Design rules of high reflectivity Bragg GaAlN mirrors for 300nm VCSELs
Résumé
Vertical Cavity Surface Emitting Lasers (VCSELs) emitting in the Ultra Violet (UV) wavelength range (300 nm) are of great importance for spectroscopy, storage, or medical applications. GaAlN based material is well appropriate to achieve this objective. We have developed a model that takes into account band-gap, absorption, refractive index of GaAlN material as function of its composition and used it to define the rules for the design of high reflectivity GaAlN based Bragg reflectors. On one hand, the index contrast between the two materials of the Bragg mirror has to be as high as possible to have a limited number of periods and high reflectivity. On the other hand, we are limited by two phenomena to reach the lowest lasing wavelength : the absorption of the material and the lattice mismatch between the two material used in the mirror. In other words, starting with AlN/GaN couple of material that offers the highest index contrast, we have to increase the Ga content in AlN layer to have a better lattice matching with GaN and to increase the content of Al in GaN layer to limit the absorption of the material. This paper will discuss the trade - off that has to be considered to get high reflectivity mirror with the lowest number of period for very short wavelength (300nm).