Using spectroscopic ellipsometry we have studied a series of virgin and germanium‐implanted crystalline silicon samples oxidized at 900 and 1000 °C. The thickness of the oxide in the virgin and implanted sample was accurately determined and found to be in agreement with the results previously obtained by single‐wavelength ellipsometry. The interface structure between the silicon substrate and the oxide layer of all the samples was investigated in detail. We find that the interposing layer between the oxide and the silicon substrate is composed of a thin layer of crystalline germanium‐silicon alloy. A brief discussion on the strain and quantum confinement of the interface that may affect the calculated composition is also presented. The interface thickness was found to be slightly larger to that obtained by high‐resolution transmission electron microscopy measurement for samples implanted at higher doses. A plausible explanation for the discrepancy is provided.