II-VI-based Bragg mirror looks good for future blue VCSELs

Distributed-Bragg-reflector gratings fabricated in II-VI semiconductor materials by researchers at the University of Bremen (Bremen, Germany) and the Romanian Academy (Bucharest, Romania) could be the basis for future II-VI-based blue and UV emitters such as vertical-cavity surface-emitting lasers (VCSELs). Grown by molecular-beam epitaxy on buffered gallium arsenide substrates (fully lattice-matched to the substrate material), the gratings show very high quality and normalized reflectivity exceeding 99% over the blue-violet spectral range.

The high-index layers are made of zinc magnesium sulfide selenide (ZnMgSSe), while the low-index layers are a short-period (1.7 nm) superlattice containing MgS and zinc cadmium selenide (ZnCdSe). The reflector contains 21 pairs of layers. Elemental proportions in the layers had to be finely tuned to avoid lowering structural quality and causing catastrophic absorption (Mg content is limited by the potential of forming rock-salt-like structures). Two compositions were tested that used either ZnS_0.06Se_0.94 or Zn_0.72Mg_0.28S_0.29Se_0.71 as the low-index material: the first reflects best from 450 to 480 nm; the second works best from 420 to 450 nm. An ellipsometric model accurately reproduced the measured reflectivity spectra. Contact Sebastian Klembt at [email protected].