We have studied the structural properties of tensile-strained Si layers grown on polished Si_1−xGe_x virtual substrates (x = 0.2, 0.3, 0.4 and 0.5) as a function of their thickness. We have used specular X-Ray Reflectivity to gain access to the sSi layer thickness (in-between 10 and 37 nm for sSi on Si_0.8Ge_0.2 and Si_0.7Ge_0.3, in-between 6 and 28 nm for sSi on Si_0.6Ge_0.4 and in-between 5 and 24 nm for sSi on Si_0.5Ge_0.5). The surfaces of the sSi layers (grown at 700 °C, 2660 Pa with SiH₂Cl₂) are characterized by a slight resurgence of the surface cross-hatch. We obtained surface root mean square roughness in-between 0.3 and 0.4 nm. The interfaces in-between sSi and SiGe are abrupt, as illustrated by High Resolution–Transmission Electron Microscopy and by the Ge decay profile in Secondary Ions Mass Spectrometry: 0.73–1.06 nm/decade. UV-Raman spectroscopy has shown that the tensile stress in the sSi layers decreased very slightly as the sSi layer thickness increased. Mean values, 1.32, 1.87, 2.58 and 3.15 GPa, are close to the ones theoretically expected for fully pseudomorphic sSi layers on Si_0.8Ge_0.2, Si_0.7Ge_0.3, Si_0.6Ge_0.4 and Si_0.5Ge_0.5 are close to the ones theoretically expected for fully pseudomorphic sSi layers. They are also very similar to those experimentally found in X-Ray Diffraction for sSi on Si_0.8Ge_0.2 and Si_0.7Ge_0.3: 1.32 GPa and 1.95 GPa. We have also studied, thanks to Secco etching, the defects present in these sSi layers. The Threading Dislocations Density (TDD) slightly increases from 1 up to 3 × 10⁵ cm^− 2 as the sSi thickness increases. It is however rather close to the TDD associated to the SiGe VS underneath (1–2 × 10⁵ cm^− 2). Meanwhile, the Line defects (i.e. stacking faults and/or misfit dislocations) Linear Density (LLD) increased from 0 up to 400, 700, 800 and 300 cm^− 1 as the sSi layer thickness on Si_0.8Ge_0.2, Si_0.7Ge_0.3, Si_0.6Ge_0.4 and Si_0.5Ge_0.5 increased from 5–10 up to 24–37 nm. Were we to adopt as critical thickness the ones for which the LLD exceeds 50 cm^− 1, we would find values around 12 nm, 10.5 nm, 6.5 nm and 9 nm for Ge contents equal to 0.2, 0.3, 0.4 and 0.5, respectively. It would seem that the LLD depends on the Ge concentration but also on the surface roughness and apparently the surface preparation prior to re-epitaxy.