Transport And Interface Manipulation Of Tungsten Disulfide Field Effect Transistors

Two-dimensional materials have attracted tremendous attention since their discovery.As a typical representative,transition-metal dichalcogenides(TMDCs)are widely used in photodetectors,transistors,and light-emitting diodes due to their exceptional semiconductor properties.Among them,tungsten disulfide(WS2)has a broad application in optoelectric devices due to its high on-off ratio,unique valley polarization characteristics and low subthreshold swing.Although excellent electrical properties are predicted by theoretical calculations,the performance of WS2 devices is still limited by the relatively high Schottky barrier and low carrier mobility.In order to solve the above problems,our work focuses on analyzing the optoelectric properties of Au-WS2 contact by the combination of first-principles calculations and experiments.The main contents are as follows:1.The structure and electronic properties of Au-WS2 heterojunction are studied systematically based on the first-principles calculations,and different defect concentration models are constructed to research the Schottky barrier height.The results show that the Schottky barrier height decreases with the increase of sulfur vacancies.By plotting the integrated density of states and the orbital-resolved density of states,we find that the electronic properties at the interface are dominated by the s-orbital of the Au atom near the Fermi level.With the increase of sulfur vacancy,the peak of s-orbital decreases gradually,and the peak of W atom appears near the valence band maximum.At the same time,we also modeled the system with S-vacancies located differently,suggesting that the location of Svacancies had less impact on the characteristics of heterojunction.Defect engineering is proven to be of great significance to the development of WS2 FETs and the improvement of its electrical performances.2.By analyzing the advantages and disadvantages of the previous WS2 experimental schemes,the traditional chemical vapor deposition(CVD)combined with the thermal evaporation coating technology was selected to prepare a large area and high-quality single-layer WS2.The precursor reactants were deposited on the substrate by the thermal evaporation coating technology,and then the key parameters of the experiments were optimized.In the process of preparation,we modulate the temperature,carrier gas flow,sulfur dose,the suitable reaction time.Finally,the single-layer WS2 crystals with high crystallinity were prepared,and a series of characterization was carried out to verify the high qualities and great uniformity of the materials.3.In order to improve the electrical properties of WS2 grown by CVD,we developed a non-destructive plasma pretreatment technology.By using low power(1 W)and short time(<16 s)argon plasma treatment,which successfully remove surface contaminants and create sulfur vacancies.The increase of sulfur vacancies density leads to the enhancement of the conductivity of the top-electrode and the bottomelectrode FET and the decrease of the turn-on voltage of the bottom-electrode devices.The theoretical results further support the mechanism of electrical performance enhancement includes reducing the effective mass of the hole,thus improving the mobility,and reducing the work function,so as to reduce the Schottky barrier.