Study On Light Emission Enhancement Of GaN-based LEDs With Micro/nano Structures

? nitrides are considered to be one of the best materials for efficient light-emitting diodes(LEDs).In recent years,thanks to the growth of high quality materials and novel device structure design,LED luminous efficiency continues to improve,widely used in backlight,lighting and other fields.With the rapid development of semiconductor lighting industry,high-efficiency LED chip demand continues to grow.However,the further improvement of luminous efficiency from GaN-based LEDs is still one of the cutting-edge issues of concern academia and industry.One the other hand,one-dimensional GaN nanomaterials with unique physical characteristics are promising as versatile building blocks for optoelectronic devices and transistors for power electronics applications.As compared,the density of threading dislocations and defects can be significantly reduced due to their large surface-to-volume ratio.In this thesis,we are focusing on improving the optical and electrical properties of ?-nitride-based LEDs.Surface plasmon polariton(SPP)coupling enhanced LED structure was prepared by ultraviolet soft nanoimprint method,and the mechanisms of SPP enhanced emission were analyzed.Meanwhile,single nanaowire(NW)InGaN/GaN LED was fabricated by using of electron beam lithography,and optical and electrical properties of single LED were investigated.The main research achievements are highlighted as follows:1.We present the fabrication of InGaN/GaN LEDs with Al-coated GaN nanorods by using soft ultravioletnanoimprint lithography.To avoid damaging the InGaN/GaN MQWs and to facilitate reciprocal vectors for the excitation of BSPs via grating coupling,we create the nanorods array only within the p-GaN layer.The presence of Al deposited on the bottom space outside the nanorods is close to the MQW structure,which enables the excitation of BSP waves that convert energy from nonradiative recombination to emitted photons.The consistency in theoretical simulation and experimental results suggests that the enhanced light emission of the Al-coated LED structure with the reduced radiative recombination lifetime is attributed to the energy transfer from excitonsin InGaN/GaN MQWs to the BSPs by strong resonant coupling.The abnormal observations of blue-shift and enhancement of the longitudinal optical(LO)phonons in the Al-coatedstructures indicates a resonant Raman scattering process that is possibly assisted by metal-induced gap states at the Al/GaNinterfaces and the surface plasmon resonance effect.2.We propose and experimentally demonstrate a linearly polarized and enhanced light emission from surface emitting InGaN/GaN green LED with bilayer Al grating.A significant enhancement in peak photoluminescence intensity and an effective enhancement of the internal quantum efficiency of the designed LED was observed.The polarization degree of photoluminescence can reach up to 0.54 at room temperature.FDTD analysis and effective mass theory were used to analyze the performance of LED structure and the results between theory and experiment are found to be excellently consistent.3.We report on the fabrication and characterization of single NW InGaN/GaN LEDs with nondestructive contacting technology.The NW array was fabricated on a planar LED substrate using top-down strategy with a two-step process.Potassium hydroxide(KOH)solution wet etch was performed after the initial inductively coupled plasma(ICP)etching to remove the etch damage while making NWs with straight and smooth faceted sidewalls.The single NW LEDs were fabricated by electron beam lithography(EBL).The peak wavelengths of single LED electroluminescence(EL)spectra remain as a constant with the increasing injection current in comparison to plane LED.This phenomenon indicates a balance between carrier induced bandgap renormalization been offset by the screening of the quantum confined Stark effect(QCSE)and the band filling effect due to strain relaxation in the NW LED.However,the abnormal red shift in emission peak of single NW LED which is attributed to resonant coupling of Fabry-Perot cavity as a result of the electrode at both ends of the NW.