| As the representative material group of the third-generation semiconductors,III-nitrides have a wide bandgap and are widely used in the optoelectronic industry,such as commercial solid-state lighting,optical fiber communications,optical detection,radio frequency,and high-power switching,deep Ultraviolet photonics,etc.Among them,the ternary alloy aluminum gallium nitride(Al Ga N)material has attracted widespread attention in ultraviolet applications for its bandgap can be continuously changed in the range of 3.4e V(Ga N)-6.2e V(Al N)with the adjust of Al composition.Due to the lack of cheap homoepitaxial substrates,sapphire has become the preferred choice of material for the heteroepitaxial growth of Al Ga N-based ultraviolet LEDs for its transparency to ultraviolet light and low cost.However,due to the mismatch of lattice constants between sapphire and Al Ga N material,the control of the crystal dislocation density and crystal quality of the epitaxial layer has become the key to improve the photoelectric performance of UV-LEDs.Therefore,the LED structure has been epitaxially grown on sputtered high-temperature annealed aluminum nitride(Al N)/sapphire template by MOCVD epitaxial technology,and the influence of the thickness of the sputtered high-temperature annealed Al N layer on the optical property of ultraviolet LEDs has been studied,which can build up a reasonable basis for the preparation of quality Al Ga N-based LEDs.In addition,we systematically investigated the influence of the strain relaxation of the n-type Al Ga N layer on the Al Ga N multiple quantum well region.Through simulation,theoretical calculation,and experimental preparation,the mechanism of the optical performance of UV LEDs changing with the difference of strain relaxation have been discussed.The main research contents of this thesis are as follows:1.By MOCVD epitaxial technology,the LED structure was epitaxially grown on sputtering high-temperature annealing Al N/sapphire templates with different thicknesses,and the influence of the thickness on the crystal quality of the Al Ga N layer and the luminescence performance of the quantum well region was studied.Through characterization and analysis,the dislocation density of the sputtering high-temperature annealing Al N/sapphire templates gradually increased with the increase of thickness,and the surface roughness of the film decreased at first and then increased with the addition of thickness.The dislocation density of the epitaxial Al Ga N layer on it shows a trend of decreasing at first and then increasing with the increase of the thickness of the Al N template,and the luminous intensity of the active region also shows the same law.It has been proved that the dislocation density and surface flatness of the Al N template has an important effect on the epitaxial layer,and there is an optimal thickness to make LED have the best performance.2.Through APSYS software simulation and theoretical calculations,the influence of the strain relaxation of the n-type Al Ga N layer on the optical performance of the multiple quantum well region is studied,and the mechanism is discussed.The method of modifying the lattice constant of the Al Ga N material is used to represent the n-type Al Ga N layer under different strain relaxation.The simulation and theoretical calculation results show that the polarization electric field strength in the quantum well region increases with the increase of strain relaxation,which intensifies the quantum confinement Stark effect(QCSE)inside the active region,and makes the emission wavelength redshift.The spatial overlap of electronhole wave functions decreases,and the luminous intensity decreases.3.Based on the sputtering high temperature annealing Al N/sapphire template with the optimal thickness,the strain relaxation is changed by inserting the Al N/Al Ga N superlattice structure with different periods between the Al N template and the Al Ga N layer.The influence of the strain relaxation of the n-type Al Ga N layer on the optical performance of the multiple quantum well region is compared with the results of simulation calculations.The test results show that the strain relaxation of the Al Ga N layer increase with the increase of the number of superlattice periods,the Al composition and dislocation density of the Al Ga N layer decrease with the rise in the strain relaxation,and the luminous intensity of the epitaxial wafer increases at first and then decreases.The maximum intensity is achieved when the relaxation degree is 45.69%.Combining the results of simulation calculation and experimental characterization,we believe that the strain relaxation is mainly affected in four aspects: strain relaxation degree will change the band structure of quantum wells and thus jeopardize the radiation recombination efficiency;the increase of strain relaxation degree will improve the crystal quality of the epitaxial layer,and the decrease of dislocation density will lead to the decrease of the non-radiative recombination rate;due to the composition pulling effect,the increase of the strain relaxation will result in the enhancement of Ga doping during the epitaxy process,thereby reducing the Al composition in the Al Ga N layer;strain relaxation will change the polarization mode of the emitted light and then affect the luminous intensity. |
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