The formation and evolution of Si nuclei on the Si(100) surface at 600 and 700 °C were observed in a microwave electron cyclotron resonance plasma chemical vapor deposition system, using both real‐timeinsitusingle wavelength and spectroscopic ellipsometry combined with high‐resolution cross‐sectional transmission electron microscopy and secondary ion mass spectroscopy. The deposited Si layers are epitaxial, and decidedly different nucleation behavior is seen at 600 and 700 °C. The experimental ellipsometry results were compared with simulations and the results show that temperature has a profound effect on the initial nucleation and growth, and the different interface structures that are observed are attributable to impurities.