Study And Optimization Of Multi-physics Field In The Preparation Processes Of GaN Thin Film

With the increasing dependence and requirements of the life in modern society for electronic devices,the traditional silicon material has become more and more difficult to meet the needs of people.As the representative of the third generation of semiconductor materials,gallium nitride(GaN)is widely used in many core fields,such as energy,power,transportation,information technology and intelligent manufacturing,due to its excellently mechanical,thermal,electrical and optical properties.Metal-organic chemical vapor deposition(MOCVD)is one of the most common methods for synthesis of GaN thin films.It owns great flexibility in producing high quality epitaxial wafers.The gas sources of MOCVD-grown GaN process are mainly ammonia and trimethylgallium or organic gallium sources,which are injected into the reactor through separate nozzles and a series of gas phase and surface chemical reactions take place in the chamber.Eventually,GaN crystals are deposited on the high temperature substrate.In order to improve the quality of GaN thin film,it is necessary to fully understand the basic phenomena of multiphysics transfer in each stage of the growth process,as well as to establish the relationship among the multi-physical mechanisms,the growth quality and the process parameters.In this dissertation,multi-physical transfer and growth quality during MOCVD-grown GaN process are systematically investigated by theoretical analysis,numerical simulation and experimental measurements.A detailed chemical mechanism of GaN growth is summarized by reviewing the related literature.Then a multi-physics coupling model with a detailed calculation of the physical properties is verified by experiment.Afterwards,influence of process parameters on the reaction path and the variation of species are analyzed,and the pyrolysis products of trimethylaluminum at different temperatures are studied by experiments.On the basis of the work,influences of the inlet status(temperature,flow rate,V/III ratio and premix conditions)and the process parameters(pressure,water-cooled temperature,susceptor temperature,rotating speed,showerhead height)on the thermal and mass transports,the GaN growth rate and the growth uniformity are studied.Finally,the growth process is optimized by orthogonal experiment,and the relationship between growth index and process parameters is revealed.Through above studies,the proposed mechanisms can provide more intermediate process information during the growth process to predict the GaN growth rate more accurately.In most cases,the decomposition reaction is the leading path of GaN growth,and the major intermediate product is dimethyl gallium.The GaN growth rate is benefited by large operating pressure,rotating speed,inlet gas temperature,and inlet flow rate,as well as by low V/III ratio and showerhead height.The showerhead height and inlet gas flow rate have the most significant influences on the growth rate and uniformity.The optimum combination of the operating factors is obtained by orthogonal experiment method.Compared with the standard conditions,the growth rate increases 114.5% for the optimal combination of the growth rate,while the standard deviation uniformity(SUP)decreases 42.7% for the optimal combination of the growth uniformity.Both can improve the growth quality significantly and provide a guidance for the optimization of process control parameters.