Study On Epitaxial Structure And Growth Of Gan-based LED With High Indium Composition

In this thesis,we studied the metal-organic chemical vapor deposition(MOCVD)epitaxial growth technology of high indium composition gallium-nitride(GaN)-based green LED.Firstly,we studied the epitaxial growth process of high In composition InGaN material.The influence of growth parameters on In composition and doping of p-type doping InGaN(p-InGaN)and the growth process of InGaN/GaN multiple quantum well(MQW)with high In composition were investigated.Then,we optimized the epitaxial structure of GaN-based green LED.We designed different epitaxial structure for GaN-based green LED to improve the quantum efficiency and light output power.The main contents of this thesis are listed as follows:1.We studied the epitaxial growth of p-InGaN and InGaN/GaN MQW with high In composition.The influence of growth temperature,flow of TMIn and pressure on the indium composition,hole concentration and mobility of p-InGaN were investigated.The concentration and mobility of holes in p-InGaN is relative to In composition.The higher the In composition is,the higher the hole concentration is and the lower the mobility is.Compared with p-InGaN with fixed In composition,p-InGaN with graded decreasing In composition could obtain higher hole concentration and mobility at the same time.As for the InGaN/GaN MQW with high In composition,the photoluminescence(PL)spectra and In composition of InGaN wells with different growth temperature and thickness were compared.In order to improve the In composition in the quantum well and maintain the high internal quantum efficiency,the ultrathin quantum well is proposed.We grew InGaN/GaN MQW with an In composition of 33.5% at 770? by decreasing the thickness of InGaN wells from 3 nm to 1.1 nm.The PL peak wavelength and internal quantum efficiency of the sample is 550 nm and 53.8%,respectively.2.We demonstrated the enhancement of light extraction efficiency(LEE)of GaN-based green LED through a nano-micro complex patterned sapphire substrate(NMCPSS).The NMCPSS was prepared by inductively coupled plasma(ICP)etching conventional microscale PSS with nickel nano-particles as the mask.Then GaN-based green LED epitaxy was grown on NMCPSS by MOCVD.The effect of NMCPSS on light output power of GaN-based green LED was investigated experimentally and numerically.The light output power at 20 mA of the LED grown on NMCPSS is 28.6% higher than that of LED grown on PSS.The experimental and simulation results show that nano-micro complex PSS can further promote the LEE of GaNbased green LED compared with conventional PSS.3.Aiming at the high polarization electric field intensity in InGaN/GaN MQW,the pndoped quantum barriers(PNBs)and the graded MQW were proposed.The PNBs was prepared by selectively doping the GaN barriers and used to replace the conventional undoped GaN barriers(UBs).According to the experimental results,the light output power of LED with PNBs increases by 20.2% compared to conventional LED with UBs.The peak wavelength of LED decreases from 521.8 nm to 505.9 nm by replacing UBs with PNBs.The simulation data demonstrates that PNBs can effectively suppress the electric field intensity in the wells and enhance the electron–hole wave function overlap,thus improving the light output power.The capacitance of LED with PNBs also decreases by 37% compared to conventional LEDs.The graded InGaN/GaN MQW has incremental indium composition and diminishing well width along the growth direction.The green LED with graded MQW shows a 13.1% enhancement in light output power and shorter peak wavelength compared to the conventional LED.Besides,the electroluminescence(EL)spectral shows that the blueshift of green LED with graded MQW is weaker than that of conventional LED when the injection current increasing.These results are attributed to the effective relief of strain in active region by graded MQW.Experimental and simulation results indicate that the graded MQW can effectively eliminate the strain in active region and thus improve the performance of GaN-based green LED.4.In order to improve the hole injection in GaN-based green LED,we proposed a graded indium composition p-InGaN conduction layer and a graded Al composition p-type AlGaN(pAlGaN)conduction layer to replace the p-AlGaN electron blocking layer(EBL)and the p-GaN layer.The indium composition of the p-InGaN layer decreases from 10.4% to 0% along the growth direction.The Al composition of the p-AlGaN layer decreases along the growth direction.These two kinds of graded composition p-InGaN and p-AlGaN conduction layer effectively enhance the hole injection and improve the radiative recombination rate,leading to higher internal quantum efficiency and light output power.Besides,they can avoid the thermal damage in MQW due to the low growth temperature.Compared to the conventional GaN-based green LED,the light intensity of the LED with a graded indium composition p-InGaN layer shows a 13.7% enhancement,and the light output power of the LED with a graded Al composition p-AlGaN layer shows a 69.2% enhancement.