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 550^oC 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 (A_1g). There are two other peak characteristics of MoS₂ which cannot be seen due to back-scattering at 286cm^-1 (E_1g), 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 550^oC, 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].

References

[1] Geim,A.K.& Grigorieva,I.V. Van der Waals Heterostructures. Nature 499,419–425 (2013).

[2] Lin,Y.-C.etal. Wafer-scale MoS₂ thin layers prepared by MoO₃ sulfurization. Nanoscale 4, 6637–6641(2012).

[3] Kang et al., Nature, 2015, 520, 656–660

[4] C. Zhou, Y. Zhao, S. Raju, Y. Wang, Z. Lin, M. Chan, Y. Chai, Adv. Funct. Mater. 2016, 26, 4223

[5] Gioele Mirabelli, Michael Schmidt, Brendan Sheehan, Karim Cherkaoui, Scott Monaghan, Ian Povey, Melissa McCarthy, Alan P Bell, Roger Nagle, Felice Crupi, Paul K Hurley, Ray Duffy, “Back-gated Nb-doped MoS2 junctionless field-effect-transistors” AIP Advances, 6, 2 , 025323 (2016)

[6] Li et al, From Bulk to Monolayer MoS₂: Evolution of Raman Scattering, Advanced Functional Materials, 22 (2012)

[7] John Y. W. Seto, The electrical properties of polycrystalline silicon films, Journal of Applied Physics 46, 5247 (1975)

[8] Saptarshi Das et al., Nb-doped single crystalline MoS₂ field effect transistor, Appl. Phys. Lett. 106, 173506 (2015)

[9] T. Hallam et al., Rhenium-doped MoS₂ films Appl. Phys. Lett. 111, 203101 (2017)