  We investigate the optomechanical properties of tensile-strained ternary
InGaP nanomembranes grown on GaAs. This material system combines the benefits
of highly strained membranes based on stoichiometric silicon nitride, with the
unique properties of thin-film semiconductor single crystals, as previously
demonstrated with suspended GaAs. Here we employ lattice mismatch in epitaxial
growth to impart an intrinsic tensile strain to a monocrystalline thin film
(approximately 30 nm thick). These structures exhibit mechanical quality
factors of 2*10^6 or beyond at room temperature and 17 K for eigenfrequencies
up to 1 MHz, yielding Q*f products of 2*10^12 Hz for a tensile stress of ~170
MPa. Incorporating such membranes in a high finesse Fabry-Perot cavity, we
extract an upper limit to the total optical loss (including both absorption and
scatter) of 40 ppm at 1064 nm and room temperature. Further reductions of the
In content of this alloy will enable tensile stress levels of 1 GPa, with the
potential for a significant increase in the Q*f product, assuming no
deterioration in the mechanical loss at this composition and strain level. This
materials system is a promising candidate for the integration of strained
semiconductor membrane structures with low-loss semiconductor mirrors and for
realizing stacks of membranes for enhanced optomechanical coupling.
