Design Of Epitaxial Structure Of GaN-based Blue LED

In recent years, GaN-based light-emitting diodes (LEDs) as a newgeneration of solid-state lighting have made great progress. GaN-based blueLED is always lively discussed and researched. The hottest issues are thepolarization effect and the efficiency droop phenomenon under high currentinjection, which strongly influence the photoelectric properties of light-emittingdiodes.In the Group III nitrides, the polarization effect of the active region willproduce strong polarization field, which will greatly change the energy bandstructure of LED. Especially in the quantum wells of active region, with aserious tilt, the electrons and holes are confined on both sides of the quantumwells respectively, which reduces the luminous probability of the carriersinvolved in radiative recombination. Therefore, the luminous efficiencydecreases in quantum wells. Polarization effect can make the light wavelengthredshift, leading to lower accuracy of material component obtained by radiationspectrum measurement. Polarization effect will also enhance the dependence ofquantum well structure on the well width and barrier thickness.At high current injection, although the number of carriers used for lightemission increases, the LED luminous efficiency does not increase with theincrease of the injected current. However, it has a downward trend. Efficiencydroop has become a bottleneck hindering the development of LED. Manystudies have pointed out that polarization effect, carrier leakage, poor holeinjection, epitaxial defects and auger recombination have been suggested asexplanations for efficiency droop. Therefore, LED luminous efficiency has been greatly restricted under the high current injection.So far, there have been a lot of excellent research team on how to improvepolarization field of the nitrides and t efficiency droop at high injection currents,hoping to effectively improve the photoelectric performance of LED. This thesiswill continue to seek the optimum design of LED device structure to improvepolarization effect and efficiency droop. The research focuses on how toeffectively reduce polarization effect of InGaN LED and improve efficiencydroop phenomenon in this thesis. The contents and the obtained results aredescribed as follows:1. The impact of multiple quantum well (MQW) structure with Incomponent gradient on optoelectric properties of InGaN blue LED isinvestigated. Simulation results indicate that MQW structure with In componentgradient can effectively reduce the polarization effect of the LED active region,which makes the wave functions overlap rate of electrons and holes increased,makes the rate of radiative recombination increased, thereby improving theinternal quantum efficiency. Therefore, the output power of LED device isgreatly enhanced. In addition, efficiency droop phenomenon and wavelengthstability has also been significantly improved.2. Based on the band structure design of electron blocking layer (EBL) inthe LED, the impact of EBL on the photoelectric properties of InGaN/GaN blueLED is investigated. And, In order to find out the optimal design scheme, a newdesign of EBL structure is systematically optimized. Simulation results indicatethat the new EBL structure can eliminate the adverse effect of the notch in theconduction band and the spike in the valence band at the last barrier (LB)/EBLinterface on the performance of LED via the tailoring of polarization to improvehole injection efficiency and electron confinement ability. Simultaneously, thesimulation results also demonstrate the effectiveness of the structure.3. Based on the design of barrier thickness in the LED active area, theimpact of different designs of barrier thickness on the luminous performance of LED is investigated. The structure with gradually decreased barrier thicknessfrom the n-side to p-side is proposed to replace the traditional structure withequal barrier thickness in this thesis. The thicker barriers located close to then-GaN layer are used for increasing the electron transport distance while thethinner barriers located close to the p-GaN layer are used for reducing the holetransport distance. Simulation results indicate that the proposed structure canmarkedly enhance luminous efficiency of InGaN blue LED and improve theefficiency droop phenomenon at high current injection due mainly to theincreased injection of holes into the QWs, leading to the effective suppression ofelectrons leaking out of the active region.