Research On Nitride Ultraviolet LED Efficiency Enhancement Mechanism And Process

The third-generation nitride wide-band semiconductors represented by gallium nitride（GaN）are now widely adopted in the applications of optoelectronic and electronic devices,with great market competitiveness.By adjusting the Al and In components in Ⅲ-V nitrides,full-spectrum luminescence from infrared to ultraviolet can be realized.GaN-based Ultraviolet（UV）Light-emitting diodes（LEDs）have been extensively studied in the past decades.With the advantages of environmental protection,miniaturization,and long lifetime,UV LEDs are now widely used in sanitization,curing,medical,and anti-counterfeiting applications.Although UV LEDs have been developed significantly,the quantum efficiency is generally low,and the efficiency droop under high current still restricts the further application of UV LEDs in high power range.The main factors affecting UV LEDs include:high dislocation density introduced by heterogeneous epitaxy reduces the internal quantum efficiency,low hole injection efficiency due to low p-type doping efficiency,low light extraction efficiency,and current crowding effect.Therefore,in view of the many challenges faced by UV LEDs,this dissertation improves the light output power and wall-plug efficiency（WPE）of UV LEDs from several aspects,such as crystal quality enhancement of the epitaxial layer,improvement of the current spreading ability of the insertion layer,and enhancement of the carrier injection efficiency by utilizing the polarization effect to introduce energy band modulation.The main research content is as follows:1.The induced nucleation technique is proposed to improve the crystal quality of GaN materials.By implanting low-dose Al ions into the sapphire substrate,the screw dislocation density is reduced from 8.4×10⁷ cm^-2 to 7.0×10⁷ cm^-2 and the edge dislocation density is reduced from 1.1×10⁹ cm^-2 to 5.5×10⁸ cm^-2 compared with the epitaxial GaN on the convention sapphire substrate.The atomic force microscope（AFM）images show that the nucleation islands are more regular and the interfaces between islands are clearer after the proper dose of Al ions is implanted.The consistent orientation of the nucleation islands proves that the ion implantation pretreatment can induce regular nucleation of the nucleation islands,avoiding the extension of the non-c-oriented GaN nucleation islands,and fundamentally reducing the possibility of tilt and twist of the islands which generates dislocations.Al ion implantation into sapphire substrates was investigated to improve the AlGaN crystal quality using the induced nucleation technique.The screw dislocation density was reduced from 8.9×10⁷ cm^-2 to 8.2×10⁶ cm^-2,and the edge dislocation density was reduced from 4.6×10¹⁰ cm^-2 to 3.3×10¹⁰ cm^-2.In addition,the effects of the implantation dose and energy on the quality of GaN crystals were systematically investigated using both C-ion and N-ion implantation on SiC substrates,and improved GaN crystal qualities based on heteroepitaxial nitrides on SiC substrates were presented.A new method of heterogeneous epitaxial nitride based on SiC substrate is proposed,and high-quality GaN epitaxial layers with X-ray diffraction（XRD）FWHM as low as 66 arcsec/96 arcsec of the（002）and（102）planes are obtained,with lower dislocation densities than those reported for epitaxial GaN on SiC substrates.2.The crystal quality,optical properties,and UV LED device performance of epitaxial UV MQWs on sapphire and SiC substrates by induced nucleation technique were investigated.Benefiting from the ion-implantation-induced nucleation,The test results of UV LEDs fabricated based on this technology show that the WPE of UV LEDs based on Al ion-implanted sapphire substrates increases from 30.7%to 37.4%at 20 mA compared to UV LEDs on conventional sapphire substrates.An increase of 11.8%in the light output power of the UV LEDs was achieved by using induced nucleation technology based on SiC substrate.The device measurement results indicates that the performance of UV LEDs can be effectively improved based on the induced nucleation technology.3.A Fe-doped insertion layer is investigated to improve the current crowding effect of UV LEDs.The performance improvement of UV LEDs is realized by inserting a high-resistance Fe-doped layer into the n-type GaN layer of UV LEDs.The test results of the prepared LED devices show that the optical output power of the UV LEDs is increased by 170.6%compared to that of the UV LEDs without Fe-doped layer at a Fe source flow rate of 100sccm,and the optical output power of the UV LEDs without Fe-doped layer is increased by85.0%compared to that of the UV LEDs without Fe-doped layer at a Fe source flow rate of800 sccm.The light distribution of the device shows that the luminous intensity distribution of the device is more uniform when doped with 100 sccm Cp₂Fe,and the calculated carrier distribution results prove that the hole concentration distribution in the quantum well is more uniform after the introduction of the Fe doping layer,which proves that Fe doping can effectively improve the current spreading of the device.4.An in-situ C doping method to improve the current spreading of UV LEDs is proposed.Using MOCVD to grow GaN,the temperature reduction can regulate the C-doping concentration,and the effects of C-doped layers grown at different temperatures on UV LEDs were investigated separately.In addition,the morphology of the C-doped insertion layer grown at reduced temperature and the effect on the crystal quality of the UV MQWs structure were also investigated.The device test results of UV LED structures prepared based on in-situ C-doped insertion layers show that the efficiency of the C-doped insertion layer UV LED structures grown at 800℃is improved by 10.7%.The technique is simple,effective and does not involve a complex process,which can be adopted in UV LEDs to alleviate the current crowding effect.5.A crystal quality improvement study was carried out for the growth of N-polar GaN epitaxial layers,based on different substrate epitaxy of N-polar GaN epitaxial layers,the improvement of N-polar GaN crystal quality was realized,and the PL test results showed that the PL intensity of N-polar UV MQWs was effectively enhanced.In addition,to address the problem of high background carrier concentration in N-polar materials,a dual superlattice UV LED structure is proposed and designed based on the N-polar surface.The advantages of N-polar LEDs compared to Ga polarity are comparatively analyzed,by employing a p-type superlattice electron-blocking layer as well as a p-type superlattice hole-injection layer in the N-polar LEDs.The results show that the optical output power of dual-superlattice N-polar UV LEDs is enhanced by 28.5%,and the internal quantum efficiency droop is reduced from 25.1%to 9.8%.In addition,to address the problem of large forward voltage in deep-ultraviolet LEDs,the study analyzes the composition of the forward voltage drop in the forward state of N-polar deep-ultraviolet LEDs,determines the source of the barrier introduced by the p-type insertion layer in the convention structure that leads to the high forward voltage,and proposes and designs a step-gradient Al-component insertion layer structure to address this,with the WPE is 2.88 times higher than the conventional device.6.The method of Al In GaN quaternary alloy insertion layer for the improvement of N-polar UV LED current crowding is investigated.By adjusting the In component as well as the Al component,lattice matching with AlGaN template can be realized,which reduces the difficulty of epitaxial growth.The simulation results show that the optical output power of the UV LED device increases by 30.9%compared with the conventional UV LED structure.The calculation results of carrier distribution show that the Al In GaN insertion layer can effectively introduce electron barriers,which promotes the lateral distribution of carriers and increases the effective radiative composite in MQWs.This approach avoids the negative effect of lattice mismatch on the epitaxial material compared to the commonly used AlGaN insertion layer.7.A nanopatterned nonpolar GaN template formed by Ni metal annealing etching is proposed to reduce the dislocation density of nonpolar GaN.To address the problems of high dislocation density and poor crystal quality in the epitaxial layer of nonpolar GaN,a nano-patterned GaN template is formed by Ni metal annealing etching,and the secondary epitaxy on the template significantly reduces the dislocation density in the nonpolar of GaN.The XRD results show that the FWHM of the（11-20）-plane rocking curves on the c-axis as well as the m-axis are significantly reduced,and the TEM images indicate that the dislocation density in the epitaxial layer based on the pattered GaN template is significantly reduced,and the reduction of dislocations in the epitaxial layer is also verified by the TEM measurements.The significant reduction of dislocations in GaN is attributed to the lateral epitaxial process introduced by the pattern.In addition,a lateral superlattice structure is proposed and designed to address the difficulty of p-type doping of nonpolar GaN.Simulation results show that p-type doping of the lateral superlattice structure can be effectively achieved by using the polarization electric field introduced by the c-plane perpendicular to the growth direction,and at the same time,a two-dimensional hole gas is formed at the AlGaN/GaN interface,which can be directly injected into the active region,realizing the suppression of the efficiency droop and the enhancement of the light output power of the UV LEDs in the nonpolar surface under the high current.The simulation results show that the light output power of the LED with the lateral superlattice structure is enhanced by 28.1%,and the WPE is improved by 31.8%.