The Role of Deposition Temperature and Substrate for Scalable and Uniform Deposition of MoS2 Grown by Vapour-Solid Chemical Reaction

Two-dimensional (2D) materials represents nowadays the new frontiers of semiconductor technology [1]. At one side, great effort has been done in research applied to graphene producing new technology that will rebuild many industrial sectors. On the other side non- graphene 2D materials like transition metal dichalcogenides (TMDs) are highly attractive because they offer complementary properties to graphene [2] but still lack a large-scale production method for high quality and well controlled layers. MoS2, one of the most studied TMDs materials, has been produced by using many techniques, but the deposition through chemical methods, i.e. based on the use of materials such as Mo or MoO3 and then the reaction with sulfur, already demonstrates to fit better the stringent requirements of lateral uniformity on the centimeter scale, vertical scalability and structural optimization as function of the growth parameters [3]. However, many details about the chemistry of the reaction between Mo precursor and sulfur needs further clarification in view of a large-area production [4]: in particular, it is not clear the role of the materials used as precursors, of the substrate type and of the deposition temperature, which drives Mo-S reaction, but also other undesired ones. This work is focused on the study of some of these fundamental parameters in the growth of MoS2 on SiO2/Si substrates by vapor-solid chemical reaction between Mo pre-deposited thin film precursor (TFP) and sulfur (figure 1(a)). We demonstrate that the control of the Mo film thickness allows us to have accurate control of the MoS2 thickness. This methodology leads to MoS2 nanosheets with a tunable number of layers and uniformly extended throughout the whole area of the support substrates (figure 1(b)). In this type of process, the role of the growth temperature and of the substrate on the structural properties of the layers becomes dramatically important. Various techniques are employed to characterize the MoS2 layers. In particular, Raman spectroscopy is the key-probe used for single- and multi-layers identification and for the validation of the high structural quality of the grown material (see figure 1(b)), but fundamental information about the chemical details, the morphology, and the structural quality are also drawn from X-ray photoelectron microscopy (XPS), atomic force microscopy (AFM), and photoluminescence (PL). The temperatures used for the study are 500, 750, and 1000 °C. All the grown films show granular morphology by AFM. Better results in term of layer quality is obtained with higher sulfurization temperature and detailed analysis of Raman spectra collected from quadrilayers demonstrates that the layer features are comparable to exfoliated MoS2 samples. We further demonstrate that SiO2/Si wafer bring intrinsic limitation as a substrate for the MoS2 growth at high deposition temperature because of the activation of silicon diffusion from the substrate and of the chemical reaction between Si and S with SiS2 byproduct formation [5]. Our study supplies some fundamental information on the chemistry of vapor-solid reaction growth, clarifying the role of temperature in giving high quality materials and putting a warning on the use of silicon substrate for this type of process [6]. References [1] Yazyev, O. V. and Chen, Y. P., Nature Nanotechnology 9, 755 (2014). [2] See editorial’s Nature Materials 13, 1073 (2014) and references therein. [3] Lee, Y.-H. et al. Adv. Mater. 24, 2320–2325 (2012); Liu, K. et al. Nano Lett. 12, 1538 (2012); Yu, Y. et al., Sci. Rep. 3, 1866 (2013); Zhan, Y. et al. Small 8, 966–971 (2012). [4] Liu, H. et al. Nanotechnology 25, 405702 (2014). [5] Niu, J. et al. Physica E 24, 278–281 (2004). [6] This research was partially supported by US-Army Contract with CNR-IMEM and CNR-IMM.