Controlled Synthesis Of Two-dimensional MoSe₂ And Its Field-effect Properties

With the development of Integrated Circuit,devices are scaled down along Moore's Law and now are approaching their physical limits.Meanwhile,the emerging two-dimensional?2D?materials provide a potential solution to continue this tendency.Recently,molybdenum diselenide?MoSe₂?,one of 2D materials,attracted tremendous research interest due to its unique properties appealing for wide ranging applications.A prerequisite for practical applications of MoSe₂ concerns controllable synthesis of large-area,high-quality MoSe₂ thin films;clarification of underlying growth mechanism;and investigation on the MoSe₂field-effect properties.In this thesis,we carry out researches on the above issues,and the obtained results are as follow.1.Controlled synthesis of large-area,high-quality MoSe₂ thin films.We explored the influence of flow rate and H₂ content on CVD-MoSe₂ properties.Through flow rate engineering and H₂ content engineering,we achieved highly crystalline MoSe₂ thin films with desired layer numbers and shapes as well as edge structures.Various characterization methods were used to confirm the high quality of as-grown MoSe₂ thin films.Our results provide a foundation for property-controlled synthesis of MoSe₂ and other transition metal dichalcogenides?TMDCs?.2.Clarification of underlying growth mechanism.The underlying growth mechanisms were proposed to explain the different MoSe₂ thin films formation process.We attributed the influence of flow rate on layer numbers and grain size to the flow rate's effect on the partial pressure of gaseous MoSe₂?PMoSe₂?and nucleation density.While the evolution in MoSe₂morphologies was attributed to the variable growth rates of two types of terminations?Mo-zz and Se-zz?that occur under different H₂ content growth conditions.Our results reveal the shape evolution during CVD-derived MoSe₂ fabrication.Since TMDCs have a similar structure,the mechanism proposed in this work was expected to be equally adaptable3.Investigation on the MoSe₂ field-effect properties.We fabricated back-gated FETs based on the as-grown MoSe₂ thin films using traditional micro-nano processing technology and its field-effect properties was systematically studied.The FETs fabricated on such as-synthesized MoSe₂ displayed different transport behaviors depending on the layer numbers,which can be attributed to the formation of Se vacancies generated during CVD synthesis process.Monolayer MoSe₂ showed n-type characteristics with an Ion/Ioff ratio of?4×10⁶ and a carrier mobility of?19 cm²V^-1s^-1,whereas bilayer MoSe₂ showed ambipolar behavior with an I_on/I_off ratio of?5×10⁵ and a higher mobility of?65 cm²V^-1s^-1 for electrons as well as an I_on/I_off ratio of 7×10⁴ and a mobility of ⁹ cm²V^-1s^-1 for holes.The FETs based on different MoSe₂ morphologies were also fabricated and analyzed.Our results may offer insight on the potential applications of MoSe₂ thin films in wide range of fields.