Study On Light Extraction Efficiency In AlGaN-based Deep-Ultraviolet Light-Emitting Diodes With Flip-Chip Structure

AlGaN-based Deep-Ultraviolet(DUV)Light-Emitting Diodes(LEDs)have attracted great interest for their potential application such as sterilization and phototherapy in the bio-medical field,air and water purification,ultraviolet(UV)curing,solid state lighting,NLOS UV communication.However,despite rapid progress has been made in improving the performance of DUV LEDs,especially in the epitaxy and fabrication technologies,the external quantum efficiency(EQE)of DUV LED is too low to justify the broad market demand.The current study shows that Light extraction efficiency(LEE)have a significant impact on EQE of AlGaN-based DUV LEDs.LEE of AlGaN-based DUV LEDs has been thought to be quite low due to UV light absorption in the GaN p-contact layer and total internal reflection(TIR)of transverse-magnetic(TM)polarized light.AlGaN-based DUV LEDs have unique optical polarization properties which is different from most LEDs operating in the visible and near-infrared wavelength range.AlGaN-based DUV LEDs is mainly TM polarized rather than transverse-electric(TE)polarized as the A1 composition increases or the wavelength decreases.The TM-polarized light propagates mainly in the lateral direction.The LEE of AlGaN-based DUV LEDs is limited by the large refractive index difference between the AlGaN film(n=2.5)and the surrounding air(n=1).Therefore,it undergoes strong effects of TIR due to the large incident angle on the interface.LEE in AlGaN-based DUV LEDs with flip-chip LED structures is simulated using finite-difference time-domain(FDTD)simulations.The main results in this thesis are highlighted as below:1.For flip-chip LED structures,LEE of TM modes is obtained to be<1%and more than ten times smaller than that of TM modes.The LEE of of TM modes is limited by the large refractive index difference between the chip and air.2.A series of simulations were did to prove the way to promote LEE of AlGaN-based DUV LEDs,such as Patterned Sapphire Substrate(PSS),patterned AlN/sapphire templates,AlGaN-based DUV LEDs are separated from sapphire substrates by laser lift-off(LLO)technique,AlGaN-based DUV LEDs are separated from AlN/sapphire templates by LLO technique,lens of AlGaN-based DUV LEDs packages.When AlN/sapphire templates of flip-chip LED structures with textured surfaces were employed and AlGaN-based DUV LEDs were separated from AlN/sapphire templates,LEE of the TM mode was greatly enhanced with lens of packages.In the case of no p-GaN layer,LEEs of the TE and TM modes are 83.9 and 47.9%,respectively.When sapphire substrate and AlN/sapphire templates of flip-chip LED structures with textured surfaces were employed,LEE of the TM mode was also greatly enhanced with lens of packages.In the case of no p-GaN layer,LEEs of the TE and TM modes are 54.7 and 43.1%,respectively.3.LEE in AlGaN-based DUV LEDs depends on the shape and angle of the patterned strips on sapphire substrate and AlN/sapphire templates.This letter investigates the shapes including trigonal prism,pyramid,hexagon pyramid,circular cone,cylinder,hemisphere,a combination of cylinder and circular cone,a combination of cylinder and hemisphere.Trigonal prism,pyramid and hexagon pyramid can cover 100 percent of sapphire substrate and AlN/sapphire templates.LEE of the TM mode was greatly enhanced in turn.In the case of no p-GaN layer of flip-chip LED structures with surfaces in the shape of hexagon pyramid,LEEs of the TE and TM modes are 70.4 and 53.1%,respectively.