Short Wavelength GaN-Based High Power Solid State Lighting Sources

With the development of technology and the expansion of application requirements,GaN based solid state lighting sources are developing towards shorter wavelength and higher optical power density.However,due to the difficulty of p-type doping,the intrinsic optical anisotropy and the strong polarization field in the MQWs active layers,the external quantum efficiency as well as the output power of the AlGaN based light emitting diodes（LEDs）with short wavelength are still insufficiently low,which hinder the utilization of AlGaN-based deep ultraviolet（DUV）LEDs in massive application areas.Meanwhile,the performance of GaN based laser diode（LD）is limited by the strong quantum confined Stark effect,composition and interface fluctuation in InGaN MQWs active layers,and the catastrophic optical damage（COD）et al.To improve the performances of Short Wavelength GaN based solid state lighting sources,we mainly focus on the theoretical design of epitaxial structure and device structure,experimental growth and device fabrication comprehensive.The major results obtained are as follows:3D-FDTD calculations were carried out firstlyto investigate TE-and TMpolarizedlight propagation behaviors in DUV LEDs with inclined sidewalls coated with SiO₂/Al.As compared with structure with vertical sidewalls,a maximum enhancement of～2.1 times is observed for TE-polarized light due to the light reflection.While TMpolarized light extraction efficiency can be enhanced by一 5 times.These results suggest that such structure is much more favorable to extract the TM-polarized lights,and has great potential to significantly enhance the total LEE of AlGaN based DUV LEDs.Then such structure was applied to fabricate two types of high-voltage（HV）DUV LEDs.At 1000 mA,the light output power as high as 118.4 mW and 145.7 mW were achieved in HV-LED1 constructed with 2×2 LEDs array and HV-LED2 constructed with 3x3 cell LEDs array,respectively.Moreover,the efficiency droop is greatly alleviated to about 5.5%in HV-LED2,owing to the low injection current density.These results demonstrate that fabrication of HV-LEDs is a good solution for achieving high lightoutput for AlGaN-based DUV LEDs.Then the effect of MQWs structure on the homogeneity of spontaneous luminescence and quantum efficiency of the high power InGaN blue LD is numerically studied.The simulated results demonstrate that,the transition energy is much uniform and the increasing rate of IQE at the initial injected current is larger in LD structure with less QW number,suggesting that the LD structure with less QW number is more likely to produce lasing,and have lower threshold current.Considering the dramatic decrease of IQE in 1QW,we believe that LD with 2QWs is the optimal choice.Accordingly,the edge emitting LD structure with 2QWs in the active region is grown on GaN substrate.To investigate the facet coating effect on the COD level,three types of LDs are coated with different materials by e-beam evaporation and ECR deposition techniques.The characterization results confirm that the single blue LD coated with theAl₂O₃ and Al₂O₃/Ta₂O₅films by ECR deposition have the highest COD level,due to the better crystal quality and interfacial quality of the coated films.The LD starts to operate at 1.04 kA/cm2 with slope efficiency of 1.8 W/A and the wavelength of 444.9 nm.As a result,the output power as high as 8.040 W is achieved at 5.5 A.Finally,the asymmetric double quantum well structure with high In-content was further designed and grown to compensate the radiation recombination wavelength effect of the difference between polarization field and injection current density.Then,based on the analysis of the main factors of high Q value of laser,designment of resonator structure with small mode volume,and optimization of the mirror reflectivity,the high-precision resonator structure with high Q value is fabricated for green LD.According the spectrums of different injection currents,it can be seen that in the highprecision resonator,with the increasing of current,the state energy is more matched with the low-level transition of laser medium,and the coupling is stronger,the lowenergy laser transition is more likely,and the peak wavelength gradually red-shift..Therefore,the green laser with lasing wavelength of 509 nm and optical power of 1.004 W is achieved.