Synthesis of MoS2 thin films by pulsed laser processing

The interest in transition metal dichalcogenides (TMDs) has significantly increased in the past decade. In particular, TMDs (e.g., MoS2, WS2, MoSe2, WSe2) forming 2D mono-layer crystals with direct bandgap presents a large number of applications in photonics and nano-electronics, as ultra-scaled (i.e., atomically thin) CMOS transistors, photodetectors, light emitting devices, flexible and transparent devices. Furthermore, thanks to the intrinsically large surface area to volume ratio of TMDs, their versatile surface chemistry and good biocompatibility, others promising applications are being developed in the biomedical sector. One of the most important challenges concerning the fabrication of devices based on TMDs is to find novel bottom-up material growth methodologies as alternative to chemical vapor deposition (CVD). In this context, sputtering has gathered a particular attention as deposition methods due to its simplicity, jointly with high reliability, large area crystal growth and repeatability. However, sputtered layers require a post-deposition thermal treatment (e.g., hight temperature furnace sulfurization) to obtain crystalline TMDs. Nevertheless, pulsed laser annealing (PLA) has recently emerged as alternative annealing technique to obtain large-scale TMDs crystals without significantly heating or affecting the underlying substrate. The principal aim of this thesis is to explore the use of sputtering deposition followed by PLA to growth large area and high-quality crystalline MoS2 (layer thickness of the order of 10 nm) on SiO2-on-Si substrates. The main advantage of this approach is that it is spatially and temporally very localised. In fact, PLA can be performed on a selected area of a sample substrate, leading to a thermal treatment confined in the top surface of the samples (few tens on nm). In addition, the UV (248 nm) pulses generated by a KrF excimer laser have a duration of about 22 ns, inducing ultra-rapid thermal treatments (few hundreds of ns). It is therefore evident that PLA is an ideal solution for extremely controlled and confined thermal processes. Furthermore, the process is easily scalable to large areas through appropriate lateral scans. In order to study the structural, morphological and electrical properties of the MoS2 layers, a large number of characterisation techniques was performed. Raman spectroscopy allowed to study the evolution of the material crystallinity varying the sputtering deposition and the PLA process parameters, finding the best material growth. Afterwards, the crystal structure was studied by x-rays diffraction (XRD), while the surface morphology was observed by atomic force microscopy (AFM). Lastly, the areal dose of both Mo and S were determined by performing Rutherford back-scattering (RBS) to correlate the layer stoichiometry with the growth/PLA conditions. In order to do investigate the electrical properties of these MoS2 thin films grown, back-gated FETs were fabricated by sputtering of Cr/Au contacts by using dedicated masks. The transistors transfer curves have been analysed to demonstrate the presence of transistor effect and to determine the material mobility. The results of this thesis confirm that it was possible to sputter and crystallize MoS2 films with very good reproducibility, within a well-defined process range. Preferential evaporation phenomena were also observed after PLA, leading to the reduction of the S areal dose in the final films. The formation of (002)-oriented planes suggest has revealed that the 2H-MoS2 phase was formed following multi-pulses annealing with energy density (ED) equals to 100 mJ/cm2. In summary, this thesis managed to investigate the capability of sputtering deposition followed by PLA to fabricate large-area crystalline MoS2 layers. Besides the very promising results achieved in optimizing the material growth processes, this thesis opens the ways to the fabrication of FETs based on laser annealed MoS2 layers.