Research On The Crystal Quality Control Of Nitride Film With Laser

In recent years,the patterned AlN,GaN and sapphire substrates to grow high-quality thin film,have attracted widespread attention.Meanwhile,annealing process during the growth of nitride film has been an effective way to improve the quality of semiconductor film and carriers activity.Femtosecond laser is a new method to fabricate micro/nano structures in various fields.In the process of femtosecond laser processing,the thermal interaction between electron and lattice is very small and could be ignored,so it can be directly etched on the material to form micro/nano patterns of small scale with extreme precision and efficiency.At the same time,the laser can also be used to induce the periodic micro/nano structures to achieve fast and efficient patterns without material removal.The film quality can be improved by annealing with laser.In this dissertation,laser control equipment for thin film quality integrated in the MBE cavity was designed,with the purpose of reducing the defects of film and improving the film performance in-situ.This dissertation mainly studied the laser patterning and local laser annealing technology,and explored the feasibility of quality control by using a laser in the process of film growth.The main research contents and conclusion are as follows:(1)Ivestigation of the patterning of AlN film directly written by femtosecond laser.The mechanisms of femtosecond laser and wide band gap material were studied.The dual-temperature model was simulated by COMSOL software,and the feasibility of femtosecond laser non-melting and non-destructive machining was analyzed.By experiments exploring the processing parameters of femtosecond laser directly writing AlN laser were explored,it was proved that when the laser spot overlapping was small,and when the pulse energy close to the processing threshold,with low heat accumulation acquired,the interaction between the electrons and the lattice thermal effect was very low and negligible.Aluminum nitride thin film after femtosecond laser treatment presented a very consistent pattern.Crystal quality and residual stress were changed slightly.As laser is applied to aluminum nitride thin film,film as a whole presented compressive stress state.The damage range caused by femtosecond laser patterning AlN was less than 1nm by TEM super resolution imaging.(2)The lateral epitaxial regrowth of AlN thin film after femtosecond laser patterning.The lateral epitaxial regrowth of AlN thin film after femtosecond laser patterning was investigated.The effect of the patterning on the film quality was analyzed by XRD and Raman.The evolution of dislocation in femtosecond laser patterned thin films was analyzed by TEM.It had been proved that femtosecond laser patterned AlN regrowth could decrease screw dislocation.On this basis,the regrowth of gallium nitride on the patterned aluminum nitride thin film with femtosecond laser was further studied,and the control effect of different patterns to the quality of the regrowth thin film was studied,which also achieved the reduction of screw dislocation.(3)Ivestigation of the laser induced periodic structure of GaN thin films.Since GaN is easy to decompose at high temperature,periodic structure(LIPSS)of GaN induced by nanosecond UV laser was studied.It was found that under the irradiation of 355 nm ns-laser,GaN droplets were easily generated and had a periodic structure.Through a variety of test and characterization techniques,a variety of micro and nanostructures with different shapes were studied,and some micro and nanostructures were identified.Femtosecond laser irradiating GaN was studied for the formation mechanism of LIPSS,and the distribution characteristics of periodic micro-nano structure induced with ultraviolet femtosecond,green femtosecond and infrared femtosecond.Formation processes of high spatial frequency LIPSS(HSFL,?< / 2)and low spatial frequency LIPSS(LSFL,?> / 2)were found: forming about 100 nm shallow pit firstly,then forming HSFL,and then transformed into LSFL+ HSFL structure,and accompaning with 100 nm shallow pits in the uniform distribution.The compressive stress of gallium nitride films increased with increasing the number of femtosecond laser irradiation pulses,and the film shrunk.Through Raman,SEM and TEM tests and time-dependent density functional theory(TDDFT)simulation,the different forming mechanisms of LIPSS with femtosecond laser and355 nm nanosecond laser were revealed.Without material being removed in the early stage with femtosecond laser inducing the gallium nitride film,the material was reorganizated at the beginning due to the defect aggregation formed by nitrogen reduction,then ripples being formed in the surface of gallium nitride.By controlling the parameters of laser processing,the direction,depth and period could be controlled.LIPSS was a very potential technology to manufacture micro-nano patterns.(4)Ivestigation of laser annealing process of AlN thin films.Annealing was a common method for film growth to improve film quality.A300 nm aluminum nitride thin film with holes was annealed by nanosecond green laser with different energy levels.The experiment showed that the holes of aluminum nitride thin film were effectively reduced.Especially in the case with tungsten transition coating,the reduction of holes was obvious.When the laser energy was high,cracks appeared in the films.Experimental results showed that nanosecond laser annealing could promote the healing of film holes and improved the film's crystal quality.The recrystallization process of aluminum nitride after laser melting was investigated by molecular dynamics simulation.The simulation results showed that the recrystallization process could effectively promote the growth of hexagonal diamond morphology(lead-zinc structure)in thin film at a cooling rate above picosecond level.The mechanism of crack formation in the thin film was identified by using one-dimensional heat conduction equation.(5)A femtosecond processing device integrated into the MBE chamber was designed.Considering the need to integrate the femtosecond laser control equipment into the MBE cavity,the overall scheme of the femtosecond laser control device was designed,and the femtosecond laser fiber transmission scheme was designed to introduce the femtosecond laser into the MBE cavity.The optical fiber conduction imaging scheme was designed to realize the imaging of the thin film substrate in MBE outside the MBE cavity,which was convenient to adjust the spot position when coducting femtosecond laser processing.The cylindrical lens combined with four-quadrant detector imaging was innovatively applied to realize femtosecond laser autofocus to ensure the consistency of processing quality.A miniaturization scheme of femtosecond laser processing device integrated in the vacuum cavity was designed to meet the spatial requirements of integration in the MBE growth cavity.