Indium-free AlGaN based distributed Bragg reflectors (DBRs) in the UV spectrum are known to have very low reflectivities due both to the low refractive index contrast as well as limitations imposed by the critical thickness of AlGaN alloys (tensile strain of ~2.41% for AlN on GaN). Near-bandedge excitonic resonances influence the real part of AlGaN’s dielectric function, which sharply increases its refractive index as the photon energy approaches the bandgap. Furthermore, heavy doping (Si: 1020 cm-3) can modify the plasma frequency of AlGaN, leading to a reduction in its refractive index. Hence, judiciously choosing the high index material to exploit excitonic resonances and using heavy doping to reduce the refractive index of the low index material can enhance the index contrast and enable growth of epitaxial DBRs with higher reflectivities. We have demonstrated this technique both experimentally and by simulations for wavelengths ranging from 240-370 nm. Typically, over 50 epitaxial pairs are needed to achieve a mirror whose reflectivity exceeds 99%, but this can be shrunk down to 20-30 epitaxial pairs by depositing silver/aluminum underneath the epitaxial DBR stack. Silver and aluminum exhibit >90% reflectivity at the AlGaN/metal interface between wavelengths ranging from >360 nm and 180-670 nm respectively. A thinner DBR stack also reduces the thermal resistance, which would allow the VCSEL to achieve higher peak output powers, and simultaneously reduce overall tensile strain.
- distributed Bragg reflectors
- ultraviolet laser diode
- vertical cavity surface emitting laser
ASJC Scopus subject areas
- Atomic and Molecular Physics, and Optics
- Condensed Matter Physics
- Electrical and Electronic Engineering