The origin of unintentionally introduced n‐type conduction at the interface of epitaxially grown layer‐InP substrate is identified. From the relation between the sheet carrier concentration and the etching depth, an n‐type conducting layer was found at the epitaxial layer‐substrate interface. The sheet carrier concentration and the sheet Si concentration at the surface of the InP substrate, which was obtained by secondary ion mass spectrometry analysis, agreed well. As a result, we determined that the Si atoms caused n‐type conduction. To make clear that origin of the Si atoms, the following possibilities were investigated. One possibility was the vessel made from silicon dioxide (SiO2), which was used for etching the substrates, but it was determined not to be the cause because no significant difference was observed between a teflon or polypropylene vessel. To investigate the contamination from the air, we used metalorganic chemical vapor deposition to prepare a sample composed of a InPcapping layer regrown on another InP layer after exposure to air. Because the Si atom sheet concentration was proportional to the time of exposure to ambient air, we conclude that the Si atoms come from air and are adsorbed on the InP substrate. We also found that the Si atoms adsorbed on InP could be removed by heating the substrate in phosphine (PH3) atmosphere just before growth. Heating makes it possible to suppress n‐type conduction.