Research On The Preparation And Plasma Modification Of Transition Metal Chalcogenides

Two-dimensional materials have become a research hotspot in the field of new generation semiconductor materials due to their excellent electrical,mechanical,thermal and optical properties.Advanced growth preparation and surface modification technologies play an important role in improving the properties of two-dimensional materials,which opens up a new dimension for the realization of their specific properties and promotes the application of these materials in the next generation of electronic products.The results show that the electrical and optical properties of two-dimensional materials can be significantly improved by chemical vapor deposition（CVD）alloying engineering or plasma surface modification（such as etching,doping and intercalation）.However,the existing growth methods have some problems,such as harsh reaction conditions,small sample size,instability and so on.The modification technology is faced with the challenges of randomness,singleness and great damage to materials,which hinder the further integration and development of two-dimensional materials and the current microelectronics industry.In view of the above problems,Capacitively Coupled Electrodeless Plasma（CCEP）and confined-spaced CVD system are utilized to realize the alloying engineering and surface modification of two-dimensional semiconductors such as Mo S₂,WS₂,etc.Firstly,large-area and high-stability monolayer WS₂was synthesized by solution precursor based CVD system,and a series of WS_2（1-x）Se_2xsamples with adjustable band gap were prepared by alloying engineering.Secondly,the doping regulation of mechanical exfoliated Mo S₂via soft ammonia plasma treatment was studied,and effective n-type or p-type doping can be achieved independently.In addition,the monolayer WS₂samples prepared by CVD were also doped by ammonia plasma treatment.The main work and achievements of this paper are as follows:1.A solution precursor based CVD method for synthesizing WS₂was proposed,and large-size,high-quality monolayer samples on Si O₂/Si substrates were achieved.The solution precursor with low melting point can avoid the addition of other auxiliary agents in the synthesis process,and thus can effectively reduce the generation of by-products and other impurities.This section provides an alternative method for the low-cost and rapid preparation of two-dimensional materials.2.The band gap and photoelectric characteristics of WS₂were regulated by alloying engineering.By introducing different content of Se into WS₂monolayer,regulating the content of hydrogen in carrier gas or the growth temperature,and the mechanism was analyzed.WS_2（1-x）Se_2xalloy samples with high stability,adjustable band gap and doping type varying from n-type to bipolar and then to p-type were successfully prepared.The photoelectric detection performances of WS_2（1-x）Se_2xalloys were studied for the first time,and the photoelectric field effect transistor（FET）with the fastest response time of 20 ms in the visible light band was achieved.3.A scheme based on soft NH₃plasma treatment was developed for the controlled doping of Mo S₂.The effects of NH₃plasma treatment on the composition,thickness,surface morphology and electrical properties of Mo S₂samples were systematically studied.The results show that the doping of N element in Mo S₂can be achieved by plasma treatment,and the doped samples possess different threshold voltage,on-state current and mobility values in comparison with the initial state.By adjusting the etching rate of H radicals in NH₃plasma and the balance of substitutional doping of N elements,the selective doping modification can be realized:the p-type doping of Mo S₂can be realized when the RF power is low and the processing time is short;the n-type doping of Mo S₂can be realized when the RF power is high and the processing time is long.4.The surface modification of self-prepared WS₂monolayers was also investigated using soft NH₃plasma treatment.The results show that the controllable p-type or n-type doping can be achieved based on soft NH₃plasma treatment.The mobility and on-state current of WS₂-FET can be realized by this method without causing excessive lattice damage to the sample.This method has the advantages of simplicity,low damage and no pollution.It is proved that such soft NH₃plasma process can be applied to other related two-dimensional materials,which provides a new method for realizing the diversity of properties of these materials.