Study On Electroluminescence Of Green LED Under High Injection Current

The applications of Light-emitting diode are very extensive in the devices such as lighting,backlight,display screen,etc.From the viewpoint of the 70%conversion efficiency between electricity and optics,the technology of the GaN-based blue LED is quite mature at present.Furthermore,green LED shows much lower quantum efficiencies compared with blue LED.Problems such as color drift,poor crystal quality,low p-type doping,which limit the quality of the green LED.It is important to research and enhance the photo-electric properties of green LED under high injection current.InGaN/GaN green LED is suffering from several problems under high injection current.On the one hand,the electro-optical conversion efficiency is very low that is called "green gap".On the other hand,external quantum efficiencies decline rapidly that is called droop effect.Chromaticity of the green LED will drift with the changing temperature or current.This behavior has a bad influence on the display screen.This paper mainly investigated the electroluminescence properties of blue and green LED under high injection current.It is difficult to measure the real-time junction temperature due to the self-heating effect.In this work,to elucidate the electroluminescence properties of green and blue LED,we use the LED with an integrated sensor unit.Peak energy(or wavelength),light output power and FWHM versus current and temperature of blue and green LED is measured and the related mechanisms are discussed.The main experimental content and results are as follows:1.The results of peak energy,light output power and FWHM versus current density and temperature with injection current ranging from 80 to 240mA and temperature ranging from 120 to 350K have been obtained.2.The translation from redshift to blue shift with temperature increasing were observed in both green and blue LEDs.Peak energy versus temperature curve of the green LED shows two emission peak,while that of the blue LED shows only one peak.We fit the value of ? are 30meV and 37.8meV for blue and green LED with Varshni's empirical formula Eg(T)=Eg(0)-?T2/?-T-?2/kT.The non-uniform distribution of In in the green LED leads to the larger ? and the stronger scatter for the exciton.It is suggested that exciton B transforms to exciton A in the green LED,which causes two peaks in the Peak energy versus temperature curve,while this behavior did not occur in the blue LED.3.The FWHM versus current is approximately linear under different inject current or different temperature,while the ratio is significantly different under the low and high temperature for the green and blue LED.For example,the proportionate parameters k of FWHM versus injection current are 0.7223E-4 eV/mA and 1.007E-4 eV/mA for blue and green LED at 250K,respectively.The k coefficient of blue LED decreases to 0.1735E-4 eV/mA,but it rises to 1.628E-4 eV/mA for green LED at 150K.We proposed that the blueshift of peak energy versus current under high temperature can be explained by the k-space band filling effect,while real-space band filling effect is responsible for the blueshift of peak versus current with respect to low temperature for the green LED.In the low temperature range,the peak energy/light output power versus current density tend to be constant for the blue LED,while the similar parameters' relationships for the green LED change significantly.We attribute the behaviors in greed LED also to the band filling effect in the real space.We think that lager In fluctuation causes the real-space band filling effect in green LED.4.We analyzed that how the increasing current influences the electroluminescence characteristics of the LED at low temperature.The result indicates that the increase of turn-on voltage is much larger than the increase of the dynamic resistance and the forbidden band width with temperature decreasing.We speculate that with increasing injection current under high injection current and low temperature,the leakage current cross over the conduction band or the valence band increases,therefore,not much more recombination occurs and little change of luminescence was shown in blue LED.