  Gallium phosphide (GaP) has recently received considerable attention as a
suitable material for building photonic integrated circuits due to its
remarkable optical and piezoelectric properties. Usually, GaP is grown
epitaxially on III-V substrates to keep its crystallinity and later transferred
to silicon wafers for further processing. Here, an alternative promising route
for the fabrication of optomechanical (OM) cavities on GaP epitaxially grown on
nominally (001)-oriented Si is introduced by using a two-step process
consisting of a low-temperature etching of GaP followed by selective etching of
the underneath silicon. The low-temperature (-30 $^o$C) during the dry-etching
of GaP hinders the lateral etching rate, preserving the pattern with a
deviation between the design and the pattern in the GaP layer lower than 5 %,
avoiding the complex process of transferring and bonding a GaP wafer to a
silicon-on-insulator wafer. To demonstrate the quality and feasibility of the
proposed fabrication route, suspended OM cavities are fabricated and
experimentally characterized. The cavities show optical quality factors between
10$^3$ and 10$^4$, and localized mechanical resonances at frequencies around
3.1 GHz. Both optical and mechanical resonances are close to those previously
reported on crystalline GaP structures. These results suggest a simple and
low-cost way to build GaP-based photonic devices directly integrated on
industry-standard Si(001) photonic wafers.
