| GaN-based light-emitting diodes(LEDs)show advantages such as small size,fast response,strong impact resistance,high luminous efficiency,long lifetime and environmental friendliness.In the past 20 years,they have been widely used in full-color display,backlight screen,general lighting and communication because of these advantages and become the fourth generation of artificial light source after incandescent,fluorescent and high-intensity gas discharge lamps.As an excellent material for LED solid-state lighting device,GaN and related alloy(such as AlGaN,InGaN)demonstrate advantages such as high thermal conductivity,high melting point,radiation resistance,chemical inertness and high hardness.In addition,they all have direct band gap and their band gap can be altered by adjusting the contents of group III elements.For example,theoretically,by adjusting the In content of InGaN ternary alloy,the band gap of InGaN ternary alloy can be changed from 0.7 eV of InN to 3.4 eV of GaN,which corresponds to the whole spectral region from near-infrared to near-ultraviolet.Therefore,as active region,InGaN/GaN multiple quantum well(MQW)structure has been widely applied in LED solid-state lighting devices.At present,in the short wavelength range(such as blue),InGaN/GaN MQW-based LEDs show the luminous efficiency above 80%due to the lower In content and better crystalline quality of epitaxial layers.However,in the long wavelength range(such as green,yellow and red),InGaN/GaN MQW-based LEDs usually present a significant reduction in luminous efficiency,which is called "green gap".The main reason for this phenomenon is that obtaining green InGaN/GaN MQW-based LEDs warrants the increase of the In content in the well layer of InGaN/GaN MQWs;however,this will further lead to the two main problems:1)In the InGaN well layer,due to large lattice mismatch(about 11%)and size difference between GaN and InN,the increase of the In content will result in the stronger content fluctuation or phase separation in the InGaN well layer.This will produce more non-radiative recombination centers and further reduce the luminous efficiency;(2)In the InGaN/GaN MQW active zone,there is large a lattice mismatch or thermal mismatch between the Gan barrier layer and InGaN well layer,which induces the generation of a polarization field.Under the influence of the polarization field,the energy band will tilt,leading to the spatial separation between electrons and holes,and the decrease in the wave function overlap of electrons and holes.This phenomenon makes the red shift of the emission peak energy,the increase of linewidth,and the decrease of luminous efficiency.This phenomenon is called quantum-confined Stark effect(QCSE),which will increase with the increase of the In content.Therefore,the temperature dependence and excitation power dependence of luminescence(photoluminescence or electroluminescence)characteristics of InGaN/GaN MQW structure with a high In content are investigated to clarify the generation,transport and recombination mechanism of carriers in this structure.The research has great significance for further improving the growth process of this structure,solving the "green gap" problem and thus realizing high-performance white InGaN/GaN MQW-based LEDs.Therefore,to realize the preparation of the InGaN/GaN MQW structure with a high luminous efficiency and long wavelength,metal-organic chemical vapor deposition(MOCVD)is used as the preparation method in this dissertation;based on the traditional growth process,improving growth process and optimizing the structural parameters are performed to achieve the stepwise increase its luminous wavelength and improve its luminous efficiency.In this dissertation,firstly,the influence mechanism of "growth interruption" during the InGaN well layer growth on the above characteristics(including luminous efficiency and luminous wavelength)was investigated;Subsequently,the impact of In Volatilization suppression technique and the gradual variation of In content in each InGaN well layer on the luminous characteristics were investigated;finally,on the basis of comprehensive analysis,using energy band cutting method,two types of InGaN/GaN multiple quantum well structures(trapezoidal quantum well and quasi trapezoidal quantum well)with a AlN low-temperature interlayer were prepared.Their optical properties were measured and compared.Through research and discussion,we have successfully prepared the long-wavelength(near red)InGaN/GaN MQW structure with an internal quantum efficiency greater than 20%.Furthermore,the influence mechanism of the sample structure on its luminescence characteristics was explored.In addition,based on the experience accumulated in this study,we also put forward the measurement method for "the luminescence measurement of the band gap in semiconductors" and its basic conditions to be met:The contents of this research main include:(1)Effect of InGaN growth interruption on photoluminescence properties of InGaN/GaN MQW structure.Two different long-wavelength(near green light)InGaN-based multiple quantum wells(MQW)samples,with and without two-step growth interruption(TSGI)during InGaN well layer growth,were prepared by MOCVD method.Their dependences of photoluminescence(PL)spectra on temperature(6-300 K)and excitation powertemperature(0.001-70 mW)were measured,and the influence of two-step growth interruption on optical properties of InGaN/GaN MQW is studied.The results show that,compared to the sample without growth interruption,the sample with two-step growth interruption has higher peak energy,narrower linewidth,weaker localization effect and weaker QCSE.This can be explained by the fact that a TSGI can reduce the average In content and result in a more homogeneous distribution of In atoms in the MQWs due to In re-evaporation and diffusion of In atoms from indium-rich to indium-poor regions(even GaN barrier layer)during the interruption duration,and thus causing a less significant In composition fluctuation and a weaker polarized electric field in the MQWs.It is also consistent with the excitation power dependence of internal quantum efficiency(IQE),in which the MQW structure with TSGI has a higher internal quantum efficiency compared with the MQW structure without TSGI,due to it having fewer non-radiative centers.However,although the TSGI method mentioned above can improve the luminescence efficiency of InGaN well layers,it is not conducive to expanding their emission wavelength towards long wavelengths.(2)Influence of In Volatilization on Photoluminescence in InGaN/GaN Multiple Quantum WellsIn order to overcome the phenomenon of shorter luminescence wavelengths caused by the volatilization and diffusion of In atoms in the above research,two different long-wavelength(green light)InGaN/GaN MQW samples,without and with an indium(In)volatilization suppression technique(IVST),were fabricated.The dependencies of the PL spectra upon temperature at different excitation powers were investigated.The results indicate that an IVST can not only increase the In content but also suppress the phase separation caused by volatilization of that In incorporated in the well layers.In addition,compared with sample B with IVST,which contains one phase structure,sample A without IVST,which contains two separate phases(i.e.an In-rich phase and an In-poor phase),exhibits higher IQE at low excitation power and lower IQE at high excitation power.The former is mainly attributed to the stronger In-rich phase-related localization effect of Structure A,because the In-rich phaserelated emission dominates the PL spectra of Structure A at a low excitation power;the latter is mainly due to the In-poor phase-related weaker localization effect of Structure A,because the In-poor phase-related emission dominates the PL spectra of Structure A at high excitation powers due to the saturation of the localized states in the In-rich phase.This conclusion is also consistent with the results of atomic force microscopy(AFM),that is,the surface roughness of the two samples is almost the same,and IQE is mainly determined by the localization effect.Therefore,the growth process used in this study(IVST)not only prevents the volatilization/diffusion of In,enhances the localization effect,but also reduces the density of non-radiation centers,thus not only increases the light-emitting wavelength of the corresponding structure,but also increases its IQE.(3)Effect of the method of In content change on the optical properties of InGaN/GaN MQWIn order to further explore other effective preparation techniques to prevent shorter wavelength caused by in volatilization/diffusion and lower internal quantum efficiency caused by QCSE,two different long-wavelength(neargreen light)InGaN/GaN MQW samples were grown by MOCVD method.They were fabricated by gradually increasing and decreasing the indium content in each InGaN well layer along the growth direction,respectively.Their temperature(6-300 K)and excitation power(0.001-70 mW)dependence of PL spectra were measured,and the influence of the method of In content change on optical properties of InGaN/GaN MQW was studied.The results of the experiment indicate that the MQWs with a gradually decreased In content have a lower peak energy,stronger phase separation,more robust carrier localization effect,and a stronger quantum-confned Stark effect(QCSE)compared to those with a gradually increased In content.These characteristics are attributed to a fact that,the latter sample(sample B)has a higher average In content in each InGaN well layer compared to the former as its volatilization of In was less significant.Additionally,the terminal region of the well layer in sample B with a lower indium content acts as a quasi-capping layer.This conclusion is also consistent with the HRXRD results.Therefore,the growth process used in this study(growth process with a gradually decrease indium content in each InGaN well layer along the growth direction)not only suppresses the volatilization/diffusion of In,enhances the localization effect,but also reduces the spatial separation of electrons and holes within the well layer,thus not only increases the luminescence wavelength of the corresponding structure,but also increases its IQE.(4)Influence of interface structure in the active region on photoluminescence in InGaN/GaN quantum wellsBased on previous research,two InGaN/GaN multiple quantum wells(MQWs)with AlN low temperature interlayer(between the GaN barrier and InGaN well layers)and different band shape were prepared by MOCVD method.One with a trapezoidal InGaN QW(including initial In composition gradient layer+intermediate layer with constant In components+terminal In composition gradient layer)and the other with a quasi-trapezoidal InGaN QW(without terminal In composition gradient region).Meanwhile,their XRC,AFM and dependences of PL spectra on the excitation power and temperature were acquired and studied.These measurements reveal almost the same dislocation density,a smoother surface,a stronger carrier localization effect,a smaller QCSE,and a higher IQE of the quasi-trapezoidal QWs compared with that of the trapezoidal QWs.This phenomenon is a result of the fact that:the characteristic structure of the quasi-trapezoidal InGaN QWs,cannot only reduce strain in the active region due to the absent of terminal In composition gradient region and its related effective straincompensated effect of the AlN LTILs,but also avoid diffusion of In atoms from the high In region(i.e.the intermediate region of the well layer including rich In clusters)into the terminal ICGR.The former decreases the piezoelectric polarization fields that arise due to lattice mismatch between the barrier and well layers.,the latter suppresses the degradation of the Inrich clusters and the generation of non-radiative centers.That is,the "quasi-trapezoidal QW structure" adopted in the research not only suppresses In diffusion and enhances localization effects,but also compensates the stress on the well/barrier interface,reduces QCSE,and thus not only increases the emission wavelength of the corresponding structure(up to the near red wavelength range),but also improves its IQE.(5)The discuss on the luminescence measurement of the band gapBased on the accumulated experience of the research involved in this thesis,several representative InGaN/GaN multiple quantum well structures are taken as the research objects,and the test conditions that need to be met for the luminescence measurement of the optical band gap of the InGaN well layer are discussed in depth.Since the InGaN well layer is a multielement alloy and is subject to stress from the GaN barrier layer,there are not only impurity/defect-related non-radiation centers in the well layer,but also localized potential fluctuation induced by composition fluctuation and quantum confinement Stark effect(QCSE)induced by polarization field.Therefore,in order to obtain a more accurate optical band gap of the InGaN well layer,the luminescence measurement should at least meet the following test conditions:1)eliminating the influence of impurities/defects on the emission process;2)eliminating the influence of the localized centers on the emission process;3)eliminating the influence of QCSE on the emission process. |
