The structural properties of Si/Mo/Si triple layers with Mo-layer thicknesses from 34.0 nm down to nominal 0.17 nm are studied by X-ray reflectometry (XRR), X-ray diffraction at glancing incidence and transmission electron microscopy (TEM). The Mo-layers, sputter deposited under various conditions, are sandwiched by 15 nm (bottom) and 18 nm (top) a-Si:H layers, which are prepared at 470 K by PCVD or magnetron sputtering in an especially developed three-chamber deposition apparatus, which is described. Depending on the deposition conditions, the discontinuous-continuous layer transition detected by XRR occurs at nominal thicknesses of the intermediate layer between 0.4 and 1 nm. In the sub-1 nm range, the density of the intermediate layer increases with thickness and reaches a maximum, which amounts to 94% of the bulk value, around 1 nm. As found by TEM, layers with nominal thicknesses around 0.7 nm consist of ellipsoidal nanoclusters in contradiction with the results of XRR. With the increase of thickness to 20 nm, dense Mo-layers (93% of bulk) with smooth interfaces (RMS roughness typically 1 nm) are grown for low pressure sputter deposition. Mo-layers prepared at increased argon pressure have a density of 62% and a porous structure with nanocolumns of typical 10 nm diameter.