Characterization Of Ultra-thin Materials By Spectroscopic Ellipsometry

With the rapid development of scientific and technological innovation,optoelectronic devices are making progress rapidly in the direction of miniaturization.Owing to the miniaturization of semiconductor chips and various instruments,ultra-thin materials,neglected in the past,have become the focus of research.Ultra-thin materials have optic and electronic properties varying with the change of thickness which different from their bulk materials.It is necessary to obtain the thickness and optical properties of ultra-thin materials accurately,whether in basic scientific research or novel photoelectric devices design and optimization.Spectroscopic ellipsometry is widely used in the measurement of geometrical parameters and optical constants of layered materials.Based on the traditional ellipsometric data analysis method,this paper studied the rapid identification of two-dimensional materials layer numbers,and measured the optical constants of ultra-thin materials with analytical calculation method.The details are as follows.First,the basic principles of spectroscopic ellipsometry for ultra-thin materials based on model fitness was studied,including the basic principle of ellipsometry,modeling of optical properties of layered materials,and measured parameters extraction method based on fitting.Optical constants of graphene were measured and extracted by the above theoretical methods.Secondly,for the problem of layer numbers identification of ultra-thin layered materials,we made numerical calculation and simulation using ellipsometric parameters as variables.This paper took graphene on transparent substrate as example to identify the layer numbers of ultra-thin layered materials,and finally,achieved the goal of identifying the number of ultra-thin layered materials layers rapidly.At last,this paper proposed a rapidly and directly forward analytical calculation method for the ellipsometric analysis of ultra-thin films without a priori knowledge of electronic transitions.The valid range of the method was discussed.In addition,it was applied to 2D materials such as graphene,molybdenum disulfide?MoS₂?and tungsten diselenide?WSe₂?.The optical constants of these ultra-thin films were obtained and the results obtained by our method were compared with traditionally analysis method by which the effectiveness of the method was proved.The theoretical methods and experimental results presented in this paper would provide guidance for improving the ability of ellipsometry to measure ultra-thin materials and pave the way for the research and application of ultra-thin materials.