Research On In-situ Measurement Technology For Two-dimensional Materials Based On DRS

Two-dimensional materials with excellent optical,electrical and mechanical properties have great potential in field effect transistors,photodetectors,supercapacitors,sensors,catalysis and other fields.It has raised extensive research on the preparation,application and industrialization of two-dimensional materials.Among many preparation methods,chemical vapor deposition(CVD)is considered to be the most potential technology for the industrial preparation.However,it is difficult to obtain high quality,large-scale,and flawless materials due to the poor controllability and unclear growth mechanism of two-dimensional materials.These factors restrict the development of two-dimensional materials in the field of industrial production.In-situ on-line measurement technology can directly study the characteristics of two-dimensional materials during the growth period.It is of great significance to explore the growth mechanism and realize the controllable preparation of two-dimensional materials.Nevertheless,the in-situ on-line measurement of two-dimensional material growth remains to be further explored.There is a difficult problem,which is to combine the material growth equipment with the detection equipment to characterize the material properties without affecting the material growth accurately and effectively.In order to solve the problem of in-situ on-line measurement during the CVD preparation for two-dimensional materials,the in-situ on-line measurement system is designed and built.This system combines differential reflectance spectroscopy(DRS)with general CVD growth equipment.It solves the difficulties caused by the changes of temperature environment and mechanical structure during the CVD preparation process.It realizes the on-line research of the two-dimensional material growth process represented by molybdenum disulfide.The main contents of this paper are as follows:(1)The DRS model for variable temperature environment is established.From the point of DRS measurement principle,the measurement model under constant temperature environment is analyzed.Three-phase model is derived from the two-phase model and is simplified.The relationship between DRS and the material thickness,optical constant and coverage is discussed.More importantly,the DRS model under variable temperature environment is proposed.The model validity and reliability for detecting material growth in variable temperature environment is proved by on-line DRS spectrum of MoS2.(2)From the perspective of two-dimensional materials prepared by CVD,the influence of experimental conditions,such as the distance between substrate and molybdenum source,argon flow rate and substrate type,on the two-dimensional materials growth is analyzed.The temperature characteristics of the substrate optical constant and its influence on the DRS detection results are analyzed theoretically.A method to suppress the error by obtaining the substrate temperature error compensation coefficient is proposed.Additionally,the temperature field and stress field of CVD reaction chamber are simulated,which provides the basis for the on-line detection system design and the layout of optical components.(3)The in-situ on-line measurement system for two-dimensional material preparation is designed and built.Based on the characteristics of general CVD equipment,a high-temperature coupled differential reflectance optical structure is designed.The in-situ on-line measurement system consists of the two-dimensional material growth module,optical measurement module,and computer with a LabVIEW program.The selection and performance experiment of devices have been completed.We also design and install the adjustable optical platform.According to the analysis of the interference signal source,the interference can be eliminated or suppressed from four aspects:the improvement of light source signal,the control of environmental variables,the suppression of spectrometer noise and the use of the variable temperature model processing.These methods improve the detection performance of this system.(4)The relationship between DRS characteristic peak intensity and coverage is analyzed.The optical properties of four regions with continuous different coverage are studied by the off-line DRS measurement device.It is found that the MoS2 thin films with sub-monolayer structure have the same energy position of the optical characteristic peak.The DRS intensity for peak B becomes greater with the coverage.In addition,the morphology distribution of MoS2 films along the air flow direction is observed by optical microscope.Raman spectroscopy and atomic force microscope characterize the monolayer thickness of the sample.The experiment proves the reliability of DRS technology for two-dimensional material characterization.(5)The growth process of MoS2 on SiO2/Si substrate is characterized by the built in-situ on-line measurement system.The formation and structural evolution mechanism of monolayer MoS2 is revealed by the change of the in-situ DRS shape.The morphology,vibration mode,absorption characteristics,and thickness of monolayer MoS2 are characterized by optical microscope,Raman spectrum,off-line DRS and atomic force microscope,respectively.The experimental results show that the in-situ measurement system in this paper is a powerful tool for in-situ characterization of two-dimensional materials growth.(6)The growth process of MoS2 on Al2O3 substrate is in-situ observed.The on-line measurement system designed in this paper is used in this experiment.Blue shifts of characteristic peak B in the growth process of MoS2 are observed through the analysis of on-line DRS.The change curve of dielectric constant with temperature is obtained.The results of on-line measurement are consistent with those of off-line characterization,such as optical microscope and Raman spectrum.The experimental results are helpful to understand the growth mechanism of two-dimensional materials on Al2O3 substrate.It emphasizes that on-line DRS technology in this paper is a non-destructive and effective method for in-situ measurement of two-dimensional material growth.