**4. Conclusions**

We showed the amplification of the SERS signal of nanodisks on a gold film by a simple addition of a silicon layer for the composition of the nanodisks. The sensitivity of these hybrid SERS substrates has been studied and compared to the results in literature obtained for regular gold nanostructures on a gold film. The EF values reached with the suggested SERS substrates (5 × 10<sup>7</sup> < EF < 6 × 107) are larger than EFs cited above. We remarked that the experimental EF values have the same behavior as those obtained with the E<sup>4</sup> model by using a generic analytical approach and numerical simulations. Our hybrid Au/Si disk-shaped nanoresonators on gold film can be optimized in order to obtain even higher enhancement factors. The obtained SERS substrates offer the possibility of being incorporated on a lab-on-chip for a label-free sensor of biochemical species in the nearest future.

**Author Contributions:** G.B. conceived the research, performed the experiments, wrote the whole paper, prepared the original draft, edited the draft; A.I. performed the simulations, wrote the Section 2.4, edited the draft; A.K.S. suggested analytical theory, performed the simulations, wrote the Section 2.4, edited the draft.

**Funding:** This research received external funding from the Russian Foundation for Basic Research (Grant No. 17-08-01448 and 18-58-00048), Russian Science Foundation (grant No. 16-14-00209), the Presidium of RAS (Program 22).

**Conflicts of Interest:** The authors declare no conflict of interest.
