Synthesis And Characterization Of Ultra-Thin-Film MoS₂ Via Chemical Vapor Deposition Method

Single layer MoS₂ is a semiconductor with a direct band gap of 1.8-1.9 eV.The band gap of MoS₂ decreases with the increasing of the crystal thickness,and gradually changes into an indirect band gap.Because of these distinctive structures,optical and electrical properties,MoS₂ can be utilized for new applications.The major methods to prepare ultra-thin MoS₂ layers include exfoliations,Li intercalation,physical vapor deposition and chemical vapor phase deposition.But it is still a great challenge to fabricate large-area and high-quality ultrathin MoS₂ layers.In this study,we report a CVD method to synthesize MoS₂ ultrathin layers in one step.In order to investigate the effects of processing parameters on the MoS₂ films,the crystal structure,surface topography and thickness distribution of MoS₂ ultra-thin layers are characterized by X-ray diffraction and atomic force microscope.In addition,we study the optical and electrical properties of the films prepared under the optimal experimental conditions.Chapter 1 presents a review of our research background,including TMDs and MoS₂,especially the structure,properties,applications and common synthesis methods of ultra-thin-film MoS₂.Chapter 2 describes the working principles and instrument structures of XRD,AFM,PL and electron transport properties testing.Chapter 3 is experiment procedures,involving instruments and materials.Chapter 4 investigates the effects of processing parameters on the crystal structure,surface topography,thickness distribution,etc.I-V curve is also discussed to study the electron transport propagates.When the tilt angle between Si substrate and horizon is 0.06°,the film is more uniform.The crystallinity of MoS₂ thin film is controlled by applied voltage and heating time.At the applied voltage of 7 V and heating time of 90s（weight of source materials is 0.003 g）,it seems that the crystallinity of MoS₂ thin film is highest and the thickness is thinnest.Thus the optimal processing parameters are applied voltage of 7 V,the heating time of 90 s,the tilt angle of 0.06? for substrates and the weight of 0.003 g for source materials.Under the condition,the surface of the film is smooth and composed of many uniformly distributed and aggregated particles,and the photoluminescence spectra（laser: 405nm）present two peaks centered at around 1.79 eV and 1.74 eV with a shoulder at 1.68 eV.The peaks at 1.79 eV and 1.74 eV correspond to the A1 and B1 direct excitonic transitions.Yet the peak at 1.68 eV corresponds to the indirect transition in multi-MoS₂.The intensity of the three peaks increases as the thickness of the thin films decreases.But the thickness of the thin films is not a constant;it decreases along the vapor transmission direction.Onother hand,it also decreases as the distance from the shadow mask V increases.Our results show that the MoS₂ atomic layers（less than 10 layers,including the MoS₂ monolayer）covers an area of more than 2 mm × 2 mm.Mobility varies from 17 to 110cm2·V·s-1,carrier concentration is4.24×10¹⁸⁷.79×10¹⁹m^-2.Chapter 5 finishes the dissertation with concluding remarks.In addition,there is a summary in the last section of every chapter.