Watching the formation of the GaP/Si(100) heterointerface in situ

III-V/Si(100) heterointerfaces are essential for future tandem solar cells. We watch their formation in situ with reflection anisotropy spectroscopy (RAS) during chemical vapor deposition and benchmark RAS to LEED and XPS. An optical anisotropy evolves during GaP nucleation on Si(100), agreeing well with that of the buried heterointerface. P and Si photoemission lines exhibit a chemically shifted component, which quantitatively corresponds to one monolayer. We attribute this to Si-P bonds at the heterointerface. We prepare ultrathin, atomically well-ordered, single-domain GaP epilayers. Sub-monolayer amounts of background residuals diffusing on Si(100) prior nucleation highly impact the formation of the heterointerface.