Research On Structure Optimization And Efficiency Improvement Technology Of GaN-based Green LED On Sapphire Substrate

As the third-generation wide-bandgap semiconductor material,Gallium nitride(Ga N)has broad application space in the field of optoelectronic devices.Light emitting diodes(LEDs)are known as the fourth-generation light source.In the context of global climate change and energy tension,the emergence of LEDs has greatly alleviated the earth’s energy problems.Nowadays,Ga N-based blue LEDs have reached a high performance level.But the external quantum efficiency(EQE)of green LEDs,which the human eye is most sensitive to,is significantly lower than that of blue LEDs,a phenomenon known as the"green gap".The preparation of high efficiency Ga N-based green LEDs is crucial to the development of high luminous efficiency semiconductor lighting and display chips.Sapphire is the most widely used substrate in the fabrication of Ga N-based LEDs.However,there exist large mismatches between sapphire and Ga N,resulting in a large number of dislocations.Also,due to the poor conductivity of sapphire,the devices prepared are mostly lateral devices with current congestion.In this dissertation,aiming at the problem of low luminous efficiency of Ga N-based green LEDs on sapphire substrates,the layer structures of green LEDs were designed and optimized.The dislocation annihilation mechanism of Ga N grown on patterned sapphire substrates(PSS)was systematically investigated and high quality Ga N was grown.Finally,the stress regulation of green multiple quantum wells(MQWs)structure was studied.The main research contents are as follows:1.Optimization of the pre-well structure of the green LED.Firstly,the nano-patterned In Ga N film was fabricated on Ga N template by self-assembled nickel(Ni)nano-mask to enhance the luminescence of green In Ga N/Ga N MQWs structure.The In Ga N nano-pattern benefited the light output by helping photons generated within the MQWs to find the escape cones.The experimental results showed that the In Ga N layer with 10%In component and the 3 nm thick Ni film were suitable to prepare the nano-patterned In Ga N layer for the green MQWs structure.Then,taking advantage of the in-situ doping C impurities when growing Ga N by metal organic chemical vapour deposition(MOCVD),an in-situ C-doped Ga N layer was inserted in green LEDs to improve the current crowding effect.As a consequence,at a current density of 20 A/cm~2,the EQE of green LED with the C-doped layer grown at 850°C was improved by 12%.2.The effects of indium(In)source flux and In Ga N well thickness on the green In Ga N/Ga N MQWs structure were investigated.During the growth of In Ga N well layer,TMIn sources with different flow rates were used.The larger the In source flux,the more the In combination in the QW,the longer the luminescence wavelength,the worse the crystal quality of sample and thus the weaker the photoluminescence intensity of the green MQWs structure.Green In Ga N/Ga N MQWs structures with different thicknesses In Ga N layers were grown.The sample with 1 nm thick In Ga N had worse crystalline quality but stronger luminescence,because the wider well confined more carriers,thereby increasing radiative recombination.The above statement was verified by simulating the band structure and carrier distribution of green MQWs structures with different thicknesses In Ga N.3.Optimization of the p-type layer of green LEDs.Green In Ga N/Ga N MQWs structures without low temperature(LT)p-Ga N and with LT p-Ga N layers with different thicknesses(16.8 nm,24 nm,36 nm)were grown.It was found that the LT p-Ga N could act as a protective layer for the active region and improved the crystal quality of green MQWs structure.The thicker the layer was,the better the effect was.But when LT p-Ga N reached a certain thickness,the improvement effect became less obvious.At the same time,too thick a LT p-Ga N layer would increase the polarization effect,hinder the injection of holes and reduced the luminescence.Finally,24 nm was the optimum thickness of the LT p-Ga N layer.Then the electron blocking layer(EBL)was optimized.The energy band simulation results showed that the large barrier height difference between In-rich In Ga N well and Ga N barrier can be used to trap electrons.Therefore,the simulated electron concentration in MQWs of green LED without EBL did not decrease much compared with the LED with Al Ga N/Ga N superlattice EBL.At the same time,without the obstruction of hole injection by EBL,the hole concentration in MQWs of green LED was increased,and the luminous efficiency was enhanced.Finally,the simulation results were verified by experiments.4.Study on the dislocation annihilation mechanism of Ga N grown on PSS.Ga N films were prepared on sputtered Al N/PSS using the“tsunami”growth modes.Under“tsunami”growth mode,Ga N grew into the shape of a truncated pyramid that promoted dislocations originated from flat area to bend towards the inclined planes then annihilate.And it was noteworthy that the propagation of dislocations in the Ga N grains on pattern surface was inhibited.By optimizing MOCVD growth process,high quality Ga N epilayer was obtained,and the full width at half maximums(FWHMs)of Ga N(002)/(102)X-ray diffraction(XRD)rocking curves were 58/90 arcsec.This was the smallest FWHM value reported so far for Ga N grown by MOCVD on sapphire substrates.Finally,green LEDs were fabricated on the obtained high quality Ga N epilayer,with a WPE of 21.1%at an injection current of 20 m A.5.Stress regulation studies on green In Ga N/Ga N MQWs structure.Double(a blue and a green)In Ga N/Ga N MQWs structures were grown on unstressed bulk Ga N and compressive stressed Ga N/PSS template.It was found that the green MQWs grown on Ga N/PSS template was of poor quality with many new dislocations,and its green luminescence was weak and inhomogeneous.While the opposite was true for green MQWs on bulk Ga N.Meanwhile,the blue MQWs on both substrates were of good quality,with uniform blue luminescence and similar brightness.Green In Ga N/Ga N MQWs structures were grown on planar and vicinal sapphire substrates.The experimental results showed that using vicinal substrate can relieve stress in green MQWs structure and was beneficial to the annihilation of dislocations.Moreover,the steps of vicinal substrate promoted the formation of a large number of V-shape MQWs,and increased the luminous area of MQWs.Thereby,the photoluminescence intensity of green MQWs structure on vicinal substrate was enhanced by about an order of magnitude.