Optical and Photoemission Studies of Amorphous Semiconductor Interfaces and Ultra Thin Layers.

In-situ polarized optical reflectivity and UV photoemission spectroscopy were used to investigate the formation and microscopic structure of individual amorphous semiconductor interfaces made by plasma CVD. The a-Si:H/a -SiO_{rm x}:H and a-Si:H/a-SiN_{rm x}:H interfaces were found atomically abrupt except for the SiO_{rm x} and Si interface which is graded over 3-4A due to the initial plasma oxidation. Extra H is incorporated in the interface region to compensate for the large lattice mismatch and pacify interface defects. The interface roughness scale was determined to be on the order of ~ 10A normal to the film. Complete energy band diagrams for these interfaces including the valence and conduction band offsets were obtained from photoemission and yield spectroscopy measurements. The quantum size effect associated with valence and conduction states in ultra thin amorphous semiconductor layers was also separately explored. The Si 3p valence electrons in a-Si:H were found to be strongly localized and contribute negligibly to the quantum size effect. Thus, the observed quantum size effect in ultra thin layers was attributed to delocalized conduction states, in agreement with results of yield spectroscopy on amorphous semiconductor layers with reduced dimensionality.