Study On The Preparation Of GaN-based Weak Polarization Electric Field LEDs On Polar Plane

As an important part of third-generation semiconductors,group â…¢ nitride semiconductors have become the core materials for new-generation optoelectronic devices.LED devices prepared based on group â…¢ nitrides have the advantages of high brightness,high efficiency,and long lifetime,and have been widely used in the fields of lighting,display,and communication.However,the development of group â…¢ nitride LEDs has been constrained by the strong polarization electric field from the device structure.Polarization electric fields greater than 1.5 MV/cm are prevalent in polar plane GaN-based blue LEDs,and can exceed 3 MV/cm in red and deep UV LEDs.This electric field caused by polarization charges in MQWs significantly bends the energy bands,leading to severe Quantum Confinement Stark effect（QCSE）.QCSE results in spatial separation of wavefunctions of electrons and holes,decreasing the radiative complexity of the carriers;enhance the carrier shielding effect,causing the LED light-emitting wavelength to shift as the injection current increases;and increase the complex lifetime of the carriers,which reduces the frequency response characteristics of the LED.These problems lead to a decrease in LED light-emitting efficiency,poor wavelength stability,and reduced modulation bandwidth,which is not conducive to the development of GaN-based LEDs in emerging fields such as Micro-LED micro-displays and visible light communications.Preparation of LED devices on semipolar or nonpolar surface GaN is an effective method to reduce the polarization electric field,but the high substrate cost and crystal quality problems make it difficult to industrialize this method in the short term.Compared with non-polar and semi-polar surface,polar surface GaN-based LEDs have higher crystal quality and mature large-size preparation process.Therefore,it is of great significance to broaden the application field of GaN-based light-emitting devices and promote the development of GaN-based light-emitting device applications if the preparation of weak polarization electric field LEDs is realized on the basis of the mature polar-plane GaN heteroepitaxial process.Quaternary InAlGaN material is the key material to realize weak polarization electric field LEDs on polar plane GaN.Since the quaternary material has higher degrees of freedom,the polarization intensity matching between the quantum well layer and the quantum barrier layer can be achieved by component control,thus eliminating the interface polarization charge and reducing the polarization electric field.However,weak polarization electric field LEDs based on quaternary InAlGaN materials also have some problems in epitaxial growth,material characterization,and structural design.Therefore,there are still challenges in realizing weak polarization electric field LEDs on polar plane GaN.In this thesis,we systematically carry out the research on the preparation of InAlGaN materials,epitaxial growth of InAlGaN-based MQWs,and the preparation of weak polarization electric field LEDs with the research objectives of preparing polar-plane GaN-based weak polarization electric field LEDs and exploring their luminescence performance enhancement and polarization electric field control methods.The main research contents are as follows:1.Design and simulation of InAlGaN-based weak polarization electric field LED structure.By calculating the polarization strength and band gap of the all-component group â…¢ nitride materials,the MQWs structures of In_0.15Ga_0.85N/InAlGaN and In_0.25Ga_0.75N/InAlGaN with weak polarization electric fields were designed.Based on the designed MQWs structure,the influence of the InAlGaN barrier on the luminescence characteristics of the MQWs is studied by simulation.For the In_0.15Ga_0.85N/InAlGaN MQWs,the polarization electric field of the MQWs decreases gradually with the increase of the In component in the InAlGaN barrier layer,which increases the overlap of the electron-hole wave function and the probability of carrier recombination.However,the increase of the In component also causes the band order of the MQWs to decrease gradually,resulting in a decrease in the active region carriers.Under the same injection current conditions,LED devices with low polarization electric fields exhibit a higher radiative recombination ratio and a lower Auger recombination rate,resulting in higher peak internal quantum efficiency（IQE）and lower efficiency droop.2.Preparation of InAlGaN-based weak polarization electric field MQWs.We used the metal-organic chemical vapor deposition（MOCVD）method to study the epitaxial growth of InAlGaN-based MQWs.Quaternary InAlGaN materials are difficult to control and characterize their components due to their more metal composition,and are prone to phase separation under low temperature growth conditions that reduce film quality.After converting InAlGaN into an equivalent quaternary InGaN/AlGaN superlattice,the In component of the InGaN layer and the Al component of the AlGaN layer can be controlled by the growth temperature and the pulse flux time of TMAl,respectively,simplifying the control and characterization process of the components.The InGaN/AlGaN superlattice prepared by pulse growth method exhibits high interfacial quality characteristics,and the atomic distribution of the InGaN layer and AlGaN layer in the superlattice is uniform.Based on the composition control law of InGaN and AlGaN,a superlattice barrier weak polarization electric field MQWs with a polarization electric field intensity of 0.5 MV/cm was prepared,and the electron-hole radiation recombination lifetime was reduced to 3.9 ns.3.Research on performance optimization of weak polarization electric field MQWs.The effects of different pulse growth modes on InGaN,AlGaN thin films and InGaN/AlGaN superlattices were studied.For InGaN films,the use of TEGa and TMIn pulse pass-in methods and the continuous flow of NH₃ can effectively increase the In component of InGaN,and further reducing the TEGa flow rate can increase the amount of In and the crystal quality of the film at the same time,thereby increasing the luminescence intensity of InGaN barrier MQWs.For the InGaN/AlGaN superlattice,the continuous introduction of NH₃ with a low flow rate in the AlGaN growth part can effectively inhibit the generation of N-related defects and In clusters in AlGaN,and improve the luminescence performance of the superlattice barrier MQWs.Based on the InGaN/AlGaN superlattice prepared under different pulse growth conditions,we propose superlattice growth models under different pulse modes,and reveal the influence mechanism of growth conditions on the structural characteristics of the superlattice.4.Modulation of weak polarization electric field MQWs.The influence of In component in the InGaN/AlGaN superlattice barrier on the polarization electric field of MQWs was studied.The polarization electric field in the MQWs decreases with the increase of the In component of the barrier layer,and is close to a linear variation relationship.However,the experimental rate of change of the polarization electric field with the In component is greater than that of the theoretical calculation.It is found that the elastic strain energy accumulated by the superlattice structure is greater than that of the equivalent component InAlGaN film under the same thickness condition,and the strain energy gap gradually increases with the increase of the In component,which leads to the deviation of the theoretical calculation results from the experimental value.Based on this,we propose a method for modifying the polarization electric field of MQWs based on the calculation of elastic strain energy based on superlattice.The calculated rate of change of the modified polarization electric field with the In component is more consistent with the experimental results,which proves that the modified method can be used to quantitatively describe the change of the polarization electric field of MQWs with the In component in the superlattice barrier.5.Research on the preparation of weak polarization electric field LEDs.Firstly,the effect of the thickness of the InGaN layer in the superlattice barrier on the luminous performance of LEDs was studied.Secondly,based on the optimal thickness of the InGaN layer,the effects of GaN barrier,InAlGaN barrier and InGaN/AlGaN superlattice barrier on LED performance were studied by simulation.Superlattice barrier LEDs benefit from lower polarization electric fields and higher wave function overlap,and exhibit weaker droop effect and stronger luminescence at high current densities.Finally,we experimentally compare the photoelectric characteristics of the traditional structure of GaN barrier LEDs and InGaN/AlGaN superlattice barrier weak polarization electric field LEDs.Compared with the traditional structure of LEDs,the peak wavelength of the weak polarization electric field LED has a blue shift of 20 nm,and the peak wavelength blue shift is 1.8 nm in the range of current density of 3～187A/cm²,which has good wavelength stability.At a large injection current density of 250A/cm²,weak polarization electric field LEDs exhibit lower efficiency droop and greater peak broadening.At the current of 60 m A,the-3d B photoelectric modulation bandwidth of weak polarization electric field LEDs is much higher than that of traditional LED devices,and the modulation bandwidth can reach 1 GHz as the device size is reduced to 20μm.Through the optimization of superlattice growth conditions,the peak EQE of weakly polarized electric field LEDs reaches 15%,and the optical output power reaches 25 m W under the condition of 250 A/cm² injection current density,which reflects its application potential in the fields of micro-display and visible light communication.