Study On Optical And Electrical Properties Improvement Of GaN-based LED By 3D Arrays And V-pits

GaN-based light emitting diodes(LEDs)have been widely used in solid state lighting,signaling and large displays because of their advantages of low energy consumption,long lifetime and high efficiency,etc.Due to total reflection and Fresnel reflection,the light extraction efficiency(LEE)of planar LEDs are limited.One of the objectives of high performance LED is to improve the LEE.Compared to planar LED,three-dimensional(3D)arrays of 3D structure LEDs can improve the LEE and regulate the optical and electrical properties of LEDs.In addition,it has been proposed that the emission performance of the c-plane MQWs(C-QWs)close to V-shaped pits(V-pits)can be enhanced by V-pits.In this thesis,nanorod 3D LED,rectangle and square 3D LED chips have been successfully fabricated by the focused ion beam(FIB)etching technique.The optical and electrical properties of 3D LED have been studied.The transport mode of guided modes for 3D arrays has been investigated by three-dimensional finite difference time domain(3D-FDTD).The improvement mechanisms of LEE for 3D LED have been clarified.The mechanisms of carrier transportation around V-pits under different temperatures and the optical and electrical properties of LED regulated by V-pits have been investigated.The contents and the obtained results are described as follows:1.Nanorod 3D LED with good periodicity has been prepared via FIB etching technique.However,after FIB etching,the photoluminescence(PL)intensity of 3D LED decreases,which can be attributed in substantial increase in defects and surface contamination.The damaged surface can be recovered by KOH etching.After KOH etching,3D array exhibits comparatively smooth sidewalls.Compared to planar LED,the light output power(LOP)of 3D LED is enhanced by 14.7% and the peak wavelength exhibits a blue-shift of 0.6 nm.Combing the PL spectra variation and 3D-FDTD simulation results,the improved optical and electrical properties of 3D LED can be attributed to stress relaxation,nano-resonator coupling and photonic crystal diffraction.2.Square 3D LED chips have been prepared via FIB etching technique.The effects of FIB beam current,KOH etch time and etch depth on the optical and electrical properties of 3D LED chips have been studied.Our results show that the sidewall surface roughness increases,the optical and electrical properties of 3D LED chips degrade with increased beam current.As the KOH etch time of 5 min,the optical and electrical properties of 3D LED chip is optimum.As the etch depth increases,the optical and electrical properties of 3D LED chip degrade.The reasons are that as the beam current increases,the leakage channels of 3D LED chip increase,the radiative recombination efficiency is reduced;the KOH etch time of 5 min provide an optimal etching strategy for sidewall surface damage removal,indicating that the FIB direct writing is an efficient way for the 3D LED chip fabrication;combing the 3D-FDTD simulation results,as the etch depth of 3D array increases,the probability of guided modes extracted by 3D array decreases,leading to the reduced LEE of 3D LED chip.3.Stripe and square 3D LED chips have been successfully fabricated by the FIB etching technique.? The optical and electrical properties of planar LED chip and 3D LED chip have been studied.The improvement mechanisms of optical and electrical properties for 3D LED chip have been clarified.Compared to the planar-chip,the LOP of stripe 3D LED chip is lower,which is attributed to reduced carrier injection efficiency due to the oversize decline of active regions.The square 3D LED chip exhibits a similar current–voltage performance compared to the planar LED chip.At the driving current of 20 mA,compared to the planar LED chip,the LOP of the square 3D LED chip is improved by 17.8%,the angular-resolved EL(AREL)intensity of the 3D LED chip is increased of 17.1% in vertical direction,which is consistent with the LOP results.Combing the 3D-FDTD simulation results,more exposed MQWs surfaces of square 3D LED chip can increase the escape opportunity of photons,so as to improve the LEE.4.The PL characteristics of the C-QWs close to V-pits have been investigated by temperature-dependent micro photoluminescence(?-TDPL).The ?-TDPL results show that the emission of the C-QWs close to V-pits broadens largely across the short-wavelength band and exhibits a rapidly blue-shift as temperature increases.The phenomenon can be mainly attributed to a higher proportion carriers generate high-energy radiative transition in C-QWs close to V-pits because of augmented carrier mobility,in which the mechanism of carriers absorbed in sidewall MQWs(S-QWs)move to the C-QWs close to V-pits acts as a critical process.The sizes of V-pits can be regulated according to the properties of high resistance for electron blocking layer and p-GaN in V-pits and high barrier for S-QWs.As the sizes of V-pits increase,higher proportion hole current can transport to C-QWs safely and the hole injection area and depth increase,leading to the improved optical and electrical properties for LED chip.