Influence of Ar impurities on optical refractive index of sputter deposited a–Si films

Abstract

Amorphous Si (a–Si) films, sputter deposited under lower Ar pressure conditions, include numerous Ar atoms and exhibit high refractive indices, higher even than that of crystalline Si (c–Si), notwithstanding their lower Si densities. Such behavior in the refractive indices of the films was inconsistent with the conventional explanation considering only the density term in the Clausius–Mossotti relation. In the present study, the contribution of the polarizability changes of the films is proposed in order to account for the result in the refractive index. The molecular orbital (MO) calculations and experiments reveal that the polarizability of the a–Si film is sensitive to change in the angular distortion of the Si–Si bonds brought on by changing deposition conditions. The incorporated Ar atoms are found to cause the distortion in the Si network, leading to higher refractive indices for the less densified films.