First-principles Study Of Multi-phonon Transition In Semiconductor Defect

The quantum transition processes of the localized states for the crystal must be accompanied by the change of the number of phonons owing to the strong lattice relaxation effect,which is called multi-phonon transition.With the development of semiconductor materials and devices into the nanometer scale,multi-phonon transition caused by the point defects becomes increasingly important to the material performance and device reliability,which also puts forward higher requirement for the specific description and analysis of multi-phonon transition process.Although the defect properties can be effectively characterized experimentally,there is still a lack of effective research method to study the physical essence of the specific defect structure and the multiphonon transition process.Theoretical calculation provides effective ways for us to understand experimental results and analyze physical processes.Meanwhile,with the aid of the rapid development of density functional theory in recent years,accurate calculations of the atomic and electronic structures for defects become possible,which greatly promotes our understanding of the multi-phonon transition process.Based on the theory of multi-phonon transition and first principles calculation method,the multi-phonon transition process associated with deep-level defects is systematically studied by taking GaN point defect structure as an example.Firstly,the importance of deep level point defects in semiconductor materials is introduced,and the basic theoretical model to explain the multiphonon transition process is reviewed.In the second chapter,we introduce the density functional theory of the first principles calculation,and elaborate the calculation method of defect transition energy level and defect phonon spectrum.The third chapter of this paper firstly reviews the theoretical controversy of multiphonon nonradiative transition between adiabatic approximation model and static coupling model,and provides the verification and analysis of these two modes base on the first principles calculation results.Meanwhile,the applicability of the high temperature and strong coupling approximation formula is given in details in this chapter.In the view of the limitation of the harmonic approximation in the accurate calculation of the nonradiative transition rate,an effective method which considering the anharmonic effect for the accurate calculation of the thermal activation energy is proposed in the fourth chapter.Furthermore,the modified calculations of the nonradiative transition rates also are given.Meanwhile,the comparison with the experimental results verifies the effectiveness of this method.In the fifth chapter,in order to figure out the microscopic origin of GaN defect luminescence,we propose that the multiphonon sidebands of the optical spectrum can be used as the defect identification fingerprint.Moreover,according to the specific GaN defect structure and the first principles calculation results,we calculate the fine structure of multiphonon sidebands generated by a single phonon frequency and mixed phonon frequency,respectively.Furthermore,the temperature dependence of multiphonon sidebands is discussed.This result provides support for defect spectrum analysis and defect structure identification.Finally,the brief summary and prospect is given in the sixth chapter.