Photoelectric Properties Of AlGaN-based Deep Ultraviolet Light Emitting Diodes Based On Anisotropic Optical Polarization

AlGaN is direct wide-bandgap,third-generation semiconductor.The bandgap can be continuously tuned from 3.4 e V to 6.2 e V by changing the Al content in AlGaN,which makes it the best candidate to fabricate deep ultraviolet light emitting diode(DUV LED).However,due to the significant lattice mismatch during the heteroepitaxial growth and the anisotropic optical polarization propety of AlGaN,the further improvement of internal quantum efficiency or light extraction efficiency is limited.Also,further improvement of the performance of DUV LED devices faces many physical problems.Therefore,how to achieve effective dislocation management and optical polarization modification are the important and critical research work in the development of high performance DUV LEDs.In this dissertation,focusing on the anisotropic optical polarization in AlGaN materials,a series of investigations are carried out,such as epitaxial growth,strain modification of the active region,surface texture and high-efficiency DUV packaging to improve the performance of DUV LED.Meanwhile,some creative results are achieved as shown below.1.Epitaxy lateral overgrowth of AlN and AlGaN were carried out on pyramidal patterned sapphire substrate(PSS)and nano-patterned AlN template.A 3-step growth method is adopted to resolve the problem of the low mobility of Al atom and the low lateral epitaxy rate of AlN.Transmission electron microscope(TEM)characterization shows a unique dual coalescence process of the AlN epilayer.The generation and annihilation mechanisms of the threading dislocations were analysed according to a model deduced from TEM characterization.Eventually,a AlN epilayer with threading dislocation density less than3×10⁸ cm^-2is achieved,which would benefit the improvement of internal quantum efficiency of DUV LEDs.Meanwhile,the homoepitaxy-like method of nano-patterned AlN template is used to achieve crack-free high quality AlN and Al_0.56Ga_0.44N epilayers,while the epitaxy strain is effectively controlled.Moreover,the interspaces generated during epitaxy lateral overgrowth can suppress the internal total reflection and increase the light extraction efficiency of DUV LEDs.2.Based on the anisotropic optical polarization of AlGaN and strain modulation theory of its energy band,the internal strain of AlGaN films were modulated by the strained heterojunction grown by MOCVD.The relationship between internal strain and the anisotropic optical polarization of AlGaN materials were investigated.By analyzing the photon energy difference between TE mode and TM mode,rearrangement of AlGaN valence subbands under strain modulation is revealed.The results show that the in-plane compressive strain pushes heavy hole band upwards and crystal field splitting band downwards.The degree of polarization(DOP)is varied from-0.69 to-0.24 when the strain changed from tensile to compressive status.The higher the in-plane compressive strain,the higher the DOP of the AlGaN materials,that is,the higher the proportion of TE mode emission,which is beneficial to enhance the light extraction efficiency of DUV LED.3.Strained AlGaN multi-quantum well structures for DUV LED active region were designed and fabricated.Combined with the temperature-dependent and polarization-dependent PL tests,a method to determine the light extraction efficiency of epitaxy structure is proposed according to the relationship between quantum efficiencies and light extraction efficiency of DUV LEDs.The internal strain shows a significant effect on the optical polarization of AlGaN multiple quantum wells.The light extraction efficiency increased by40%when a-0.96%compressive strain is introduced.Besides,surface texture of a periodic cylindrical microstructure pattern array was prepared on the surface of the strained quantum well samples to increase the scattering of the UV emission.The combined effect of strain modulation and surface texture scattering leads to a nearly 2-fold enhancement of photoluminescence intensity of the multiple quantum wells.4.Inspired by the concept of surface texture,periodic nano-structure arrays are prepared on the surface of a special fluororesin film with high transmittance in ultraviolet spectrum.Due to its flexible characteristics,it can be firmly attached to the surface of the DUV LED chip to realize surface texture of both the front surface and side walls of the DUV LED chip.The result shows that this packaging strategy increases the light intensity of TE mode and TM mode by 20.5%and 21.8%,respectively,which is of great significance for the improvement of light extraction efficiency of DUV LED.