  We systematically investigated the physical properties of amorphous Mo$_{\rm
x}$Si$_{1-x}$ films deposited by the magnetron co-sputtering technique. The
critical temperature $T_C$ of Mo$_{\rm x}$ Si$_{1-x}$ films increases gradually
with the stoichiometry x, and the highest $T_C$=7.9 K was found in Mo$_{\rm
0.83}$ Si$_{0.17}$. Beyond $x$=0.83, preformed Cooper pairs and superconducting
domains persist in the films, despite the superconducting state with perfect
zero-resistivity is absent. The thick films of Mo$_{\rm 0.83}$ Si$_{0.17}$ show
surprising degradation in which the onset of zero-resistivity is suppressed
below 2 K. The thin Mo$_{\rm 0.83}$ Si$_{0.17}$ films, however, reveal robust
superconductivity even with thickness d$\leq$1 nm. We also characterized wide
microwires based on the 2 nm thin Mo$_{\rm 0.8}$ Si$_{0.2}$ films with widths
40 and 60 $\mu$m, which show single-photon sensitivity at 780 nm and 1550 nm
wavelength
