Design And Fabrication Of The P-type Electrode For Vertical-structure Light-emitting Diode

The application market of light-emitting diodes has been developed for the direction of high-power or super-current driving.The traditional lateral-structure LED chip on sapphire substrate has been gradually replaced by vertical-structure LED chip on silicon for its initial disadvantages such as current-crowding effect,poor heat-dissipation,lighrt-absorption and expensive price itself.Reflector process is the one of the most important process in vertical-structure LED fabrication.Depositon metal mirrors on p-type GaN make V-LED possess superiority of high light extraction efficiency and single side light-exit compared with lateral-structure LED chip.The reflector metal generally picks silver,because it has the highest reflectivity at wavelength visible light of all the metal.However,the silver also has the poor adhesiveness,low work function to form good ohmic contact and meanwhile,it is also easily oxidized and agglomerated during annealing in oxidizing atmosphere in RTA.And these phenomena could finally result into the degeneration of Ag-based reflercor and even the LED chip.Thus various methods have been proposed to solve these problems such as inserting an interlayer under silver layer or overlayer among silver,even the entering other metal into silver as the alloy.With these methods most problems were solved,but the light-absorption by interlayers and the agglomeration phenomena exists during annealing process still trouble ourselves.So we aims to solve theses residual problems,and acutally abtain a reflercor not only has the high reflectivity but also with low resistivity,and we have got several achievement.We propose the Ni/Ag/Ni sandwich structure reflertor to be baseline to experiment.,and we do experiment from following three aspects.Fristly,ptimize contact layer: we ensured the best nickel thickness is 2 angstorm and best grain diameteris at the range of 100 to 120 nanometer,at this time,the absorption comes to the minimum value,and the corresponding LOP of V-LED reach the top in the gradient.On the basis of this,we prepared the nickel oxide nanodots as interlayer to reduce the light-absorption issue by annealing the nickel nanodots in pure oxygen atmosphere,the corresponding LOP of V-LED improves three percents further,comes near the limiting conditions.Meantime,we found the grain size of nickel dots reduce,thickness reduce,which restore the morphology of the p-type GaN substrate,and intuitively explain the mechanism of the loss absorption.Secondly,ptimize reflective layer: we put foeward the mechanism of adjusting the residual stress in silver layer by controlling the thickness silver.And obtained the minimum residual stress and maximum reflectivity at 75nm-thickness.Using this method,we got V-LED chip with 2.95 V forward voltage and 122lm/W light output power at 350 mA operating current.Meantime,we continue discuss the annealing condition of Ni/Ag/Ni based reflector,and we finally obtain the optimal annealing temperature and annealing atmosphere are 430°C and mixture of nitrogen and oxygen(N/O ratio is 4:1).Finally,ptimize the preparation method of reflector: we put forward the innovation two-step process to prepare the silver-based reflector with both high-reflectivity and low-resistivity combine with wet etching and double annealing.Solving the classic difficulties.Using this method,we improved 4.1 percents about the LOP of V-LED chip with L-LED,and stabilize the forward voltage below 3.2V,and stabilize the yield of reverse leakage current above 86%.