An improvement in the intrinsic properties of deposited semi-conductor layers is provided by an efficient barrier against ion migration while maintaining or enhancing critical optical properties. Other coatings can act as buffer layers between the semi-conductor layers.

Al₂O₃ has excellent ion barrier properties. It is more common to use SiO₂ for this propose, but Al₂O₃ is superior, and may render the glass more stable to exposure of super elevated temperatures (above 600o C).