Research On UVA GaN-based Vertical Cavity Surface Emitting Laser

The vertical cavity surface emitting laser(VCSEL)is featured with excellent output beam characteristics including circular symmertry beam profile and small divergence angle..It is capable of be two-dimensional device array integration,on-chip testing,and efficient coupling with optical fibers.In addition,VCSEL has the advantages of low power consumption,low threshold,high modulation rate,and high temperature stability.These properties make VCSEL promising for the application fields such as optical communication,optical sensing,laser lighting and display,biomedicine,and optical atomic clocks.The GaN-based material system belongs to the third generation of semiconductors,and the emission wavelength can be continuously tuned covering from the deep ultraviolet to infrared.Therefore,they are excellent materials for light-emitting devices.GaN-based VCSELs have been a hot area of research for recent years.At present,great progresses have been made in GaN-based VCSELs emitting in the visible light band,and device performance has approached the level of practical application.However,in the ultraviolet band,GaN-based VCSELs are still facing challenges such as low reflectivity of the distributed Bragg reflector(DBR),high absorption losses,and the difficulty in growing AlGaN with high Al composition.In this thesis,we designed the epitaxial material and device structure for the UVA band GaN-based VCSELs.Low-threshold UVA band GaN-based VCSELs with periodic gain structure,gradual cavity length,tunable emission wavelength and covering from 375 to 410 nm are successfully fabricated.The main work includes:1.The differences between the nitride DBR and the dielectric film DBR are analyzed,and the dielectric DBR with high reflectivity is designed and fabricated in UVA band by using TiO2/SiO2.The longitudinal mode cahracteristics and the standing wave field distribution inside the cavity are analyzed.To increase the gain enhancement factor of the device,a periodic gain structure is designed and a phase adjustment layer is inserted to the DBR to make sure the antinode of standing wave overlap with the active region.The threshold gain of the UVA VCSEL was also calculated.2.The InGaN quantum well epitaxial wafer with periodic gain structure was grown by MOCVD,and the fabrication process of GaN-based VCSEL in the UVA band was developed.The key technologies in device fabrication and characterization were explained in detail.3.The optical characteristics of InGaN/GaN multiple quantum well epitaxial wafers with periodic gain structure and common structure were characterized.The internal quantum efficiency(IQE)of the two structures were estimated to be 50.63%and 31.48%,respectively,indicating that the periodic gain structure is featured with higher crystal quality.The device performance of the UVA band GaN-based VCSEL was studied.The lasing wavelength of the devices can be modulated by the cavity length,and covers the spectral range from 375 to 410 nm.The threshold energy of the device is as low as 390 kW/cm2,which is the lowest value in the UVA band VCSEL with dual dielectric DBRs.