Numerical Simulation And Stress Analysis Of GaN Crystal Growth By MOCVD Method

As one of the III-nitride compound semiconductor materials,GaN is commonly used in bright light-emitting diodes(LEDs),with advantages of wide bandgap,high heat capacity and large thermal conductivity.Growth uniformity of GaN influences reliability,light output and durability of the LED devices.Metal-organic Chemical Vapor Deposition(MOCVD)has been the most popular method for growing high-quality GaN thin films.Although the technology has already been industrialized,its fundamental understanding remains a hot topic in the research.Three-dimensional models,coupling fluid flow and heat transfer,have been adopted to analyze influences of the process parameters on the temperature uniformity in an industrial MOCVD reactor.Important factors,such as the inlet gas flow,the susceptor rotation,the heater power,the distance between the heat shield and the susceptor(d1),as well as the distance between the heater and the susceptor(d2),have been investigated carefully.The system heating condition is characterized by temperature uniformity denoted as the standard deviation of temperature,and by thermal efficiency expressed as a combination parameter of the dissipated energy.The results reveal that decrease of the gas flow rate at the inlet,decrease of the susceptor rotation rate,as well as increase of the distance d1,could monotonically enhance the temperature uniformity.The results also show that decrease of the above three parameters could improve the thermal efficiency.Futhermore,increase of the distance d2 enhances the temperature uniformity,and reduces the thermal efficiency slightly.The influences of the parameters on the uniformity vary at the different locations of the susceptor as divided into Zone A,Zone B and Zone C according to the distance to the susceptor center.Zone C has worse temperature uniformity and thermal efficiency than Zone A and Zone B do.The conclusions help the growth engineer optimize the system design and process conditions of the reactor with a balance between temperature uniformity and thermal efficiency.Thermal stress due to the difference of thermal expansion coefficients in GaN heterostructure has been studied.This paper uses the finite element method to analyze the generation and evolution of thermal stress in thin film during cooling down from the growth temperature to the room temperature.The effects of substrate material,buffer material,thin film thickness,buffer thickness and processing conditions on the stress distribution were explored in detail.It has been found that under certain conditions,the film thermal stress decreases with inserting buffer layer between the substrate and GaN layer.The release of thermal stress weakens with the growth temperature decreasing.The film thermal stress intensifies with the deposition temperature rising,but decreases with the film thickness and buffer layer thickness increasing.Futhermore,the GaN layer becomes compressive and the wafer bends to be convex on the GaN layer side when sapphire,ZnO or SiC are substrates,and that the thermal stress decreases in turn.Instead,the GaN layer turns into tensile as Si becomes substrate.When InN,Al N or SiC insert between the substrate and GaN layer as buffer layer,the film thermal stress decreases in turn which means increased release of thermal stress.