  We present high-performance megahertz micromechanical oscillators based on
freestanding epitaxial AlGaAs distributed Bragg reflectors. Compared with
dielectric reflectors, the low mechanical loss of the monocrystalline
heterostructure gives rise to significant improvements in the achievable
mechanical quality factor Q while simultaneously exhibiting near unity
reflectivity. Experimental characterization yields an optical reflectivity
exceeding 99.98% and mechanical quality factors up to 20 000 at 4 K. This
materials system is not only an interesting candidate for optical coatings with
ultralow thermal noise, but also provides a promising path toward quantum
optical control of massive micromechanical mirrors.
