Preparation Of Monolayer MoS₂ Film And Its Application In TFT

In recent years,two-dimensional materials with its atomic thickness,high carrier mobility,adjustable band gap and other aspects of the outstanding performance are drawing more and more attentions.The most typical two-dimensional material is MoS₂,because of its superior physical and electrical properties,in the field of new optoelectronic devices and integrated circuits,MoS₂ is expected to replace the traditional semiconductor materialsOver the past few years,many studies have reported using the CVD to produce MoS₂over the years,however it is less seen in the aspect of controllable preparation.In this paper,a large-area monolayer MoS₂ crystal is fabricated by designing a temperature control device with a conventional single-temperature zone tube furnace CVD system to achieve the control of the size,density and thickness of MoS₂.The temperature distribution at the edge of the tube furnace was obtained by analytical tests and the location of the reactant sulfur powder was determined to reliably form sulfur vapor.The airtightness of the system is also optimized.The advantage of the system is that it can change the position of the sulfur powder in the quartz tube to change the evaporation temperature of the sulfur powder and the reaction temperature of the sulfur powder and MoO₃ to achieve the control of the size and thickness of MoS₂.The main factors affecting the formation of MoS₂ were studied experimentally:the temperature of sulfur powder,the reaction temperature and the distance between the upper and lower substrates.It was found that the evaporation temperature and the reaction temperature of the sulfur powder together were determined by whether the MoS₂ and the MoS₂ can be generated and the size and intensity of the MoS₂ are determined when the MoO₃ temperature is constant.The distance between the upper and lower chips determines the thickness of the MoS₂ and the distribution range.The optimum reaction conditions were determined by considering the influence of each factor.Under these conditions,the MoS₂ shape was changed from hexagonal to triangular,and except the center of the substrate,the other parts of the MoS₂ are single layer.The maximum size of MoS₂ is103.6μm,the difference of peak value of A_g¹ and E¹_2g in Raman spectrum is 20.2 cm^-1,and the peak of PL spectrum is located at 628.5nm（1.97eV）and 679.9nm（1.82eV）,respectively.The thickness of the monolayer MoS₂ is 0.68 nm and the double layer is 1.4nm measured by AFM.In this paper,a bottom gate type thin film transistor（TFT）is fabricated with large size MoS2,in which the gate insulating layer is SiO₂ and the thickness is 300 nm.The source/drain electrode is a Ti-Au composite electrode,which is obtained by electron beam evaporation,wherein the Ti film has a thickness of 10 nm and Au is 50 nm.The prepared TFT exhibits typical output characteristics and transfer characteristic curves,resulting in a threshold voltage of-15.7 V for the MoS₂ TFT,a carrier mobility of 24.4cm²/（V·s）,a subthreshold swing SS of 4.85 V/decade,when V_DS=1 V,the on/off current ratio is about 10⁵.