The Study Of Current Spreading On AlGaInP LED With Carbon Nanotubes Transparent Conductive Layer

Compared with conventional light sources, the Light-emitting diode(Light-Emitting Diode, LED)have advantages of substantial energy saving, highefficiency, low voltage, high reliability, long life and others. It has been widely usedin displays, instructions and lighting and other fields, it is promote preferred lightinglow-carbon life and expected to replace the current lighting in the area dominated byfluorescent and incandescent lamps, so it’s draw attention by the world. The Effect ofcurrent spreading for the optical power in AlGaInP red LED was studied in this paper.We simulated current spreading of the traditional AlGaInP red LED, and prepared theAlGaInP LED with carbon nanotubes(CNT) as a transparent conductive layer. We testits electro-optical properties, the simulation results and experimental results have agood agreement with each other. The main contents of this paper include thefollowing aspects:(1) Simulation and calculation about the current spreading of AlGaInP LED.Firstly, according to Kirchhoff ’s law and E.F. Schubert’s theory about the currentspreading, we deduced the formula of current spreading in the vertical AlGaInP LED,we get the solution of current spreading length LSand electrode edge current densityJ0. We simulate the electrical characteristics of the traditional AlGaInP LED, andaccording to the the theory of current spreading, we calculated J0and LS, thecalculated results were compared with the simulation results and we obtained goodagreement with the results. Through the analysis we gives the correspondingdifference is due to the length of the device make the current density accumulated atthe edge of device and make the simulation results get a deviation from the theorycalculation.(2) The analysis of Current spreading simulation.We simulated electrical characteristics of a AlGaInP LED which the area is300×300μm2, get the results of the current density distribution inside the device, thecurrent density away from the electrode edge would be decayed by exponential. As most of the injection current and carrier are located under the electrode, the opaqueelectrode would block the most of photon from the device, and only little carrier havecontribution to the output light power. So the effective carriers were defined here asan active electrode outside carriers. According to the simulation results, the effectivecarriers were calculated and we found effective carrier decrease with the increasing ofinjection current and sheet resistance of current spreading layer. Effective carrier has asignificant impact on the output light power.(3) CNT AlGaInP LED testing and simulation comparisonWe prepared two transparent conductive layer: CNT and CNT with2nm Auevaporate in surface(Au-CNT). These two samples were tested by electrical andoptical, measured the sheet resistance by four-probe method, CNT sheet resistance inabout1000Ω/sq, and sheet resistance of Au-CNT is about130Ω/sq; we measured thetransmittance of CNT was92%and Au-CNT light transmittance of81%at thewavelength range of620nm. We prepared CNT AlGaInP LED and Au-CNT AlGaInPLED, tested their I-V curves and L-I curves and near-field light distribution. L-I curveindicate that the output light power of Au-CNT system is higher than the normaldevice about30%. The effective carrier compare simulation results, Au-CNT deviceshave a good agreement between effective carriers and output light power, but the CNTdevice is not. The reason is because the uneven distribution pipelines in the CNT andmake a big junction resistance. The results of Au-CNT LED show the effective carrieris an important factor in the optical power increases.