High-Transmittance Phase-Change Media with TiO₂ Film for Rewritable Dual-Layer Blu-ray Discs

Abstract

We have developed a semitransparent layer structure that is ideally suited to the laser-incident-side (front-side) recording layer of a rewritable dual-layer Blu-ray Disc (BD-RE). A TiO 2 top film and a memory film of GeTe–Sb 2 Te 3 characterize this layer structure. The TiO 2 film has a large refractive index, which works to increase the transmittance of the entire recording layer to more than 50%. It simultaneously increases the reflectivity contrast, defined by ( R cry - R amo )/( R cry + R amo ), where R cry and R amo ( R cry > R amo ) are the reflectivities of the optical disc in the crystalline and amorphous parts, respectively. The GeTe–Sb 2 Te 3 memory film has optical constants that are appropriate for balancing the transmittance between the crystalline part ( T cry ) and the amorphous part ( T amo ) in the laser wavelength region of approximately 405 nm. Our experimental dual-layer disc shows high transmittances of T cry =51.6% and T amo =52.0% and a high reflectivity contrast of 0.71 in the laser-incident-side layer. Dynamic examination showed that both recording layers have a good jitter value, satisfying the BD-RE specifications; more specifically, the laser-incident-side layer jitter value is 7.4% (spec.: 8.5%) and the back-side layer jitter value is 6.4% (spec.: 6.5%) through the front-side layer.