Investigation Of High Quality GaN Based Green Micro-Cavity And Light-emitting Devices

GaN based micro cavity(MC)light emitters including resonate cavity LED(RCLED)and vertical cavity surface emitting laser(VCSEL)are featured with high modulation rate,good directivity,high emission efficiency,and excellent spectral purity when compared with the traditional light emitting sources.By adjusting the alloy composition,the emission of GaN based MC light emitter can cover a wide spectral range,from ultraviolet to near infrared.These advantages make it promising for applications such as solid-state lighting,optical storage,visible light communication,biomedical and high-resolution mobile projectors.Benefiting from the large exciton binding energy and oscillator strength,GaN based MC light emitter is also a good platform for the study of cavity quantum electrodynamics(CQED)a at room temperature,and is rationally expected to be promising candidates for novel opticalelectronic devices including highly efficient single photon source and polariton lasers.However,there are still several challenges in GaN based MC light emitters,including the fabrication of electrically injected GaN MC with high Q factor,lateral optical confinement inner the cavity,thermal dissipation,and the low efficiency in green emitting devices.In respect of the issues,a systematical study on structural design,process developing,material growth,device fabrication and characterization of green emitting GaN based MC light emitter were carried out in this thesis.The main work is as follows:1.The basic device principle of GaN MC light emitter was analyzed.The reflectivity and the penetration depth of the DBRs based on different material systems,as well as the electric field distribution inner the cavity were calculated.The effect under different coupling condition between the active layer and the electric field of the cavity mode was discussed.2.Green emitting GaN based RCLED with high Q factor was fabricated.The optical loss inner the cavity was greatly reduced by optimizing device structure and fabrication processes.Q factor of as high as 6039 was realized for an electrically injected green emitting RCLED,being the highest value in electrically injected GaN based MC to the best of our knowledge.In addition,by applying a buried AIN lateral guiding layer,3D confinement of optical field was realized in the cavity.And a systematical measurement on the 3D confined optical states was conducted.3.Tunable GaN based RCLED was fabricated.The modulation effect on the cavity mode emission property by the gain enhancement factor was studied theoretically and experimentally.By applying this modulation effect,together with InGaN quantum dots(QDs)active region with a broad emission spectrum,tunable GaN based RCLED was successfully fabricated.The emission spectrum of the device can be tuned by injected current from yellow-green(564 nm)to violet(435 nm).The tuning range turned out to be 129 nm,which is the largest value in GaN based MC light emitter.4.Low threshold green emitting InGaN QD VCSEL was successfully fabricated.Threshold current density of GaN based VCSELs and its dependence on several important structural parameters were studied theoretically,and green emitting InGaN QDs with high quality were grown.With InGaN QD active region and an optimized device structure,room temperature continuous wave lasing in green region was successfully realized.The emission wavelengths of the devices range from 479 nm to 565nm,covering most of the "green gap",and the threshold current density is as low as 0.66 kAcm-2.5.Thermal dissipation in GaN based VCSEL was systematically investigated.We calculated the temperature distribution in GaN based VCSELs with three typical structures.The dependences of thermal resistance on structural parameters including cavity length,DBR,mesa size,and contacting electrode were systematically studied and effective methods to improve thermal dissipation were proposed.This work provides a better understanding of the thermal characteristics of GaN based VCSEL and is useful in optimizing the structure design and improving the device performance.The main innovations of this paper could be summarized as follows:(1)3D optical confinement was successfully realized in an electrically injected GaN based FP microcavity by introducing a buried AlN guiding structure.3D confined optical states were clearly observed and lateral losses of the optical field inner the cavity was effectively reduced by this method.(2)Tunable RCLED was realized without the necessary to fabricate a tunable microcavity,which is usually realized by complicated MEMS processes.By adopting QD active region with tunable gain,together with the modulation effect on the cavity mode emission property by the gain enhancement factor,tunable GaN based RCLED with large tuning range was realized.This method greatly reduced the difficulty in fabrication of tunable RCLED.(3)By utlizig InGaN QDs as active region,room temperature CW lasing of electrically injected VCSELs emitting in"green gap" were realized.(4)Effective method to improve thermal dissipation in GaN based VCSELs was proposed,which is important to realize GaN based VCSELs with large output power.