Chemical vapor deposition of ultra-thin molybdenum ditelluride films and their morphological and optical characterization

Graphene has been one of the most exploited 2D materials in various fields. However, it is not the most suitable candidate for many electronic devices unless its bandgap structure is tuned through functionalization. Recently, among the post-graphene 2D materials, transition metal ditellurides, like MoTe2, have gained increasing research interest due to their unique physical properties. They offer two stable allotropic states, namely the semiconducting 2H phase and the metallic 1T’ phase, with a low energy difference of 44meV. Consequently, the phase transition between its semiconducting and metallic phase is likely to happen, thus making them suitable candidates for "phase-change" related applications, like 2D non-volatile memory devices and memristors. Applications require the growth of broad-area and high-quality samples, which is a non-trivial challenge due to tellurium’s low reactivity. In this thesis, I dedicated my efforts to a systematic study on the synthesis of ultrathin MoTe2 films through chemical vapor deposition (CVD), aiming to obtain centimeter-scale deposition with high crystalline quality (large grain size, phase uniformity, etc..). I developed a protocol for the growth of few-layer MoTe2 based on a tellurization approach, which takes advantage of a thin metal molybdenum film pre-deposited on the SiO2/Si substrate exposed to tellurium vapours. A vital aspect consisted of determining the thermodynamics and kinetics constraints in the CVD reaction, enabling the material’s growth in one specific allotropic phase. In this context, different experimental parameters such as temperature, carrier gas flux, boat distance, substrate configuration, and growth time were tuned to optimize the growth. Aiming at optimizing the deposition condition, I also developed a simulation tool for modeling the precursor gradients, concentration, carrier gas fluxes, and temperature distribution starting from Navier-Stokes and mass-transport equation solved in the COMSOL environment. The material quality assessment was performed by investigating morphological and physical properties at the nanoscale using atomic force microscopy (AFM) operating in different modes: tapping, Kelvin-probe, and electrostatic force. The structural and optoelectronic investigations of the films were carried out by confocal micro-Raman spectroscopy. Moreover, before the characterization of the grown material, I used the AFM and Raman techniques to investigate MoTe2 crystal flakes obtained by mechanical exfoliation of commercial bulk samples. I used the measurements on the flakes as a reference set of data to assess the quality of the grown material. In this work, I will show that the adopted CVD approach allows the MoTe2 deposition uniformly on the 4cm SiO2(50nm)/Si substrate in the 1T’ dominant phase with ultra-scaled (7 nm) thickness.