THE OPTICAL BANDGAP OF MULTILAYERS OF A-SI-H AND ALLOYS - A COMPARISON OF EXPERIMENT AND THIN-FILM OPTICS CALCULATIONS

The optical transmission of compositional multilayers of a-Si:H and alloys with Ge and C has been calculated using coherent multilayer thin-film optics. The optical bandgap in dependence on different electronic well and barrier widths was derived from the calculated spectra in the same way as from experimental ones. Si-SiGe and Si-SiC multilayers of about 1-mu-m total thickness and consisting of up to 400 periods were deposited by fast gas switching. Their periodicity was verified by X-ray scattering, and the optical gaps compared to those derived from calculated spectra. Observed shifts of the gap in calculations agree well with experimental values without taking into account a quantum size effect. Remaining differences may be explained by an interfacial transition layer between well and barrier of about 10 angstrom and/or a possible contribution to the shift due to redistribution of hydrogen in its bonding configuration. X-ray and IR data support the view that the width of the transition region is clearly more than the inter-atomic spacing.