ABSTRACT Tunable bimetallic Ag-Au nanoparticles (Ag-AuNPs) and hot spot regions were created through the reduction of Ag-Au ions onto nanohillock silicon surfaces. A set of as-prepared textured silicon substrates with different nanohillock topographies was synthesised by a wet potassium hydroxide (KOH) chemical etching process on crystalline silicon for incubation times from 4 to 10 min. These structures were explored as a substrate for optimizing the plasmonic characteristics of Ag-AuNPs/nanohillocks Si surface-enhanced Raman scattering (SERS) heterostructures. The goal of this paper is to create sufficient SERS heterostructures with a high enhancement and exceptional reproducibility. The nanohillock Si substrate was employed to create nearly homogeneously distributed AgAuNPs and hot spot regions with extraordinary specific surface area (S.S.A.) values. Plasmonic characteristics of the created Ag-AuNPs were investigated and analysed based on the surface features of the substrate via atomic force microscopy (AFM), field emission scanning electron microscopy (FE-SEM), X-ray diffraction spectroscopy (XRD), energydispersive X-ray spectroscopy (EDS) and Raman measurements. Tunable Ag-AuNPs sizes and hot spot regions were synthesized by controlling the incubation time of the wet KOH chemical etching process. The density and size distribution, and hence the plasmonic characteristics of the Ag-AuNPs and hot spot regions, were improved significantly with increasing surface roughness and average hillock height of the substrate and specific surface area of the Ag-AuNPs. A high enhancement factor of 3.7×1013 with a minimum reproducibility deviation of about 4% was attained in 10−14 M rhodamine 6G dye for an 8 min etching incubation time.