Effects Of Micro-Nano Patterned Reflector On The Light Extraction Efficiency For Deep Ultraviolet Light-Emitting Diodes

AlGaN-based deep ultraviolet LEDs(DUV LEDs)have great application potentials in these scopes such as sterilization,medical treatment,UV curing,and agriculture.However,DUV LEDs with a high external quantum efficiency(EQE)are still difficult to be obtained.The development of EQE is strongly limited by a lower than 10%of light extraction efficiency(LEE).Many factors lead to the low LEE.It is well known that the LEE is substantially influenced by total internal reflection(TIR)and Fresnel loss,which is caused by the large refractive index contrast between AlGaN and air(n_air=1 and n _AlGaN=2.6).Besides,the increase of Al content in AlGaN-based quantum wells yields the dominance of transverse magnetic(TM)-polarized light,which is difficult to propagate into escape cone and be extracted from the DUV LEDs.Another obstacle limiting the LEE arises from the absorptive p-Ga N contact layer because it is difficult to form Ohmic contact for Al-rich p-AlGaN layer with low hole concentration.To enhance the LEE,on one hand,various technologies including patterned sapphire substrates,inclined sidewalls,and surface plasma polaritons(SPPs)have been extensively applied.And by doing so,the scattering effect can be enhanced and helps increase the escape probability from the sapphire substrate for photons.On the other hand,technologies such as the distributed Bragg reflector(DBR)/omni-directional reflector(ODR)are proposed in DUV LEDs.These methods improve the LEE through increasing the reflectivity of the electrodes and reducing the optical absorption that is caused by the p-Ga N layer.Our work focuses on improving the LEE of the DUV LEDs through the design of the reflector structure.And the p-type electrode contact and inclined sidewall scattering structure in the flip-chip DUV LEDs are researched in detail.A series of research results have been achieved,and the specific work is as follows:1.The effect of meshed p-electrode on the LEE for DUV LED is systematically investigated.A meshed hybrid p-Ga N/p-AlGaN with a small inclined angle as p-type electrode contact is proposed to enhance the LEE.The proposed structure not only reduces the optical absorption from p-Ga N but also increases the scattering ability of the reflector.For meshed p-Ga N nanorods contact,the LEE does not always increase with the spacing of nanorods decreases.This is mainly due to the competition among scattering effect,Ga N absorption and metal absorption when DUV photons propagate in the structure.The large spacing of nanorods results in severely decreased scattering effect,while p-Ga N absorption and SPPs resonance absorption of metal will be promoted by too small spacing.Therefore,maintaining a specific spacing and a best optical scattering ability is more beneficial to enhance LEE.On this basis,we further propose to pattern the p-Ga N/p-AlGaN contact into truncated cone structure.And it can be observed that the optical scattering ability is stronger for the p-AlGaN truncated cone with the smaller inclined angle.Moreover,the truncated cone-shaped p-AlGaN have a significant effect on improving TM-polarized LEE.2.The ODR structure on the inclined sidewall structure for the DUV LED is studied carefully.A new design strategy about the ODR on the inclined sidewall is proposed.It is found that it is far more important to select a low-refractive-index dielectric layer with a thickness of over one wavelength than the choice of metal.We observe that although the reflectivity of Ag is far lower than that of Al and Mg,the LEE for inclined sidewall DUV LED with Ag reflector is not always lower than that with Al and Mg reflector.The reason for this phenomenon is that the special ODR structure on the inclined sidewall DUV LED uses TIR to improve the reflectivity of the mirror.In addition,the design of the thickness and the material of the passivation dielectric layer is also different from that of the ODR for planar DUV LED.For a thin dielectric layer,the dielectric layer material with a high refractive index should be applied to avoid the SPPs resonance absorption induced by evanescent wave from TIR;for a thick dielectric layer,the dielectric material with a lower refractive index should be used to increase the possibility of total internal reflection.The energy of the evanescent wave propagating to the metal surface is almost zero due to the thick dielectric layer,and then the SPPs resonance absorption can be neglected.