Defects Nucleation And Propagation Induced By Stress In Gallium Nitride

Gallium nitride(GaN),as a third-generation direct-band semiconductor material,has various applications in different fields,especially in illumination industry.GaN-based lighting emitting diodes(LEDs)are capable of being ideal luminaires with high brightness,high luminous efficiency and long lifetime.However,due to current producing technologies that induce high inner stress,large density of defects in GaN limits further improvement of efficiency and lifetime of LEDs.A deep understanding of defect formation and evolution in GaN will help reduce defect density,and be benefit for future application and dissemination of LEDs.Because of an affordable computational load,molecular dynamics(MD)is widely adopted to investigate microscale characteristics of defects,such as cracking and dislocations.Combining MD simulation with theoretical analysis and experimental investigation makes it possible to connect microscale features of defects with macroscopical process parameters of GaN production.The thesis adopts molecular dynamics simulation,theoretical analysis and experimental investigation to systematically study cracking mode,dislocation nucleation,defects'generation,and growth uniformity of GaN thin films.Based on molecular dynamics study,it is found that cracking in GaN has a very brittle feature,and that tilt grain boundaries tend to nucleate initial cracks easily.With thickness of GaN increasing,nucleated initial cracks will propagate from tilt boundaries to {1010} crystallographic planes.Further theoretical analysis reveals a relation between inner stress and critical growth thickness of crack-free GaN.In addition,MD simulation demonstrates 60° edge structures formed during GaN islands merging are ideal sites for {1010} surface shuffle-set dislocations to nucleate.Reduction of activation energy of the dislocation is found when either pressure or steer stress in GaN films is increased.Based on simulation results,a theoretical formula is proposed to describe the effect of both pressure and sheer stress on the activation energy.Then,a curve to indicate critical stress for preventing dislocation nucleation avalanche is presented.Moreover,since cooling process would induce pressure in GaN films due to thermal mismatch,further analysis of the formula also indicates that the higher the cooling rate is,the smaller the nucleation rate will be.Transmission electron microscope(TEM)and double-crystal X-ray diffraction(DCXRD)measurements had been performed to investigate stress-induced defects in the whole structure of the LED layers deposited on different substrates.TEM results show that patterned substrate surface is able to effectively bend dislocations,thus decreasing defect's density in GaN thin films.Temperature dependence of dislocation formation is further investigated.The result implies that an optimized range of the growth temperature could be obtained to improve quality of the LED layers.Finally,by testing growth uniformities of LED wafers produced by an industrial reactor,the thesis reveals the principles common to the reactor technique,and provides a basic for further optimization of the reactor to improve the growth uniformity of GaN-based LED wafers.