For pt.II see ibid. vol.73, p.7690 (1993). In order to investigate the interface between polycrystalline-silicon (poly-Si) and crystalline silicon (c-Si), which is of crucial importance for the passivation of high-voltage devices, an infrared diagnostic method has been developed which is based on a modified attenuated total reflection configuration. This interface is shown to consist of silicon oxides (mainly SiO/sub 2/) in the monolayer range with a thickness of 7+or-2 AA. The interpretation of the experimental results is based on a direct comparison of the infrared reflectivity spectrum of the interface to be studied with that of a reference sample containing a 100-AA thick SiO/sub 2/ interface layer, as well as on extensive computer calculations. Such calculations have been performed for a three-layer system as well as for a simplified system consisting of a single absorbing layer sandwiched between two transparent half-spaces. The latter system can be solved analytically and provides detailed insight into the physics of the interaction of light with the vibrational excitations of the interface layer. The existence and properties of such a silicon oxide interface layer are compatible with secondary ion mass spectrometry experiments (oxygen segregation at the interface) and its thickness is in excellent agreement with the thickness of the amorphous interface layer observed by transmission electron microscopy.