High-transmittance phase-change media with TiO2 film for rewritable dual-layer blu-ray discs

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 TiO2 top film and a memory film of GeTe-Sb2Te3 characterize this layer structure. The TiO2 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-Sb2Te3 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.