Large Area Growth of MoS2 By Chemical Vapour Deposition
Authors: Coleman Emma , Monaghan Scott , Gity Farzan , Schmidt Michael , Connolly James , Lin Jun , Walsh Lee , Cherkaoui Karim , O'Neill Katie , McEvoy Niall , O' Coileain Cormac , Buckley Darragh , O'Dwyer Colm , Hurley P. K. , Duesberg Georg S. 
Published: February 2020

Transition Metal Dichalcogenindes (TMD) have considerable potential for applications spanning electronics, sensors and optoelectronics due to the wide ranging electronic and optical properties which are displayed by this class of 2D layered materials [1]. Research is focused on issues such as: large area growth [2, 3], stable approaches to doping [4] and achieving required values of specific contact resistivity [5]. We are contributing to the research effort by investigating the structural, optical and electronic properties of crystalline molybdenum disulfide (MoS2) grown by chemical vapour deposition (CVD) in a commercial 300mm atomic layer deposition reactor. In this work we report on the properties of monolayer and multilayer MoS2 growth at 550oC using Mo(CO)6 and H2S precursors on a number of different substrates, including SiO2, sapphire and amorphous alumina. This work focuses on the topology, Raman response and electronic properties of the CVD grown MoS2 thin films.

Figure 1 (a) shows the Raman spectrum for an as-grown 10nm MoS2 on a SiO2/Si substrate. MoS2 can be determined by the peaks at approximately 385cm-1 (E1
2g) and 410cm-1 (A1g). There are two other peak characteristics of MoS2 which cannot be seen due to back-scattering at 286cm-1 (E1g), and lack of sensitivity below 200cm-1 at 32cm-1 (E2
2g )[6]. Additional peaks are detected at wavenumbers between 2000-3000cm-1, seen in Fig. 1 (b). Further investigation is needed to establish if these peaks are from photoluminescence, contaminants within the structure or purely a surface effect. Fig. 1 (c) shows an AFM image of an annealed 1.2nm MoS2 sample on a SiO2/Si substrate. The blue marks indicate areas on the material that are above a height of 0.6nm. Further results will be presented as a function of the number of MoS2 monolayers using Conductive AFM (C-AFM) and Kelvin Probe analysis. XTEM images seen in Fig. 1 (d), (e) show MoS2 grown on SiO2/Si and sapphire substrates respectively. They show that polycrystalline and layered MoS2 is formed at the growth temperature of 550oC, with no subsequent post growth annealing. There is no interfacial layer formed at the MoS2/SiO2 interface, but an amorphous interfacial layer of ~0.5nm is observed between MoS2 and sapphire, which is still being investigated. Plan view TEM analysis (not shown) confirms aligned MoS2 with grain sizes (over a local area of around 100 nm x 100nm) in the range 5nm to 20nm.

The carrier concentration, carrier type and carrier mobility were studied with Hall measurements carried out at room temperature using a Van der Pauw structure (1cm x 1 cm). Excellent ohmic behavior is achieved on MoS2 (nominally 10nm) deposited on both sapphire and a-Al2O3/sapphire substrates. Table 1 provides a summary of the Hall analysis, showing that the non-intentionally doped MoS2 grown by CVD is n-type with very low carrier concentrations on the order of ~1014cm-3, electron mobility in the range 3.3-16.7cm2/V.s. Mobility values up to ~ 15 cm2/Vs for a grain size in the 10nm to 60nm range, is an interesting result, as in the work of K. Kang et al., [3], the monolayer grain size is around 1 mm with an associated electron mobility of 30 cm2/V. These results suggests that grain boundary defects in 2D MoS2 may not be the main factor limiting carrier mobility, as is typically the case in polycrystalline 3D semiconductors (see for example [7]). In addition, the unintentional n type doping in the CVD grown MoS2 is low, with values around 1-3x1014 cm-3. This low value of unintentional doping provides a useful baseline in-situ for doping studies with elements such as Nb [8] and Re [9].

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Figure 1