Study On Fabrication Of Ultraviolet LED With Distributed Bragg Reflectors

With the research and development of GaN-based light emitting diode(LED)deepening,innovation and application of LED technology is also expanding.Ultraviolet(UV)LED with AlGaN as the active region has become the focus of research.UV LED based AlGaN has important application value in curing,medical,short-range communication and other fields.Compared with the traditional ultraviolet light source,the utility model has the advantages of high efficiency,energy saving,environmental protection without mercury,reliability,durability,small size,etc..In addition,distributed Bragg reflectors(DBR)can be used to reflect the specific wavelength of light,which is widely used in the LED structure to improve the efficiency of light extraction.In this paper,we firstly analyzed the relationship between layer cracking critical thickness of AlxGa1-xN thin films grown on sapphire substrate/Ga N template and aluminum component x.20 pairs of Al0.32Ga0.68N/GaN DBRs were prepared on sapphire substrates by metal organic chemical vapor deposition(MOCVD),and the surface of DBR was found to be full of cracks.The experimental results show that it is difficult to get high reflectivity AlGaN/GaN DBR with no crack because of the high stress in the DBR structure grown on GaN template layer on sapphire substrate.Then the crack-free AlGaN/GaN DBR was prepared on sapphire substrate by introducing a low temperature AlN insert layer to adjust the internal stress of the films.The effect of AlN intercalation layer on the DBR characteristics was investigated by means of the preparation of different pairs of AlGaN/GaN DBR.It is found that the introduction of the low temperature AlN insert layer prevents the cracks from occurring,and introduces the arc surface defects in the epitaxial layer resulting in the degradation of the DBR surface,which may be caused by the micro cracks in the AlN insert layer.With the growth of DBR pairs increasing,arc surface defects gradually disappear,and DBR surface roughness decrease gradually.Meanwhile,the experimental reflectivity spectra get closer to the simulated results.Finally,we obtain a smooth-surface 25-pair Al0.32Ga0.68N/GaN DBRs with a reflectance of 94% at 390 nm and a 16 nm stop-band bandwidth.The preparation of high-quality DBRs lays the foundation for the future development of high efficiency resonant cavity UV LEDs.Compared with sapphire substrate,the lattice mismatch between Si C substrate and AlGaN is small,which can effectively improve the quality of AlGa N epitaxial layer.At the same time,the simulation results of SILVACO TCAD show that the vertical conduction structure LED can improve the utilization ratio of the active region compared with the horizontal conduction device.And the Si C substrate has good conductivity,which can be used to prepare the vertical structure LED.However,because SiC absorbs UV light,it is difficult to prepare UV LED with high light extraction efficiency.Introduction of a DBR may be a good approach for resolving this problem.Silicon-doped Al0.19Ga0.81N/Al0.37Ga0.63 N DBR for the UV region were grown on n-type 6H-SiC(0001)substrates by MOCVD.To suppress the generation of cracks,a low-temperature AlN pre-deposition layer on 6H-SiC substrate was used as buffer.A smooth-surface 15-pair electrically conducting DBR with a reflectance of 68% at 369 nm is obtained.The stop-band bandwidth and the RMS value of DBR are 10 nm and 0.4 nm,respectively.Furthermore,we prepared vertical structure UV LED with and without n-DBR on 6H-SiC substrates.By comparing EL spectra,it is shown that the introduction of DBR structure can effectively improve the UV light emission.