Regulation Of Carrier Conduction By Composition Change In Mo_1-xW_xTe₂ Ternary Alloy

The discovery of two-dimensional materials represented by graphene has opened up a brand-new field for the study of physics,but the zero band gap structure of graphene seriously restricts its application range.The two-dimensional transition metal chalcogenides(TMDs)represented by Mo S₂,WS₂,and MoTe₂ have semiconductors,semi-metallic and other structural phases,and exhibit rich and unique physical and chemical properties.They are applied in electronics,optoelectronics,photocatalysis,and sensing.Their potential has aroused great concern and has become one of the hot research topics in the field of materials physics today.According to the growth environment,the electronic structure of TMDs can be adjusted according to the chemical composition,and the composition change can be easily achieved by substitutional doping/alloying.Alloy doping can effectively achieve the performance optimization of adjusting the energy band structure of TMDs,reducing the Schottky barrier,and increasing the mobility.The ternary TMDs alloy materials substituted by anions or cations are widely used in electronics and other fields.However,the performance of TMDs modulated by doping reported in the literature is based on more than 10%doping.High doping will cause significant crystal defects,such as lattice mismatch,defect vacancies,and grain boundaries,which seriously limit device performance.Compared with other two-dimensional TMDs,semiconductor phase 2H and semi-metallic phase Td of MoTe₂ are stable at room temperature,and according to literature,the structure of its 2H phase is more sensitive to doping,where a small proportion of W ions(9%)substituting Mo can realize the structural phase transition from 2H to Td phase,which provides the possibility to realize the adjustment of electrical properties under low doping while avoiding serious crystal defects.In this paper,by doping W ions in various proportions in the MoTe₂ structure,two-dimensional Mo_1-xW_xTe₂ ternary alloy materials in various proportions have been grown;the effects of doping on the morphology and structure of the material have been characterized and analyzed.Series of field effect transistor(FET)devices were prepared.We studied the relationship between doping ratio and electrical transport performance in detail,and realized the modulation of the devices'bipolar electrical transport with low concentration doping.Carrier mobility and other parameters of doping devices are better than the undoped results.Our work may provide new ideas and methods for improving the performance of TMDs electrical devices.The main contents of this paper are summarized as follows:(1)A variety of doping ratios of two-dimensional Mo_1-xW_xTe₂ alloy thin films were prepared and detailed physical and chemical characterization were carried out.First,Mo_1-xW_xTe₂ crystals with different doping ratios were grown by Chemical Vapor Transport(CVT).Then using the improved mechanical exfoliation,the bulk Mo_1-xW_xTe₂material was thinned to a size of 20?m,and the thickness of the sheet was measured by AFM to be2-10nm.Energy Dispersive X-Ray Spectroscopy,Transmission Electron Microscope,Raman spectroscopy,two-photon fluorescence spectroscopy and other means were used to analyze the microscopic morphology,Raman vibration mode frequency shift,and energy band structure changes,etc.The exact ratio of elements in the material was determined.it was confirmed that as the W concentration increased,the phase transitions among 2H phase,1T'phase and Td phase occurred,which corresponds to the phase diagrams of the existing literature.(2)Using semiconductor micro processing technology,FET devices based on 2H phase,1T?phase and Td phase with few layers of Mo_1-xW_xTe₂(doping range 5%-100%)were prepared,and further measured.According to the results of the 2H semiconductor phase devices,the performance of the devices under low doping(<10%)is significantly better than that of the undoped MoTe₂,such as Mo_0.95W_0.05Te₂ under 0.05%doping can reach 20.2cm²V^-1s^-1 of mobility,which is significantly higher than that of pure MoTe₂ of9.1 cm²V^-1s^-1under the same experimental condition,indicating that the low doping concentration has few defect states without bad effect on device performance.At the same time,it is found that in the range of 2H phase,the threshold voltage of the device increases with the increase of the W concentration,and the p-type dominant bipolar is gradually modulated to p-type,which shows that the addition of W is p type doping for the material.Based on the experimental results,we further studied the effect of doping on the Fermi level modulation in principle,and gave a physical image of the Schottky barrier between the doped modulated semiconductor and metal electrode.(3)The Mo_1-xW_xTe₂ field effect transistor devices with high doping(>10%)1T?phase and Td phase were tested in detail,proving that they have good semi-metallic properties.It is expected to be used as electrode materials to form homojunction with 2H phase Mo_1-xW_xTe₂ FET devices to further improve performance.