Investigation Of Dislocations And Defects Evolutions In GaN Films

Gallium nitride(GaN)film has a wide range of applications in the field of optoelectronics.There are high-density dislocations in GaN films grown by existing methods,which are harmful for the performance of GaN-based electronic devices.Therefore,it is important to study on the nucleation and evolution of dislocation defects in the films to reduce the defect density,thus improving the quality of films and the performance of electronic devices.Molecular dynamics(MD)method is able to make a detailed analysis of formation mechanism and evolution of dislocations at the atomic level;thus,it is a powerful tool for studying the behavior of dislocation defects.In this paper,GaN films with a surface step are studied by molecular dynamics method.The nucleation and evolution of dislocations are analyzed.Influences of temperature,step height,surface polarity and step surface anisotropy on the dislocation nucleation have been analyzed in detail.As a result,theoretical guidance for improving the quality of the thin films in the industrial production process have been proposed.In addition,behaviors of the edge dislocations,the screw dislocations,and the vacancy structures under different stress conditions are investigated theoretically.Evolutions of the single-edge dislocations as well as the double-edge dislocations under tensile stress load are analyzed.The reproduction process of the dislocations is elaborated systematically.The related factors affecting evolution of the edge dislocations are discussed.Moreover,the method of climbing nudged elastic band is adopted to analyze the process of dislocation reproduction and its energy barrier.The evolutions of GaN film screw dislocation under tensile,compressive and shear stress,as well as the vacancy model for GaN films under tensile stress,are analyzed.In addition,we further make a detailed analysis of the cracking process and the interaction between cracks and dislocations.The simulation study presented in the thesis describes the evolutionary behaviors of the dislocation defects and their interactions from a microscopic perspective,and elucidates the surface step nucleation in GaN film originally,which provides a unique view for the study of dislocation behavior during the film growth.The results show that the surface morphology should be improved during the growth to prevent the generation of large steps.Furthermore,large stress in the film,especially tensile stress,should be averted to reduce defects density.