Electrical Injection-induced Stress And Quantum State Variation In ?-nitride Quantum Wells

InGaN-based light-emitting diodes(LEDs)are revolutionizing lighting applications toward realizing high efficiency.LED efficiency is increasingly becoming a major obstacle because of efficiency loss under high current injection with the advancement of solid-state lighting.Owing to lack of direct measurement,identification of the reasons for efficiency droop has been restricted.AlGaN-based deep ultraviolet(UV)light-emitting diodes(LEDs)are attracting much interest for their promising application in water purification,data storage,medical curing,bio-chemical agent detection,and photolithography.However,even though the tremendous progress on defect density,carrier confinement and p-type doping has been made,the low external quantum efficiency is still an obstacle to wildly use.One major limitation to the external quantum efficiency is the poor light extraction efficiency arised from the fundamental valence band crossover issue.In this thesis,the electrical injection-induced stress on interband transitions have been performed by first-principle calculation,a direct measurement technique for stress and metal-organic vapor phase epitaxy(MOVPE)technique.The major results are as follows:A direct measurement technique is developed in this study for the stress in GaN-based blue LEDs under electrical injection,which provides important insights into the fundamental processes that are necessary for improvement of quantum efficiency.Characterized by Raman spectroscopy measurements,the Raman shift of the GaN E2 mode decreases as the current increases and reaches 565.5 cm-1 with a downshift of 4.4 cn-i when the driving current on GaN-based LEDs increases to 700 mA,indicating that the biaxial stress vary by approximately 1.03 GPa.The largest EQE droop occurs while the compressive stress of 0.65 GPa is released stress-free.As the compressive stress becomes tensile,the EQE declines slower even under the biaxial tensile of 0.38 GPa.The first-principles calculation results show that the lattice of the MQW expands and the biaxial tensile strain reaches 2.3%,while the electrons accumulate to 5 because of the high current injection.The intensity of Im(?xx+?yy)decreases dramatically as the biaxial compressive strain is released,and then slowly declines as the biaxial tensile strain builds up,whose tendency is consistent with the experimentally observed stress-dependent efficiency droop.According to the charge analysis,as the strain varies from the compressive state to the tensile state,the charge densities of HH and LH both greatly reduce,while the charge density of 1e remains invariant,indicating that less hole quantum states is responsible for the decrease of Im(?xx+?yy)and the efficiency droop.The rule of stress-dependent efficiency is further examined by comparing the light-output powers of LEDs on sapphire substrates with different thicknesses.Based on the efficiency dependence on stress,stress control may produce a new degree of freedom in the design of LEDs with improvement with regard to efficiency droop.The effect of electrical injection-induced stress on interband transition as well as the optical polarization in high Al content AlGaN MQWs have been studied.Characterized by the angle-dependent EL measurements,as the injection current increases,the integrated intensity of electroluminescence increases slowly around the surface normal direction(0<30°);while it increases dramatically at 0 larger than 50°.These phenomenon agree well with the previous reported results,suggesting that the TM(E//c)polarized emission becomes stronger as the injection-current increases.the proportion of TM polarized emission increases with increasing the injection-current.The Raman peaks of the E2(GaN-like)and E2(AlN-like)are found to gradually shift toward lower frequencies as the driving current increases,indicating a tensile stress builds up.Since it is hardly to distinguish the strain variation from quantum well and barrier layers,the average influence in MQWs with the average Al content of 0.52.The E2(GaN-like)phonon frequency of unstressed A10.52Ga0.48N was evaluated to locate at 596.5 cm-1 according to the tendency of E2(GaN-like)mode.Before current injection,the active layer suffers tensile stress of 0.90 GPa.As the current increases,the E2(GaN-like)phonon frequency shift to lower frequency,and the tensile stress increases monotonically.The tensile stress increases by 0.42 GPa when the driving current increases to 200 mA.The first-principles calculations demonstrate that electron accumulation is responsible for the stress variation.Further close inspection of strain dependent electronic structures of high Al-content MQWs reveal that,as the tensile strain increases,the CH band become more dominant at the top of valence bands and exhibits stronger dispersion along the ?-A([0001])direction,leading to deconfinement of hole states and decrease of the degree of polarization.Consequently,it can be inferred that the increase of proportion of TM polarized emission is mainly attributed to the electrical injection-induced tensile stress in MQWs active region.Furthermore,as the MQWs relax to unstressed state or suffer from a compressive strain,the top of valence band recovers to be discrete quantum states for heavy holes and light holes,and the total spontaneous emission rate Rsp increases dramatically.These results suggest that controlling the strain status of high Al-content AlGaN MQWs would be a promising way to improve the TE polarized emission as well as quantum efficiency in deep UV optoelectronic devices.