, predominantly the WS2 crystals in the second monolayer continue to grow in lateral dimensions up to ~40 nm. These crystals reach larger lateral dimensions compared to the crystals in the bottom, first layer due to low reactivity for WF6 adsorption on the WS2 basal plane compared to Al2O3. Presumably, they grow laterally by precursor species that adsorb on and diffuse across the WS2 surface, before being incorporated at the more reactive edges of the WS2 crystals in the second layer. Such process proceeds slowly with only up to 40 % surface coverage of the second WS2 layer after 150 minutes of CVD reaction. The CVD reaction is mediated by the starting surface: WF6 precursor preferentially adsorbs on Al2O3, whereas adsorption is not observed on SiO2. Nevertheless, WS2 grows on SiO2 in close proximity with Al2O3 in 90 nm pitch Al2O3/SiO2 line patterns. Hence, functionalization of the starting surface (e.g., SiO2 with Al2O3) can provide opportunities to grow monolayer-thin WS2 crystals at predetermined locations by selective, lateral growth with tunable crystal size, even at low deposition temperatures.

Gas-Source CVD Growth of Atomic Layered WS2 from WF6 and H2S Precursors with High Grain Size Uniformity

“…S2a), which is less than 10% the size of the WS 2 grains grown with NaCl. This result is consistent with previous findings 35 . Furthermore, observed FWHM values from the PL spectrum (Fig.…”

“…Therefore, we believe that alkali metals, not chloride, drive the alkali-metal assistance method. A plausible overall chemical equation 35 for WS 2 synthesis from H 2 S and WF 6 is…”

“…Equation (5) also has a negative Δ G value. However, this reaction is negligible, since the reactivity between SiO 2 and WF 6 is poor 35 . Furthermore, the surface of the substrate was not roughened after the growth, where root-mean-square roughness of the substrate was changed from 0.197 to 0.205 nm with the growth.…”

“…The use of H 2 S and WF 6 provide a number of additional advantages over solid precursors: i) gas-phase supply makes large-scale growth easy (e.g. WS 2 CVD and ALD growth from these precursors onto 300-mm Si wafers with a dielectric coating 35–38 ); ii) they are already widely used in semiconductor manufacturing (e.g. WF 6 is used for low-temperature CVD growth of W films on Si wafers to form W-plugs 40–42 ) and iii) their high reactivity allows low-temperature WS 2 growth (250–450 °C), which enhances their applicability 34–39 .…”

Optimisation of processing conditions during CVD growth of 2D WS2 films from a chloride precursor

“…Tungsten halide precursors have also been widely investigated [26,29,[36][37][38] due to their relatively low decomposition temperatures and the absence of hydrocarbon reaction products which could incorporate as impurities. In this paper, we describe investigations of the growth of WS 2 on c-plane sapphire using the metal halide WCl 6 as the tungsten source in a cold-wall CVD reactor.…”