Development of High-Reflectivity and Antireflection Dielectric Multilayer Mirrors for AlGaN-Based Ultraviolet-B Laser Diodes and their Device Applications

Fabrication techniques for high-reflectivity (HR) and antireflection (AR) dielectric multilayer mirrors for AlGaN-based ultraviolet-B (UV-B) laser diodes are developed. After depositing several dielectric materials and evaluating their complex refractive indices via ellipsometry, it is determined that SiO2 as a low-refractive-index material and Ta2O5 as a high-refractive-index material are appropriate material combinations in the UV-B region at a light wavelength of approximate to 300 nm due to their low extinction coefficients and large refractive index difference. Based on these results, HR mirror with a reflectance of >99% in the UV-B region at a center wavelength of 310 nm and an AR mirror with a reflectance of approximate to 8% in the same wavelength range are demonstrated; a mirror with reflectance that is almost equal to the designed value is demonstrated. Furthermore, these mirrors are coated on the respective edge surfaces of the UV-B laser diodes. A comparison of the characteristics of the same device before and after edge coating reveals a reduction in the threshold current density of laser oscillation, whereas, simultaneously, an increase in slope efficiency and external differential quantum efficiency is observed. The improvement of these device characteristics, estimated from the above reflectance values, is confirmed to be almost theoretically explainable.