Spectroscopic ellipsometry studies on ultrathin hydrogenated amorphous silicon films prepared by thermal chemical vapor deposition

Ultrathin amorphous silicon films prepared by the thermal chemical vapor deposition (CVD) have been investigated using a spectroscopic ellipsometer. The analysis of ultraviolet-visible (UV-VIS) spectroscopic ellipsometric data reveals the morphology of the ultrathin films. To determine the optical functions of such films from ellipsometric data, a new parameterization, i.e., the Sellmeier law with four Lorentz oscillators, has been successfully introduced. A direct correlation has been made between the new parameters and the change of optical functions with the thickness of ultrathin a-Si:H films. By the analysis of ellipsometric:data, it has been shown that the formation of dense Si matrices with low structural disorder is possible when the thickness of a-Si:H is more than about 8 nm; and the film with a thickness of less than around 3.5 nm develops voids.