| Quantum well structures are the basis and bricks of many kinds of GaN/AlGaN based optoelectronic device due to their controllable quantum confinement effect and mature fabrication technique.On one hand,quantum well infrared detector and terahertz detector can be made utilizing the intraband transition in the quantum well.On the other hand,based on the interband transition,ultraviolet light-emitting diode,laser and detector can be fabricated by using GaN/AlGaN quantum well structures.However,due to the intrinsic material property and crystal structure of wurtzite III nitrides,GaN/AlGaN quantum well structures have a very unique characteristic,that is polarization effect,including spontaneous polarization and piezoelectric polarization.This polarization can induce a very strong polarization field in the quantum well,which can be as high as several MV/cm.The transition energy and probability of electrons in the quantum well will be impacted by this polarization-induced electric field,which will significantly affect the performance of the optoelectronic devices.Considering the importance of polarization effect in GaN based optoelectronic devices,this thesis will focus on the polarization field,systematically and detailedly investigate the effect of polarization field on the transition,how to manipulate the polarization field and the optical property of the GaN/AlGaN quantum well structures.The main research results are as follows:(1)A theoretical calculation model of GaN/AlGaN quantum well structure has been created via effective mass and envelop function approximation method.In this model,the behaviors of electron and hole are described by the Schrodinger equation,and the potential is determined by the Poisson equation.The spontaneous and piezoelectric polarization effects can be taken into account separately or simultaneously.The Schrodinger and Poisson equations have been self-consistently solved by the finite differential method.The wavefunctions and energy levels of electrons and holes in the quantum well structures have been obtained by the calculations,which lays a basis for the study of the electronic transitions and optical property of the quantum well structures.(2)A step quantum well structures have been designed to obtain THz intersubband transitions,and the effects of the polarization field on the intersubband transitions have been investigated detailedly through theoretical calculations.The results show that,by manipulating the polarization field with inserting a step quantum well,a flat potential profile can be obtained in the as-tilted quantum well.Thus,the intersubband transitions can reach THz range.Meanwhile,it has been found that the intersubband transitions can reach THz range only when the width of the quantum well is between 1.7 and 4.3 nm,and the width of step quantum well is larger than 7 nm.In addition,it has been found that the intersubband transition energy is minimum and within THz range when the Al composition of quantum barrier is twice of that of step quantum well.(3)We have found that the intersubband transition based second order nonlinear effect is very strong for the GaN/AlGaN quantum well structures.Through comparable investigations,it has been found that the second order nonlinear optical susceptibility of step quantum well structure can be enhanced by 10 times compared with conventional quantum well structure,which is very beneficial to the nonlinear optical devices such as frequency-double optical element.In addition,it has been found that a double-resonance three energy level system can be achieved in step quantum well structure via the manipulation of the polarization field,which can further increase the second order nonlinear optical susceptibility to the highest value of 4×10-6m/V.(4)Due to the absence of polarization field in nonpolar a-plane GaN/AlGaN quantum well,in theory the electron and hole wavefunctions will be fully overlapped,leading to a highest radiative recombination rate.Thus,several a-plane GaN/AlGaN quantum well structures with different well width and Al composition of barrier have been grown on r-plane sapphire substrate by MOCVD.The temperature dependent PL measurements indicate that the peak of the exciton emission from the quantum well first red-shifts then blue-shifts and finally red-shifts with increasing the temperature,this is the so called "S-shape" phenomenon,which suggests the localization of the excitons.The localization energy has been obtained by Varshni fitting of the temperature dependent PL peak variation.It has been found that,the stronger the quantum confinement effect,the larger the localization energy and the higher the emission efficiency of the quantum well.Combining the transmission electron microscopy(TEM)characterizations and the theoretical calculations,the localization of excitons has been attributed to the potential fluctuation induced by the uneven widths of the quantum well.(5)The step quantum well structure has been utilized for the first time in the multi-quantum wells active region of GaN based ultraviolet LED(UV LED).The theoretical calculations indicate that the polarization field can be effectively manipulated and reduced by inserting a step quantum well,leading to a significant improvement of the electron and hole wavefunction overlap.GaN/AlGaN multi-quantum well structures with different quantum wells have been grown by metal organic chemical vapor deposition(MOCVD).The internal quantum efficiency(IQE)of the samples has been estimated by temperature dependence photoluminescence(PL)measurements.The results show that the IQE of step GaN/AlGaN quantum well structure is as high as 82.4%,enhanced by more than 2 times compared with conventional quantum well structure.Low temperature time-resolved photoluminescence(TRPL)measurements show that the radiative recombination lifetimes of carriers in quantum well structure are in the range of 230 to 300 ps.It has been found that the radiative recombination lifetime of carriers in step quantum well structure is shorter than that in conventional quantum well structure,which demonstrates that the radiative recombination rate can be enhanced by inserting a step quantum well,thus leading to a higher IQE. |
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