Effect Of AlGaN Electron Blocking Layer On Photoelectric Properties Of GaN-based Green LED On Silicon Substrate

With the continuous progress and breakthrough of technology,GaN-based semiconductor materials are widely used in LED devices.Its wavelength can be adjusted from ultraviolet to blue,green and yellow.Compared with blue LED,MQW of green LED is a kind of InGaN material with higher In component,while InGaN material with high quality and In component is difficult to grow,which results in low luminescence efficiency of green LED.GaN-based LED devices have high light efficiency at low current density,but efficiency Droop will occur at high current density.Usually,efficiency Droop of green LED is more obvious than that of blue LED.The factors leading to the efficiency Droop include the leakage of electrons from the active region to the p-layer,the low injection efficiency and the nonuniform distribution of holes.In order to further improve the luminescence efficiency of green LED,this paper improves the hole injection efficiency by changing the AlGaN electron blocking layer.The effect of AlGaN electron blocking layer on the photoelectric properties of GaN-based green LED with large V-pit structure on silicon substrate was studied.The following results were obtained:1.An additional low-doped p-AlGaN layer after the conventional p-AlGaN electron blocking layers was adopted,the effects of different low-doped p-AlGaN thickness on the luminescent properties of GaN-based green LEDs on silicon substrate were studied.The results show that the external quantum efficiency of green LED increases first and then decreases with the increase of p-AlGaN thickness from 0nm to38nm at 35A/cm² current density.When the thickness of low-doped p-AlGaN is 25nm,external quantum efficiency and output power of 520nm green LED reach up to 43.6%and 362.3mW.This is the highest record reported so far.When the thickness is further increased to 38nm,the external quantum efficiency and output power decrease.The reason is that the change of low-doped p-AlGaN thickness can regulate the injection route of holes.With the increase of thickness,the proportion of holes injected via V-shaped increases,which can be injected into deeper quantum wells close to n-layer,making the distribution of holes more uniform,thus improving quantum efficiency.However,accompanied by the increase of p-AlGaN thickness,the side wall thickness of that will also increase,which will have a greater blocking effect on the holes,so the efficiency gets worse.The electroluminescence phenomena at low temperature are different from those at room temperature.With the increase of the thickness of p-AlGaN,not only the emission peak near the p-GaN layer but also the emission peak near the n-GaN layer appear at high current.This further shows that the thickness of low-doped p-AlGaN can regulate the injection path of holes,which is conducive to improving the injection efficiency of holes via V-pits.2.By adjusting the growth pressure of p-AlGaN layer,the effects of different growth pressure on the quality of p-AlGaN crystal,carbon concentration distribution and photoelectric properties of GaN-based green LED were studied.When the growth pressure increased from 100mbar to 200mbar,the C concentration decreased from 2.3×10¹⁸atom/cm³ to 1.2×10¹⁷atom/cm³.The reason is that the growth pressure is low,the activity of molecules and atoms decreases,and it is difficult to react to form CH₄gas molecule.The C atom separated from the surface of epitaxial wafer decreases,which leads to the increase of C concentration.With the increase of growth pressure,both forward-bias and reverse-bias leakage current of devices increase,which can be attributed to the suppression of heat-driven convection and parasitic reactions in the gas phase at lower growth pressure,the improvement of the steepness of the p-AlGaN interface,and the reduction of defects,so the tunneling current decreases.With the decrease of defect,the holes is compensated less by defect,so EQE increases at low current density.Under the current density of 35A/cm²,the external quantum efficiency of green LED increases from 35.7%to 40.7%with the decrease of growth pressure.At high current density,EQE increases obviously,which can be attributed to the fact that the change of C concentration in p-AlGaN layer can also adjust the hole injection pathway.The increase of C concentration can improve the injection efficiency of hole via V pit,so the luminescence efficiency get improved.3.The structure of the last barrier of MQW was adjusted.The 10nm GaN barrier was replaced by 5nm GaN and 5nm Al_0.5Ga_0.5N without doping.The effect of the last barrier Al_0.5Ga_0.5N on the luminescent properties of GaN-based green LED on silicon substrate was studied.Because the last barrier Al_0.5Ga_0.5N is not doped,and is a high Al component,it will form a high resistance region,which has a large blocking effect on the holes,resulting in the increase of the series resistance and the significant increase of the voltage.When the Al_0.5Ga_0.5N layer is added to the last barrier,the external quantum efficiency of the LED is improved at low and high current density.At low current,the enhancement of luminescent efficiency can be attributed to the blocking effect of Al_0.5Ga_0.5N on holes,which makes it difficult for holes to be injected into more quantum wells through c-plane,but in a single quantum well,the hole concentration is higher,the effective current density is higher,so EQE is improved.With the increase of the effective current density,the FWHM increases and the PW decreases.At35A/cm² current density,the external quantum efficiency of green LED increases from39%to 43%.At high current density,the obvious enhancement of luminescence efficiency can be attributed to the fact that the last barrier Al_0.5Ga_0.5N also plays a role in regulating the hole injection pathway.With the increase of the proportion of hole injection from V-shaped pit,it can be injected into deeper quantum wells close to n-layer,thus EQE can be enhanced.Holes can be injected into more quantum wells,the effective density of states will decrease,the band filling effect will alss decrease,so the FWHM decreases.At low temperature,with the increase of current density,there are not only the emission peak with 10nm barrier close to p-GaN,but also the emission peak with 13nm barrier close to n-GaN and sidewall quantum well emission peaks.This further shows that the last barrier Al_0.5Ga_0.5N layer can effectively improve the holes injection efficiency via V-pit.In this paper,the AlGaN layer is studied,and the luminous efficiency of GaN-based green LED on silicon substrate with large V-shaped pit structure is obviously improved.However,in order to further improve the luminescence efficiency,it is necessary to optimize the epitaxy growth parameters and structure design of the AlGaN layer:to reduce the growth rate of the AlGaN layer in V pit,to reduce the Al component of the AlGaN layer in V pit,and to increase the Mg component of the AlGaN layer in V pit,etc.so as to further improve the injection efficiency of holes from V pit.