  High-precision resistance noise measurements indicate that the epitaxial
CoSi$_2$/Si hetero-structures at 150 K and 2 K (slightly above its
superconducting transition temperature $T_c$ of 1.54 K) exhibit an unusually
low 1/f noise level in the frequency range of 0.008-0.2 Hz. This corresponds to
an upper limit of Hooge constant $\gamma \leq 3 \times 10^{-6}$, about 100
times lower than that of single-crystalline aluminum films on SiO$_2$ capped Si
substrates. Supported by high-resolution cross-sectional transmission electron
microscopy studies, our analysis reveals that the 1/f noise is dominated by
excess interfacial Si atoms and their dimer reconstruction induced fluctuators.
Unbonded orbitals (i.e., dangling bonds) on excess Si atoms are intrinsically
rare at the epitaxial CoSi$_2$/Si(100) interface, giving limited
trapping-detrapping centers for localized charges. With its excellent
normal-state properties, CoSi$_2$ has been used in silicon-based integrated
circuits for decades. The intrinsically low noise properties discovered in this
work could be utilized for developing quiet qubits and scalable superconducting
circuits for future quantum computing.
