Insituphosphorous‐doped Si layers have been deposited in three different ways: (i) by low‐pressure chemical vapor deposition (CVD) at 615 °C resulting in polycrystalline Si; (ii) by radio‐frequency plasma‐enhanced CVD at 250–300 °C resulting in either epitaxial Si, or (iii) in amorphous Si which is subsequently crystallized at 600 °C, resulting in epitaxially regrown Si. From secondary ion mass spectrometry and spreading resistance profile measurements, the P appears in cases (i) and (iii) to be diffused into the underlying substrate across several hundreds of nanometers, much further than normal diffusivities would forecast. An explanation of this anomalous behavior, based on precipitation of P causing supersaturated self‐interstitial concentrations is proposed.