【作者】 李鑫；

【导师】 段曦东；

【作者基本信息】 湖南大学 ， 化学工程（专业学位）， 2022， 硕士

【摘要】 自2004年石墨烯被发现以来,人们对二维（2D）材料进行了广泛的探索。二维过渡金属硫族化合物（2D-TMDs）由于其优异的性能,在电子和光电子器件领域很受欢迎。二维材料由于其原子厚度和表面无悬挂键,可以通过垂直堆叠或横向外延形成异质结或同质结,从而获得新的性能。在传统的二维材料异质结以及二维材料器件中,接触在连接二维材料及其器件与三维世界中起着桥梁的作用。接触的质量通常受限于接触材料本征特性（如金属材料功函数）或者接触界面质量的影响。金属-半导体结（Metal-semiconducter Junction,MSJ）是电子、光电子器件的关键组成部分,它极大程度上决定了器件性能。传统的蒸镀电极由于存在界面诱导间隙态和界面缺陷,会导致半导体材料存在强烈的费米能级钉扎（Fermi level pinning）现象。研究发现,通过形成范德华尔斯金属-半导体接触（vd W MSJ）是防止费米能级钉扎的有效措施。但是固定功函数的金属电极已经一定程度上确定了接触的势垒。因此,接触金属功函数的调控能够极大程度上改善MSJ的接触势垒,提高器件性能。为此,本文主要展开了以下工做:（1）采用传统化学气相沉积法合成了金属型VS_2xSe_2（1-x）合金纳米片,通过对硫粉温度的调控,能得到不同含量的VS_2xSe_2（1-x）合金纳米片,X-射线衍射（XRD）、X-射线光电子能谱（XPS）、透射电子显微镜（TEM）、扫描电子显微镜（SEM）、原子力显微镜（AFM）,表明了所合成的VS_2xSe_2（1-x）合金纳米片为六方晶系,并且具有极高的质量。并且讨论了XRD,XPS,TEM随着S含量增大产生的相应变化。开尔文探针力显微镜（KPFM）显示,通过VS_2xSe_2（1-x）合金纳米片含量的连续调控（0≤x≤1）,功函数相应地从4.79 e V（x=0）到4.62 e V（x=1）的连续变化。（2）VS_2xSe_2（1-x）/WSe₂垂直范德华异质结（vdWH）制备。本文通过改进的化学气相沉积法成功合成了双层WSe₂大单晶,通过光学显微镜（OM）和原子力显微镜（AFM）表征了其形貌和厚度,拉曼（Raman）和荧光（PL）显示了WSe₂具有极高的结晶质量。通过两步CVD法,通过在WSe₂基底上化学气相沉积VS_2xSe_2（1-x）成功合成了不同含量二维VS_2xSe_2（1-x）/WSe₂vdWH。（3）VS_2xSe_2（1-x）/WSe₂vdWH场效应晶体管（FET）的电学特性研究。采用电子术曝光和热蒸发技术成功制备了不同含量的VS_2xSe_2（1-x）/WSe₂vdWH FET,通过对不同含量器件性能进行测试,我们发现VS_2xSe_2（1-x）/WSe₂vdWH FET性能会明显强于In/Au接触的WSe₂FET。并且我们通过调节接触金属VS_2xSe_2（1-x）的功函数,FET性能得到进一步提升,在x=0.0时的VS_2xSe_2（1-x）/WSe₂vdWH FET最大导通电流达到39μA/μm,迁移率高达72 cm²V^-1s^-1,开关比约为10⁷。更多还原

【Abstract】 Since graphene was discovered in 2004,two-dimensional（2D）materials have been extensively explored.Two-dimensional transition metal choriocarbides（2D-TMDs）are popular in the field of electronics and optoelectronic devices due to their excellent properties.Due to their atomic thickness and absence of suspended bonds on their surfaces,2D materials can be vertically stacked or laterally epitaxial to form heterojunctions or homojunctions,obtaining new properties.In traditional 2D material heterojunctions and 2D material devices,contact acts as a bridge between 2D materials,their devices and the 3D world.The quality of contact properties is usually limited to the intrinsic properties of the contact material（such as the work function of the metal）or the quality of the contact interface.The metal-semiconductor junction（MSJ）is a key component of electronic and optoelectronic devices,which largely determines device performance.There are strong Fermi level pinning phenomena in semiconductor materials due to interfacial gap states and defects in conventional evaporation electrodes.It is found that the formation of van der Waals metal-semiconductor junction（vd W MSJ）is an effective measure to prevent Fermi level pinling.But the metal electrodes with a constant work function already determine the contact barrier and limit the performance of devices.Therefore,adjusting the work function of contact metal of vd W MSJ can greatly reduce the contact barrier of vd W MSJ and improve the device performance.To this end,this paper mainly carried out the following work:（1）Metallic VS_2xSe_2（1-x）alloy nanosheets were synthesized by traditional chemical vapor deposition method.VS_2xSe_2（1-x）alloy nanosheets with different components could be obtained by adjusting the temperature of sulfur powder source.X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）,transmission electron microscopy（TEM）,scanning electron microscopy（SEM）and atomic force microscopy（AFM）showed that VS_2xSe_2（1-x）alloy nanosheets were hexagonal and possessed high quality.The corresponding changes in the results of XRD,XPS,and TEM with the increasing S component are discussed.Kelvin probe force microscopy（KPFM）displays a continuous change of the work function from 4.79 e V（x=0）to4.62 e V（x=1）by continuous regulation of the S components x（0≤x≤1）of the VS_2xSe_2（1-x）alloy nanosheets.（2）Preparation of VS_2xSe_2（1-x）/WSe₂van der Waals heterojunctions（vdWH）.In this paper,the bilayer WSe₂large single crystal was successfully synthesized by improved chemical vapor deposition method,and its morphology and thickness were characterized by optical microscopy（OM）and atomic force microscopy（AFM）.Raman and fluorescence（PL）showed the extremely high crystal quality of WSe₂.The VS_2xSe_2（1-x）nanoplates with different components was grown on pre-growed bilayer WSe₂nanosheet,forming VS_2xSe_2（1-x）/WSe₂vdWH successfully through a two-step CVD process.（3）The electrical properties of VS_2xSe_2（1-x）/WSe₂vdWH field effect transistor（FET）.The VS_2xSe_2（1-x）/WSe₂vdWH FET devices were fabricated from VS_2xSe_2（1-x）/WSe₂vdWH with different S component by electronic exposure and thermal evaporation techniques.Consequently,we found that the performance of VS_2xSe_2（1-x）/WSe₂vdWH FET will be significantly better than the In/Au contacted WSe₂FET.And the vdWH FET performance is further improved by adjusting the work function of the contact metal VS_2xSe_2（1-x）.The VS_2xSe_2（1-x）/WSe₂（x=0.0）vdWH FET displays a excellent device performance with a high ON-current density of up to39μA/μm,a mobility of up to 72 cm²V^-1s^-1,and a on/off ratio of about 10⁷.更多还原