(Invited) Manufacturable Deposition of Two-Dimensional Tungsten Disulfide for Logic Applications
Authors: Groven Benjamin , Shi Y. , Morin P. , Caymax Matty 
Published: May 2020

The two-dimensional (2D) semiconducting molybdenum and tungsten disulfide (MoS2 and WS2) hold promise as ultra-scaled metal-oxide-semiconductor (MOS) channel material for low power and/or high-performance logic applications. Manufacturable approaches that develop highly crystalline MX2 layers, tailor the layer number down to the atomic level and remain compatible with temperature sensitive structures, are essential to unlock the desired material functionality. However, fundamental understanding is lacking on how to design chemical deposition processes for 2D MX2, such as chemical vapor deposition (CVD).

Therefore, our current research efforts focus on how to control the crystallinity, structure and morphology of WS2 crystals in the first, single layer through understanding the growth behaviour during a metal-organic (MO-)CVD process. WS2 is grown from tungsten hexacarbonyl (W(CO)6) and dihydrogen sulfide (H2S) precursors on 300 mm Si substrates covered with amorphous SiO2 and on single crystalline sapphire templates. A commercial 300 mm state-of-the-art Si and SiGe epitaxial reactor has been modified to deposit MX2 materials.

In order to construct a qualitative model for relevant growth processes during WS2 MOCVD, the reaction kinetics are studied. Insight in the growth mechanisms is captured from the evolution in morphology of the WS2 crystals at different stages during the MOCVD process. Based on a statistical and morphological analysis of crystals in the first, single WS2 layer, two figures of merit describing the MOCVD process are extracted: the areal density of WS2 crystals and median lateral growth rate (lateral GR in nm2/(min∙cm2)).

The WS2 MOCVD process is a profoundly thermally activated deposition process, with both nucleation rate and lateral GR dependent on deposition temperature. From an Arrhenius graph of WS2 inter-nucleus distance, the activation energy of surface diffusion is 10 kcal/mole. The areal density of WS2 crystals decreases over two orders of magnitude with deposition temperature, from 2x1010 /cm2 at 550 °C to 1x108 /cm2 at 1000 °C. However, it does not vary significantly with W(CO)6 partial pressure nor ratio between chalcogen and metal precursor partial pressure. Hence, the type and areal density of active surface sites of the starting surface determines the areal density of WS2 crystals, rather than the dose of metal precursor supplied to the starting surface. This opens opportunities to further control the areal density of WS2 crystals, for example through surface pretreatment. In contrast, the lateral growth rate of WS2 crystals in first, single layer increases most profoundly with deposition temperature and metal precursor partial pressure, with an activation energy of lateral growth approaching 31 kcal/mole. From the experimentally determined activation energies, the MOCVD process is likely governed by diffusion of reagents and reaction products (e.g., CO) across the boundary layer, and the dissociative physi-sorption of W(CO)6 precursor on starting surface.

Based on these insights, the MOCVD process has been optimized and WS2 crystals with a median crystal size of 500 nm have been grown on amorphous SiO2. That learning is also applied to single crystalline sapphire substrates for their epitaxial seeding capability, considered to date the preferred approach to obtain state-of-the-art intrinsic material quality. In contrast to WS2 layers on SiO2, the WS2 crystals on the pretreated sapphire substrate develop a preferential in-plane crystalline orientation. The combination of the epitaxial seeding capability with the control over the areal density of WS2 crystals down to 1.7 x109 /cm2, implies that neighboring crystals can merge without forming a defective grain boundary yielding in principle micrometer-size crystals in the first, single layer.