In Situ Investigation Of Chemical Vapor Deposition To Reveal Growth Mechanisms Of Monolayer Molybdenum Di-sulfide

Monolayer transition metal sulfide molybdenum disulfide（MoS₂）is a two-dimensional semiconductor material with the atomic thickness.Because of the excellent optical and electrical properties,the applications of monolayer MoS₂,such as the field effect transistor,photodetector,capacitor,catalyst and other aspects,have been widely concerned.Chemical vapor deposition（CVD）is considered as a suitable method for growing large-sized monolayer MoS₂.Many studies have been devoted to the technology optimization of monolayer MoS₂ CVD growth and the investigations of growth mechanism.However,the in-situ investigation was still a great challenge for the monolayer MoS₂ CVD growth,as the thermal radiation at high temperature,serious corrosion of the reduction atmosphere and volatile raw materials.As a result,the crystal growth details were still unclear,and the reaction kinetic and mechanism analysis could not be supported by the available experiment data.For this case,this research paper conducted the in-situ investigations on monolayer MoS₂ crystal CVD growth using a specially designed probing system.We first observed the in-situ images of monolayer MoS₂ growth via CVD at the high-temperature,and then estimated the real-time growth rate of monolayer MoS₂;finally proposed the growth kinetics and nucleation mechanism of monolayer MoS₂ by CVD method.The research results of this paper are as follows:（1）In situ investigation the gasification process of molybdenum trioxide precursor.Evaporation and sublimation of molybdenum trioxide precursor were obtained by real-time in-situ image.The gasification way of precursor is controlled by the carrier gas velocity and the heating rate:Lower carrier gas velocity can make molybdenum trioxide provides molybdenum raw materials for the reaction via the form of evaporation and higher carrier gas velocity can make molybdenum trioxide provides molybdenum raw materials for the reaction via the form of sublimation.When the carrier gas velocity is moderate,molybdenum trioxide will adjust the gasification mode with the change of the heating rate.（2）High-temperature spectrum proved the existence of molybdenum dioxide intermediates in monolayer MoS₂ CVD grown,and obtained the effect of molybdenum dioxide on monolayer MoS₂ growth.The Raman signal of molybdenum dioxide intermediates was obtained for the first time during solid phase reaction by real-time high temperature spectra.During the gas reaction of molybdenum trioxide and sulfur（S）,the Raman spectra and X-ray photoelectron spectroscopy data were analyzed,and the specific conditions for intermediate formation were obtained.The morphological changes of molybdenum dioxide intermediates were observed for the first time in the process of CVD,and the behavior of intermediates in the growth process of monolayer MoS₂ was explored.（3）The CVD growth of monolayer MoS₂ crystals was observed in situ.The growth pattern and reaction path of MoS₂ crystals were analyzed and the real-time growth rate of the crystals was obtained.The CVD growth process of MoS₂ single crystal was observed in situ for the first time,and two modes of monolayer MoS₂ growth were found by CVD method,and the growth rate of the crystal was measured in real time.It was found that the concentration of sulfur in the reaction process would affect the growth habit of MoS₂.MoS₂ was thermodynamically restricted growth in a sulfur-rich environment,and diffused restricted growth in a sulfur-poor environment.Finally,on the basis of the above studies,we proposed a three-step kinetic model for the monolayer MoS₂ crystals CVD growth.