Epitaxial Growth And LED Fabrication Of InGaN/GaN Quantum Wells With Different Polarities

Group-III nitride semiconductor materials represented by Ga N are widely used in the field of optoelectronic due to their excellent properties.Representative ones are Ga N-based light-emitting diodes（LEDs）,lasers（LDs）,photodetectors,and high electron mobility devices（HEMTs）.There are two different polarities of Ga N materials,gallium polarity and nitrogen polarity,due to the lack of inversion symmetry along the c-axis of the Ga N crystal structure.The research on gallium-polar（Ga-polar）Ga N-based materials is relatively mature,and Ga N-based light-emitting devices are generally fabricated based on Ga-polar materials currently.However,for Ga-polar light-emitting diodes（LEDs）using In Ga N/Ga N quantum wells（QWs）as active region.Due to the lower decomposition temperature of the In-N bond,the required growth temperature of the In Ga N QW layer is lower with the increased In content of the QWs,and the corresponding material quality is significantly reduced.The increase of the In content in the QWs also leads to an increase in the lattice mismatch between the In Ga N QW and the Ga N quantum barrier（QB）,which intensifies the quantum confinement Stark effect and thus reduces the luminous efficiency of the LED.Besides,In Ga N-based LEDs have a serious efficiency drop under high current density,namely“efficiency droop”.Therefore,it is necessary to investigate the structure optimization of Ga-polar In Ga N/Ga N QWs structure and LED devices to improve the performance of the LED.Compared with Ga-polar materials,nitrogen-polar（N-polar）materials have some unique superiority.For instants,N-polar In Ga N has higher In incorporation efficiency,enabling the fabrication of In Ga N/Ga N QWs at higher growth temperatures to improve the crystal quality and luminous efficiency.In addition,the reverse polarization electric field direction in N-polar LEDs compared with Ga-polar ones results in an increase in carrier injection and a suppression of electron overflow,which improves the radiative recombination efficiency and effectively alleviates efficiency droop.However,limited by the quality of N-polar Ga N-based materials,N-polar LEDs fail to realize their theoretical advantages.Therefore,further study on N-polar materials and Ga N-based LEDs is needed.Focusing on the current issues of In Ga N-based LEDs,this paper carried out research on the preparation of In Ga N/Ga N QWs and LEDs from two aspects of gallium polarity and nitrogen polarity,using metal-organic chemical vapor deposition（MOCVD）technology.The specific research contents are as follows:1.The fabrication and study of Ga-polar In Ga N/Ga N QWs.Firstly,we study the influence of different Al N and Al Ga N buffer layer structures on the crystal quality,surface morphology,and strain property of Ga-polar Ga N films.Ga-polar Ga N film with high quality was obtained,the polarity of which was determined to be gallium polarity by KOH solution wet etching.Based on the optimal Ga N film as template,the influence of growth pressure on the structural properties,optical properties,and surface morphologies of Ga-polar In Ga N/Ga N QWs was studied.The results show that increasing the growth pressure is beneficial to improving the In incorporation efficiency in the QWs.A higher growth pressure can lead to a stronger localization effect.In addition,the size and density of V-shaped pits in the QWs were reduced with the increase of growth pressure.2.The fabrication and study of Ga-polar In Ga N-based green LEDs.The effect of Al Ga N/Ga N superlattices（SLs）interlayer on the properties of Ga-polar In Ga N/Ga N QW LEDs was studied.Our results demonstrate that the electroluminescence（EL）intensity and external quantum efficiency（EQE）of LED with Al Ga N/Ga N SLs interlayer are significantly higher than LED without SLs interlayer.Meanwhile,the efficiency droop effect of LED is remarkably alleviated by inserting Al Ga N/Ga N SLs interlayer.We carried out simulation design research and experimental verification of Ga-polar In Ga N/Ga N green LEDs’electron blocking layer（EBL）.Firstly,LEDs with different EBL structures were simulated by the software of APSYS.Compared with the normal Al Ga N EBL with fixed Al content of 0.3,the use of an EBL with Al content linearly changed from 0.3 to 0 can effectively improve the overflow barrier of electrons and the injection efficiency of holes.Therefore,the radiative recombination efficiency of carriers in the active region is enhanced.In order to verify the simulation results,we epitaxially fabricated Ga-polar In Ga N/Ga N green LEDs with the same structures as used in the simulation.EL measurement results indicate that the LED with graded Al Ga N EBL exhibits an improved EQE,and the efficiency droop effect of which is weakened,which is consistent with the simulation results.3.Epitaxial growth of N-polar Ga N and In Ga N films.We epitaxially grew N-polar Ga N by a two-step high-temperature growth method.The prepared N-polar Ga N film exhibits a flat surface with screw and edge dislocation densities of 3.45×10⁷ cm^-2and8.08×10⁸ cm^-2,respectively.The residual stress in the N-polar Ga N film is only 0.025GPa.The effect of pulse growth mode on the characteristics of N-polar In Ga N films was studied.The results show that using the pulse growth mode of alternately injecting Ga and N sources while injecting In source continuously can effectively improve the mobility of group-III adatoms,thereby eliminating the zincblende-phase inclusions and reducing the surface roughness of the N-polar In Ga N film,while reducing the screw dislocation density of N-polar In Ga N.4.The fabrication and study of N-polar In Ga N/Ga N QWs.We studied the influence of Ga N QB growth temperature,hydrogen flow rate in QB growth carrier gas,In Ga N QW growth temperature,and the period number of QWs on the characteristics of N-polar In Ga N/Ga N QWs.The results show that increasing the QB growth temperature from 845°C to 945°C can eliminate the zincblende-phase inclusions in the N-polar QWs,thereby significantly improving the surface morphology and luminescence properties of the QWs.Introducing an appropriate amount of H₂ into the N₂ carrier gas for the growth of QB can effectively suppress the formation of hexagonal hillocks and improve the luminescence properties of the QWs.However,the triangular step edge density on the surface of QWs will increase when introducing excess H₂into the carrier gas.Within a certain growth temperature range,the emission wavelengths of N-polar In Ga N/Ga N QWs exhibit an inversely proportional linear relationship with the QW growth temperature,and is significantly longer than that of Ga-polar QWs grown at the same temperature owing to a higher In incorporation efficiency in N-polar In Ga N.Increasing the period number of N-polar QWs leads to an increased fluctuation of the triangular step edge on the surface,which increase the amount of In atoms gathered at step edges and form localization states with deeper energy level.Therefore,the light emitted from the deep and shallow energy levels in the same QWs is macroscopically separated,and then the dual-wavelength emission phenomenon occurs.The luminescence properties and surface morphology of the N-polar QWs are optimal when the period number is 2.5.The fabrication and study of N-polar In Ga N-based LEDs.N-polar In Ga N/Ga N QWs LEDs were fabricated with the QWs before and after optimization as active regions,respectively.Due to the improved crystal quality and surface morphology of the QWs after optimization,the series resistance of optimized LED is reduced from 111Ωto 72Ω.The EL measurement results show that the peak EQE of optimized LED is higher than that of LED before optimization by～92%.This is mainly because the luminous properties of the N-polar QWs have been improved after optimization,and the carrier injection efficiency was improved by the reduction of LED series resistance.At the same time,due to the enhanced confine ability for carriers of QWs after optimization,the efficiency droop effect of LED is greatly alleviated.