Layerwise reaction at a buried interface

X-ray diffraction was used to monitor the in situ reaction of Pd deposited on Si(111) at room temperature. An ordered silicide forms spontaneously beneath a poorly ordered overlayer. It is commensurate and strained at low coverage, but relaxes to an unstrained state above a critical thickness of 18 . During both phases of growth sustained intensity oscillations are seen that correspond to a layerwise consumption of the substrate at the buried interface.