Epitaxial Structure Design And Material Growth Of AlGaN Based Deep Ultraviolet LED

Deep ultraviolet light emitting diodes(DUV LEDs)have the advantages of small size,low power consumption,long life,adjustable wavelength,and environmental friendliness.DUV LEDs are expected to replace the traditional ultraviolet mercury lamp light source,and play an important role in water,air purification,surface sterilization,biological detection and other fields.However,due to the high dislocation density,strong polarization effect,low light extraction and carrier injection efficiency problems,the current light output power and quantum efficiency of DUV LEDs are still low.In this dissertation,the surface evolution and dislocation filtering mechanism of nitride materials under large mismatch stress is deeply studied.By focusing on the key scientific issues in the preparation and application of AlGaN-based DUV LEDs,high-quality AlN and AlGaN materials are realized.On this basis,the quantum well structure design and carrier regulation mechanism of DUV LEDs under strong polarization electric fields have been explored,and a series of results have been obtained.The main research contents and results of this dissertation are as follows:(1)The dislocation filter layer method has been developed on the flat sapphire substrate to control the dislocation density of the AlN film.The atomic force microscope(AFM)results show that the dislocation filter layer can increase the coalescence depth of the AlN film from 0.4?m to about 2?m.During the coalescence process,more dislocations are annihilated by bending.Finally,the full width at half maxima(FWHMs)of the(002)and(102)rocking curves of AlN with dislocation filter layer are 140 and 267 arcsec,respectively,and the corresponding dislocation density is about 7.6×10⁸ cm^-2.The turn-on voltage of the DUV LED device grown on high-quality AlN template with dislocation filter layer was reduced by 0.3 V,and the light output power was increased by 40.4%.The effect of the grain orientation control layer on the nucleation and growth of AlN films on patterned substrates is also studied.The characterization results show that the grain orientation control layer can reduce the anisotropy of AlN material growth in the pattern area of the substrate and eliminate the hexagonal pillars on the surface of the AlN film.Finally,the FWHMs of the(002)and(102)rocking curves of AlN with grain orientation control layer are reduced from300 and 309 arcsec to 165 and 185 arcsec,corresponding to a dislocation density of 3×10⁸cm^-2.(2)In terms of the epitaxial growth of AlGaN materials on the polar plane,this chapter proposes a composition-graded insertion layer to reduce the lattice mismatch interface stress at the AlN/AlGaN film interface,and reduce the dislocation density and phase separation effects of AlGaN materials.The FWHMs of the(002)and(102)planes rocking curve of the AlGaN film are reduced from 447 and 594 arcsec to 334 and 441 arcsec,and the corresponding dislocation density is reduced from 3.5×10⁹ cm^-2 to 1.7×10⁹ cm^-2,respectively,the surface roughness is reduced from 3.21 nm to 1.20 nm.Besides,the influence of the SiN_x insertion layer on the material properties of the non-polar AlGaN material is also studied.The SiN_x mask layer can change the growth mode of the AlGaN material,thereby reducing the in-plane anisotropy of surface roughness and crystal quality.(3)The influence of the thickness of quantum well on the light output power of the DUV LED under the condition of a strong polarization electric field is systematically studied.The DUV LED with a quantum well thickness of 2 nm has the highest external quantum efficiency.Also,a DUV AlN/GaN quantum well with a peak wavelength of 249nm was grown on the c-plane sapphire substrate by MOCVD.Polarized photoluminescence results show that the light emission mode of the ultrathin GaN quantum well is dominated by transverse electric polarizations.When the thickness of the GaN quantum well is reduced,the strong quantum confinement effect will change the microstrip structure arrangement in the quantum well,thereby increasing the effective bandwidth of the quantum well.Finally,DUV LED with AlGaN/GaN/AlGaN composite electron barrier structure has been designed and grown.The experiment results show that the light output power of the deep ultraviolet LED with a composite electron barrier structure is increased by 25.3%.The simulation results show that the composite electron blocking layer structure can reduce the hole injection barrier of DUV LED from 409.3 meV to 378.6 meV,which allows more holes to be injected into the quantum well,thereby increasing the radiation recombination rate of the quantum well.