  We report on MoSi-based superconducting nanowire single-photon detectors on a
gallium arsenide substrate. MoSi deposited on a passivated GaAs surface has the
same critical temperature as MoSi deposited on silicon. The critical
temperature decreases slightly on depositing MoSi directly on the native oxide
of GaAs. Hence, MoSi works well as a thin-film superconductor on GaAs. We
propose that the amorphous structure of MoSi ensures compatibility with the
GaAs matrix. Superconducting nanowire single-photon detectors (SNSPDs) are
fabricated with MoSi on GaAs using a meander-wire design. The SNSPD metrics are
very similar to those of devices fabricated with the same procedure on a
silicon substrate. We observe a plateau in the response-versus-bias curve
signalling a saturated internal quantum efficiency. The plateau remains even at
an elevated temperature, 2.2 K, at a wavelength of 980 nm. We achieve a timing
jitter of 50 ps and a recovery time of 29 ns. These results point to the
promise of integrating MoSi SNSPDs with GaAs photonic circuits.
