Study On Carrier Dynamics Of InGaN/GaN Multiple Quantum Well Structure Under Stress Control

As the representative material of the third-generation semiconductor,Ga N and its ternary alloy In Ga N have gradually occupied a leading position in the lighting field due to their excellent photoelectric properties.At the same time,this series of materials also have huge development potential and application prospects in the fields of aerospace,medical and wireless communications.With the deepening of study,the study on blue LEDs with InGaN/GaN multiple quantum well(MQW)structure as the core has shifted from material growth and device structure improvement to exploring internal mechanisms to solve problems such as efficiency droop,thereby further improve device performance.Previous studies have proved that the quantum confinement Stark effect(QCSE)is one of the main reasons that affect the internal quantum efficiency of InGaN/GaN MQW blue LEDs.The cause is mainly due to the built-in piezoelectric field caused by the lattice mismatch between the epitaxial film and the substrate.Improving the QCSE situation obviously needs to compensate or weaken the built-in piezoelectric field.Based on this,in this paper,two thin film samples with different structures were prepared by etching on the basis of commercial InGaN/GaN MQW epitaxial thin films as the study object.The measurement and analysis of these two thin film samples with different structures under different stress conditions were carried out using multispectral technology.And made an in-depth study of its internal carrier dynamics mechanism.This work has guiding significance for the performance improvement of InGaN/GaN MQW LEDs and the preparation of new devices in the future.The research of this thesis mainly has the following two aspects:(1)Based on the InGaN/GaN MQW film epitaxially grown on a sapphire substrate,a film sample with a micro-pillar array structure etched on its surface is the study object.Using photoluminescence spectroscopy to analyze and study the difference between the micro-pillar array structure sample and the control sample.Among them,the internal quantum efficiency of the micro-pillar array structure sample is affected by a large number of defects introduced by the etching process,which is significantly lower than that of the control sample.The photoluminescence intensity at room temperature is significantly weaker than that of the control sample.The light extraction efficiency is improved due to the change in surface structure that provides more light exit channels and destroys the total reflection conditions,which shows that the photoluminescence intensity is stronger under low temperature conditions.Through the measurement of the carrier decay lifetime and the analysis of the integrated intensity of the photoluminescence,it is found that the non-radiative recombination thermal activation energy of the carriers becomes larger after etching,which means that the dominant non-radiative recombination form has changed.These results reveal that the etching through the quantum well layer releases part of the stress inside the quantum well layer,which improves the QCSE situation to a certain extent.However,the concentration of non-radiative recombination centers formed by defects introduced in the etching process is too high,and the improvement of QCSE is not enough to make up for its negative effects,resulting in a decline in LED performance.(2)Based on the InGaN/GaN MQW film epitaxially grown on the Si substrate,the thin film samples that were etched away from the Si substrate and transferred to the flexible substrate is the study objects.Use spectroscopy to analyze it.The results of Raman spectroscopy show that external stress can effectively control the internal stress caused by lattice mismatch in the device.The application of external stress enables the film sample to obtain a maximum increase of 19% in photoluminescence intensity.However,this modulation is not monotonous.As the external stress increases further,the photoluminescence intensity will decay again.Through the analysis of the carrier decay lifetime,it can be known that the modulation principle of this external stress is that the new polarized charge generated by the external stress compensates the built-in piezoelectric field of the device.It restores the energy band of the inclined state without external stress to the horizontal state,improves the spatial separation of the wave functions of electrons and holes,and increases the probability of radiation recombination.It is manifested by the decrease of carrier decay lifetime and the increase of photoluminescence intensity.However,as the external stress continues to increase,over-compensation will occur.That is,the excessive accumulation of polarization charges forms a new reverse polarization piezoelectric field.As a result,the energy band is tilted again in the opposite direction to the initial tilt direction,and the wave functions of electrons and holes are separated in space again,the probability of radiation recombination decreases,the carrier decay lifetime increases again,and the photoluminescence intensity decreases.The above results reveal how the internal stress in the InGaN/GaN MQW structure affects the transition dynamics of carriers,which in turn affects the internal quantum efficiency of the device,and has guiding significance for the efficiency improvement of LED devices in the future.