On The Impact Of Defects On The Carrier Recombination And Carrier Injection For Blue InGaN/GaN Light-emitting Diodes

At present,great progress has been made for III-V nitride based light-emitting diodes(LEDs),especially for blue LEDs.The efficiency for LEDs is higher than that for traditional tungsten filament lamps and mercury-based light tubes by more than 20 times.Therefore,a worldwide application of LEDs can save resources and energy,reduce the emission amount of CO₂and SO₂.Moreover,the replacement of mercury based light tubes by solid-state LEDs can avoid the pollution of mercury,which helps to improve the eco-system for our earth.However,in spite of the well-developed technology for Ga N based LEDs,there is still space to further understand the device physics and enhance the internal quantum efficiency.This thesis has conducted an in-depth study regarding the impact of the defects on the carrier injection and thus the internal quantum efficiency.We find that the internal quantum efficiency increases but the efficiency droop is big when the defect density is reduced.Meanwhile,the internal quantum efficiency decreases with the suppressed efficiency droop when the defect density is increased.Therefore,the defect-induced Shockley-Read-Hall(SRH)nonradiative recombination is an important factor affecting both the internal quantum efficiency and the efficiency droop.As is well known,the defects are electrically charged which will cause the energy minimum for carriers.Therefore,the defects can capture carriers and the nonradiative recombination occurs therein.Ultimately,the defects will affect the carrier injection and the carrier concentration in the active region for LEDs.Our studies indicate that the hole injection is more likely to be reduced because of the presence of defects.Therefore,enhancing the hole injection for Ga N based LEDs always has a long way to go.This thesis numerically shows the impact of the defect concentration and defect energy level on the carrier injection and carrier recombination for the In Ga N/Ga N blue light-emitting diodes by using advanced simulation programs.We find that the electron injection is less impacted by the defect density and the defect energy level.However,the hole injection can be more affected by the defect density and the defect energy.For those shallow-level defects,the holes still have the chance of escaping from the defects after being traps,and the escaped carriers can get involved into the radiative recombination in the active region.Our studies also indicate that the hole capture and the nonradiative recombination can be mostly obtained when the defect energy is close to the bandgap center.Other energy levels apart from the energy band center have negligible effect in influencing the hole injection.For deep-level defects,the holes have less chance of escaping from the defects after being trapped,and hence the nonradiative recombination occurs.Moreover,as the defect density increases,the leakage current for the LEDs also increases.The defects provide the current leakage paths and serve as the shunt resistance.