Influence Of Dislocations And Other Caystal Defects On The Thermal Conductivity Of Gallium Nitride

Gallium nitride(GaN),with wide and direct bandgap,has been widely used in various optoelectronic devices.At present,GaN-based materials are usually fabricated by Metalorganic chemical vapor deposition(MOCVD)on heterogeneous substrates.Due to the mismatch of lattice parameters and thermal expansion coefficients between epitaxial layer and substrate,as well as the coalescence of three-dimensional island,the GaN epitaxial layer often grows accompanied by a large number of crystal defects,such as vacancies,dislocations,low-angle boundaries,stacking faults,etc.Those defects in GaN can seriously affect the quality of optical and electronic devices.The thermal transport properties of GaN play an important role in controlling stability and lifetime of related devices.In this thesis,the molecular dynamics(MD)simulation and Stillinger-Weber potential are employed for investigating the influence of crystal defects on the thermal transport properties of GaN.The thermal conductivity of GaN with edge dislocation is calculated using both equilibrium molecular dynamics(EMD)simulations and non-equilibrium molecular dynamics(NEMD)simulations.Results show that edge dislocations can reduce the thermal conductivity of GaN severely in directions perpendicular and parallel to the dislocation lines,between which the perpendicular direction is deeper influenced.The thermal boundary resistances of low-angle grain boundaries and stacking faults are calculated by NEMD simulations.And the influence of system length and temperature on the thermal boundary resistances have also been analyzed.It shows that the thermal boundary resistances of low-angle grain boundaries and stacking faults decrease with the increase of system length and temperature.In addition,it is found that the thermal boundary resistances increase when the thickness of stacking faults in c-plane of GaN increases.Moreover,the local phonon density,atomic potential distribution,the stress distribution of dislocation and stacking fault regions in GaN crystal are calculated and addressed by molecular dynamics.Results reveal that local phonon density of dislocation structure in GaN is obviously reduced,and atomic potential energy of the dislocation and its nearby region increases with obvious local stress.The existence of stress reduces the vlocity and specific heat of the phonon group and thus reduces its thermal conductivity.Besides,a-plane stacking faults only reduce the atom and phonon density slightly on the interface of fault zone,with energy peak and stress concentration.And the local phonon density,the potential distribution and stress distribution in c-plane stacking fault regions are not changed significantly.